Ionic conductors (i.e., materials that can selectively transport ions, also called electrolytes) are widely used in batteries, fuel cells, sensors and in gas separation technologies. Solid electrolytes with conducting ions of O²⁻, H⁺, Li⁺, K⁺, Na⁺, Ag⁺, F⁻, Cu⁺, or Cl⁻ have been reported in the last decades (Hamann et al., 1998; Vayenas et al., 2001; Vielstich et al., 2003; Vernoux et al., 2013). The importance of these materials as catalytic carriers in heterogeneous catalysis has been obvious since the demonstration by Vayenas and coworkers (Vayenas et al., 2001; Vayenas, 2011; Vernoux et al., 2013) that the conducting ionic species can significantly promote the catalytic activity of metal or metal oxide nanoparticles, via the non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) effect, also called Electrochemical Promotion of Catalysis (EPOC).

In EPOC studies, the catalyst active phase (conductive catalyst-electrode) is in contact with an ionic conductor-support (in an electrochemical cell configuration). Application of electrical currents or imposition of potentials between the catalyst film and a counter electrode (catalytically inactive) can cause significant alterations in catalytic properties (Vayenas et al., 2001; Vayenas, 2011; Vernoux et al., 2013).

A key parameter in EPOC studies is the catalyst potential U_WR. The subscript “WR” denotes the potential of the working (“W”) electrode (which also serves as the catalyst) with respect to a reference “R” electrode. Experimental (i.e., in situ work function measurements) (Vayenas et al., 1990, 2001; Tsiplakides and Vayenas, 2001) and theoretical studies (Tsiplakides and Vayenas, 2001; Riess and Vayenas, 2003) have shown that there exists (over wide temperature ranges) an one-to-one correlation between the change in U_WR and the concomitant change in the work function, Φ, of the gas exposed, i.e., catalytically active, catalyst surface: (1)ΔΦ=eΔUWR

Thus, in electrochemical promotion studies, upon varying the catalyst potential, U_WR, the catalyst work function, Φ, is also modified. Increasing coverage of electron acceptor (electronegative) promoting species increases the catalyst potential and work function while increasing coverage of electron donor (electropositive) promoting species decreases both the catalyst potential and the work function (Stoukides and Vayenas, 1981; Vayenas et al., 2001).

Since the discovery of EPOC in the early 80 s (Stoukides and Vayenas, 1981), more than 100 different catalytic systems (oxidations, hydrogenations, dehydrogenations, isomerizations, decompositions) have been electrochemically promoted on Pt, Pd, Rh, Ag, Au, Ni, IrO₂, RuO₂ catalysts deposited on O²⁻ (YSZ), Na⁺ (β″-Al₂O₃, NaSiCon), K⁺ (β″-Al₂O₃), H⁺ (CaZr_0.9In_0.1O_3−α, Nafion), F⁻ (CaF₂), aqueous, molten salt and mixed ionic-electronic (TiO₂, CeO₂, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ/Ce_0.9Gd_0.1O_1.95 composite) conductors (Vayenas et al., 2001; Katsaounis, 2010; Kambolis et al., 2012; Vernoux et al., 2013). EPOC does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, therefore, the investigation of new materials regarding the electrolyte, the promoting agent and the active catalyst phase, is very challenging. Moreover, the knowledge obtained from EPOC studies can be transferred in dispersed catalysts in terms of choosing the support for a specific reaction by choosing the appropriate ionic agent (Vernoux et al., 2009, 2013).

Silver catalysts have found excellent application to ethylene epoxidation (Dadyburjor et al., 1979; Ayame et al., 2003) and hold great promise for the SCR of NOx with hydrocarbons (Burch et al., 2002; Shimizu and Satsuma, 2006). However, sintering is a serious issue for silver catalysts (Presland et al., 1972; Plummer et al., 1987; Ruckenstein and Lee, 1988), and a complex array of factors have been linked to sintering, including the reaction gas environment (Ruckenstein and Lee, 1988), support effects (Bird et al., 2000), and surface binding and mobility (Meyer et al., 2007).

This work presents, for the first time, attempts to develop an electrochemical catalyst which allows transportation of Ag⁺ between the solid electrolyte and the Pt film. This provides a controlled, reversible, and reproducible way of altering the surface composition. The idea of this study is to utilize the phenomenon of EPOC to electrochemically control the transportation of Ag⁺ cations between the solid electrolyte and the catalyst film. Propane combustion was used as model reaction. This reaction is well-studied in the field of EPOC on Pt electrochemical catalysts deposited on O²⁻ and Na⁺ conductors. Therefore, a Pt catalyst film was deposited on β″-Al₂O₃ (Ag⁺). Moreover, the use of Pt film instead of Ag allows the discrimination of species coming from the support.

