JOURNAL OF CATALYSIS 94,69-78 (1985)

X-Ray Photoelectron Spectroscopy Study of Silica-Alumina

Catalysts Used for a New Pyridine Synthesis1

R. BICKER,* H. DEGER,* W. HERZOG,* K. RIESER, t H. PULM,:j: G. HOHLNEICHER,:j:AND H.-J. FREUND:j:2

* Hoechst A.G., 6230 Frankfurt-am-Main 80, Germany; tGrace GmbH, 6520 Worms, West Germany; and

:f:lnstitutfiir Organische Chemie, Universitiit zu K6/n, Greinstrasse 4,5000 K6/n 4/, West Germany

Received August 2, 1984; revised December 31, 1984

Alumina-doped silica gels used as catalysts for a new pyridine synthesis from cyclopentadieneand ammonia have been investigated using X-ray photoelectron spectroscopy. Two different typesof aluminum species are identified. Their chemical nature is assigned to a "normal" Al oxide and amore electron-rich AI species (Br~nsted centers). The distribution of the two species within thesamples as a function of distance from the surface is not homogeneous and is strongly dependentupon the preparation of the catalyst. The concentration of the Br~nsted centers in the proximity ofthe catalyst surface seems to correlate with the activity of the catalyst in the pyridine synthe-sis. C 1985 Academic Press, Inc.

INTRODUCTION ily available at low cost from the Cs-cut ofrefined oil:

Pyridine, a valuable intermediate in a va-riety of industrial processes, is manufac-tured today either by fractional distillationof coal tar or by condensation of acetalde-hyde and ammonia (1). These technicalroutes for the production of pyridine arelimited. For example, coal tar contains onlylittle (0.1 %) pyridine, and its isolation andpurification are difficult and very expen-sive. In addition coking plants are disap-pearing more and more since the demandfor coke and coking gas is decreasing. Inthe aldehyde-ammonia process the forma-tion of pyridine is always accompanied byside products (picolines, lutidine, collidine)which limit the economical feasibility ofthis process.

As an alternative to these processes wehave investigated the heterogeneously cata-lyzed formation of pyridine from cyclopen-tadiene (2). The latter hydrocarbon is read-

I Dedicated to Prof. Dr. R. Sammet on the occasion

of his 65th birthday.2 To whom correspondence should be addressed.

Permanent address: Institut fUr Physikalische undTheoretische Chemie der Universitat Erlangen-Niim-berg, Egerlandstr. 3, 8520 Erlangen, Germany.

69

0021-9517/85 $3.00Copyright Q 1985 by Academic Press, Inc.

All rights of reproduction in any form reserved.

0 , NH3/020~N

A variety of catalysts have been examined.Alumina-doped silica gels were found to bemost active in this process. The catalystswere prepared by incorporating aluminumsalts during silica gel formation. Their ac-tivity could be varied by adding the saltsduring different stages of silica gel develop-ment. In order to characterize the catalystand to investigate the chemical state of thealuminum, which is possibly the activemetal species, we have applied X-ray pho-toelectron spectroscopy (XPS). In contrastto other methods often used, (such as IRspectroscopy (3) and titration methods (4»,XPS allows one to specify directly in a sur-face sensitive manner the "chemical state"of the operative metal species in the vicin-ity of the catalyst surface. The present pa-per is intended to give preliminary experi-mental hints about the chemical nature ofthe metal species and to trigger further ex-perimental woFk on these materials.

The paper is organized as follows. We

70 BICKER ET AL.

first briefly present the experimental condi-tions of the study and the applied proce-dures. In a second part the results are out-lined and discussed with respect to an iden-tification of the active species. A final partsummarizes the conclusions of our study.

EXPERIMENTAL

The pyridine synthesis was carried out ina stainless-steel reactor (16 x 70 mm) filledwith the catalyst (pellets, 2-mm diam.). Thereactor was heated in a salt bath (T =300°C) and exposed to a gas stream consist-ing of cyclopentadiene (0.4 g/h)/NH3 (5 li-ters/h)/H2O (10 g/h)/air (7.5 liters/h). Thegas was analyzed by gas chromatography(210°C, 1.5-m Purpak QS column) beforeand after the reaction. From the differencethe yield of pyridine was determined.

