Counting Au catalytic sites for the water–gas shift reaction

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 2.3 wt.%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 ± 0.5 s−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 2.3 wt.%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 2100 cm−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.

Graphical abstractAddition of bromine corresponding to 16% of the Au surface atoms decreases the water–gas shift (WGS) rate of a 2.3 wt.%Au/TiO2 catalyst by six times and does not result in an appreciable change in apparent activation energy and orders of reaction, implying that unsaturated metallic corner Au atoms (Au atoms marked blue in the figure below) are the dominant active sites for WGS on Au/TiO2 catalysts.Figure optionsDownload full-size imageDownload high-quality image (198 K)Download as PowerPoint slideHighlights► Bromine poisons the active Au sites of a 2.3 wt.%Au/TiO2 catalyst but does not significantly change its apparent kinetic parameters. ► The WGS TOF for Au/TiO2 catalysts with and without bromine, based on the operating active sites, is 1.6 ± 0.5 s−1 under 6.8% CO, 8.5% CO2, 11.0% H2O, 37.4% H2 at 120 °C. ► The dominant active sites on Au/TiO2 catalysts for the WGS reaction are the metallic corner Au sites with Au–Au coordination number of 4.