A systematic study of the synthesis conditions for the preparation of highly active gold catalysts

Applied Catalysis A: General 226 (2002) 1–13

A systematic study of the synthesis conditions for the preparation of highly active gold catalysts Anke Wolf, Ferdi Schüth∗ MPI für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim a.d. Ruhr, Germany Received 8 May 2001; received in revised form 30 July 2001; accepted 2 August 2001

Abstract Supported gold catalysts were prepared by a deposition–precipitation method in order to investigate the influence of the synthesis conditions on the difference in catalytic activities for CO oxidation. The optimization of the synthesis parameters resulted in highly active Au/TiO2 , Au/Co3 O4 , Au/Al2 O3 and Au/ZrO2 catalysts. SiO2 was found to be an unsuitable support material for the deposition–precipitation method. With increasing pH value during precipitation and decreasing temperature of calcination increasing catalytic activity was observed. The optimum pH was in the range of eight to nine and slightly dependent on the nature of the support. The optimum temperature of calcination was 200 ◦ C. According to XRD and TEM the increasing catalytic activity could be attributed to a decrease in the gold particle size. However, comparing two samples with similar gold particle sizes, Au/TiO2 is more active than Au/Al2 O3 . This indicates that the catalytic activity is not only a particle size effect, but the role of the metal oxide is more than just the stabilization of the gold particles. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Gold catalysts; CO oxidation; Catalyst preparation

1. Introduction Supported gold catalysts show a high catalytic activity in several reactions like the reduction of nitrogen oxides [1,2], the epoxidation of propene [3,4] and the low temperature oxidation of CO [5,6]. The high catalytic activity of gold is unexpected as bulk gold is quite inert and even reactive molecules like CO and H2 do not adsorb on the surface of gold [7]. This behavior changes, if gold is highly dispersed as nanosized particles on certain metal oxides. Although the catalytic activity of gold catalysts in the low temperature CO oxidation has been intensively studied during the last decade, the nature of the active species is still ∗ Corresponding author. Tel.: +49-208-3062373; fax: +49-208-3062995. E-mail address: [email protected] (F. Schüth).

discussed controversially. It has been suggested that the role of the metal oxide is the stabilization of the gold nanoparticles and that the reaction takes place on the gold surface [5,8–10]. Other authors proposed that the reaction takes place at the gold/metal oxide interface and that the metal oxide could act as a source of oxygen [11–13]. Also the electronic structure of gold in active catalysts is unclear. Park and Lee concluded from XPS investigations that oxidic gold is the active species [14]. Minicò et al. [15] found a correlation between the occurrence of an IR band, which can be assigned to the adsorption of CO on Au+ , and the catalytic activity of coprecipitated Au/Fe2 O3 catalysts. In contrast, other authors suggest metallic gold to be the active species [5,11,16,17]. The synthesis of highly dispersed small gold particles is highly sensitive towards the preparation method. Haruta and coworkers [5,6] showed that the

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 7 7 2 - 4

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incipient wetness impregnation is unsuitable to produce highly dispersed gold catalysts, and that in order to obtain high activity gold catalysts, the materials have to be prepared via coprecipitation or deposition– precipitation. Other methods like cosputtering [18], chemical vapor deposition (CVD) [19], adsorption of gold colloids on metal oxides [20] or supporting gold phosphine complexes on as-precipitated wet metal hydroxides [11] are also suitable techniques. The preparation method which can be applied to the widest range of different support materials is the deposition–precipitation. Beside the preparation method also the synthesis conditions, like pH value during precipitation, temperature of calcination and the pretreatment conditions (air, vacuum, hydrogen), have a significant effect on the properties of gold catalysts. However, the results reported in the literature concerning these investigations are often contradictory [5,14,21,22]. Although rarely expressed in publications, it is well known that the reproducibility of highly active gold catalysts is typically very low. This might be the reason why, despite of the great influence of the preparation on the catalytic activity of gold catalysts, no systematic study concerning the synthesis conditions has been published yet. Here we present a detailed investigation of the influence of the synthesis parameters on the catalytic activity of supported gold catalysts, which includes more than 300 samples. Au/TiO2 , Au/ZrO2 , Au/Al2 O3 , Au/Co3 O4 and Au/SiO2 were prepared by deposition–precipitation and low temperature CO oxidation was used as catalytic test reaction. Also during this study reproducibility problems were encountered, but due to the high number of catalysts synthesized and experiments conducted, general trends can be deduced.

