Catalytic reduction of nitric oxide: Studies using ammonia and methane as reducing agents

Nitric oxide emissions are responsible for acid rain and vegetation decline. Reduction of nitric oxide emissions from stationary and mobile sources continues to be an important challenge for catalyst scientists. While several researchers have investigated the current NOx reduction technology, i.e. selective catalytic reduction (SCR) using ammonia as a reducing agent, there is also an increasing interest for alternative reducing agents, such as hydrocarbons, that would replace ammonia in the present technology. Methane being most abundant and cheaply available offers a very lucrative alternative.; In this study ammonia and methane have been investigated as reducing agents for nitric oxide reduction. Vanadia/titania catalyst was used for NO reduction with ammonia as a reducing agent while Pd/titania catalyst was used for usage of methane as a reducing agent. A steady-state fixed bed reactor system was used for activity and selectivity measurements. Identification and quantification reaction effluents were achieved using an analytical scheme that combined gas chromatography, mass spectrometry, chemiluminescence, and wet chemistry techniques.; Catalysts used in this study were characterized using BET surface area technique, XRD, XPS, Raman spectroscopy, SEM, 3-D imaging, and thermal analysis techniques. Steady state as well as unsteady state isotopic transient tracer techniques were used to investigate the mechanism of NO reduction reactions. Activity and selectivity data was combined with the characterization results to obtain a fundamental understanding of the reaction pathways.; Based on the isotopic tracer studies using labeled N-15 and O-18 over vanadia/titania catalysts, it was concluded that there were at least two types of sites available for ammonia adsorption. The stronger sites (longer surface life) led to the formation of nitrogen whereas the sites with shorter surface life led to the formation of nitric oxide and nitrous oxide. Over Pd/titania catalysts it was realized that conversion of oxidic phase to metallic phase was the key to NO reduction activity with methane as a reducing agent. Based on the isotopic labeling studies it was concluded that nitrogen formation went through direct participation of methane whereas nitrous oxide was formed from decomposition of NO through a dimer-type of species. Oscillatory behavior of the reaction was observed under certain conditions investigated and was explained through the cyclic transformation between the metallic and oxidic phases of palladium.