methods for preparation of catalytic materials

7/27/2019 Methods for Preparation of Catalytic Materials

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Chem. Rev. 1995, 95,477-51 0 477

Methods for Preparation of Catalytic MaterialsJames A . Schwarz*

Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244-1190Cristian Contescu and Adriana Contescu

Institute of Physical Chemistty, Romanian Academy, Spl. lndependentei 202, Bucharest 77208,RomaniaReceived May 2, 1994 (Revised Manuscript Received October 31, 1994)

ContentsI. IntroductionII. Three-Dimensional ChemistryA. Liquid-Liquid Blending

1 Precipitation2. Coprecipitation3. Complexation4. Gelation5. Crystallization8. Solid-Solid BlendingC. Liquid-Solid Blending

A. Epitaxial Metallic FilmsB. Unsupported Bulk MetalsC. Amorphous AlloysD. Colloidal MetalsIV. Two-Dimensional ChemistryA. Mounting Dissolved Precursors from AqueousPhase1 Impregnation2. Homogeneous Deposition-Precipitation3. ton Exchange4. Colloidal Events: Electrostatic Adsorption5. Coordinative Events: Grafting by6. Molecular Events: Formation of Chemical

B. Mounting Dissolved Precursors from OrganicMediaC. Mounting Precursors from the Vapor PhaseD. Mounting Precursors from the Solid PhaseE. Mounting Preformed Active Phases

111 Solid Transformations

Hydroxyl InteractionsCompounds

V. The Next DimensionVI. AcknowledgmentsVII. References

477480480481482483484486489490490491491492492493494495496498499500501502503503504505506506

I. IntroductionCatalytic materials exist in various forms and theirpreparation involves different protocols with a mul-titude of possible preparation schemes, many timeslarger than the number of known catalysts. More-over, preparation of any catalyst involvesa sequenceof several complex processes, many of them notcompletely understood. As a result, subtle changesin the preparative details may result in dramaticalteration in the properties of the final catalyst. Ourobjective in this review is to provide the various

0009-2665/95/0795-0477$15.50/0

preparative procedures available t o create catalyticmaterials. To accomplish this objective, we soughtthe most recent literature. Our review, therefore,focuses on research reported mainly in the past fiveyears.The goal of a catalyst manufacturer is t o produceand reproduce a commercial product which can beused as a stable, active, and selective catalyst. Toachieve this goal, the best preparative solution issought which results in sufficiently high surface area,good porosity, and suitable mechanical strength. Thefirst of these, surface area, is an essential require-ment in that reactants should be accessible t o amaximum number of active sites. The properties ofa good catalyst for industria l use may be divided, atleast for the purpose of easy classification, into twocategories: 1)properties which determine directlycatalytic activity and selectivity, here such factors asbulk and surface chemical composition, local micro-structure, and phase composition are important; and2) properties which ensure their successful imple-mentation in the catalytic process, here thermal and

mechanical stability, porosity, shape, and dimensionof catalyst particles enter. The requirements whichare fundamental for catalyst performance generallyrequire a compromise in order t o produce a materialwhich meets the contradictory demands imposed byindustrial processes. An acceptable solution is typi-cally ascertained by a trial-and-error route. Catalyticmaterials become catalysts when they are used inindustrial pr0cesses.l A way this can be realizedoccurs when the variety of methods used t o preparecatalytic materials are viewed in relation to theirsuccessful implementation in commercial applica-tions.In our attempt t o develop the elements for a

scientific basis for catalyst preparation, we return t othe fundamental blending and mounting proce-dures used to prepare catalytic materials. Figure 1is a simplified diagram which summarizes the tra-ditional methods used for the preparation of hetero-geneous catalysts. The vertical ordering takes intoaccount the fact that the final catalyst is a solid phasewith new properties which have t o be acquired andstabilized during the preparation process while thehorizontal delineation depicts the various methodsfor blending and mounting o produce the cata-lytic material. A noticeable discontinuity does de-velop here, however, because some preparative pro-cedures can fit into both cases.0 995 American Chemical Society


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470 Chemical Reviews, 1995, VoI. 95, No. 3

Jim Schwarz was born in the early 194Os, experienced the 'Sputnik eraof the 1950% and finally grew up in California in the 1960s where hereceived his Ph.D. from Stanford University. After a series of post doctoralpositions at Cambridge University, U.C. Berkeley, and (back again)Stanford, he established his industrial credentials at Chevron Researchand then Exxon. in 1979. he was appointed Associate Professor ofChemical Engineering and Materials Science at Syracuse University. Ina timely fashion, his title was changed to Professor; his scientific interestsfocus on phenomena occurring at interfaces. He fills his daily life with abalance between pursuits of the mind and the body.

Cristian Contescu was born in Galati and raised in Tulcea, Romania, twocnies on the border of the Danube. AHer he received a B.A. i n PhysicalChemistry from the University of Bucharest (1971), he joined the Instituteof Physical Chemistry in Bucharest and received a Ph.D. from thePolyiechnical Institute in Bucharest (1979). His interest has alwaysfocused on the study of interfaces but has shined from phenomena atthe gas-solid intelface studied by field emission microscopy to theproblems of catalyst preparation and phenomena at the solid-liquidinterface. In 1992, he joined Professor Schwarz's group at SyracuseUniversity as a visiting research associate. He enjoys classical music,literature, and fine arts.In Figure 1, two preparation routes define theextremes of tradi tional procedures used in cata lystpreparation: precipitation (with the variant of co-

precipitation) and impregnation (with such variantsas ion exchange, deposition, and grafting). In theprecipitation route, a new solid phase is obtained bythe blending of proper reagents (precipitatingagents)from a liquid medium; the resulting precipitate istransformed in subsequent preparation stages intothe active catalyst. During these transformations,both the mechanical properties of the catalyst andthose intrinsically related t o the catalysts' perfor-mance have t o be considered simultaneously. Incontrast, in the impregnation route, a solid phasepreformed in a separate process is used as a support,and the catalytically active material is mounted andstabilized on it. In this way, at least a part of the

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Adriana Contescu was born in Argetoaia, Romania, and raised in Oltenia,a rich wu nt y in southern Romania. She received her B.A. in InorganicChemistry from Bucharest University (1971) and a Ph.D. from thePolytechnical Institute in Bucharest (1984). AHer joining the Institute ofPhysical Chemistry in Bucharest in 1979, her research concentrated onthe chemistry of polynuclear inorganic complexes and nonconventionalroutes for the preparation of mixed oxides. This review paper is a resultof her 1993 visit to Syracuse University. Her spare time pursuits includeneedlework, gardening, and playing with her dog.mechanical properties of the final catalyst'are con-trolled by the preexisting support, and the prepara-tion process is basically focused o n the introductionof the catalytic compound(s). Between these twoextremes there lies methods which are best charac-terized as solid transformation. Here physical andchemical processes are used to reconstruct a solid intoa form that meets the demands imposed by theprocesses in which they will be used.To establish guidelines for the development of ascientific basis for catalyst preparation is perhaps avery ambitious goal. We would be required first toanswer the following rhetorical questions:*What are the properties which determine theperformance of a catalytic material?

How can these properties be introduced, devel-oped, and/or improved during preparation?The answer to these questions involves a compre-hensive discussion of the theories of catalysis, whichis beyond the scope of our review. We will attempt,instead, to provide a rationale for each reader toanswer these questions on the basis of hisher owninterests. We star t our discussion by describing thefundamental steps in producing bulk catalysts and/or catalyst supports. The fundamental processesinvolved are those derived from traditional three-dimensional chemistry. The topic areas will includesingle-component and multicomponent metal oxides.Unsupported metallic catalysts are formed by trans-formations involving physical or chemical processes,and the preparation methods for this class of materi-als will be discussed next. Our attention will thenturn t o the preparation of supported catalytic materi-als. The main topics to be discussed will be thoserelated to the interaction between the support andthe active phase when they are put together t ogenerate the catalyst. In this approach, we exploitthe virtually unexplored field of surface, or two-dimensional, physical chemistry. The materials con-sidered include dispersed metals and alloys andcomposite oxides.We recognize that this organization might seemarbitrary and tha t the reader might equally propose


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Methods for Preparation of Catalytic Materials Chemical Reviews, 1995, Vol. 95, No. 3 479

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480 Chemical Reviews, 1995, Vol. 95, No. 3a different classification scheme. The differencebetween supported and unsupported (or bulk) cata-lysts is not always apparent . Even bulk catalysts orsupports, usually thought to have uniform chemicalcomposition, may present a multiphase structure thatmight be the result of either doping, promoting,surface o r bulk segregation, or even the effect of thereaction environment on the catalyst. We prefer theabove organization in that, from one point of view,there are only two main routes for the preparationof almost all catalysts. These can be divided into thetwo categories: methods in which the catalyticallyactive phase is generated as a new solid phase byeither precipitation or a decomposition reaction, andmethods in which the active phase is introduced andfixed onto a preexisting solid by a process which isintrinsically dependent on the surface of the support.

Schwarz et al.

