We present a combined theoretical and experimental study of CO hydrogenation on a Ni(110) surface, including studies of the role of gas-phase atomic hydrogen, surface hydrogen, and subsurface hydrogen reacting with adsorbed CO. Reaction mechanisms leading both to methane and methanol are considered. In the reaction involving surface or subsurface hydrogen, we investigate four possible pathways, using density functional theory to characterize the relative energetics of each intermediate, including the importance of further hydrogenation versus C-O bond breaking, where the latter may lead to methane production. The most energetically favorable outcome is the production of methanol along a pathway involving the sequential hydrogenation of CO to a HCO* intermediate, followed by a final hydrogenation to give methanol. In addition, we find that subsurface hydrogen noticeably alters reaction barriers, both passively and through the energy released by diffusion to the surface. Indeed, the effective reaction barriers are even lower than for CO methanolation on Cu(211) and Cu(111) than for Ni(110). In studies of gas-phase H atoms impinging on a CO-adsorbed Ni(110) surface, Born-Oppenheimer molecular dynamics simulations show that direct impact of H is unlikely to result in hydrogenation of CO. This means that Eley-Rideal or hot-atom mechanisms are not important; thus, thermal reactions involving subsurface hydrogen are the primary reaction mechanisms leading to methanol. Finally, we demonstrate experimentally for the first time the production of methanol and formaldehyde from CO hydrogenation on Ni(110) and confirm the role of subsurface hydrogen in the mechanism of this reaction.

Acetic acid adsorption and reactions at multiple surface coverage values on Ni(110) were studied with temperatureprogrammed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) at 90−500 K. The experimental measurements were interpreted with density functional theory (DFT) calculations that provided information on adsorbate geometries, energies, and vibrational modes. Below the monolayer saturation coverage of 0.36 ML at 90 K, acetic acid adsorbs mostly molecularly. Above this coverage, a physisorbed layer is formed with dimers and catemers, without detectable monomers. Dimers and catemers desorb as molecular acetic acid at 157 and 172 K, respectively. Between 90 and 200 K, the O−H bond in acetic acid breaks to form bridge-bonded bidentate acetate that becomes the dominant surface species. Desorption-limited hydrogen evolution is observed at 265 K. However, even after the acetate formation, acetic acid desorbs molecularly at 200−300 K due to recombination. Minor surface species observed at 200 K, acetyls or acetates with a carbonyl group, decompose below 350 K and generate adsorbed carbon monoxide. At 350 K, the surface likely undergoes restructuring, the extent of which increases with acetic acid coverage. The initial dominant bridge-bonded bidentate acetate species formed below 200 K remain on the surface, but they now mostly adsorb on the restructured sites. The acetates and all other remaining hydrocarbon species decompose simultaneously at 425 K in a narrow temperature range with concurrent evolution of hydrogen, carbon monoxide, and carbon dioxide. Above 425 K, only carbon remains on the surface.

No abstract

Guaiacol (2-methoxyphenol, C6H4(OH)­(OCH3)) adsorption and reactions on a Pt(100) surface were studied with infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements at different surface coverage values from 100 to 800 K. In addition, density functional theory (DFT) calculations were used to determine geometries, adsorption energies, and vibrational frequencies for adsorption structures. Depending on surface coverage, guaiacol formed one or two physisorbed states. At low coverage, a single state with a desorption peak at 225 K was observed. At high coverage, two physisorbed states were observed with desorption peaks at 195 and 225 K. At temperatures above 225 K, after the desorption of physisorbed layers, a dissociatively adsorbed structure, C6H4O­(OCH3) + H, was observed. Recombinative molecular guaiacol desorption was detected at 320 K. The dissociatively adsorbed structure was stable up to 337 K when C–O bonds began to break. Molecularly adsorbed guaiacol in horizontal (flat-lying) configurations bound through its benzene ring was not observed under all tested conditions. Similarities of vibrational spectra and desorption measurements for a Pt(100) surface in this study and a Pt(111) surface reported previously demonstrate that the obtained results are generally valid for low-index Pt crystal planes and, more importantly, for catalytic Pt nanoparticles.

No abstract