The kinetics and mechanism of spillover

The rate of reduction r of WO3 to HxWO3 (x ~ 0.35) by H2 at room temperature was studied in mechanical mixtures of WO3 with supported or unsupported Pt catalysts and in the presence of co-catalysts with varying proton affinity (PA). As PA increased, r decreased in the order: H2O, CH3OH, C2H5OH, n-C3H7OH, n-C4H9OH, t-C4H9OH. For these co-catalysts, r did not change with Pt surface area S. It did not change as the amount n of adsorbed co-catalyst was decreased to about a monolayer below which r fell linearly with n. For a co-catalyst with low PA, such as CH3COOH, r depended on S. For co-catalysts with high PA, log r was proportional to PA over three decades of r. It appears that a solvated proton is formed on the metal. If PA is low, this is the rate determining process (rdp). But if PA is high, the rdp is shifted to the end of the reduction following the last surface migration of the proton in the adsorbed monolayer of co-catalyst, as the proton is released to the WO3 surface. This model explains why H2 spillover can be a fast process at low temperatures.