Photoelectron and Auger spectra wererecorded using a modified Leybold-He-raeus LHS 10 electron spectrometer in the£\B/E = constant mode (5). The X-ray tubewas equipped with a Si anode (6) (1739.5eV) to allow ionization of Al Is electrons.For characterization purposes we comparein Fig. 1 the Si Ka- and Mg Ka-excitedspectra of a gold foil in the region of the Au

Au" I

.t.I, ,: V/ Vy .Mg-K~ \...-.

1~v

~ I ~.V ' .~. \ .12

"\Si-K(1 v-.-JV .

\ ~I I.5 -~ 4'0 ,"VI

FIG. 1. Mg Ka (1254.6 eV)- and Si Ka (1739.5 eV)-induced Au 4f spectra of a polycrystalline gold foilThe Au 4frn-line maximum is given as energy zero.

4f spin-orbit doublet. Although the separa-tion of the two components of Si Ka radia-tion is larger than for Mg Ka radiation theobserved shapes of the Au 41 doublets areidentical. Differences appear at lower bind-ing energies due to the different satellitelines associated with Si Ka radiation ascompared to Mg Ka radiation.

The spectrometer is interfaced (7) with aPDP 11/10 computer (Digital Equipment).Data were taken every 0.1 eV with a dwelltime of 2 s/channel. To keep the signal to areasonable noise level, spectra were aver-aged up to 150 scans/spectrum in thosecases in which signals of small intensitiesare of significance. The spectrometer en-ergy scale was calibrated using the knownbinding energies of Au 4f and Cu 2p (8).Charging was checked upon using previ-ously described methods (9, 10). To ana-lyze our data, however, we use the Augerparameter which is independent of samplecharging.

The silica-alumina catalysts were pre-pared by incorporating aluminum via alumi-num salts into the silica gel during differentstages of the silica gel development. Someof the dry catalysts prepared in this waywere annealed at 300°C. The stage of silicagel development during which aluminum isdoped and the amount of incorporated alu-minum govern the activity of these silica-alumina catalysts, as seen in Table 1. Werefer to the samples by the roman numeralsgiven in Table 1. The relative activities ofthe samples are

II < IV < V < I < III < VI.

The activity of VII has not been measured.Samples V and VII were prepared by an-

nealing samples IV and VI at 300°C for 24h. This procedure has been applied to studythe effect of loss of volatile components ofthe catalysts on the observed spectra. Asreference we used the metal and oxide sig-nals of an oxidized aluminum foil.

The silica-aluminas of types I to V werestudied as received. In particular they havenot been sputtered before recording the

71X-RAY SPECTROSCOPY OF SiO2-AI2O3 IN PYRIDINE SYNTHESIS

TABLE 1

Characterization of Catalyst Samples

No." Amountof AI

(AI2O3)(%)

Stage of dopingthe aluminum

1(1) 2.2 36.7 During an intermediatestage before the develop-ment of the internal silicagel structure

During an early stage of thesilica gel development

During a late stage after thedevelopment of theinternal silica gel struc-ture

During an intermediatestage before the develop-ment of the internal silicagel structure

No. IV heated 24 h at 300.CDuring a late stage after the

development of theinternal silica gel struc-ture

No. VI heated 24 h at 300.C

6.5

1.2

20.7

45.5ill (3)

3.8 27.2

vVI (5)

3.80.9

36.149.5

~!

vn 0.9

Note. The AI contents in the silica gels are determined asAI2O). The last column contains specific remarks on the indi-vidual preparation of the sample before spectroscopic study.

Q Silica-alumina cogels from Grace GmbH, Worms; (1) No.

4, (3) No. 10, (5) No.8, (2) No.3, (4) No.6.

: :'--!:!.!::-X~, ,, ', :, '80 : : 7S: i 70 EBloVl-I ..I'! I -.

