Contributions of Electron Microscopy to Understanding CO Adsorption on Powder Au/Ceria–Zirconia Catalysts

Abstract

The influence of the highly dispersed gold phase on the CO-support interaction occurring in two 2.5 wt % Au/Ce(0.62)Zr(0.38)O(2) catalysts with medium (Au/CZ-MD) and high (Au/CZ-HD) metal dispersion is quantitatively assessed. For this purpose, we have followed an approach in which high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), computer modelling, volumetric adsorption and FTIR spectroscopy studies are combined. This approach has already been fruitfully applied to the investigation of the specific CO-metal adsorption in Au/ceria-zirconia catalysts. As deduced from the experimental studies reported herein, the presence of gold dramatically increases the amount of CO strongly chemisorbed on the support. Moreover, this amount is sensitive to the metal dispersion, thus suggesting the occurrence of a mechanism in which the CO molecules that are initially adsorbed on the gold nanoparticles are further transferred to the support by means of a spillover process. An annular model is proposed for the growth of the CO phase adsorbed on the ceria-zirconia mixed oxide in the presence of Au. By assuming this model, we have estimated the width of the annulus, Delta r, of the adsorbed CO grown around the Au nanoparticles in Au/CZ-MD and Au/CZ-HD catalysts. This value is found to be very close to Delta r approximately 2 nm in both cases, the coincidence lending some additional support to the model. To further confirm this proposal, we have investigated the influence of CO pre-adsorption on the D(2)-Au/CZ-MD interaction, at 298 K. As revealed by FTIR spectroscopy, the kinetics of the deuterium spillover is significantly disturbed by the pre-adsorbed CO, which is fully consistent with an annular model for the CO adsorption. We conclude from the global analysis of the results reported here and those already available on CO-Au adsorption that the appropriate combination of nanostructural, computer modelling and chemical techniques is a powerful tool allowing us to gain a comprehensive picture of the complex series of processes involved in the CO adsorption on this relevant family of gold catalysts.

Recent Progress in Chemical Characterization of Supported Gold Catalysts: CO Adsorption on Au/Ceria–Zirconia

“…41 Figure 1 summarizes an example of application to a Au/CZ powder catalyst of the routine discussed above. 42 In accordance with the difference isotherm (23) reported in part B of Figure 1, the experimental range of P(CO) at 308 K is wide enough as to reach the saturation coverage of gold. Similar results have also been reported for other samples of this family of powdered Au/CZ catalysts.…”

“…b) The kinetics of the process is dramatically modified by the presence of gold, the Au nanoparticle size distribution shown by the catalysts playing also a very relevant role. 42 To our knowledge, no quantitative data evidencing such a strong influence of the metal phase on the chemical behavior of the oxide support had earlier been reported. Moreover, it is obvious from this study that the support contribution to the total amount of CO chemisorbed on Au/CZ catalysts cannot be determined from parallel adsorption studies carried out on the bare support.…”

“…The observations above clearly indicate that a mechanism implying the initial CO adsorption on the metal and its further transfer to the support (spillover) is much faster than the direct COCZ process. 42 Also interesting, the dependence of the CO adsorption on the Au nanoparticle size distribution suggests that the growth of the CO phase strongly chemisorbed on the support follows an annular model, the metal nanoparticles being located at the center of the annuli. 42 To confirm this proposal, the radius of these annuli (r a ) was estimated for Au/CZ-1 and Au/CZ-2 by application to the corresponding Au nanoparticle size distributions an adsorption model based on the following assumptions: a) r a is constant and independent of the size of the Au nanoparticles; b) the surface of the support consists of [111] facets, the one showing the highest density of O 2¹ ions and thermodynamic stability; 57 and c) the adsorption process involves two surface O 2¹ sites per CO molecule.…”

“…42 a) The CO adsorption on the CZ support is kinetically controlled. b) The kinetics of the process is dramatically modified by the presence of gold, the Au nanoparticle size distribution shown by the catalysts playing also a very relevant role.…”

“…Actually, the second isotherm would measure the joint contribution of the two weakly adsorbed forms, whereas the difference between the first and second isotherms would account for the strong, irreversible CO adsorption on the support. 41,42 To separate the contributions of the weakly adsorbed forms due to the metal and support, the experimental routine applied to Au/CZ was also used in the investigation of the pure CZ sample. 41 By analogy with the interpretation proposed for the Au/CZ data, the second isotherm recorded for CZ would account for the CO weakly adsorbed on the support; therefore, the difference between the second isotherms for Au/CZ and CZ would measure the amount of CO specifically chemisorbed on the gold nanoparticles.…”