Understanding the catalytic mechanisms of CO2 hydrogenation to methanol on unsupported and supported Ga-Ni clusters. (1st November 2019) - Tang, Qingli
Graphical abstract: Highlights: Adsorption energies show intermediates and active sites dependence. The barrier of rate-limiting step on Ga6 Ni10 is the lowest with the value of 1.04 eV. d-band centers and exposed active sites can well explain the activities of clusters. SiO2 or γ- Al2 O3 support lowers the electron transfer ability of Ga6 Ni10 cluster. Abstract: Although experimental studies have shown that Ga3 Ni5 is a promising catalyst for carbon dioxide (CO2 ) hydrogenation to methanol (CH3 OH), the roles that cluster size and support play in the reaction are not clear. This research was set to study the quantitative and qualitative impacts of the size and support of Ga-Ni clusters on CO2 hydrogenation to CH3 OH at electronic and molecular levels by using density functional theory (DFT). Ga3 Ni5, Ga6 Ni10, Ga12 Ni20, Ga15 Ni25, and Ga24 Ni40 nanoclusters, and γ-Al2 O3 - and SiO2 - supported Ga6 Ni10 were chosen as the study objects. Results show that adsorption energies of intermediates are highly related to intermediates' characteristics and clusters' active sites. Moreover, cluster size has no linear relation with the adsorption strengths of intermediates, while it has significant impact on the activation barrier of the rate-limiting step of hydrogenation process. Ga6 Ni10 cluster has the lowest activation barrier of 1.04 eV due to the d-band center location and the exposed active sites of clusters. Compared with unsupported Ga6 Ni10 clusters, Ga6 Ni10 /SiO2 and Ga6 Ni10 / γ- Al2 O3 increase the adsorption energies of the intermediates and the activation barriers of the rate-limiting steps due to the lower electron transfer ability of Ga6 Ni10 on SiO2 and γ- Al2 O3 supports. Therefore, a support that can increase the electron transfer abilities of catalysts are preferable. The findings will be very beneficial for preparing new Ga-Ni catalysts for CO2 hydrogenation to CH3 OH.