and

no

mi

ab,c, T

Hall,

stitute

igashicDepartment of Applied Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama,

e three metals tested, ruthenium revealed the most attractive catalytic

t of metal could be lowered from 2.0 to 0.1% of the weight of support

provided that appropriate catalysts are employed.

Among the catalysts reported, those derived from LDH

Applied Catalysis A: General 27precursors demonstrated a more pronounced catalytic perfor-

mance compared to catalysts synthesized by conventional

techniques [2128]. Traditionally, LDHs are prepared by

* Corresponding author. Tel.: +1 613 562 5800x6042;

fax: +1 613 562 5170.

E-mail address: andrey_tsyganok@yahoo.com (A.I. Tsyganok),

t.hayakawa@aist.go.jp (T. Hayakawa).

0926-860X/$ see front matter # 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2004.07.030without any decrease in catalytic activity or in selectivity to syngas product.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Methane oxidation (partial); Dry reforming; Synthesis gas; Noble metal catalysts; Layered double hydroxides

1. Introduction

Production of hydrogen and synthesis gas from natural

gas- or methane as its major component over supported

metal catalysts represents an industrial process of utmost

significance [17]. Although the recent priority has been

given to nickel catalysts due to their low cost, the precious

metals still remain under consideration, as they offer not

only high catalytic performance but also a lower coking

capacity and therefore a longer operation lifetime in

reactions of methane reforming to syngas with steam,

carbon dioxide, and by partial oxidation [819].

The possibility of uniting a slightly exothermic partial

oxidation of methane (reaction 1 in Table 1) with a highly

endothermic reaction of dry reforming (reaction 2) has already

been proposed and experimentally demonstrated in the

literature [20]. Such a combined reaction (for example,

reaction 3 that was studied in this work) needs a considerably

lower amount of external heat to keep a catalytic reactor at a

high temperature. Such information suggests that synthesis

gas from methane feedstock can be produced at a lower cost,ethylenediaminetetraacetate (EDTA) to anionic species. Among th

performance toward PODRM reaction. It was shown that amounReceived 18 February 2004; received in revised form 8 July 2004; accepted 22 July 2004

Available online 1 September 2004

Abstract

Combined partial oxidation and dry reforming of methane (PODRM) to synthesis gas at 850 8C over noble metals, supported on MgAlmixed oxide, has been studied. Ruthenium, rhodium, and platinum catalysts were prepared from layered double hydroxide (LDH) precursors,

synthesized by a reconstruction reaction of MgAl mixed oxide. Due to a memory effect of a calcined hydrotalcite, a layered structure of

catalyst precursors was recreated upon exposure of the mixed oxide to an aqueous solution of noble metals, which were pre-chelated withHigashi-Hiroshima 739-8527, JapanCombined partial oxidation

to synthesis gas over

on MgAl

Andrey I. Tsyganoka,*, Mieko InabKatsuomi Takehira

aDepartment of Chemistry, University of Ottawa, DIoriobInstitute for Materials and Chemical Process, National In

Tsukuba Central 5, 1-1-1 Hdry reforming of methane

ble metals supported

xed oxide

Tatsuo Tsunodab, Kunio Suzukib,akashi Hayakawab

10 Marie Curie Street, Ottawa, Ont., Canada K1N 6N5

of Advanced Industrial Science and Technology (AIST),

, Tsukuba 305-8565, Japan

www.elsevier.com/locate/apcata

5 (2004) 149155

coprecipitation of Mg(II) and Al(III) cations at basic pH

[2932]. An additional (guest M) metal is introduced as a

cationic species, which results in a partial substitution of Mg

or Al in brucite-like layers and therefore a formation of a solid

with lamellar structure similar to that of a hydrotalcite

[Mg6Al2(OH)16]CO34H2O (MgAlCO3; HT) [3336] or ameixnerite [Mg6Al2(OH)16](OH)24H2O (MgAlOH) [3740].

