Suchergebnisse

Despite the intense investigation on the NH -SCR-NOx reaction mechanism catalyzed by small pore Cu-CHA zeolites, neither the rate-determining step of the process nor the exact nature of the active sites under reaction conditions are clearly established. In this work, in situ EPR and IR techniques combined with DFT calculations are applied to the study of the oxidation half-cycle of the NH -SCR-NOx reaction on Cu-SSZ-13 and Cu-SAPO-34 catalysts. EPR and IR spectroscopies unambiguously show that Cu is oxidized to Cu at room temperature in the presence of the reaction mixture (NO, O , and NH ) or NO and O , producing adsorbed NO , nitrites, and nitrates. Several pathways are proposed from DFT calculations to oxidize Cu cations placed in the plane of the 6R ring units of SSZ-13 and SAPO-34 to Cu , either by NO alone or by a mixture of NO and O , with activation energy barriers lower than 70 kJ mol . The results reported here demonstrate that a reaction mechanism invoking the formation of nitrate/nitrite intermediates on copper cations attached to the zeolite framework can be operational in the low-temperature region (T < 350 °C). Moreover, different intermediates, nitrites versus nitrates, are preferentially stabilized, depending on the catalyst composition, silicoaluminophosphate vs aluminosilicate. ; This work was supported by the Spanish Government through "Severo Ochoa Program" (Nos. SEV 2012-0267; SEV-2016-0683), No. MAT2015-71261-R, and No. CTQ2015-68951-C3-1-R, and by the European Union through No. ERC-AdG-2014-671093 (SynCatMatch). Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de Valencia are gratefully acknowledged for computational resources and technical support. R.M. acknowledges "La Caixa–Severo Ochoa" International PhD Fellowships (call 2015).