Il. Three-Dimensional ChemistryRecent years have witnessed marked progress inthe preparation of stable catalytic materials, manyof them with potential applications as catalysts. Thissuccess has been achieved by either the selection of

a suitable support or by choosing a proper method ofpreparation, or by a combination of both approaches.The simplest kinds of catalysts, from a structuralpoint of view, are single phase catalysts, such as bulkmetals and alloys, bulk oxides, sulfides, carbides,borides, and nitrides. These materials are, more orless, uniform solids at the molecular level that exhibitcatalytic properties on their external surface. There-fore, these materials are preferably used in a physicalform which allows for a maximum development ofcontact of the surface of the material with itsenvironment. To this end, preparation methods areselected which avoid excessive heat treatments whichwould result in the system acquiring a more stablelower surface energy state a t the expense of its activesurface.Bulk oxide catalysts, either single metal or multi-metal, used in industrial processes are usually in theform of powders, pellets, o r tablets, with eitheramorphous or polycrystalline structure. The mostcommon method used for preparation of bulk oxidecatalysts is the (co)precipitationof a precursor phase,followed by thermal transformation that leads t o theoxidic phase. The ceramic method involving grindingand firing mixtures of oxides is not very convenientfor preparation of oxide catalyst because of the hightemperatures needed. Thus, the trend in the devel-opment of preparation methods has witnessed effortst o eliminate the high-temperature treatments of thecoprecipitated materials (such as calcination of mix-tures of hydroxides and decomposition t o oxides)which affect the solid state reactions that producethe intimately mixed oxide phase that acts as acatalyst. Several alternative preparation routes thatenable a better mixing of the components have beenproposed. A method of continuous homogeneousprecipitation was developed, wherein th e precipitat-ing agent (hydroxyl ions in the classical coprecipita-tion method) is slowly and continuously generatedin the synthesis medium by a controlled hydrolysisprocess (such as hydrolysis of urea). The advantageof slow precipitation is a more efficient mixing of the

components in the precipitated product. The sol-gel method, although related t o the coprecipitationmethod, provides better control of the texture of theresulting catalyst and ensures an increased unifor-mity of the product. The method consists in forma-tion of a colloidal dispersion of the metal constituents,usually by hydrolysis of metal alkoxides. The col-loidal solution is then subjected t o gelation by eitherchanging the pH, the temperature, or the electrolyte.The resulting gel is then heat treated to remove thesolvent. Decomposition of coordination compounds,including polynuclear compounds, is another pre-parative route that start s from a precursor where themetallic elements are intimately mixed at the mo-lecular o r at the atomic level. Among the metalcomplBxes that can be decomposed at relatively lowtemperatures are oxalates, formates, citrates, andcarbonyls.Bulk sulfide catalysts and mixed sulfide catalystsare prepared most commonly by either direct sulfi-dation (i.e., reaction with hydrogen sulfide) of oxides,halides, or other metal salts. The direct method mayrequire the use of high temperatures. A secondvariant is the decomposition of a sulfur-containingprecursor, such as a thiosalt, which is obtained bylow-temperature precipitation. A type of low-tem-perature coprecipitation is homogeneous sulfide pre-cipitation, wherein the mixing of the metal salts ismade before any addition of the precipitant.Recently,a new genre of single phase catalysts hasemerged in which the entire solid rather than justthe external surface is involved in catalysis. The newmaterials are crystalline solids which contain activesites uniformly distributed throughout their bulk atthe intracrystalline level. This family of uniformheterogeneous catalysts, generally referred to asmolecular sieves, includes microporous zeolites, alu-minum phosphates, with metal- and silicon-substi-tuted analogs, layered compounds such as clays andtheir pillared variants, layered oxides with perovskitestructures, and heteropolyacids with a liquid-likebehavior. The possibilities for preparation of materi-als in this class are vast since they exploit thevirtually unlimited number of ways to link togetheratomic units in a crystalline or polymeric structure .Their methods of preparation consist of a combina-tion of chemical (precipitation, leaching) and physical(supercritical crystallization) procedures.The common features of all the preparation meth-ods summarized above for bulk catalytic materialsis the use of traditional methods and techniques frompreparative chemistry, such as precipitation, hy-drolysis, and thermal decomposition. The chemistryinvolved during these preparation steps does notdiffer much from that taught in classical handbooksof analytical o r inorganic chemistry. These processesinvolve mixing of solutions, blending of solids, pre-cipitation, filtration, drying, calcination, granulation,tableting, and extrusion. In other words, the chem-istry involved is three dimensional with the meaningthat it is isotropic with respect t o the container inwhich it is done.A . Liquid-Liquid Blending

The method of precipitation is the best known andmost widely used procedure for synthesis of both


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Methods for Preparation of Catalytic Materialsmonometallic and multimetallic oxides. Precipitationresults in a new solid phase (precipitate) that isformed discontinuously (i.e., with phase separation)from a homogeneous liquid solution. A variety ofprocedures, such as addition of bases or acids, addi-tion of complex-forming agents, and changes of tem-perature and solvents, might be used t o form aprecipitate.

The term coprecipitation is usually reserved forpreparation of multicomponent precipitates, whichoften are the precursors of binary or multimetallicoxidic catalysts. The same term is sometimes im-properly used for precipitation processes which areconducted in the presence of suspended solids.Depending on the particular application, the newlyformed solid phase may be further subjected t ovarious treatments, such as aging and hydrothermaltransformation, washing, filtration, drying, grinding,tableting, impregnation, mixing, and calcination.During all these preparative steps, physicochemicaltransformations occur which can profoundly affectthe structure and composition of the catalyst surfaceand even its bulk composition. If the adage thecatalyst remembershow it was prepared, even afterbeing subjected t o various heat treatments at el-evated temperatures is valid, then any cause-and-effect correlations that can eventually be madebetween the precipitation procedures and the finalcharacteristics of the catalyst becomes significant.

1. PrecipitationA scientific approach t o the preparation of catalystsby precipitation routes was introduced by M a r ~ i l l y . ~ , ~The formation of the precipitate from a homogeneousjiquid phase may occur as a result of physicaltransformations (change of temperature or of solvent,solvent evaporation) but most often is determined by

chemical processes (addition of bases or acids, use ofcomplex forming agents). In almost all cases, theformation of a new solid phase in a liquid mediumresults from two elementary processes which occursimultaneously or sequentially: 1)nucleation, i.e.,formation of the smallest elementary particles of thenew phase which are stable under the precipitationconditions; and (2) growth or agglomeration of theparticles.Marcilly2a stressed the importance of supersatu-ration, among other factors such as pH, temperature,nature of reagents, presence of impurities, andmethod of precipitation in determining the morphol-ogy, the texture and the structure of the precipitates.

For example, under conditions of high supersatura-tion, the ra te of nucleation of solid particles is muchhigher than the rate of crystal growth and leads tothe formation of numerous but very small particles.Under the condition when the critical nucleation sizeis very small, only a metastable and poorly organizedphase can develop; this may further change t o a morestable phase during the hydrothermal treatment ofthe precipitates.Obtaining high supersaturation conditions is adifficult task in practice because of the naturalevolution of the system toward a decrease of super-saturation by nucleation of solid particles and con-sumption of reagents. High levels of supersaturation

Chemical Reviews, 1995, Vol. 95, No. 3 481can only be obtained for a short time and withinlimited volumes of solution. The problem of obtain-ing a homogeneous precipitate with respect to thesize and structure of the particles reduces t o th at ofachieving a uniformly high level of supersaturationthroughout the liquid before the nucleation starts,which may be quite difficult because of mass and heattransport 1imitationse2

The chemical and physical properties of the pre-cipitates kept in contact with their mother liquor maychange, often substantially, due t o secondary pro-cesses taking place in the suspension. One of theseprocesses, known as Ostwald ripening, leads t o anincrease in the particle size of a precipitate. Becausethe solubility increases with decreasing particle size,small particles begin t o dissolve and large crystalscontinue t o grow. Another process which takes placeduring aging of precipitates is agglomeration ofcolliding particles as a result of either Brownianmotion or imposed mechanical forces.The most common catalysts derived from precipita-tion are aluminas. In order to emphasize the sig-nificance of the variables described above on the

physicochemical properties of the finished material,we will devote some effort t o outline the proceduresused to formulate aluminas. Because of its industrialimportance, the preparation of aluminas of controlledporosity and surface area continues to be the focusof a large number of investigation^.^-^Studies on the preparation of alumina in theabsence of additives showed that the pore sizedistribution and the surface area are determinedmainly by conditions of precipitation and aging.Development of these properties is due t o the inter-conversion of amorphous hydroxide, pseudoboehmite,and bayerite formed during pre ~ipi ta ti on .~Washing and drying were found t o have little

influence on texture for samples precipitated fromammonia and aluminum nitrate but contributed t othe enlargement of pores when NaOH was used inprecipitation. More control is possible by the use ofadditives.* Alcohols added before precipitation ofaluminum hydroxides only had an effect on the poresize when their adsorption on the precipitate wasstrong enough t o replace t he solvent barrier at thesurface of precipitates. With these additives, thesolubility of precipitates is decreased, leading t odecreased Ostwald ripening and thus encouragingaggregation by particle bridging. Alcohol washingafter precipitation produces higher surface areas andhigher mesoporosity due to lower surface tension andless pore collapse during ~al ci na ti on .~The thermal and physical characterization of theconversion of pseudoboehmite to y-Al203 were re-viewed: and the relationship with the manufacturingroute of the pseudoboehmite powder was shown. Ingeneral, physical properties like particle size andshape, crystallinity, and porosity have a distinctinfluence on the thermal behavior of pseudoboehmitepowders. Better characterization of thermal andphysical properties has allowed one t o improvecatalyst manufacturing at the industria l level. Unitoperations such as mix-mulling, extrusion, drying,and calcination are clearly affected by powder char-acteristics. o


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482 Chemical Reviews, 1995, Vol. 95, No. 3The calcination step may induce fur ther changesin the texture of the finished supports. Underconventional operation conditions, y-Al203 is stable,but at temperatures between 1250 and 1350 K aphase transformation through metastable 6- and

O-Al203 leads to formation of a - A l 2 0 3 . The processis accelerated by steamll and results in a drastic dropin surface area which is caused by sintering ofprimary particles. The versatility of alumina t o beproduced with a broad range of surface areas andpore size distributions is in part due t o the phasetransformations during calcination.A systematic s t ~ d y ~ , ~f aluminum oxides obtainedby heat treating of y - A l 2 0 3 showed th at a monodis-perse structure is preserved below 1375 K, with aslight increase in the average pore radius. Formationof the a -A l 2 0 3 phase is related t o the generation of anew system of wider pores that again becomesmonodispersed when the a-phase is completelyformed.6 A relationship between porosity and me-chanical strength was proposed for alumina catalyst

