Mechanistic Insights into the NH3 Oxidation Rate and Selectivity Hysteresis on Pt/Al2O3 Catalysts

Steady-state and transient kinetic measurements are combined with a suite of characterization techniques and a microkinetic model to provide insights into the mechanistic details of NH3 oxidation on Pt/Al2O3 samples of different Pt particle sizes and pretreatment history. Larger Pt particles show NH3 oxidation rates (per Pt) that are higher than those of smaller Pt particles. In situ X-ray absorption spectra reveal that smaller Pt particles are more susceptible to bulk oxidation state changes than large Pt particles, in part leading to an influence of the Pt particle size on the observed reactivity. A microkinetic model was developed that describes the dependence of product selectivity on ammonia conversion and rationalizes the different selectivity observed with respect to the catalyst particle size and condition history. Regardless of the Pt particle size, all samples show a temperature-dependent (450-550 K) hysteresis in reaction rates and product selectivities, with higher rates and N2O selectivity observed when the reaction temperature was previously at a higher temperature rather than a lower temperature. Differential kinetic and characterization data measured under conditions corresponding to the upper and lower branches of the rate hysteresis on Pt/Al2O3 suggest sample-history-dependent changes in the surface structure and active site characteristics, beyond those expected solely from changes to particle dispersion and the number of surface active sites.