Effect of surface hydroxyls on selective CO 2 hydrogenation over Ni 4/γ-Al 2O 3: A density functional theory study

Abstract

Catalytic hydrogenation of CO 2 to valuable chemicals or liquid fuels is a promising way to recycle and utilize CO 2. In the present study, elementary steps leading to the formation of formate and CO, two important intermediates in CO 2 hydrogenation on Ni/γ-Al 2O 3, have been explored using the density functional theory (DFT) slab calculations. Two systems: Ni 4 cluster supported on the dry γ-Al 2O 3(1 1 0) surface, D(Ni 4), and on the hydroxylated γ-Al 2O 3(1 1 0) surface, H(Ni 4), have been used to model Ni/γ-Al 2O 3. On D(Ni 4), the reaction energy and activation barrier for formate formation are −0.23 eV and 1.25 eV, respectively, whereas those for CO formation are −0.48 eV and 2.13 eV, respectively. As such, formate formation is preferred kinetically while CO formation is more facile thermodynamically. On H(Ni 4), the reaction energy and activation barrier for formate formation are −0.36 eV and 2.32 eV, respectively, whereas those for CO formation are −0.67 eV and 0.69 eV, respectively. Consequently, CO formation becomes more favorable both kinetically and thermodynamically. These results indicate that hydroxylation of the γ-Al 2O 3 support alters the pathway, and ultimately, the selectivity of CO 2 hydrogenation on Ni/γ-Al 2O 3. This conclusion supports the fact that varying the reaction environment such as water partial pressure is often used to improve the selectivity of a reaction.