Theoretical Investigation of Surface Reactions of Lactic Acid on MgO Clusters

Interaction of lactic acid with various sizes of magnesium oxide clusters (MgO)x (x = 2, 4, 6, 8, 9, 12, 15, 16, 18) is investigated. Density functional theory with the PBE functional and a polarized double-ζ basis set is employed to optimize the structures. For MgO dimer, optimizations are also performed at the PBE/DZP, PBE/TZP, and MP2/TZV(d,p) levels of theory, and single-point CCSD(T)/TZV(d,p) calculations are computed at the PBE/TZP optimized geometries. CCSD(T)/TZV(d,p) calculations show that the PBE/DZP results are off by no more than 5 kcal/mol. Dissociative adsorption of a proton bound to oxygen is possible for the carboxylic acid group, the hydroxyl group, and for a simultaneous interaction of the carboxylic acid and hydroxyl groups. Associative adsorption of various functional groups is also possible, although these represent higher energy structures. All of the adsorptions are calculated to be exothermic. Dissociative adsorption of the carboxylic acid group of lactic acid at the lowest coordinated magnesium sites is determined to be the lowest energy structure. Adsorption energies are found to decrease in magnitude as the size of MgO increases. The geometry of the magnesium oxide cluster changes to a nanorod-like structure when lactic acid interacts with higher coordinated magnesium atoms in double layer systems, but remains simple cubic when a third layer is added. The coordination environment rather than the size of the MgO cluster appears to dominate the adsorption energy when the size becomes larger than (MgO)12.