A number of studies report methods t o increase thethermal stability of y-Al203 particles by introducingvarious additives. The subject was recently reviewedin relation to preparation of stable materials for high-temperature combustion.14 For example, it wasreported15J6 hat several ions (In3+,Ga3+,and Mg2+)have an accelerating influence while others Z1R+,Ca2+,Th4+,La3+)have an inhibiting action duringthe transformation of aluminas t o the a-phase. Theeffect of thermal stabilizing modifiers is due t osurface nucleation of stable compounds, which byinteraction with the underlying alumina,17J8 esultsin the formation of an aluminate surface layer whichprevents transformation of y t o a aluminalgbut mayalso modify the Lewis acidity of alumina.20 Accordingto other results,21,22ddition of alkaline ear th metals(Ca, Sr, Ba) increased the ability t o preserve a highsurface area 2 5 m2g-l) after calcination at 1700 K.Since small Ba0.6Al203 crystallites prepared througha coprecipitation route had a similar sintering resis-tance, it was concluded that formation of bariumhexaaluminate is a promising option for stabilizingcombustion catalyst supports.14

An alternate method t o achieve the thermal sta-bilization of alumina without foreign additives wasalso reported.23 Since the transition of metastablephases t o a - A l 2 0 3 occurs predominantly at the con-tact between primary particles, the key for suppress-ing the rate of sintering without additives is prepar-ing active aluminas in a morphological state in whichthe area of contact between primary particles isminimized. Alumina prepared by fume pyrolysis ofsols consists of fibrillar boehmite, approximately 100nm in length and 10 nm in diameter. After calcina-tion at 1473 K for 30 h, the material maintained asurface area of 50 m2 g-l and still consisted of fibrils.This was ascribed t o the suppression of the phasetransformation t o a - A l 2 0 3 .

New classes of catalyst supports, which are usedin demanding reactions, are beginning t o receiveattention. For example, the conditions for prepara-tion of magnesium oxide, which is the catalyst usedfor oxidative coupling of methane t o ethane and

supports.12J3

Schwarz et al.ethylene, has gained the recent attention of severalresearcher^.^^-^^ In the case of Mg(OH)2 as thestarting reagent, washing with alcohol of the hydrox-ide precipitate leads to a drastic decrease in surfacearea of the calcined MgO. The effect was ascribedt o formation of surface alkoxides and induction ofparticle-particle bridges through surface condensa-tion reactions.25 This process favors the developmentof order in the precipitate. The bridges formedduring washing were maintained through the calci-nation step. Also, the morphology of Mg(OH)2 pre-cipitates was found t o be dependent on whether thepH during precipitation and aging was above orbelow the isoelectric point (pH = 12). This demon-strates the influence of the electric charge of primaryparticles on their tendency toward aggregation.2. Coprecipitation

In the synthesis of multicomponent systems, theproblems are even more complex. Coprecipitationrarely allows one t o obtain good macroscopic homo-geneity. In a system with two or more metalliccompounds, the composition of the precipitate de-pends on the differences in solubility between thecomponents and the chemistry occurring duringprecipitation. Generally, under the conditions ofeither a slow precipitation rate or poor mixing withinthe reaction medium, coprecipitation is selective andthe coprecipitate is heterogeneous in composition.Subsequent t o formation of the coprecipitate, hydro-thermal treatments which transform amorphousprecipitates t o crystalline materials with improvedthermal stability and surface acidity may be carriedThis procedure is widely applied to preparemolecular sieves.

Depending on the composition of the precipitateformed, two chemical routes should be distinguishedin the coprecipitation procedures. The simplest caseis that of sequential precipitation of separate chemi-c a l compounds. This occurs whenever there is a largedifference in the solubility products of the compoundsinvolved. The so-called coprecipitates of hydrox-ides, hydroxo carbonates, oxalates, and formatescontaining two o r more different metals are generallynonhomogeneous in composition and only very sel-dom generate a homogeneous mixed oxide28by solidphase reactions at high calcination temperature.Doping or substitution of ions in these precipitatesis difficult because of the different reactivities in-volved.

The second possibility is the formation by copre-cipitation of a well-defined chemical compound whichmight serve as a chemical precursor from which thefinal catalyst is ~btained.~~-~lhe intermediatecompound must be easily decomposed under mildcalcination. This route is preferred whenever abetter intimate mixing of the catalyst components isdesired. The metal ratio in the precursor compoundis, however, restricted t o a quite rigid stoichiometry.Crystalline stoichiometric precipitates formed byseveral metal oxyanions (vanadates, chromates, tung-states, and molybdates) and a second metal cationmay be further used t o obtain an intimate interdis-persion of the two metals. As an example, theactivity of Cd Cr catalysts depends on the amount of


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Methods for Preparation of Catalytic Materialscopper chromate, CuCr04, formed during their prepa-ration, and this is the precursor of the most activesites of the final catalyst.32 In another example,precipitation from nitrate solutions of Fe, Co, and Biwith ammonium heptamolybdate gave molybdenumheteropolycompounds containing Keggin-type anions,[MMo6024H6], where M = Bi or Fe, which are theprecursors of the corresponding molybdates by solidphase reactions.33

Other intermediates extensively studied as catalystprecursors comprise the class of mixed-metal hy-drated hydroxo carbonates with a layered st ructure.Thus, during preparation of active copper catalystsused for synthesis of hydrocarbons or methanol byhydrogenation of C0,34 the copper phase must beobtained highly interdispersed with at least one otheroxide component. This structure stabilizes the verysmall Cuo particles and favors further interaction ofcopper with the host oxide.35 A high degree ofhomogeneity at the atomic level for this class ofcatalysts may be achieved by decomposition of singlephase precursors, such as either the hydroxo carbon-ates with the aurichalcite structure , (Cu,Zn)5(C02)2-(OH)6, from which binary Cd Zn catalysts are pre-pared or with the lamellar hydrotalcite-type crystallinefor preparation of highly active ternary catalysts, C dZn/(Al,Cr,Ga,Sc). The details of precipitation of thesesingle phase precursors involve a careful control ofpH and the rate of pr e~ ip it at io n; ~~he morphologyof the precursor may influence the degree of inter-dispersion of the final multiphase catalyst.37 In thesesystems, because all elements are homogeneouslydistributed in the hydrotalcite phase, no surfacesegregation is observed and pseudomorphic thermaldecomp~sit ion~~eads t o a spinel-type oxide. Thelimits between which the ratio of metals in thecatalyst may be varied depends on the stoichiometryand structure of the single phase precursor: for theCu/Zn binary aurichalcite, it may be changed be-tween quite large limits (0.02 t o 0.301, while thestructure of the ternary hydrotalcite allows the C dZn/Me**I atio t o be varied within much narrowerlimits; a typical value for single phase precursors ofcopper-based catalysts is 30/45/25.39 The hydrotal-cite-like coprecipitated precursors were recently usedas intermediates for the preparation of other non-stoichiometric spinel-type catalysts, with the generalformula M1+~Cr2-2~/304M = Zn, Cu, Co); they areemployed as catalysts for specific hydrogenationreactions.38,40

With a proper selection of metals and complexingagents, precipitation of mixed-metal polynuclearcoordination compounds is possible. The use ofcoordination compounds as raw materials is a non-conventional procedure t o prepare mixed oxides bya mild thermal deco mp~ sit ion .*~ -~~ecent literaturein inorganic chemistry often makes reference t osynthesis and characterization of several types ofbinuclear coordination compounds with molecularlyorganized structures th at contain metals of interestfor preparation of catalysts. For example, mixed-metal complexes in the general series {NBu4[MCr-OX ^]}^ (where NBu4+= tetrabutylammonium ion,ox2- = oxalate ion, and M = Mn2+,Fe2+,Co2+,Ni2+,

structure, (CU,Zn)s(A1,Cr,Ga,SC)z(C03)(oH)~6,sed

Chemical Reviews, 1995, Vol. 95, No. 3 483Cu2+,Zn2+) orm a three-dimensional structure com-prised of alternate arrays of Cr(II1) and M(I1) met-a l ~ . ~ ~lso, in the series of p-oxo-trinuclear mixed-metal carboxylate complexes,45M2TT1M110(ac)6L31nL,where MII1= Fe, Co, Cr, MI1= Fe, Co, Ni, Zn, Mn,Mg, and L = py, H20, the molecular structure iscomposed of trinuclear, oxo-centered M2111M110 nits.It is k n o w n that thermal decomposition of poly-nuclear coordination compounds of the latter typeyields mixed oxides with spinel structure^.^^ Thissuggests the use of other coordination compoundssuch as those mentioned above as potential precur-sors for the binary mixed oxides in the system CuO-ZnO-Cr203. For the moment, preparation of ternarymixed-metal compounds remains a more difficulttask.Polynuclear mixed metal complexes deserve moreattention as precursors for the preparation of cata-lysts. This methodology has the advantage tha t thecomponent metal ions are intimately bound in themolecular structure of the polynuclear compound anda homogeneous mixed-oxide phase or a compositeoxide is more easily formed after either a milddecomposition or a hydrothermal treatment at mod-erate temperatures.Finally, a new approach to the precipitation methodis the use of organic solvents as precipitation media.The colloid chemistry is not easily extrapolated fromthe aqueous phase t o organic systems. In addition,organic solvents pose practical problems t o catalystmanufacturing, but these difficulties can be offset bythe special properties of catalysts precipitated fromorganic so1vents.l As an example, two procedures canbe summarized for the preparation of VPO catalystsfor selective oxidation of butane t o maleic anhydride:47 (a) reduction of V205 o r NH4V03 in aqueousmedium, followed by addition of H3P04 or (b) reduc-tion of V205 in a n organic medium, using isobutyl o risopropyl alcohol, followed by addition of H3P04. Thecatalysts prepared in organic media are more active.A possible explanation is that the precursor obtainedby precipitating v205 and HBPOI in an organicmedium has a macrostructure consisting of sponge-like spherical particles which are not obtained byprecipitation in aqueous media.48 Since the catalystis activated through a topotactic transformation:

