Dynamic behavior of activated carbon catalysts during ozone decomposition at room temperature
The catalytic decomposition of ozone (200-1600 ppm) to molecular oxygen was investigated over activated carbons in the form of woven fibre fabrics (ACF) or granules (ACG) at room temperature. The dynamics of carbon activity was characterised by two distinct regions. First the "high activity" towards ozone decomposition was observed, which was mainly due to chemical interaction of ozone with carbon. This interaction resulted in the formation of oxygen containing surface groups on carbon until saturation. Then the conversion was sharply decreased and carbons went to "low activity" region. The ozone decomposition to molecular oxygen takes place in this region following a catalytic route. The carbon activity in dry atmosphere was compared with the activity in the presence of water vapour and NOx Water vapour diminished the catalytic activity, but in the presence of NO, carbons were observed to be more active due to the change in the C-surface functionality. The surface functional groups were modified in two ways: by boiling in diluted HNO3 or by thermal treatment in He at temperatures up to 1273 K. The acid pre-treatment was found to increase the activity of carbons under the quasi steady-state, while the thermal treatment at 1273 K renders catalysts with lower activity. The ozone decomposition toward gasification of carbon producing CO, took place with the selectivity less then 25%. The catalysts were characterised by temperature-programmed decomposition of surface functional groups, X-ray photo-electron and IR-spectroscopy. Mechanistic aspects of the reaction are discussed.