Density Functional Theory Study of the Partial Oxidation of Methane to Methanol on Au and Pd Surfaces

The partial oxidation of methane to methanol has been a goal of heterogeneous catalysis for many years. Recent experimental investigations have shown how AuPd nanoparticle catalysts can give good selectivity to methanol with only limited total oxidation of CH4 using hydrogen peroxide as an oxidant in aqueous media. Interestingly, the use of colloidal nanoparticles alone, without a support material, leads to efficient use of the oxidant and the possibility of introducing oxygen from O2(g) into the CH3O2H primary product. This observation indicates that a radical mechanism is being initiated by H2O2 but then the oxygen addition step, catalyzed by these nanoparticles, can incorporate O2(ads). In this contribution, we use density functional theory (DFT) to study the elementary steps in the partial oxidation of methane to methanol using H2O2 as a radical initiator and molecular oxygen as an oxidant over the low index surfaces of Pd and Au. We are able to show that pure Pd nanoparticles are prone to oxidation by O2(g), whereas the competitive adsorption of water on Au surfaces limits the availability of O2(ads). Calculations with Au added to Pd or vice versa show that both effects can be alleviated by using mixed metal surfaces. This provides a rationalization of the need to use alloy nanoparticles experimentally, and the insights from these results will aid future catalyst development

Au-ZSM-5 catalyses the selective oxidation of CH4 to CH3OH and CH3COOH using O2

The oxidation of methane, the main component of natural gas, to selectively form oxygenated chemical feedstocks using molecular oxygen has been a long-standing grand challenge in catalysis. Here, using gold nanoparticles supported on the zeolite ZSM-5, we introduce a method to oxidize methane to methanol and acetic acid in water at temperatures between 120 and 240 °C using molecular oxygen in the absence of any added coreductant. Electron microscopy reveals that the catalyst does not contain gold atoms or clusters, but rather gold nanoparticles are the active component, while a mechanism involving surface adsorbed species is proposed in which methanol and acetic acid are formed via parallel pathways. [Figure not available: see fulltext.]

Materials and Molecular Modelling at the Exascale

Progression of computational resources towards exascale computing makes possible simulations of unprecedented accuracy and complexity in the fields of materials and molecular modelling (MMM), allowing high fidelity in silico experiments on complex materials of real technological interest. However, this presents demanding challenges for the software used, especially the exploitation of the huge degree of parallelism available on exascale hardware, and the associated problems of developing effective workflows and data management on such platforms. As part of the UKs ExCALIBUR exascale computing initiative, the UK-led MMM Design and Development Working Group has worked with the broad MMM community to identify a set of high priority application case studies which will drive future exascale software developments. We present an overview of these case studies, categorized by the methodological challenges which will be required to realize them on exascale platforms, and discuss the exascale requirements, software challenges and impact of each application area

Analysis of bonding in cyclopentadienyl transition-metal boryl complexes

The chemical bonding in transition-metal boryl complexes of the type [(C5R5)M(CO)2BX2] was analyzed using density functional methods. The relative contributions to the metal boryl linkage from ionic and covalent interactions were quantified. Covalent terms were found to be roughly twice as important to the overall instantaneous metal-boryl interactions than were ionic terms

Carbonyl analogues? Analysis of Fe-E (E=B, Al, Ga) bonding in cationic terminal diyl complexes by density functional theory.

A series of DFT calculations has been carried out with the aim of characterizing the metal-group 13 element interaction in the novel cationic borylene complex [(eta5-C2Me5)Fe(CO)2(BMes)]+ (1) and related species of the type [eta5-C5R5)M(L)2(EX)]n+. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems (e.g. cationic metal carbonyls, carbenes and vinylidenes) for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of (i) molecular orbital composition; (ii) bond dissociation energies (BDEs) and the ratio of ionic to covalent contributions (DeltaEelstat/DeltaEorb); and (iii) sigma and pi symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1 and for related cationic and neutral systems [e.g.[(eta5-C5H5)Fe(PMe3)2(BMes)]+ and (eta5-C5H5)Mn(CO2(BMes)]] are consistent with the presence of bonding interactions between metal and borylene fragments of both sigma and pi symmetry. Furthermore, on the basis of BDEs, DeltaEorb values and sigma/pi covalent ratios, the bonding in cationic terminal borylene complexes such as 1 appears to have as much right to be termed a M=E double bond as does that in archetypal Fischer carbene and related complexes such as [(eta5-C5R5)Fe(CO)2(CCMe2)]+ and [(eta5-C5R5)Fe(CO)2(CH2)]+

Cationic terminal borylenes by halide abstraction: synthesis and spectroscopic and structural characterization of an Fe=B double bond.

The synthesis and the spectroscopic and structural characterization of the cationic terminal borylene complex [Cp*Fe(CO)2(BMes)]+ are reported. Halide abstraction from the corresponding bromoboryl species using Na[BAr f4] generates the borylene as the [BAr f4]- salt in ca. 50% yield. Analyses of IR, NMR, crystallographic, and DFT data are consistent with the presence of an Fe=B double bond

Linking of metal centres through boryl ligands: synthesis, spectroscopic and structural characterisation of a symmetrically bridged boryl complex CpFe(CO)(2)BO2C6H2O2BFe(CO)(2)Cp

The synthesis, spectroscopic and structural characterisation of the symmetrically bridged boryl complex CpFe(CO)2BO2C6H2O2BFe(CO)2Cp are reported, together with analysis of the bonding by structural and DFT methods

A group 13/group 17 analogue of CO and N2: coordinative trapping of the GaI molecule.

The first row diatomic molecules N2 and CO have played central roles in developing fundamental models of electronic structure, and their interactions with transition metals have been widely investigated. By contrast, the valence isoelectronic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a simple metal complex. Here, we find that iodide abstraction from Cp*Fe(dppe)(GaI2) yields the stable complex [Cp*Fe(dppe)(GaI)]+ which features a terminally bound GaI ligand, characterized by a near linear (but readily deformed) Fe-Ga-I geometry and by very short Fe-Ga and Ga-I distances. Chemical and computational evidence reveal a relatively weak metal-ligand bond similar in strength to that found for dinitrogen analogues