Philip C. H. Mitchell

Vibrational spectroscopy with neutrons: with applications in chemistry, biology, materials science and catalysis

The Theory of Inelastic Neutron Scattering Spectroscopy Instrumentation and Experimental Methods Computational Modelling of Inelastic Neutron Scattering Spectra Interpreting Inelastic Neutron Scattering Spectra Dihydrogen and Hydrides Hydrogen Bonding Non-Hydrogenous Materials and Carbon Soft Condensed Matter: Polymers and Biology Vibrations of Atoms in Surfaces and Adsorbed Species Organic and Organometallic Compounds.

Ru/alumina and Ru−Mo/alumina catalysts: an XPS study

RuAl2O3 and RuMoAl2O3 catalysts have been prepared by impregnating respectively γ-Al2O3 and calcined MoAl2O3 with aqueous solutions of ruthenium (III) acetate. XPS spectra of catalysts dried at 393 K, calcined in air at 873 K, and sulfided in H2H2S are reported. Ru and Mo species in the catalysts, according to the respective binding energies, were ruthenium dioxide and oxomolybdenum (VI) in catalysts calcined at 873 K and a ruthenium sulfide and MoS2 in the sulfided catalysts. Unusually low Ru binding energies were found for some catalysts. The properties of the catalysts in thiophene HDS are interpreted with the aid of the XPS results.

Hydrolysis of the oxovanadium(IV) ion and the stability of its complexes with the 1,2-dihydroxybenzenato(2–) ion

A technique which eliminates effects of changes in asymmetry potential has been adapted for use in pH titrations with high-resistance glass electrodes. A study, using cells without liquid junctions, has been made of the hydrolysis of the oxovanadium(IV) ion and of its complexes with the 1,2-dihydroxybenzenato(2–) ion. Equilibrium constants are reported.

Reactions of carbon monoxide with hydrogen over molybdenum/charcoal catalysts

Abstract Molybdenum-on-charcoal catalysts have been prepared and characterised and their activities and selectivities determined for the reaction of carbon monoxide with hydrogen at temperatures in the range 573 to 673 K, and at pressures from 0.16 to 1.6 MPa. The catalysts have been activated either by calcination under nitrogen at 773 K followed by heating in hydrogen at 773 K, or by reduction in hydrogen at 1073 K. The former procedure led to the formation of MoO 2 as the major phase, whereas the latter resulted in the formation of Mo 2 C. The influence of potassium, either present as an impurity in the charcoal or added during preparation of the catalysts, was investigated. The results show that the activities and selectivities of the MoO 2 /C and Mo 2 C/C catalysts are comparable, which may indicate that the active sites in both catalysts are similar. Potassium was found to seriously poison both types of catalyst, a result which is in contrast to previous published observations. Comparison of the catalytic results with literature data shows that the catalysts have quite high activity and stability, but are very susceptible to impurities such as potassium. This emphasises the importance of the type of charcoal chosen as the support for molybdenum catalysts.

Bone mineral : evidence for hydroxy groups by inelastic neutron scattering

Inelastic neutron scattering (INS) spectroscopy which is especially sensitive to vibrations of groups with hydrogen has been used to resolve the extent of substitutions at the hydroxy group site in bone. Bone is a complex material consisting of a mineral apatite (Ca5(PO4)3 OH), in an extracellular protein matrix. Many aspects concerning the composition of the mineral are controversial. It has been suggested that there is complete substitution by carbonate at the hydroxy site. Bands assigned to hydroxy vibrations are often not resolved in the infrared and Raman spectra. We show, by INS, that in ox femur bone 40–50% of the hydroxy groups are not substituted. The inelastic neutron scattering experiment opens up new opportunities to monitor changes in bone composition which are important in bone ageing and some pathological conditions.

Hydrated sites in biogenic amorphous calcium phosphates: An infrared, Raman, and inelastic neutron scattering study

Amorphous minerals are widely distributed in biology, and occur as carbonates, phosphates, and silica. Calcium and magnesium are the major cations in the phosphate deposits, but in addition, there is usually an organic component, and the minerals are hydrated with up to 20% water. Such deposits are found as intracellular granules in a variety of invertebrates such as the shore crab Carcinus maenas. These intracellular granules and synthetic amorphous calcium phosphates and crystalline apatite have been studied by infrared, Raman, and inelastic neutron scattering to establish the protonation of the phosphates and the structure of the water in these deposits. Monetite and newberyite were used as model compounds for comparison. It is concluded that the water occurs in regions that are only loosely associated with the cations in these solids.

