High temperature proton conductors: Applications in catalytic processes

Solid State Ionics 178 (2007) 507 – 513 www.elsevier.com/locate/ssi

High temperature proton conductors: Applications in catalytic processes Christos Kokkofitis, Martha Ouzounidou, Aglaia Skodra, Michael Stoukides ⁎ Chemical Engineering Department, Aristotle University and Chemical Process Engineering Research Institute, U. Box 1517, University Campus, Thessaloniki 54124, Greece Received 26 August 2006; received in revised form 30 October 2006; accepted 20 November 2006

Abstract In addition to their basic applications, i.e. sensors, separators, fuel cells and hydrogen pumps, high temperature proton conductors have been also used in the construction of proton conducting membrane cell-reactors in which catalytic hydro- and dehydrogenations are optimized in terms of their yield or selectivity to the desired product. The fundamental operating principles, results in technologically important reactions, and the hurdles to be overcome in order to promote these systems into an industrial scale, are discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemical membrane reactors; Solid state proton conductors; NEMCA

1. Introduction In 1981, Iwahara and coworkers [1] reported that some perovskite-type materials based on SrCeO3 exhibited protonic conductivity at elevated temperatures (500–1000 °C). Since then, a large number of proton conducting ceramics have been synthesized and their proton conduction properties have been studied [2–7]. The various applications of high temperature proton conductors (HTPC) have been described in previous review articles and include sensors, fuel cells, hydrogen separators, proton conducting membrane reactors, separation of hydrogen isotopes and steam electrolysers [7–12]. The present review is restricted to only one of the above applications, namely the proton conducting membrane reactors (PCMR). Because the above classification of applications is not very strict, it is possible that one work may belong partly to more than one category. For example, the reaction of steam reforming of a hydrocarbon may take place at the anode of a PCMR with simultaneous separation of the produced hydrogen from the reacting mixture. Also, a hydrogen separator in which hydrogen is immediately used as a reactant, may be considered as a PCMR as well. Therefore, the criterion used in order to include studies herein was that the primary goal of the PCMR ⁎ Corresponding author. Tel.: +30 231 0996165; fax: +30 231 0996145. E-mail address: [email protected] (M. Stoukides). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.11.010

was the production of a compound through a chemical or electrochemical reaction. Since this topic is related to several scientific fields, (Chemical Reaction Engineering, Heterogeneous Catalysis, Electrochemistry), the scope of this paper is to provide a survey of relevant studies and also to present the characteristic features, methods and techniques used in this class of reactors. 2. Operation modes and designs of a PCMR Fig. 1 shows schematically the configuration of a PCMR. The cell-reactor consists of a dense solid state proton conducting membrane and two porous electrodes. For simplicity, the overall chemical reaction that takes place can be written stoichiometrically as: A , m B þ n H2

ð1Þ

The two electrodes are connected to a voltmeter (case a), or to an external resistive load (case b), or to an external power source (case c). As long as the chemical potential of hydrogen is different at the two sides (electrodes) of the cell, a driving force for hydrogen transport across the HTPC exists and the cell may operate in one of the following modes: a) In the open-circuit operation, (case a). There is no net current through the electrolyte. The difference in chemical potential

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is converted into the open-circuit voltage (OCV) of the cell. Reaction kinetics can be combined with OCV data in order to elucidate the reaction mechanism. Several catalytic systems, primarily oxidation reactions, have been studied with the use of this technique [13–17], which, however, can be extended to the study of hydro- and dehydrogenation reactions [17]. b) In the closed-circuit operation (cases b and c of Fig. 1), A is converted into B and H+: A , m B þ 2n Hþ þ 2n e−

ð2Þ

In the form of H+, hydrogen is electrochemically transferred to the cathode, where 2 protons combine to produce H2: 2n Hþ þ 2n e , n H2

