Electrochemical dissolution of surface alloys in acids: Thermodynamic trends from first-principles calculations

Kinetic aspect of CO2 reforming of CH4 on Ni(111): A density functional theory calculation

The solvent effect on catalyst activity and selectivity for the liquid-phase hydrogenation of acetophenone (AP) to 1-phenylethanol was thoroughly investigated over Ni/SiO2. Solvents of different nature were used: protic (C1–C3 primary and secondary alcohols), aprotic polar (tetrahydrofuran, γ-butyrolactone, and acetonitrile) and apolar solvents (cyclohexane, toluene, and benzene). The solvent had a strong influence on the AP hydrogenation rate but did not modify significantly the selectivity to 1-phenylethanol that was always higher than 92%. The AP hydrogenation activity followed the order: C2–C3 alcohols > cyclohexane > toluene > tetrahydrofuran > γ-butyrolactone > methanol ≫ benzene ≅ acetonitrile. In order to explain this activity pattern, the solvent–AP, solvent–H2 and solvent–catalyst interactions were analyzed. For the analysis of the solvent–AP interactions in liquid phase, both classical measures of polarity and others based on different solvatochromic scales were considered. The H2 availability in the liquid phase was estimated from the H2 solubility at reaction conditions. Solvent–catalyst interactions were characterized by means of the adsorption enthalpies measured calorimetrically. A reasonable correlation between the catalyst activity and some solvatochromic parameters was found only when solvents of similar nature were compared. For protic solvents, the AP hydrogenation rate decreased with the solvent polarity and its ability for H-bond formation with AP. Instead, the solvent–AP interactions were weak when using apolar solvents and thereby the activity pattern was essentially determined by the strength of solvent–catalyst interactions. In the case of aprotic polar solvents, both the solvent–AP interactions in the liquid phase and the solvent adsorption strength on the catalyst surface influenced the hydrogenation activity. The highest catalytic activities were obtained when using C2–C3 alcohol solvents. These protic solvents adsorbed dissociatively on metal nickel surface increasing the number of active H available for the hydrogenation reaction; this effect was much more important in the case of 2-propanol.

Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO2: A comprehensive study of the phenomenon

A comprehensive review of hydrogen production from methanol thermochemical conversion for sustainability

CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.

CO2 reforming of CH4 on Ni(111): a density functional theory calculation.

The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory−generalized gradient approximation (DFT−GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O−H bond breaking in CH3OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH3O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C−O and C−H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study.

Based on the gradient-density functional theory, calculation results of methoxy adsorption on Au(111), Ag(111), Cu(111), Pt(111), Pd(111), Ni(111), Rh(111), and Fe(100) surfaces are presented, and a consistent picture for some key physical properties determining the reactivity of metals appears. These eight metals belong to two groups: either with filled d electrons (group IB) or with unfilled but more than half filled d electrons (group VIII). The calculated adsorption energies are quite in agreement with the experimental data as well as the previous theoretical calculation results. Importantly, using the analysis of B. Hammer and J. K. Norskov, Nature (London) 376, 232 (1995) and in Chemisorption and Reactivity on Supported Clusters and Thin Films, edited by R. M. Lambert and G. Pacchioni (Kluwer Academic, Dordrecht, 1997), pp. 285–351, the binding energies have selectively been linearly correlated to the d-band center and to the size of the metal d-band orbital overlapping with the adsorbate (coupling matrix element) for these two groups of metals. And by analyzing the nature of the adsorption bonding, the possible reason of this difference is suggested.

Yu-Hua Zhou

Papers

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study.

Characterization of methoxy adsorption on some transition metals: A first principles density functional theory study