Nitrogen oxides, in particular NO and NO2 (NOx), resulting from combustion processes continue to be a major source of air pollution. They contribute to photochemical smog, acid rain, ozone depletion, ground level ozone, and greenhouse effects. More than 95% of nitrogen oxide emissions are derived from two sources: ˜49% from mobile sources, such as vehicles, and ˜46% from stationary sources, such as power plants. Many technologies have been developed in an attempt to decrease such emissions.
Three-way catalysis is very effective for removing emissions from gasoline engines, where narrow band oxygen sensors afford closed loop control with an air:fuel ratio of about 14.07. Diesel engines, on the other hand, operate very lean and with a wide-band air:fuel ratio of about 14 to about 24. While diesel engines have considerable benefits to gasoline engines, due to the nature of diesel fuel and the compression ignition combustion process, diesel engines emit a high quantity of particulate matter and nitrogen oxide emissions. Many catalysts useful for gasoline engines are not suitable for use in a diesel engine exhaust stream as a wider operating temperature window is required.
The current commercially available technology for reducing NOx emissions from stationary sources is selective catalytic reduction (SCR). Ammonia (NH3) is widely accepted as the reducing agent of choice. Similar SCR technology is also effectively applied to mobile sources, where NH3 is usually generated by the thermal decomposition of urea. However, there are many commercial and logistical drawbacks, namely: (1) a separate tank and injection system is required, (2) several issues exist relating to NH3 slip, (3) the difficulty of handling urea solutions during cold conditions, and (4) as of yet, no real infrastructure exists to widely deploy the necessary urea solution. These factors indicate the desirability of the development of an active NOx reduction catalyst that makes use of other reductants, such as hydrogen. Hydrogen has been shown to be a promising reductant for NOx under lean burn conditions and will most likely be available in automobiles from fuel processors for fuel cell applications, on-board reforming of diesel fuel, or the like.