Crystal and molecular structure of three oxovanadium(IV) porphyrins: oxovanadium tetraphenylporphyrin(I), oxovanadium(IV) etioporphyrin(II) and the 1:2 adduct of (II) with 1,4-dihydroxybenzene(III) ― Hydrogen bonding involving the VO group ― Relevance to catalytic demetallisation

Abstract The structures of the three title compounds have been determined by X-ray Crystallography. (I) is tetragonal, spacegroup [4/m with Z = 2, a = 13.345(7), c = 9.745(8) A. (II) is triclinic, spacegroup P 1 , Z = 2, a = 10.52(1), b = 11.70(1), c = 12.97(1) A, α = 70.7(1), β = 113.8(1), γ = 108.8(1)°; (III) is orthorhombic, spacegroup Pnaa, Z = 8, a = 17.72(1), b = 18.14(1), c = 18.57(1) A. Data for all three compounds have been collected on a diffractometer and 556 (I), 2316 (II), 1263 (III) above background reflections have been refined to R (0.099, 0.086, 0.082) respectively. (I) has disordered C4h symmetry following a well-known structure type. The features of the oxovanadium(IV) etioporphyrin structures in (II) and (III) are similar. Compound (III), the adduct with 1,4-dihydroxybenzene (quinol, H2Q) [VO(ETP)]2[H2Q], which crystallised from a solution of [VO(ETP)] in tetrahydrofuran stabilized with O.1% quinol, consists of [VO(ETP)] molecules linked by H-bonds through the VO groups to the quinol OH groups. We discuss the general phenomenon of H-bonding to VO groups and its possible relevance to binding of VO porphyrins to OH groups at catalyst surfaces.

Experimental and theoretical studies of fuel cell catalysts: Density functional theory calculations of H2 dissociation and CO chemisorption on fuel cell metal dimers

Abstract Gradient-corrected density functional theory (GC-DFT) calculations have been performed for molecular hydrogen, carbon monoxide and the metal dimers PtPt, PtNi, and PtRu. The dissociative adsorption of molecular hydrogen on these metal dimers has been modelled. Derived potential energy surfaces for the dissociation of molecular hydrogen on these dimers are presented. Furthermore, the interaction with carbon monoxide has been studied. Here we find that Pt2 binds CO significantly stronger than PtNi and PtRu. We present equilibrium geometries, calculated binding energies, Mulliken charge distributions, and orbital energies. Our results correlate well with our experimental studies of the hydrogen electro-oxidation reaction in a proton-exchange fuel cell where the fuel to the anode ( Pt Pt alloy on carbon electrode) is hydrogen, prepared by reforming of methane, which contains trace amounts of carbon monoxide.

Co-Mo-Al2o3 hydrodesulphurisation catalysts: Correlation of activities with properties of the catalysts☆

Abstract The UV-visible reflectance spectra, magnetic susceptibilities and adsorption-desorption behaviour of cobalt and molybdate ions at surfaces in contact with an aqueous phase have been measured for a series of Co-Mo-Al 2 O 3 hydrodesulphurisation (HDS) catalysts. The object was to discover correlations between properties of the catalysts and their activities in thiophene desulphurisation. Two ways in which cobalt acts as a promoter are identified: (a) in the presence of cobalt, the fraction of the surface of the alumina carrier covered by molybdate increases and with it the catalytic activity and (b) the initial rate of butane formation correlates with the concentration of cobalt at the catalyst surface; cobalt promotes hydrogenation reactions during HDS. The significance of the results in terms of the formation of Co-promoted MoS 2 crystallites, which are the catalytic species, is discussed.

The interaction of vanadium and nickel porphyrins and metal-free porphyrins with molybdenum-based hydroprocessing catalysts: relevance to catalyst deactivation and catalytic demetallization

Abstract The poisoning of a Co-Mo-alumina hydrodesulphurization (HDS) catalyst by metal-free, vanadium and nickel etio- and tetraphenyl porphyrins and the binding and reactivity of the porphyrins on oxide and sulfide surfaces is described. The surface interactions of porphyrins were studied by adsorption from solution, and UV-visible diffuse-reflectance and ESR spectroscopy. Metalloporphyrins undergo demetallation at acidic sites. The porphyrin ring is oxidized at a meso-carbon by oxomolybdenum(VI) species and adsorbed oxygen to a cation radical and an oxophlorin. During HDS porphyrin bound to the catalyst is hydrogenated at the meso-carbon to phlorin. Interaction of the porphyrins with surfaces is similar to that of polynuclear aromatic hydrocarbons. The relevance of these results to catalyst deactivation and catalytic demetallation is discussed.