Experimental and Density Functional Theory Studies on the Zeolite-Based Fe–Ni–W Trimetallic Catalyst for High-Temperature NOx Selective Catalytic Reduction: Identification of Active Sites Suppressing Ammonia Over-oxidation

The selective catalytic reduction (SCR) of NO x with NH 3 at high temperature ( T > 500 °C) usually has low conversion efficiency due to the undesired NH 3 over-oxidation. To suppress the oxidation of NH 3 to NO x, we proposed a Fe-exchanged zeolite catalyst with the modification of Ni and W for the NH 3 -SCR process, which was investigated in a wide temperature range (200–850 °C). It was found that the NH 3 over-oxidation was significantly suppressed on the Fe–Ni–W trimetallic catalyst, leading to an improvement in the SCR performance. The catalyst characterizations suggest that the thermal tolerance of the acidity sites was enhanced and the active species in the multi-metal combined catalyst showed a higher valence state. To further identify the roles of active sites during NH 3 oxidation, dehydrogenation, and transformation into intermediates, time-resolved in situ diffuse reflectance Fourier transform spectroscopy and density functional theory calculations were conducted. We found that the enhanced activity of catalysts at high temperature was ascribed to the Brønsted acid sites on the ZSM-5 lattice and extra-framework exchanged metal center that was affected by the topology confinement from zeolite. A high activation barrier for the rate-determining step of the NH 3 oxidation process was observed on these active sites, thus inhibiting the oxidation side reaction. The findings provide rational strategies to modify mononuclear sites and synergetic restriction effect of multi-metal centers over zeolite-based catalysts for effective SCR at high temperatures.