Sci Total Environ 2019 Jul 1;673:266-271. Epub 2019 Apr 1.
State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100049, China.
Nitrous oxide (NO) has been the most serious ozone-depleting species throughout the 21st century. Zeolite-based catalysis is a highly promising method for NO removal in large-scale industrial applications. However, the exchanged transition metal species in zeolites greatly influence the performance of catalysts. The primary factor governing the catalytic activity is a fiercely debated topic and remains highly uncertain. Here we synthesize a series of transition-metal ion (Fe, Co, Ni, Cu)-exchanged ZSM-5 zeolite catalysts. Both experiments and density functional theory (DFT) calculations demonstrate that the activity for NO decomposition follows the order Fe ≈ Co > Ni > Cu. Analysis of the electronic structure properties reveals that the catalytic activity of the transition-metal ion-exchanged ZSM-5 zeolites is governed by the local softness of active sites and the composition of their HOMOs. The higher the local softness and the proportion of 4s orbitals in the HOMO, the higher the catalytic activity, which facilitates electron transfer in the redox process and thereby reduces the reaction barriers for NO decomposition into N and O. Clarification of the nature of catalytic activity advances the understanding of the principles of zeolite-based catalysis, and is helpful for the design of highly efficient zeolite catalysts for pollutant removals.