The role of cationic Au 3+ and nonionic Au 0 species in the low-temperature water–gas shift reaction on Au/CeO 2 catalysts

Abstract

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO 2 catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au 3+ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au 0 species by thermal decomposition of Au oxides (Au 3+) and subsequent nucleation and growth of very small Au 0 aggregates and metallic Au 0 nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au 3+), 84.0–84.6 eV (up-shifted signal of small Au 0 aggregates), and 84.0 eV (metallic Au 0). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO 2 catalysts is discussed.