N2O decomposition over doubly-promoted Pt(K)/Al2O3–(CeO2–La2O3) structured catalysts: On the combined effects of promotion and feed composition

Abstract

In the present study the feasibility of enhancing the N2O decomposition activity of Pt supported on Al2O3, by means of combined structural and surface promotion is explored by employing: (i) support-mediated promotion by rare earth oxides (CeO2, La2O3) incorporation into the Al2O3 support and (ii) surface-induced promotion by deposition of potassium. The study is conducted over structured catalysts suitable for practical applications, i.e., honeycomb cordierite monoliths washcoated with the catalyst composite. The chemical formula of the low active metal loaded (0.5wt% Pt) doubly-promoted catalyst composite is: 0.5wt%Pt(0–2wt%K)/Al2O3–(16wt%CeO2+4wt%La2O3). Assessment of the de-N2O catalysts performance, either in the presence or absence of O2, CO and H2O in the feed steam, reveals the superiority of the doubly-promoted Pt(K)/Al2O3–(CeO2+La2O3) catalysts in comparison to the bare Pt/Al2O3. Specifically, the de-N2O performance of Pt/Al2O3 is dramatically enhanced by incorporation of 20wt% CeO2–La2O3 mixed oxide on the Al2O3 support and then further improved by 2wt%K addition: the initial very poor de-N2O efficiency of ca. 35% N2O conversion at 600°C over Pt/Al2O3 is led to 100% N2O conversion at ca. 500°C over the structural-promoted Pt/Al2O3–(CeO2+La2O3), which is finally achieved at ca. 440°C over the doubly-promoted Pt(2wt%K)/Al2O3–(CeO2+La2O3) monolith. In all cases (bare, structural- or doubly-promoted catalysts), the de-N2O efficiency is strongly affected by the reaction conditions, following the sequence N2O+CO≫N2O>N2O+O2>N2O+O2+CO. Water vapor and/or (H2+O2) in the feed strongly suppresses the de-N2O performance in a partially reversible manner. The de-N2O efficiency, however, can be totally restored by H2 addition into N2O+H2O feed.