syntheses. The synthesis could be transferred to the larger scale without difficulties in all cases. The catalysts were prepared by deposition–precipitation according to the following procedures. Method A: A solution of HAuCl4 was added to an aqueous suspension of the support. The pH of the suspension was adjusted to the desired pH value (5–10) with Na2 CO3 (Fluka). Method B: A solution of HAuCl4 was adjusted to the desired pH before the addition of the support material. For both methods the temperature and duration of the aging was varied between room temperature and 70 ◦ C and 2–12 h. The suspension was then filtered, washed and dried at 90 ◦ C before calcination at 200, 300, 400 or 500 ◦ C. As supports we used P25 (Degussa; 70% anatase; 30% rutile BET surface area 56 m2 /g), Hombifine N (Sachtleben; anatase, BET surface area 391 m2 /g), amorphous ZrO2 (MEL; BET surface area 234 m2 /g), tetragonal and monoclinic ZrO2 which was precipitated from ZrO(NO3 )2 ·xH2 O (BET 70 surface area m2 /g, 50 m2 /g) according to [24], Pural Ox (Condea, ␦/␥-Al2 O3 , BET surface area 118 m2 /g), Aluminiumoxid C (Degussa, ␦-Al2 O3 , BET surface area 130 m2 /g), Co3 O4 which was precipitated from cobalt nitrate (BET surface area 50 m2 /g), aerosil 200 (Degussa) as well as unconventional supports like a CoAl2 O4 spinell with a surface area of 190 m2 /g. Some Au/Co3 O4 catalysts were also prepared by coprecipitation. For this, a combined solution of 0.5 M Co(NO3 )2 ·(H2 O)6 (Fluka) and freshly prepared HAuCl4 was added dropwise to 1 M Na2 CO3 (Fluka). The suspension formed was stirred for 2 h and then filtered, washed and dried in air at 90 ◦ C. The as-made material was calcined at 400 ◦ C.

2. Experimental 2.2. Characterization 2.1. Preparation of catalysts The syntheses were partly carried out with a modified Gilson XL 232 automated dispenser, which enables the production of a large number of samples with high reproducibility [23]. As the amount of one sample which can be produced by the roboter is limited, and for a detailed characterization more substance was needed, we also carried out manual

The gold content of the catalysts was determined by inductively-coupled plasma atom emission spectroscopy (ICP-AES) using a UNICAM PU 700 spectrometer. Energy dispersive X-ray analysis (EDX) was performed on a Hitachi S-3500N scanning electron microscope equipped with an OXFORD EDX system to examine possible contaminations by chloride and sodium. The sodium and chloride contents

A. Wolf, F. Schüth / Applied Catalysis A: General 226 (2002) 1–13

of the catalysts were found to be below the detection level (0.1 wt.%). X-ray diffraction (XRD) measurements were carried out on a Stoe STADI P transmission diffractometer and a Stoe STADI P reflection diffractometer using Cu K␣ radiation. Transmission electron microscopy (TEM) was performed on a Hitachi-HF-200 electron microscope with field emission source. At least 200 particles were chosen to determine the mean diameter of gold particles. The BET surface areas of the support materials and the catalysts were determined by N2 -sorption at 77 K using a Micromeritics-ASAP-2010 instrument. 2.3. CO oxidation In the first step, the catalytic test reaction was carried out in a high-throughput-reactor which was developed in our group [23]. The reactor allows the parallel testing of 16 catalysts close to conventional methods. The 16 separate channels were loaded with 46 mg of powdered catalyst each. The reaction gas containing 1% CO in air was passed over the catalyst beds at a total flow rate of 250 ml/min, which results in a space velocity of 20,000 ml/h gcat for each sample. The most promising candidates were also tested in a conventional plug flow reactor, which can operate at temperatures of −100 to 300 ◦ C. In this reactor 200 mg of sieved catalyst (250–500 ␮m) was used at a flow rate of 67 ml/min resulting in the same space velocity as in the parallel test. In both cases the catalysts were pretreated in air at 150 ◦ C for 1 h prior to starting the catalytic reaction. The concentration of CO2 was analyzed with non-dispersive IR spectroscopy using a URAS 3E (Hartmann and Braun). Previous studies showed that the deviations in the ignition curves obtained in the parallel and conventional plug flow reactor do not exceed 10 K [23].

