Combination of Density Functional Theory and Microkinetic Study to the Mn-Doped CeO₂ Catalysts for CO Oxidation: A Case Study to Understand the Doping Metal Content
2018
Song, Weiyu | Chen, Lulu | Deng, Jianlin | Jing, Meizan | Zheng, Huiling | Liu, Jian | Zhao, Zhen
CeO₂ doped with metal is employed in many catalytic reactions. Optimizing the content of metal is a routine step to improve the activity. In low doping metal content, the catalytic performance improves with the increase of the doping metal content. However, the catalytic performance decreases beyond the optimal doping metal content. To our best knowledge, the molecular level understanding to such phenomenon has never been reached. In the present study, Mn-doped CeO₂ for CO oxidation has been taken as a case study toward this aim. Three different models, in which one, two, and three Mn atoms replace Ce atoms in CeO₂(111) 3 × 3 supercell, were constructed. The oxidation state for all Mn atoms is +3. Because of charge imbalance, O₂²– and O₂– species would form spontaneously via a combination of lattice O on Mn₂Ceₓ–₂O₂ₓ(111) and Mn₃Ceₓ–₃O₂ₓ(111) surface, respectively. On all these three surfaces, CO oxidation follows the MvK mechanism which involves lattice oxygen atom. The main steps include CO physisorption, CO chemisorption, the formation and desorption of CO₂, O₂ adsorption, a second CO adsorption and the reaction between CO and adsorbed O₂–. Microkinetics simulations show that CO oxidation rates and optimal temperatures are different for these three surfaces. Optimal temperatures for CO oxidation depend on oxygen adsorption energy. The CO oxidation rate is highly influenced by the adsorption energy of the second CO molecule. This work provides insights into the influence of different Mn doping on the catalytic performance of CeO₂.
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