Electrochemical activation of reduction of nitric oxide by carbon monoxide on noble metals

A comparative study of the electrochemical activation of the reduction of NO by CO on platinum, palladium and rhodium was performed. The catalyst was supported on a yttria-stabilized zirconia (YSZ) electrolyte, in the form of a 1 cm diameter disc suspended in a glass reactor. It was found that rate and nitrogen selectivity, SN2, could be modified by application of external potential. The promotion effect is non-Faradaic, that is, modification of rate is up to three orders of magnitude higher than the rate of transfer of O2- from the solid electrolyte. All three catalysts exhibited electrophilic enhancement for the application of a negative potential. Rate enhancement ratios for palladium, platinum, and rhodium were 2, 3.5, and 7.9, respectively. Enhancement was attributed to increased back donation and increased NO increased dissociation, the rate limiting step. On rhodium and palladium maximum enhancement occurred at or near equal reactant partial pressures. On platinum CO inhibited the reaction, and the response for negative potential switched from electrophobic to electrophilic as PNO was increased. This was due to the changing of the rate determining step form NO adsorption to NO dissociation as PNO, and thetaNO, increased. Platinum and rhodium also exhibited electrophobic enhancement when positive potential was applied due to the removal of CO and NO, respectively. The effect was much more gradual than electrophilic enhancement due to spreading out of Odelta- from the three phase boundary. Palladium exhibited no electrophobic enhancement. The reaction on platinum initially underwent rate oscillation due to the rapid formation and destruction of surface oxides on the surface. Application of a negative potential suppressed oscillation and promoted the rate. On reopening the circuit the rate continued to oscillate. A comparison was also done between the fuel cell and single pellet configurations for palladium. It was seen that promotion was qualitatively the same and that electrochemical promotion is the change in potential, and work function, and not a function of applied potential.