Applied Catalysis A: General 202 (2000) 91–97

Influence of the microscopic properties of the support on the catalyticactivity of Au/ZnO, Au/ZrO2, Au/Fe2O3, Au/Fe2O3–ZnO,

Au/Fe2O3–ZrO2 catalysts for the WGS reaction

Tatyana Tabakova, Vasko Idakiev, Donka Andreeva∗, Ivan MitovInstitute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev street, bl.11, 1113 Sofia, Bulgaria

Received 14 April 1999; received in revised form 17 January 2000; accepted 17 January 2000

Abstract

It has been established that the gold catalysts on well crystallized supports, Au/Fe2O3 and Au/ZrO2, display higher catalyticactivity in the water gas shift (WGS) reaction in comparison with the samples on amorphous and not well crystallized supports— Au/ZnO, Au/ZrO2, Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2. It could be concluded that the catalytic activity of the gold/metaloxide catalysts depends strongly not only on the dispersion of the gold particles but also on the state and the structure of thesupports. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Gold catalysts; WGS reaction; State and structure of the support

1. Introduction

During the last few years, many studies have beenreported on gold-containing catalytic systems becauseof their high catalytic activity in a series of importantreactions, such as CO and H2 oxidation, catalyticcombustion of hydrocarbons etc. [1,2]. The additionof gold to some oxidic systems leads to an increase intheir catalytic activity which is probably due to syn-ergism between the gold and the metal oxide species.The peculiar structure and properties of the gold/metaloxide interface are the result of this specific inter-action. The nature of the support exerts a decisiveinfluence on the catalytic activity of gold-containingcatalysts. Our own investigations showed recently thatAu/Fe2O3 and Au/TiO2 catalysts manifest a high cat-

∗ Corresponding author. Fax:+35-92-756116.E-mail address:andreev@ic.bas.bg (D. Andreeva)

alytic activity in the low temperature water gas shift(WGS) reaction [3–6]. Depending on the preparationtechnique, the Au/TiO2 samples drastically changetheir catalytic activity [6]. When freshly preparedTi(OH)4 was used as a support, the activity of thissample was significantly lower in comparison withthe sample obtained by deposition–precipitation onanatase. There are data available in the literature onthe influence of the support state and structure on thecatalytic properties. Baiker et al. showed that the de-gree of crystallization of the zirconia supports stronglyinfluenced the catalytic activity in the reverse WGSreaction and methanol synthesis and it was establishedthat the crystallization of amorphous ZrO2 resultsin significantly lower activity and selectivity in theabove-mentioned reactions [7]. The gold-containingcatalysts displayed, however, insufficient stability inmany cases and their activity decreased during thecatalytic operation. Baiker et al. applied some nontra-

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92 T. Tabakova et al. / Applied Catalysis A: General 202 (2000) 91–97

ditional methods for the preparation of efficient andstable CO oxidation zirconia-based gold catalysts us-ing Au–Ag–Zr or Au–Fe–Zr amorphous metal alloysas precursors [8].

The present study is motivated by the need for se-lecting better supports for the gold catalysts and toelucidate the influence of the state of the support ma-terials on the catalytic activity in the WGS reaction.

2. Experimental

2.1. Sample preparation

The samples were prepared in a ‘Contalab’ lab-oratory reactor (Contraves AG, Switzerland) un-der complete control of all the parameters: tem-perature −60◦C, pH=8.0, stirrer speed 250 rpm,reactant feed flow rate 8 ml/min etc. All the chemi-cals used, Fe(NO3)3·9H2O, Zn(NO3)2·6H2O, ZrCl4,HAuCl4·3H2O and Na2CO3, had ‘analytical grade’purity. All the samples were prepared by thedeposition–precipitation method [4]. This involves thedeposition of gold hydroxide onto the support througha chemical interaction between HAuCl4·3H2O(Merck) and Na2CO3 in aqueous solution. The1Au/ZrO2 sample was prepared on well crystallizedcommercial zirconia (Reachim). For the samples, de-noted as Au/Fe2O3, Au/ZnO and 2Au/ZrO2, freshprecipitates of the corresponding metal hydroxideswere used as supports. The samples, denoted asAu/Fe2O3–ZnO and Au/Fe2O3–ZrO2, were obtainedon freshly coprecipitated mixed metal hydroxides. Inall cases, the precipitates were aged for 1 h at 60◦C,filtered and washed carefully until the disappearanceof NO3

− and Cl− ions. Then, the samples were driedunder vacuum at 80◦C and calcined in air at 400◦Cfor 2 h.

2.2. Sample characterization

The BET surface area of the samples was deter-mined on a ‘Flow Sorb II-2300’ device.