3. Results 3.1. General synthesis parameters The investigation of the reproducibility led to good results if the experiments were performed under exactly the same synthesis conditions on the same day. The difference in the temperature of 50% conversion (T1/2 ) did not exceed 20 K for both automated and

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manual synthesis. However, repeating the experiments at a later time, that means at different ambient pressure, temperature, moisture, drying and calcination in a different batch, etc. could lead to deviations of T1/2 of more than 100 ◦ C. To optimize the reproducibility we therefore first tried to identify crucial factors by modifying synthesis parameters, such as the order of supplying the components, the medium and temperature during precipitation, and the period of aging. Following method A which was mainly used in the automated preparation, the chloroauric acid is added to an aqueous suspension of the support and the pH is then adjusted to the desired pH value. In case that the precipitation is fast, the pH during precipitation is not well defined using this method. To avoid this problem, a solution of the chloroauric acid was adjusted to the desired pH before addition of the support (method B). Although the addition of the support can lead to a slight change in the pH, this change is small compared to the pH change in method A. The experiments revealed that with both preparation methods comparable results concerning the catalytic activity of the materials were obtained. The reproducibility could not be improved by the pH adjustment. In order to extend the investigation of the influence of the method by which precipitation was induced, also homogeneous precipitation was studied. In this variation urea is added to a suspension of the support and the chloroauric acid. Heating the reaction mixture to 80 ◦ C leads to the decomposition of the urea and a slow and uniform formation of hydroxide ions. Although this method also resulted in active gold catalysts, it could not improve the reproducibility of the experiments. Also the variation of the precipitation reagent, using NaOH or NH3 instead of Na2 CO3 in order to elucidate the effect of sodium and carbonate ions, did not influence the catalytic activity and reproducibility of the synthesized materials. Changing the aging period from 2 to 12 h neither resulted in a different gold content nor in a different gold particle size of the materials. The repetition of the experiments led to the same deviations in the catalytic activity as reported above. Also the temperature during precipitation could not influence the reproducibility, although a slight improvement of the catalytic activity could be achieved by raising the temperature during precipitation from room temperature to 70 ◦ C. We therefore conclude, that the pH adjustment, the medium of precipitation

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as well as the temperature and period of aging do not influence reproducibility. Other factors at present beyond our control seem to be decisive. 3.2. pH value during precipitation Fig. 1 shows the catalytic activity of Au/TiO2 catalysts which had been precipitated at different pH values. The catalyst which was precipitated at pH 5 is only active for the oxidation of CO at elevated temperatures. The temperature for 50% conversion (T1/2 ) is 136 ◦ C. Increasing the pH value during the synthesis to pH 6.5 leads to a catalyst with higher catalytic activity (T1/2 = 60 ◦ C). Further increase of the pH results in more active catalysts until a maximum of activity is achieved with materials that have been synthesized in the pH range of pH 7.8–8.8. Those catalysts convert 50% of CO to CO2 at only −32 ◦ C. The precipitation of catalysts at higher pH values than pH 8.8 resulted in a slight increase in the temperature for 50% conversion. The increasing catalytic activity with increasing pH value could always be reproduced within a series of experiments performed on the same day, although the actual temperature of 50% conversion for experiments conducted at the same pH, but at a later time, could deviate by about 100 ◦ C, which will be discussed later. According to ICP-AES the catalysts shown in Fig. 1 contain less than 1.5 wt.% gold. At this low gold content the determination of the gold particle size from X-ray diffraction line broadening

becomes very difficult. Furthermore, the gold peak with the highest intensity (Au(1 1 1)) is superimposed by a strong anatase reflection. For the sample that was precipitated at pH 5, the average gold particle size could be calculated from the Au(2 0 0) peak using the Scherrer equation as 13 nm. The X-ray diffraction patterns of the other samples did not show a reflection at 44.4◦ (Au(2 0 0)). This is probably due to the small particle size of the gold leading to strong broadening of the reflections, which does not allow to discriminate them from the background. The small particle sizes could be confirmed by TEM. The sample which was precipitated at pH 7.8 showed highly dispersed gold particles with particle sizes between 2 and 5 nm. One would not expect to detect such small particles via XRD at a loading level of ca. 1 wt.%. A similar dependence of the catalytic activity on the precipitation pH was also found for Au/Al2 O3 and Au/ZrO2 . For Au/Al2 O3 catalysts a shift in the T1/2 from >130 to 2 ◦ C could be observed in the pH range of pH 6.5–8.6. Fig. 2 shows the X-ray diffraction pattern of four Au/Al2 O3 samples. With increasing pH values a broadening of the Au(1 1 1) and Au(2 0 0) reflections can be observed. This clearly indicates that higher pH values during the precipitation lead to smaller gold particles. As the gold reflections are superimposed by the support reflections, it was not attempted to calculate the particle size from X-ray diffraction line broadening. TEM investigations of the most active catalyst

Fig. 1. Catalytic activity of Au/TiO2 catalysts in dependence on the pH during precipitation: (a) yield of CO2 ; (b) temperature of 50% conversion.

