CO2 methanation over Ni and its structure sensitivity A computational study

Abstract

The methanation of CO2 is a reaction with the potential to reduce the negative environmental impact of CO2 point sources and at the same time increase the large scale applicability of renewable electricity. CO¬2 emitted at point sources, such as cement or steel factories, can be captured and catalytically converted towards methane over supported nickel catalysts with the use of renewably produced hydrogen. Interestingly, CO2 hydrogenation over supported nickel nanoparticles is a structure sensitive reaction and the reaction intermediates are IR-active. This makes CO2 hydrogenation over nickel an excellent model reaction to gain a more thorough understanding of the mechanisms behind structure sensitivity. However, the reaction mechanism is not yet fully understood. Three reaction mechanisms are postulated to be active in CO2 hydrogenation over nickel. Firstly, direct CO2 dissociation, secondly H-assisted CO dissociation via alcohol intermediates and thirdly H-assisted CO dissociation via formate intermediates.

The goal of this theoretical study is to explain observations from previously performed FR-IR-experiments, and thereby unravel the reaction mechanism through which CO2 methanation over nickel is energetically most favorable. Finally, by understanding which nickel facet is most favorable in CO2 hydrogenation, the aim is to understand observed structure sensitive effects.

An extensive DFT study is performed of all possible reaction intermediates in Ni-catalyzed CO2 hydrogenation on four different facets; Ni(111), Ni(100), Ni(110) and Ni(211). In this way, sets of stable geometries of each reaction intermediate were obtained, which were used to study each elementary reaction step of the three reaction mechanisms (carbide, formate, alcohol) on the four nickel facets.

The results demonstrate the CO2 hydrogenation likely proceeds via the carbide mechanism, with hydrogen-assisted CO* dissociation. CO* dissociation was found to be most facile via COH* on Ni(100), Ni(110) and Ni(211) and via HCO* on Ni(111). CO2 hydrogenation was found to be energetically least demanding over Ni(110) with a rate limiting step of 110 kJ/mol. However, a combination of the four nickel facets results in a mechanism with the overall lowest energy profile with a rate limiting step of 99 kJ/mol.