Fj. Williams et al., Electrochemical promotion of rhodium-catalyzed NO reduction by CO and by propene in the presence of oxygen, J PHYS CH B, 105(14), 2001, pp. 2800-2808
The catalytic performance of a rhodium thin film in contact with the solid
electrolyte Na-beta" alumina can be greatly enhanced by the reversible elec
trochemically controlled transport of sodium from the electrolyte to the me
tal surface. By this means, the reduction of nitric oxide by carbon monoxid
e or by propene can be promoted, even in the presence of oxygen. The effect
is due to the Na-enhanced dissociation of adsorbed NO, the key reaction-in
itiating step. XP and Auger spectroscopies show that, under promoted condit
ions, the alkali-metal surface phase consists of carbonate, nitrate, or bot
h, depending on the gas composition. To a first approximation, the chemical
identity of the counterion appears not to play a significant role. With in
creasing oxygen partial pressure the promotional effects of sodium are prog
ressively decreased, and the markedly different behavior of CO and propane
as reductants is due to the opposite effects of coadsorbed alkali metal on
the electronegative or electropositive adsorbate, respectively. At the high
est oxygen partial pressures and alkali-metal coverages, drastic poisoning
by sodium is due to strong alkali-metal inhibition of propene adsorption, e
xcessive formation of Na2O, and oxidation of Rh to Rh2O3.