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 NOx with NH3 at high temperature (T > 500 degrees C) usually has low conversion efficiency due to the undesired NH3 over-oxidation. To suppress the oxidation of NH3 to NOx we proposed a Fe-exchanged zeolite catalyst with the modification of Ni and W for the NH3-SCR process, which was investigated in a wide temperature range (200-850 degrees C). It was found that the NH3 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 NH3 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 Bronsted 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 NH3 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.

Automated summary

The Fe-exchanged zeolite catalyst with Ni and W modification effectively suppresses the over-oxidation of NH3 and improves the SCR performance at high temperatures. This is attributed to enhanced thermal tolerance of acidity sites and a higher valence state of active species in the multi-metal catalyst.