Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and Dependence on Oxide, Adsorbate Pair, and Density Functional
Journal article, 2017
Open-shell molecules on metal oxide surfaces frequently display cooperative adsorption mechanisms, where pairs of adsorbates are significantly more stable than the isolated species. In this work, density functional theory is used to investigate the cooperative adsorption of OH-H and NO2-NO on rocksalt BaO(100), rutile TiO2(110), fluorite CeO2(111), and tetragonal PdO(101) surfaces. The OH and NO2 adsorbates are considered to be located at the metal sites, whereas H and NO are situated on the oxygen sites. Despite differences in adsorption mechanisms, the pairing is found to be consistently exothermic. The pairing is most pronounced on BaO(100) and weakest on PdO(101). OH and H form more stable pairs than do NO2 and NO. In all cases except CeO2(111), the hybrid HSE06 functional predicts a stronger pairing than the semilocal PBE functional. The absolute pairing energy is a convoluted measure of charge transfer energies, electrostatic interactions, and ionic relaxations, which is analyzed in detail using a thermodynamic cycle. Adsorbate pairing has marked effects on the reactivity, which is exemplified by studying the reaction of CO with OH on CeO2(111).