Counting Au catalytic sites for the water–gas shift reaction

Abstract

We have developed various techniques to count catalytic sites of Au/TiO2 catalysts for the water–gas shift (WGS) reaction. Addition of Br in an amount that is only 16% of the moles of the surface Au on a 2.3wt.%Au/TiO2 catalyst decreases the majority of its WGS reaction rate per total mole of Au but does not result in an appreciable change in the average Au particle size, Au particle shape, apparent activation energy, or reaction orders. From transient isotopic switch experiments, the WGS turnover frequency (TOF) for Au/TiO2 catalysts with and without Br, based on the operating active sites counted in the experiment, is 1.6±0.5s−1 under 6.8% CO, 8.5% CO2, 11.0% H2O, 37.4% H2 at 120°C. The estimated number of potential active sites, ∼2% of the total amount of Au on the 2.3wt.%Au/TiO2 catalyst, best correlates with the Au corner atoms (∼2%) of the cubo-octrahedral particles. From operando FTIR spectroscopy, the normalized IR peak area of CO adsorbed on Au0 near 2100cm−1 is proportional to the WGS reaction rate for Au/TiO2 catalysts with and without Br. Thus, the dominant active sites on Au/TiO2 catalysts for the WGS reaction are taken to be the metallic corner Au sites with Au–Au coordination number of 4.