Gold in CO oxidation and PROX: The role of reaction exothermicity and nanometer-scale particle size

Gold is a promising catalyst for various processes. There have been extensive studies in catalysis using Au-containing catalysts, but their results are ambiguous, prompting researchers to revise the data available from the literature. In this work, we analyze publications on CO oxidation and PROX (preferential CO oxidation) over various supported gold catalysts, focusing on two important aspects: effect of high exothermicity of the reaction and of nanometer-scale particle of gold. Under certain conditions, exothermicity brings the reaction into the external diffusion control regime, causing the so-called ignition of the catalyst surface. The transition of the reaction to this regime is accompanied by a number of effects (sharp increase in conversion as a result of increasing temperature, CO conversion hysteresis, appearance of a hot spot in the catalyst bed, change in activation energy, and precursor self-activation), which are misinterpreted by researchers sometimes. The behavior of the catalyst in the steady-state external diffusion control regime and on return from this regime is discussed. In the synthesis of gold catalysts by conventional methods (precipitation, deposition precipitation, impregnation), the precursors dried at 100 degrees C have gold mostly in its cationic form. The gold cations turn into Au-0 as the calcination temperature is raised or as the catalyst is kept in a reductive atmosphere. This yields gold metal particles on the support surface. Their size distribution is usually asymmetric, with the maximum shifted to smaller sizes, which is typical of lognormal distribution. A critical review is made of the data concerning the size effect, specifically, the existence of an extremum of catalytic activity as a function of the mean particle size. It is demonstrated that the variation of gold content from one sample to another is often disregarded in the interpretation of these data, while catalytic activity and particle size correlate with gold content. (c) 2014 Elsevier B.V.. All rights reserved.