Non Faraday Electrochemical Modification of Catalysts Activity pot

Non Faraday Electrochemical Modification of Catalysts Activity CG Vayenas and S Brosda, University of Patras, Patras, Greece & 2009 Elsevier B.V. All rights reserved. Introduction The term NEMCA refers to nonfaradaic electrochemical modification of catalytic activity. The NEMCA effect is also known as electrochemical promotion or electro- chemical promotion of catalysis (EPOC) or electro- promotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on elec- tronically conductive catalysts deposited on ionic (or mixed ionic–electronic) supports upon application of electric current or potential (typically 72 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The NEMCA effect was first reported in 1981 for the case of the oxidation of ethylene (C 2 H 4 ) by gaseous oxygen on porous silver catalyst films deposited on Y 2 O 3 - stabilized ZrO 2 (YSZ), an O 2À conductor, and has been described in the literature for more than 80 catalytic systems on a variety of catalysts (platinum, palladium, rhodium, silver, iridium(IV) oxide (IrO 2 ), ruthenium(IV) oxide (RuO 2 ), nickel, copper, gold) using numerous anionic (YSZ, calcium fluoride (CaF 2 )), cationic (b 00 - Al 2 O 3 ,Na 3 Zr 2 Si 2 PO 12 ,K 2 YZr(PO 4 ) 3 , CaZr 0.9 In 0.1 O 3–a , CsHSO 4 , Nafion), or mixed electronic–ionic (titanium di-oxide (TiO 2 ), cerium(IV) oxide (CeO 2 ), YZrTi) sup- ports, and also aqueous and molten salt electrolytes. The observed change in catalytic rate is typically 5–10 5 times larger than the electrochemical reaction rate (i.e., the rate of ionic transport in the support – the rate of ion supply to or ion removal from the catalyst); thus, the effect is strong ly nonfaradaic. The electropromoted catalytic reaction rate is typically 2–500 times larger than the open-circuit (i.e., unpromoted) catalytic rate. Basic Phenomenology The basic phenomenology of this effect when using O 2À , Na þ , and H þ conducting solid electrolytes is shown in Figures 1–3. The (usually porous) metal catalyst elec- trode, typically 0.5–5 mm thick, is deposited on the solid electrolyte and under open circuit (I ¼ 0, no electro- chemical rate) produces a catalytic rate r 0 for ethylene oxidation (Figures 1 and 3) or carbon monoxide oxi- dation (Figure 2). Application of an electrical current, I, or potential (7 2 V) between the catalyst and a coun- terelectrode causes very pronounced and nonfaradaic (i.e., DrcI/2F ) alterations to the catalytic rate, r, and, quite often, to the product selectivity ( Figure 4 ). The rate of the catalytic reaction, r, can become up to 500 times larger tha n the open-circuit rate, r 0 , and up to 3  10 5 times larger than the faradaic rate (I/2F for O 2À , À I/F for Na þ and H þ ) of ion supply (or removal) to (or from) the catalyst electrode. Up to 2007, more than 70 different catalytic reactions (oxidations, hydrogenations, dehydrogenations, iso- merizations, decompositions) have been electrochemically promoted on platinum, palladium, rhodium, silver, gold, nickel, iridium(IV) oxide, and ruthenium(IV) oxide cata- lysts deposited on O 2À (YSZ), Na þ (b 00 -Al 2 O 3 ), H þ (CaZr 0.9 In 0.1 O 3 À a , Nafion), F À (calcium fluoride), aque- ous, molten salt, and mixed ionic–electronic (titanium dioxide, cerium(IV) oxide) conductors. Clearly, EPOC is not limited to any particular class of conductive catalyst, catalytic reaction, or ionic support. Definitions and Some General Characteristics The magnitude of electrochemical promotion for a given catalytic reaction is described by three parameters: 1. The faradaic efficiency, L, is expressed as L ¼ðr À r 0 Þ=ðI =nFÞ½1 where r is the electrochemically promoted catalytic rate, r 0 the unpromoted (open-circuit) catalytic rate, I the applied current, n the charge of the promoting ion, and F (96 460 C mol À1 ) Faraday’s constant. A reaction is electrochemically promoted when |L|>1. For |L|r1 electrocatalysis occurs. Values of L as high as 3  10 5 or as low as À 3  10 4 have been measured (Table 1). For oxidation reactions, L>1 implies electrophobic behavior ð@r =@U WR > 0Þ and Lo–1 implies electrophilic be- havior ð@r=@U WR o0Þ where U WR is the catalyst (working electrode, ‘W’) potential with respect to the reference (‘R’) electrode. For oxidation reactions on metals supported on O 2À conductors, L also expresses the ratio of the lifetimes of the promoting O 2À species and of normally che- misorbed O on the catalyst surface. 2. The rate enhancement ratio, r, is expressed as r ¼ r=r 0 ½2 Values of r as high as 500 or as low as zero (complete catalyst poisoning) have been measured. 64 . Non Faraday Electrochemical Modification of Catalysts Activity CG Vayenas and S Brosda, University of Patras, Patras, Greece & 2009 Elsevier B.V term NEMCA refers to nonfaradaic electrochemical modification of catalytic activity. The NEMCA effect is also known as electrochemical promotion or electro- chemical promotion of catalysis (EPOC). and selectivities of catalytic reactions taking place on elec- tronically conductive catalysts deposited on ionic (or mixed ionic–electronic) supports upon application of electric current or potential