Recent developments and trends in the electrochemical promotion of catalysis (EPOC)

Abstract

Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is one of the most exciting discoveries in Electrochemistry with great impact on many catalytic and electrocatalytic processes. According to the words of John O'M. Bockris, EPOC is a triumph, and the latest in a series of advances in electrochemistry which have come about in the last 30 years. It has been shown with more than 80 different catalytic systems that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in a very pronounced, reversible and, to some extent, predictable manner by applying electrical currents or potentials (typically up to ±2 V) between the catalyst and a second electronic conductor (counter electrode) also deposited on the solid electrolyte. The induced steady-state change in catalytic rate can be up to 135 9 10 3 % higher than the normal (open-circuit) catalytic rate and up to 3 9 10 5 higher than the steady-state rate of ion supply. EPOC studies in the last 7 years mainly focus on the following four areas: Catalytic reactions with environmental impact (such as reduction of NO x and oxidation of light hydrocarbons), mechanistic studies on the origin of EPOC (using mainly oxygen ion conductors), scale-up pf EPOC reactors for potential commercialization via development of novel compact monolithic reactors and application of EPOC in high or low temperature fuel cells via introduction of the concept of triode fuel cell. The most recent EPOC studies in these areas are discussed in the present review and some of the future trends and aims of EPOC research are presented.

Enhancing the catalytic activity and selectivity of the partial oxidation of methanol by electrochemical promotion

“…This phenomenon is based on the modification of the catalytic properties of an heterogeneous catalyst by the electrochemical pumping of promoter ions from an electro-active catalyst support, which is a solid electrolyte material. Until now, this phenomenon has been demonstrated in more than 80 catalytic systems with several important technological possibilities, particularly in industrial product selectivity [12,13]. For instance, previous works have demonstrated the interest of EPOC in enhancing the selectivity of several catalytic processes such as the selective catalytic reduction of NO x [14,15], CO 2 hydrogenation [16], Fischer-Tropsch synthesis [17], or CO preferential oxidation [18].…”

Electrochemical promotion of CO 2 hydrogenation on Ru catalyst–electrodes supported on a K–β″–Al 2 O 3 solid electrolyte

“…A parallel approach to classical chemical promotion is the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA effect) [17][18][19][20][21][22][23][24][25][26] which can be used to promote metal catalyst films, also acting as electrodes which are deposited on solid electrolyte supports. Electrochemical promotion allows for continuous in situ control of the coverage of promoting species (O dÀ , Na d+ , K d+ , H d+ ) on the catalyst surface.…”

“…Numerous surface science and electrochemical techniques have shown that EPOC is due to a electrochemically controlled migration (reverse-spillover or backspillover) of promoting ionic species (O 2À in the case of YSZ, TiO 2 and CeO 2 , Na + or K + in the case of b 00 -Al 2 O 3 , H + in the case of Nafion, CZI (CaZr 0.9 In 0.1 O 3Àa ) and BCN18 (Ba 3 Ca 1.18 Nb 1.82 O 9Àa ), etc.) between the ionic or mixed ionic-electronic conductorsupport and the gas exposed catalyst surface, through the catalystgas-electrolyte three phase boundaries (tpb) [17][18][19][20][21][22]. Upon application of a change, DU WR , to the potential, U WR , of the catalyst (working electrode) with respect to a reference electrode, these backspillover species, accompanied by their compensating charge in the metal, migrate to the metal-gas interface, creating an overall neutral double layer, termed effective double layer.…”

“…This ion backspillover causes the catalyst work function to change by DF = eDU WR and accordingly the chemisorptive bond strengths of reactants and intermediates are modified [27][28][29]. Thus, both catalytic activity and selectivity are affected in a pronounced, reversible and to some extent predictable manner [17][18][19][20].…”

Electrochemically induced oxygen spillover and diffusion on Pt(111): PEEM imaging and kinetic modelling

“…2 Despite its importance in EPOC, the nature and the properties of this species are not completely clarified yet, 5 and the detailed mechanism of EPOC is still under debate. [6][7][8][9][10][11] In the following, the term ''spillover oxygen'' refers to oxygen which is formed at the three-phase boundary (TPB) of the solid electrolyte (yttria stabilized zirconia, YSZ), the electrode (platinum, Pt) and the gas phase during anodic polarization and diffuses onto the electrode surface-without making any statement on its chemical and electronical nature.…”