Applied Catalysis B: Environmental, Vol.206, 308-318, 2017
Selective catalytic reduction of NO by H-2/C3H6 over Pt/Ce1-xZrxO2-delta: The synergy effect studied by transient techniques
A series of Pt/CexZr1-xO2-delta (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 C3H6 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/C3H6 synergy effect towards steady-state selective reduction of NO into N-2 at T >400 degrees C. In particular, the chemical structure of adsorbed active and inactive (spectator) NOx species formed under C3H6-SCR, H-2-SCR and H-2/C3H6-SCR of NO and the surface coverage and site formation of active NOx were probed. The Pt/Ce1-xZrxO2-delta catalysts present significant differences in their H-2-SCR performance (NO conversion and N-2-selectivity) in the low-temperature range of 120-180 degrees C but practically the same catalytic behavior at higher temperatures. It was proved that the active NOx 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% H2O in the H-2-SCR feed stream applied over the 0.1 wt% Pt/Ce0.6Zr0.5O2 catalyst results in a 25% decrease in the concentration of active NOx, thus partly explaining the drop in activity observed when compared to the H-2-SCR in the absence of H2O. A remarkable activity and N-2-selectivity enhancement was observed at T >400 degrees C when both H-2 and C3H6 reducing agents were used compared to H-2-SCR or C3H6-SCR alone. This synergy effect was explained to arise mainly because of the increase of OH 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 H-2-SCR. (C) 2017 Elsevier B.V. All rights reserved.