2VOHPO,.(H,O),~, - V0),P,O7 + 2H,O 1)it is conceivable th at the catalyst obtained in organicmedia has more accessible active sites than thecatalyst obtained from the aqueous route. The dif-ference in the texture corresponds t o a high-perfor-mance catalyst which has found commercial recog-nition. In this example, the precursor acts as animproved matrix for the crystalline growth of theoxide active phase during the topotactic transforma-t i ~ n . ~ ~he properties th at the solid will develop asa final catalyst are strongly affected by all structuraland morphological changes which occur during thetopotactic tran~formation.~O-~~3. Complexation

The composition of the homogeneous phase can beused to exploit the high binding affinity of metal ions


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484 Chemical Reviews, 1995, Vol. 95, No. 3which will result in catalytic materials with proper-ties that differ from those derived from (co)precipi-tation procedures. The complexation method makesuse of chemical reactions which transform slowly andwithout physical discontinuity (i.e., without phaseseparation) the homogeneous solution of catalystprecursors into a homogeneous, amorphous phase,with either a glassy, jelly-like, o r foamy appearance.This precursor is then dried and decomposed to yieldbetter intermixed and more highly dispersed oxidesthan those prepared by the usual precipitation routes.The procedure has also been called the method of anamorphous intermediate.53

To obtain a smooth gelation of the homogeneoussolution of catalyst precursors, they are complexedo r chelated with multifunctional organic reagentsthat are capable of entering in a successive series ofintermolecular polycondensation reactions. The ge-lation process results in a three-dimensional organicnetwork with the metallic components entrapped inthis st ructure. The organic matrix is responsible forthe textural properties such as pore structure orsurface area of the catalyst.The search for the appropriate catalyst precursors,solvents, and complexing agents lead t o the develop-ment of variants of the chemical mixing method.According t o the original proposal by Courty andM a r ~ i l l y , ~ , ~ ~he metallic elements are added aswater-soluble salts, and various a-hydroxy acids areused as complexing agents. Because citric acid wasmore frequently used (several other acids, such asmalic, tartaric, glycolic, and lactic can also be equallyemployed), the complexing method has also beencalled the citrate method. This method was re-cently used t o prepare oxide solid solution Laog-

Sr0.~01.45atalysts for the methane coupling reac-t i ~ n . ~ ~n the ternary Cu/Co/Al system, catalystswere prepared with atomic ratios (Cu + Co)/Al andCu/Co covering a much broader range than thatallowed by the rigid stoichiometry of the hydrotalciteprecursors that could be formed by c~ pr ec ip it at ion. ~~

A more general procedure, named by the authorsthe chemical mixing method, was proposed byMizukami and N i ~ a . ~ ~ , ~ ~hey introduced metalliccomponents as either soluble salts (nitrates, chlo-rides, acetates) o r metal-organic compounds (alkox-ides o r P-diketone complexes) and used polar solventswith at least two complex-forming functionalities(diols, keto alcohols, and amino alcohols) t o obtainhomogeneous solutions. In the gelation step, coagu-lation occurs by hydrolysis and intermolecular con-densation reactions and a three-dimensional poly-meric network is finally produced. At this stage, thecomponents are uniformly incorporated within eachother and the homogeneity of the initial solution ismaintained (as schematically shown in Figure 2). Themixed oxides obtained by decomposition were muchmore effective catalysts than their counterpartsprepared by conventional precipitation o r kneading.

A related procedure was used by Busca andL o r e n ~ e l l i ~ ~ - ~ ~o prepare amorphous alumina withzeolite-type microporosity. They reacted aluminumnitrate with organic agents (glycerol, tartaric acid)and decomposed the spongy bulky solid that resulted.Addition of other glass-forming elements, such as

Schwarz et al.

I I IOROHO R * O M ~ X R O H 0 H X :[ H OH O R OH 0- - O 0- MI-0 4 0-lk- 0- M- 0- M- OROH

Figure 2. Mixed-metal gel depicting homogeneity ofstructure.phosphorous or boron, inhibited both the crystalliza-tion and the y - to a-alumina phase transition. Highsurface area, amor hous aluminas, with controlledprepared by this method.

A common characteristic of all the above variantsof the complexingo r chemical mixing methods isthe use of organic molecules with multiple chemicalfunctionalities as templates for formation of thethree-dimensional network during the gelation step.Removal of the organic ingredient is a critical stepduring catalyst activation. A highly exothermic,uncontrolled decomposition was reported for glassyprecursors containing metals active for oxidationreactions.61 This could be avoided if decompositionwas carried out under an inert at m ~ sp he r e . ~ ~n-complete burning of the organic ingredient duringthermal decomposition of the amorphous precursorusually results in uncontrolled carbon contaminationof the catalyst.53 These inconveniences can be elimi-nated by using sol-gel methods, in which the gela-tion step is better controlled.

pore size (10-15 and surface acidity, can be

4 GelationIn contrast with the (co)precipitation route, which

is a discontinuous transformation, the gelation route(also known as the sol-gel method) is a homogeneousprocess which results in a continuous transformationof a solution into a hydrated solid precursor (hydro-gel). Sol-gel methods have been recognized for theirversatility which allows control of the texture, com-position, homogeneity, and structural properties ofthe finished solids. The applications of the sol-gelmethod to catalyst preparation were reviewed re-cently62with special emphasis on the broad range ofpossibilities offered by this method t o prepare tai-lored materials, such as dispersed metals, oxidiccatalysts, and chemically modified supports.The nanoscale chemistry63 involved in sol-gel

niethods is a more straightforward way t o preparehighly divided materials. Within the general contextof sol-gel methods, it is possible t o find examples ofall major types of catalysts. Hydrosols are formedduring precipitation of hydrous oxides. The networkthat results by aggregation of primary sol particlesmay either extend quasiinfinitely throughout thevolume of the specimen (gel)o r may be discontinuous(flocculates). Gels dried by simple evaporation of theliquid which interpenetrates their framework alwayssuffer from pore shrinkage with a concomitant ir-reversible reduction of their surface area (xerogels).The collapse of the pore structure is caused bymechanical forces due t o retreating water menisci in


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Methods for Preparation of Catalytic Materialsthe pores. It can be avoided by either replacing waterwith another liquid with lower surface tension beforedrying or by removing water vapor a t a temperaturehigher than its critical point (aerogels). The majorbreakthrough in the sol-gel methods for catalystsynthesis came with the discovery of new and fastermethods t o produce aerogels which subsequently leadto their application in large-scale synthesis.The method proposed in 1968 by Nicolaon andT e i ~ h n e r ~ ~or preparation of silica aerogels wasimmediately extended t o the synthesis of many othersystems of mono- or multicomponent oxides. Themethod is based on the hydrolysis and gelling (forinstance by controlled addition of water) of alkoxidesor other reactive compoundsin alcoholic solution^.^^-^^The chemistry of the processes which occur duringthe sol-gel synthesis can be represented by thefollowing sequence of acid- o r base-catalyzed nucleo-philic additions or substitutions:68hydrolysis (hydroxylation) of the metal alkoxides

(M = metal or Si; R = alkyl) (2)

hydroxy bridges)

( X = H o r R ) (3)

M-OR + H,O M-OH + R-OHolation (condensation with formation of

M-OH + M-OHX *= M-OH-M + X-OH

Chemical Reviews, 1995, Vol. 95, No. 3 485(which is that of the vessel in which they wereprepared), aerogels may easily be obtained with thesame atomic density as gases at standard conditions.In this very open structure, practically all atoms areexposed t o the ambient atmosphere. Other specialproperties include their extremely low thermal con-ductivity and very good textural and structuralstability at high temperatures.

Inherent in the preparation processes of aerogelsis control of their structure and morphology from themacroscopic level (preforming he material into mono-liths, powders, lumps) down to the mesoscopic one(usually referred t o as the porous structure, whichis controlled by changing various parameters duringpreparation, such as pH, solvent, amount of wateradded for reaction) and finally to the microscopic level(complete atomic exposure).69The homogeneity of the gels depends on the solu-bility of reagents in the solvent used, the sequenceof addition of reactants, the temperature, and the pH.Network forming elements, such as Si, metals ofprincipal groups, lanthanides, and early transitionmetals, must be used to obtain fairly homogeneous

solids. The usual precursors which are readily avail-able commercially for preparation of oxide aerogelsare organic alkoxides, acetates, or acetylacetonates,as well as inorganic salts, such as chlorides. Amongthe classes of solvents, alcohols are largely used, butother solvents (benzene) may also be used for somealkoxides. The catalysts introduced in the polycon-densation stage are volatile acids (acetic acid) orbases (ammonia).The versatility of the sol-gel process is so extensivetha t the number of catalytic materials prepared asaerogels has increased rapidly. In Table 1,we updatethe list of aerogels prepared as potential catalysts orcatalyst supports based on data reported in two very

recent comprehensive reviews.'O The method ofpreparing aerogel materials can easily be applied t oobtain single-metal oxides customarily used as cata-lyst supports. As an example, preparation of ther-moresistant, impurity-free alumina supports withhigh surface area and variable ~r ys ta ll in it y~ l- ~~fvery pure magnesium oxide,74of amorphous sili-c ~ a l u m i n a t e s , ~ ~ ? ~ ~nd of uniform nanosize silicaparticle^^^,^^ has been reported. In addition, the sol-gel method was extended to preparation of multi-component metal oxides. A detailed discussion of thepreparative details may be found in Pajonk's re-view.70 synthesis is based on cogelation of suitablemetal derivatives in appropriate organic solvents,using stoichiometric amounts of water and volatileacids and bases as catalysts. Supercritical drying isthen conducted at the highest critical temperatureof either the organic solvents or dispersing agentsused. Materials with new properties could be pre-pared in this way. For example, incorporation ofMoo3 in alumina led to a modified structure of thealumina network, which preserves a high surfacearea and prevents segregation of Moo3 even a t highloading.72 In binary oxidic systems, dispersion of onecomponent in the matrix of the second oxide mayeither stabilize metastable crystalline phases (cubicZrOz in ZrOz-Si02 system79) r may prevent crystal-lization of the second oxide (niobia in Nb~05-AlzO3~~

oxolation (condensation with formation ofoxygen bridges)

