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The activity and selectivity of Pd catalysts supported on YSZ and dosed with different amounts of Na promoter has been investigated for the reduction of NO by alkenes (C 3H6) or alkanes (CH4). It is found that Na strongly promotes the reduction of NO by C 3H6. Rate increases by an order of magnitude are achievable, while the N2-selectivity is improved from ~75% over the unpromoted Pd catalyst to >95% over the optimally Na-promoted catalyst. With CH 4 as the reductant, a very different behaviour is observed: for all loadings, Na induces only poisoning. The experimental data indicate that Na increases the strength of NO chemisorption relative to the hydrocarbons. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is proposed as the critical reaction-initiating step. According to this model the promoting or poisoning effect of Na depends on the interaction strength of the catalyst surface with the hydrocarbon. The different behaviour of propene and methane reflects the weaker interaction of alkanes with metal surfaces compared to that of alkenes. XPS and Auger data demonstrate that Na coverage increases monotonically with promoter loading and that there is no significant tendency for the promoter to agglomerate with increasing promoter loading. However, a very small but constant tendency to accumulate subsurface or dissolved Na was observed as the promoter loading is increased.

Promotion by Sodium in emission control catalysis: The difference between alkanes and alkenes in the Pd-catalysed reduction of NO by hydrocarbons