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Fig. 2. XRD of a series of Au/Al2 O3 catalysts precipitated at different pH values: (a) pH 6.3; (b) pH 6.8; (c) pH 7.7; (d) pH 8.1; (e) support.

show highly dispersed gold particles with a narrow particle size distribution and a maximum at around 2 nm. Fig. 3 shows the fraction of gold offered which was found by ICP-AES to be incorporated in the catalyst, in dependence of the pH during precipitation for several Au/TiO2 catalysts with a nominal gold loading of 2.4 wt.%. The maximum deposition of gold was found for pH values below about 8 with above 50% of the amount offered. With increasing pH the fraction of gold deposited on the support

Fig. 3. Fraction of gold offered found by ICP-AES to be incorporated in the samples in dependence on the pH during precipitation for Au/TiO2 catalysts with a nominal gold loading of 2.4 wt.%.

obviously decreases. Therefore, the observed decreasing gold particle size with increasing pH value seems to correlate with the decreasing amount of gold and could be explained by agglomeration of the gold particles at higher gold loadings. However, for most systems investigated, the changes in the gold loading that are induced by a different pH during precipitation are not high enough to explain the differences in the catalytic activity, as will be described in Section 3.4. The difficulties in the reproducibility of the gold catalysts, that were mentioned before are illustrated in Fig. 4. Here, the temperatures of 50% conversion of several series of experiments are plotted versus pH. For all the three supports the same trend can be observed, namely the increasing catalytic activity with increasing pH values during the precipitation. However, one can see that the actual value of the T1/2 of a catalyst precipitated at the same pH can deviate about more than 100 ◦ C. In order to determine a critical pH value for the synthesis of active gold catalysts for each support, we tried to find a curve which fits the data of Fig. 4 for each of the systems. We could find a similar pH for Au/TiO2 and Au/ZrO2 (pH 6.8) and a slightly higher pH for Au/Al2 O3 (pH 7.5). Considering the isoelectric points of the support materials, this trend seems to correspond to the higher isoelectric point (IEP) of

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Fig. 4. Temperature of 50% conversion for Au/TiO2 , Au/Al2 O3 and Au/ZrO2 catalysts in dependence on the pH during precipitation: (䉬) Au/TiO2 (Hombifine N); (䉫) Au/TiO2 (P25); (䊉) Au/ZrO2 ; (䉱) Au/Al2 O3 .

Al2 O3 (∼9) compared to those of TiO2 and ZrO2 (∼6–7) [25]. The strongest reproducibility problems were encountered around precipitation pH in the transition region between active and less active catalysts. The increase of the catalytic activity with increasing pH correlates with the decreasing size of the gold particles according to XRD and TEM. The influence of the pH during precipitation on the gold particle size has already been described in [5]. However, Haruta reported that for Au/TiO2 in the pH range 6–10 the particle size was below 5 nm in diameter and did not change significantly, whereas our results indicate that in order to obtain highly active catalysts the pH should be higher than pH 8.

3.3. Temperature of calcination Several publications report that highly active gold catalysts have to be calcined at temperatures above 300 ◦ C [5,26]. However, other examples exist were increasing calcination temperatures led to a decrease in the catalytic activity [14,21,27,28]. We investigated the influence of the calcination temperature on the catalytic activity by treating the as-made materials, which had been dried at 90 ◦ C at different temperatures, namely 200, 300, 400 or 500 ◦ C. With increasing temperature of calcination a significant decrease in the catalytic activity could be observed for all investigated systems (Table 1). The catalysts calcined at 200 ◦ C consistently showed the best catalytic

Table 1 Influence of the temperature of calcination on the catalytic activity Sample

T1/2 (◦ C) after calcination at Uncalcined

∼1 wt.% Au/TiO2 ∼1 wt.% Au/TiO2 ∼2.5 wt.% Au/TiO2 2.6 wt.% Au/TiO2 Au/Co3 O4 (DP)a Au/Co3 O4 (CP)b Au/ZrO2 Au/Al2 O3 a b

DP: deposition–precipitation. CP: coprecipitation.

−52 <25 <25 −20 −30

200 ◦ C

300 ◦ C

<25 −46 <25 −60 −45 <25 −18 −30

30 11 70 50 12 50 31

400 ◦ C 50 133 93 33 60

500 ◦ C 60

115

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Fig. 5. XRD of Au/TiO2 calcined at different temperatures: (a) uncalcined; (b) 200 ◦ C; (c) 300 ◦ C; (d) 400 ◦ C; (e) 500 ◦ C.

