Elsevier

Journal of Catalysis

Volume 262, Issue 1, 15 February 2009, Pages 169-176
Journal of Catalysis

New insight on the nature of catalytically active gold sites: Quantitative CO chemisorption data and analysis of FTIR spectra of adsorbed CO and of isotopic mixtures

https://doi.org/10.1016/j.jcat.2008.12.017Get rights and content

Abstract

FTIR absorption spectra of CO, of 12CO–13CO isotopic mixtures and of CO–18O2 interaction on gold catalysts supported on group IV oxides are reported together with CO quantitative chemisorption data. On Au/TiO2 two kinds of metallic gold surface sites, mutually interacting, adsorb CO in spite of the low CO/Au ratio (0.03). On Au/ZrO2, where a CO/Au ratio of 0.30 has been determined, mutually interacting corner sites on non-metallic gold nanoclusters are present. Finally, isolated and negatively charged gold nanoclusters have been evidenced on Au/CeO2. Different absorption coefficients have been found. On all samples, by contacting CO–18O2 at 90 K, only C16O18O is produced: gold sites are involved in the activation of both molecules. The largest amount of C16O18O is produced on the CeO2 supported sample, as a consequence of an easier activation of the oxygen on the anionic gold clusters and on the support.

Graphical abstract

12CO and 12CO–13CO FTIR absorption spectra combined with quantitative chemisorption data pointed out the relationship between the nanostructure and the physical/chemical properties of Au catalysts supported on group IV oxides. On Au/TiO2, the CO adsorbing sites are mutually interacting edge and corner sites of Au metallic particles, while on Au/ZrO2, the adsorbing sites are mutually interacting corners of non metallic Au nanoclusters. Finally, almost isolated and negatively charged Au nanoclusters have been evidenced on CeO2. The differences between the absorption coefficients of CO on Au sites have been discussed. On all samples, by contacting CO–18O2 at 90 K, only C16O18O is produced, suggesting that only the gold sites are involved in the activation of both molecules.

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Introduction

Gold catalysts are very active in the CO oxidation, in the PROX and in the WGS reaction. Therefore, they are good candidates for the catalytic production and purification of hydrogen for fuel cell applications [1]. Experimental and theoretical works suggest that for the PROX reaction the active sites for CO and oxygen activation may be uncoordinated sites of metallic gold nanoparticles [2]. Moreover, a role of cationic [3] and anionic [4] Au has been proposed and widely discussed in the last years. As for the WGS reaction, a very recent work on model samples [5] shows that water dissociates on oxygen vacancies of the oxide nanoparticles, in close contact with the gold sites. However, up to now key questions concerning the reaction mechanisms and the nature of the active sites are still without shared answers, possibly because the real catalysts have many of potential active centres, as stated very recently by Hutchings [6].

When studying real catalysts, it is useful to utilise different combined experimental methods, in order to distinguish and qualitatively determine the different gold sites exposed at the surface and also to find methods for quantitative estimation of these sites.

The analysis of the FTIR spectra after CO and CO isotopic mixtures adsorption at different temperatures on samples differently pretreated and of the FTIR spectra obtained after the CO interaction with 18O2 can be very useful to get qualitative information on the nature and the structure of the gold active species in the CO oxidation. In addition, the relation between a defined amount of sites and the catalytic activity can be achieved by the analysis of CO quantitative adsorption volumetric measurements in specific experimental conditions, chosen on the basis of the spectroscopic experiments.