ð3Þ

It can be seen that Eqs. (2) and (3) can be added together to produce the same overall reaction, i.e. Reaction (1). If the goal is the production of electricity, chemical energy can be converted directly into electrical energy (case b). If, on the other hand, the primary goal is the production of product B, the external power source can be used to impose a current (and equivalently a hydrogen flux) through the cell in the desired direction (case c). This mode of operation is called electrochemical hydrogen “pumping”. In fact the term “pumping” should be used only in cases where hydrogen is forced to flow in the direction opposite to the thermodynamically expected. It has been also used, however, to show that the flux is driven externally, regardless of the spontaneous direction of the flow. Depending on if the two electrodes are gas-tight separated from each other, two reactor designs may exist, the doublechamber and the single-chamber reactor. A schematic diagram of a typical double-chamber PCMR is shown in Fig. 2. In this design, anode and cathode are exposed to different gaseous

Fig. 2. Schematic of a double-chamber PCMR. The two electrodes are gas-tightly separated.

mixtures. Fig. 3 is a schematic of a single-chamber PCMR. Unlike the other cell-reactors, this cell does not provide for separate feed in two chambers. The HTPC disk is suspended in a flow of the reacting mixture. A big advantage of this design is that it is easy to apply to existing catalytic processes as it does not require reactants to be separated. The proton conducting ceramic simply replaces the conventional catalyst support. PCMRs can be used to electrochemically promote catalytic reaction rates. If the catalyst to be promoted is one of the electrodes, protons can be “pumped” to or away from the catalyst during reaction. This can alter the catalytic activity and/ or the selectivity of the reaction under study. The effect of electrochemical pumping can be quantitatively expressed by the use of two dimensionless parameters [15]: K ¼ Dr=ðI=2FÞ;

ð4Þ

and q ¼ r=r0

ð5Þ

where I is the imposed current, F is Faraday's constant, r is the catalytic reaction rate obtained at closed circuit, r0 is the opencircuit catalytic rate and Δr = r − r0. If Λ = 1, the effect is Faradaic, i.e. the increase in reaction rate equals the rate of

Fig. 1. Schematic of a Proton Conducting Membrane Reactor (PCMR). (a): open circuit operation, (b): fuel cell mode, (c): H+ “pumping” mode.

Fig. 3. Schematic of a single-chamber PCMR. The proton conducting disk is suspended in a flow of the reacting mixture.

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proton transport through the electrolyte. In many cases, a strong non-Faradaic effect has been observed with Λ values of the order of 105 and ρ values of the order of 103 [15]. This is the NEMCA (Non-Faradaic Electrochemical Modification of Catalytic Activity) phenomenon. Details can be found in Ref. [15]. NEMCA can have a significant impact on catalytic research because a) unlike traditional catalytic promoters, the surface modification can be monitored electrochemically, and b) the product yield may be altered by imposing an electric current orders of magnitude lower than the stoichiometrically required. Eq. (4) is valid as is if the solid electrolyte is a pure proton conductor. If not, it has to be modified to include the proton transport number, PTN, which is the fraction of the total current that is carried through the solid electrolyte in the form of H+: PTN ¼ ðmoles of H2 =s transfered through the proton conductorÞ=ðI=2FÞ

ð6Þ

Hence, in the general case (0 b PTN b 1.0), Eq. (4) is modified as: K ¼ Dr=ðI=2FÞdPTN

ð7Þ

3. Catalytic studies in PCMRs Table 1 contains the catalytic reaction systems that have been studied in PCMRs. The first column on the left-hand side contains abbreviated symbols of the type of HTPC that was used. The exact formula of these electrolytes can be found at the bottom of the table. The second column shows the catalyst used, which was also the working electrode (WE) of the PCMR. In certain studies [18,19], there was no working electrode because the HTPC was a mixed conductor (e.g. H+ and e−) and therefore the cell was self-short-circuited. The next two columns show the reactants and products for med on the WE and the counter electrode (CE), respectively. It can be seen that in most of these studies, the reaction of interest was catalyzed on the anodic electrode, i.e., where protons were produced. The next column shows the reactor type (DC = double-chamber, SC = singlechamber) and the last column contains the reference(s) of each work. A summary of the most important findings for each reaction system is given below. 3.1. Methane conversion to C2 hydrocarbons The direct conversion of methane to versatile industrial raw materials, such as ethane or ethylene, is advantageous and financially preferable. In the 1980s, numerous investigators searched for the appropriate catalyst that would favor the oxidative coupling of methane by successfully suppressing the complete oxidation to CO2 and Ç2O. Several alternatives to the traditional reaction of gaseous methane and oxygen were examined including membrane reactors of all types [20,21]. The discovery of HTPCs directed several research groups in to a different route based on methane dimerization via dehydrogenation rather than partial oxidation. Hence, the reactions