The samples for the X-ray diffraction (XRD) stud-ies were prepared by means of Synocryl 9122X in adiluted toluene solution and exposed to irradiation for6 h in a Guinier-De Wolff-Nonius camera Mark IV,using FeKa1 radiation.

Transmission electron microscopy (TEM) char-acterization was performed on a Hitachi-H-600-2electron microscope. The samples were dispersedin bidistilled water by ultrasound. A drop was thentransferred to a carbon-coated bronze mesh and dried.

Derivatograph-1500 was used for DTA-DTG mea-surements; the rate of heating was 10◦C/min in airmedium.

A ‘Carlo Erba’ mercury porosimeter was used forthe pore structure measurements.

The Mössbauer spectra were obtained with an elec-tromechanical spectrometer (Wissenschaftliche Elek-tronik GmbH) operating under constant acceleration atroom temperature. A57Co/Cr source and ana-Fe stan-dard were used. The experimentally obtained spectrawere processed mathematically by the least squaresmethod. The parameters of hyperfine interaction suchas isomeric shift (IS), quadrupole splitting (QS) andeffective internal magnetic field (Heff ) as well as theline widths (FWHM) and the relative weight of thespectral partial components (G) were determined.

2.3. Catalytic activity

The catalytic activity of the samples in the WGSreaction was measured in a flow reactor at atmo-spheric pressure in the 140–360◦C temperature rangeunder the following conditions: catalyst bed volume0.5 cm3 (0.63–0.80 mm sieve fraction), space veloc-ity 4000 h−1, and vapour/gas ratio=0.7. Preliminaryreduction of the samples was performed at 250◦C for12 h in a hydrogen/argon mixture (1% H2). The reac-tant gaseous mixture, fed into the reactor, contained4.03 vol.% CO, the rest being argon. The analysis ofthe converted mixture composition at the reactor out-let was carried out on an ‘Infralyt 2000’ gas analyzerwith respect to CO and CO2 content. The CO conver-sion degree was accepted to be the measure for thecatalytic activity.

3. Results

3.1. Sample characterization

The chemical composition of the investigatedsamples, the BET surface area and the pore volumedata are listed in Table 1. As it could be expected

T. Tabakova et al. / Applied Catalysis A: General 202 (2000) 91–97 93

Table 1Chemical and textural properties of the samples

Samples Atomic ratio, Au:Me BET surface area (m2/g) Pore volume

Fresh Spent∑

Vp (cm3/g) rmax (Å)∑

Vp (cm3/g) rmax (Å)

Au/Fe2O3 1:22.6 59 0.69 42 0.37 60Au/ZnO 1:26.0 70 2.10 103 0.41 1201Au/ZrO2 1:24.0 26 0.27 69 0.26 752Au/ZrO2 1:24.0 85 0.23 60 0.10 50Au/Fe2O3/ZnO 1:21.4:1.3 69 0.51 60 0.46 200Au/Fe2O3/ZrO2 1:21.4:1.2 74 0.63 46 0.12 60

in advance, the BET surface area of the sample onamorphous zirconia (2Au/ZrO2) was the largest incomparison with the sample on well crystallized zir-conia (1Au/ZrO2) and other studied samples. Thesample on poorly crystallized zinc oxide as well asthe samples on iron oxide modified with zinc or zir-conium oxides possess very close values and they arelarger in comparison with the sample on well crystal-lized hematite. A similar tendency is followed by thepore volume values. The samples obtained on amor-phous zirconia display a low value of the total porevolume prior to catalytic operation, which is an indi-cation that some sintering is occurring as early as inthe dehydroxylation process (during the pretreatment,all samples were calcinated at 400◦C for 2 h). Aftercatalytic operation, the pore volume of the samples,containing amorphous ZrO2 or poorly crystallizedZnO, is additionally decreased as a result of sintering.The sintering effect is also observed with the sam-ples on the mixed oxide systems Au/Fe2O3–ZnO andAu/Fe2O3–ZrO2.

X-ray and TEM analyses of the samples give addi-tional information on the structure of the investigatedsamples. The samples studied contain finely dividedgold particles deposited on the support and the aver-age size for all the catalysts is below 5 nm, accordingto the data from XRD and TEM.

The sample Au/Fe2O3 shows an intensive XRD pat-tern of a-Fe2O3, and during the catalytic operation,the hematite is being transformed into magnetite, ac-cording to the X-ray data [4,5].

The TEM micrographs of the fresh sample Au/ZnOshow that it contains ZnO crystallites of approximately10–20 nm size (Fig. 1a), which is confirmed by the

broadened X-ray reflections. The TEM micrograph ofthe spent Au/ZnO sample gives a rather clear pictureof sintered ZnO crystallites — about 60 nm and theX-ray reflections are less broadened.

