Mechanisms of Mo₂C(101)-Catalyzed Furfural Selective Hydrodeoxygenation to 2‑Methylfuran from Computation

The selective formation of 2-methylfuran (F-CH₃) and furan from furfural (F-CHO) hydrogenation and hydrodeoxygenation on clean and 4H precovered Mo₂C­(101) surfaces has been systematically computed on the basis of periodic density functional theory including dispersion correction (PBE-D3). The clean Mo₂C­(101) surface has two distinct surface sites: unsaturated C and Mo sites for the adsorption of H and furfural, respectively. The selectivity comes from the different preference of furfural hydrogenation and dissociation (F-CHO + H = F-CH₂O vs F-CHO = F-CO + H) under the variation of H₂ partial pressure. On the basis of the computed minimum energy path on the clean surface, microkinetics shows that high H₂ partial pressure can promote 2-methylfuran formation and suppress furan formation. To verify this proposed selectivity trend of 2-methylfuran at high H₂ partial pressure, the 4H precovered Mo₂C­(101) surface (0.25 monolayer hydrogen coverage), which provides neighboring hydrogens for promoting furfural hydrogenation and blocks the active sites for suppressing furfural dissociation, has been used. The computed results are in full agreement with the experimentally observed selective formation of 2-methylfuran and the H₂ reaction order of one half as well as rationalize the need for a high H₂/furfural ratio (400/1). On the basis of these results, a new two-step protocol for experiments is proposed: i.e., the first step is the pretreatment of the catalyst with hydrogen, and the second step is furfural hydrogenation on H precovered catalysts

Mechanisms of CO Activation, Surface Oxygen Removal, Surface Carbon Hydrogenation, and C–C Coupling on the Stepped Fe(710) Surface from Computation

To understand the initial steps of Fe-based Fischer–Tropsch synthesis, systematic periodic density functional theory computations have been performed on the single-atom stepped Fe(710) surface, composed by <i>p</i>(3 × 3) Fe(100)-like terrace and <i>p</i>(3 × 1) Fe(110)-like step. It is found that CO direct dissociation into surface C and O is more favored kinetically and thermodynamically than the H-assisted activation via HCO and COH formation. Accordingly, surface O removal by hydrogen via H₂O formation is the only way. On the basis of surface CH_<i>x</i> hydrogenation (<i>x</i> = 0, 1, 2, 3), surface CH_<i>x</i> + CH_<i>x</i> coupling and CO + CH_<i>x</i> insertion resulting in CH_<i>x</i>CO formation followed by C–O dissociation, surface C hydrogenation toward CH₃ formation is more favored kinetically than the formation of CH_<i>x</i>-CH_<i>x</i> and CH_<i>x</i>CO, as well as thermodynamically. Starting from CH₃, the formation of CH₄ and CH₃CO has similar barriers and endothermic reaction energies, while CH₃CO dissociation into CH₃C + O has low barrier and is highly exothermic. Therefore, turning the H₂/CO ratio should change the selectivity toward C–C formation and propagation

Mechanisms of H₂O and CO₂ Formation from Surface Oxygen Reduction on Co(0001)

Surface O removal by H and CO on Co(0001) has been studied using periodic density functional method (revised Perdew–Burke–Ernzerhof ; RPBE) and ab initio atomistic thermodynamics. On the basis of the quantitative agreement in the H₂O formation barrier between experiment (1.34 ± 0.07 eV) and theory (1.32 eV), H₂O formation undergoes a consecutive hydrogenation process [O + 2H → OH + H → H₂O], while the barrier of H₂O formation from OH disproportionation [2OH → H₂O + O] is much lower (0.72 eV). The computed desorption temperatures of H₂ and H₂O under ultrahigh vacuum conditions agree perfectly with the experiment. Surface O removal by CO has a high barrier (1.41 eV) and is strongly endothermic (0.94 eV). Precovered O and OH species do not significantly affect the barriers of H₂O and CO₂ formation. All of these results indicate that the present RPBE method and the larger surface model are more suitable for studying cobalt systems

Reactions of CO, H₂O, CO₂, and H₂ on the Clean and Precovered Fe(110) Surfaces – A DFT Investigation

The reactions of CO and H₂O on the clean Fe(110) surface as well as surfaces with 0.25 monolayer O, OH, and H precoverage have been computed on the basis of density functional theory (GGA-PBE). Under the considerations of the reductive nature of CO as reactant and H₂ as product as well as the oxidative nature of CO₂ and H₂O, we have studied the potential activity of metallic iron in the water-gas shift reaction. On the clean surface, CO oxidation following the redox mechanism has a similar barrier as CO dissociation; however, CO dissociation is much more favorable thermodynamically. Furthermore, surfaces with 0.25 monolayer O, OH, and H precoverage promote CO hydrogenation, while they suppress CO oxidation and dissociation. On the surfaces with different CO and H₂O ratios, CO hydrogenation is promoted. On all of these surfaces, COOH formation is not favorable. Considering the reverse reaction, CO₂ dissociation is much favorable kinetically and thermodynamically on all of these surfaces, and CO₂ hydrogenation should be favorable. Finally, metallic iron is not an appropriate catalyst for the water-gas shift reaction

Adsorption Structures and Energies of Cu_<i>n</i> Clusters on the Fe(110) and Fe₃C(001) Surfaces

