Selective catalytic reduction of NO by H2/C3H6 over Pt/Ce1-xZrxO2-δ: The synergy effect studied by transient techniques

Abstract A series of Pt/Ce x Zr 1-x O 2-δ (x = 0.4–0.6) solids were synthesized and evaluated for the SCR of NO under lean burn conditions (2.5 vol% O 2 ) using C 3 H 6 and H 2 as reducing agents. SSITKA-Mass Spectrometry, SSITKA-DRIFTS and other in situ DRIFTS experiments were conducted for the first time to gather fundamental information in explaining the remarkable H 2 /C 3 H 6 synergy effect towards steady-state selective reduction of NO into N 2 at T > 400 °C. In particular, the chemical structure of adsorbed active and inactive (spectator) NO x species formed under C 3 H 6 -SCR, H 2 -SCR and H 2 /C 3 H 6 -SCR of NO and the surface coverage and site formation of active NO x were probed. The Pt/Ce 1-x Zr x O 2-δ catalysts present significant differences in their H 2 -SCR performance (NO conversion and N 2 -selectivity) in the low-temperature range of 120–180 °C but practically the same catalytic behavior at higher temperatures. It was proved that the active NO x of the H 2 -SCR path reside within a reactive zone around each Pt nanoparticle which extends to less than one lattice constant within the support surface. The chemical structure of the active intermediate was proved to be the chelating nitrite , whereas nitrosyl, monodentate and bidentate nitrates were considered as inactive species (spectators). It was illustrated for the first time that the presence of 15 vol% H 2 O in the H 2 -SCR feed stream applied over the 0.1 wt% Pt/Ce 0.5 Zr 0.5 O 2 catalyst results in a 25% decrease in the concentration of active NO x , thus partly explaining the drop in activity observed when compared to the H 2 -SCR in the absence of H 2 O. A remarkable activity and N 2 -selectivity enhancement was observed at T > 400 °C when both H 2 and C 3 H 6 reducing agents were used compared to H 2 -SCR or C 3 H 6 -SCR alone. This synergy effect was explained to arise mainly because of the increase of θ Η by the presence of –CH x species derived from adsorbed propylene decomposition on Pt, which block sites of oxygen chemisorption, and of the increase of surface oxygen vacant sites that promote the formation of a more active chelating nitrite (NO 2 − ) species compared to the case of H 2 -SCR.