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assumed to occur at the anode and the cathode of a PCMR could be: 2CH4 ⇔C2 H6 þ 2Hþ þ 2e− ðor C2 H4 þ 4Hþ þ 4e− Þ

ð8Þ

2Hþ þ 2e− ⇔H2

ð9Þ

respectively. The first report of methane dimerization in a PCMR was in 1987 [22,23]. K. Mori used a SCYb (SrCe0.95Yb0.05O3−a) HTPC and Pt electrodes. By imposing a current through the cell, hydrogen was abstracted from a methane stream and was electrochemically transported to the cathode. At the same time, methane was dimerized to ethylene. Two products were simultaneously produced and separated, hydrogen at the cathode and ethylene at the anode. H. Iwahara et al. [24], used Pt electrodes and operated their PCMR as a chemical cogenerative fuel cell at 800–1000 °C. A mixture of steam and methane was flowing over the anode and was partially converted to C2 hydrocarbons. At the cathode, hydrogen was oxidized to water by an oxygen stream. Other research groups [25–29] have also studied the conversion of methane to C2 hydrocarbons using PCMRs as H+ pumps. It should be mentioned that the proton transference number of Sr–Ce and Ba–Ce perovskites may vary and depending primarily on the partial pressures of hydrogen and oxygen, these membranes may exhibit almost pure protonic [23,24,27], mixed protonic-electronic [18,25,30] and even mixed oxygen ion-protonic-electronic [30] conductivity. Hence, mixed conducting membranes, either with [25,30] or without electrodes [18,19,31], have been also utilized in an effort to achieve acceptably high yields to C2 products. Worth noticing is that several mixed conducting perovskites (e.g. strontium cerates) are very efficient methane coupling catalysts and have been used as such in regular catalytic reactors [32]. In all cases, however, low C2 yields were reported (less than 2%). One of the reasons for this is that without oxygen, the reaction of methane pyrolysis inevitably takes place: CH4 ⇔C þ 2H2

ð10Þ

3.2. Other reactions of methane activation The conversion of methane into synthesis gas (CO and H2) was tested by Iwahara, Uchida and Morimoto [24], who used a Nd-doped barium cerate as a HTPC and Pt electrodes to construct a mixed-conducting (H+ and O2−) PCMR. Mixtures of methane with steam were fed in at 900–1000 °C and high conversion to syngas was observed. At the cathode, H+ and O2 reacted to produce H2O. The advantage of this cell is that both conducting ions contribute to the formation of the desired products. K. Mori used a SCYb HTPC and Pt electrodes to decompose CH4 in to C and H2 at the anode and then transfer and produce H2 at the cathode [22]. Belyaev et al., used a PCMR with Pt electrodes to oxidize CH4–O2 mixtures to CO2 at the cathode and simultaneously electrolyze steam at the anode

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Table 1 Catalytic studies in high temperature proton conducting membrane reactors HTPC

Reactants and products on W.E.

Reactants and products on C.E.

PCMR Type

Ref(s)

Methane conversion to C2 hydrocarbons SCYb Pt SCYb Pt SCYb La0.6Sr0.4 MnO3 SCYb Ag SCYb Ag BCM – SCYb SrTi0.4Mg0.6O3 SCYb –

Catalyst (W.E.)