( X = H o r R ) 4)The overall process produces a highly reticulate,

metastable polymer with an open structure in whichthe primary uni ts a re held together by either chemi-cal bonds, hydrogen bonds, dipole forces, or van derWaals interactions. This framework is imbibed bythe solvent. In order t o transfer this structure intothe solid phase, the liquid within the gel must beremoved in such a way that a liquidvapor interfaceis not formed. Aerogels are obtained when the gel isdried by supercritical extraction. This procedure isconducted by high-pressure heating, which trans-forms the liquid contained in the gel into supercriticalvapors, and eventually is followed by graduallydiminishing the pressure at a constant supercriticaltemperature. Under supercritical conditions, thestructure of the gel is conserved in the solid statewithout collapsing. Another way t o avoid a liquidvapor interface requires that the liquid first be frozenand then sublimed. The resul tant aerogel is calleda cryogel, since cryogenic conditions are normallyinvolved in drying.The three-dimensional network in the aerogelstructure consists of tetrahedrally coordinated units,M04, in a loosely packed configuration. One of themost striking properties of aerogels is their very lowatomic density (as low as lozo~ m - ~ ,s compared withcm-3 for typical condensed matter). This isobviously related to a very high porosity and highspecific area. Although characterized by a shape

M-OH + M-OX-M-O-M +X-OH


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486 Chemical Reviews, 1995, Vol. 95, No. 3 Schwarz et al.Table 1. Application of Aerogel Techniques forPreparation of Catalytic Materials

single oxides refls) single oxides refls)7462Si02 62,77,78 MgOA 2 0 3 62,63,73 MoOzZrOz 62 NiO 62Ti02 62 vzo5 62,81ThOz 62 CUO 62Cr203 62 PbO 62Fez03 62 CeOz 82

refis)626262626262,7171626262

binary oxides reffs)vzo5-Tio2 81Nbzo5-~zo3 80Si02-Al203 75,76M003-&03 72TiO2-Si02 aSiOz-TiOz bTiOz-CeOz 82ZrOz-SiO2 79CuO-Si02 c

ternary oxides ref ternary oxides refNiO-SiOz-Al203NiO -A1203 -MgONiO- SiOz- MgONiO-Fe~03-Al203NiO-Vz05-MgONiO-MgO-Al203

metal oxide

62 Fe203-NiO-Al20362 Cr203-Al~03-MgO62 Cr203-Fe203-MgO62 Cr203-Fe203-Al20362 V205-TiO~-Si0~62 M003-C00-&03

ref metal oxide

626262627381ref

Pt-Si02 62 Pd-Si02 87,dNi-Si02 62 Cu-ZrO2 62Ni-Al203 62 Cu-Zr02-Al203 62Ni-SiOz-Alz03 62 Cu-ZnO-Al203 62Ni-SiO2-MgO 62 Rh-SiOz-Al203 76cU- o3 62 Rh-TiOz-SiOz aCu-Si02 62 Rh-MgO 91CUO-MgO 62 Rh-Ti02 91Ni-Moo2 62 Ru-Si02 90,89Pt-MoOz 62 Ru-Al203 89Pt-Si02 92 d e Pt- Sn-Al203 fPd-&03 62a Cauqui, M. A.; Calvino, J. J.; Cifredo, G.; Esquivias, L.;Rodriguez-Izquierdo, J. M.Non-Cryst. Sol ids 1992,14711 48,758. Ingo, G. M.; Dire, S.; Babonneau, F. Appl. Surf Sci.1993, 701 71, 23 0. van der Grift, C. J. G.; Mulder, A.; Geus,J. W. Coll. Surf 1991, 53, 223. Lopez, T.; Moran, M.;Navarette, J.; Herrera, L.; Gomez, R. J. Non-Cryst. Solids1992,1471148,753. e Lopez, T.;Romero, A.; Gomez, R. J . Non-Cryst. So li ds 1991 ,127 ,105 . f Gomez, R.; Bert in, V.; Ramirez,M. A.; Zamudio, T.; Bosch, P.; Schifter, I.; Lopez, T. J . Non-Cryst. Solids 1992 , 1471148, 748.

and vanadia in V205-Ti02-A120381). Moreover, newcompounds could be formed under mild conditions( C e T i o ~ , ~ ~l N b 0 4 8 3 .The reducing properties of most alcohols usedeither as solvents o r dispersion media combined withthe autoclave conditions required for supercriticaldrying results in the possibility of preparing in situreduced metal catalysts on aerogel supports. Thisis achieved by cogelling precursors of easily reducibleoxides (NiO, CuO) together with those of theircarrier, eventually under a hydrogen a tm o~ ph er e. ~~For platinic metal catalysts, the incorporation of themetal precursor (salts , inorganic complexes) duringthe gelation step results in additional interactionsbetween terminal hydroxyls of the gel network andthe metallic precursor and eventually in the incor-poration of the latter into the gel structure. Lopez

and c o - w o r k e r ~ ~ ~ - ~ ~emonstrated in a series ofpapers that Pt, Pd, Ru, and Rh introduced aschlorides or chloro amines interact strongly in thecoordination sphere with silanol groups of the freshsilica, e.g.

[ E S i - O - S i = ] - O H + trans--[ Pt(NH&C12]-b)PH 3I [ P~C~doH),(SIO)z] 5)x + y + z = 4 )

(C)PH9- Pt(NH3)dOH)ASIO)Z]x + y + z = 4 )

where process a is only a weak surface interaction,while processes b and c represent strong interactionswith the bulk of the silica support.g1 The fact thatthe sol-gel method generates either structures likeand Ru/Si02-OHg0 could explain the novel behaworof the catalysts, such as high selectivity in hydroge-nation of acetylenics and high resistivity t o cokedeposition. However, Gonzalezg2 eports, on the basisof a TEM study of the catalyst prepared by thisimproved sol-gel method, that these catalysts are notalways highly dispersed. Thus, it appears otherexplanations are necessary t o account for the robustactivity of these sol-gel catalysts and their abilityt o resist coke formation.5.Crystallization

The method of crystallization has found wideapplications in the preparation of homogeneous mi-croporous solids, a class of monophase crystallinesolids in which the active phase is distributed uni-formly. They comprise the general class of materialsdesignated as molecular sieves. In 1932, McBaing3proposed this t erm t o describe materials that exhib-ited selective adsorption both in terms of the size andthe shape of the adsorbates. Since tha t time morethan 200 molecular sieve structures have been dis-covered, and their organization has been made on thebasis of both structure and elemental compositions.These microporous crystalline solids are structurallybased on frameworks formed by linked TO4 tetrahe-dra with each oxygen shared between two T ele-ments.

One of the most important factors in the synthesisof molecular sieves is the chemical compositionof thegel from which the crystalline products are separated.A wide range of organic and inorganic compoundshave been used t o modify the crystallization proce-dure. A simplified mechanismg4depicted in Figure3 shows the essential features of the steps involved.In addition t o the gross composition and the reactionmixture, time and temperature also influence thestructure of the final product. When the crystalgrowth is carried out in aqueous solution above ornear 375 K, the conditions are designated as hydro-thermal; this has proven t o be the most efficient waythus far t o produce these microporous materials.

those shown above or of the type [Si0211&~zOH,l1


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Methods for Preparation of Catalytic Materials Chemical Reviews, 1995, Vol. 95, No. 3 487SOURCES OF Si, A I P, Ga, Ge, B,...OTHER CATIONSBA SIC REAGENTS H 2 0

11 ZEOLITE CRYSTALSAMORPHOUSHYDROGEL

STRUCTURING AGI+ TMAINERALIZINGAGENT OH, F)I PRIMARY Si or AI)BUILDING UNITS

Zeolites are distinguished from other molecularsieves on the basis that zeolites are a crystallinealuminosilicate with a framework based on a three-dimensional network of oxygen ions with Si4+andA13+ons occupying the tetrahedral sites formed bythe oxygens. The A104 tetrahedra determine theframework charge which is balanced by cationsoccupying nonframework positions. Thus, a repre-sentative empirical formula for a zeolite is M2,,O*Al203xSiO2yH20, where M represents the exchange-able cation (also including nonmetal and/or organiccations), n is its valence and x is a number equal t oo r greater than 2 because A13+ does not occupyadjacent tetrahedral sites. Typical cations includealkali metals, alkaline earth cations, NH4+, H30+(H+), etramethylammonium (TMA),other nitrogen-containing organic cations, rare earth ions, and noblemetal ions. The crystalline framework structure ofzeolites contains voids and channels of discrete size.These may be divided into three major groups ac-cording t o their pore/channel system. A listing basedon the largest pore opening is given in Table 2.The shape of the 8-membered oxygen rings variesfrom circular to puckered to elliptical. Straight chainmolecules such as n-paraffins, olefins, and primaryalcohols can be adsorbed by this group. The pore/channel systems of these zeolites also contain inter-connecting supercages which are much larger thanthe connecting windows.Almost all members of the 10-membered oxygenring systems are synthetic. This framework struc-ture contains 5-membered oxygen rings and thus aremore siliceous than previouslyknown zeolites. Dwy-er and Jenkinsg5have considered them as silicateswith framework substitution by small quantities ofalumina. As in the case of the small pore zeolites,the shape and precise size of the 10-memberedoxygen rings also varies from one structural type toanother. Among the zeolites in this group, ZSM-5and ZSM-11 have bidirectional intersecting channels.The H-form of these zeolites are very stable acidic