The electrochemical catalyst (Figure 1) consisted of a β″-Al₂O₃ pellet (Ionotec Ltd) Ag⁺ conductor, covered on both sides by three electrodes; the working-catalyst, the counter and the reference electrode. The Pt working electrode (0.8 cm² geometric surface area and 1.9 mg loading) was deposited by application of metalorganic platinum paste (Engelhard-Clal 6926) and catalytically inactive gold counter (symmetrical to working). Reference and counter electrodes were deposited on the other side of the pellet by application of metalorganic paste, followed by calcination in air for 2 h at 450°C. The reference electrode was deposited close enough (about 3 mm) to the counter-electrode avoiding any electrical perturbation. A galvanostat/potentiostat Voltalab PGP 201 (Radiometer) was used in order to apply and measure both potential and current in the electrochemical catalyst. Voltage and current were applied and measured according to the procedure generally used in conventional three-electrode electrochemical cells. The catalyst potential, U_WR, was measured between the working electrode and the reference electrode (Au) which may be considered as a pseudoreference since its potential does not vary significantly with the composition of the gaseous mixture, as experimentally checked.

The presence of silver in the interface of the Pt porous catalytic film and the β″-Al₂O₃ support was confirmed by SEM-EDX analysis carried out over the sample after catalyst performances measurements. Before the SEM-EDX analysis, the sample was cooled down under the reaction conditions from 400°C to room temperature. During this period, the sample was under positive polarization of U_WR = +2V. Figure 1 shows a SEM micrograph of the Pt film and a zoom in areas where the Pt film is open to the support. Figure 1 also includes a mapping of two regions for Pt, Ag, Al, and O₂. From the mapping of elements, one can conclude that silver particles cover the area between the Pt and the β″-Al₂O₃. The presence of silver species in this interface can be attributed to surface diffusion during annealing the catalyst which is enhanced by the presence of oxygen (Presland et al., 1972; Baker and Skiba, 1977).

The catalytic performances of the electrochemical catalyst was evaluated in a quartz reactor, which was designed to facilitate the connection between the electrodes and the galvanostat/potentiostat, allowing the reactive mixture to reach the electrocatalyst surface. The temperature was measured with a K-type thermocouple placed in proximity to the working electrode surface. The reaction gases were mixtures of C₃H₈ (Air Liquide, 8000 ± 80 ppm), O₂ (Air Liquide, 99.95%) and He (Air Liquide, 99.999%) as the vector gas. The gas composition was controlled by mass flow controllers (Brooks), with accuracy of 1%. The reaction products were analyzed by an on-line micro gas-chromatograph (R3000 SRA Instruments) and a CO₂ IR analyzer (Horiba VA 3000). The C₃H₈ partial pressure in the feed was held constant during all the experiments at 0.24 kPa. The O₂ partial pressure in the feed was kept at 1.2 kPa with total gas flow rate of 12 L/h. Carbon monoxide was never detected, according to our 10 ppm lower detection limit.

In order to quantify the magnitude of EPOC, three parameters are commonly used. First, the rate enhancement ratio, ρ: (2)ρ=r/ro second the apparent Faradaic efficiency, Λ: (3)Λ=(r−ro)/(I/nF) and third the promotional index, PI_Ag: (4)PIAg=(ρ−1)/ΔθAg where r is the electropromoted catalytic rate, r_o the open-circuit catalytic rate, I the applied current, and n the number of exchanged electrons during the electrode reaction, F is the Faraday's constant and θ_Ag the coverage of the Ag coverage onto the catalyst surface induced by the backspillover of Ag⁺ ions from the electrolyte.

When the promoting species can take part in the reaction (e.g., O²⁻ in an oxidation reaction or H⁺ in a hydrogenation reaction), the Faradaic efficiency reveals if the process is (sub)Faradaic (i.e., Λ ≤ 1) or if the catalytic reaction exhibits NEMCA behavior (i.e., |Λ| > 1). However, in the case of promoting species that cannot participate in the reaction (e.g., K⁺, Na⁺, or Ag⁺), the Faradaic efficiency has no significant meaning and the promotional index is used. Upon applying a positive (negative) current, the Ag⁺ cations are removed (supplied) from (to) the Pt catalyst surface and, in this way, the θ_Ag is decreased (increased).

The development of a new kind of electrochemical catalyst, which permits in situ control of Ag⁺ species during propane combustion, was investigated. Encouraging results have shown that significant electropromotion of the catalytic performance can be achieved (rate increase up to a factor of 4). During galvanostatic transient, response of catalytic rate is in excellent agreement with NEMCA theory. Indeed, exponential increase of the rate is observed when the potential linearly increases with time for extremely low currents. The origin of the electrochemical activation upon positive polarizations was tentatively attributed to the electrochemical decomposition of silver oxide which could generate promoting ionic oxygen species.