Structure-Activity Relation Of Morphology-Dependent NbO_x/CeO₂ Catalysts In Oxidative Dehydrogenation Of Propane

Coal, petroleum, natural gas is the main source ot the world’s energy. With the gradual depletion of petrochemical resources and the deterioration of the environmental problems, reasonable use of the relatively abundant light alkanes in the petrochemical resources into more valuable chemical raw materials is a major challenge in the field of catalysis. Propane, as one of the major components of natural gas which is relatively abundant, is cheap and readily available and propylene, as one of the important chemical raw materials, is mainly obtained by fluid catalytic cracking which dependents on petroleum of declining reserves. Therefore, the oxidative dehydrogenation of propane to propylene reaction is worthy of further study.Oxide catalyst in the catalytic oxidative dehydrogenation of propane to propylene reaction showed good catalytic performance. Recently, the morphology regulation of the oxide nanocrystals became an effective method to optimize the catalytic properties without changing the catalyst composition. At the same time, because of its relatively uniform surface structure, it is also conducive to the understanding of the catalytic reaction mechanism and structure-property relationships. Oxide-supported CeO2 catalysts are typical catalysts of the oxidative dehydrogenation of propane to propylene reaction, preparation of CeO2 nanocrystals with different morphologies is also very mature. A series of NbOx/CeO2 catalysts were synthesized employing CeO2 cubes (c-CeO2), CeO2 rods calcined at 500 ℃ (r-CeO2-500) and CeO2 rods calcined at 700 ℃ (r-CeO2-700) as the supports, and their structures and catalytic performances in the oxidative dehydrogenation of propane reaction were studied. Strong CeO2 morphology-dependent NbOx-CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts were observed. The supported NbOx species evolve with the increasing Nb loading from the monomeric Nb species to the polymeric Nb species and the CeNbO4 species. The monomeric Nb5+ species interacting with oxygen vacancy/Ce3+ on CeO2 can be reduced to form the monomeric Nb4+species. The formed monomeric Nb species follows the order of NbOx/r-CeO2-500> NbOx/c-CeO2> NbOx/r-CeO2-700. The loading of NbOx suppresses the surface reduction of c-CeO2 and r-CeO2-500 but promotes the surface reduction of r-CeO2-700. R-CeO2-500 and r-CeO2-700 are more catalytic active than c-CeO2 in the oxidative dehydrogenation of propane reaction. The loading of NbOx slightly enhances the catalytic activity of c-CeO2but decreases the catalytic activity of r-CeO2-700. A loading of 0.6 Nb/nm2 much enhances the catalytic activity of r-CeO2-500 at 200 ℃, but further increasing the Nb loading results in the decrease of the C3H6 conversion. These results add a solid example to vindicate the morphology engineering strategy to modify the structure and catalytic performance of CeO2-based catalysts.