I TTEA)-

Figure 3. Simplified schematic of the steps involved during synthesis of molecular sieves.Table 2. Classification of Zeolite Structures as aFunction of the Number of TO1 Units That Shape thePore Opening

8 ring 10 ring 12 ringbikitaitebrewsteritechabaziteedingtoniteerionitegismondineheulanditelevynemerlionitenatrolitepaulingitephillipsiterhothomsoniteType A, ZK-5yugawaraliteTMA-E (AB)

dachiarditeepistilbiteferrieritelaumontitestilbiteZSM-5 (silical ite)ZSM-22 (the ta-1)ZSM-11ZSM-23ZSM-48 (Eu-2)ZSM-50 (Eu-1)

betacancrinitefaujasite (TypeX, Ygmelinitemazzitemordeniteoffretiteomegatype LZSM-12

catalysts. Furthermore, they have pores of uniformdimensions with no large supercages containingsmaller size windows. Three factors are probablyresponsible for the successful industrial applicationof these zeolites: high silica t o alumina ratios,geometrical constraint imposed by the 10-memberedoxygen-ring-sized pores, and the absence of bottle-necks in their pore system which precludes theinclusion of large polynuclear hydrocarbons respon-sible for coking and irreversible deac ti ~ a ti on .~~Zeolites containing dual pore systems have inter-connecting channels of either 12- and 8-memberedoxygen rings openings o r 10-and 8-membered oxygenring openings. Acidic zeolite catalysts in this classhave a tendency t o coke and deactivate readilybecause of their intersecting channels of differentsizes. Large 12-membered oxygen ring openings orsupercages deactivate more rapidly than medium o rsmall pore materials during acid catalyzed reactions.A typical zeolite synthesis involves mixing togetheralkali, sources of AlO2 and SiO2, water, and other


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488 Chemical Reviews, 1995, Vol. 95, No. 3Table 3. The Effects of Several Synthesis Variableson the Properties of the Final Products in ZeoliteCrystallization

Schwarz et al.

composition of influence onreaction mixture crystallized productsSi02/A1203 rat io framework compositionH20/Si02 ratio rat e and mechanism ofcrystallizationOH-/Si02 rat io molecular weightinorganic cations/SiOz ratioorganic additivesISiO2 ratio

zeolite structure andcation distributionzeolite structure andcontent of framework A13+

components in appropriate proportions; the resultinggel is then subjected t o elevated temperatures (gen-erally less than 625 K). It is of interest to know whatspecies are present at the beginning of the reaction.Aluminate and silicate solutions have been studiedindividually. Aluminate solutions contain only onetype of ion at high pH; the tetrahedra l Al(OH)d- ionis the important species for normal zeolite synthe-S ~ S . ~ ~hen the pH moves toward neutral, otherpolymeric ions appear. Silicate ions at high pHcontain a range of small silicate polymers formed bycorner-sharing tetrahedral Si04 units. Rings andcages are the preferred form of silicate species.Depending on the temperature and composition, theoptimum crystallization time can range from severalhours to several weeks. During this time period, thesystem is in a highly disordered s tate with a higherentropy than its ordered counterpart, the crystallizedzeolite product. Ostwald's rule of successive trans-formations generally governs the formation of thefinal product, but changes in the hydroxide ionconcentration and/or the presence of certain anions((21-, Sod2-, NOS-) can also play a factor. One canfollow the course of a crystallization either by stop-ping the crystallization at various times and sam-pling the batch, by taking samples while the processis occurring, or by running the process in a series ofidentical crystallization vessels charged with thesame batch of starting gel. The latter has been themost extensively used.

Zeolite synthesis, unfortunately, enjoys the desig-nation of an art to the uninitiated. A major reasonfor this is that not only does each component of thereaction mixture contribute to the crystallization ofa particular zeolite structure, but all of the compo-nents are interrelated. Thus, changing two compo-nents together can influence the final product in away different from that achieved by varying compo-nents individually. Despite this, it is possible t oenumerate some general guidelines a s t o the effectsof individual components of the mixture. These arepresented in Table 3 and discussed in more detail inthe following text.

The SiOdA1203 atio in the gel places a constrainton the framework composition. Table 4 presents thegeneral effects of changing the ratio on the physicalproperties of the zeolite product. For catalytic ap-plications such as cracking and isomerization, zeo-lites improve with increasing SiOdAl203 ratio. Re-sistance to acids and heat treatment are also improvedin this manner. On the other hand, for eitheradsorption or ion exchange processes, a decrease in

Table 4. Influence of SiOdAl203 Ratio on the PhysicalProperties of Zeoliteshigh SiOZ/A1203 ratio low SiOZ/A1203ratio

improved acid resistance increased hydrophilicityimproved thermal stability high cation exchange capacityincreased hydrophobicitylow affinity for polarlow cation exchangeadsorbentscapacity

this ratio is favored because of the required increasein cationic content for charge neutralization.The methodologies employed for changing the SiOdA 1 2 0 3 ratio in the gel phase can also be important inachieving exceptionally high SiOdAl203 ratios. Modi-fication procedures include addition of organic addi-tives, use of novel sources of silica such as H2SiF6,and addition of complexing agents for the aluminum.The hydroxide ion concentration influences, amongother things, the nature of the polymeric speciespresent in the reaction mixture and the rate at whichthese species interconvert by hydrolysis. Increase inthe hydroxide concentration accelerates crystal growthand shortens the induction period preceding crystalformation. One explanation for this is that it acts t ofacilitate transport of the silicate and aluminatespecies by an enhanced solubility of the reactants athigher pH. The reactan ts will nucleate and growfaster because of the increase in the collision fre-quency between the more concentrated precursorspecies in the solution phase.In addition t o serving as charge compensators,inorganic cations present in the reaction mixtureoften appear as the dominant factor which controlsthe zeolite structure obtained; they can influencecrystal morphology, crystallinity, and yield.97 Theeffect of the added cations is, indeed, complex andmay be due t o many factors. The presence of differ-ent cations (as well as amounts) will modify the pHof the mixture with crystallization time. Anotherpossibility has been described as a template theory.An ion (or neutral species) is considered to be atemplate or crystal-directing agent if, upon its addi-tion to the reaction mixture, crystallization is inducedof a specific structure that would not have beenformed in the absence of the template. The processhas been analyzed as one in which the zeolitestructure grows around the template; thus stabilizingcertain pore structures or subunits. The theory isnot only limited t o explaining the effects of inorganiccations; it has been shown that neutral and ionicorganic amines also follow a similar templating effectalthough other explanations have been suggested.The water content of the starting mixture alsoplays an important role in determining the s tructureof the zeolitic product. Water has been proposed t ointeract strongly with cations present in solution andbecomes itself a sort of template for structure control.The role played by water is reinforced by resultsobtained from systems in which the crystallizingmedium was not a q u e o u ~ . ~ ~ ~ ~ ~ompared with thelarge number of structures formed in aqueous sol-vents, few zeolites have been found t o crystallize fromnonaqueous solvents. Solvents such as hexanol, pro-panol, glycol, sulfolane, and pyridine have been used.


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Methods for Preparation of Catalytic MaterialsCharge imbalance due t o the number of silicon andaluminum ions in the framework of zeolites gives riset o active sites; they a re classified as either conven-tional Lewis o r Br~ nste d cids. Classical Branstedand Lewis acid models are described by Br~nstedacidity which is proton donating and Lewis aciditywhich is electron accepting acidity. The formeroccurs in the zeolites when the cations balancing theframework anionic charge are protons; the latterwhen an aluminum atom is trigonally coordinated

resulting in a n electronic deficiency, and thus i t canaccommodate an electron pair.Zeolitic acid catalysts are produced when thecations present in the freshly synthesized materialare replaced with protons. Difficulties associatedwith the process are (1) arge organic quaternaryamine cations which are used in common synthesisroutes today are difficult to remove from the poresystem; (2) everal exchanges are generally needed;and (3) irect proton exchange using acids results inleaching of aluminum ions from the framework.Ion exchange is normally accomplished using anaqueous ammonium salt, and the resulting materialis calcined t o produce the acid form.lo0-lo7 Directtreatment with HC1 can also be U S ~ ~ . ~ ~ ~ J ~ ~ - ~ ~ ~ J ~ ~thas been shown th at the activation method t o intro-duce acidity also can induce variation in the reactiveproperties of the activated catalysts.lo8 In summary,three major factors influence the activation processfor the final acid catalysts: (1)he type of exchangetreatment;(2)he degree of ion exchange; and (3)hecondition of calcination subsequent t o the exchange.More than 10 years ago a new class of microporousmaterials became important in both industrial andresearch 1aborat0ries.l~~hey are known as AlPOs,, Os, MeAF'Os, and ElAPOs (El = As, B, Be, Ga,Ge, Li, Ti). They are derived by isomorphous sub-stitution of AlPO4. More than 27 different structureshave been found, and 15 elements other than Al andP with oxidation states ranging from 1 t o +5 havebeen incorporated into the Alp04 framework.The synthesis of this wide scope of materials ischaracterized by109J1 mildly acidic t o mildly basicslurries (pH 3-10), narrow P/Al (0.8-1.7) omposi-tion, the common use of amines o r alkylammoniumions as templating agents, a synthesis success (yieldperformance) strongly dependent on source of reac-tants and stirring and aging of gel, and a high degreeof isomorphous substitution during synthesis.The net charge on the Alp04 molecular sieves iszero because the framework A l 0 2 - and P02+ units

exist in equal amounts in their structure. Thus, theAlPOs have no ion exchange capacity; however, theydo exhibit a reasonable attraction toward water duet o the polar nature of the Al-0-P structure.Recently, the first molecular sieve with ringshaving greater than 12 T atoms was synthesized.lThe so-called VPI-5 is a family of aluminophosphatesbased molecular sieves possessing the same threedimensional topology. The extra-large pores of VPI-5contain unidimensional channels circumscribed byrings which have 18 T atoms and possess freediameters of approximately 12A. While this mate-rial is interesting from the standpoint of its structure,its applications t o catalysis have been limited. One

Chemical Reviews, 1995, Vol. 95, No. 3 489reason is its hydrophobicity. Modification of acidityis accomplished through various dealumination tech-niques and/or doping with silicon.6. Solid-Solid Blending

One of the most important requirements in thepreparation of single metallic o r multimetallic oxidecatalysts is obtaining a good interdispersion of dif-ferent phases and components which constitute thecatalyst. Its importance arises in order t o achievethe desired spatial distribution of the components foruse in catalytic reactions where selectivity dependson the diffusion of the reactants. In an attempt toimprove the homogeneity of the catalysts at themolecular o r atomic scale, different procedures weredeveloped. A common characteristic of these meth-ods is the use, separately or combined, of bothchemical and physical factors in order t o control theglobal chemical reaction and t o achieve a state ofintimate interdispersion and mixing of either re-agents or of reaction products.

