Catalytic activities and surface properties of zeolite - supported molybdenum nitrides for
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Abstract
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 H2-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.