Cleaning With Catalysts

To eliminate these compounds, they used catalyticoxidation, i.e. using oxygen from the atmosphere and, with the help ofa catalyst, burning off the prejudicial compounds.

Catalysts, inthis process, provide a number of advantages. On the one hand, theyreduce the energy necessary for burning the contaminant compounds and,on the other, each type of contaminant gas has its specific catalyst.Nevertheless, for the combustion process to be effective, the catalystshave to comply with a series of requirements.

Where and when?

Catalystsusually have two basic components: the base and the active phase. Thebase usually involves a porous solid (in order to achieve the maximumsurface for reaction). As regards the active phase, this is thecompound that initiates the reaction, normally a metal such as platinumor palladium or, as in the case of the Leioa research, a zeolite.

Inthe process of catalysis, the contaminant gas enters the pores of thecatalyst and it is there that it reacts with the active phase. Water,carbon dioxide and a halogenated compound are produced as a result ofthis reaction. This last product is subsequently neutralised in ashower of caustic soda. Thus, only H20 and CO2 are liberated into theatmosphere.

In this case the researchers worked with zeolitecatalysts and the results obtained showed that the canals presentwithin the structure of the zeolites are of great importance in thereaction.

Zeolites may have parallel or intercrossing; and itwould seem that the zeolites with the second structure are better forthis type of reaction. This is because an obstacle in a zeolitestructure of parallel canals will prevent the gas passing through whilethis does not happen in the case of zeolites with a structure ofintercrossing canals.

The kinetics of the reaction

Butapart from the type of canal there are other characteristics thatinfluence the reaction. Very important, for example, is the amount ofcontaminant gas, the quantity of catalyst used and the temperature ofcombustion. Two more factors are the kinetics of the reaction and itsevolution.

Temperature, in fact, is one of the most importantparameters. The lower the temperature of combustion, the less will bethe energy spent.

The researchers are currently trying toestablish the durability of the catalysts, i.e. to ascertain whenactive phase compound has to be replaced or renewed so that the“cleaning” of the volatile organic compounds continues to operate.