Steady-state multiplicity phenomena during the electrochemical promotion of NO reduction by C2H4 in presence of O2 on thin Rh and Pt catalyst-electrodes in a monolithic electropromoted reactor

The electrochemical promotion of catalysis (EPOC) was used to promote the selective reduction of NO by hydrocarbons in presence of oxygen using thin (40 nm) porous Rh and Pt catalyst layers sputtered on the opposite surfaces of thin (0.25 mm) solid electrolyte (YSZ) plates serving as electrocatalytic elements of a monolithic electrochemically promoted reactor (MEPR). Using 22 Rh/YSZ/Pt type cells it was found that the reduction of NO in presence of 1.1 kPa O₂ and 0.36 kPa C₂H₄ can be efficiently electropromoted with 340% rate enhancement, reaching 95% NO conversion with 100% selectivity to N₂ in the temperature range from 280 to 340 °C. The apparent Faradaic efficiency is larger than unity for both the NO reduction and the C₂H₄ oxidation reaction.At elevated temperatures (≥300 °C) and high reactant conversions it was found that after current interruption, the catalytic rates do not return to their initial values but remain in a new highly active steady state. It appears that this highly active state is not a genuine intrinsic permanent NEMCA state but is manifestation of steady-state multiplicity in the monolithic reactor resulting from near complete gaseous O₂ consumption. Thus the low and high activity steady states corresponding to zero applied potential appear to correspond to high and low average P_O2 in the reactor. The latter is the result of the near complete reactant conversion under the preceding electropromoted operation. These highly active permanent NEMCA states may be quite useful for practical applications.