Catalytic activities and surface properties of zeolite-supported molybdenum nitrides for NO reduction with H₂

A series of zeolite (H-ZSM-5)-supported molybdenum nitride catalysts with Mo loading ranging from 2 to 30 wt.% were synthesized by temperature-programmed nitridation in a flow of NH₃. The surface properties of the nitride samples were characterized by XPS, H₂-TPR, and XRD techniques and their catalytic activities were evaluated for NO reduction with H₂. For the fresh samples, molybdenum nitrides coexisted with oxides on the zeolite. With the increase of Mo loading from 2 to 30 wt.%, the degree of nitridation increased linearly with the increase of Mo loading. We observed that a catalyst with higher Mo loading exhibited higher initial activities. The nitrided 2 wt.% Mo/H-ZSM-5 catalyst was the most stable and NO conversion to N₂ remained unchanged within a test period of 15 h. For the catalysts with Mo loading above 2 wt.%, catalytic activities decreased with time on stream. After 15 h, the nitrided 2 wt.% Mo/H-ZSM-5 catalyst was the most active among the tested catalysts. The results of H₂-TPR measurements for the used and oxygen-saturated catalysts revealed that catalyst deactivation was a result of oxygen incorporation into the nitride lattices. The strong interaction between the molybdenum species and H-ZSM-5 zeolite as well as the lowering of H ₂-reduction temperature of surface oxygen might be the reasons for the good performance of the nitrided 2 wt.% Mo/H-ZSM-5 catalyst for NO reduction with H₂.