In 1978, Sheppard and Nguyen elegantly demonstrated the correlation that exists between the bonding geometry of adsorbed carbon monoxide on metals and the wavenumber of the C–O stretching vibrations [7]. They also drew attention to two other factors which can influence this wavenumber: the coexistence of surface atoms having different numbers of nearest neighbours and the dipolar interaction between adsorbed molecules. These factors are of particular importance in spectra from supported catalysts since coordinatively unsaturated edge and defect sites represent a large proportion of all the available adsorption sites on small particles. Dipolar interactions lead to coverage-dependent values of the wavenumber, since the predominant effect is to couple the vibrations of individual molecules to produce collective oscillations having infrared active modes at vibrational frequencies higher than the isolated singleton molecule. In subsequent years, these factors have been extensively studied on single crystal substrates. In particular, investigations using stepped surfaces have allowed more controlled studies on the influence exercised by the coordination changing of the surface atom to which a CO molecule is bonded. It has been found that molecules at step sites do indeed exhibit different wave numbers from those on terrace sites, but the direction of the shift depends on the nature of the substrate. For example, on copper surfaces, step sites give rise to bands ≈15 cm−1 higher than terrace sites do [8], while on platinum the step site bands are shifted by a similar amount, but in the opposite side [9]. This difference has been rationalised in terms of the relative balance of σ and π bonding of the two surfaces: on both metals, bonding at step sites is stronger than at terrace ones, but on platinum the increased bonding interaction is associated with increased back-donation of metal electrons into the molecule's 2π orbital, and hence with a fall in wavenumber, whereas on copper and gold it is believed that the predominant change is a reduction in the occupancy of the σ orbital, with a concomitant shift to higher wavenumber [10]. It has also been found that the observation of bands due to molecules on the two types of site is complicated by the dipolar coupling interactions which tend to transfer intensity from low wavenumber bands to their higher wavenumber counterparts. As a consequence, the FTIR intensities of CO absorption bands are useful for a qualitative determination of the nature of the adsorbing active sites. In a number of cases, isotopic mixtures can be used to modify these interactions in order to clarify the vibrational spectra. Up to now such kind of data have been discussed for small gold particles [11], but were not yet available for small supported clusters nor for cationic and anionic species. Here we will present these data. Moreover, independent volumetric quantitative data of the gold adsorbing sites on some different catalysts will be presented, in order to compare more deeply different catalysts.

The absorption changes, both in position and shape, observed with the temperature and with the isotopic mixtures composition will be examined and interpreted. The assignments to the adsorption on corner, edge, borderline or cluster sites in gold nano-structured catalysts will be discussed, looking at the related quantitative chemisorption data.

Section snippets

Materials

The examined catalysts, reported in Table 1, were all prepared by the deposition–precipitation method (dp). The Au/TiO2 sample is an Au/TiO2 reference catalysts provided by the World Gold Council [12]. As for the Au/ZrO2 sample, the support was prepared by precipitation from ZrOCl28H2O (Fluka) at constant pH (pH = 8.6), aged under reflux conditions for 20 h, washed free from chloride (AgNO3 test) and dried at 383 K overnight. Then, the support was heated up to 923 K in flowing air for 6 h,

Spectroscopic data and quantitative chemisorptions of CO adsorbed on gold supported on titania, zirconia and ceria

In Fig. 1 the spectra of CO adsorbed on gold supported on the titania, zirconia and ceria, after reduction at 423 K, hydration at r.t. and successive cooling down to 120 K are shown (fine curves) together with the CO spectra recorded after heating up to 157 K. To our experience [15], this pretreatment is the most suitable one for a comparison of different gold catalysts on different supports, since the mild reduction eliminates the oxygen bonded to the surface of very small gold particles [17],

Conclusions

The combined analysis of 12CO and 12CO–13CO FTIR absorption spectra and of quantitative chemisorption data allowed a deeper understanding of the relationship between the nanostructure and the physical and chemical properties of gold catalysts supported on the group IV oxides. The main points are:

  • (i)

    two kinds of mutually interacting sites (corners and edges) are the CO adsorbing sites on the gold metallic particles (mean size 3.8 nm) supported on titania. In spite of the low CO/Au ratio (0.03) the

Acknowledgments

The authors gratefully acknowledge the MIUR for financial support (COFIN 2006033400_002).

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