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

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 (C3H6) or alkanes (CH4). It is found that
Na strongly promotes the reduction of NO by C3H6. 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 CH4 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.