Transition metal modified titanium dioxide photocatalysts for the removal of nitric oxide
Photocatalytic NO decomposition and reduction reactions, using CO as a reducing gas, have been investigated over Ti02, Ag-Ti02 and Rh-Ti02 photocatalysts, using a purpose built continuous flow photoreactor. The transition metal modified Ti02 photocatalysts were prepared using wet impregnation techniques, and the effect of thermal processing parameters on their photocatalytic behaviour was studied. Prepared photocatalysts were characterised using a number of complementary techniques, including XRD, TEM, DSC, and XPS. The findings from these techniques were used to explain the observed photocatalytic properties. The activity and selectivity of the photocatalysts were found to be dependant on a number of factors; thermal pretreatment temperature, type and amount of the modifying element, chemical nature of the modifying element and the reaction conditions used. It was found, for Ti02 photocatalysts, that increasing the pretreatment calcination temperature resulted in lower NO conversion rates, due to removal of surface bound hydroxyl groups. A similar trend was observed for Ag-P25 photocatalysts, but the reduction in activity was greater due to the presence of larger silver clusters, which acted as recombination centres for photogenerated electron-hole pairs. The activity of the Ag-P25 photocatalysts decreased as the silver loadings increased, whilst the activity of the Rh-P25 photocatalysts remained largely unaffected by the metal concentration. Over Ti02 and Ag-Ti02 systems, the NO conversion rate was lower for the reduction reactions compared to decomposition reactions. This was attributed to the preferential adsorption of the CO molecules, blocking NO adsorption sites. Contrasting behaviour was observed over Rh-P25 systems and NO conversions as high as 87 % were recorded in the presence of CO. Silver modified catalysts were highly selective for N2 formation (90 %) whilst rhodium modified catalysts were more selective for N20 formation. These results are discussed with respect to the possible surface reactions and the chemical intermediates that may be formed.