Computational studies of Lewis acidic gas adsorption to transition metal oxide nanoclusters and metal organic frameworks
Computational studies of the interaction of Lewis acid gases with metal oxide clusters and metal organic frameworks show how these gases interact with and degrade these materials at the molecular level. The calculations were done at the levels of density functional theory and correlated molecular orbital theory ((CCSD(T))). Group VI metal oxides clusters physisorb CO_2 near or below to 298K, and chemisorption of CO_2 by carbonate formation is an endothermic process. SO_2 physisorbs to Group VI clusters near or below 298K. Group VI metal oxides chemisorb SO_2 by forming sulfites with positive free energies of binding at 298K. The formation of sulfates is thermodynamically allowed for Cr clusters because Cr clusters have a higher reducibility than do Mo or W clusters. Group IV metal oxide clusters prefer chemisorption of both gases by carbonate and sulfite formation. Mo and W oxides may function as long lived sorbents for these gases, whereas Cr and Group IV metal oxides would degrade upon exposure to these gases as sulfites, sulfates, or carbonates form on their surfaces. Uranium trioxide clusters are predicted to chemisorb CO_2 by uranyl carbonate formation. The exposure of nuclear waste to CO_2 could cause uranium oxides to degrade leading to ground water contamination. The physisorption of the Lewis acid gases (CO_2, SO_2, H_2O, H_2S, CO, and NO_2) to M-MOF-2 systems (M = Zn, Cu, Co), was investigated. The MOFs are predicted to bind H_2O, H_2S, and SO_2 more strongly than the other gases. The binding energies are larger for Zn and Co than for Cu. Zn-MOF-2 clusters will degrade faster than Cu.