FIG. 3. Al 2p spectra of samples I-VII. The peakareas have not been norInalized. For colnparison aspectruln of an oxidized AI foil (AI(ox» is shown at thebottOIn.

spectra. Figure 2 shows a 200-e V -wide scanof the region of Si 25, Si 2p, and AI2p signalsto illustrate the relative signal intensities ofSi and AI ionizations. The reproduced spec-trum reflects the relative amount of AI andSi within the escape depth of sample II,which contained the largest amount of alu-

minum. Our discussion in the next sectionis restricted to the electronic structure ofthe aluminum sites. Therefore we show inFigs. 3-5 the AI Is, AI 2p, and the mostintense peaks of the AI KLL photoinducedAuger spectrum (14) of samples I to VII.We include for comparison the spectra ofan oxidized Al foil. Figure 6 shows the fullphotoinduced AI KLL spectrum of the oxi-dized aluminum foil, excluding the 's stateat lowest energy originating from the 2S~p6double-hole-state configuration.

Sih

FIG. 2. Wide scan (200 eV) of a representative sam-ple (II) below 50-eV binding energy. The peaks in-duced by the X-ray satellites of the source are markedby "Sat." The peaks of the Si KLL Auger spectrumare characterized by their term symbols.

RESULTS AND DISCUSSION

Figure 3 shows the spectra of the AI 2psignals as determined for samples I to VIItogether with the signal of the oxidized alu-minum foil which is included at the bottomof Fig. 3.

Before discussing the spectra of Fig. 3 wenote that ionization of a 2p electron out of a

Yield ofpyridine

(%)

72 BICKER ET AL.

VII with the spectrum of the oxidized AIfoil it is obvious that those spectra exhibitlinewidths much too large to be due to asingle species. Clearly, spectra I, II, IV, V,and VI indicate a doublet structure. The po-sitions of the two most important compo-nents are marked AI I and AI 2. They donot coincide with metallic aluminum (AI)and aluminum oxide (Alox). All has a bind-ing energy of 74.4 eV, intermediate be-tween metallic aluminum and aluminum ox-ide. AI 2 has a binding energy of 76.8 eV,which is greater than that of aluminum ox-ide. Although the spectra of samples III andVII do not show a pronounced doubletstructure the widths of the signals is consis-tent with the superposition of spectra oftwo (or more) different chemical species inthe energy range between 73- and 77-eVbinding energy. Particularly, spectrum IIseems to indicate that there is relativelymore of AI 2 present in the probed surface

,.,i

: :, ', '

i :, ': I,: '

"KIN[eI/] 1380 1385 1390"". 1 'I ' \

FIG. 4. AI K~L3 IDz Auger spectra of samplesI-VII. The peak areas have not been normalized. Forcomparison a spectrum of an oxidized AI foil (AI(ox»is shown at the bottom.

filled 2p shell leads to two final states withtotal spin i and ! (2PI/2 and 2P3/2) and inten-sity ratio 2PI/2/2p3/2 = i. The splitting ofthese two states, however, is £1/2.3/2 = 0.4eV (11) and cannot be resolved with thechosen spectrometer setting, which allowsfor a resolution of 1.5 eV. Therefore thetwo lines observed for the oxidized foil aredue to two different chemical species,namely ionization of metallic aluminum(AI) at. 72.8 e V and aluminum oxide (Alox)at 76-e V binding energies. Satellite struc-ture can be excluded since the relative in-tensity of the two lines depends on thechemical composition of the sample, whichcan be varied by sputtering. Upon compari-son of the AI 2p spectra of our samples I to

FIG. 5. AI Is spectra of samples I-Vll. The peakareas have not been nonnalized. For CoInparison aspectruln of an oxidized AI foil (AI(ox» is shown at thebottOln.

73X-RAY SPECTROSCOPY OF SiOZ-AlzO3 IN PYRmINE SYNTHESIS

FIG. 6. AI KLL Auger spectrum of a sputtered oxidized AI foil. The peaks are assigned by their term

symbols.

region with depth of approximately 20 A. Inaddition, if we measure the aluminum con-tent using the (Al2p)/(Si 2p) intensity ratio(corrected for differences in cross section(15) we find, e.g., for samples I, II, and VI:0.24,0.30, and 0.20, respectively. Thesevalues deviate considerably from those forthe bulk (Table 1). Although one has to usethis quantitative information with cautionwe conclude that within the probed samplevolume the concentration of Al is higherthan expected from the bulk values.

