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WIREs Comput Mol Sci
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The promotional role of water in heterogeneous catalysis: mechanism insights from computational modeling

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Water is the key to life in our planet. As one of the most abundant resources on the earth, water may play important roles in chemical reactions in addition to being a reaction medium. In this review, we highlight recent advances in experimental observations and mechanistic understanding of the promotional role of water in chemical reactions, with an emphasis on the essential effects of water in heterogeneous catalysis. As water may exist in molecular state or dissociate to hydroxyl (OH) and hydrogen (H) species on catalyst surfaces, we have outlined the roles of water from three aspects: (1) the promotional role of molecular water including the solvation‐like effect and water‐mediated H‐transfer, (2) the promotional role of OH/OH and H/H+ species, and (3) some miscellaneous effects of water, such as water‐assisted carbon removal, surface reconstruction, and active sites blocking. The results discussed here provide a fundamental understanding of the promotional role of water in heterogeneous reactions and may inspire the theoretical studies of water effects on other fields of chemistry and atmospheric science as well. WIREs Comput Mol Sci 2016, 6:679–693. doi: 10.1002/wcms.1272

Dependence of the CO oxidation reaction rate on the moisture concentration in the reactant gas at 270 K over Au/TiO2 . (Reprinted with permission from Ref . Copyright 2001 Academic Press)
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DFT prediction of energy profile. The energies for removal of chemisorbed carbon species on BaO/Ni(111) are relative to gas‐phase H2O and an adsorbed carbon species on BaO/Ni(111). (Reprinted with permission from Ref . Copyright 2011 Macmillan Publishers Limited)
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The structure of initial states, transition states, and final states of O2 * + H2O *OOH * + OH * on (a) Au10 and (b) Au38 clusters. E ad refers to the co‐adsorption energy of H2O and O2 on gold clusters, and E a refers to the reaction barrier. (Adapted with permission from Ref . Copyright 2010 Tsinghua University Press and Springer‐Verlag Berlin Heidelberg)
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H‐tunneling in an H2O tetramer. (a) Schematic showing manipulation of the H‐tunneling by a Cl‐terminated tip. (b) Current trace recorded during the H‐tunneling manipulation shown in (a). O, H, Au, Cl, and Na+ are denoted by red, white, golden, cyan, and blue spheres, respectively. (Reprinted with permission from Ref . Copyright 2015 Macmillan Publishers Limited)
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Energy profile for hydrogen atom diffusion on FeO/Pt(111): H2O‐mediated (blue solid line) and intrinsic (red dashed line); insets provide side views of the initial state (IS), transition state (TS), and final state (FS) for the H2O‐mediated diffusion. (Reprinted with permission from Ref . Copyright 2012 American Association for the Advancement of Science)
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Schematic of the promotional role of molecular water: solvation‐like effect (in black) and water‐mediated H‐transfer (in red). A refers to reactants of hydrogenation reactions, and H–A refers to reactants of dehydrogenation reactions.
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Partial dissociation of H2O on Ru(0001): STM images of (a) intact water stripes after deposition at 145 K and its transformation to (b) partially dissociated H2O–OH stripes after 30 min annealing at 145 K. In the DFT optimized structures shown, the OH group is highlighted by an orange O atom. (Reprinted with permission from Ref . Copyright 2014 APS and Ref . Copyright 2015 American Chemical Society)
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CO consumption rate (△ or ▴), CH4 selectivity (□ or ▪), and C5 + selectivity (○ or ●) as a function of H2O partial pressure on Ru/SiO2 . Open symbols: space velocity changes; closed symbols: H2O‐addition. (Reprinted with permission from Ref . Copyright 2013 Wiley‐VCH)
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Enhancing effect of H2O on propene epoxidation with O2 over Au/TS‐1‐K1. Step 1: without H2O; step 2: H2O was added to the feed gas; step 3: the addition of H2O was stopped; step 4: H2O was added to the feed gas again. (Adapted with permission from Ref . Copyright 2009 Wiley‐VCH)
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