Kinetics of two-step catalytic sequence on nanoclusters with limited cluster occupancy

Two-step catalytic sequence was considered for cooperative kinetics of heterogeneous catalytic reactions on nanoclusters with a limited number of adsorbed species due to size restrictions allowing for derivation of the corresponding rate expressions. Dependence of the kinetic parameters on the number of adsorbed species was considered for different models of biographical (intrinsic) and induced non uniform surfaces. Explicit expressions were derived for the rates on evenly nonuniform surfaces exhibiting a linear decrease in the Gibbs energy with site occupancy. An expression for the apparent activation energy was obtained for the two step sequence with both irreversible steps. Simulations were performed for several expressions for the forward reaction rate in a two-step sequence with different number of maximal adsorbed species. The analysis illustrated that the reaction rates in the forward direction pass through maxima even without any competition of both reactants. Strong nonuniformity or lateral interactions make the rate maxima less prominent. Larger clusters, which can accommodate more adsorbed intermediates per cluster, exhibit rather wide maxima in the rates expanding over a broad range of concentrations. The experimental data on resazurin reduction with hydroxylamine over gold clusters bearing 15 to 25 gold atoms were used to illustrate applicability of the developed theoretical approach showing a dramatic switch in kinetic regularities between Au15 and Au18 clusters, which could be attributed to changes in the number of adsorbed species per cluster.