In order to obtain further evidenceabout the chemical nature and number ofspecies involved in the spectra, we recordedthe Al KLL spectra of all samples. The dataare collected in Fig. 4. Again the spectrumof the. oxidized aluminum foil is shown atthe bottom of the figure. Note that the en-ergy scale now refers to the kinetic energyof the Auger electrons (14). To point outclearly the part of the Al KLL spectrum towhich we refer, Fig. 6 shows a more com-plete spectrum of an oxidized aluminum foilwhich has been sputtered slightly longerthan the one which leads to the spectrum inFig. 4. Due to the reduced escape depth ofelectrons with approximately 1400-eV ki-netic energy (-15 A), a more extended

cleaning process was necessary to producean Auger spectrum which shows the peaksof both aluminum metal and oxide.

The assignment given in Fig. 6 is straight-forward on the basis of literature data (12,14) and it is not appropriate to discuss it inthis context. We shall only compare in thefollowing the most intense multiplet com-ponent, marked as IDz, originating from theconfiguration AI2sz2p4 (14). The importantpoint is that the chemical shift between themetal I Dz state and the corresponding peakof the oxide is 7.2 e V, in agreement withdata from the literature (12). This is largerby a factor of 2 than the chemical shift ob-served for AI 2p hole states. Briefly, thisenhanced shift is a consequence of in-creased screening in the final double-holestate of the system (16).

If we return to Fig. 4 we find that theAuger spectra (IDz band) of the samples ex-hibit two bands (with the exception of I andVII with only one band) which do not coin-cide with Alox and AI (which is not even onthe energy scale of the section shown). Ac-cording to their energy positions the lineshave to be assigned to AI 2, which is themost intense one at low kinetic energies,and to AI 1 which is situated at higher ki-

74 BICKER ET AL.

0 ".,... U"U "I.' ..,...

c silicates

.silicates. tetrahedral Al

.acti.e catalysts

~,10

~~

,~:c

m'"+

N~'".1-~

:cz~

...

K

netic energies. The chemical shift betweenthe two species is now approximately 5 e V,which is about 2 eV more than observed forthe AI2p hole states. The intensities of thetwo component bands vary rather stronglywith sample preparation. As stated before,All is not observable for VII and I while itseems to be most intense for III. Theshaded areas in Fig. 4 attached to thedashed AI 1 line are included to indicatethat the energy position of the AI 2 band isnot as uniform as was suggested from theanalysis of Fig. 3. With the higher sensitiv-ity of the Auger final state to chemical shiftsthis can be resolved. For II, III, and IV theshift is slightly smaller, and for V it isslightly larger, than is indicated by thedashed line. Spectrum III seems to indicatethat there is possibly some Alox present inthe probed region of the sample volume.The relative intensities of Al 1 and AI 2 asobserved in Auger electron spectroscopy(AES) are different from the relative inten-sities in XPS. There are several reasons forthis behavior which are hard to separate.Thus, the sample is inhomogeneous andeven a small reduction in escape depth forAl KLL in comparison to Al 2p may con-tribute to the observed changes. In addi-tion, it is known (15) that lineshape changesobserved for Auger signals of a specific ele-ment can influence relative intensities. Wenote, however, that samples with high AIlintensities in Fig. 3 also exhibit the largestAl 1 signal in the Auger spectra. In thisstudy the Auger signal intensities are not ofprimary interest. The important informa-tion is the relative chemical shift of the Au-ger signal in comparison with AI 2p chemi-cal shifts as observed by XPS to determinethe chemical nature of the species givingrise to AI 1 and AI 2 signals.

To obtain information on the distributionof AI 1 and Al 2 we use the AI Is spectrashown in Fig. 5 in comparison with the Al2p spectra in Fig. 3. For reference purposesthe spectrum of a sputtered, oxidized AIfoil is shown at the bottom of Fig. 5. Thebinding energies are in the range 1550 to

..~~

1570 eV. The kinetic energy of the photo-electrons is therefore between 150 and 200eV. With respect to the AI 2p spectra thismeans a considerable reduction in escapedepth (to a value below 10 A). It is clearthat the predominant species is AI 2. Infact, a significant amount of AI 1 is onlypresent in sample VI, and some is indicatedin sample III. Obviously the topmost sur-face region is free of AI 1 in most cases.

