Article pubs.acs.org/JPCC
H2O Adsorption/Dissociation and H2 Generation by the Reaction of H2O with Al2O3 Materials: A First-Principles Investigation Yu-Huan Lu, Shiuan-Yau Wu, and Hsin-Tsung Chen* Department of Chemistry and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan ABSTRACT: The microscopic reaction mechanisms for the water adsorption/dissociation and hydrogen generation processes on the α-Al2O3(0001) surface are clarified by using spinpolarized density functional theory with the projected augmented wave approach. The adsorptions of OH, O, and H species are also examined. Calculations show that the H2O, OH, O, and H species prefer to adsorb at the Al(II)-top, Al(I, II)-top, Al(I, II)-bridge, and Al(II)-top sites with adsorption energies of −1.34, −5.91, −8.22, and −3.14 eV on the Al-terminated surface, whereas those are Al-top, Al-top, Al-top, and O-top sites with adsorption energies of −1.11, −2.79, −2.00, and −2.23 eV for the Al, O-terminated surface. Geometries of the molecular adsorbed intermediates, transition states, and the hydroxylated products as well as the energetic reaction routes are fully elucidated. Hydrogen generation and full dissociation of water are found to occur on the Al-terminated surface with overall exothermicities of 2.37 and 4.22 eV, whereas only the production of coadsorbed H(ads) + OH(ads) is observed on the Al, O-terminated surface with an overall exothermicity of 1.06−1.64 eV. In addition, the local density of states and Bader charge calculations are carried out to study the interaction between the adsorbate and surface along the reaction.
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INTRODUCTION The interaction of a water molecule with a metal-oxide surface has engaged a lot of interests in the experiment and theory. The ceramic material, alumina, is one of the most important oxide widely used as an adsorbent, support, and catalyst in industrial and environmental applications.1−3 We have great interest in the H2O adsorption and dissociation on the aluminum oxide surface because these are the critical steps for hydrogen generation by the Al/water system.4−11 A large amount of studies have focused on H2O adsorption/dissociation on the γ-Al2O3 surface with high surface area.1,12−18 However, the interaction between H2O and the α-Al2O3 surface is less studied. In this study, we concentrate on the reaction of H2O on the α-Al2O3 surface. Experimental studies have used vibrational spectroscopy, highresolution electron energy loss spectroscopy, thermal and laserinduced desorption, LEED, AFM, and dynamic-mode SFM to investigate the interaction between water and alumina surface.19−24 The experiments have reported that the adsorption energy of molecular H2O on the α-Al2O3(0001) surface is in the broad range from −8 to −64 kcal/mol and H2O can then dissociate on the Al-terminated Al2O3(0001) surface or on the defect sites of the surface.19−36 The dissociative adsorption of water on the α-Al2O3(0001) single-crystal surface with the adsorption energy of −23 to − 41 kcal/mol was clearly observed by Nelson et al.21 Theoretical studies37−48 have employed both empirical and quantum-mechanical methods to investigate the reaction mechanism of water dissociation at various Al2O3 clusters © 2016 American Chemical Society
and the surface models. Wittbrodt et al. used various AlnOm clusters to study the reaction of H2O adsorption/dissociation at the B3LYP/6-31(d) level of theory.47 They demonstrated that the H2O can molecularly adsorb on the AlnOm clusters and dissociate to H + OH by two different pathways named 1,2-H and 1,4-H migration in which 1,2-H migration is more favorable with a lower barrier. Contrarily, the periodic slab calculations with the Car−Parrinello methodology by Hass et al. reported that the 1,4-H migration is strongly favored kinetically.45,48 Ranea et al. applied the GGA-PW91 to calculate the 1,2-dissociation pathway and found a barrier of 4.4 kcal/mol.49 However, they did not consider the 1,4-dissociation pathway. Recently, Wang et al. employed the density functional theory with the all-electron triple numerical polarized basis sets to investigate the H2O adsorption/dissociation on the α-Al2O3(0001) surface. Calculations showed that the 1,2-H and 1,4-H migration pathways are competitive and the 1,2-H migration is more preferable thermodynamically.18 However, the interaction of water−α-Al2O3 surface has not been clearly understood in the molecular-level interpretations. In this work, we investigate the mechanisms of H2O adsorption/dissociation, as well as the hydrogen production on the α-Al2O3(0001) surface by using the spin-polarized density functional theory (DFT) calculations. Received: July 18, 2016 Revised: August 26, 2016 Published: August 27, 2016 21561
DOI: 10.1021/acs.jpcc.6b07191 J. Phys. Chem. C 2016, 120, 21561−21570
Article
The Journal of Physical Chemistry C
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COMPUTATION METHODS The spin-polarized density functional theory calculations are carried out by the Vienna Ab initio Simulation Package (VASP).50−53 The electron exchange-correlation functional is chosen as the Perdew−Burke−Ernzerhof (PBE) density functional treated within the generalized gradient approximation (GGA).54 The GGA-PBE functional has been demonstrated to work well for the γ-Al2O3 surface.17 The interaction between ions and electrons is described by the projector augmented wave (PAW) approach with a plane-wave basis set.55 The Brillouin zone is sampled with the Monkhorst−Pack grid of (4 × 4 × 4) and (4 × 4 × 1) k-points for bulk and surface calculations, respectively.56 A cutoff energy of 400 eV is applied to allow the total energy converge to 0.01 eV. For the surface calculations, the bottom four atomic layers are frozen while the remaining four layers and the adsorbed species are fully relaxed. In this work, the adsorption energies are computed by the equation below
Figure 1. Slabs models for the α-Al2O3(0001) surfaces: (a, c) side view, (b, d) the relevant coordinatively unsaturated surface sites: Al(I), Al(II) for Al-terminated α-Al2O3(0001) surface; Al and O for Al, O-terminated α-Al2O3(0001) surface.
ΔEads = E[total] − (E[surface] + E[adsorbate])
where E[total], E[surface], and E[adsorbate] are the calculated electronic energies of adsorbed species on an α-Al2O3(0001) surface, a clean (0001) surface, and a gas-phase species, respectively. One should note that we have carried out calculations for H2O adsorption on the (1 × 1), (2 × 2), and (3 × 3) adlayer surfaces. The size effect can be neglected due to the small difference of the adsorption energy (
