1 Introduction

Promoters play a key role in heterogeneous catalysis and their use is of paramount importance in the design of successful commercial catalysts for industrial applications [1]. Two kinds of promoters can be used in heterogeneous catalysis: structural promoters and electronic promoters. The former enhance and stabilize the dispersion of the active phase on the catalyst support. The latter enhance the catalytic properties of the catalytic phase itself. This review is are focus on the second kind of promoters, which are typically alkali ions that can be added chemically or electrochemically to a catalyst [2]. The effect of these promoters is based on their ability to modify chemisorption properties of the catalyst and hence to significantly alter the chemisorptive bond strength of reactants and intermediates. These effects may be originated from either electronic interactions, i.e., the modification of electron density of alkali-promoted metal surfaces, and/or from electrostatic interactions associated with alkali-metal ions, and/or from site blockage [3]. There is a very rich literature and comprehensive book [4] on the role of promoters in heterogeneous catalysis. In the majority of cases, it has been found that alkali addition results in a significant improvement of catalytic performance and that the promotional effect, which often depends on the size of the dopant atom, is maximized for a certain alkali content on the catalytic surface [5, 6]. Classical examples of industrially important catalytic reactions promoted by alkalis are the Fischer–Tropsch synthesis of hydrocarbons and the ammonia synthesis from N2 and H2 [7]. Commercial catalysts for ethylene epoxidation are also typically promoted by alkali-metal salts that are incorporated during the catalyst preparation step [8]. One can also find in literature a great number of research studies of alkali promotion for a wide range of catalytic systems. For instance, recent reports have shown substantial improvements in the water gas shift reaction when doping alkali (Na, Li, K) to Pt/CeO2 [9, 10], Pt/TiO2 [11–13], Pt/ZrO2 [14, 15], Pt/Al2O3 [16] and Pt supported on carbon nanotubes [17]. The selective catalytic reduction of NO x (SCR) has also widely studied in literature by Pt-alkali promoted systems. Hence, it has been shown that NO reduction rate increases by up to two orders of magnitude accompanied by large increases of N2-selectivity over optimally promoted Pt catalysts by Li, Na, K, Rb, Cs or Ba, [18–21]. The promotional effect of alkali ions has also been demonstrated in other metal catalysts such as Pd [22], Rh [23, 24], Ru [25], Cu [26], Au [27], on a wide variety of catalytic reactions. These are only several examples of the very rich literature related to alkali promotion in heterogeneous catalyst. However, in all these previous examples a fixed amount of alkali promoter is chemically added during the preparation step of the catalyst. In the last years the phenomenon of electrochemical promotion of catalysis (EPOC) has shown to be an useful tool for the electrochemical addition of promoter ions to a catalyst surface [2]. In this case the addition of the promoter to the catalyst can be done in situ, during the catalytic reaction itself. Hence, the amount of alkali promoter can be easily modified (and thus controlled) by the external imposition of an applied electrical potential or current, which allows the electrochemical pumping of promoter ions from the catalyst support (solid electrolyte). This innovative concept that has been developed by Vayenas and co-workers [2] occurs when a catalytic reaction takes place on a catalyst film which is in contact with a solid electrolyte support. These kinds of materials have the ability to selectively transport ions in a wide temperature range between 25 and 1,000 °C. If the solid electrolyte is a cationic conductor material such as (NASICON, Na–βAl2O3 or K–βAl2O3), the application of a cathodic polarization between the catalyst film (which also behaves as a working electrode) and a second electrode (counter electrode) deposited on the other side of the solid electrolyte allows the electrochemical migration of the alkali ions (Na+ or K+) to the catalyst surface (phenomenon denoted as back-spillover). This concept represented on the Fig. 1, allows the electrochemical addition of electronic promoters, anionic ions (O2−) or cationic ions as in Fig. 1 (K+), to a catalyst surface during working conditions, instead of its chemical addition during the preparation step of the catalyst. One can find in literature very interesting reviews from more fundamental to more applied aspects related to the EPOC phenomenon [2, 28–35]. However, these previous reviews have been mainly focus on the use of anionic ions (O2−) conductors materials such as yttria-stabilized zirconia (YSZ) as a solid electrolyte. Due to the wide use of alkali ions as electronic promoters in heterogeneous catalysis the aim of this review is to bring closer the EPOC phenomenon to the catalysis scientific community by the analysis of the most important and recent contributions of electrochemical promotion of catalytic reactions with alkaline conductors (Na+ and K+ conductors). Hence, this manuscript summarizes the most relevant aspects to understand the alkali electro-promotional effect of catalysts from its origin and mechanism to finally present high impact recent contributions of the alkali-EPOC phenomenon to the catalysis field.