CH4 → H+, C2H4, C2H6, e− CH4 (H2O) → H+, C2H4, C2H6 (+COx), e− CH4 (H2O) → H+, C2H4, C2H6 (+COx), e− CH4 → H+, C2H4, C2H6, e− CH4 → H+, C2H4, C2H6, e− CH4 → H+, C2H4, C2H6, e− CH4, H2O → H+ C2H4, C2H6, COx, e− CH4 → H+, C2H4, C2H6, e−

e−, H+ → H2 e−, H+, O2 → H2O e−, H+, (O2) → H2 (H2O) e−, H+, (O2) → H2 (H2O) e−, H+ → H2 e−, H+, (O2) → H2 (H2O) e−, H+, O2 → H2O e−, H+, O2 → H2O

DC DC DC SC DC DC DC DC

[22,23] [24,25] [26] [27–29] [30] [18] [31] [19]

Other reactions of methane activation SCYb Pt SCYb Pt BCN Pt SCYb Pt SCD Pt BCY Ag BCY Pt

CH4 → H+, C, e− CH4, H2O → H+, CO, e− CH4, H2O → H+, CO, e− H+, CH4, CO2, e− → CO2, H2O, C2O, C2H4, C2H6 H+, CH4, O2 e− → CO2, H2O CH4, O2 → H+, CO2, e− CH4 (H2O) → H+, CO2, e−

e−, H+ → H2 e−, H+ O2 → H2O e−, H+, O2 → H2O H2 → e−, H+ H2O → e−, H+, O2 e−, H+ → H2 e−, H+, O2 → H2O

DC DC DC DC DC DC DC

[23] [24] [69] [34] [33] [37] [35,36]

Decomposition of alcohols SCYb Pt SCYb, BCN Pt

C2H5OH → H+, CO, H2, e− CH3OH → H+, CO, H2, e−

e−, H+, O2 → H2O e−, H+, O2 → H2O

DC DC

[38] [24]

Reactions with alkanes and alkenes BZY Pt SCYb Pt, Ni CZI Pt BCaN Pt SCYb Pt, Pd SCYb Pd

C2H4, O2 → H+, CO2, H2O, e− C2H6 → H+, C2H4, H2, e− C2H4, O2 → H+, CO2, H2O, e− C2H4, O2 → H+, CO2, H2O, e− C3H8 → H+, C3H6, H2, e− C3H8, H2O → H+, C3H6, H2, e−

e−, H+, O2 → H2O e−, H+, O2 → H2O e−, H+, O2 → H2O e−, H+, O2 → H2O e−, H+ → H2 e−, H+ → H2

SC DC SC SC DC DC

[17] [38] [41] [42] [39] [40]

Forward and reverse water gas shift reactions SCYb Pt CO, H2O → H+, CO2, H2, e− SCYb Pt CO, H2, H2O → H+, CO2, e− SZY Cu H+, CO2, e− → CO, H2 SCYb Pd CO, H2O → H+, CO2, H2, e− BCY Pt CO (H2O) → H+, CO2, e−

e−, H+, O2 → H2O e−, H+ → H2 H2 → e−, H+ e−, H+ → H2 e−, H+, O2 → H2O

DC DC DC DC DC

[38] [44] [47] [46] [36]

Steam electrolysis SCYb SCYb BCY SCY,SZYb SCYb, SZY

e−, H+ → H2 H+, O2, e− → H2O e−, H+ → H2 e−, H+ → H2 e−, H+ → H2

DC DC DC DC DC

[44,48] [51] [50] [49,52] [45]

Pt Pt Pt,Ag Pt Pt

H2O → H+, O2, H2O → H+, O2, H2O → H+, O2, H2O → H+, O2, H2O → H+, O2,

e− e− e− e− e−

Decomposition of H2S SCYb, LS Pt

H2S → H+, S, e−

e−, H+ → H2

DC

[53,54]

Decomposition and Reduction of NOx SZYb Sr/Al2O3 SCYb, SZYb Pt/Ba/Al2O3

H+, NO, e− → N2, H2O H+, NO, e− → N2, H2O

H2O → e−, H+, O2 H2O → e−, H+, O2

DC DC

[55] [56,57]

Synthesis and Decomposition of NH3 SZI Fe SCYb Pd SZI Fe LCZ Pd–Ag BCaN,BCG Pd–Ag CM Pd–Ag SCYb Ru

NH3 → H+, N2, e− H+, N2, e− → NH3 H+, N2, e− → NH3 H+,N2, e− → NH3 H+,N2, e− → NH3 H+, N2, e− → NH3 NH3 → H+, N2, e−

e−, H+ → H2 H2 → e−, H+ H2 → e−, H+ H2 → e−, H+ H2 → e−, H+ H2 → e−, H+ e−, H+ → H2

