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

The turn-over-rate (TOR) for the water gas shift (WGS) reaction at 200 1C, 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 A ˚ and 0.13 A ˚ 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 DXANES, 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.