performance. The uncalcined materials have almost the same activity as the materials that were calcined at 200 ◦ C. However, all catalysts were pretreated in air at 150 ◦ C for 1 h in the catalytic reactor before exposing them to the reaction gas mixture. Thus, the pretreatment temperatures for the uncalcined and the 200 ◦ C calcined sample were not that different. In Fig. 5, the XRD patterns of the 2.6 wt.% Au/TiO2 catalysts (Table 1) are illustrated. For the uncalcined material and the catalyst calcined at 200 ◦ C no reflections of gold could be detected. After calcination at 300 ◦ C the appearance of the Au(1 1 1) and Au(2 0 0) reflections can be observed and they become more significant after calcination at 400 and 500 ◦ C. As described before, the gold particle size could not be determined from the line broadening of the gold reflections because of superimposition of the reflections of the support. Nevertheless, the decreasing FWHM of the gold reflections with higher temperature of calcination can be interpreted by a higher average gold particle size. This observation could be confirmed by TEM investigations (Fig. 6). The uncalcined material shows a homogeneous dispersion of gold particles with particle sizes below 2 nm. After calcination at 200 ◦ C the major fraction of the gold particles are still smaller than 2 nm in size, however, at some parts agglomeration to larger gold particles of 3–10 nm in size can be observed. After calcination at 300 ◦ C almost no 2 nm gold particles exist and now the average particle size lies between 4

and 8 nm. Calcination at 400 and 500 ◦ C leads to a further agglomeration of the gold particles to an average gold particle size higher than 10 and 15 nm. According to XRD and TEM, the decreasing catalytic activity at higher temperature of calcination can therefore be explained by an increasing size of the gold particles. In the standard calcination procedure we did not use a controlled heating rate. The materials were directly put in the calcination oven which was preset to the desired temperature. To investigate the influence of different heating rates during the calcination we also calcined some samples with a heating rate of 1 ◦ C/min. However, catalytic activities for samples heated with a rate of 1 ◦ C/min and at the maximum rate were almost identical. The heating rate is thus not important in governing the catalytic properties of such catalysts. As described in Section 2 the coprecipitated Au/Co3 O4 catalysts were calcined at 400 ◦ C because the spinell phase of the cobalt oxide is not formed at lower temperatures. As the Au/Co3 O4 catalysts prepared via deposition–precipitation showed the best results after calcination at 200 ◦ C, we also calcined some coprecipitated samples at only 200 ◦ C. The results show that also the catalytic activity of the coprecipitated Au/Co3 O4 catalysts could be improved by decreasing the temperature of calcination. While the catalysts calcined at 400 ◦ C only converted 20% of CO to CO2 at room temperature, the CO conversion could be increased to 90% for the material calcined

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Fig. 6. TEM of the Au/TiO2 from Fig. 5 calcined at different temperatures: (a) uncalcined; (b) 200 ◦ C; (c) 300 ◦ C; (d) 400 ◦ C; (e) 500 ◦ C.

at 200 ◦ C. Fig. 7 shows the X-ray diffraction pattern of the two gold/cobalt oxide catalysts. In the sample calcined at 200 ◦ C both the reflections of gold and Co3 O4 are broader compared to the sample calcined at 400 ◦ C. This indicates that at 200 ◦ C the sample consists of small particles of Co3 O4 or that the Co3 O4 phase is not completely developed. The Au(1 1 1) reflection at 38.18◦ is superimposed by the Co3 O4 reflection. However, one can see that in the sample calcined at 400 ◦ C the FWHM is smaller which corresponds to larger gold particles. Therefore, also in case of coprecipitated catalysts the higher catalytic activity at lower temperature of calcination can be traced back to the gold particle size. Park and Lee [14] found a correlation between the catalytic activity of Au/TiO2 , Au/Al2 O3 and Au/Fe3 O4 and the transition of Au(OH)3 over Au2 O3

to metallic gold, which was achieved with increasing calcination temperature. They therefore claimed that oxidized gold is more active than metallic gold. In contrast, Haruta [5] who also found oxidic gold by EXAFS in the presence of metallic gold, could not see a correlation with the catalytic activity and concluded that metallic gold has to be the active gold species. From our investigations, we cannot exclude the presence of oxidic gold, if present as small or amorphous particles in low concentration and the reason for the decreasing catalytic activity observed with increasing temperature of calcination could therefore also be the change of oxidic to metallic gold instead of the increasing gold particle size. However, one should remember that after calcination at 400 ◦ C according to [6] the gold should be present as metallic gold. Nevertheless, the gold

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Fig. 7. XRD of Au/Co3 O4 : (a) calcined at 200 ◦ C; (b) calcined at 400 ◦ C.