Fig. 1
figure 1

The scheme of the electrochemical cell for electrochemical promotion studies with an alkali ionic conductor (K+ conductor) solid electrolyte

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2 The Effect of Chemical and Electrochemical Addition of Alkalis on the Chemisorption Properties of Catalysts

Catalysis and promotion are intimately related to the phenomenon of chemisorption. For a catalytic reaction to take place on a catalyst surface:

$$ {\text{A }} + {\text{ B }} \to {\text{Product}} $$

it is necessary that at least one of the reactants, and usually both, can chemisorb on the catalyst surface:

$$ {\text{A }} + {\text{ M}} \leftrightarrow {\text{A}} - {\text{M}} $$

where M is a site on the catalyst surface. Langmuir was the first to model chemisorption phenomenon and to relate the surface coverage of the adsorbate with the catalytic activity leading to the well known Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics for catalytic process [36]. The addition of promoters can affect the electronic state of the catalyst leading to significant changes in the work function, ϕ, of the catalyst surface and hence on its chemisorption properties. The work function ϕ (eV/atom), is one of the fundamental electronic properties of a metal surface and it can be defined such as the minimum energy required to extract one electron from the surface, when the surface is electrically neutral; i.e. the energy to bring an electron from the metal Fermi level to a distance of a few micrometers outside the metal surface so that image force interactions are negligible. Particularly in the case of alkali ions (Na+, K+), electropositive promoters, its addition to a metal particle cause a decrease in the work function of surfaces while electronegative promoters (e.g. O2− ions) increase it. An example of this is the effect of alkali addition on Ru (0001) and Ru (1010) single crystals [37]. For low alkali metal coverage an initial decrease of the work function (by almost 4 eV) is observed. Thus, upon adding electronic promoters (e.g. alkali ions) on the catalyst surface, the chemisorptive bond strength of reactants and intermediates is modified and the catalytic kinetics are usually changed dramatically.

For the case of electrochemical addition of promoters to an heterogeneous catalyst, through the concept of EPOC, the effect on catalyst chemisorption and on reaction kinetics is exactly the same as the chemical addition of electronic promoters [2, 29]. Hence, it has been reported that the EPOC phenomena can be used to control the work function (ϕ) of gas-exposed metal surfaces. In 1990 Vayenas et al. [38] used the Kelvin probe technique to in situ measure the controlled variation of catalyst work function under EPOC conditions and to investigate the metal-solid electrolyte interface. They showed that work function changes on a catalyst supported on a solid electrolyte (as represented on Fig. 1) upon changing the ohmic-drop free change in the working electrode potential (UWR) can be calculated by the Eq. (1):

$$ \varDelta \phi = {\text{ e}}\varDelta {\text{U}}_{\text{WR}} $$
(1)

In the last two decades, the Eq. (1) has been confirmed both theoretically and experimentally by several techniques [2]. It has been concluded that changes in the work function are due to the electrochemically induced and controlled migration of ions from the solid electrolyte onto the gas-exposed catalyst electrode surface. These back-spillover species form an effective double layer, which can thus dramatically influence catalytic reactions taking place at that gas-exposed interface, leading to the effect of electrochemical promotion.

Then, as commented above, upon varying the work function (by chemical or electrochemical addition of an electronic promoter) chemisorption properties of metal surface are changed. At the molecular level, this is a consequence of direct electrostatic interactions between adsorbed molecules and the electric field created by the coadsorbed promoter and indirect interactions due to the modification of the binding state of adsorbed reactants near the Fermi level of the metal. Hence, two rules derived by considering lateral attractive and repulsive interactions of co-adsorbed reactants and promoters were established [39, 40]:

-Increasing work function (by addition of electronegative promoters) strengthens the chemisorptive bond of electron donor adsorbates (D) and weakens the chemisorptive bond of electron acceptor adsorbates (A).

-Decreasing work function (by addition of electropositive promoters) weakens the chemisorptive bond of electron donor adsorbates (D) and strengthens the chemisorptive bond of electron acceptor adsorbates (A).

Alkali ions are electropositive promoters and, therefore, may act by enhancing chemisorption of electron acceptor species (decreasing work function), for example carbon monoxide and oxygen, and/or by suppressing chemisorption of electron donors, such as olefins and hydrogen [2]. A recent study related to the effect of electrochemical addition of alkali ions on the chemisorption properties of Pt catalyst can be found on reference  [41]. In this work the chemisorption properties of a Pt catalyst deposited on K–βAl2O3 solid electrolyte (source of electro-positive potassium ions) were investigated at varying catalyst potential (VWR) i.e., at varying potassium coverage on the Pt surface, via the isothermal titration technique (Fig. 2). In this experiment carried out at 250 °C, the Pt catalyst was firstly exposed to O2 for 15 min (at fixed potential). Then, the reactor was purged with pure He for 4 min to remove gaseous O2. Subsequently, the reactor was fed up with C3H6, and the amount of oxygen remaining (adsorbed) on the Pt surface (N) was obtained by integrating the area of the CO2 peak in the reactor effluent. The variation of the CO2 formation vs. time during the purge with C3H6 for each catalyst potential is shown on the Fig. 2. By integrating the area of the CO2 peak, one can measure the oxygen remaining on the surface (N) versus the applied catalyst potential (inset Fig. 2). It can be clearly observed at the same time that the catalyst potential decreased, the CO2 formation during the purge with C3H6 as well as the width of the CO2 peak strongly increased. Thus, as the catalyst potential decreased and hence the potassium coverage on the Pt catalyst increased, there was an enhancement of the strength of the Pt–O bond during the isothermal titration experiment. It leds to both an increase of the amount of oxygen remaining on the surface (N) after the desorption step, and a time prolongation in the CO2 formation during the purge with propene. Then is clear that the presence of potassium ions on the Pt catalyst surface strongly modified chemisorption properties of the Pt catalyst, e.g., O2 chemisorbed molecules increased by approximately a factor 20 on the Pt electro-promoted surface (by K+ ions). One can find in literature many other studies related to the effect of electrochemical addition of alkali ions on chemisorption properties of a catalyst. For instance Williams et al. [42] studied the electro-promotional effect of Cu catalyst deposited on Na–βAl2O3 on the catalytic reduction of NO by CO. The spectroscopy data clearly showed that the origin of the observed promotional effect by electrochemical pumping of Na+ to the Cu catalyst (increasing the NO reduction activity and N2 selectivity) was due to the strengthening of the metal-N bond (increasing the NO coverage) and the weakening of the N–O bond (facilitating the NO dissociation). Guided by these results Konsolakis et al. [19] synthesized conventional dispersed Pt/Al2O3 catalysts that when optimally promoted by Na, exhibited very large enhancements of both activity and selectivity showing the analogy between the two ways of promotion.

