New gold catalysts supported on mixed ceria-titania ... - Springer Link

Jointly published by Akadémiai Kiadó, Budapest and Springer, Dordrecht

React.Kinet.Catal.Lett. Vol. 91, No. 2, 213−221 (2007) 10.1007/s11144-007-5084-6

RKCL5084 NEW GOLD CATALYSTS SUPPORTED ON MIXED CERIA-TITANIA OXIDES FOR WATER-GAS SHIFT AND PREFERENTIAL CO OXIDATION REACTIONS Maela Manzolia, Floriana Vindignia, Anna Chiorinoa, Tatyana Tabakovab*, Vasko Idakievb and Flora Boccuzzia a

Department of Chemistry IFM, University of Torino, via P. Giuria 7, 10125 Torino, Italy, b Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 11, 1113 Sofia, Bulgaria

Received November 20, 2006, accepted March 30, 2007

Abstract New gold catalysts supported on mixed CeO2-TiO2 with different ceria loading have been prepared. BET, HRTEM and FTIR characterization has been carried out to reveal the effect of ceria on the WGS and PROX activity. Keywords: Gold catalysts, ceria, titania, WGSR

INTRODUCTION Catalytic systems containing highly dispersed gold have received great interest from both experimental and theoretical points of view. Due to their high catalytic activity, particularly in CO oxidation at low temperature [1,2] they are considered promising candidates for hydrogen production, through methanol decomposition and water-gas shift (WGS) reactions, and purification, through preferential CO oxidation (PROX). The nature of the support on which gold is dispersed plays a crucial role in determining the catalytic activity. Recent studies have shown that ceria supported gold catalysts are stable and exhibit a _________________________ * Corresponding author. Tel.: +3592 979 2528; Fax: +3592 971 2967 E-mail: [email protected] 0133-1736/2007/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved

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high activity for the WGS reaction over a wide temperature range [3], at different space velocities and H2O/CO ratios [4]. However, there is no consensus on the nature of the active sites for WGSR over this type of catalysts. Some of us have used FTIR and concluded that metallic gold particles in close contact with defective ceria play an essential role for the high WGS activity of Au/CeO2 [5]. Wang et al. have indicated that the rate determining steps for WGSR occur at the gold-ceria interface, with the active sites being small gold clusters and O vacancies [6]. In contrast, Fu et al. have reported that metal gold nanoparticles do not participate in the WGSR and provided evidence that the active site for the WGS reaction was cationic gold bound strongly to the ceria surface [7]. In order to get an interpretation of the effect of ceria, characterizations by HRTEM and FTIR and catalytic activity tests have been undertaken on new gold catalysts supported on CeO2-TiO2 mixed oxides with different ceria loadings (20 and 50 wt.%). Additionally, the relationship between dispersion and oxidation state of gold and WGS activity of Au/CeO2-TiO2 catalysts has been discussed. EXPERIMENTAL The catalysts were prepared by deposition-precipitation of gold on CeO2TiO2 supports (with different ratio of ceria and titania, 20:80 (AuCe0.2Ti0.8) and 50:50 (AuCe0.5Ti0.5), respectively) suspended in water, via interaction of HAuCl4⋅3H2O and K2CO3 at a constant pH = 7.0 and at 333 K. After aging for 1 h, the precipitates were washed, dried in vacuum at 353 K and calcined under air at 673 K for 2 h. The gold loading for each catalyst was 3 wt.%. The syntheses of the supports and gold catalysts were carried out in a "Contalab" laboratory reactor enabling complete control of the reaction parameters (pH, temperature, stirrer speed, reactant feed flow, etc.) and ensuring high reproducibility. HRTEM analysis of the powdered samples deposited on a copper grid was performed using a Jeol 2000 EX (200 kV) microscope. Specific surface area was determined by N2 adsorption at 77 K using a gas volumetric apparatus ASAP 2010 Micromeritics. FTIR spectra were taken on a Perkin-Elmer 1760 spectrometer (equipped with a MCT detector) with pellettized samples introduced in a cell allowing thermal treatment in controlled atmospheres and spectrum scanning at controlled temperatures (from 120 to 300 K). Band integration was carried out by "Curvefit", in Spectra Calc (Galactic Industries Co.) by means of Lorentzian curves. From each spectrum, the spectrum of the sample before the inlet of CO was subtracted. The spectra were normalized in respect to the gold content of each sample.


