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“Promotion by sodium in emission control catalysis: The difference betweenalkanes and alkenes in the Pd-catalysed reduction of NO by hydrocarbons

Yentekakis I.V. , Konsolakis Michail, Kiousis V. , Lambert R.M., Tikhov M.S.

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Summary

The activity and selectivity of Pd catalysts supported on YSZ and dosed with different amounts of Na promoterhas been investigated for the reduction of NO by alkenes (C3H6) or alkanes (CH4). It is found thatNa 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 theoptimally Na-promoted catalyst. With CH4 as the reductant, a very different behaviour is observed: for allloadings, Na induces only poisoning. The experimental data indicate that Na increases the strength of NOchemisorption relative to the hydrocarbons. This is accompanied by weakening of the N-O bond, thusfacilitating NO dissociation, which is proposed as the critical reaction-initiating step. According to thismodel the promoting or poisoning effect of Na depends on the interaction strength of the catalyst surfacewith the hydrocarbon. The different behaviour of propene and methane reflects the weaker interaction ofalkanes with metal surfaces compared to that of alkenes. XPS and Auger data demonstrate that Na coverageincreases monotonically with promoter loading and that there is no significant tendency for the promoterto agglomerate with increasing promoter loading. However, a very small but constant tendency toaccumulate subsurface or dissolved Na was observed as the promoter loading is increased.

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