An alternative technique of introducing metals into LDH

structure has also been reported [41,42]. It was based on a

well-known capacity of metal cations to form highly stable

EDTA chelates that are negatively charged (MY) [43,44];these were further introduced into LDH via coprecipitation,

anion-exchange, and reconstruction reactions [45]. Versa-

tility of a novel approach was then demonstrated by

synthesizing meixnerite-like LDHs bearing chelates of

memory effect of calcined HT, i.e. on the ability of

Mg(Al)Ox mixed oxide to re-create (or reconstruct) a

layered structure upon rehydration with aqueous solutions

[4749]. Results of catalytic activity tests and characteriza-

tion of catalysts and catalyst precursors by X-ray powder

diffraction (XRD), thermogravimetry and differential

thermal analysis (TG-DTA), and transmission electron

microscopy (TEM) techniques will be presented and

discussed. It will be shown that the amount of a supported

noble metal can be substantially lowered without viewable

reduction in the activity/selectivity of a catalyst.

2. Experimental

2.1. Reagents and solutions

Chemicals from Wako Pure Chemical Industries, Ltd.

included the nitrates of Mg(II) and Al(III); sodium

hydroxide, carbonate, and edetate; and platinum(IV)

A.I. Tsyganok et al. / Applied Catalysis A: General 275 (2004) 149155150

Table 1

Enthalpy values for reactions of methane studied in this work

Reaction DH0 (kJ per mole of

CH4) at temperature

25 8C 850 8C

(1) CH4 + 1/2O2 = 2H2 + CO 35.65 21.79(2) CH4 + CO2 = 2H2 + 2CO 247.34 260.35

(3) 11CH4 + 5O2 + CO2 = 22H2 + 12CO 9.92 3.86precious metals [46]. For the test reaction of dry reforming

of methane, a ruthenium (2.0 wt.%) catalyst was found the

most suitable among all the noble metals tested.

In this work, we present details of preparing ruthenium,

rhodium, and platinum catalysts, supported on MgAl

mixed oxide. The method of synthesis was based on aFig. 1. Powder XRD patterns of MgAl double hydroxides bearing noble

metal EDTA-chelates (pattern of a reference meixnerite is shown for

comparison).Fig. 2. Thermal behavior under oxidative conditions of MgAl LDHs

having EDTA-chelated noble metals: MgAlRuY (A), MgAlRhY (B),

MgAlPtY (C), and a reference meixnerite (D). LDH loading: 0.040.05 g;air flow: 20 cm3 min1; rate of temperature increase: 5.0 8C min1.

chloride dissolved in a hydrochloric acid. The nitrate of

ruthenium(III) nitrosyl was purchased from Sigma-Aldrich

Co., the nitrate of rhodium(III) was purchased from Strem

Chemicals Inc. All the reagents had 9899% purity and were

used without additional purification. Distilled and deionized

water was used throughout the work.

Amounts of precious metals corresponded to 2 wt.% to

the weight of Mg(Al)Ox support unless indicated otherwise.

Equimolar amounts of sodium edetate were taken for

preparing metal complexes. Chelation of noble metals with

EDTA was carried out in an aqueous solution at room

temperature.

Methane and carbon dioxide were from Tokyo Gas

Chemical Co. and Showa Tansan Co., respectively. Oxygen

as well as nitrogen to dilute a feeding gas mixture was

from a pipeline supply. All gases employed were of purity

higher than 99.99%.

2.2. Synthesis of LDHs

ground up to a fine powder, which was further calcined in air

at 500 8C for 16 h to produce Mg(Al)Ox mixed oxide.Preparation of a reference material of meixnerite

involved rehydration of HT-derived MgAl mixed oxide

with degassed and deionized water. The oxide powder (2 g)

was placed into a 100 cm3 conical flask with a Teflon-

coated stirring magnet bar. Then the flask was filled with

water to almost no headspace volume and tightly capped

in order to minimize contamination of the synthesized

solid with carbonate that would come from CO2 of air.