Fig. 2
figure 2

The effect of the catalyst potential on CO2 formation during the isothermal surface titration of oxygen by C3H6 at 250 °C on Pt–KβAl2O3. Inset influence of the catalyst potential on the mass of reactive oxygen adsorbed on the Pt surface. Reprinted with permission from Ref. [41]. Copyright 2014 Elsevier

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3 Electrochemical Promotion with Alkaline Ionic Conductors

3.1 Background

The phenomenon of electrochemical promotion with alkali ions has been widely studied in literature. A great number of catalysts and chemical reactions have been investigated with different kinds of alkaline ionic conductors materials (Na+, K+ conductors), especially by using Na–β–Al2O3, K–β–Al2O3 and NASICON (Sodium Super Ionic Conductor) derived materials. One can find an excellent review on the use of these kind of ionic conductor materials as catalyst supports [35].

The Na–β-alumina family, a series of compounds in the Na2O–Al2O3 system, is one of the most important groups of solid electrolytes that have been used in alkali electro-promotion studies. This material of excellent Na+ ionic conductivity has been used in several reactions such as propane oxidation [43], CO oxidation [44], ethylene epoxidation [45], NO reduction [46–48] and hydrogenations [49, 50]. The NASICON family, Na3Zr2Si2PO12, another kind of Na+ ionic conductor was also used for electrochemical promotion studies in ethylene [51] and propylene oxidation [52] and also NO reduction [53]. In 1997, K+ conductor was used for the first time for NEMCA experiments. Pitselis et al. [54] investigated the decomposition of NH3 on Fe films deposited on K2YZr(PO4)3 and they found an electro-promotional effect at temperatures near 500 °C. More recently, de Lucas-Consuegra introduced a different kind of material as a conductor of K+ ions, K–β–Al2O3, to electrochemically promote Pt catalyst on propylene oxidation [41], CO oxidation [55], NO x [56] and N2O reduction [57]. These are only several examples on the great number of studies of alkali electro-promotion. Table 1 summarizes the most relevant contributions along the history of EPOC with alkaline ionic conductors materials as solid electrolytes.

Table 1 Some of the most relevant contributions in the history of EPOC with cationic conductors
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It can be observed that from the pioneering work in 1991 on ethylene oxidation on Pt deposited on Na–βAl2O3 [58] several important contributions have been made in the field from both: fundamental and more applied point of view. Since this first report of alkali electro-promotion, several research efforts have been carried out leading to a thorough understanding of the phenomenon. So far the most important achievement has been the understanding of its origin and mechanism and its contribution to further clarify the effect of alkali promoters in heterogeneous catalysis [62]. The latest progress has been impressive pointing out technological researches in terms of catalyst-electrodes development and its investigation in novel catalytic reaction of environmental and industrial interest [41, 64–68].