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WGS activity measurements were carried out in a flow reactor at atmospheric pressure. The reactant gas mixture fed into the reactor contained 4.42 vol. % CO, the rest being argon. The following conditions were applied: catalyst bed volume = 0.5 cm3 (0.63−0.80 mm pellets), space velocity = 4000 h -1, partial pressure of water vapor = 31.1 kPa. The samples were previously subjected to a reduction treatment at 383 K for 1 h in a 1% H2/Ar mixture. The analysis of the mixture converted was carried out at the reactor outlet on an “URAS-3G” (Hartmann & Braun AG) gas analyzer with respect to the CO content. The catalytic activity was expressed by the degree of CO conversion. RESULTS AND DISCUSSION The analysis of BET, HRTEM and FTIR measurements pointed out a quite different morphology and structure of the samples. BET measurements indicated that AuCe0.5Ti0.5 has a specific surface area significantly higher than that of Au/TiO2 (104 m2/g vs. 86 m2/g, respectively), while the specific surface area of AuCe0.2Ti0.8 is 77 m2/g. HRTEM analysis evidenced a large interdispersion of ceria and titania in the AuCe0.5Ti0.5 catalyst. Ceria appears as highly crystalline nanoparticles roundly shaped and having an average diameter around 4.5 nm, while the titania particles are larger. Therefore, in this sample, the ceria is responsible for the higher specific surface area. Two easily recognizable distinct regions have been evidenced in AuCe0.2Ti0.8: one of them, appearing darker, mainly composed by ceria and another mainly by titania. The analysis of the diffraction fringes showed regular planes exposed by the two oxides. In this sample, the ceria particles are larger than in AuCe0.5Ti0.5. As a consequence, the presence of ceria on AuCe0.2Ti0.8 does not play a positive effect on the specific surface area which remains close to that of pure titania supported catalysts. It is worth noting that, because of the low difference in the atomic weight, it is difficult to distinguish the metallic gold particles from CeO2. In AuCe0.5Ti0.5 catalyst with higher content of ceria, only few gold particles with a size around 2 nm have been detected. Most of the gold particles, however, are not detected by HRTEM as a consequence of dispersion to atomic level of gold atoms interacting with defective ceria. Gold particles are hardly observable on the ceria-rich region of the AuCe0.2Ti0.8 catalyst, while they are easily seen as dark dots on the surface of the TiO2-rich region. In this sample the average diameter of gold particle was estimated to be ≈ 2 nm, with a size distribution between 1 and 3 nm. Deposition of gold could lead to atomically dispersed gold in the regions of the supports rich in defective ceria, and to small gold particles in the titania-rich regions. A confirmation to this hypothesis is given by the analysis of the FTIR data that sheds some light on the nature and abundance of the adsorption sites exposed at the surface of the two catalysts.

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Fig. 1. FTIR spectra of AuCe0.5Ti0.5 (section a) and AuCe0.2Ti0.8 (section b) oxidized at 673 K after adsorption of 15 mbar of CO at RT (dashed line), at decreasing pressures (fine lines) and under outgassing at RT (bold line)

Figure 1 shows the FTIR spectra produced by the adsorption of 15 mbar of CO at room temperature (RT) and at decreasing coverage at the same temperature on previously at 673 K oxidized catalysts - AuCe0.5Ti0.5 (section a) and AuCe0.2Ti0.8 (section b). Besides the usual bands related to CO on the supports and on metallic gold, that will be discussed later on, an unusual and broad absorption band at about 2166 cm-1 is observed on both catalysts. This band is almost completely irreversible to the outgassing at RT, giving evidence of a strong bond between CO and the involved adsorption sites. It is interesting to observe that this feature has never been observed before on supported gold. Looking at the paper of Wu et al. [8] and Fielicke et al. [9], the band at about 2166 cm-1 may be related to CO adsorbed on cationic gold clusters, Aun(CO)m+, stabilized on the mixed oxide surface. This adsorption band shows also a different intensity on the two samples. In particular, it is three times more intense on AuCe0.5Ti0.5 (section a) than on AuCe0.2Ti0.8. This difference indicates that gold clusters are more abundant on AuCe0.5Ti0.5 than on AuCe0.2Ti0.8, i.e. their abundance appears related to the amount of ceria in the


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samples. It could be suggested that gold clusters are stabilized by an interaction with nanodispersed ceria. The band at 2116 cm-1 is due to CO molecules adsorbed on mildly oxidized Au sites at the surface of small particles [10,11]. The ratio between the integrated areas of the 2116 cm-1 band in the two samples is 1.3, and it indicates that the amount of sites exposed at the surface of the small metallic gold particles is larger on AuCe0.2Ti0.8. As to the absorptions observed at 2205 cm-1 and 2186 cm-1 (Figs 1a and 1b), they are due to CO adsorbed on Ti4+ surface sites, to the most uncoordinated ones and to ions exposed on regular (001) or (010) planes [12,13] respectively. It appears that on the AuCe0.5Ti0.5 sample, where these bands are less intense, the ceria deposition strongly perturbs the titania sites.

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Fig. 2. FTIR spectra of AuCe0.5Ti0.5 (section a) and AuCe0.2Ti0.8 (section b) reduced in H2 at 473 K after adsorption of 15 mbar of CO at RT (dashed line), at decreasing pressures (fine lines) and under outgassing at RT (bold line)