Before we discuss the results of our mea-surements in connection with the catalyticactivity of the samples, let us try to identifythe chemical nature of the two species, sofar only characterized as AIl and AI 2. Thiscan be done by comparison with spectra ofa variety of other aluminum-containing ma-terials using the two-dimensional plot inFig. 7. This method has been put forwardby Wagner (17,18). The binding energies ofthe AI2p bands are plotted versus the mea-sured kinetic energies of the AI KLL IDzAuger band. The diagonal lines refer to theright-hand ordinate where the modified Au-ger parameter a, as defined by Wagner (17,

.AI metGL ~~- --~ ~.~---,~-

.ALAs

A ALN

1395

1390

§.!J'~N 1385

:i(

z..¥

78 76 74 72

EB AL 2p

FIG. 7. Chemical state plot of aluminum-containingmaterials as taken from Ref. (19) with the differencesdiscussed in the text.

X-RAY SPECTROSCOPY OF SiO2-AI2O3 IN PYRIDINE SYNTHESIS 75

18), is plotted. a is defined as the differencebetween the Auger kinetic energy and thephotoelectron kinetic energy. In order toavoid sign changes in this parameter, de-pending on whether the kinetic energy ofthe Auger electron or the kinetic energy ofthe photoelectron is the larger absolutenumber, the difference is always added tothe photon energy hv. a is constant alongthe diagonal lines and has the advantage ofbeing free from charging effects, since bothphotoelectron kinetic energy and Augerelectron energy are influenced by the samesurface potential if taken on the same sam-ple. In other words, in the presence ofcharging the data points move along the di-agonal. Most of the entries [36] shown inFig. 7 have been taken from Wagner et al.(19). The values taken from the presentwork are also indicated as filled circles.Disregarding our own values, Fig. 7 is areproduction of Fig. I and 4 in Ref. (19)with one difference. We have shifted all en-tries except the ones for AIN and AlAs withrespect to abscissa and ordinate along thediagonal in such a way that the binding en-ergy separation between oxidized and me-tallic aluminum agrees with our results (3.2eV) of Fig. 3. The binding energy separa-tion between AI and Alox shown in ourspectrum is in agreement with literaturedata (12, 20). This inconsistency in Fig. 1of Ref. (19) has also been recognized byWagner (21). This point is not important forthe future analysis but is mentioned herefor completeness.

Let us now turn to the details of Fig. 7.The entries fall into three groups, namelyaround 76-eV binding energy, between 73-and 75-eV binding energy, and metallic alu-minum below 73-e V binding energy. Thegroup around 76-eV binding energy fallsinto two subgroups, the tetrahedrally coor-dinated aluminum centers in aluminum sili-cates (8), namely zeolites, with an Augerparameter between 1460 and 1461 eV andoctahedrally coordinated aluminum centersin oxides, hydroxides (0), and silicates (0)with an Auger parameter around 1462 e V

(19). The binding energy derived from alu-minum oxide layers on top of metallic alu-minum is shown as a filled circle (8) withinthe region of aluminum oxides, i.e., octahe-drally coordinated aluminum. The Al2 spe-cies with an Auger parameter of 1461.5 eV(filled circle with arrow) is found in the re-gion of octahedrally coordinated aluminumin silicates as expected. Since the values forAl 2 are almost independent of samplepreparation the single entry represents thevalues for all samples. The values for Al 1lie within the second group of values above74-eV binding energy and vary with respectto the Auger parameter within a range ofslightly more than 1 e V. Five entries arepresented because for the other two sam-ples an Auger peak could not be observed,as pointed out in connection with Fig. 5.The variation of the Auger parameterclearly shows that AI 1 does not represent asingle unique chemical species, while Al 2could be suspected to be unique. Note thatAl 1 and Al 2 originate from one sample,which means that there are definitely twodifferent chemical species present, whilefor the other aluminum-containing mate-rials only one line has been observed, ex-cept with H-Zeolon (marked with a curvedbracket). In the latter case the line wasbroader, so that two species might havebeen present (19,21).