3.2 Electrochemical Promotion versus Classical Promotion of Alkali Ions in Heterogeneous Catalysis

As already analyzed in Sect. 2 the chemisorption properties of a metal catalyst can be modified in the same way by the chemical or the electrochemical addition (back-spillover) of alkali ions. For example Lambert and coworkers [59] demonstrated via XPS that the state of Na is the same when it is introduced via evaporation on Pt interfaced with Na–βAl2O3 or via cathodic potential application (through the EPOC phenomenon). A more recent example which shows the same origin and effect of the two ways of promotion has been reported by Pekridis et al. [69]. In this case two different methods of catalyst promotion, were comparatively applied on a catalytic system of significant environmental and practical importance: the N2O reduction by hydrocarbons (alkanes and alkenes), over Pd catalysts. Results obtained clearly demonstrate the closed similarities that exist between the two methods of promotion, including the response of N2O and hydrocarbons conversion on K-promotion and reaction conditions. Similar values of the optimum promoter loading were also obtained in both cases [69]. The mode of action of electropositive promoters in conventional, highly dispersed, catalysts have also been recently studied by means of in situ diffuse reflectance infrared Fourier transform spectroscopy [70–72]. Results shown that the addition of alkalis (Na or K) to Pt and Pd based catalyst leads to electron-enriched metal sites, in which the chemisorption bond of electron acceptor adsorbates is enhanced. For instance in Ref. [70], Konsolakis and Yentekakis have analyzed with detail the relative population of adsorbed species formed on the surface of unpromoted and Na-promoted Pt/γ–Al2O3 catalysts, during the NO reduction by propene. It was found that under steady-state reaction conditions the surface of the unpromoted catalyst is mainly covered by carbon containing species, such as hydrocarbon fragments, carboxylates and cyanides. On the opposite, the surface of Na-promoted catalysts is predominantly covered by NO x adsorbed species, carbonyls and isocyanates, implying the significant effect of Na promoter on surface species formation. The further and clever investigation of the system by IR spectra under different transient atmospheres \( {\text{NO}} \to {\text{He}} \to {\text{C}}_{3} {\text{H}}_{6} \to {\text{NO}} \) revealed very interesting information regarding the formation of different kind of reaction intermediates on the unpromoted or Na-promoted Pt/γ–Al2O3 catalyst. The results showed that over Na-free catalysts the interaction of propene with NO x pre-adsorbed species (mainly nitrates) leads to the formation of strongly bonded carboxylates and cyanides, which are inactive towards NO. In contrast, NO interaction with the surface of Na-promoted catalysts results in the formation of nitro/nitrite species as well as to nitrosyls, which are highly active towards propene, leading to active intermediates such as isocyanates (NCO). These results clearly explained the strong promotional effect of alkali (Na) dossing to Pt catalyst that have been found in a large number of studies with conventional [18–21] and electrochemical [2, 29, 46, 47, 53] catalyst systems. Then is clear that the unique difference between electrochemical promoting with alkali ions a catalyst surface and the classical promotion (by chemical addition of alkali promoter during the preparation step of the catalyst) is that in the first case one can in situ control the amount of promoter on the catalyst surface. Then the two ways of promotion are equivalent and the unique difference between them is operational and not functional [2]. In fact, the effect of promoters on the overall kinetics of the reaction process only depends on the reaction order of the adsorbates involved in the reaction, their electronic nature (electronegative or electropositive) and the kind of ionic promoters supplied, cations (electropositive promoters) or anions (electronegative promoters). In 2001, simple, rigorous and general rules were developed by Vayenas et al. [62], which described the dependence of catalytic rates on catalyst work function. These rules are in very good agreement with the electrochemical and conventional promotion literature and they are able to predict the rate dependence on work function or promoter concentration on the basis of the rate dependence on the electron donor (D) or electron acceptor (A) reactant partial pressures onto the unpromoted catalyst surface [29]. These rules were progressively established on the basis of experimental observations on electrochemical promotion studies for more than 100 different catalytic systems, using different metals and solid electrolytes [35]. Hence one can distinguish four main types of global promotional behaviors:

Electrophobic Reactions: Those that show an increase in reaction rate for positive polarization. This type of behavior occurs when the kinetics are positive order in electron donor reactant and zero or negative order in the electron acceptor reactant. Electron donor is more weakly adsorbed on the catalyst surface, and therefore an increase in its chemisorption favors the overall kinetic process.

Electrophilic Reactions: Those that show an increase in reaction rate for negative polarization. This type of behavior occurs when the kinetics are positive order in electron acceptor reactant and zero or negative order in the electron donor reactant. Electron acceptor is more weakly adsorbed on the catalyst surface, and therefore an increase in its chemisorption favors the overall kinetic process.

Volcano-type Reactions: Those that exhibit a maximum in the reaction rate with respect to the applied potential. This type of behavior occurs when both electron donor and acceptor reactants are strongly adsorbed on the catalyst surface.

Inverted Volcano-type Reactions: Those that exhibit a minimum in the reaction rate with respect to the applied potential. This type of behavior occurs when both electron donor and acceptor reactants are weakly adsorbed on the catalyst surface.

Despite of the functional identity and origin between two ways of promotion (chemical and electrochemical) which follow the same general rules the usefulness of the latter is clear. Hence, Table 2 summarizes some of the most important advantages of electrochemical addition of alkali promoters versus its chemical addition to an heterogeneous catalyst. Among the different advantages presented here one of the most interesting one from a fundamental point of view may be the better interpretation of the metal-support interaction effect (MSI) described in literature on conventional dispersed catalysts [73]. It is well established that many supports of heterogeneous catalysts, besides altering physical properties of the catalytic system, such as providing high dispersion and surface area for the catalyst, have also a chemical role in catalysis and are not inert as initially thought. Even when the active phase has the same dispersion or average crystallite size, the support can have a dramatic effect on the catalytic activity or selectivity due to the phenomenon of MSI. Even if the origin of MSI is not fully clarified, recent studies were performed to compare the magnitude of MSI with that of EPOC by using metal-supported catalyst on oxygen ionic conductors such as TiO2 [74]. These experiments seem to demonstrate that the two phenomena can be explained by a similar process, that is, the backspillover of O2− ionic species from the support onto the metallic particles. These experiments of self-driven EPOC without any electrical polarization demonstrate that supported dispersed catalysts can also be electropromoted if the support is an ionic conductor. Because O2− backspillover can take place over micrometer distances on the porous Pt films used for EPOC studies with YSZ and TiO2, it can certainly take place over nanometer distances on the supported Pt nanoparticles of commercial highly dispersed YSZ and TiO2 supported catalysts. These experiments seem to indicate that the mechanism of MSI promotion of catalytic oxidations can be similar to the mechanism of self-driven electrochemical promotion systems and is based on self-induced migration of promoting O2− species on the nanoparticle catalyst surface. Nevertheless, the EPOC effect allows to control the backspillover of the ionic species and can be reproduced on highly dispersed catalyst which may be of significant importance for the development of more efficient catalyst formulations.

