David Chadwick

Direct synthesis of hydrogen peroxide from H2 and O2 using Pd and Au catalysts

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of supported metal catalysts is described and discussed. A detailed study of the factors influencing the formation and decomposition of hydrogen peroxide is presented for a Pd/sulfonated carbon catalyst in a methanol/water solvent. The use of low temperatures (1–2 °C) and short reaction (residence) time are identified as the key factors that favour high selectivity to hydrogen peroxide. Decomposition of hydrogen peroxide, mainly via further hydrogenation, prevents the formation of high concentrations of hydrogen peroxide. Combustion of hydrogen to water is a competing reaction that becomes significant at higher temperatures, but this can be partially inhibited by the addition of HBr. A second set of supported Pd and Au catalysts are evaluated for the direct synthesis of hydrogen peroxide using supercritical CO2 as a solvent. The use of supercritical CO2 is shown to be beneficial when compared with hydrogen peroxide formation at a temperature just below the critical temperature for CO2. However, at the critical temperature of CO2 (31.1 °C), the decomposition of hydrogen peroxide is rapid and only low rates of hydrogen peroxide formation are observed. At low temperature (2 °C) supported Au catalysts are shown to be very selective for the synthesis of hydrogen peroxide. The rate of hydrogen peroxide synthesis is enhanced markedly when Pd is present with Au and a detailed scanning transmission electron microscopy study shows that the 2–9 nm metal nanoparticles present in this supported catalyst are a Au∶Pd alloy.

Modelling of an indirect internal reforming solid oxide fuel cell

Creation of an autothermal system by coupling an endothermic to an exothermic reaction demands the matching of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal reforming (IIR) of methane, whereby the endothermic steam reforming reaction is thermally coupled to the exothermic oxidation reactions. A steady-state model of an IIR-SOFC has been developed to study the mismatch between the thermal load associated with the rate of steam reforming at typical SOFC temperatures and the local amount of heat available from the fuel cell reactions. Results have shown a local cooling effect, undesirable for ceramic fuel cells, close to the reformer entrance. The system behaviour towards changes in catalyst activity, fuel inlet temperature, current density, and operating pressure has been studied. Increasing the operating pressure is shown to be an effective way of reducing both the local cooling caused by the reforming reactions and the overall temperature increase across the cell. Simulations for both counter-flow and co-flow configurations have been performed and compared.

Kinetics and modelling of dimethyl ether synthesis from synthesis gas

The kinetics of the dual catalytic methanol and dimethyl ether (DME) synthesis process over a commercial CuO/ZnO/Al2O3 (methanol forming) and a γ-alumina (dehydration) catalyst have been investigated at 250°C and 5 MPa using a gradientless, internal-recycle-type reactor. A kinetic model for the combined methanol+DME synthesis based on a methanol synthesis model proposed by Vanden Bussche and Froment (1996) J. Catal., 161, 1–10) and a methanol dehydration model by Bercic and Levec (1993) Ind. Engng Chem. Res., 31, 1035–1040) has been tested using results obtained from a wide range of CO2 : CO feed ratios. Results at different COx : H2 ratio and catalyst loading ratios were also obtained. Catalyst deactivation was observed during DME synthesis at high space velocities and a large ratio of dehydration catalyst.

Layered double hydroxides supported on multi-walled carbon nanotubes: preparation and CO2 adsorption characteristics

Layered double hydroxides (LDHs) are promising materials for CO2 sorption, although improvements in performance are required for practical applications. In the current study, the CO2 sorption capacity and multi-cycle stability were both increased by introducing an open supporting framework of multi-walled carbon nanotubes (MWNTs). This nanostructured inert network provides a high surface area, maximizing the gas accessibility and minimizing coarsening effects. Specifically, LDH nanoparticles were precipitated directly onto MWNTs, initially oxidised to ensure a favourable electrostatic interaction and hence a good dispersion. The dependence of the structural and physical properties of the Mg–Al LDH grown on MWNT supports has been studied, using electron microscopy, X-ray diffraction, thermogravimetric analysis (TGA), and BET surface area, and correlated with the CO2 sorption capacity, established via TGA and temperature programmed desorption measurements. The use of a MWNT support was found to improve the absolute capacity and cycle stability of the hybrid adsorbent under dry conditions.

