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

•Strong synergy effect in C3H6-SCR using H2/C3H6 at T > 400 °C over Pt/Ce0.5Zr0.5O2.•H2-SCR and C3H6/H2-SCR pass through an active chelating nitrite (NO2−) species.•Labile oxygen of Ce0.5Zr0.5O2 support at T > 400 °C facilitates removal of -CHx on Pt.•Vacant sites of support is responsible for the creation of more active chelating nitrite species under C3H6/H2-SCR.•Active NOx in H2-SCR are located within a zone around Pt nanoparticles which extends to less than one lattice constant of support.

A series of Pt/CexZr1-xO2-δ (x = 0.4-0.6) solids were synthesized and evaluated for the SCR of NO under lean burn conditions (2.5 vol% O2) using C3H6 and H2 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 H2/C3H6 synergy effect towards steady-state selective reduction of NO into N2 at T > 400 °C. In particular, the chemical structure of adsorbed active and inactive (spectator) NOx species formed under C3H6-SCR, H2-SCR and H2/C3H6-SCR of NO and the surface coverage and site formation of active NOx were probed. The Pt/Ce1-xZrxO2-δ catalysts present significant differences in their H2-SCR performance (NO conversion and N2-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 NOx of the H2-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% H2O in the H2-SCR feed stream applied over the 0.1 wt% Pt/Ce0.5Zr0.5O2 catalyst results in a 25% decrease in the concentration of active NOx, thus partly explaining the drop in activity observed when compared to the H2-SCR in the absence of H2O. A remarkable activity and N2-selectivity enhancement was observed at T > 400 °C when both H2 and C3H6 reducing agents were used compared to H2-SCR or C3H6-SCR alone. This synergy effect was explained to arise mainly because of the increase of θΗ by the presence of -CHx 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 (NO2−) species compared to the case of H2-SCR.

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