The first sample on zirconia (1Au/ZrO2) shows awell crystallized ZrO2 type baddeleyite, while thesecond one (2Au/ZrO2) is completely amorphous asindicated by the XRD and TEM data (Fig. 2a).

The picture of the samples on the mixed oxides ismore complicated. Both samples containa-Fe2O3,and after catalytic operation, magnetite, according tothe data of XRD and TEM microscopy (Figs. 1b and2b). The results from the Mössbauer spectra analysisof the fresh samples show that the sextet parametersare close to those fora-Fe2O3 and correspond to theones in high spin Fe3+ ions with octahedral coor-dination. The broad, apparently nonLorentzian linesas well as the downgradedHeff values (the magneticfield is distributed in the region of 450–500 kOe)demonstrate the high dispersity of the dominantphase. The second doublet component in the spec-tra can be attributed to the iron ions included inparticles with superparamagnetic behaviour (SPM).The SPM component relative weight is higher forthe zinc oxide-containing sample compared to theFe2O3–ZrO2-based sample. The Mössbauer spectraof the spent catalysts also display sextet and paramag-netic component lines, but the prevailing phase in thespectra is Fe3O4, while lines of tetrahedral (Fe3+

tetra)and octahedral (Fe2.5+

octa ) coordinated ions were alsodetected. The area ratio Fe2.5+

octa /Fe3+tetra is approxi-

mately 2 for the ZnO-containing sample and about1.46 for the ZrO2-containing sample. A nonstoichio-metric magnetite Fe3−xO4 is evident in the second

94 T. Tabakova et al. / Applied Catalysis A: General 202 (2000) 91–97

Fig. 1. TEM photographs of the samples: (a) Au/ZnO; (b)Au/Fe2O3–ZnO. Fig. 2. TEM photographs of the samples: (a) 2Au/ZrO2; (b)

Au/Fe2O3–ZrO2.

T. Tabakova et al. / Applied Catalysis A: General 202 (2000) 91–97 95

Fig. 3. DTA-DTG of the samples: (1) 2Au/ZrO2; (2) Au/Fe2O3;(3) Au/Fe2O3–ZrO2.

case. The amount of vacancies located in octahedralplaces, as determined by the method described in [9],is x=0.109. The XRD data confirm the above results.The absence of the specific peaks of ZnO or ZrO2in the XRD patterns and product interaction withthe major phase (iron oxide) can be attributed to theamorphous state of the above-mentioned oxides. TheXRD lines are wide-ranging to a greater extent in thefresh samples.

Fig. 3 shows the DTA-DTG curves of thevacuum-dried Au/Fe2O3, 2Au/ZrO2 and Au/Fe2O3–ZrO2 samples. The higher thermal stability of zirconiais illustrated in comparison with the sample on ironoxide and mixed iron–zirconia oxide. It can also beseen that the dehydroxylation of the iron oxide-basedsamples is proceeding more gradually in comparisonwith the abrupt dehydroxylation of zirconia. This isobviously the reason for the sintering of the samplescontaining amorphous phases, even as early as duringthe process of pretreatment of the samples. This addi-tional sintering process is growing deeper during thecatalytic operation of the samples, containing amor-phous zirconia or not well crystallized ZnO (Table 1,samples 2Au/ZrO2 and Au/ZnO).

Fig. 4. Temperature dependence of the catalytic activity (degreeof conversion) of the samples: (1) Au/Fe2O3; (2) 1Au/ZrO2; (3)Au/Fe2O3–ZnO; (4) Au/Fe2O3–ZrO2; (5) 2Au/ZrO2; (6) Au/ZnO.

3.2. Catalytic activity

The catalytic activity of the samples was evaluatedin the WGS reaction. The results from the catalyticactivity measurements of the investigated samples arerepresented in Fig. 4. The promoting effect of the goldis appearing even at 140◦C, whereupon the differencesare most distinct within the low-temperature range160–240◦C [3,4]. It can be seen that the highest cat-alytic activity is manifested by the samples Au/Fe2O3and 1Au/ZrO2 (on well crystallized ZrO2) in compar-ison with all other studied samples. The lowest cat-alytic activity is shown by the samples on amorphouszirconia and on poorly crystallized zinc oxide. Thecatalytic activity of the samples on the mixed oxidesupports is lower in the low temperature range and thesample Au/Fe2O3–ZnO displays a higher conversiondegree compared to Au/Fe2O3–ZrO2.

4. Discussion

More recent studies were focused on the hot prob-lem of the day to find some more stable and efficientsupports for gold-based catalysts. The use of zirconiaas a support satisfies these requirements to the greatestextent [10]. Baiker et al. used a method for the prepa-ration of some stable gold-based catalysts by ‘in situ’oxidation of Au–Ag–Zr and Au–Fe–Zr alloys, but inour opinion, it is rather complicated and difficult toapply [8].

