Stabilization of Reactive MgO Surfaces by Ni Doping

Ni-MgO solid solutions are promising materials for catalytic reduction of CO$_2$ and dry reforming of CH$_4$. To explain the catalytic activity, an {\it{ab initio}} study of Ni-substitutional defects in MgO (Ni$_{\rm Mg}$) has been performed. At first, the validation of the theory level was done. We compared results of CCSD(T) embedded-cluster calculations of Ni$_{\rm Mg}$ formation energies and adsorption energies of CO, CO$_2$ and H$_2$ on them to the HSE($\alpha$) hybrid DFT functional with the fraction of the exact exchange $\alpha$ varied between 0 and 1 [1]. HSE(0.3) was found to be the best compromise in this study. Our periodic HSE(0.3) calculations show that Ni$_{\rm Mg}$ defects are most stable at corner sites, followed by steps, and are least stable at (001) terraces. Thus, Ni-doping stabilizes stepped MgO surfaces. The dissociative adsorption of H$_2$ on the terrace is found to be endothermic ($+1.1$~eV), whereas on (110) surface with Ni$_{\rm Mg}$ it is highly exothermic ($-1.6$~eV). Adsorbed CO$_2$ is also significantly stabilized ($-0.6$ vs. $-2.2$~eV). These findings explain recent microcalorimetry measurements of H$_2$ and CO$_2$ adsorption at doped Ni-MgO samples.---[1] A. Mazheika and S.V. Levchenko, DOI: 10.1021/acs.jpcc.6b09505.