Kinetics of NH3 oxidation on Pt/Al2O3: Rate enhancement and NH3 inhibition

We describe a combined experimental kinetics and modeling study for NH3 oxidation on Pt/Al2O3 and development of a predictive microkinetic model valid over a range of conditions. The NH3 oxidation rate (TOF) dependence is reported for NH3 concentrations between 10 and 25,000 ppm and temperatures below 250°C for Pt/Al2O3 powder and washcoated monoliths. The data reveal a shift from positive- to negative-order rate dependence on NH3 with the rate maximum dependent on the processing of Pt/Al2O3; unmilled Pt/Al2O3 powder exhibits a rate maximum at 500 ppm NH3, while ball-milled Pt/Al2O3 has a maximum at ~10,000 ppm. This unexpected enhancement of Pt/Al2O3 activity from milling results in a lowering of the light-off temperature (T50) by up to 100°C. Further examination rules out the extent of pore diffusion limitations as the root cause, but rather an increase in the fraction of stepped crystalline planes and destabilization of Pt oxide shown from XRD and H2-TPR characterization. The dependence of a shift in the rate maximum to higher NH3 concentrations with milling extent is shown to require single- and dual-site reaction pathways. The kinetic scheme also captures a subtle shift in the apparent NH3 reaction order for both unmilled and milled Pt/Al2O3. The microkinetic scheme is incorporated into fixed-bed and monolith reactor models which show excellent agreement with the NH3 conversion and product distribution data.