Insight into the structure–activity relationship of In-modified SSZ-13 zeolite for selective catalytic reduction of nitrogen oxides with methane

Given the excellent CH4 activation capability of In and the outstanding hydrothermal stability of SSZ-13 zeolite, the In/SSZ-13 system represents a promising catalyst for CH4-SCR in lean-burn natural gas vehicles. However, the underlying interaction mechanism between In species and SSZ-13 remains insufficiently understood. This study systematically investigated the structure–activity relationship and catalytic mechanism of In-modified SSZ-13 catalysts, focusing on the effects of In loading on structure, metal coordination, metal chemical states, adsorption behaviour, catalytic performance, CH4 utilisation efficiency (CUE) and promotion mechanism. The results reveal that InO+ serves as the primary active site for CH4-SCR, facilitating chemisorption of NOX as nitrosonium, nitrosyl, nitro, and various nitrate species. Simultaneously, CH4 is activated at InO+ sites to form reactive methyl groups, which subsequently react with adjacent nitrates to generate key intermediates such as nitromethane or methyl nitrite, thereby promoting the overall reaction. In2O3 exists as agglomerates dispersed across the zeolite surface and within the pores, diminishing reactant-catalyst contact efficiency and CUE at high temperatures. Low In loading provides insufficient InO+ sites for effective NOX adsorption and CH4 activation, whereas excessive In loading increases the proportion of In2O3/(InO++In2O3), adversely affecting NOX conversion at high temperatures. An In loading of 15 wt% offers an optimal balance among CH4 activation, NOX adsorption, and CUE for the catalyst, achieving a remarkable NOx conversion for a monometallic CH4-SCR catalyst.