Catalytic Nitrous Oxide Abatement

Nitrous oxide (N2O) is a non-flammable gas with important applications in fields ranging from anaesthesia to high power rocketry. It is a greenhouse gas, however, with a global warming potential (GWP) 264 times that of CO2 and also known to deplete stratospheric ozone. The environmental emissions of this useful chemical therefore need to be carefully controlled.
The electronics industry uses significant quantities of N2O, commonly as an oxidant in the deposition of silicon-based dielectric layers, and sales to this sector are predicted to double in the next 5 years. As a result, there is an urgent need to develop new technologies that can mitigate the environmental impact associated with the expected increase in use. In the chemical vapour deposition of silicon dioxide and related materials for microprocessor and flat panel display manufacture, N2O is introduced into the process chamber, under vacuum, where it reacts with a silicon precursor on the surface of the wafer to form a dielectric layer. Excess gas is then pumped away and treated prior to release into the environment. Typically, this treatment involves a heating step followed by wet scrubbing, which is effective for reducing concentrations of most process gases and associated by-products to below threshold values. In the case of N2O, however, using a combustor is less effective as this serves to convert a few percent of the N2O to NO and NO2, which are also pollutants and not sufficiently soluble to be wet scrubbed. An alternative solution for N2O removal is therefore required. All oxides of nitrogen can be chemically reduced to nitrogen and an oxide that is representative of the reducing agent. These fall into two classes: non-selective reductants, which also react with oxygen, and selective reductants which do not. Ammonia is typically employed as the reagent in both catalytic and non-catalytic selective reduction of NO and NO2. However, to perform this reaction at point of use (the process chamber exhaust) is problematic as it requires piping the chemical to many sites within the facility, which incurs significant extra cost. Furthermore, area or site-wide abatement is inappropriate, as there exists a risk that incompatible chemicals will react in the ducting on way to the process facility. A particularly attractive alternative is the catalytic decomposition of N2O to nitrogen and oxygen, as this requires no additional reagents and forms innocuous molecules. Numerous studies have been conducted on the catalytic decomposition of NO, as this constitutes the major component (90-95%) of the total NOx formed from combustion of fossil fuels. In comparison, the study of this reaction with N2O is virtually unexplored, but is likely to proceed by a similar mechanism. We propose to explore and better understand the heterogeneous catalytic decomposition of N2O on a range of simple, binary and complex metal oxides/fluorides, as well as zeolites and metal-organic frameworks. These studies will be focused on developing a technology that will either replace or compliment the current point-of-use abatement systems that are employed in the semiconductor and flat panel display industries. Furthermore, they may also be applicable to the reduction of NOx from fossil fuel combustion.