catalysts calcined at this temperature or even higher still show a considerable catalytic activity in the CO oxidation. This aspect supports the hypothesis that metallic gold is the active species and that the influence of the temperature of calcination observed in our investigations can rather be attributed to a particle size effect than to a change in the oxidation state of gold. 3.4. Gold content The influence of the gold loading on the catalytic activity was investigated for catalysts prepared by deposition–precipitation as well as for coprecipitated catalysts. The results of two series of catalysts are listed in Table 2. For the Au/TiO2 catalysts which had been precipitated at pH 7 the gold contents determined by ICP-AES were 0.9–10.7 wt.%. The catalysts were calcined at two different temperatures which had a considerable effect on the resulting influence of the gold loading on the catalytic activity. After calcination at 200 ◦ C the catalytic activity did not significantly depend on the loading with gold. In contrast, for the catalysts calcined at 300 ◦ C a decreasing catalytic activity with increasing gold loading was observed. While the 0.9 wt.% Au/TiO2 showed 100% conversion at room temperature, the T1/2 for the 10.7 wt.% Au/TiO2 was only 159 ◦ C. As we showed in Section

3.3, the temperature of calcination can affect the size of the gold particles. Indeed, the investigations by XRD and TEM revealed that while the average gold particle sizes for the catalysts calcined at 200 ◦ C were below 5 nm (no reflections of gold in the XRD), after calcination at 300 ◦ C a significant increase in the gold particle size with increasing gold content could be observed (∼8 nm for 2.6 wt.%, ∼15 nm for 4.3 wt.%, ∼17 nm for 10.3 wt.%). Therefore, in the case of the Au/TiO2 catalysts calcined at 300 ◦ C the decreasing

Table 2 Influence of the gold loading on the catalytic activity of Au/TiO2 catalysts prepared by deposition–precipitation and coprecipitated Au/Co3 O4 catalysts Au/TiO2

Au/Co3 O4 (◦ C)

Au (wt.%)a

T1/2

200 ◦ C

300 ◦ C

0.9 2.2 2.6 4.1 10.7

−49 −39 −60 −39 −28

<25 1 50 135 159

a

after calcination at

Found by ICP-AES.

Au CO2 (%) (wt.%)a at 25 ◦ C 0.4 1.0 1.6 3.4 6.4 9.8 12.3 25.9

8 10 28 36 40 51 72 93

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catalytic activity with higher gold content correlates again with the gold particle size. These observations could be confirmed by other series of catalysts. For example, in a series of Au/TiO2 catalysts with a gold loading ranging from 0.8 to 2.6 wt.% the gold particle sizes were smaller than 5 nm even after calcination at 300 ◦ C, which resulted in comparable catalytic activities (−44 ◦ C < T1/2 < −37 ◦ C). We can therefore conclude that for the catalysts precipitated via deposition–precipitation the gold content does not significantly affect the catalytic activity as long as the gold particles have comparable sizes. At higher gold contents the distance between separate gold particles becomes smaller and agglomeration can lead to larger gold particle sizes, which will then lead to a decrease in the catalytic activity. In case of the coprecipitated Au/Co3 O4 the influence of the gold content on the catalytic activity is completely different (Table 2). Increasing the gold content from 0.4 to 25 wt.% increases the catalytic activity from 8% CO conversion to 93% at 25 ◦ C. The increasing catalytic activity with increasing gold content could be reproduced, although in some series of experiments we could find an optimum in the gold content in the range of 5.7–23.3 wt.% and a decreasing catalytic activity with a further increase in the amount of gold. The transmission electron micrographs of the coprecipitated catalysts show an inhomogeneous distribution of gold particles for all catalysts, even for the most active ones. The most active catalyst showed 93% conversion of CO at room temperature and a broad particle size distribution with gold particles between 2 and 200 nm. As the catalytic activity of the catalysts is usually attributed to the presence of gold particles below 5 nm, it would be expected that the fraction of these small gold particles increases with increasing gold content. Unfortunately, the TEM of the coprecipitated Au/Co3 O4 did not allow the exact determination of a particle size distribution, since the contrast between the gold and the support is not very pronounced. 3.5. Influence of washing procedure The catalysts have to be washed carefully to remove chloride, which is known to be a poison in oxidation catalysis. As the freshly precipitated materials, especially in the case of the coprecipitation, are sticky and

therefore difficult to wash, we decided to carry out a second washing step after calcination. The calcined materials were dispersed in water and stirred for 1 h at 50 ◦ C, filtered and dried at 90 ◦ C. For the materials that were prepared by deposition–precipitation the second washing after the calcination did not lead to significant changes in the catalytic activity. In contrast, for coprecipitated Au/Co3 O4 catalysts the second washing step improved the activity substantially. Washing the coprecipitated catalysts after calcination typically shifted the temperature of 50% conversion by more than 100 ◦ C [23]. 3.6. Influence of different support materials It has been reported that the nature of the support material as well as the physical state of the support can influence the catalytic activity of the resulting gold catalysts [11,13,17,29]. In Section 3.2 we have shown that the optimum pH value during precipitation was slightly different for Au/TiO2 , Au/ZrO2 and Au/Al2 O3 catalysts. Also the precipitation of gold at the optimum pH found for each of the materials led to a different catalytic activity in dependence on the nature of the support. In Table 3 the temperatures of 50% conversion of the best catalysts obtained during this investigation as well as the isoelectric points of the support materials are summarized. The highest catalytic activity in the CO oxidation could be achieved for Au/TiO2 and Au/Co3 O4 , prepared by deposition–precipitation, followed by Au/Al2 O3 and Au/ZrO2 . Also the use of unconventional support materials like CoAl2 O4 resulted in highly active gold catalysts. The highest catalytic activity obtained for coprecipitated Au/Co3 O4 (CP) was 93% conversion of CO at room temperature Table 3 Temperature of 50% conversion obtained for the best catalysts Sample

