In-depth insights into N2O formation over Rh- and Pt-based LNT catalysts

Abstract In this paper mechanistic aspects involved in the formation of N 2 O over model Pt-Ba/Al 2 O 3 and Rh-Ba/Al 2 O 3 LNT catalysts are discussed. The reactivity of both gaseous NO and of stored NO x (nitrates) has been studied, with simultaneous surface characterization by operando FT-IR spectroscopy, using different reductants (i.e. H 2 , CO, CO + H 2 , CO + H 2 O) both under isothermal conditions and temperature programming. The results show that N 2 O formation may occur during both the lean/rich and rich/lean switches (primary and secondary N 2 O, respectively). In particular: i) primary N 2 O formation involves the presence of gas-phase NO and partially reduced metal sites; ii) N 2 O formation increases in the presence of CO because the reduction of the metal sites is slower, thus favoring N 2 O formation upon the lean/rich transition; iii) residual reducing species onto the surface (i.e. NCO − , CO) can react with NO giving the secondary N 2 O peak. A reaction pathway for N 2 O formation is suggested where metal sites (Pt or Rh) catalyse the NO dissociation reaction into N- and O-adatoms; N-species further interact with undissociated NO molecules leading to the formation of N 2 O (primary N 2 O). In additions, isocyanates formed during the NO x reduction in the presence of CO may participate in the N 2 O formation upon reaction with NO during the lean phase (secondary N 2 O). Pt- and Rh-based catalysts show similar behavior even if Rh-based catalyst is less reactive than Pt-based sample likely due to the lower dispersion of the noble metal.