A Novel Application of Trapping Catalysts for the Selective Low-Temperature Oxidation of NH3 to N2 in Simulated Biogas

Abstract

The low-temperature selective oxidation of NH3 to N2 in simulated biogas containing a large excess of CO and H2 has been examined using a novel NH3 and a standard NOx trapping catalyst. The N2 selectivity during NH3 oxidation at 200°C for a 1%Pt–20%BaO–Al2O3 NOx trapping material, with typical lean/rich switches, was initially good (>90%) but decreased markedly over a small number of cycles. In contrast, the N2 yield obtained using a novel NH3 trapping material (1%Pt–20%CuO–Al2O3) with rich/lean fuel switching exceeded 95% and was stable over many switching cycles, while an unmodified 1%Pt–Al2O3 catalyst displayed poor N2 selectivity under all conditions. The data obtained from probe reactions between the various potential adsorbates and gaseous species of the reaction indicate that the N2 yields obtained from the 1% Pt–20% CuO–Al2O3 catalyst are formed via an Internal Selective Catalytic Reduction (iSCR) between NHx species adsorbed on the trapping component and NO formed from NH3 total oxidation on the Pt during the lean cycle of operation. For the 1%Pt–20%BaO–Al2O3 catalyst, NOx, formed during lean operation, is reduced to N2 in the rich cycle by a combination of reactions with NH3, CO, and H2. The use of a hybrid catalyst, based upon a combination of iSCR and NOx trapping processes, gave a peak N2 yield of >95% and an integrated N2 production over the entire rich/lean cycle of 75%. These results reflect the potentially dramatic improvements possible by rational design of catalyst systems based upon a fundamental knowledge of the processes involved.