Water Adsorption and Decomposition on Co(0001) Surface: A Computational Study

Spectroscopic investigation of a Co(0001) model catalyst during exposure to H2 and CO at near-ambient pressures

Cobalt catalysts, although already used industrially for Fischer-Tropsch synthesis, are prone to a number of deactivation mechanisms such as oxidation of the active phase, and the deposition of carbon and reaction products. We have performed near-ambient-pressure X-ray photoelectron spectroscopy on Co(0001) model catalysts during exposure to gases relevant to Fischer-Tropsch synthesis, i.e., CO and H2, at 0.25 mbar total pressure. At this pressure, CO seems to be more efficient at keeping the Co(0001) surface metallic than H2, which is the opposite behavior as reported in the literature for other pressure ranges. We offer an interpretation of these differences based on the preferred adsorption and dissociation sites of CO and H2 compared to the oxidizing agent water (present as impurity in the gas feed and one of the products of the reaction). Additionally, detailed carbon spectra measured at the HIPPIE beamline of MAX IV allow for the distinction of different adsorbed species: CO and COx species are present in correlation to the presence of oxygen on the surface. Carbidic carbon and graphitic carbon can both be removed by hydrogen, whereas adsorbed hydrocarbons possibly poison the surface.

Coverage-dependent adsorption and dissociation of H2O on Al surfaces

The adsorption and dissociation of H₂O on Al surfaces including crystal planes and nanoparticles (ANPs) are systematically investigated by using density functional theory (DFT) calculations. H₂O adsorption strength follows the order ANPs > Al(110) > Al(111) > Al(100). Due to the smaller cluster deformation caused by the moderate H₂O adsorption, the relative magnitude of H₂O adsorption strength on ANPs and crystal planes is opposite to the trend of adatoms like O* and/or N*. The energy barrier for the decomposition of H₂O into H* and OH* is larger on ANPs than on crystal planes, and it decreases with the increasing cluster size. Due to the competition between the hydrogen (H) bonding among water molecules and the interaction between the water molecules and the substrate, the adsorption strength of H₂O first increases and then decreases with the increase of water coverage. Moreover, each H₂O molecule can efficiently form up to two H bonds with two H₂O molecules. As a result, H₂O molecules tend to aggregate into cyclic structures rather than chains on Al surfaces. Furthermore, the dissociation energy barrier of H₂O drops with the increasing water coverage due to the presence of H bonds. Our results provide insight into interactions between water and Al, which can be extended to understand the interaction between water and other metal surfaces.

Mechanistic insight into methane dry reforming over cobalt: a density functional theory study

Cobalt-based catalysts are a potential candidate among non-noble metal catalysts in dry reformation of methane (DRM), while the detailed mechanism of the DRM reaction is still largely unknown. In this contribution, the rather complicated reaction network for DRM is explored by density functional theory calculations. The most favorable adsorption structures of all species involved in the DRM reaction over Co(0001) have been identified. For CO2 activation, its direct dissociation to generate CO and O is the dominant reaction pathway. For CH4 direct dissociation, CH dehydrogenation into atomic C and H is the rate-determining step (RDS). It is predicted that the CH is the most abundant species among CHx (x = 0-3) over Co(0001). O acts as an oxidant and reacts with CH to produce CHO, and subsequently, CHO decomposes into CO and H. Atomic C may directly react with O to produce CO, or be oxidized by OH to COH, followed by the COH decomposition to CO and H. Thus, three possible pathways for DRM over the Co(0001) surface are proposed in our study, and the oxidation step is suggested as the RDS. The dominant route is identified as CH4 successive dissociation into CH, and CH oxidizing by O to form CHO, then CHO decomposition to CO and H.