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The water gas shift (WGS) reaction is an important reaction system and has wide applications in several processes. However, the mechanism of the reaction is still in dispute. In this paper we have investigated the reaction mechanism on the model Cu(1 1 1) system using the density functional method and slab models. We have characterized the kinetics and the thermodynamics of the four reaction pathways containing 24 elementary steps and computed the reaction potential energy surfaces. Calculations show that the formate (HCOO) intermediate mechanism (CO + OH → HCOO → CO 2 + H) and the associative mechanism (CO + OH → CO 2 + H) are kinetically unlikely because of the high formation barrier. On the other hand, the carboxyl (HOCO) intermediate mechanism (CO + OH → HOCO → CO 2 + H) and the redox mechanism (CO + O → CO 2 ) are demonstrated to be feasible. Our calculations also indicate that surface oxygen atoms can reduce the barriers of both water dissociation and HOCO decomposition significantly. The calculated potential energy surfaces show that the water dissociation which produces OH groups is the rate-determining step at the initial stage of the reaction or in the absence of surface oxygen atoms. With the development of the reaction or in the presence of oxygen atoms on the surface, CO + OH → HOCO and CO + O → CO 2 become the rate-limiting step for the carboxyl and redox mechanisms, respectively.

Density functional and periodic slab model calculations are performed to study adsorption of water on various Cu surfaces, focusing on monomers and dimers at the planar Cu surfaces and monomers at stepped ones. Single water molecules tend to weakly bind to atop positions with the molecular plane basically parallel to the substrate surface on the planar surfaces or the step plane on the stepped surfaces with negligible structural deformation of water. The experimental adsorption energies of water on the (1 1 1) and (1 0 0) surfaces are about twice as large as the theoretical values of monomerically adsorbed water. This phenomenon is demonstrated to be due to formation of water clusters and/or existence of surface defects. It is revealed that the most favorable hexagonal ring superstructure on Cu(1 1 0) is a four-layer structure, not the commonly accepted bi-layer configuration. We found that the adsorption energy of monomeric water correlates linearly with following quantities, respectively: the bond length and the stretching frequency of the Cu–O bond, the coordination number of the surface Cu atom, the surface work function of the clean surface and the 1b1 MO energy shift with respect to the value in the gas phase.

To provide information about the chemistry of water on Pd surfaces, we performed density functional slab model studies on water adsorption and decomposition at Pd(1 1 1) surface. We located transition states of a series of elementary steps and calculated activation energies and rate constants with and without quantum tunneling effect included. Water was found to weakly bind to the Pd surface. Co-adsorbed species OH and O that are derivable from H2O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H2O and OH, respectively. Calculated kinetic parameters indicate that dehydrogenation of water is unlikely on the clean regular Pd(1 1 1) surface. The barrier for the hydrogen abstraction of H2O at the OH covered surface is approximately 0.2–0.3 eV higher than the value at the clean surface. Similar trend is computed for the hydroxyl group dissociation at H2O or O covered surfaces. In contrast, the O–H bond breaking of water on oxygen covered Pd surfaces, H2Oad + Oad → 2OHad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OHad → H2Oad + Oad, is also found to be very feasible with a barrier of ∼0.1 eV. These results show that on oxygen-covered surfaces production of hydroxyl species is highly likely, supporting previous experimental findings.

Surface segregation of Zn and Pd in PdZn alloys was investigated based on density functional calculations using periodic slab models. The segregation propensity of Pd–Zn films was found to be support-dependent: Zn showed a stronger propensity to segregate on Zn support than on Pd support; likewise, for Pd the propensity was stronger on Pd support than on Zn support. In self-supported 1:1 alloy films, segregation of neither Zn nor Pd was favorable. Segregation was suppressed due to the fact that Pd–Zn heteronuclear bonds are stronger than the corresponding homonuclear bonds Pd–Pd and Zn–Zn. However, in films rich in Zn or Pd, segregation of the dominant component was possible. For instance, when a monolayer of Pd is deposited on a Zn(0 0 0 1) film, segregation of Zn is calculated energetically favorable, in agreement with experimental observations. With increasing Zn concentration in the top surface layer, the corrugation of alloy films was reduced.