Interaction Of The Oxide With Supported Noble Metal And The Effect On Its Catalytic Activity

NO is one of the main pollutants in automobile exhaust. Emissions of nitrogen oxides have grown rapidly in recent years, which was caused by the development of automobile industry and sharp increasing in the quantity of vehicles, resulting in a serious deterioration of our living environment and a great harm to our health. Therefore, the elimination of NO is of great significance to alleviate air pollution and solve the environmental crisis. Precious metals have been widely used in these studies, due to their good catalytic activity for automobile exhaust gas. It has been found that Pd catalysts can effectively maintain the size of the grains in the catalytic process, rather than losing the activity caused by sintering in the long-term reaction. Pd also has a cheaper price than Pt and other noble metal, and it has a good performance in the heat.In this paper, the deposition characteristics and migration characteristics of single atoms and clusters of Pd and Pt on the carrier BaCeO3 have been compared. Pd13 clusters, which have stable icosahedral structures, have been selected as the research object. In my paper, density functional theory(DFT) method was used. Depend on this method, the neutral, anionic and cationic Pd13 clusters for adsorption of NO, differences in the electronic structures, the NO catalytic decomposition reaction path and the reaction energy barriers have been analyzed. Then the effects of the charged Pd on the automobile exhaust catalytic decomposition of NO have been explored. The results are summarized as follows:(1) Comparing the adsorption energy of Pd and Pt on BaCeO3 surface, it can be found that either Pd or Pt adsorbed more strongly on BaO surface than on CeO2 surface. At different adsorption positions, the adsorption capacity of Pt was bigger than that of Pd on the surface of the BaCeO3. As indicated by the results of cNEB calculations, Pt had higher energy barriers than that of Pd atoms on BaCeO3 surface while Pd and Pt atoms migrated on CeO2 and BaO surface. It suggested the Pt atom was more difficult to migrate on the surface. From another point of view, it reflected that the degree of dispersion of Pt on BaCeO3 surface was higher than Pd. On the contrary, the energy barriers of Pd were lower, so Pd was easy to gather to clusters.(2) The state charge of noble metal has a great impact on the adsorption energy. The hollow sites on Pd13 clusters were identified as the most preferred site whether NO was on the neutral, anionic or cationic Pd13 clusters, and NO was in the form of N-terminal contacting with Pd atoms. The absolute value of adsorption energy of NO on anionic Pd13 clusters was greater than on other clusters. The value of absorption energy would reduce as the number of the charge was increased, so the adsorption capacity of NO on Pd-1 cluster was much better than on others.(3) Through the quantitative and qualitative analysis of the electronic structure of the system, we found d-band center of anionic Pd13 clusters was closer to the Fermi level, which could be considered as a reflection of the better catalytic activity of anionic clusters than neutral and cationic Pd13 clusters. It was also in agreement with the results of adsorption energies and bond lengths. Based on the difference of charge density, it could be found that the charge transfer mainly concentrated on the N atoms and Pd atoms bonding with the N atom.(4) By the study of the decomposition path of NO on Pd13 clusters, we found that: the path of the NO decomposition prefered migrating between the two diagonal hollow sites of adjacent Pd13 clusters. When the N-O bond was broken, migration only happened on O atom. Because of the lower NO decomposition energy barriers on the anionic Pd13 clusters and fewer external energy required for, NO decomposition was more likely to occur on anionic Pd13 clusters. And, with the number of electrons of Pd13 clusters increased, the energy barriers would become increasingly lower and lower, the energy barriers on Pd-1, Pd-2, Pd-3 clusters were 2.52 eV, 2.46 eV, and 2.44 e V.