Various methods based on solid-solid blending arefrequently used for the preparation of mixed oxidecatalysts; some were borrowed by the catalyticcommunity from ceramic manufacturers.In the high-temperature ceramic method, the mixed-oxide phase results from heating intimately mixedpowders at temperatures high enough t o allow in-terdiffusion and solid sta te reactions. The methodhas the advantage of the extreme simplicity, and it suse is essential for preparing mixed oxides, such asperovskites,l12 with special morphologies such assingle crystals o r thin layers. A major shortcomingof the ceramic method is the lack of homogeneity ofthe materials prepared, because the solid state reac-tion between the precursor oxides occurs with veryslow rates. The high temperatures (1300K or above)required t o complete solid state reactions betweenoxides lead t o a drastic decrease in surface area ofthe resulting material by sintering. This severelylimits the use of the ceramic method in preparationof catalysts designed for most low temperatureprocesses. To overcome this problem, precursorcompounds, such as carbonates and oxalates, thatdecompose at lower temperature, have been usedinstead of the corresponding oxides. Another strat-egy has been proposed and tested.The method of temperature-programmed synthesiswas used successfully t o prepare carbides and ni-

trides with high surface area starting from precursorswith very low specific surface area.l13 As shown bythe schemes in Figure 4, he starting material wasWOs. This was converted t o P-W2N by using am-monia as the reducing gas and a very slow temper-ature ramp (ca. 0.10 K s-l). In a second step, twomodifications of tungsten carbide were prepared bycarburizing either WO3 o r P-WzN with a CH4:H2 =4:l mixture. The key t o this method is that thetemperature must be increased very slowly duringnitridation o r carburization, in order t o preserve thetopotactic character of transformation;l this leads t oa porous solid (either tungsten nitride or carbide)with specific surface area in the range of 100-200


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490 Chemical Reviews, 1995, Vol. 95, No. 3 Schwarz et al.NH3 C H 4+ H

TPR 700-1000 K) @ PR 700-1000 K)CH,+ H

1100 K 1000 K(* excess polym eric carbon )

Figure 4. Scheme for producing tungsten carbides from tungsten oxide using temperature-programmed procedures.m2 g-l. The catalytic materials obtained, if devoidof excess polymeric carbon, are very active for reac-tions of hydrogenolysis and dehydrogenation of al-kanes and alcohols and hydrogenation of alkenes,reactions th at also occur on group VI11 metals.l14The ceramic method finds application in the prepa-ration of low surface area catalysts (0.5-10m2 g-l)which must resist deactivation in reactions at hightemperatures. An interesting application of thismethod in preparation of oxide systems with valence-controlled dopants was described by Klier.35 In theoxidative coupling of methane, ZnO is a low activitycatalyst, but its reactivity is controlled by oxygenvacancies which can be generated by doping withlower valence ions, such as Cu+. On the other hand,introduction of a redox Lewis acid such as Fe3+ isexpected t o change the reaction mechanism by oxi-dizing the methyl radicals to formaldehyde. Doubledoping of ZnO with Cu+/Cu2+ nd Fe2+/Fe3+ edoxcouples was beneficial: Cu+ acted as an oxygenactivator for ZnO and Fe3+as a selectivity switch t of0rma1dehyde.l~The doped ZnO catalyst was pre-pared by the ceramic method. At the high temper-ature employed, substitutional Cufzn and Fe3+znonsdiffuse together in the ZnO lattice and enrich thesurface region of the catalyst. The two substi tuteions which were mutually attracted by Coulombicforces as Cu+ and Fe3+, represent, respectively, anegative and a positive charge with respect t o theZn2+ attice.In an attempt to obtain homogeneously uniformsolids, avoiding the imperfections of the coprecipita-tion method or the severe heat treatments of theceramic method, several other procedures were de-veloped. The homogeneity of the solid product de-pends on the homogeneity of initial reagents. Oneof the simplest ways t o obtain a homogeneous dis-tribution of cations is t o have them in solution. Insome instances, the final solid product may beobtained without very severe heating, simply byremoving the solvent. Different methods based onliquid-to-solid transformations were explored; theydiffer by the way in which the solvent is removed.The simplest method is that of ry evaporation ofa homogeneous solution that contains the precursorsalts. This technique resembles the ceramic methodbecause it may result in a nonhomogeneous soliddepending on the crystallization rates of the variouscomponents. The homogeneity of the original solu-tion is best conserved, for example, by increasing therate of evaporation of the liquid. In the spray-dryingtechnique, a solution is dispersed as fine droplets ina hot chamber. Very fine particles are formed anddried quickly, and then the product is collected as apowder.

A related method is that of freeze-drying in whichthe solution containing the desired decomposablecompounds is sprayed into liquid nitrogen. In thisway, very small particles are formed by rapid freez-ing, and the homogeneity of the initial solution ispreserved. Removing the solvent by vacuum dryinghas a similar effect. This procedure may be used t odry solids with low melting temperatures and inmany instances to preserve their amorphous orglassy character. These last techniques were usedfor the preparation of mixed oxides with perovskitestructures and surface areas in the range of 10-50m2 g-l.l16-llSC. Liquid-Solid Blending

Many commercial catalysts are manufactured bythe co-mulling technique, a technique borrowedfrom ceramics. This procedure consists of blendinga powder of dry aluminum hydroxide with a smallamount of water, which may also contain otherprecursors of active ingredients of the catalyst, anda peptizing agent. A homogeneous paste is formedby kneading which is further extruded; the extru-dates are dried and calcined. Extrusion permitsproduction of catalysts and supports with smallerdimensions and at lower cost than ~e1 le t ing . l~~Extrusion has been used for manufacturing ofceramic materials, but development of the technologyhas remained fairly stagnant because the approachhas been almost always empirical. Extrudable pastesare two-component systems that contain a particulatephase (a powder) and a continuous phase (a liquid).The selection of these two phases is critical t o thesuccessof the process, which depends on the rheologyof the paste; it must have high viscosity at low shearrates and low viscosity at high shear rates.120 Inspite of its importance for large-scale preparation ofcatalysts, only a few papers addressed the problemfrom a theoretical viewpoint.120-122 mpirical con-tributions have come from the field of ceramicmaterials science.123 Unger124J25ompared variouscommercial aluminas in relation t o the manufactur-ing of porous supports by extrusion. Jiratovallgshowed that the effects of different peptizing acidson the properties of the extrudates can be generalizedusing the Hammet acidity function of the peptizingsolution. Luck126used a battery of spectroscopicmethods t o compare NiO-Mo03-Al203 hydrotreat-ing catalysts prepared by kneading and by conven-tional impregnation.Ill. Solid Transformations

A number of different procedures are used to formcatalytic materials that do not fall into either the


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Methods for Preparationof Catalytic Materialsblendingor the mounting classes. The commonelement that perhaps best describes these proceduresis the transformation of a solid phase either byphysical o r chemical processes. In both cases, thepreparation of unsupported metals and alloys ascatalysts, either on the industrial or on the laboratoryscale, is conditioned by the ability t o stabilize metalsin a physical form characterized by a large surfacearea.

Chemical Reviews, 1995, Vol. 95, No. 3 491different from those of either the bulk material orthe substrate.The formation of a thin film by any of the experi-mental methods mentioned is not a simple process.Atoms or molecules which are incident on a surfacemay either diffuse over the surface, diffuse into thebulk of the substrate, o r be desorbed. In typicalcases, one or more of these processes dominates thefilm formation process.B. Unsupported Bulk Metals

The preparation and characterization of unsup-ported metal catalysts was recently reviewed;lZ9platinum group metals are the widest used becauseof their high activity. A list, with examples, includesthe Pt-Rh wire gauze used for oxidationof ammonia,the Pd-Au alloy wires used for selective hydrogena-tion of hydrocarbons, the Pt-Rh gauze used as acatalyst in the synthesis of hydrogen cyanide frommethane and ammonia, and the wire and granularsilver catalyst used for selective oxidation of metha-nol t o formaldehyde. Palladium membranes havefound application as a hydrogen permeable catalystwhich integrates a hydrogenation and a dehydroge-nation reaction in a unique catalytic re a ~ t o r . l ~ ~ J ~ lheuse of an inorganic oxide guard phase (a layer oftitania) was found to be effective in preventing thepalladium surface from rearrangement under theaction of temperature and reaction mixture.130Preparation of bulk metal catalysts in the form ofwires, foils, gauzes is fairly simple. Introduction ofa second metal component was sometimes used as apractical means to vary systematically the propertiesof the resulting system.132Bulk bimetallic catalystsand alloys play a major role in fundamental researchwhere the catalytic influence of the second metalcomponent is studied. Recent results on the growthmode of evaporated bimetallic films (Pd-W, Pd-and on their catalytic properties (Pt-Re139demonstrate tha t bimetallic systems are catalyticallyinteresting because of both geometrical and electroniceffects. 37Methods to prepare alloys in the powder form use,for example, reduction of mixtures of either salts(chlorides, nitrides, carbonates) o r hydroxides of themetals in question. The reducing medium is usuallyhydrogen, and the homogeneity of the alloy is en-sured by either physically mixing the salts o r calcin-ing the hydroxides before reduction. Obtaining ahigh surface area is possible by keeping the reductiontemperature as low as possible. The use of liquidmedia (either aqueous o r nonaqueous) for reduction,such as hydrazine, formaldehyde, and sodium boro-hydride solutions might be preferred because of thelower temperature needed.Metals can also be prepared in small-particleskeletal forms and used in either fixed-bed or slurryoperating reactors. In this state their preferredmorphology would consist of small particles, more orless separated from one another, and protectedagainst s intering by an oxide stabilizer. The prin-ciple of al loykg with aluminum, which is laterselectively dissolved in very alkaline solutions, formsthe basis of preparing Raney-type catalysts and hasbeen applied t o several metals, such as Ni, Co, Cu,

