A DFT Study On The Structure And Catalytic Performance Of Ni/α-Al₂O₃ Catalyst In POM

Because of the rapid reaction and the short contact time (<10-2 s), especially the high temperature, it's difficult to determine the intermediate in the partial oxidation of methane, so there are still many arguments on the reaction mechanics, and the selectivity and stability of the catalyst still need more improvements. The quantum chemical calculation could provide the researchers with the structure and energy messages about the reactants, intermediates and products, and give instructions to the modification of catalysts and the improvement of the technology.This work has studied theα-Al₂O-3 supported Ni clusters(Nin, n=1-4) with the Dmol3 module based on the DFT theory. And then the catalytic properties of the Ni4/α-Al₂O-3(0001) system in POM were studied.The interaction between metal and the support has an effect on the performance of the catalyst. The adsorption and growth of the Ni clusters are simulated onα-Al₂O-3(0001) surface. This work has found that the Ni clusters tend to form the three-dimensional structures on the supportα-Al₂O-3. This is because that the Ni-surface interaction is reduced as the cluster size gets bigger, and the Ni-Ni interaction gets stronger and has more influence on the clustering energy, which means the Ni cluster tends to form three-dimensional structures onα-Al₂O-3(0001) surface. This work has also found that the activation energy for Ni atom to migrate on the surface is low. And charge transfer is found after Ni clusters adsorb on the surface, which indicates that Ni clusters and the surface are combined by loacal polarization.This work has also studied the catalytic performance of the Ni4/α-Al₂O-3(0001) catalyst in the partial oxidation of methane(POM). The result has indicated that CH4 will chemically adsorb on the Ni4/α-Al₂O-3(0001) surface while undergoing the first step of the dissociation, and the activation energy of this step is 240.2 kJ/mol. The carbon species and H species tend to adsorb on the edge of the Ni clusters, and the priority adsorption sequence for the species on Ni4/α-Al₂O-3(0001) surface is H > O2 > CH3 >> CH4 > H2. The O2 molecule is largely activated on the Ni4 cluster supported onα-Al₂O-3(0001) surface, and the formation of surface O species becomes more favorable because of this, which can accelerate the oxidation of carbon species. The support was also found to have reduced the chemical adsorption of the H2, and made the desorption of H2 easier, which means more yield for the product H2.This work has calculated the interaction between the Ni cluster and the support and their catalytic performance in POM based on the DFT theory. The results could provide theory data on the structure and catalytic properties of the supported Ni catalyst, which can help the reserchers to study the reaction mechanics in the partial oxidation of methane, and can give instructions for designing more effective catalysts in this reaction.