David O. Hayward

Effects of microwave dielectric heating on heterogeneous catalysis

The effect of microwave dielectric heating on both endothermic and exothermic reactions was investigated. Apparent equilibrium shifts for both reactions were observed which were attributed to the formation of spatial hot spots in the catalyst bed. The possible location of remarkable temperature gradients was examined experimentally and theoretically.

Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts

The catalytic reduction of sulfur dioxide with methane to form carbon dioxide and sulfur has been studied over MoS2/Al2O3 catalysts. The reaction has been found to occur with microwave (2.45 GHz) heating at recorded temperatures as much as 200 ◦ C lower than those required when conventional heating was used. An activation energy of 117 kJ mol −1 has been calculated for the conventionally heated reaction, but an Arrhenius analysis of the data obtained with microwave heating was not possible, probably because of temperature variations in the catalyst bed. The existence of hot spots in the catalysts heated by microwave radiation has been verified by the detection of -alumina at a recorded temperature some 200 ◦ C lower than the temperature at which the -t o-alumina phase transition is normally observed. Among four catalysts prepared in different ways, a mechanically mixed catalyst showed the highest conversion of SO2 and CH4 for microwave heating at a given temperature. Supported catalysts, sulfided either by conventional heating or under microwave conditions, showed little difference in the extent of SO2 and CH4 conversions. The highest conversions to carbon dioxide and sulfur, combined with low production of undesirable side products, was obtained when the molar ratio of SO2 to CH4 was equal to two, the stoichiometric ratio.

Carbon Dioxide Reforming of Methane with Pt Catalysts Using Microwave Dielectric Heating

Microwave heating was applied to the catalytic reforming reaction of methane with carbon dioxide over platinum catalysts. It was found that CO2 and CH4 conversions and the product selectivity (H2/CO) were generally higher under microwave conditions than that obtained with conventional heating at the same measured temperature. The effect of microwave heating was attributed to the formation of hot spots with higher temperature than that measured in the bulk catalyst bed.

Oscillatory behaviour during the oxidation of methane over palladium metal catalysts

Oscillatory reactions over palladium foil and wire catalysts during the oxidation of methane have been investigated over a wide range of reaction temperatures and argon/methane/oxygen feed gas compositions. Characterisation of the catalyst has also been carried out using scanning electron microscopy (SEM) techniques, which revealed the presence of a porous surface. This suggested that the metal surface has undergone a change since the reaction commenced, and using X-ray powder diffraction (XRD) techniques the palladium phase was shown to be the dominant phase present. Hysteresis phenomena were observed in the activity of the reaction as the temperature was cycled up and down, showing that the metal surface was continually changing throughout the reaction. The activation energies of the reaction during the high reactivity mode, PdO, and low reactivity mode, Pd, were also calculated. Oscillation rates were observed to depend on the dominant surface. Oscillations were frequent when the high reactivity mode was dominant while the activation energy of this mode was found to be low. When the low reactivity mode was dominant, the oscillations were slower and the activation energy was three times larger. The results obtained imply that the behaviour of the palladium surface, switching back and forth from the reduced state to the oxidised state, is responsible for the oscillatory behaviour seen in this system.

Further Studies on Oscillations over Nickel Wires During the Partial Oxidation of Methane

Oscillatory reactions over nickel wires during the partial oxidation of methane were investigated in a tubular continuous flow reactor made of quartz at atmospheric pressure. A modified thermocouple was designed to measure the temperature while the interposed nickel coil worked as a catalyst. Significant effects on the oscillations were observed by varying the system temperature and the feed gas composition, and by cutting off one of the reactant gases temporarily.

Rate oscillations during partial oxidation of methane over chromel–alumel thermocouples

A chromel–alumel thermocouple has been found to catalyse the methane/oxygen reaction, the main products being CO, H2 with some CO2 and H2O. Regular oscillations in both reactants, products and temperature have been observed at temperatures around 700 ○C. Similar behaviour has been obtained using nickel wires.

Oscillatory Behavior During the Partial Oxidation of Methane Over Nickel Foils

The study of the partial oxidation of methane over nickel foils has been carried out over a wide range of reactor temperatures (748-930 °C) and feed-gas compositions (Ar/CH4/O2), ranging from 30:29:1 to 30:12:18 cm3 min-1 at atmospheric pressure. The product formation shows pronounced oscillations. A modified thermocouple was designed to measure the temperature while the interposed nickel foil worked as a catalyst. The oscillations have been attributed to the cyclic reduction and oxidation of the nickel surface in the specific reaction environment at elevated temperatures.

Oscillatory behaviour during the partial oxidation of methane over cobalt wires and foils

The oscillatory behaviour over cobalt wire and foil catalysts was examined at atmospheric pressure under various reaction temperatures and argon/methane/oxygen feed gas compositions for the partial oxidation of methane. Rough and porous oxide layers on the catalyst surface were confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The results obtained from the experimental studies indicated that the oscillatory behaviour exhibited during methane oxidation was related to the behaviour of the catalyst surface switching back and forth from the reduced state to the oxidised state. The reduction of the oxide layer followed by the almost immediate re-oxidation of the metal can be accounted for only if the oxide layer formed by oxidation is different from the oxide layer existing during reduction.

Dielectric properties of MoS2 and Pt catalysts: effects of temperature and microwave frequency

The effects of temperature and microwave frequency on the dielectric properties of MoS2 and Pt catalysts together with an aluminum oxide support were investigated. Dielectric constants and dielectric loss factors were measured in a temperature range of 200-800 °C by a cavity resonator technique with a cylindrical copper cavity resonating in TM0n0 modes (n = 1, 2, ... ,5), which corresponded to frequencies of 0.615, 1.413, 2.216, 3.020, and 3.825 GHz.