Effect of surface hydroxyls on selective CO2 hydrogenation over Ni4/γ-Al2O3: A density functional theory study

Catalytic hydrogenation of CO2 to valuable chemicals or liquid fuels is a promising way to recycle and utilize CO2. In the present study, elementary steps leading to the formation of formate and CO, two important intermediates in CO2 hydrogenation on Ni/γ-Al2O3, have been explored using the density functional theory (DFT) slab calculations. Two systems: Ni4 cluster supported on the dry γ-Al2O3(1 1 0) surface, D(Ni4), and on the hydroxylated γ-Al2O3(1 1 0) surface, H(Ni4), have been used to model Ni/γ-Al2O3. On D(Ni4), 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(Ni4), 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-Al2O3 support alters the pathway, and ultimately, the selectivity of CO2 hydrogenation on Ni/γ-Al2O3. 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.