Promotional mechanism of tungstation on selective catalytic reduction of NOx by methane over In/WO3/ZrO2

The catalytic performance and promotional mechanism of tungstation for selective catalytic reduction of NO by methane over In-loaded tungstated zirconia (In/WZr) were investigated. A clear improvement of catalytic activity was found over In/WZr catalysts. The highest NO conversion of 70% was achieved over a 1% In/WZr catalyst at 450 °C and 12,000 h−1. In contrast, the maximum NO conversions of WZr, ZrO2 and In/ZrO2 were only 12%, 31% and 20%, achieved at 500, 650 and 600 °C, respectively. Tungstation was observed to influence the properties of catalysts in three aspects: (i) modify the existent state of surface indium species; (ii) determine the activation species of CH4; (iii) cooperate with the loading of indium to enhance the formation and reduction of the intermediates. Differences in the existent state of indium and the activation species of CH4 result in different catalytic activities and mechanisms for CH4-SCR of NO. X-ray photoelectron spectroscopy measurements and Py-IR analysis showed that indium species on the In/ZrO2 catalyst was in the In2O3 bulk phase due to the lack of Brønsted acid sites. However, the tungstated In/WZr catalyst possessed strong Brønsted acid sites, which was beneficial to the formation of active InO+ species. DRIFTS studies further revealed the reaction intermediates of CH4-SCR of NO on the tungstated and the untungstated catalysts. On In/ZrO2 and ZrO2, the activation species of CH4 was the fully oxidized products CO2 and H2O. While on WZr and In/WZr, HCOO−, the real reductant for CH4-SCR of NO, was detected as the main intermediate species. In addition, the formation of HCOO− and the reduction of nitrate species were greatly accelerated by the synergistic effect between InO+ and tungstation, which might explain the higher catalytic activity of In/WZr in comparison with WZr and In/ZrO2.