Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction

Abstract

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 °C, 7% CO, 9% CO₂, 22% H₂O, 37% H₂ and balance Ar, of 1.4 nm Au/Al₂O₃ is approximately 20 times higher than that of 1.6 nm Pt/Al₂O₃. Operando EXAFS experiments at both the Au and Pt L₃ edges reveal that under reaction conditions, the catalysts are fully metallic. In the absence of adsorbates, the metal–metal bond distances of Pt and Au catalysts are 0.07 Å and 0.13 Å smaller than those of bulk Pt and Au foils, respectively. Adsorption of H₂ or CO on the Pt catalysts leads to significantly longer Pt–Pt bond distances; while there is little change in Au–Au bond distance with adsorbates. Adsorption of CO, H₂ and H₂O leads to changes in the XANES spectra that can be used to determine the surface coverage of each adsorbate under reaction conditions. During WGS, the coverage of CO, H₂O, and H₂ are obtained by the linear combination fitting of the difference XANES, or ΔXANES, spectra. Pt catalysts adsorb CO, H₂, and H₂O more strongly than the Au, in agreement with the lower CO reaction order and higher reaction temperatures.