Electrochemical promotion of conventional and bipolar reactor configurations for NO reduction

Abstract

The reduction of NO by CO or C3H6 in the presence of O2, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst films using a single-chamber and a wireless bipolar cell configuration. In the latter case the Rh catalyst films were deposited on the inner side of a YSZ tube, while two Au films deposited on the outer side of the tubes were used to polarize the Rh catalyst surface. The experimental conditions used in this study were close to those in the exhaust of a lean burn or diesel engine, i.e., high flowrates and space velocities and in some cases, considerable excess of oxygen. It was found that both direct (conventional) and indirect (wireless) polarization of the catalyst causes significant enhancement in the reaction rates (up to a factor of 20) and in the reactant conversion. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, A, in the order of 100, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst films were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Rh films were found to exhibit higher catalytic activity than the unpromoted films, with a considerable decrease in their light-off temperature. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Rh catalysts. The results showed that the effect of Chemical and Electrochemical Promotion on the catalyst performance can be synergetic and their combination may lead to interesting practical applications. This is further supported by the fact that such bipolar tube configurations: (a) do not need electrical connection to the catalyst and (b) can be adapted easier to commercial exhaust units.