Selective hydrogenation of CO on Fe3S4{111}: a computational study

Fischer-Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as a FT catalyst, among them, iron presented several phases under reaction conditions, oxide and carbides, as active sites for the FT and reverse water gas shift reaction. We present the CO reduction on an iron sulfide phase with spinel structure, Fe3S4, considering also the pathways where C-O dissociates leaving CHX species on the surface, which may feed longer aliphatic chains via FT process. We analysed the thermodynamic and kinetic availability of each step leading to O, OH species co-adsorbed on the surface as well as the formation of H2O from the hydrogenation of the alcohol group in the molecule. This detailed analysis let to energy profiles, on both active sites of the surface, and conclude that this Fe3S4 surface is high selectivity towards the formation of methanol, in full agreement with experimental results. These findings point out that the C-C bond formation on greigite takes place through an hydroxycarbene FT mechanism

Structures and properties of phosphate-based bioactive glasses from computer simulation: a review

Phosphate-based bioactive glasses (PBGs) dissolve harmlessly in the body with a dissolution rate which depends sensitively on composition. This makes them proposed vectors for e.g. drug delivery, or other applications where an active component needs to be delivered at a therapeutically appropriate rate. Molecular dynamics (MD) simulations provide atomic-level structural information about PBG compositions. We review recent work to show that MD is an excellent tool to unravel the connections between the PBG glass composition, its atomic structure, and its dissolution rate, which can help to optimise PBGs for specific medical applications

Liquid phase hydrogenation of CO2 to formate using palladium and ruthenium nanoparticles supported on molybdenum carbide

We report the development of palladium nanoparticles supported on Mo2C as an active catalyst for the liquid-phase hydrogenation of CO2 to formate under mild reaction conditions (100 °C and 2.0 MPa of a 1:1 CO2:H2 mixture). A series of Pd/Mo2C catalysts were synthesised via the modified wet-impregnation (MIm) and sol-immobilization (SIm) techniques and evaluated for CO2 hydrogenation, in an aqueous 1M NaOH solution. MIm catalysts synthesised using PdCl2 dissolved in a 2M HCl solution gave the highest formate yield with turnover numbers of up to 109 after 19 h. We further report the crucial role of base and the pH of the reaction medium for the hydrogenation of CO2 to formate. Based on stability studies, electron microscopic characterisation and density functional theory calculations we found that Ru has a stronger affinity than Pd to Mo2C resulting in the development of a stable bimetallic RuPd/Mo2C catalyst for the hydrogenation of CO2 to formate salt

Surface simulation studies of the hydration of white rust Fe(OH)2, goethite [alpha]-FeO(OH) and hematite [alpha]-Fe2O3

Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)2, α-FeO(OH) and α-Fe2O3. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol−1 for Fe(OH)2, 26 to 80 kJ mol−1 for goethite and 40 to 85 kJ mol−1 for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol−1 for goethite and 76 to 190 kJ mol−1 for hematite. The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars

A computer modelling study of the uptake, structure and distribution of carbonate defects in hydroxy-apatite

Computer modelling techniques have been employed to qualitatively and quantitatively investigate the uptake and distribution of carbonate groups in the hydroxyapatite lattice. Two substitutional defects are considered: the type-A defect, where the carbonate group is located in the hydroxy channel, and the type-B defect, where the carbonate group is located at the position of a phosphate group. A combined type A–B defect is also considered and different charge compensations have been taken into account. The lowest energy configuration of the A-type carbonate has the O–C–O axis aligned with the channel in the c-direction of the apatite lattice and the third oxygen atom lying in the a/b plane. The orientation of the carbonate of the B-type defect is strongly affected by the composition of the apatite material, varying from a position (almost) flat in the a/b plane to being orientated with its plane in the b/c plane. However, Ca–O interactions are always maximised and charge compensating ions are located near the carbonate ion. When we make a direct comparison of the energies per substitutional carbonate group, the results of the different defect simulations show that the type-A defect where two hydroxy groups are replaced by one carbonate group is energetically preferred View the MathML source, followed by the combined A–B defect, where both a phosphate and a hydroxy group are replaced by two carbonate groups View the MathML source. The type-B defect, where we have replaced a phosphate group by both a carbonate group and another hydroxy group in the same location is energetically neutral View the MathML source, but when the replacement of the phosphate group by a carbonate is charge compensated by the substitution of a sodium or potassium ion for a calcium ion, the resulting type-B defect is energetically favourable View the MathML source and its formation is also promoted by A-type defects present in the lattice. Our simulations suggest that it is energetically possible for all substitutions to occur, which are calculated as ion-exchange reactions from aqueous solution. Carbonate defects are widely found in biological hydroxy-apatite and our simulations, showing that incorporation of carbonate from solution into the hydroxyapatite lattice is thermodynamically feasible, hence agree with experiment

A computer modeling study of redox processes on the FeSbO4 (100) surface

Computer modeling techniques, based on the density functional theory, were used to investigate the oxidation–reduction behavior of the (100) surface of iron antimony oxide, FeSbO4. We calculated the geometries and stabilities of different surface compositions with deficiency or excess of oxygen and also with two different distributions of cations, a bulk-like distribution with Fe and Sb alternation in the [001] direction and a segregated distribution with an Sb-rich surface. We found that the creation of oxygen vacancies in the surface led to the formation of surface Sb3+ species, with lone pairs pointing at the vacancy site. The electrons localized on the iron to form Fe2+ species only if there were no Sb5+ cations present to reduce preferentially. The formation of oxygen vacancies was more favorable on the Sb-segregated surface than on the nonsegregated surface because of the strong relaxation that occurs in the former on vacancy formation. Molecular oxygen can adsorb at an oxygen vacancy in the form of a peroxy ion, taking the electrons from the reduced Sb3+ cation near the vacancy. But this adsorbed peroxy species is only metastable, as the oxidized surface is thermodynamically unfavorable with respect to the stoichiometric surface

Mechanisms of carbon dioxide reduction on strontium titanate perovskites

Strontium titanate (SrTiO3) is a promising material for the light-driven conversion of carbon dioxide (CO2) into renewable fuels. However, the mechanisms of the relevant reactions are not yet well understood. In this work, we have used density functional theory calculations to explore CO2 reduction on the (001) surface of the SrTiO3 photocatalyst. Our results indicate that, in contrast to COOH, the formation of a HCOO or CO2− intermediate is thermodynamically hindered, which is consistent with the fact that formic acid (HCOOH) is not a major product in the experiments reported in the literature. We show that a pathway to carbon monoxide (CO) is instead possible, and that the formation of COOH is the rate-limiting step. Finally, we suggest that substitutional doping of Sr ions represents a promising approach to lower the energy barrier of the COOH formation