Bridging the Gap between CO Adsorption Studies on Gold Model Surfaces and Supported Nanoparticles

Abstract

An in-depth understanding of CO adsorption on highly dispersed gold nanoparticles (AuNPs) is critically important to fully interpret the catalytic behavior of supported gold systems in processes such as CO oxidation, PROX (selective oxidation of CO in presence of a large excess of H2), [5–7] or LT-WGS (low-temperature water gas-shift) reactions. Despite its relevance, the quantitative data and fundamental information presently available on the CO–Au interaction mainly comes from studies carried out on model single-crystal and thin-film surfaces under experimental conditions far from those at which catalytic assays on supported gold systems are typically run. Probably because of the very weak and singular nature of the CO–Au interaction, which on the basis of both theoretical and experimental studies is generally acknowledged to take place on low-coordination surface sites, and the additional contribution of the support, a few studies have been carried out that were aimed at estimating the amount of CO adsorbed at low temperature on powdered or model supported AuNPs. To our knowledge, however, none of these have arrived at a detailed quantitative description of this process under conditions close to those occurring in real catalytic reactions. To bridge this gap, we have developed an approach in which AuNP size distributions, as determined from HAADFSTEM (high-angle annular dark-field scanning transmission electron microscopy) and quantitative CO adsorption data, as determined from volumetric adsorption at 308 K, under CO partial pressures ranging from 6.65 10 Pa to 3.99 10 Pa, are jointly analyzed with the help of a nanostructural model for the gold particles. This model could be deduced from the analysis of images recorded in a parallel HRTEM (highresolution transmission electron microscopy) study. As discussed herein, this approach gives a substantial experimental support to the extension to supported gold catalysts of the chemical principles governing the CO adsorption on model surfaces. We investigated two catalyst samples, 2.5 wt% Au/ Ce0.62Zr0.38O2 (Au/CZ) and 1.5 wt% Au/Ce0.50Tb0.12Zr0.38O2 x (Au/CTZ), which have significantly different gold particle size distributions. Two consecutive CO volumetric adsorption isotherms were recorded on the gold catalysts and the corresponding supports. Prior to running the second isotherms, samples were evacuated (residual pressure Pres< 1.33 10 4 Pa) for 30 min at 308 K. By processing the volumetric data in a similar way to earlier studies (for details, see the Supporting Information), the amounts of CO adsorbed on the AuNPs, on the surface cations of the supports (weak adsorption), and on the surface anions of the supports, which mainly consist of strongly chemisorbed carbonate species, could be determined from the difference of the two isotherms. The amount of CO adsorbed on the AuNPs at PCO= 1.33 10 4 Pa (100 Torr) was used as a measurement of the saturation coverage. The corresponding data are reported in Table 1 and the Supporting Information, Figure S1. Gold particle size distributions were determined for each of the investigated catalysts from the analysis of series of experimental ultra-high-resolution HAADF-STEM images (Supporting Information, Figure S2). In accordance with the physical principles lying behind the mechanism of image formation, this technique is particularly suitable for obtaining reliable metal particle size distributions in oxide-supported metal catalysts. Moreover, as recently shown, this technique can be fruitfully applied to a very fine characterization of AuNPs dispersed on mixed oxides of heavy elements, as is the case of those investigated herein. (Size distributions for Au/CZ and Au/CTZ catalysts are shown in the Supporting Information, Figure S2). [*] M. L pez-Haro, Dr. J. J. Delgado, J. M. Cies, E. del Rio, S. Bernal, Dr. M. A. Cauqui, Dr. S. Trasobares, Dr. J. A. P rez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016-288 E-mail: jose.calvino@uca.es