Table 2 Electrochemical promotion (EPOC) versus classical promotion with alkali ions
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These previous mentioned impact points of EPOC are clear and very interesting from both fundamental and technological point of view. However, considering that an external electric potential or current are required for the case of EPOC (in contrast with chemical promotion), the reader may ask about the electrical energy consumption for electrochemical promoting a catalyst surface. It has been reported that the cost of the energy to electrochemically supply the promoter ions is lower and typically negligible than other operational costs [2]. This is because a short amount of electrical energy is required, just to supply a low amount of promoter to the catalyst surface. Typical current density values of the order of μA/cm2 have been reported in literature at catalyst potentials typically bellow/1 V/ [41, 53] and maximum power density values required to obtain significant electropromoted states of 0.5 mW/cm2 of catalyst-electrode have been also reported [35]. Considering typical operating conditions of the EPOC studies reported, the energy required for heating the gaseous stream can be estimated around 1.54 mW/K. In terms of electrical energy, a previous work has reported values of the order of 10−3 J/cm2 to electrochemically promote a Pt–C catalyst film deposited on K–βAl2O3 solid electrolyte [67]. By this way, the permanent EPOC effect observed with cationic solid conductor also allows the saving of additional energy to electrochemically promoted a catalyst as will be analysed bellow [41]. This permanent effect along with other important recent findings of alkali electro-promotion will be analysed next.

3.3 Recent Findings of Alkali Electro-Promotion in Heterogeneous Catalysis

In this section the most relevant recent findings of electrochemical promotion with alkaline ion conductors are analyzed. A recent published experimental data of each demonstrated concept will be shown.

3.3.1 In-Situ Optimization of Alkali Promoter Coverage

As already mentioned in the introduction section of the paper a common feature of most of the previous works of alkali chemical and electrochemical promotion is that the catalytic rate is maximized for a certain alkali content on the catalytic surface [5, 6]. In addition the optimal promoter coverage on the catalyst surface depends on the reaction conditions: the temperature and the reaction mixture composition. This is a direct consequence of the origin of the promotional effect and the competitive chemisorption of reactants at varying reaction conditions. In fact it is well known that the promotional behavior depends on the reaction conditions. For instance, Vernoux et al. [52] showed that for the case of C3H6 oxidation reaction on Pt/NASICON near the C3H6/O2 stoichiometric ratio, a strong promotional effect is observed for negative overpotential (electrophilic NEMCA effect) while the opposite trend (electrophobic behaviour), was observed under large excess of O2. Hence, authors found an optimum sodium promoter coverage in the Pt catalyst under stoichiometric conditions under application of ΔV WR = −0.2 V, which corresponds to a sodium promoter coverage on the catalyst surface of θ Na = 0.0363. The possibility of in situ optimize the promoter coverage on the catalyst surface is of relevant importance in a wide variety of catalytic systems, for instance in non stationary process (e.g., automotive catalysis). A previous work demonstrated the possibility of optimizing the behavior of Pt/K–βAl2O3 in the selective catalytic reduction of NO x by C3H6 at varying reaction conditions such as those expected for the exhaust of an engine [56]. A more recent example comes from the group of Prof. Vayenas in the hydrogenation of CO2 over Ru-catalyst electrodes deposited on Na–βAl2O3 [75] (experiment of Fig. 3). Figure 3 shows the transient effect of the CO2 hydrogenation products (CH4 and CO) under a constant applied current and versus the corresponding Na coverage as well as the catalyst potential response (UWR), at T = 320 °C and P = 1 bar under highly reducing conditions (P H2/P CO2 = 14, P CO2 = 0.5 kPa). It can be observed that the application of a negative current (I = −10 μA) promotes the catalytic rates of CH4 and CO formation. Although the rate of CO production increases monotonically with the Na promoter coverage, the rate of CH4 formation firstly increases and then it decreases after a certain coverage of sodium (θ Na = 0.23) is achieved. Then this coverage of promoter allows to optimize the CH4 formation according to the kinetic order of the reactants molecules at the explored conditions. Other experiments carried out in the same study [75] (not shown here) showed that upon increasing the partial pressure of CO2 to 3 kPa at atmospheric pressure, the maximum rate of CH4 formation was strongly suppressed and displaced to lower Na+ coverage (0.05 instead of 0.23), because, in this case, the methanation rate is near to zero order in CO2. Upon increasing the pressure to five bar the rate of CO formation vanishes and CH4 was the unique product. Under those conditions, the maximum rate shifted to higher Na coverage levels (θ Na = 0.15). Therefore it is clear that the EPOC phenomena allows to rapidly explore and supply the optimum promoter amount at different reaction conditions. This is not possible in conventional catalytic promotion (chemical) where each catalyst has a fixed amount of promoter. One can find in literature other similar recent examples in a wide variety of catalytic systems such as the steam reforming and partial oxidation of methanol [67], water gas shift reaction [76], or ethylene epoxidation [45], etc.

