Insight into the Nature of Active Species of Pt/Al2O3 Catalysts for low Temperature NH3 Oxidation

Abstract

Two series of Pt/γ-Al2O3 catalysts for low temperature NH3 oxidation were prepared using Pt(NO3)4 and H2PtCl6 precursors. Using both precursors results in the formation of small Pt particles (d<1.5 nm), however, Cl-containing Pt precursors give a higher fraction of highly dispersed Pt species. Such species show high stability against thermal or H2 treatment probably due to the presence of a substantial amount of chlorine on the surface. Treatment of the samples prepared from Pt(NO3)4 with H2 leads to the formation of metallic Pt nanoparticles accompanied by the improvement of catalytic activity in NH3 oxidation at T<200 °C. The main products of ammonia oxidation at temperatures below 250 °C were molecular nitrogen and nitrous oxide with the N2 selectivity reaching 80 %. Operando XANES/EXAFS revealed that even after H2 pretreatment at least 40 % of Pt surface remains in oxidized state under reaction conditions resulting in the appearance of N2O as a by-product.