The Effect of nitric oxide on the photocatalytic oxidation of small hydrocarbons over titania

The catalytic UV photo-oxidation of NO in the absence and presence of ethane, ethene, propane, propene, and n-butane over TiO2 in the presence of excess oxygen was studied in the temperature range 21–150 °C. It was confirmed in our system that in the absence of hydrocarbon NO was photocatalytically oxidised by oxygen to NO2 over TiO2 and was strongly absorbed. Both NO and hydrocarbon could be simultaneously photo-converted with the conversion varying considerably with both NO and hydrocarbon concentration and the nature of the hydrocarbon. In some instances the presence of NO in the feed gas enhanced hydrocarbon oxidation via reactions involving NO2 that is a powerful oxidant. The extent of this effect depended on the relative strengths of adsorption on TiO2 of the reactants and products. To reduce surface coverage of hydrocarbon most reactions were run at 150 °C, and it was shown that at this temperature NOx adsorbed on titania could be reduced by photogenerated hydrocarbon surface species to N2O and N2 under these conditions. The formation of N2 was confirmed using 15NO with helium as carrier gas. By contrast, at room temperature in the presence of propene NO was converted to NO2.