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Figure 2 shows the FTIR spectra of CO adsorption at RT on the reduced catalysts AuCe0.5Ti0.5 (section a) and AuCe0.2Ti0.8 (section b). After reduction in hydrogen at 473 K, the band at 2166 cm-1 is totally depleted on both samples. This feature can support the assignment of the band to cationic gold clusters present only at the surface of the oxidized samples. The main feature for AuCe0.5Ti0.5 (section a) is a broad band due to CO adsorbed on gold sites at 2084 cm-1, red shifted from the usual frequency. This evidence, together with the HRTEM observation that most of the gold cannot be seen in AuCe0.5Ti0.5, indicates that in this sample all the gold is highly dispersed and is in good contact with the small reduced ceria particles. After the reductive treatment in H2, an electron transfer from Ce3+ to Au can occur. Due to this feature and on the basis of our previous results on other gold catalysts supported on ceria, we can infer that, on AuCe0.5Ti0.5, negatively charged gold clusters or flattened, thin gold layers are present at the surface of defective ceria. The observation of a weak band at 2110 cm-1 (Fig. 2a), assigned to CO adsorbed on Au0 step sites, indicates that a small fraction of gold is present as tridimensional particles on the titania surface. It can be assumed that after reduction, Au spreads on the reduced ceria and also the gold particles, to which the absorption band of CO at 2110 cm-1 is related, may become very thin. Such phenomenon is observed by Akita et al. [14] in a HRTEM study of gold nanoparticles supported on a model CeO2 exposing low index flat faces. On the contrary, on AuCe0.2Ti0.8 catalyst, a significant fraction of the gold particles, possibly in contact with titania, does not undergo significant changes in the reducing atmosphere, as indicated by the presence of the usual band of CO on Au0 at 2107 cm-1. However, the presence of the shoulder at 2090 cm-1 is an indication that the rest of the gold is in contact with ceria and it is small enough to be negatively charged and flattened after reduction. Figure 3 shows the catalytic activity of five catalysts in the WGS reaction. The activity of the new samples was compared with that of Au/TiO2 and Au/CeO2 catalysts prepared in the same laboratory and with Au/TiO2 Ref. sample, provided by the World Gold Council [15]. The catalytic data showed that at low temperature AuCe0.2Ti0.8 is more active than the other catalysts; moreover, its performance is close to Au/TiO2 and to Au/TiO2 Ref. On the contrary, AuCe0.5Ti0.5 demonstrated higher activity than AuCe0.2Ti0.8 above 523 K, and its behavior is similar to Au/CeO2. The lower catalytic activity of AuCe0.5Ti0.5 at temperatures below 523 K could be related to the large amount of carbonate-like species that covered the surface of the catalysts due to the basic character of ceria and its ability to bind strongly carbonate entities (see also Fig. 4). A temperature higher than 523 K is necessary to clean the surface and to enable the sites for WGS reaction. In order to find experimental confirmation for such suggestion and to show the applicability of these new catalysts for preferential CO oxidation, the FTIR


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100 WGS equilibrium

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Fig. 3. Temperature dependence of CO conversion in the WGS reaction on different catalysts: AuCe0.5Ti0.5 (■); AuCe0.2Ti0.8 (□); Au/TiO2 (▲); Au/CeO2 () and Au/TiO2 Ref. ()

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Fig. 4. FTIR spectra of AuCe0.5Ti0.5 (bold line) and AuCe0.2Ti0.8 (dotted line) oxidized at 673 K in contact with PROX reaction mixture at RT

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spectra of previously oxidized AuCe0.5Ti0.5 (bold curve) and of AuCe0.2Ti0.8 (dotted curve) in contact with a PROX reaction mixture at RT have been collected. The inlet of H2 has been done before the inlet of the CO and O2 (1:1) mixture. AuCe0.5Ti0.5 is more active than AuCe0.2Ti0.8, as confirmed by the higher intensity of the band at 2350 cm-1 due to the asymmetric stretching of the CO2 produced. High absorption in the carbonate-like region can be observed on AuCe0.5Ti0.5, due to the presence of the larger amount of ceria. On the contrary, a broad band in the OH-stretching region is evident on AuCe0.2Ti0.8. This feature can be explained by the ability of metallic gold nanoparticles, present on titania, to break the H-H bond. As previously observed by TEM, a large amount of very small gold particles is present on the AuCe0.2Ti0.8. On this catalyst, the breaking of the hydrogen molecular bond with the subsequent spill-over of the atomic hydrogen onto the surface and OH groups’ formation occurs, in contrast to AuCe0.5Ti0.5, where mainly gold clusters embedded in the ceria are present. CONCLUSIONS New gold catalysts were prepared by deposition-precipitation on mixed CeO2-TiO2 supports with different content of ceria. Two different kinds of gold sites have been evidenced by FTIR measurements: small clusters and ultrasmall gold particles. The relative amount and the nature of the exposed sites on the two samples are different and strongly depending on the support composition and on the pretreatment. The CO interaction at RT well evidenced the presence of either cationic clusters, or small metallic gold particles or small negatively charged flat particles. In particular, special sites - cationic clusters where CO is bonded irreversibly to the outgassing at RT, are present in both oxidized Au/CeO2-TiO2 samples. These sites are three times more on AuCe0.5Ti0.5 than on AuCe0.2Ti0.8. On the contrary, a large amount of sites exposed at the surface of the small metallic gold particles is present in AuCe0.2Ti0.8. This kind of sites could be associated to the catalytic activity in the WGS reaction and be responsible for the good performances of AuCe0.2Ti0.8 that shows a higher activity at lower temperature. REFERENCES 1. 2. 3. 4.

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