Rules and mathematical modeling of electrochemical and chemical promotion 1. Reaction classification promotional rules

The effect of electrochemical promotion has already been studied for more t han 60 different catalytic systems. Its origin is electrochemically control led introduction of promoting species on catalyst surfaces interfaced with solid electrolytes. The promoting mechanism is thus similar to classic (che mical) promotion where the promoting species is added during catalyst prepa ration or from the gas phase. In this work simple and rigorous rules are de rived that govern the promotional action, i.e., the catalytic rate dependen ce on promoter concentration or on work function, Phi, of the catalyst surf ace. These rules enable one to predict the rate, r, dependence on Phi or on promoter concentration on the basis of the rate dependence on the electron donor (D) or electron acceptor (A) reactant partial pressures, P-D and P-A , on the unpromoted catalyst surface. The four main types of promotional be havior, i.e., electrophobic (partial derivativer/partial derivative Phi > 0 ), electrophilic (partial derivativer/partial derivative Phi < 0), volcano type, and inverted volcano type, are shown to correspond to strong adsorpti on of A, strong adsorption of D, strong adsorption of A and D, and weak ads orption of A and D, respectively. The rules are in excellent agreement with both the electrochemical and classic promotion literature and can be summa rized by the inequalities (partial derivativer/partial derivative Phi)(PA.P D) (partial derivativer/partial derivative (PD))(Phi .PA) > 0 and (partial derivativer/partial derivative Phi)(PA.PD) (partial derivativer/partial der ivative (PA))(Phi .PD) < 0, which dictate that for a reaction involving a s trongly adsorbed electron donor/acceptor reactant, an electropositive (elec tron donor)/electronegative (electron acceptor) promoter is to be recommend ed. (C) 2001 Academic Press.