Mesoscale study of crystal-plane effects of Ni catalysts on CO2 hydrogenation.

Abstract

Crystal-plane effects have pivotal roles in the design of catalysts. In this study, a branched Ni (Ni-BN) catalyst was mainly exposed at the Ni(322) surface and was synthesized in the presence of H2. A Ni nanoparticle (Ni-NP) catalyst was mainly exposed at Ni(111) and Ni(100) surfaces and was synthesized without H2. The Ni-BN catalyst showed higher CO2 conversion and methane selectivity than the Ni-NP catalyst. DRIFTS revealed that, unlike the formate route for methanation over the Ni-BN catalyst, the main methanation pathway over the Ni-NP catalyst was the CO2 direct dissociation route, which revealed that the diversity of reaction mechanisms of CO2 methanation on different crystal planes led to the disparity in catalyst activity. DFT calculation of the CO2 hydrogenation reaction over various surfaces showed that the energy barriers on Ni(110) and Ni(322) surfaces were lower than those of Ni(111) and Ni(100) surfaces, which was also related to different pathways of the reaction mechanism. Microkinetic analysis showed the reaction rates on Ni(110) and Ni(322) surfaces were higher than those of other surfaces, and CH4 was the main product on all calculated surfaces, whereas the yields of CO on Ni(111) and Ni(100) surfaces were higher. Kinetic Monte Carlo simulations revealed that the Ni(322) surface with stepped sites was responsible for CH4 generation, and that simulated methane selectivity was consistent with experimental results. The crystal-plane effects of the two morphologies of Ni nanocrystals explained why the reaction activity of the Ni-BN catalyst was greater than that of the Ni-NP catalyst.