Fig. 3
figure 3

The transient effect of a constant applied current and corresponding Na coverage on rates of CH4 and CO formation on Ru/β“–Al2O3 and on the catalyst potential UWR. P t = 1 bar, P H2 = 7 kPa; PCO2 = 0.5 kPa, T = 320 °C. Reprinted with permission from Ref. [75]

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3.3.2 Electrochemical Activation of a Metal Catalyst at Lower Reaction Temperatures

The activation of metal catalyst at lower reaction temperatures has a great technological importance especially in environmental catalysis. For example in the treatment of car exhaust engine emissions where most of the pollutant emissions occur during the cold start engine regime. A previous work of de Lucas-Consuegra et al. studied CO oxidation activities on Pt/K–βAl2O3 electrochemical catalyst through temperature-programmed reaction experiments (light-off measurements) under application of five different catalyst potentials [55]. Results obtained showed that as the catalyst potential decreased to lower values (electrochemically supplying K+ ions to the Pt catalyst) the activity of the catalyst began at lower reaction temperatures. For example under application of −2 V, the value of the light off temperature (Temperature at which the catalyst achieved 50 % of conversion) dropped by almost 40 °C in comparison to the un-promoted Pt catalyst (under application of 2 V). Another important point is that the temperature corresponding to the beginning of the catalytic activity (5 % of CO conversion) decreased from about 240 °C without promoters (+2 V) to 210 °C in the presence K+ cations (−2 V). Recently a similar example have been reported for the selective catalytic reduction of N2O by C3H6 also using a Pt/K–βAl2O3 electrochemical catalyst [57]. The SCR activity of the system was also investigated through temperature-programmed reaction experiments (light-off measurements) under application of four different catalyst potentials (Fig. 4). Experiments were carried out at a fixed gas composition of C3H6/N2O/O2: 2,000/1,000/2,000 ppm between 200 and 600 °C with a constant heating rate of 2 °C/min. It can be observed, that as the catalyst potential decreased, the system could be strongly promoted above 340 °C. While in the case of unpromoted catalyst (V WR = 2 V) the N2O conversion did not exceed 17 % at high temperatures (580 °C), the electrochemically promoted ones (V WR <2 V) exhibited higher conversions at much lower temperatures. For instance, for an applied catalyst potential of V WR = −2 V (which allowed to supply the optimal promoter coverage for the explored reaction conditions), the temperature corresponding to 15 % of N2O conversion (T15) decreased by 167 °C in comparison to the unpromoted catalyst (V WR = 2 V). These results demonstrate the interest of EPOC for activating a catalyst at lower reaction temperatures which may also have strong importance for reactor energy savings in industrial processes.

Fig. 4
figure 4

N2O conversion versus the reaction temperature at different catalyst potential on Pt–KβAl2O3. Conditions: C3H6/N2O/O2: 2,000 ppm/1,000 ppm/2,000 ppm, He balance. Reprinted with permission from Ref. [57]. Copyright 2014 Elsevier

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3.3.3 Enhancement of Catalytic Selectivity

One of the most promising features of alkali-electropromotion is related to product selectivity modification and a great number of examples can be found in literature. Particularly interesting is the case of the selective catalytic reduction of NO x , where the production of N2 instead of N2O (a greenhouse effect gas) is one of the key aspects for the design of effective catalyst that allows to reduce pollutants concentration in future legislation. For instance Vernoux et al. [53] studied the SCR of NO x on Pt deposited on NASICON under real oxidizing conditions. They found an increase in the N2 selectivity at 295 °C from 30 to 65 % under cathodic potential application (electrochemically supplying Na+ ions to the Pt catalyst). Similar results have also been reported on the SCR of NO x under similar oxidizing conditions and lower reaction temperatures on Pt/Na–βAl2O3. In this work N2 selectivity was increased from 20 to 45 % under Pt modified by Na+ surface [47]. Even under wet reaction conditions and large excess of Oxygen (more representative conditions from a real exhaust of a car engine), the N2 selectivity was increased from 40 to almost 100 % of N2 selectivity on Pt/K–βAl2O3 electrochemical catalyst [56]. A similar electro-promotional effect has been reported in the SCR of NO x for a wide range of reaction conditions and reductant agents such as CO [44, 48], ethylene [59] and even by using non noble metal catalysts such as Cu [42]. The importance of Electrochemical Promotion with alkali ions in catalyst selectivity has also been widely investigated and demonstrated in other catalytic process of great industrial interest. For instance in the Selective Hydrogenation of Acetylene [77], Fischer–Tropsch synthesis [64] or CO2 hydrogenation [75]. A more recent and impressive example can be found on Ref. [68] for the synthesis of fuels by CO2 hydrogenation on Cu depossited on K–βAl2O3 (at bench scale). Figure 5 shows the effect of the response of CO2 conversion and selectivity to CH3OH, C2H5OH and C2H6O to different applied potentials (between 4 and −2 V). Experiments were carried out at 325 °C and using a H2/CO2 ratio of 3. It can be observed that the CO2 hydrogenation is significantly affected by the applied potential, with selectivity to CH3OH, C2H5OH and C2H6O enhanced up to 27.7, 27.9 and 47.1 %, respectively, under optimal potential conditions (optimal potassium coverage on the Cu catalyst). These results demonstrate the interest of EPOC with cationic conductors materials in this case for the production of different target fuels whose selectivity can be modulated by modifying applied potential under given operating conditions. Its application in other catalytic fields, for example in fine chemistry, may open novel opportunities for the improvement of catalytic processes.

