The dependence of catalytic activity for N2O decomposition on the exchange extent of cobalt or copper in Na-MOR, H-MOR and Na-MFI

Catalytic decomposition of N2O was studied on Na-MOR, H-MOR, and Na-MFI samples exchanged to various extents with cobalt or copper. Co-MOR samples were characterized by FTIR and volumetric measurements of NO adsorption. The most abundant species on Co-MOR was Co2+(NO)2. In agreement, the volumetric data yielded NO/Co = 1.8 ± 0.2. On Co-MOR, N2O conversion progressively increased as the cobalt content increased. All samples yielded similar apparent activation energy, Ea=75 ± 5 kJ mol-1. The reaction order was 0.9 ± 0.1 for N2O, and 0.0 ± 0.1 for O2. For samples having a Co-exchange percentage up to 61%, the turnover frequency per total Co atom was independent of the cobalt content and was significantly lower for more extensively exchanged samples. On all Co-MOR samples, the turnover frequency per isolated Co atom was nearly constant, indicating isolated Co2+ as the active site. On Cu-MOR and Cu-MFI samples, N2O conversion markedly increased with the copper content. Samples having a Cu-exchange percentage up to 62% yielded higher Ea than more extensively exchanged samples (150 ± 5 kJ mol-1 vs. 100 ± 5 kJ mol-1). The reaction order was 0.5 ± 0.1 for N2O, and 0.0 ± 0.1 for O2. We conclude that in Co-MOR and Co-MFI catalysts the active site for N2O decomposition is isolated Co2+, whereas in Cu-MOR and Cu-MFI isolated Cu2+ is nearly inactive. In extensively exchanged Cu-MOR and Cu-MFI, the active site for N2O decomposition is most probably Cu1+. A similar reaction mechanism for N2O decomposition operates over Co-zeolites and extensively exchanged Cu-zeolites.