Hydrogenation of carbon dioxide to methanol over palladium-promoted Cu/ZnO/A12O3 catalysts

Abstract The effect of Pd on a Cu/ZnO/A1 2 O 3 catalyst for methanol synthesis from CO 2 /H 2 has been investigated. Activities of impregnated catalysts and physical mixtures were studied in an internal recycle reactor under 5 MPa, 250°C and a range of conversions. In all cases, the promotion of methanol production was greater at higher flow rates (lower conversions). The promotion achieved by use of Pd/A1 2 O 3 + Cu/ZnO/Al 2 O 3 physical mixtures was found to increase with Pd content. Greater promotion was observed over the Pd impregnated Cu/ZnO/Al 2 O 3 catalysts, although this was insensitive to the particular Pd loadings used. The results are consistent with the proposal that hydrogen spillover is responsible for the observed promotion. The effectiveness of Pd as a promoter for the reduction of CuO in the catalysts was studied by TPR and was found to be related to the level of promotion in methanol production.

Methane conversion over Nb-doped ceria

Methane steam reforming and dry methane conversion over ceria, and ceria doped with 1.4 and 5% Nb cation has been investigated at 900 °C (a typical solid oxide fuel cell temperature). The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. Use of reducing conditions leads to significantly lower crystallite size, higher specific surface area and greater Nb solubility. Nb-doping lowers activity mainly as a consequence of strong segregation of Nb to the ceria surface. Kinetics of steam reforming are interpreted in terms of a redox mechanism.

One-step dehydration and isomerisation of n-butanol to iso-butene over zeolite catalysts

The efficient one-step conversion of n-butanol to iso-butene over zeolite catalysts by combined dehydration and isomerisation has been demonstrated. The medium pore-size unidirectional channel zeolites Theta-1 and ZSM-23 show high conversion and stable selectivity to iso-butene.

The influence of point defects on the resistance of ceria to carbon deposition in hydrocarbon catalysis

The reactivity of methane with ceria, doped with either 10 cation% Gd, 1.4 cation% Nb, 5 cation% Nb, or undoped has been investigated using micro-reactor techniques. Temperature programmed reduction (TPR) was carried out in a mixture of 5% hydrogen in argon and both temperature programmed reaction (TPRx) and isothermal reaction at 900°C were carried out in 5% methane in argon. Oxidation of deposited carbon was investigated by temperature programmed oxidation (TPO) in 10% oxygen in helium. All cerias had good catalytic activity for conversion of methane to hydrogen and carbon monoxide until oxygen exhaustion. Exposure to methane resulted in only small amounts of carbon deposition. Doping was found to have a relatively minor effect on reaction with methane, but Nb-doped ceria was capable of oxidizing carbon deposits at lower temperatures than undoped or Gd-doped ceria.

Influence Of Phosphorus On The Hds Activity Of Ni-Mo/γ-al2O3 Catalysts

A series of coimpregnated P-Ni-Mo/γ-Al 2 O 3 catalysts have been prepared with various phosphorus loadings and their activities for thiophene HDS measured. HDS activity is found to increase with phosphorus content reaching a maximum at about 1% wt P. The catalysts have been characterised by a number of techniques including XPS. XPS studies show that the phosphorus is in monolayer form and that it influences the repartition of Mo in the catalysts. No evidence was found for the involvement of phosphorus in a sulphide species.

The importance of water in the mechanism of methanol decomposition on ZnO

The decomposition of methanol on ZnO has been investigated by temperature programmed desorption. The main decomposition products were CO and H2 with only a small amount of CO2. The influence of water on methanol decomposition has been studied by a series of quantitative co-adsorption experiments. As the amount of coadsorbed water was increased, the CO yield decreased whereas that of the CO2 increased and went through a maximum. This indicated that water and methanol competed for the same adsorption sites. The results suggest that methanol (or dehydrogenated intermediate) reacts with coadsorbed water to form CO2 and H2. Preadsorbed or residual water give similar results.