SC DC, SC SC DC DC DC DC, SC

[65] [58,59] [60] [61] [62,63] [64] [66]

BCaN = BaCa1.18Nb1.82O3−a; BCG = BaCe0.8Gd0.2O3−a; BCM = BaCe0.95Mn0.05O3−a; BCN = BaCe0.90 Nd0.10O3−a; BCY = BaCe0.90Y0.10O3−a; BZY = BaCe0.90Y0.10O3−a; CZI = CaZr0.90In0.1O3−a; CM= Ce0.8M0.2O2−a (M = La, Y, Ga, Sm); LS= Li2SO4; LCZ = La1.9Ca0.1Zr2O6.95; SCYb = SrCe0.95Yb0.05O3−a; SCD = SrC e0.92Dy0.08O3−a; SZYb = SrZr0.90Yb0.1O3−a; SZY = SrZr0.90Y0.1O3−a.

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[33]. Hibino et al., passed CH4–CO2 mixtures over the anode and pumped H+ away, to accelerated both, the reforming of methane and its conversion to C2 hydrocarbons [34]. W.G. Coors studied the methane steam reforming reaction in a 10% yttrium-barium cerate PCMR operating in the fuel cell mode. Ambipolar steam permeation from the cathode to the anode, eliminated the requirement for steam injection [35,36]. In a recent study, S. Yamaguchi et al. [37] converted CH4–O2 mixtures to CO2 and H+ at the anode, with the latter transported through the HTPC to produce pure H2 at the cathode. 3.3. Decomposition of alcohols. reactions of alkanes and alkenes H. Iwahara and co-workers examined the operation of PCMRs in the fuel cell mode with methanol-steam or ethanolsteam mixtures decomposing at the anode and H+ reacting with O2 at the cathode to produce H2O [24,38]. Strontium and barium-doped cerates were tested as HTPCs. The overall conductivity of barium cerates was higher but, at the same time, these materials exhibited mixed H+ and O2− conductivity [38]. The same research group studied the performance of a PCMR in which ethane was converted to ethylene with simultaneous generation of electrical energy. The cell operated at 800–1000 °C and the anode materials tested were platinum and nickel. Water vapor was added to the ethane stream for improved performance [38]. Recently, Karagiannakis et al., studied the conversion of propane to propene and hydrogen at the anode of a SCYb PCMR in which hydrogen was separated from the reacting mixture [39,40]. Polycrystalline Pt and Pd electrodes were used as anodes. When, instead of pure propane, propane-steam mixtures were introduced in the PCMR, an up to 90% of the produced hydrogen could be electrochemically separated [40]. The oxidation of C2H4 on Pt was studied in single-chamber cells by Makri et al. [41], Thursfield et al. [42] and Poulidi et al. [17], using CZI (CaZr0.90In0.1O3−a), BCaN (BaCa1.18Nb1.82O3−a) and BZY (BaCe0.90Y0.10O3−a) conductors, respectively. A NEMCA effect was observed by all three groups. Values of ρ and Λ as high as 12 and 1000, respectively, were obtained upon “pumping” protons to the catalyst [42]. 3.4. Forward and reverse water gas Shift The water gas shif t (WGS) reaction, (CO + H2O ⇔ CO2 + H2), is one of the key technologies in the hydrogen purification processes of syngases obtained by steam reforming or partial oxidation of hydrocarbons. PCMRs can be used to selectively remove hydrogen from the reacting mixture and at the same time, shift the reaction equilibrium to the right and obtain higher conversions [7–9]. Furthermore, this has been proposed as a novel method for CO2 separation from flue and fuel gases [43]. H. Iwahara et al. [38] and H. Matsumoto et al. [44], used SCYb and Pt electrodes and operated their PCMRs as a fuel cell and as a hydrogen separator, respectively. This solid electrolyte, however, was not ad equate because of its reaction with CO2. H. Matsumoto et al. [45], tested zirconate-based electrolytes which have durability to CO2 and also proposed alternatives to the expensive Pt electrodes. C. Kokkofitis et al. [46], studied the