Fig. 5
figure 5

The influence of the applied potential on (closed square) CO2 conversion and on selectivity to (closed circle) CH3OH, (closed triangle) C2H5OH and (closed inverted triangle) C2H6O, (H2/CO2 = 3, T = 325 °C) on on Cu–KβAl2O3. Reprinted with permission from Ref. [68]. Copyright 2014 Elsevier

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3.3.4 Permanent Electrochemical Promotion

The permanent EPOC effect is one of the most interesting and potentially important characteristics of the electrochemical promotion from a practical point of view [2]. It allows to keep the electro-promotional effect on the catalytic system upon current interruption (under open circuit conditions). By this way an initial polarization is only required to electrochemically supply promoter ions which allows to reduce electrical energy requirements, polarizing the catalyst only for a short period of time. This permanent EPOC effect was firstly discovered by Comninellis for the case of (O2−) conductors [78] and latter it has also been reported for the case of K+ conductor materials [41, 79]. Hence, for the case of alkali electro-promotion the origin of this phenomenon has been explained considering the stability of the promoter species formed by the electrochemical supplied ions and the co-adsorbed reactants molecules. Hence, as already demonstrated with different in situ and ex situ characterization techniques, the electrochemically supplied promoting ions under EPOC reaction conditions form surface compounds with the different chemisorbed reactants molecules under reaction conditions [42, 52, 56, 63, 65, 80]. Resultant promotional phases some times are very stable and remain in the catalyst surface even under open circuit potential conditions. For instance Fig. 6 shows an example of the permanent EPOC effect for the case of propene oxidation reaction with Pt/K–βAl2O3 electrochemical catalyst under C3H6/O2: 2,000 ppm/1 % at 270 °C [41]. This experiment depicts the response of the propene conversion versus time to step changes in applied potential. Apart from the strong promotional effect observed during the negative potential application of −2 V, the most important point of this experiment is that after the negative polarization, at t = 7 h, under open circuit conditions (O.C.V.), the catalyst showed just a slight decrease on its catalytic activity. Indeed, after 5 h, the propene conversion decrease was lower than ten points. The investigation by a variety of in situ (cyclic voltammetry, FTIR) and ex situ techniques (SEM-EDX) have shown strong evidences that the permanent NEMCA effect was due to the electrochemical creation and stabilization of promoter oxides and superoxides on the catalyst surface such as potassium oxides and peroxides [41]. These kind of species are formed on the catalyst surface during the negative polarization step and are stable under open circuit conditions leading to the observed permanent effect.

Fig. 6
figure 6

Propene conversion response versus time to step changes in applied catalyst potential: 2,000 ppm C3H6, 1 % O2, T = 270 °C on Pt–KβAl2O3. Reprinted with permission from Ref. [41]. Copyright 2014 Elsevier

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3.3.5 Electrochemical Storage of Surface Compounds

The possibility of electrochemical store surface compounds on a porous catalyst film via electrochemical supply of alkali ions has opened a novel application of cationic electrochemical catalyst. Hence the possibility of store large amount of surface compounds was firstly reported by de Lucas-Consuegra et al. [65]. In this previous work a tubular Pt/K–βAl2O3 electrochemical cell was used as NO x storage/reduction catalyst that can operate over a range of reaction conditions for the effective removal of NO x . In the first step under negative polarization (electrochemical pumping of potassium ions), NO x was stored on the catalyst surface in form of potassium nitrates. In this initial step potassium ions electrochemically transferred to the Pt catalyst have a double role in the NSR process: as a promoter for the NO oxidation reaction and as storing sites by the formation of potassium nitrates. In a second step, under positive polarization the catalyst was regenerated, and stored nitrates were efficiently desorbed and reduced to N2. The variation of the current under the applied polarizations allowed monitoring the progress of both: storing and regeneration phases, and therefore to optimize the duration of both sequences in a feasible technical way. Moreover, the possibility of electrochemical regeneration of the catalyst surface allowed to work under a fixed lean gas composition, which implies an important technological advance for the NSR process [81]. A more recent example on the use of a cationic electrochemical catalyst to store and remove pollutants has also been developed by Ruiz et al. for the case of CO2 capture [82]. In this research, authors reported a bench-scale study of the electropromoted CO2 capture over Pt on K–βAl2O3, at high flow rate and using simplified exhaust gas compositions (CO2/N2 and CO2/O2/N2). Hence Fig. 7, shows the response of current density (j) and CO2 concentration versus time (t) and potential (E) during an anodic voltammetry (catalyst regeneration at 400 °C), after an adsorption step at −1 V in the presence of CO2 (12.6 %) and O2 (5.3 %) at 400 °C. As can be observed on this figure, two anodic (positive) current peaks were obtained during the voltammetry. It seems evident that the sharper peak at 1.8 V, which coincides with the maximum in released CO2, corresponds to decomposition of potassium carbonate species previously stored during negative polarization. Therefore, it seems to confirm the possibility of monitoring the decomposition of captured compounds by a simple electrical measurement without the experimental analysis of the exiting CO2 content. The overall amount of stored CO2 calculated from the experiment of Fig. 7 was around 1,130 mg CO2/g Pt. Additionally the authors reported that CO2 capture is enhanced on decreasing Pt particle size, in the presence of O2 and at higher temperatures up to a certain value which depends on the thermal stability of the species formed. The Pt/K–βAl2O3 system can be regenerated, allowing CO2 separation, by electro-chemical decomposition of previously stored compounds without increasing temperature.

