Selective catalytic reduction (SCR) of NO, by ammonia is an efficient technology for removal or nitrogen oxides from diesel engine exhaust gas. The commercial catalysts used today are V2O5-doped TiO2, using WO3 as promoter. The active component V2O5 in commercial catalysts is toxic and melts at 650 °C.
In this work an alternative catalyst system, based on iron as active component, was developed.
For this purpose a number of supports were taken and impregnated with Fe2O3 and Fe2O3 / WO3 by incipient wetness technique. All the catalysts were characterised by surface area measurement, temperature program desorption, scanning electron microscopy, energy-dispersive X-ray analysis and powder X-ray diffraction analysis. The synthesized catalysts were tested for SCR reaction of diesel model exhaust gas using temperature programmed reduction technique. After screening it was found that Fe/W/Zr catalyst system exhibits the highest SCR activity. In the temperature range between 300 and 500 °C quantitative NO, conversion was obtained. The conversion of NOx took place almost completely to nitrogen and water.
The catalysts with WO3 less than 2.3 mol% is noticeably deactivated by thermal ageing, but at contents being higher than 6.2 mol% fresh and aged material reveal the same activity. Hence, for the latter WO3 loads the catalyst is considered to be thermally stable. Prolonged heat treatment of the exhausted 1.4Fe/7.OW/Zr catalyst conducted for further 24 h at 600 °C does also not affect SCR activity. It was concluded that tungsten oxide acts as a bifunctional component. On the one hand it promotes the catalytic activity of Fe2O3, while on the other hand it reduces the susceptibility of catalyst to thermal ageing.
The SCR performance with 5 and 10 vol.-% CO2 leads to a slight decrease in the activity whereas stronger deactivation is observed when water was added to the feed.
NH3 is strongly adsorbed on the Fe/W/Zr catalyst system. As opposite to NH3, NO does not adsorb appreciably on the catalyst surface. This is in line with a reaction mechanism involving a gas phase NO and a strongly adsorbed ammonia species, according to an Eley-Rideal pathway. This was confirmed by DRIFTS analysis that NH3 is strongly adsorbed on the catalyst while NO react from the gas phase. The proposed mechanism includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH3 that leads to partial reduction of the iron as well as production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N2 and H2O. In the final step, the reduced Fe sites are regenerated by oxidation with 02. As a side reaction of SCR, imide species, originated from the decomposition of amide, are oxidized by NO2 or
O2 into NO.
A comparative study was also made on V2O5-based TiO2 with WO3 and it was found that this catalyst was significantly deactivated by thermal ageing. SCR performance of iron based alumina and zeolite ZSM-5 was also examined for comparison and it was observed that the addition of WO3 to both systems cause a decrease in SCR activity. The thermal treatment of 1.4Fe/AI results in a significant decrease in SCR activity. However in case of 2.6Fe/ZSM-5 the activity was found to improve by thermal ageing.