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reaction in a SZY (SrZr0.90Y0.1O3−a) PCMR. The working (cathodic) electrode was Pd. A moderate NEMCA effect was observed. The maximum ρ and Λ obtained was 2.0 and 8.0, respectively. G. Karagiannakis et al. [47], studied the reverse WGS reaction (CO2 + H2 ⇔ CO + H2O) on Cu electrodes of a SZY PCMR. Upon “pumping” protons to the Cu surface, the intrinsic catalytic activity was increased by up to a factor of 10. 3.5. Steam electrolysis H. Iwahara and coworkers [48,49], used PCMRs for steam electrolysis and hydrogen separation and discussed the advantages of using either a purely H+, a purely O2−, or a mixed conductor, for high temperature electrolysis of water. Guan et al. [50], used a mixed (H+–e−) conductor and examined its performance as a hydrogen separator operating at relatively low temperatures (500–800 °C). Iwahara, Hibino and Sunano tested the operation of a PCMR as an electrochemical steam pump [51]. Water decomposed at the anode, H+ was transported through the SCYb electrolyte and it reacted with oxygen at the cathode to produce H2O again. This device was used to extract traces of water vapor from a gaseous stream [49]. Recently, steam electrolysis was also studied by T. Kobayashi et al. [52] and H. Matsumoto et al. [45] in an effort to increase the current efficiency by testing new materials as HTPCs. 3.6. Decomposition of H2S and NOx Kirk and Winnick, constructed and tested studied O2− and H fuel cells with H2S used as fuel at 650–750 °C [53]. The performance of the O2− fuel cell was better in terms of the maximum generated current density. Both electrolytes, however, (strontium and barium cerates) suffered from gradual degradation and conversion to sulphates. Also, the use of the PCMR eliminated the possibility of SO2 production. For these reasons, Peterson and Winnick tested a HTPC composed of Li2SO4. The latter operated stably without degradation and with a maximum current density of 12 mA/cm2 [54]. The reduction of NOx was studied in PCMRs by T. Kobayashi et al. [55–57]. Steam was electrolyzed at the anode and the produced H+ reacted with NOx at the cathode to produce N2 and H2O. Strontium cerates and strontium zirconates were tested as HTPCs. When Pt/Ba/Al2O3 or Pt/Sr/ Al2O3 were used as working electrodes, it was possible to reduce the NOx even in the presence of excess O2 [55,57]. +

3.7. Synthesis and decomposition of ammonia The dominant process for ÍÇ3 synthesis is the Haber process: the reaction of gaseous nitrogen and hydrogen on a Fe-based catalyst at high pressures. At its early stages of development, the required pressures were in the range of 500–1000 bar. The continuous search for more active catalysts made it possible to operate at lower temperatures and consequently at lower pressures as well. Also, parallel to the catalyst optimization, several alternative processes have been proposed including the electrochemical synthesis in PCMRs [58–64].

