Multi-functionality of rhodium-loaded MOR zeolite: production of H2via the water gas shift reaction and its use in the formation of NH3

Abstract

Rh-loaded mordenite (RhMOR) zeolite was investigated as a catalyst that can use CO + H₂O as a H₂ source for NO reduction. Operando IR measurements showed that CO was captured in the form of Rh dicarbonyl species ([Rh(CO)₂]⁺) in zeolite, which reacts with H₂O to form H₂in situ via the water gas shift (WGS) reaction (CO + H₂O → CO₂ + H₂) at >350 °C. Temperature-programmed surface reaction (TPSR) measurements under a flow of NO + CO + H₂O monitored both the surface ad-species and outlet gas components. At >250 °C, the decrease in the outlet NO and CO as well as the increase in the IR intensity corresponding to NH₄⁺ in zeolite were observed, indicating that the formed H₂ was directly used for the reduction of NO into NH₃, which was stored at the Brønsted acid sites (BAS). The mechanism of the WGS reaction was theoretically investigated; the predicted rate-determining step was the dissociation of H₂O on [Rh(CO)₂]⁺ to give [Rh(CO)(COOH)(H)]⁺ and its activation barrier was 176 kJ mol⁻¹. The present study demonstrates the effective use of the multi-functionality of the isolated active metal anchored in zeolite as well as the potential utilization of CO + H₂O as a H₂ source.