CBE Seminar: Fabio Ribeiro, Purdue University
On the Reaction Mechanism and the Nature of the Active Site for Standard Selective Catalytic Reduction of NOx on Cu/SSZ-13 Zeolites
Cu/SSZ-13 zeolites are used commercially in mobile applications for the selective reduction of NOx with ammonia. Our data suggest that the catalytic cycle is a redox one and occurs on a single Cu site. Complete reduction to Cu(I) was observed only in the absence of O2 and with both NO+NH3 present, which led to the formation of mobile diamminecopper(I) species. The order with respect to NH3 was ~0, indicating strong adsorption of ammonia on Cu(I) and Cu(II). The effect of NO partial pressure indicates that NO was involved in both reduction and oxidation half-reactions.
X-ray absorption spectra for all samples at ambient conditions showed the presence of hydrated Cu2+ ions with a coordination number of ~4. Brønsted acid titrations and FTIR spectroscopy revealed that the ion exchanged Cu preferentially exchanged as isolated Cu2+ ions charge-balanced by two framework Al atoms followed by [CuOH]+ ions charge-balanced by one framework Al atom. Exposure to different gases such as He at 400 °C or H2 at 250 °C revealed differences between these two sites and showed that the [CuOH]+ sites were more reducible compared to the Cu2+ sites. In spite of this apparent chemical difference, the two sites had similar standard SCR reaction kinetics, while operando XAS experiments showed similar Cu(I) – Cu(II) fractions at steady state, demonstrating their similarity for SCR redox chemistry. The chemical state of Cu was dependent on the gas treatments used. Consequently, it is essential that the catalyst be studied under differential reaction conditions with known concentration of reactants and products.
Friday, March 24, 2017 at 10:00am to 11:00am
Colburn Lab, Room 102
University of Delaware- Colburn Lab, University of Delaware, 150 Academy St, Newark, DE 19716-3196, USA