image: The superior catalytic performance of ZrO2 in catalytic propane dehydrogenation is attributed to be the unsaturated coordination zirconium (Zrcus) near the oxygen vacancy. However, the calculations revealed the double-edged role of oxygen vacancy in reaction, which significantly reduced C-H bond activation barrier but hinder the facile desorption of products due to strong binding. Moreover, the localized electron within oxygen vacancy is identified as the driving force for the C-H bond activation. The complete two different mechanisms are also noted for pristine and defective surfaces which also resulted from the double-edges effects of oxygen vacancy.
Credit: Chinese Journal of Catalysis
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).
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Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top one journals in Applied Chemistry with a current SCI impact factor of 15.7. The Editors-in-Chief are Profs. Can Li and Tao Zhang.
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Journal
Chinese Journal of Catalysis
Article Title
Revisiting the origin of the superior performance of defective zirconium oxide catalysts in propane dehydrogenation: Double-edged oxygen vacancy
Article Publication Date
9-Jan-2025