A . Epitaxial Metallic FilmsMassive metals, either polycrystalline o r in theform of thin films and single crystals have limitedapplications as practical catalysts, while they doserve as excellent model systems in laboratory stud-ies. Recent studies on surface catalytic reactions onthin expitaxial films approximately one monatomiclayer thick formed on particular metal substratesshow that the catalytic reactivity of the surface for aparticular chemical reaction may increase signifi-~ a n t l y . l ~ ~ J ~ ~hus, by using thin film techniques analmost limitless range of model surfaces, with theirdistinct surface chemistry, can be devised.Epitaxy is a term that refers to the oriented growthof one material, the overgrowth, on a crystallinesubstrate. Those planes and directions which givethe best lattice fit generally determine the orientationof the film with respect to the substrate. Misfit thatoccurs produces stra in, which, if large enough, maygenerate line defects called misfit dislocations at theinterface between the film and the substrate. Thesedislocations tend to reduce the misfit s train.The processes that are involved in forming anepitaxial overgrowth may involve the solid, liquid,and vapor state while the growth of solid phases asalloys is controlled by interfacial relationships. Inthe liquid state, epitaxial overgrowth can be formed

by electrodeposition or by a process called liquidphase epitaxy (LPE), whereby a satura ted solutionplates out a material on a particular solid substrate.Vapor phase methods are probably the most commonand include 1) acuum evaporation from a hot sourceonto a colder substrate (molecular beam epitaxy,MBE), 2) chemical vapor deposition (CVD), hichinvolves surface chemical reactions of gases at pres-sures near atmospheric (e.g., thermal decompositionof a gas on a hot substrate or polymerization ofmonomers), and 3) ion sputtering processes wherebyions produced in a gas discharge (or by other gaseousionization methods) are accelerated toward a target.The subsequent interaction by momentum inter-change results in the emission of atoms or moleculesfrom the target material which then are permittedt o strike a substrate on which the film of interestgrows.The properties of epitaxial films can be made t ovary widely because of the high reactivity of indi-vidual atoms and molecules. Thus, in combining theatoms t o form a thin film, numerous physical andchemical processes may be involved, thereby makingpossible an almost limitless variety of properties, e.g.,microstructure (i.e., defect content), orientation, com-position, and topography. This result is especiallytrue for very thin films. In this case, their physical,chemical, and mechanical properties may be widely


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492 Chemical Reviews, 1995, Vol. 95, No. 3Fe, Ag, and Re. However, the use of Raney-typecatalysts is often limited by their propensity for self-heating and self-ignition in air, which is determinedby the presence of a large amount of active hydrogendissolved in the highly dispersed, lattice distorted,active meta1.13* The stability of Raney Ni catalystscan be increased by mild surface oxidation139whichleads to formation of a protective film of nonstoichio-metric nickel oxide.Progress in the preparation of Raney-type catalystswas made by the technology of making activatedmetal supports and metal catalysts, using any con-figuration into which metals can be fabricated (pel-lets, sheets, foils, single crystals, etc.). The methodentails the steps of high-temperature deposition anddiffusion of volatile metal compounds (halides) on thebase of a metal substrate, followed by high-temper-ature diffusion t o form an intermetallic compound onthe base metal surface. Selective removal of thediffused metal leaves behind the base metal with adepleted lattice and a high and active surface area.The Baldi-Damiani technology140 rovides a meanst o synthesize metal and support in any size, shape,or form. Because these supports are thermallyconductive and can be made in almost any configu-ration, the Baldi-Damiani activated metals shouldfind application in highly exothermic reactions andin processes in which significant pressure drops mustbe avoided.

Schwarz et al.

C. Amorphous AlloysA technique for producing metallic alloys by rapidlyquenching melts has a ttracted the attention of met-allurgists, physicists, and recently, catalytic chem-i s t ~ ~ ~ ~ - ~ ~ ~ecause of the exceptional properties of thematerials obtained. They have neither long-rangeorder nor complete amorphous character. Materials

prepared by metal quenching methods are referredt o as amorphous metal alloys or metallic glasses.These cognomers underscore the fact that suchmaterials are never pure metals, but alloys with arigid structure and short-range ordering. Severalreviews appeared recently on their catalytic applica-t i o n ~ . ~ ~ ~ , ~ ~ ~lthough many elements of the periodictable can form a variety of alloys with glassy struc-ture, only certain compositions have been studied fortheir catalytic properties. One groups is that ofmetal-metalloid alloys comprised of a late transitionmetal (Ni, Co, Fe, Pd, Au, about 80 atom percent)and a metalloid (B, C, Si, Ge, P) which contributest o the lowering of the melting point. A second groupis that of metal-metal glasses with typically 1:lcomposition (e.g., Ni-Ti, Cu-Ti, Ni-AI, Pd-Zr, Cu-Z r , Ni-Nb, Ti-Be, Ca-Mg).Obtaining noncrystalline metal alloys with a meta-stable structure requires cooling of the melt at a ratehigh enough th at crystallization does not occur. Inthe melt-spinning technique, this is achieved byrapidly increasing the melt surface area and trans-forming it into ribbons o r tapes. Most catalyticstudies have concentrated on ribbon samples pre-pared by this In many cases, theas-prepared alloys have low surface area and mini-mal catalytic activity, so that activation of thecatalyst surface is needed. Activation of amorphous

metal catalysts uses procedures th at are common t othose of traditional metal catalysts, such as reductionin hydrogen at elevated temperatures and oxidationby acid etching followed by reduction with hydrogen.Activation by a leaching procedure may result in theformation of a rough, Raney-type porous surface, aswas revealed by electron microscopy studies of amor-phous alloys containing Zr.149 Another route t oprepare highly active catalysts from pretreated me-tallic glasses consists of selective oxidization of themore electropositive metal, which results, after re-duction, in finely dispersed transition metal particlesembedded within a partially crystalline oxide ma-triX.150Other techniques, such as vapor and sputter depo-sition, flash evaporation, and chemical reduction,were used t o produce amorphous alloys. A promisingalternative was recently developed to produce amor-phous metal catalysts with high surface area. In thespark erosion technique, a repetitive spark is main-tained between two electrodes of the material t o bequenched, while it is immersed in an organic dielec-t r i c f l ~ i d . l ~ l J ~ ~t the extremely high cooling ratesprovided by this method, amorphous metallic pow-ders were prepared th at were found t o be active formethanol (Cu-Zr, Cu-Zn, Cu-Zn-Al) o r Fischer-Tropsch (Fe-B) synthesis. This technique overcomestwo major shortcomings of amorphous ribbons whenused as catalysts: their low surface area and theirsurface nonhomogeneity.The use of metallic glasses in catalytic applicationsis limited t o low temperatures because the amor-phous state is thermodynamically unstable and tendst o crystallize. Once exposed t o temperatures abovetheir crystallization point, the amorphous charactermay be lost and the catalytic activity may be drasti-cally changed. However, there are indications tha ta small amount of crystallinity seems t o improve theproperties of m e t a l l i c g l a ~ s e s . l ~ ~ J ~ ~he use of anorganic liquid as a sparkling medium for the powdersobtained by the spark erosion method leads t o theformation of a carbon matrix of high surface area thathas a stabilizing effect for dispersed amorphousmetallic pa r t i c l e ~ . ~~ ' J ~~D. Colloidal Metals

The ultimate dispersion s tate in which metals canbe prepared without major alteration of their proper-ties is tha t of metal sols. Colloidal metals have foundnumerous applications in catalysis, especially forcatalysis in solutions, and are also used for thepreparation of supported metal catalysts.155 Colloidalmetal particles prepared by growth from molecularprecursors are usually small and exhibit a narrowsize distribution. The chemical reaction most suit-able for obtaining colloidal metals by this method isthe reduction of metallic ions. Many reducing agentshave been used, the most popular being formalde-hyde, alcohol, carbon monoxide, hydrogen peroxide,and hydrazine. Metallic particles dispersed in watercarry electric charges and must be protected againstaggregation, The presence of electrolytes destabilizessuch colloidal dispersions and addition of a syntheticpolymer, both in soluble and insoluble form, protectssuch dispersions from coagulation. In practice, re-


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Methods for Preparation of Catalytic Materialsduction can either precede or follow the interactionof the colloid with the polymer. The simplest methodof preparation is refluxing of alcoholic solutions ofmetal salts in the presence of the protective polymer.Bimetallic systems can be prepared by the sameprocedure from solutions containing two metal com-p o u n d ~ . ~ ~ ~ , ~ ~ ~list of metals which have beenprepared and characterized in colloidal solutionsincludes P t, Pd, Ru, Rh, Os, Au, Ag, and bimetallicsystems such as Pd/Pt.158

The unsupported metallic catalysts derived fromdifferent transformation routes do serve to producepractical materials suitable for use in industrialprocesses. However, mounted catalysts offer spe-cial advantages as well as disadvantages a s will bediscussed in the following sections. Our focus ispragmatic; we emphasize the methodologies and theunderlying physicochemical processes tha t

methods for preparation of catalytic materials

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