Quantitative determination of gold active sites on Au/ZrO2 catalysts

In the last years the catalytic properties of finely dispersed gold particles were extensively investigated, and exploration of performances of Au nanoparticles on the basis of sizes was the main focus of research in this field. The size of Au particles is mainly determined by TEM, but it is an expensive method and hardly applicable for highly dispersed samples. A technique more widely diffuse both in academic and industrial laboratories, reproducible and fast, such as chemisorption test, should be very useful for these systems. The problem is that Au does not chemisorb many molecules easily, while the support could do it under the traditional experimental conditions. We have recently demonstrated that CO chemisorption, by a well defined procedure, can be taken as a reproducible technique for determining the concentration of Au active sites on Au/TiO2, Au/Fe2O3 and Au/CeO2 samples. In order to extend our previous study, we investigate here the quantitative determination of gold active sites on Au/ZrO2 samples, that are of significant interest as catalysts for CO oxidation, butadiene hydrogenation, epoxidation of styrene and WGS reaction. Au/ZrO2 samples were prepared by various methods, with different metal contents and various particle sizes, and were characterized by N2 adsorption analysis, pulse-flow CO chemisorption, FTIR spectroscopy, HRTEM. Volumetric measurements of CO chemisorption were performed on mildly reduced catalysts and after saturation of the surface with a proper amount of water, in order to avoid CO chemisorption on uncoordinated support ions, as demonstrated by FTIR results. Also the possible formation of formates or other non carbonylic species on the zirconia support was tested by FTIR on the different samples, looking at the mid-IR region spectra. We have selected the temperature of 157K to perform pulse flow CO chemisorption measurements on Au/ZrO2 samples, checking by spectroscopic analysis that at this temperature and with a reducing+hydrating pretreatment, the chemisorption data are related only to chemisorption on gold. The Au/ZrO2 catalysts have shown various molCO/molAu ratio (from 0.07 to 0.31 molCO/molAu). FTIR CO chemisorption measurements, both on reduced and reduced+hydrated samples at the same temperature were also performed. These last data indicate that the selected temperature and pre-treatment are suitable for the quantitative gold sites determination. In summary, this work demonstrates that by the use of a pulse flow system and after a proper pretreatment, that is presaturation of the surface with water, the CO chemisorption at 157K can be taken as a method for the quantitative determination of the gold active sites on Au/ZrO2 catalysts.