Rules and Mathematical Modeling of Electrochemical and Chemical Promotion: 1. Reaction Classification and Promotional Rules

The effect of electrochemical promotion has already been studied for more than 60 different catalytic systems. Its origin is electrochemically controlled introduction of promoting species on catalyst surfaces interfaced with solid electrolytes. The promoting mechanism is thus similar to classic (chemical) promotion where the promoting species is added during catalyst preparation or from the gas phase. In this work simple and rigorous rules are derived that govern the promotional action, i.e., the catalytic rate dependence on promoter concentration or on work function, Φ, of the catalyst surface. These rules enable one to predict the rate, r, dependence on Φ or on promoter concentration on the basis of the rate dependence on the electron donor (D) or electron acceptor (A) reactant partial pressures, pD and pA, on the unpromoted catalyst surface. The four main types of promotional behavior, i.e., electrophobic (∂r/∂Φ>0), electrophilic (∂r/∂Φ<0), volcano type, and inverted volcano type, are shown to correspond to strong adsorption of A, strong adsorption of D, strong adsorption of A and D, and weak adsorption of A and D, respectively. The rules are in excellent agreement with both the electrochemical and classic promotion literature and can be summarized by the inequalities (∂r/∂Φ)pA, pD(∂r/∂pD)Φ, pA>0 and (∂r/∂Φ)pA, pD(∂r/∂pA)Φ, pD<0, which dictate that for a reaction involving a strongly adsorbed electron donor/acceptor reactant, an electropositive (electron donor)/electronegative (electron acceptor) promoter is to be reco mmended.