Fig. 7
figure 7

CO2 concentration and current density variation (j) versus the applied potential/time during linear voltammetry in N2 at 400 °C over electrocatalyst Pt/K–βAl2O3. Adsorption step at −1 V in the presence of CO2 (12.6 %) and O2 (5.3 %) at 400 °C. Reprinted with permission from Ref. [82]. Copyright 2014 Elsevier

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3.3.6 In Situ Electrochemical Regeneration of a Deactivated Catalyst at Fixed Conditions

One of the typical common limitations of heterogeneous catalyst is its deactivation by carbon deposition for instance in hydrocarbon reforming and partial oxidation reactions. It leads to a progressive decrease in a number of catalytic active sites with the consequent deactivation of the catalyst. Thus, current intense research efforts are being placed to develop new catalyst configurations, which are able to overcome the above mentioned limitations for on-board H2 production from methane. In a recent work [66] a new electrochemical catalyst (Pt–Pt/YSZ/Na–βAl2O3) has been prepared and characterized for the H2 production at low temperature from CH4 with a high CO2 selectivity. It has been tested under Electrochemical Promotion conditions under different reaction systems: steam reforming, partial oxidation and autothermal steam reforming. Among them, the latest system seems to be the most suitable one for the operation of the electrochemical catalyst in a cyclical way between positive and negative polarizations. Hence, Fig. 8 shows an example on the mode of operation of the electrochemical catalyst. This figure shows the variation of H2, CO and CO2 production rates with time on stream under different cycles of positive and negative potentials under ATR conditions (CH4/H2O/O2: 1 %/4//0.2 %, N2 balance at 500 °C). In this experiment, a negative potential of −1 V was applied for 4 h and then a positive polarization of 2 V was applied for 2 h. It can be observed that under positive polarization, the electrochemical catalyst produced a high amount of H2 however a strong deactivation was also observed due to the deposition of carbon. Upon the application of this positive potential increased the chemisorption of CH x-species was increased, enhancing the production of H2 but leading to a fast deactivation due to the low coverage of water and oxygen molecules responsible from the removal of carbon. Nevertheless, the subsequent application of negative polarization leds to a strong increase in the coverage of O2 and H2O, electron acceptor molecules (induced by the electrochemical supply of Na+ ions) that caused the removal of the deposited carbon and, hence, the regeneration of the catalyst. This system showed a reproducible behaviour along different cycles demonstrating that the phenomenon of electrochemical promotion with alkali ionic conductors could be used to regenerate, in an unlimited way, the catalytic performance of the system for the low temperature H2 production from CH4. In addition, this catalyst regeneration could be carried out under fixed reaction conditions, which would avoid the requirement of modifying the reaction atmosphere in contrast with conventional catalytic systems.

Fig. 8
figure 8

The Influence of the applied potential on H2, CO and CO2 production rates during the reproducibility experiment under ATR conditions (CH4/H2O/O2: 1/4/0.2 %) at 500 °C on on Pt–Pt/YSZ–NaβAl2O3. Reprinted with permission from ref [66]. Copyright 2014 Elsevier

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4 Conclusions and Prospects

Electronic promoters such as alkali ions play a key role in the improvement and development of novel catalyst configurations. Thus, upon adding promoters on the catalyst surface (chemically or electrochemically), the chemisorptive bond strength of reactants and intermediates is modified and the catalytic kinetic is usually changed dramatically.

The possibility of in situ control the alkali promoter coverage on the catalyst surface under working conditions through the EPOC concept opens new opportunities in the field of heterogeneous catalysis. Related to environmental catalysis, the optimization of the catalytic activity at varying reaction conditions or the activation of metal catalyst at lower reaction temperatures are of great technological importance especially in the treatment of car exhaust engine emissions. The improvement of catalytic selectivity, the possibility of regenerating a catalyst at fixed reaction conditions or the electrochemically assisted synthesis of renewable fuels from CO2 could also have a strong importance in fine chemistry and in large scale industrial catalytic processes.

The phenomenon of Electrochemical Promotion has completely opened a new research field to explore novel alternatives in order to improve chemical and environmental industrial processes in the future. The technological progress of this phenomenon has to be focused on finding new industrial and environmental reactions and new ways of operating catalytic systems. It is also challenging to investigate EPOC in potential catalytic reactions with a non well established market in order to assist the technology entrance. In addition, further fundamental studies should be done to extend chances for EPOC commercial applications. The multidisciplinary approach of EPOC combining heterogeneous catalysis, solid state electrochemistry and materials science has certainly contributed to the wide utilization of ionically conducting ceramics in catalysis. The understanding of the interactions of metallic active sites and ionically conducting support through the EPOC phenomenon may contribute with no doubt to the development of more efficient catalysts formulations. Moreover, the development of new materials: solid electrolytes of higher ionic conductivities at lower temperatures, and new catalyst-electrodes of higher catalytic activity could be a good contribution to broaden applications of this phenomenon.