S. Bebelis

Non-faradaic electrochemical modification of catalytic activity: 1. The case of ethylene oxidation on Pt

Abstract It was found that the catalytic activity of polycrystalline Pt for the oxidation of ethylene to CO 2 and H 2 O can be increased by up to a factor of 50 when oxygen anions O 2− are electrochemically pumped onto the Pt catalyst surface. The experiments were conducted using a stabilized zirconia solid electrolyte at temperatures of 550 to 725 K. The steady-state increase in the catalytic reaction rate is typically 10 5 higher than the rate of O 2− transport to the catalyst surface. Over a wide range of conditions the catalytic reaction rate increases exponentially with the catalyst-solid electrolyte overpotential η which is proportional to the change in catalyst work function. The reaction activation energy decreases toward zero with increasing η. The phenomena are completely reversible and show that catalyst work function and catalytic activity can be varied at will by adjusting the catalyst potential. A model which takes into account the change in catalyst work function with changing catalyst activation overpotential and the consequent changes in the bonding strength of chemisorbed species is proposed. The model is in semiquantitative agreement with experiment.

Non-faradaic electrochemical modification of catalytic activity 4. The use of β″-Al2O3 as the solid electrolyte

Abstract It was found that the catalytic activity of polycrystalline Pt films deposited on β″-Al 2 O 3 , a Na + -conducting solid electrolyte, can be altered dramatically and reversibly by polarizing the catalyst-solid electrolyte interface. The observed decrease in the rate of ethylene oxidation on Pt is typically 103 − 5 × 10 4 times larger than the rate of supply of Na + to the catalyst-electrolyte interface. A Na coverage of 0.015 suffices to cause a 70% decrease in catalytic rate which varies exponentially with catalyst work function. The latter was measured both potentiometrically and by means of a Kelvin probe. The behavior is qualitatively similar to that observed when using O 2− -conducting solid electrolytes and shows that the newly found effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) is not restricted to any particular metal or solid electrolyte. This supports the proposed explanation of the effect which is based on the change in catalyst work function and concomitant change in chemisorptive bond strengths as a result of ion spillover upon polarization of the catalyst-solid electrolyte interface.

The Electrochemical Activation of Catalytic Reactions

The use of electrochemistry to activate and precisely tune heterogeneous catalytic processes is a new development1-7 which originally emerged due to the existence of solid electrolytes. Depending on their composition, these specific anionic or cationic conductor materials exhibit substantial electrical conductivity at temperatures between 25 and 1000°C. Within this broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used as reversible in situ promoter donors or poison acceptors to affect the catalytic activity and product selectivity of metals deposited on solid electrolytes in a very pronounced, reversible, and, to some extent, predictable manner.

Atomic resolution STM imaging of electrochemically controlled reversible promoter dosing of catalysts

Abstract Reversible electrochemically controlled dosing (back-spillover) of sodium on Pt(111) at atmospheric pressure was imaged via atomically resolved STM. The Pt(111) monocrystal was interfaced with a flat polycrystalline sample of β″-Al 2 O 3 , a Na + conductor. Application of an electrical current between the Pt(111) monocrystal and a counterelectrode also in contact with the β″-Al 2 O 3 Na + -conducting solid electrolyte causes reversible migration (back-spillover and spillover) of sodium, which forms a (12 × 12) hexagonal structure on the Pt(111) surface. In addition to explaining the phenomenon of electrochemical promotion in heterogeneous catalysis, these observations provide the first STM confirmation of spillover phenomena which play a key role in numerous catalytic systems.

Electrochemical promotion (NEMCA) of CH4 and C2H4 oxidation on Pd/YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy

Abstract The catalytic activity of Pd for the oxidation of methane to CO2 can be markedly and reversibly affected using the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP), i.e. by interfacing polycrystalline Pd films with Y2O3-stabilized ZrO2 (YSZ) and varying Pd catalyst-electrode potential in galvanic cells of the type: CH4, O2, CO2, Pd|YSZ|Au, CH4, O2, CO2. It was found that by applying positive overpotentials or currents and thus, supplying O2− onto the catalyst surface, up to 70-fold increase in the catalytic rate of CH4 oxidation can be obtained, compared to the open circuit (unpromoted) catalytic rate. Electrochemical oxygen removal from the Pd catalyst-electrode surface, following negative overpotential or current application, also enhances the catalytic rate by up to a factor of 10. The induced changes in catalytic rate were typically two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. Faradaic efficiency Λ values of the order of 150 were attained. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. The existence of charged back-spillover species giving rise to catalyst work function change for positive overpotential application is manifest in this work for the first time using AC impedance spectroscopy.

Non-Faradaic Electrochemical Modification of Catalytic Activity in Solid Electrolyte Cells

The catalytic activity and selectivity of metal catalysts used as electrodes in high temperature solid electrolyte cells can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized metal-solid electrolyte interfaces. Oxygen anions, forced electrochemically to adsorb on the metal catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases as high as 4000%. Changes in catalytic rates typically exceed the rate of O2− transport to or from the catalyst surface by 102-3 · 105. Significant changes in product selectivity have been also observed. The case of several catalytic reactions in which this new phenomenon has been observed is presented and the origin of the phenomenon is discussed.

Electrochemical promotion of heterogeneous catalysis

Abstract The catalytic activity and selectivity of metals interfaced with solid electrolytes can be altered dramatically and reversibly via potential application. The increase in catalytic rate can be several orders of magnitude higher than that anticipated from Faradays Law. This new phenomenon of electrochemical promotion is of considerable theoretical and potentially practical importance in heterogeneous catalysis. In this paper the main phenomenological features of electrochemical promotion (or NEMCA effect) are surveyed and the origin of the effect is discussed in view of recent surface spectroscopic and quantum mechanical studies.

Electrochemical activation of catalytic reactions using anionic, cationic and mixed conductors

Abstract The effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion has been investigated for the oxidation of ethylene on Pt using several types of solid electrolytes and mixed conductors. It was found that, despite the different nature of the solid electrolyte support and promoting ion, there exist important similarities in the NEMCA behavior of ethylene oxidation. Thus, in general, the rate of oxidation increases with increasing catalyst-electrode potential (electrophobic behavior) at high oxygen to ethylene ratios and increases with decreasing potential (electrophilic behavior) at low oxygen to ethylene ratios. There exist, however, significant differences in the magnitude of the observed rate enhancement, which is up to sixty-fold in the case of ZrO 2 (Y 2 O 3 ) and in the apparent Faradaic efficiency, which is up to 3·10 5 for the cases of ZrO 2 (Y 2 O 3 ) and CeO 2 and even higher for the Na + conductors. The common features and differences are summarized and discussed together with the underlying electrochemical promotion mechanism on the basis of recent experimental and theoretical studies.

Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ Cermet Anodes

AbstractThe kinetics of the steam reforming reaction of CH4 were investigated at temperatures 750 to 950°C under both open-circuit and closed-circuit conditions on Ni-YSZ (Yttria Stabilized Zirconia) solid oxide fuel cell (SOFC) anodes and polycrystalline Ni film SOFC anodes of measured Ni surface area. It was found that the rate of methane reforming on the Ni surface exhibits a Langmuir-Hinshelwood type dependence on