The suspension was kept under vigorous stirring for 4 h at

room temperature. Then followed the steps of filtering a

solid out, rinsing it with water, and drying in air as for HT

synthesis.

Incorporation of noble metals into MgAl LDHs was

done by reconstruction reactions using aqueous solutions of

pre-chelated metals. Each metal salt and sodium edetate,

taken in equimolar amounts, were first placed into a conical

flask (100 cm3 capacity) and dissolved in 80 cm3 of water.

Then pH of the solution was adjusted to 10.5 by dropwise

A.I. Tsyganok et al. / Applied Catalysis A: General 275 (2004) 149155 151To prepare synthetic hydrotalcite, we first dissolved

Mg(II) and Al(III) nitrates (150 and 50 mmol) in distilled

and deionized water (100 cm3). Then this solution was

dropwisely added to a 200 cm3 aqueous solution of sodium

carbonate (200 mmol), which was poured into a 3-neck

round-bottom reaction flask and pre-heated to 6063 8C.During coprecipitation, the slurry was vigorously stirred

with a Teflon-coated magnet bar and kept at pH 10 by

dropwise addition of 1 M solution of NaOH. After complete

addition of the metal nitrates solution, the suspension was

stirred at 6063 8C for an hour, followed by ageing for 18 hwithout stirring at the same temperature. Thus synthesized

colorless solid was filtered out, rinsed with a large volume of

water, and dried in air at 80 8C for 16 h. The dried solid was

Fig. 3. Powder XRD patterns of ruthenium-containing MgAl mixed

oxides, obtained from LDHs by calcination in air at 1000 8C: Ru2.0%/Mg(Al)Ox (A), Ru

0.5%/Mg(Al)Ox (B), and Mg(Al)Ox from a referencemeixnerite (C).Fig. 4. Combined partial oxidation and dry reforming of methane over

noble metal catalysts: reaction temperature, 850 8C; catalyst loading,0.03 g; gas feed, CH4/O2/CO2/N2 = 35.2:16.2:3.18:45.8 (vol%); F/W spacevelocity, 226666 cm3 g1 h1; pressure, 1 atm.

addition of 1 M NaOH solution. After that, a fine powder of

Mg(Al)Ox mixed oxide (2 g) was put into the solution, and

the slurry was vigorously stirred by Teflon-coated magnet

bar at room temperature. After stirring for 4 h, the solid was

filtered out, rinsed with 1000 cm3 of water, and dried in air at

80 8C for 16 h.

2.3. Characterization techniques

XRD patterns of powdered samples were recorded at

room temperature under air using a MacScience MXP18

diffractometer with a Cu Ka irradiation source (l =1.54056 A) at voltage 40 kV and current 50 mA.

TG-DTA studies of solids were carried out under a flow of

air (20 cm3 min1) with TGA-50 and DTA-50 analyzers(both from Shimadzu Co.) using 0.04 to 0.05 g of a sample

and a 5 8C min1 temperature increase rate.Specific surface area of solids calcined was measured by

N2 adsorption at liquid nitrogen temperature using a

Quantachrome Automated Gas Sorption System and

powdered samples (0.120.13 g), which were outgassed at

320 8C for 3 h.

A.I. Tsyganok et al. / Applied Catalysis A: General 275 (2004) 149155152

Table 2

Surface areas of the mixed oxides derived from MgAl LDHs

Starting LDH material Treatment Specific surface area (m2 g1)a

(1) MgAlRuY (i.e. precursor of Ru2.0%/Mg(Al)Ox catalyst) Calcination in air at 1000 8C for 5 h 70(2) MgAlOH Calcination in air at 1000 8C for 6 h 80(3) MgAlCO3 Calcination in air at 1000 8C for 6 h 82

a Measured by BET method.Fig. 5. High-resolution TEM micrograph image of a spent Ru2.0%/Mg(Al)Ox cataly

as in Fig. 4.st after PODRM reaction at 850 8C for 5 h: reaction conditions are the same

approached the value, the sampling of post-reactor gas phase

average cationcation distance in the brucite-like layers

remained unchanged regardless of the kind of anionic

complex introduced.