Having thus established the different na-ture of Al 1 and Al 2 we can use Fig. 7 todraw conclusions about the electroniccharge on the aluminum centers All and Al2. This cannot be done by just using thedifference in binding energy of the Al 2plines, because the observed shift in thespectrum is a superposition of initial-state(electronic charge) and final-state effects,as discussed by various authors (22). TheAuger parameter, however, which is inde-pendent of sample charging, can be used todetermine in an approximate manner thecontribution of the final state. This has beenshown by Wagner et at. (17, 18), Wertheimet at. (23), and recently by Pulm et at. (6,24). The difference between the observed

76 BICKER ET Al

TABLE 2

Contribution of the Electron Density Difference inthe Neutral Ground States to the Shift in Binding

Energy Observed in the XPS Experiment

Auger parameter and a reference value, forwhich we use the Auger parameter of me-tallic aluminum, is approximately equal totwice the relaxation contribution to theshift. Since relaxation always stabilizes thefinal state, the observed shift has to be re-duced by the relaxation contribution. Ap-plying this procedure to our data we end upwith binding energy differences due to ini-tial-state effects as shown in Table 2.

On the basis of Table 2 AI 1 obviouslycarries a higher electron density than AI 2.Since our samples only contain silicon, alu-minum, oxygen, and hydrogen we concludefrom Fig. 7 that AI 2 is due to photoemis-sion from oxygen-coordinated aluminumcenters. It is not clear whether the alumi-num centers are octahedrally or tetrahe-drally coordinated although the observedAuger. parameter is closer to the one foundfor octahedrally coordinated aluminumcenters in silicates. For the species thatgives rise to the AI 1 emission we proposealuminum centers of Br~nsted type.Clearly, this assignment can only be tenta-tively based solely on our XPS results. Theapplication of other experimental tech-niques is highly desirable to provide fur-ther conclusive evidence. However, the as-signment of AI 2 to a species whosechemical nature is different from an oxidecenter (AI 2) is reasonable considering thedifference in the observed Auger parame-ter. The Br~nsted centers are formed from

Shift(eV)

0.02.61.020.981.350.970.98

Species

Alme!Al2AlIaAllbAllcAlldAIle

the reaction of aluminum centers in three-fold oxygen coordination with hydroxylions from water (25). If the OH bond disso-ciates, the remaining oxygen ion is only co-ordinated to one aluminum atom and candonate more electron density toward thealuminum atom. This in turn leads to thesmaller chemical shift with respect to me-tallic aluminum.

Summarizing the spectroscopic analysisso far, we find evidence for the presence oftwo different aluminum species, which canbe identified as the "normal" octahedrallycoordinated aluminum oxide species usu-ally found in silicates, and an electron-richspecies, tentatively assigned to Br!1instedcenters. The distribution of the two ob-served aluminum species as a function ofdistance from the surface is not homoge-neous. The presence of the Br!1insted cen-ters in the proximity of the sample surfacestrongly depends on the preparation proce-dure of the catalyst material. The amount ofBr!1insted centers close to the surface doesnot scale with the amount of aluminum inthe sample. In fact, almost the inverse istrue. Sample VI which was impregnatedwith the smallest amount of aluminumshows the largest Br!1insted aluminum sig-nal in the AI Is spectrum, indicating a highconcentration close to the sample surface.This same sample leads to the highest pyri-dine yield in the catalyzed reaction asshown in Table 1. Sample III with secondlowest aluminum load shows the mostasymmetric peak in Fig. 5 and we suspect arather high Br!1insted center concentrationin surface proximity. Again, this sampleleads to a rather high pyridine yield.

We therefore deduce from our study,qualitatively that there is a correlation be-tween catalyst activity and AI 1 concentra-tion in the surface region. Figure 5 allowsus to draw this conclusion since in this caseonly a small region below the sample sur-face is probed while for the spectra shownin Fig. 3 the escape depth of the measuredelectrons is too large to give us the relevantinformation directly. Figure 3 leads via Fig.

X-RAY SPECTROSCOPY OF SiOZ-AlZO3 IN PYRIDINE SYNTHESIS 77

able to yield high Brtpnsted center concen-tration close to the catalyst surface.

