Helen Daly

Interaction of water, hydrogen and their mixtures with SnO2 based materials: the role of surface hydroxyl groups in detection mechanisms

DRIFTS, TGA and resistance measurements have been used to study the mechanism of water and hydrogen interaction accompanied by a resistance change (sensor signal) of blank and Pd doped SnO(2). It was found that a highly hydroxylated surface of blank SnO(2) reacts with gases through bridging hydroxyl groups, whereas the Pd doped materials interact with hydrogen and water through bridging oxygen. In the case of blank SnO(2) the sensor signal maximum towards H(2) in dry air (R(0)/R(g)) is observed at approximately 345 degrees C, and towards water, at approximately 180 degrees C, which results in high selectivity to hydrogen in the presence of water vapors (minor humidity effect). In contrast, on doping with Pd the response to hydrogen in dry air and to water occurred in the same temperature region (ca. 140 degrees C) leading to low selectivity with a high effect of humidity. An increase in water concentration in the gas phase changes the hydrogen interaction mechanism of Pd doped materials, while that of blank SnO(2) is unchanged. The interaction of hydrogen with the catalyst doped SnO(2) occurs predominantly through hydroxyl groups when the volumetric concentration of water in the gas phase is higher than that of H(2) by a factor of 1000.

Heterogeneous chemistry of toluene, kerosene and diesel soots.

Soot samples as potential mimics of atmospheric aerosols have been produced from the combustion of toluene, kerosene and diesel in order to compare the nature of soot produced from a simpler material, toluene, with soots from the fuels kerosene and diesel. Characterisation of the soots using elemental analysis, infrared spectroscopy, solvent extraction, thermal desorption and electron microscopy techniques before and after reaction with ozone allows assessment of the reactivity of soots from these different fuels. Despite the production of toluene and kerosene soots from identical combustion conditions, strong differences in structure and reactivity are observed in terms of their reaction with ozone. However, toluene soot is a much better mimic of diesel soot. It is proposed that the differing reactivities of the soots is related to the nature of the organic carbon and structure of the elemental carbon which vary with soots from the different fuels.

Improved Efficiency for Partial Oxidation of Methane by Controlled Copper Deposition on Surface‐Modified ZSM‐5

The mono(μ‐oxo) dicopper cores present in the pores of Cu‐ZSM‐5 are active for the partial oxidation of methane to methanol. However, copper on the external surface reduces the ratio of active, selective sites to unselective sites. More efficient catalysts are obtained by controlling the copper deposition during synthesis. Herein, the external exchange sites of ZSM‐5 samples were passivated by bis(trimethylsilyl) trifluoroacetamide (BSTFA) followed by calcination, promoting selective deposition of intraporous copper during aqueous copper ion exchange. At an optimum level of 1–2 wt % SiO2, IR studies showed a 64 % relative reduction in external copper species and temperature‐programmed oxidation analysis showed an associated increase in the formation of methanol compared with unmodified Cu‐ZSM‐5 samples. It is, therefore, reported that the modified zeolites contained a significantly higher proportion of active, selective copper species than their unmodified counterparts with activity for partial methane oxidation to methanol.

Continuous flow gas phase photoreforming of methanol at elevated reaction temperatures sensitised by Pt/TiO2

Gas phase photoreforming of methanol using a Pt/TiO2 photocatalyst has been performed under flow conditions at elevated temperatures. Comparing the activity of the reforming process as a function of temperature under dark and irradiated conditions shows a significant enhancement in the rate of H2 production using the photo-assisted conditions at temperatures between 100–140 °C. At higher temperatures, the effect of irradiation is small with the process dominated by the thermal process. Deactivation of the catalyst was observed under irradiation but the catalyst was easily regenerated using an oxygen treatment at 120 °C. Diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) showed that the activity of the catalyst could be correlated with the presence of the photogenerated trapped electrons. In addition, lower amounts of CO adsorbed on Pt, compared to those observed in the dark reaction, were found for the UV-irradiated systems. It is proposed that CO and adsorbed intermediates, such as formate, can act as inhibitors in the photoreforming process and this is further supported by the observation that, before and after the regeneration process in O2, the CO and surface adsorbed organic intermediate products are removed and the activity is recovered.