Spin-polarized density functional theory computations have been carried out to investigate the adsorption configurations of Cu_<i>n</i> (<i>n</i> = 1–7, 13) on the most stable Fe(110) and Fe₃C­(001) surfaces. On both surfaces the adsorbed Cu_<i>n</i> clusters favor aggregation over dispersion, and monolayer adsorption configurations are more favored thermodynamically than the two-layer adsorbed structures because of the stronger Fe–Cu interaction over the Cu–Cu bonding. On the basis of the computed adsorption energies the Fe(110) surface has stronger Cu affinity than the Fe₃C­(001) surface, in agreement with the experimental results. The Fe(110) surface also has stronger Cu_<i>n</i> aggregation energies and more pronounced charge transfer from surface to adsorbed Cu_<i>n</i> clusters than the Fe₃C­(001) surface. Different Cu_<i>n</i> growth modes have been discussed accordingly

DFT+U Study of Molecular and Dissociative Water Adsorptions on the Fe₃O₄(110) Surface

Spin-polarized density functional theory method (GGA+U) and periodic supercell model have been used to study water adsorption properties on the Fe₃O₄(110) surface, which has A and B terminations in close surface energy. The adsorption of one and two water molecules is molecular on the A termination, while dissociative on the B termination. For the adsorption of three and four water molecules, mixed dissociative and molecular coadsorption is preferred on the A termination, and fully dissociative adsorption as well as mixed molecular and dissociative coadsorptions are preferred on the B terminations. The stepwise adsorption energies show that the full monolayer water adsorption on both terminations is thermodynamically possible. Further analysis shows that surface iron atoms and hydrogen bonding contribute to the adsorption energies. The adsorption mechanism has been analyzed on the basis of projected density of states (PDOS)

Dissociative Hydrogen Adsorption on the Hexagonal Mo₂C Phase at High Coverage

Hydrogen adsorption on the primarily exposed (001), (100), (101), and (201) surfaces of the hexagonal Mo₂C phase at different coverage has been investigated at the level of density functional theory and using ab initio thermodynamics. On the Mo-terminated (001) and (100) as well as mixed Mo/C-terminated (101) and (201) surfaces, dissociative H₂ adsorption is favored both kinetically and thermodynamically. At high coverage, each surface can have several types of adsorption configurations coexisting, and these types are different from surface to surface. The stable coverage as a function of temperature and partial pressure provides useful information not only for surface science studies at ultrahigh vacuum condition but also for practical applications at high temperature and pressure in monitoring reactions. The differences in the adsorbed H atom numbers and energies of these surfaces indicate their different potential hydrotreating abilities. The relationship between surface stability and stable hydrogen coverage has been discussed

Adsorption Equilibria of CO Coverage on β-Mo₂C Surfaces

Adsorption and surface coverage of CO on the (001), (101), and (201) surfaces of β-Mo₂C were computed at the level of density functional theory under the consideration of the temperature and CO partial pressure by using the ab initio atomistic thermodynamic method. On the basis of the computed Gibbs free energies, the relationship between CO coverage on the surfaces and temperature as well as CO partial pressure has been established, and excellent agreements have been found between the predicated CO desorption temperatures and the experimentally recorded temperature programmed desorption (TPD) spectra. These computed phase diagrams show that a stable CO coverage can be obtained within a range of temperature and partial pressure; different surfaces can have different coverage at the same conditions, and different partial pressure has a different desorption temperature. In addition, these phase diagrams provide useful information for adjusting the balance between temperature and CO partial pressure for a stable CO coverage and for identifying the active surface and the initial states under given conditions. These results should also be very interesting for surface science under ultra high vacuum conditions

Exploring Furfural Catalytic Conversion on Cu(111) from Computation

The full potential energy surface of the catalytic conversion of furfural to 2-methylfuran on the Cu(111) surface has been systematically computed on the basis of density functional theory, including dispersion and zero-point energy corrections. For furfuryl alcohol formation, the more favorable step is the first H addition to the carbon atom of the CO group, forming an alkoxyl intermediate (F-CHO +H → F-CH₂O); the second H atom addition, leading to furfuryl alcohol formation (F-CH₂O + H → F-CH₂OH), is the rate-determining step. For 2-methylfuran formation from furfuryl alcohol dissociation into surface alkyl (F-CH₂) and OH groups, H₂O formation is the rate-determining step (OH + H → H₂O). Our results explain perfectly the experimentally observed selective formation of furfuryl alcohol and the equilibrium of furfural/furfuryl alcohol conversion under hydrogen-rich conditions as well as the effect of H₂O suppressing furfural conversion. In addition, it is found that dispersion correction (PBE-D3) overestimates the adsorption energies of furfural, furfuryl alcohol, and 2-methylfuran considerably, whereas those of H₂ and H₂O can be reproduced nearly quantitatively. Our results provide insights into Cu-catalyzed furfural selective conversion and broaden our fundamental understanding into deoxygenation reactions of oxygenates involved in the refining of biomass-derived oils

Copper Promotion in CO Adsorption and Dissociation on the Fe(100) Surface

Spin-polarized density functional theory computations have been carried out to study the adsorption and dissociation of CO on clean as well as <i>n</i>Cu-adsorbed and <i>n</i>Cu-substituted Fe(100) surfaces (<i>n</i> = 1–3) at different coverage to explore the Cu promotion effect in CO activation. Increasing Cu content not only lowers CO dissociation energies but also increases CO dissociation barriers as well as making CO dissociation thermodynamically less favorable, and the clean Fe(100) surface is most active in CO adsorption and dissociation. The <i>n</i>Cu-substituted Fe(100) surface can suppress CO adsorption and dissociation more strongly than the <i>n</i>Cu-adsorbed Fe(100) surface. CO stretching frequencies at different coverages have been computed for assisting experimental investigations