Mechanism of the selective reduction of NOx by methane over Co-ZSM-5

The selective catalytic reduction of NOx by methane (CH4-SCR) was studied on Co-ZSM-5 catalysts. Different Co-ZSM-5 and Co,Na-ZSM-5 catalysts were prepared by ion exchange, characterized by different methods, and were tested in CH4-SCR. The catalytic performance depends on the Co content where catalysts with Co loadings between 2.5-3.0 wt.% showed the best. For mechanistic studies operando DRIFTS/UV-vis spectroscopy was employed enabling the simultaneous observation of surface adsorbates (DRIFTS), changes of Co oxidation state and coordination sphere (UV-vis), and product formation (MS). These measurements reveal that preadsorbed nitrato/nitrito (NOy) species formed by oxidation of NO at Co3+ species present in oxide-like clusters are reduced by gaseous CH4 corresponding to an Eley-Rideal mechanism. Isocyanate species were identified as possible intermediates which react with the NOy species or NO2 to form N-2. This could also be confirmed by TG-DSC-MS investigation of Co-ZSM-5 impregnated with (NaNO3)-N-15 and NaOCN, where (NN)-N-15-N-14 was already formed at 288 degrees C in an exothermic reaction of (NO3-)-N-15 with OCN- catalyzed by Co because on a Co-free sample the (NN)-N-15-N-14 formation proceeds at significant higher temperature. The isolated Co2+ cations located at exchange positions play an indirect role, facilitating chemisorption of NO and, thus, practically act as storage sites for NO.