As (i) the amount of metal complexes introduced was

considerably lower than that allowable by stoichiometry, (ii)

the time of LDH syntheses was shortened to 4 h, and (iii) due

to a high rate of LDH reconstruction from MgAl mixed

oxide [50], the synthesized LDHs were most probably

similar to a meixnerite, with introduced species being

located on the outer surface of the reconstructed LDH

crystallites.

Similar thermal behavior of reconstructed LDHs and of a

reference MgAl-OH was observed as well. As TG analysis

revealed, all the synthesized solids experienced a 2-step

reduction in weight, with a final weight loss of 43.0 to

44.6%. In turn, the reference MgAl-OH revealed a 36.8%

reduction in weight. Two endothermic peaks appeared in

DTA profiles of all the synthesized solids and a reference

meixnerite, which was typical of MgAl LDHs (Fig. 2). The

first peak at 220230 8C was due to the release of structural

alysis A: General 275 (2004) 149155 153was started. Analysis of reaction products was carried out by

gas chromatography (GC) using columns packed with

Porapak Q and molecular sieve 5A. After 5 h time on stream

at 850 8C, admission of CH4, O2, and CO2 was stopped, andreactor was cooled to room temperature under flow of

nitrogen. Spent catalysts were removed from the reactor and

some of them were characterized with XRD and TEM

techniques.

3. Results and discussion

3.1. Incorporation of noble metals into the structure of

MgAl double hydroxide

Solutions of all three noble metal chelates were

intensively colored. This made it simple to control the

efficiency of chelates incorporation into reconstructed

LDH. In fact, after 4 h stirring at room temperature, the

synthesized solids acquired the color of chelates, while an

aqueous solution, employed for rehydration, was colorless

after filtration. Both findings indicated the complete

involvement of noble metal chelates into a solid matrix of

the synthesized materials.

Formation of a layered structure of the rehydrated solids

was confirmed by the results of XRD studies. Diffraction

patterns very similar to that of a meixnerite MgAl-OH were

recorded for all the prepared solids (Fig. 1). The presence of

a set of three reflection peaks at 2Q 11.08, 22.78, and 34.58indicated that the solids had a regular layered structure with

a 3R symmetry for layers stacking as in MgAl-OH and HT.

No shift was observed in the peak position at 2Q 60.48TEM was done with a JEOL JEM-2010F machine

equipped with a Gatan slow-scan camera for high-resolution

observation. The accelerating voltage applied was 200 kV.

Specimens for TEM were prepared by standard techniques.

Element composition of TEM specimens was measured by

registering an emission of X-rays from specimen with an

attached Link ISIS energy dispersive X-ray spectrometer

(EDS) from Oxford Instruments plc.

2.4. Catalytic activity tests

Prior to catalytic tests, every LDH was pressed into a

tablet and ground up to sieve a 212425 mm fraction ofparticles. These were calcined in air at 1000 8C for 5 h. Aresultant mixed oxide (0.03 or 0.15 g in loading) was diluted

with Wakogel silica G (0.15 g; 300600 mm), and themixture was placed into a quartz tube reactor (8 mm i.d.). No

reductive activation pretreatment was applied. A reaction

gas mixture CH4/O2/CO2/N2 (35.2:16.2:3.18:45.8 vol%)

was introduced with a 6800 cm3 h1 total flow rate at1 atm pressure. Then the reactor was heated from room

temperature to 850 8C within an hour. When the temperature

A.I. Tsyganok et al. / Applied Catattributed to reflection from (1 1 0) plane indicating that theFig. 6. Combined partial oxidation and dry reforming of methane, cata-

lyzed by supported ruthenium: reaction temperature, 850 8C; catalystloading, 0.15 g; gas feed, CH4/O2/CO2/N2 = 35.2:16.2:3.18:45.8 (vol%);F/W space velocity, 45333 cm3 g1 h1; pressure, 1 atm.

water, while the second peak (around 400 8C) was attributedto the dehydroxylation of Mg and Al hydroxides and

decomposition of the acquired metal complexes. A small

exothermic peak was registered at 361 8C for LDH bearingRhY species (Fig. 2(B)); this was ascribed to the combustion

of EDTA ligand itself [42].

