Catalytic Reduction of NO and N2O with Hydrogen in Water by Polymer-Protected Palladium Nanoparticles

Nitric oxide (NO) and nitrous oxide (N2O) are important components of the global nitrogen cycle. They are intermediates or end products for natural bacterial processes such as nitrification and denitrification, and are formed in increasing amounts from anthropogenic sources such as increased bacterial denitrification of fertilizers. Both are contributors to ozone destruction, and N2O is a potent greenhouse gas. Reduction of both gases to harmless dinitrogen by nanoparticulate palladium catalysts is a promising method for treating NO and N2O formed in water. However, creating a better understanding of the method of catalytic activity and corresponding selectivity is critical for developing an optimal process. We have synthesized a water-soluble Pd/PVP colloid (PVP= poly(N-vinyl-2-pyrrolidone) by an alcohol reduction method. Nitric oxide and nitrous oxide reduction activities were determined by monitoring the headspace composition in a reaction flask by GC-MS. NO reduction rates were considerably faster than N2O reduction rates, and essentially no production of N2 was observed until all of the NO had been reduced to N2O. This is consistent with expectations that the binding constant of NO to the palladium nanoparticle surface is considerably larger than that of N2O. Molar balances and post-experimental ammonia analysis confirmed that no side production of ammonia was occurring. Further studies showed that no reaction occurred with an air-exposed catalyst until hydrogen added in a helium atmosphere was sufficient to remove oxygen from the surface. Hydrogen held by the Pd nanoparticles was modified by controlled heating under helium in order to confirm the need for hydrogen in N2 production from N2O