Size and Support Effects for the Water-Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al2O3 and TiO2

The water-gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO2, 22% H2O, and 37% H-2 at 1 atm for Au/Al2O3 catalysts at 180 degrees C and Au/TiO2 catalysts at 120 degrees C varies with the number average Au particle size (d) as d(-2.2+/-0.2) and d(-2.7+/-0.1), respectively. The use of nonporous and crystalline, model Al2O3, and TiO2 supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au-0 (near 2,100 cm(-1)) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al2O3 and Au/TiO2 catalysts, respectively, and 10 times more active for Au on TiO2 versus Al2O3. From operando Fourier transform infrared spectroscopy (FTIR) experiments, it is determined that the active Au sites are metallic in nature. In addition, Au/Al2O3 catalysts have a higher apparent H2O order (0.63) and lower apparent activation energy (9 kJ mol(-1)) than Au/TiO2 catalysts with apparent H2O order of -0.42 to -0.21 and activation energy of 45-60 kJ mol(-1) at near 120 degrees C. From these data, we conclude that the support directly participates by activating H2O molecules.