IEP

T1/2 (◦ C)

Au/TiO2 Au/ZrO2 Au/Al2 O3 Au/Co3 O4 (DP)a Au/Co3 O4 (CP)b Au/CoAl2 O3 Au/SiO2

∼6.0 ∼6.7 ∼9.0

−60 −20 −44 −62 ∼10 −32 >200

a b

1.0–2.0

DP: deposition–precipitation. CP: coprecipitation.

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and therefore significantly lower than for Au/Co3 O4 (DP), which is in contrast to the literature, where the coprecipitation is reported to be the favorable synthesis procedure for Au/Co3 O4 . The catalytic activity drastically changes when SiO2 is used as a support. The best Au/SiO2 catalyst obtained only converts 18% of CO to CO2 at 180 ◦ C. Okumura et al. [30] could prepare highly dispersed small gold particles on SiO2 by chemical vapor deposition (CVD), which showed a high catalytic activity in the CO oxidation. This indicates that the reason for the low catalytic activity of the Au/SiO2 cannot be simply attributed to the nature of the support, but that the synthesis procedure is crucial. The comparison of the isoelectric points of the supports shows that the materials with an IEP between 6 and 9 result in active gold catalysts while SiO2 with a IEP below 2 is found to be an unsuitable support material for gold catalysts prepared via deposition–precipitation. This suggests, that in the pH range needed to precipitate Au(OH)3 the highly negatively charged surface of SiO2 does not allow the adsorption of [Au(OH)n Cl4−n ]− species onto the support surface, which is necessary for the formation and stabilization of small gold particles. Several authors found the activity of Au/Al2 O3 in the CO oxidation substantially lower than for Au/TiO2 and Au/Co3 O4 [13,14]. Park and Lee suggested that TiO2 forms an interface with the gold more easily than Al2 O3 . This seemed at first sight reasonable as TiO2 is known to form strong metal support interactions while Al2 O3 is an inert support material. However, the strong metal support interactions reported for noble metal/TiO2 catalysts only develop after reduction at ∼500 ◦ C, which in the case of gold leads to less active catalysts. Schubert et al. [13] explained the observed differences in the catalytic activities of Au/TiO2 , Au/Co3 O4 and Au/Al2 O3 by differences in the reaction mechanism. While “active” supports like TiO2 and Co3 O4 act as oxygen suppliers, in case of “inactive” supports like Al2 O3 the oxygen has to be adsorbed on the gold surface. Nevertheless, in this investigation we almost could obtain the same catalytic activity for Au/Al2 O3 as for Au/TiO2 and Au/Co3 O4 (DP), which would suggest that the stabilization of the gold particles is the most important role of the support. At closer inspection, TEM investigations reveal, that a Au/TiO2 and a Au/Al2 O3 , which have comparable gold particle size distributions (Fig. 8) result in

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Fig. 8. Gold particle size distributions determined by TEM.

significant differences in the catalytic activity in CO oxidation. While the temperature for 50% conversion is −44 ◦ C for the Au/TiO2 sample, T1/2 is only 2 ◦ C for the Au/Al2 O3 catalyst. This result shows that the catalytic activity is not only a question of the gold particle size but also of the nature of the support, which however, influences the performance by a mechanism so far not clear. The catalysts prepared during this study show excellent catalytic activity compared to literature data. The temperature of 50% conversion reported in literature for catalysts tested under the same reaction conditions (SV = 20,000 ml/h gcat , 1% CO in air) was −36 ◦ C for Au/TiO2 prepared by deposition–precipitation, −65 ◦ C for coprecipitated Au/Co3 O4 and 17 ◦ C for Au/Al2 O3 prepared by deposition–precipitation [6,30], which is comparable with the data obtained in this work (Table 3). Iwasawa et al. [11,31] found that the phase of iron oxide supports plays an important role in the synthesis of gold catalysts using gold phosphine complexes and as-precipitated wet iron hydroxide. To investigate the influence of the support for the deposition–precipitation method, we used two different TiO2 , P25 (Degussa) (TiO2 1) which contains

