Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and Dependence on Oxide, Adsorbate Pair, and Density Functional

Abstract

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).

Role of the Band Gap for the Interaction Energy of Coadsorbed Fragments

“…The magnitude of the interaction energy is influenced by many factors, such as the element forming the oxide, the ionization potential, or the electron affinity of the fragments, i.e., the ability to donate or accept a charge (strength of the fragment). The amount of charge transfer is also influenced by a structural relaxation of the oxide (polaronic distortions), 38 the nature of the oxide (reducible or nonreducible), the surface exposed, and the presence of defects. 50 Despite these, we can identify a one-to-one scaling between the interaction energy and the band gap (the discrepancies from the one-to-one scaling are caused by the effects mentioned above).…”

“…This effect has been pointed out for a few systems, for example, NO x adsorption on alkaline-earth oxide surfaces, ,− halogen and halogen hydrides on CeO 2 (111) and La 2 O 3 (001), , methane activation on La 2 O 3 (001) and PdO, and H–OH pair on CeO 2 (111), BaO(100), TiO 2 (110), and PdO(101) . Forward steps have also been made in understanding the mechanism of the cooperative adsorption by investigating, for example, charge transfers, electrostatic interactions, and ionic relaxations …”

“…30 This effect has been pointed out for few systems, for example, NO x adsorption on alkalineearth oxide surfaces, 25,[31][32][33][34][35][36] halogen and halogen hydrides on CeO 2 (111) and La 2 O 3 (001), 28,29 methane activation on La 2 O 3 (001) and PdO, 37 and H-OH pair on CeO 2 (111), BaO(100), TiO 2 (110), and PdO(101). 38 Forward steps are also done in understanding the mechanism of the cooperative adsorption by investigating, for example, charge transfers, electrostatic interactions, and ionic relaxations. 38 Here, we report an understanding of the adsorption on oxide semiconducting surfaces and which can be extremely useful in the catalytic screening process by reducing the computational time for these type of surfaces.…”

“…38 Forward steps have also been made in understanding the mechanism of the cooperative adsorption by investigating, for example, charge transfers, electrostatic interactions, and ionic relaxations. 38 Here, we report an understanding of the adsorption on oxide semiconducting surfaces, which can be extremely useful in the catalytic screening process by reducing the computational time for these types of surfaces. We find that the band gap directly affects the binding energies of electron donor and acceptor fragments, and we generalize the observations found for Lewis acid−base pairs.…”

CO2 adsorption on hydroxylated In2O3(110)

“…The five-fold coordinated In 3 and In 4 show a small positive increase in their BVS upon hydroxylation, see the ESI. † Changes of oxidation state upon surface hydroxylation are well known for ceria (CeO 2 ), 50,54 which is a reducible oxide. The adsorption of hydrogen CeO 2 (111) has been reported to be preferentially homolytic 50,51 at a coverage of 50% and the oxidation state in this case changes from +4 to +3.…”