S. Brosda

The double-layer approach to promotion, electrocatalysis, electrochemical promotion, and metal–support interactions

Abstract Recent progress in understanding and modeling promotion, electrocatalysis, electrochemical promotion, and metal–support interactions is surveyed. It is shown that via the action of spillover and via the concept of the sacrificial promoter, the phenomena of promotion, electrochemical promotion, and metal–support interactions are functionally identical and only operationally different, as they all correspond to catalysis in presence of a controllable double layer. This is then utilized to derive adsorption isotherms and kinetic expressions which account explicitly for the electrostatic interactions between the double layer and the adsorbed reactants, intermediates, and products. The resulting analytical expressions are shown to be in excellent semiquantitative agreement with experiment and with the recently established promotional rules.

Electrochemical Promotion of Ethylene Oxidation over Rh Catalyst Thin Films Sputtered on YSZ and TiO2 / YSZ Supports

The effect of electrochem. promotion of catalysis or nonfaradaic electrochem. modification of catalytic activity or electropromotion was studied for the model reaction of ethylene oxidn. on sputter-coated Rh films. The thin (40 nm) Rh films were deposited on Y2O3-stabilized-ZrO2 (YSZ) and on YSZ coated with a thin porous TiO2 layer. The catalytic activity of Rh for C2H4 oxidn. can be reversibly enhanced via anodic current or potential application by up to a factor of 80 and the increase in the oxidn. rate is up to 2000 times larger than the rate of supply of O2- to the Rh catalyst-electrode. Smaller anodic currents cause periodic catalytic rate and potential oscillations. The TiO2 layer was found to enhance the open-circuit catalytic activity and to stabilize the electrochem. promoted catalyst state. The obsd. pronounced electrochem. promotion behavior is due to the anodically controlled migration (back spillover) of O2- species from YSZ to the Rh/gas interface and the concomitant destabilization, via repulsive lateral interactions, of the formation of surface Rh2O3. The electropromotion of such thin metal catalyst films with metal dispersion near 10% is of significant importance for the practical use of the electrochem. promotion of catalysis. [on SciFinder (R)]

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.

Electrochemical Promotion of Pt Catalyst Electrodes Deposited on Na3Zr2Si2 PO 12 during Ethylene Oxidation

The electrochemical promotion of Pt catalyst electrode films interfaced with Na 3 Zr 2 Si 2 PO 12 (NASICON), a sodium ion conductor, was investigated during ethylene oxidation. At 430°C, electrochemical Na + supply to the Pt catalyst under near-stoichiometric ethylene-to-oxygen ratios causes an up to tenfold catalytic rate enhancement for Na coverages of 0.03-0.08. The behavior of the system is discussed in terms of previous electrochemical promotion studies using sodium-ion conductors. On the basis of the kinetic data the promotional role of sodium is attributed to enhanced oxygen chemisorption on the Pt catalyst-electrode surface.

Electrochemical promotion of an oxidation reaction using a proton conductor

A proton conductor., Ba 3 Ca 1.18 Nb 1.82 O 9-α , has been used to electrochemically promote, via potential-controlled spillover of protons, a Pt catalyst-electrode for the model reaction of ethene oxidation at temperature 250-350 C. The rate enhancement is up to 12 times that of the open-circuit rate and typically 10 3 times larger than the rate of proton supply to the catalyst. At the highest temperatures and most oxidizing conditions employed an up to threefold permanent electrochemical enhancement in catalytic rate was also observed. In situ a.c. impedance spectra reveal a buildup of sacrificial promoter, most likely OH formed by reaction of spillover protons with chemisorbed oxygen, under electrochemical promotion conditions. The observed pronounced electrophilic (∂r/∂U ≤ 0) behavior, in conjunction with the reaction kinetics, is in good agreement with the recently established rules of electrochemical and classical promotion.

Electrochemical promotion: experiment, rules and mathematical modeling

Electrochemical and conventional promotion are closely related since the former is due to electrochemically controlled migration (backspillover) of promoting ionic species on metal catalyst surfaces. In both cases, the promoting species alter the catalyst surface work function Φ. Here, we develop simple and rigorous rules, which describe the dependence of catalytic rates on catalyst work function. These rules are in very good agreement with the electrochemical and conventional promotion literature. These experimentally derived rules also follow from a mathematical model, which takes into account lateral electrostatic adsorbate interactions.

Electron Donation–Backdonation and the Rules of Catalytic Promotion

The rules of classical and electrochemical promotion of catalysis, which allow for the prediction of the effect of electropositive and electronegative promoters on catalytic rate and selectivity on the basis of the catalytic reaction kinetics, are compared with experiment for all relevant catalytic studies during the last two decades and are used to show that the rate versus catalyst work function dependence always parallels the rate versus electron donor reactant dependence. This generalized rule is rationalized both by considering the interaction of the electric field in the effective double layer at the metal catalyst–gas interface with the electric dipoles of the adsorbed reactants and also by considering the electron donation–backdonation between the adsorbed reactants and the catalyst. This generalized promotional rule allows for promoter selection on the basis of the reaction kinetics.

Reaction Kinetic-Induced Changes in the Electrochemically Promoted C2H4 Oxidation on Pt/YSZ

The electrochemical promotion of C2H4 oxidation reaction has been investigated under various feed gas compositions and temperatures over a Pt/Y2O3-stabilized ZrO2 catalyst electrode. It was found that under oxidizing conditions the reaction exhibits electrophobic behavior, i.e. the catalytic rate of CO2 formation increases with anodic potential application, while it shifts to electrophilic behaviour, i.e. the rate increases with cathodic potential application, under near stoichiometric feed conditions. At intermediate C2H4 to O2 ratios and low temperatures volcano type behavior is observed, i.e. the rate is poisoned both with anodic and cathodic potential application. The effect of feed gas composition on the electrochemical promotion behaviour is related to changes in the reaction kinetic order with respect to C2H4 and O2. The results are in excellent agreement with the rules of electrochemical and chemical promotion.Graphical Abstract