The Surface Chemistry of Water on Fe(100): A Density Functional Theory Study

Abstract

The formation of water by hydrogenation of atomic oxygen is studied using density functional theory. Atomic oxygen preferentially adsorbs at the four-fold hollow site, the hydroxyl group prefers the bridge site in a tilted configuration, and water is most stable when adsorbed at the top site with the two O-H bonds parallel to the Fe surface. Water formation by the hydrogenation of oxygen is a highly activated process on the Fe(100) surface, with similar activation energies, in the order of 1.1 eV, for the first and second hydrogen additions. A more favourable route for the addition of the second hydrogen atom involves the disproportionation of hydroxyl groups to form water and adsorbed oxygen. Dissociation of the OH is also likely since the activation energy is similar to that for disproportionation of 0.65 eV. Furthermore, the results show that the dissociation of water on Fe(100) is a non-activated process: 0.16 eV for the zero-coverage limit and 0.03 eV when surface oxygen is present. Herein, adsorption energies, structures and vibrational frequencies are presented for several adsorption states at 0.25 ML coverage, as well as the potential energy surface for water formation on Fe(100).

Chemistry in glow discharges of H2/O2mixtures: diagnostics and modelling

“…In previous works [37][38][39], adsorptions of neutral molecules were considered negligible as compared with those of atoms and radicals. Literature calculations suggest that the adsorption of water on surface iron atoms can take place without a barrier [89,90]. However, model simulations with this assumption, including the subsequent reactions of adsorbed water, lead to relevant discrepancies with our experimental results, which are accounted for much better by assuming no H 2 O adsorption.…”

“…However, model simulations with this assumption, including the subsequent reactions of adsorbed water, lead to relevant discrepancies with our experimental results, which are accounted for much better by assuming no H 2 O adsorption. Likewise the LangmuirHinshelwood heterogeneous recombination of adsorbed reactants like OH(s) + H(s) or 2 OH(s) for water production, with predicted energy barriers of ~ 1.1 eV and ~ 0.65 eV [89], respectively, have been considered minor, as compared with the barrierless Eley-Rideal processes listed in table 2 and have not been included in the final modelling.…”

Methane, formaldehyde and methanol formation pathways from carbon monoxide and hydrogen on the (0 0 1) surface of the iron carbide χ-Fe5C2

“…These are higher compared to previously reported values for the Fe(1 1 0) surface [57], which are $60 kJ/mol and 70 kJ/mol for the first and second H additions. Govender et al demonstrated that water formation on Fe(1 0 0) can also occur by reaction of two OH groups [27]. This step appears less likely on the carbide surfaces.…”

“…Among the mixture of carbide and oxide phases [5][6][7][8] present under process conditions, the Hägg carbide (v-Fe 5 C 2 ) is generally regarded as the active phase for FTS [2,5,6,[9][10][11][12][13][14][15][16][17][18], although other phases such as e-Fe 2.2 C or h-Fe 3 C may exhibit activity as well [9]. Mechanistic studies on CO hydrogenation and FTS on metallic iron surfaces are available in the literature [4,14,[19][20][21][22][23][24][25][26][27][28][29], but studies on the catalytic properties of iron carbides in FTS are scarce, we refer to de Smit and Weckhuysen [13] for a recent review and to other literature [3,5,[8][9][10][11]13,15,24,[30][31][32][33][34][35]…”

First-Principles Modeling of Direct versus Oxygen-Assisted Water Dissociation on Fe(100) Surfaces

“…For the O species, Govender et al [35] have found that the hcp and bridge sites were equally stable. On the other hand, Błoński et al [60] and Lu et al [30] suggested that O atom only occupied the hollow hcp sites.…”