7 to an identification of the chemical nature

of the involved species. To what extent the

presence of the second observed chemical

aluminum species besides the Br.0nsted

centers are relevant to the catalytic process

cannot be deduced from this study. Usually

it is assumed, however, that a single type of

catalytic center is active in processes onalumino-silicate surfaces (25). REFERENCES

The mechanism of the catalytic process, 1. Budzinski, H., Chern. Ind. (London) 33, 529and in particular the question about the spe- (1981); Beschke, H., Aldri~himi~a Acta 14, 13cies that is produced by interaction of am- (1981); Beschke, H., and Fnedench, H., Chem.-

. / ..Ztg. 101, 377 (1977); Beschke, H., and Kleemann,

moma and or cyclopentadlene with the alu- A. "Ullmanns Ency klo pa" die de T h . h...,r ecmcen

DllnosIlIcate surface, cannot be answered Chemie," Vol. 19, p, 591. Verlag Chemie,

from the study presented here. It is sus- Weinheim, 1981.pected, however, from infrared studies 2. EP 46897 (Hoechst AG); DOS 3244 032 (Hoechst

that, by interaction of NH3 with Br.0nsted 3 ~G).h M Od T Ik b ..t al .. 1 ...me ara, ., a, ., e e, Y., and Hlshida,

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tern so that the pyridine heterocycle can be 31, 435 (1981).formed 5. Pollaschegg, H. D., Appl. Phosp. 4,63 (1974).

.6. Pulm, H., thesis. Koln, 1984.

CONCLUSIONS 7. Gonska, H., and Baum, H. G., unpublished.8. Johannson, G., Hedman, J., Berndsson, A.,

We have presented results of an X-ray Klasson, M., and Nilsson, R., J. Electron Spec-photoelectron spectroscopic study of sil- trosc, Relat. Phenom. 2, 295 (1973). .

. al . tal t d c .9. Freund, H.-J., Gonska, H., Lohnels, H., andIca- umma ca ys s use lor a new pyn- Hohlnel .cher G J El t S t R I t' ., .ec ran pec rosc. ea.

dme synthesIs. The results mdlcate the Phenom. 12,425 (1977).

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ferent depth profiles within the catalysts. (1977),Th h . al t f th t .' 11. Eberhardt, W., Kaloffen, G., and Kunz, C., Solid

e c emiC na ure 0 e wo speCIes IS State Commun, 32 901 (1979).

identified as "normal" aluminum oxide 12. Castle, J. E., and Wert, R.H., J, Electron Spec-

centers and species with higher electron trosc. Relat. Phenom. 18, 355 (1980).density on the metal center. We tentatively 13. Ertl, G., and Kiippers, J., "Low Energy Elec-assign the latter species to Brjiinsted cen- trans and Sw:ace Chemistry" ~H. F. Ebel, Ed.),t Th t t ' f th B d t d Monographs In Modern Chemistry. Verlag Che-

ers. e concen ra Ion 0 e r",ns e mie Weinheim 1974.

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face seems to correlate with the activity of copy." Wiley, New York, 1972.the catalysts in the pyridine synthesis. 15. Briggs, D., and Reviere, J. C., in "Practical Sur-

The results of our study explain why a face Analysis by Auger and XPS" (D. Briggs and

1 1 d . f al .. uffi . t t M. P. Seah, Eds.).Wiley, New York, 1983.ow oa mg 0 UDllnUm IS S Clen 0 16. Aksela, S., J. Electron Spectrosc. Relat. Phenom.

yield high pyridine yields when we choose 24, 91 (1981).the silica gel phase after development of its 17. Wagner, C. D., Faraday Discuss. Chern. Sac, 60,

internal structure for doping the aluminum. 291 (1975),A higher aluminum loading at an earlier 18. Wagner, C. D., J. Electron Spectrosc. Relat. Phe-

d 1 f h . li 1 b . 1 nom. 10, 305 (1977).eve opment stage 0 t eSI cage 0 VIOUS Y 19. Wagner, C. D., Passoja, D. E., Hillery, H. F.,

influences the incorporation of the alumi- Kinisky, T. G., Six, H. A., Jansen, W. T., and

num into the gel in a way that is unfavor- Taylor, J. A., J. Vac. Sci. Technol. 21,933 (1982).

ACKNOWLEDGMENT

The Hoechst group kindly thanks Professor H. Jen-sen for continuous support of this work. H.J.F.thanks the "Fonds der Chemischen Industrie" for fi-nancial support.

78 BICKER ET AL.

ter, H. U., Drews, J., and Eberz, U., Less Com-mon Metals, in press; G. Hohlneicher, H. Pulm,H.-J. Freund, to be published.

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