Propane dehydrogenation (PDH), as an efficient catalytic production process to obtain propylene, has developed rapidly in recent years. Previous studies have shown that zirconia exhibited excellent performance in the PDH, with the coordination-unsaturated zirconium (Zrcus) around the oxygen vacancy being the active site in the reaction. However, the critical role of oxygen vacancy is still remaining elusive, and lacked a rationale to establish a relation between structure and performance. Moreover, the strong binding of propene and hydrogen molecules shadowed the facile desorption at defective sites for good yields.
To resolve these issues, recently a research team led by Prof. Zhen Zhao and Prof. Bo Li from ShenYang Normal University (China) carried out a combined DFT and microkinetic simulation to explore the propane catalytic dehydrogenation on the tetragonal zirconia (001) and (100) pristine surfaces and surfaces containing oxygen vacancies, (001)-vac and (100)-vac surfaces, and make a direct comparison between pristine and defective surfaces.
Density of states (DOS) analysis shows that compared with the pristine surface, the generation of oxygen vacancies on the defective surface induced a new electron localized state close to Fermi level, which caused the rearrangement of charge density. And the orbital wave function analysis further confirmed that these electrons were mainly confined within the oxygen vacancy. The localized electronic state made Zrcus a good electron donor, thereby promoting the activation of C-H bonds in propane.
The PDH reaction pathway includes two successive dehydrogenation and desorption of the product (propylene and hydrogen molecules). On the pristine crystal surface, metal-oxygen pair is used as the active site of the propane C-H bond activation, and the propane decomposition products, C3H7 and H, are adsorbed on the zirconium and oxygen sites respectively. The active site changes to Zrcus on the defective surface containing oxygen vacancy. Compared with the pristine surface, the interaction between propane and Zrcus is significantly increased, which is conducive to the subsequent dehydrogenation step. Compared with the pristine surface, the reaction energy barrier of the first C-H bond on the defective surface is greatly reduced. However, due to the unsaturated coordination, propylene and hydrogen are strongly adsorbed at the Zrcus site, which hinders the effective desorption of the product.
The turnover frequency (TOF) results show that the defective surface has better catalytic performance than the pristine surface under typical reaction temperature, indicating that presence of oxygen vacancies indeed increase the performance. The degree of rate control (DRC) analysis show that the first C-H bond activation is the rate control step on the pristine surface, while the hydrogen formation is the rate control step on the defective surface, which confirmed that the pristine and the defective surface obey the different reaction mechanism. The results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(24)60163-4).