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70% anatase and 30% rutile and has a surface area of 56 m2 /g and an anatase (Sachtleben) (TiO2 2) with a surface area of 390 m2 /g. For catalysts with a nominal gold loading of 2.4 wt.% that have been precipitated in the pH range of 6–8 and calcined at 300 ◦ C, the average T1/2 out of 15 samples is 38 ± 74 ◦ C for Au/TiO2 1 and 16 ± 30 ◦ C for Au/TiO2 2. Although at first sight the anatase seems to be the favored TiO2 support, it has to be noticed that the standard deviation in case of the Au/TiO2 1 is much higher and that the best catalyst showed a higher catalytic activity (T1/2 = −42 ◦ C) than the Au/TiO2 2 (T1/2 = −32 ◦ C). Also the direct comparison of two samples prepared at the same day at pH 8.0 and calcined at 200 ◦ C led to similar catalytic activities (T1/2 = −30 ◦ C for Au/TiO2 1 and T1/2 = −36 ◦ C for Au/TiO2 2). We therefore conclude that the two investigated TiO2 supports did not lead to significant differences in the catalytic activity of the gold catalysts. Furthermore it did not matter weather the anatase was calcined at 300 ◦ C for 4 h before the synthesis, which leads to a decrease in the surface area from 390 to 140 m2 /g. This result indicates that the surface area of the support material does not significantly affect the catalytic activity. In contrast, the use of two different Al2 O3 supports led to catalysts with significant differences in the catalytic activity. We used a ␦/␥-Al2 O3 , (Pural Ox, Condea) (Al2 O3 1) with a BET surface area of 145 m2 /g and a ␦-Al2 O3 (Aluminiumoxid C, Degussa) (Al2 O3 2) with a surface area of 117 m2 /g. Fig. 9 shows two series of experiments with catalysts that have been prepared at the same day in an

automated synthesis. This means, that the samples have been dried and calcined under exactly the same conditions, which usually led to a good reproducibility. The conversions of Au/Al2 O3 2 are significantly lower than for Au/Al2 O3 1. In another series of samples which had been synthesized according to method B, again a lower catalytic activity was obtained for gold supported on Al2 O3 2, namely a difference in the temperature of 50% conversion of about 50 ◦ C after calcination at 200 ◦ C and more than 100 ◦ C after calcination at 300 ◦ C. Although we mentioned before that the reproducibility of TiO2 catalysts could differ that strongly, we never found that discrepancy when preparing two catalysts on one day. Therefore, we assume that in case of the Au/Al2 O3 system the differences in catalytic activity are not a question of reproducibility. The use of the two different alumina supports really results in different gold catalysts. According to X-ray diffraction measurements, the differences in the catalytic activity of the two Au/Al2 O3 systems could again be attributed to the gold particle sizes. Precipitation at the same pH value led to larger gold particles for the Au/Al2 O3 2 material. Therefore, the influence of the alumina support on the catalytic activity is not a real support effect, but can be attributed to the synthesis procedure. This example indicates that the physical and chemical state of a metal oxide can influence the particle size of gold and therefore the catalytic activity of the resulting catalyst. Besides the low reproducibility of the gold catalysts, the differences in the physical state of a chosen support might therefore be another reason for the discrepancies reported in literature.

Fig. 9. Yield of CO2 at four temperatures determined in the parallel reactor over two series of Au/Al2 O3 catalysts with different supports: (a) Au/Al2 O3 (Pural Ox); (b) Au/Al2 O3 (Aluminiumoxid C).

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4. Conclusion Highly active gold catalysts based on TiO2 , ZrO2 , Al2 O3 and Co3 O4 were prepared by deposition– precipitation. SiO2 was found to be an unsuitable support for the deposition–precipitation method. Au/Co3 O4 catalysts prepared by deposition– precipitation showed much better catalytic activity than the coprecipitated Au/Co3 O4 catalysts. The catalytic activity of gold/metal oxide catalysts depends strongly on the pH during precipitation and the temperature of calcination. The optimum pH value is slightly dependent on the support and lies between pH 8 and 9. With increasing temperature of calcination the catalytic activity in the CO oxidation decreased. According to XRD and TEM the increased catalytic activity with increasing pH value and decreasing calcination temperature could be attributed to a decrease in the gold particle size. The same gold particle size led to much higher catalytic activity for Au/TiO2 than for Au/Al2 O3 . The increasing catalytic activity is therefore not only a particle size effect. We could show that as long as the isoelectric point of the support lies between 6 and 9, we can obtain highly active gold catalysts by optimization of the synthesis conditions on a large number of supports. Therefore, the nature of the support should only play a secondary role.

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