Determination of the Active Site for Nanoparticle Gold Water-Gas Shift Catalysts Using a Model Support

The rate per mole of gold for the water-gas shift (WGS) reaction was measured for a series of gold catalysts supported on model rutile TiO₂.  The use of a model support provides an enhanced ability in resolving supported gold nanoclusters using Transmission Electron Microscopy (TEM).  A physical model of the gold clusters as truncated cubo-octahedra was used to determine the fraction of atoms in a gold cluster that have a specific number of gold neighbors.  The rate of each sample was then obtained by assuming a constant rate per active site on each cluster imaged by TEM and obtaining a value for the constant rate to best fit the experimental data.  Results indicate that the number of active sites on each cluster is constant irrespective of the gold nanocluster size in the range of samples studied.  DRIFTS and transmission FTIR experiments were used to show how different surface intermediates are related to the WGS rate under in situ conditions and during isotope switching experiments.  The rate on a Au/TiO₂ catalyst with a 1.2 nm average particle size (1.4x10^-1 s^-1) is two orders of magnitude higher than the rate on an industrial Cu/ZnO/Al₂O₃ catalyst (5x10^-4s^-1) (moles CO reacted per surface mole of Au or Cu at 120^oC, 6.8% CO, 21.9% H₂O, 8.5% CO₂, 37.4% H₂, and balance Ar) further emphasizing the potential practical and scientific interest of nanogold WGS catalysts.