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Our investigations on gold-based catalytic systemsclearly demonstrated that Fe2O3 is one of the mostsuitable supports for the preparation of highly activeWGS reaction catalysts [3–6]. The presence of goldenhances the formation of nonstoichiometricg-Fe2O3spinel phase, which manifests a higher catalytic ac-tivity in the WGS reaction compared to magnetite[11]. The catalytic stability tests, however, show thatthe activity of the gold-containing catalysts is de-creasing during the catalytic operations [5]. It wasfound by Kozlova et al. that only the use of amor-phous as-precipitated iron-hydroxide Fe(OH)3

∗ as asupport for Au(PPh3)(NO3) enables one to obtaintremendously active catalysts for low-temperature COoxidation [12]. Moreover, the amorphous materialmay contain many surface defects that should alsostrengthen Au–support interactions, and therefore,prevent gold agglomerating into larger gold particles.As was mentioned above, there are a limited numberof papers studying the stability of gold-based catalysts[5,8].

The present study is an attempt to obtain somehighly active and more stable gold catalytic systemsfor WGS reaction by the modification of iron(III) ox-ide support by ZnO and ZrO2 and the results obtainedare compared with those with the Au/Fe2O3, Au/ZnOand Au/ZrO2 catalysts. On one hand, the addition ofZnO and ZrO2 to Fe2O3 by coprecipitation allows oneto obtain a nonstoichiometric spinel phase which re-sults in the increase in the number of Fe3+

octa sites. It isknown that the Fe3+

octa sites play an essential role in in-creasing the WGS reaction activity [11]. On the otherhand, these additives preserved, to a greater extent, theamorphous character of the support.

The catalytic activity measurements carried out onthe samples in this study, using mixed oxide supports,showed significantly lower activity in comparison withthose on the well crystallized iron or zirconium oxides.The analysis of the XRD and Mössbauer spectroscop-ical data showed that, during the thermal treatment ofthe Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2 samples, theamorphous character of the samples was preserved.During catalytic operation, however, a fast sintering ofthe catalysts used was occurring. Obviously, the de-sirable effect of increased activity and stability by themodification of iron oxide with zinc and zirconiumoxides for the WGS reaction was not achieved.TheAu/ZnO sample does not exhibit a high WGS reaction

activity too, in spite of the fact that the ZnO supportobtained had a very small particle size with amorphousnature. A low WGS reaction activity is also exhibitedby the sample on amorphous zirconia.

As was shown in Fig. 4, the highest WGS reactionactivity is manifested by the Au/Fe2O3 and 1Au/ZrO2(on well crystallized ZrO2) samples. It is interestingto note that ZrO2 itself exhibits no activity in theWGS reaction (it was not shown in Fig. 4 for thesake of brevity). Obviously, the high WGS reactionactivity observed for the 1Au/ZrO2 catalyst is due tosome specific interaction between Au and ZrO2 Thevery good synergetic effect between Au–Fe2O3 andAu–TiO2 [3–6] is valid also for the Au–ZrO2 cat-alytic system. The results obtained in the comparativestudy of 1Au/ZrO2 and 2Au/ZrO2 show doubtlesslythat the sample on amorphous zirconia is not activein the low-temperature WGS reaction. The analysisof the DTA-DTG results showed that the amorphouszirconia support sample (2Au/ZrO2) was dehydroxy-lated abrubtly at 447◦C. This fact is an indication that,even during the thermal treatment of the 2Au/ZrO2sample, a process of sintering has started and it is con-tinuing in the course of the catalytic operation. Wehave observed the same behaviour with the Au de-posited on amorphous titania [6]. The gold catalystsbased on well crystallized titania manifested signif-icantly higher WGS reaction activity in comparisonwith that of the gold samples on amorphous titania.

5. Conclusions

• The catalytic activity of gold/metal oxide catalystsdepends strongly not only on the dispersion of goldparticles but also on the nature and textural structureof the supports.

• The highest catalytic activity and stability in theWGS reaction is manifested by the catalysts on thebasis of the well crystallized Fe2O3 and ZrO2.

• The catalytic activity in the WGS reaction is de-creased when gold is deposited on amorphous ornot well crystallized ZrO2, ZnO, Fe2O3–ZnO andFe2O3–ZrO2.

• The analysis of textural structure and proper-ties of the fresh and spent 2Au/ZrO2, Au/ZnO,Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2 samplesshowed a substantial reduction of the pore volume,

T. Tabakova et al. / Applied Catalysis A: General 202 (2000) 91–97 97

which was reflected in the lower catalytic activityand fast deactivation of the catalysts in the WGSreaction.

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