Calcination of LDHs in air at 1000 8C caused drasticchanges in phase composition of the resultant mixed oxides.

Very similar XRD patterns were recorded for the mixed

oxides obtained from catalyst precursors and for those from

reference LDHs. Due to a low content, no phases involving

noble metals were detected by XRD. The patterns recorded

for two Ru-containing mixed oxides and a calcined

meixnerite are shown in Fig. 3. Additionally to a periclase

MgO, a spinel structure of MgAl2O4 was clearly detected.

One should here mention that, despite the high temperature

for LDH calcination in air, the produced mixed oxides

retained sufficiently high surface areas (Table 2).

3.2. Combined partial oxidation and dry reforming of

methane over noble metal catalysts

Results of activity tests for catalysts containing 2 wt.% of

noble metals in PODRM at 850 8C are shown in Fig. 4.Ru- and Rh-catalysts revealed activity values markedly higher

than that of platinum, as well as higher yields of syngas

formation of catalytically active metallic phase occurred in

situ under the reaction conditions. It is seen that the Ru- and

Rh-catalysts approached their maximal activity, which

appeared to be limited by equilibrium, since the beginning

of the test runs. One can here propose that reduction of noble

metal oxides to metallic phase could occur at a lower

temperature or in the very beginning of test reaction at 850 8C.However, the formation of noble metal particles could not be

detected with either XRD or high-resolution TEM (Fig. 5).

Also, TEM observation revealed the absence of any

carbonaceous deposits on catalyst and the presence of a

crystalline material of support without any spots potentially

attributable to supported noble metal clusters. At the same

time, EDS analysis confirmed the presence of noble metals in

specimens. One can here assume that noble metal clusters

were not observed with TEM due to their very small size.

Another reason could be a complete conversion of metallic

clusters back into a supported oxide due to contact with

oxygen of air, as the samples of spent catalysts were removed

from the reactor and kept under air after reaction.

Among the noble metals tested, the supported ruthenium

revealed the highest yield of syngas and H2-to-CO ratio. It

was therefore selected for the next step of studies aimed

at determining the minimal amount of supported metal

required for high catalytic performance. Additionally to

Ru2.0%/MgAlOx catalyst, four more samples with various

A.I. Tsyganok et al. / Applied Catalysis A: General 275 (2004) 149155154components. It should be mentioned that the mixed metal

catalysts were not reductively pretreated and thus theFig. 7. Synthetic strategy of preparing noble metal catalysts, supported oRu-contents were prepared from LDH precursors and tested

in PODRM reaction at 850 8C. Results obtained are shownn MgAl mixed oxide (explanation for steps 14 is given in text).

Mg(Al)Ox structure and of MgAl2O4 spinel (step 3). The

metal introduced was supposed to be present in the form of

and sufficiently general concept for fabricating mono- and

multi-component heterogeneous catalysts for various reac-

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Acknowledgements

The support of this work by The Japan Society for the

Promotion of Science (JSPS) organization and The Japan

Research and Development Center for Metals (JRCM) is

gratefully acknowledged.

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Combined partial oxidation and dry reforming of methane to synthesis gas over noble metals supported on Mg-Al mixed oxideIntroductionExperimentalReagents and solutionsSynthesis of LDHsCharacterization techniquesCatalytic activity tests

Results and discussionIncorporation of noble metals into the structure of Mg-Al double hydroxideCombined partial oxidation and dry reforming of methane over noble metal catalysts

ConclusionAcknowledgementsReferences