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The reaction was first studied by Marnellos et al. [58,59] on Pd electrodes and with a SCYb HTPC. The conversion of H+ into ÍÇ3 was as high as 78% [58]. The Λ and ρ values observed were quite low, i.e. Λ b 2.0, ρ b 1.5 [59]. Yokari et al. [60], used a single-chamber reactor and studied the reaction on a commercial Fe catalyst. Upon “pumping” H+ to the catalyst surface, the reaction rate could increase by as much as 1300% (ρ = 13). Upon “pumping” H+ away from the catalyst, a decrease and eventually, a complete loss of the catalytic activity was observed. J.D. Wang et al. [61–64], concluded that the rate of ÍÇ3 synthesis is limited by the conductivity of the HTPC and thus tested a very large number of protonic conductors. A Pd–Ag alloy served as the working electrode in all their studies. The decomposition of ÍÇ3 in PCMRs is a potential method to produce very pure hydrogen for fuel cell applications. G. Pitselis et al. [65] studied the reaction on a Fe catalyst at 500–600 °C. A consider able decrease in the reaction rate was observed at negative currents (pumping H+ to the catalyst surface). The Λ and ρ factors were up to 150 and 3.5 respectively. A. Skodra et al. [66], studied the decomposition of ÍÇ3 on Ru. An increase in the catalytic rate at positive currents was observed (maximum Λ = 4.0), while at negative currents there was essentially no effect. An up to 75% decrease in the reaction activation energy was observed. 4. Current experience and future directions Table 1 shows that a large number of industrially important reactions have been studied in PCMRs. The goal in these works was one or more of the following: 1) Investigate the mechanism of the catalytic reaction by operating under both, open and closed circuit. The intrinsic reaction kinetics were compared to those obtained under proton pumping to or from the catalyst surface. The observed changes in the activation energy and/or in the dependence of the reaction rate on the reactant gases, could reveal valuable information about the rate determining step. Similarly, the appearance and the intensity of the NEMCA effect provided insight on the bond strength of the species adsorbed on the surface during reaction and on the work function changes upon H+ pumping [15]. 2) Accumulate the necessary knowledge in order to scale up and bring these PCMR processes into industrial practice. There are several advantageous characteristics of PCMRs that can make them economically more attractive than conventional catalytic reactors. One such characteristic is the ability to run a dehydrogenation reaction (e.g. C3H8 to C3H6) on the working electrode and produce pure H2 at the counter electrode. Thus, two valuable products are produced and separated at the same time. Another possibility is to operate the PCMR in the fuel cell mode and co-generate electricity and useful chemicals. For example, at the anode, C3H8 is converted to C3H6 and H+ and at the cathode, H+ reacts with O2 to produce H2O. A third possibility, offered by NEMCA, is to modify the intrinsic catalytic activity in the desired direction. A large Λ value means that the electrochemical promotion can be achieved with a minimal consumption of electrical energy [15]. The forth advantage is that the requirement for high pressures (e.g. ÍÇ3 synthesis), can be replaced by the supply of

electrical energy. Additionally, because in the PCMR hydrogen is electrochemically supplied as H+, impurities that could poison the working electrode are avoided. Therefore there is no need for extensive purification of H2 before entering the reactor. Finally, when a mixed (H+–e−) instead of a pure H+ conductor is used, there is no need for electrodes. It is easier for the industrial sector to adopt these “wireless” PCMRs rather than the classical electrochemical cells. Given the above advantages, it is perhaps puzzling that PCMRs have not been used extensively in industrial processes. The answer is not straightforward. On one hand, as already discussed in previous reviews [11,13,20,67], there are several hurdles that these reactors have to over come on their way to commercialization: high cost of capital investment, low protonic conductivity of the HTPCs, immature technology (very few processes have been tested for over 1000 h) and low price of the produced chemicals. On the other hand, it should be pointed out that very few detailed techno-economic evaluations of these processes can be found in the literature [13,20,68]. The above mentioned inhibiting reasons are quite general and the effects on each candidate PCMR process may vary considerably. Safe conclusions on the economic feasibility of a process can be drawn only if the specific parameters for the particular system are used. Hence, in the last decade, a number of applications disregarded in the past, are revisited with the possibility for further development. For example, among the various membrane processes considered for methane coupling, Liu et al. [21] proposed the mixed (H+–e-) conductors as the most promising. The conversion of alkanes into alkenes (e.g. propane in to propene) with simultaneous separation and production of pure hydrogen, is another promising application. Hydrogen is considered the energy currency of the future and therefore, its production technology is going to have a considerable impact on energy economy. Along these lines, processes that involve the hydrogen production-separation together with the production of an industrially important compound may be brought closer to commercialization in the near future. 5. Conclusions In the last 20 years, PCMRs have been used to both study and influence the rates of catalytic hydro- and dehydrogenation reactions. For several reactions, remarkable advantages of a PCMR vs. a conventional catalytic reactor have been identified. This advantageous performance, however, has not yet been sufficient enough to promote these processes into a larger scale. It should not be considered, however, that all this research work was done without any benefit. First, either with the open-circuit or with the closed-circuit operation, the PCMR studies provided information very useful in elucidating the reaction mechanism, information that could not have been obtained other wise. Second, research in this particular field has provided the industrial world with a number of potential alternatives to existing catalytic routes. Third, with the continuous progress in materials science and solid state ionics, one should expect that in the near future, economic factors currently inhibiting the scale up of these processes, will be substantially decreased to allow large scale applications.

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