(327f) Activity Enhancement and Kinetics of NH3 oxidation on Pt/Al2O3:Rate Enhancement By Ball Milling and Structure Senstivity Effects

Ammonia-oxidation(AMOX) over Pt-containing catalysts is an important reaction for eliminating NH₃ in diesel-engine exhaust. Kinetic models have been developed for Pt-single crystals under ultra-high vaccum[1] and for Pt-gauze for HNO₃ production[2,3] but these are of limited value for NH₃-oxidation on Pt-supported catalysts for emission control. Here we describe a kinetic study for Pt/Al₂O₃ at atmospheric pressure and the development of a microkinetic model. We measured reaction-rate(TOF) dependence over wide range of NH₃ concentration from 10 – 40,000ppm for Pt/Al₂O₃ powder and washcoated-monoliths. The data in Figs.1(a) and (b) show the presence of rate-maximum at 500ppm for unmilled-Pt/Al₂O₃ but at 10,000ppm for ball-milled Pt/Al₂O₃ powder and washcoated-monolith. Another unexpected observation is the enhanced activity of Pt/Al₂O₃ samples upon ball-milling, which resulted in lowering of light-off temperature(T₅₀) by 50°C[Fig.1(c)]. Ball-milling reduces particle size, in-turn decreases the extent of diffusion-limitations. However, steady-state AMOX done over ball-milled Al₂O₃ followed by Pt addition did not show the same enhanced activity ruling out diffusion-limitations as the main cause of the activity enhancement. Further characterization of Pt/Al₂O₃ samples using XRD[Fig.1(e)] reveals an alteration in the distribution of Pt crystalline-planes; notably, Pt<211> increased and Pt<111> decreased. These data suggest that the higher activity of ball milled-Pt/Al₂O₃ is a result of this crystalline-plane transformation. Microkinetics by Rebrov[1] assumed Pt<111>plane which predicted no rate variation with NH₃ concentration. This is due to NH₃ and O₂ adsorbing on independent sites ‘b’ and ‘a’ respectively. Contrary, experiments showed ‘+’ to ‘-’ NH₃ reaction-order variation over wide range of NH₃ concentration indicates site-competition[Fig-1(d)]. The model was modified by introducing site-competition between NH₃ and O₂ on site-a in addition to NH₃ adsorption on site-b which best described the experimental data and kinetics. These findings show the close dependence of kinetics and reactivity on Pt crystal-planes.
References:
[1]Rebrov.E.V et al.,Chem.Eng.J,90,61-76(2002)
[2]Hass.M et al.,ISCRE-25(2018)
[3]Baerns.M et al.,J.Catal,232,226-238(2005)