Electrochemical promotion of the hydrogenation of CO2 on Ru deposited on a BZY proton conductor

Journal of Catalysis 331 (2015) 98–109

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Electrochemical promotion of the hydrogenation of CO2 on Ru deposited on a BZY proton conductor I. Kalaitzidou a, A. Katsaounis a, T. Norby b, C.G. Vayenas a,c,⇑ a

Department of Chemical Engineering, Caratheodory 1, St. University of Patras, GR-26504 Patras, Greece Department of Chemistry, University of Oslo, FERMiO, Gaustadalléen 21, NO-0349 Oslo, Norway c Academy of Athens, Panepistimiou 28 Ave., GR-10679 Athens, Greece b

a r t i c l e

i n f o

Article history: Received 8 April 2015 Revised 7 August 2015 Accepted 29 August 2015

Keywords: Hydrogenation of CO2 CO2 methanation Ru catalyst RWGS reaction BZY proton conducting support Selectivity modification Electrochemical promotion of catalysis (EPOC) Non-faradaic electrochemical modification of catalytic activity (NEMCA effect)

a b s t r a c t The kinetics and the electrochemical promotion of the hydrogenation of CO2 on polycrystalline Ru deposited on BZY (BaZr0.85Y0.15O3
a + 1 wt% NiO), a proton conductor in wet atmospheres, with a  0.075, was investigated at temperatures 300–450 °C and atmospheric pressure. Methane and CO were the only detectable products and the selectivity to CH4 could be reversibly controlled between 15% and 65% by varying the catalyst potential by less than 1.2 V. The rate and the selectivity to CH4 are very significantly enhanced by proton removal from the catalyst via electrochemically controlled spillover of atomic H from the catalyst surface to the proton-conducting support. The effect is strongly non-Faradaic and the apparent Faradaic efficiency of methanation takes values up to 500 and depends strongly on the porous Ru catalyst film thickness. The observed strong promotional effect, in conjunction with the observed reaction kinetics, is in good agreement with the rules of electrochemical and chemical promotion. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The hydrogenation of CO2 to hydrocarbons or alcohols is among the most important chemical conversions of CO2 not only for the production of renewable fuels but also as a possible means for decreasing the overall CO2 emissions [1–5]. In addition to Cu, which is the main component of current commercial catalysts for the industrially important CO2 hydrogenation to CH3OH [4–7], several other metals (e.g. Pt, Rh, Pd, Ru, Fe, Co, Ni) have been investigated as catalysts on a variety of supports (e.g. Nb2O5, ZrO2, Al2O3, SiO2) with several alkali-based promoters. Work in this area has been reviewed recently [1,3,4]. The hydrogenation of CO2 on Ru, which is known to give only CH4 and CO as products, has received considerable attention in recent years [8–10]. When co-feeding CO2 and H2 over a hydrogenation catalyst there are two main reactions that can take place (Eqs. (1) and (2)):

CO2 þ H2 ! CO þ H2 O

ð1Þ

xCO2 þ ð2x
z þ y=2ÞH2 ! Cx Hy Oz þ ð2x
zÞH2 O

ð2Þ

⇑ Corresponding author at: Department of Chemical Engineering, Caratheodory 1, St. University of Patras, GR-26504 Patras, Greece. E-mail address: [email protected] (C.G. Vayenas). http://dx.doi.org/10.1016/j.jcat.2015.08.023 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

The former is the reverse water–gas shift reaction (RWGS), which is a redox reaction, while the latter is a synthesis reaction leading to the formation of hydrocarbons and/ or alcohols. The adsorption of H2 on Ru(0 0 1) and other Ru crystallographic planes is dissociative and has been investigated thoroughly in the past [11–16] using a variety of techniques including TPD [11–14], work function measurements [12], low energy electron diffraction (LEED) [13,14], molecular beams [15] and high-resolution electron loss spectroscopy (HREELS) [13,16]. The desorption energy decreases from 110 kJ/mol to 46 kJ/mol as the H coverage increases from 0 to 0.8 [11]. The adsorption of CO2 on Ru/TiO2 and its coadsorption with CO and H2 on the same catalysts have been investigated by Grätzel and coworkers using Fourier transform infrared (FTIR) spectroscopy as well as 13CO2 isotopic studies [17]. They reported that even though CO2 methanation occurs via CO as an intermediate, the nature of the transition species was different in the hydrogenation of CO and CO2. Thus whereas adsorption/hydrogenation of CO gives rise to multiple CO binding states corresponding to RuOx(CO)n where x = 0–2 and n varies from 1 to 3, only monocarbonyl species, a precursor to methanation, is formed during CO2 hydrogenation [17].

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The dissociative adsorption of CO2 to CO and O has been suggested in the past to be the initial step reaction for CO2 hydrogenation. According to this mechanism CO is further decomposed to C and O followed by the hydrogenation of C to CH4. An alternative mechanism has been also proposed [18,19] introducing the formation of formate at the metal–support interface which acts as intermediate for methane and CO production. Marwood et al. [20] observed these formate species by infrared spectroscopy (IRS). They appeared to be mainly on the support while metal-supported CO(ads) was also detected. Even though the formate adsorption sites were located on the support, the rate of formation of this species was a function of the metal loading, which implied that the formate had to migrate from the metal, or the metal–support interface (three-phase-boundaries (tpb)), to the support. Panagiotopoulou et al. [21,22] suggested that hydrogen adsorbed on Ru migrates to the metal–support–gas three phase boundaries and reacts with CO2 to yield formate and finally RuCO species. Prairie et al. [9] used dispersive in situ infrared spectroscopy and detected formate species which again appeared to be mainly on the support. Based on their results they proposed the following reaction scheme:

CO2ðgÞ $ CO2i

ð3Þ

H2ðgÞ $ 2Hm

ð4Þ

CO2i þ 2Hm $ HCOOHðiÞ

ð5Þ

HCOOHðiÞ $ HþðsÞ þ HCOO
ðsÞ

ð6Þ

HCOOHðiÞ $ COðmÞ þ H2 O

ð7Þ

COðmÞ þ 6Hm ! CH4 þ H2 O

ð8Þ

where ‘‘m”, ‘‘s” and ‘‘i” denote metal, support and unspecified (possibly interfacial) adsorption sites. In the same study [9] the mobility and accumulation of the formate on the support were also demonstrated by IR spectroscopy. Tada et al. [19] demonstrated via Fourier transform IRS measurements the presence of formate and carbonate species and that the decomposition of formate species was fast over Ru/CeO2 and Ru/CeO2/Al2O3 in contrast to Ru/Al2O3, leading to improvement of CO2 reduction to CO. It is worth noting that the above mechanism does not involve any electrochemical steps. It nevertheless suggests that by interfacing the metal catalyst with a H+ conducting support and by controlling, via electrical potential application, of the electrochemical potential of H+ at the support/catalyst interface one may be able to influence via step (6) the coverages of formic acid and formate ion on the catalyst surface and thus also the catalytic activity and selectivity. This is an idea pursued in the present work. A parallel approach to classical chemical promotion is the electrochemical promotion of catalysis (EPOC), also known as nonfaradaic electrochemical modification of catalytic activity (NEMCA effect) [23–31], which can be used to reversibly promote metal catalyst films deposited on solid electrolyte supports, such as yttriastabilized-ZrO2 (YSZ, an O2
conductor) or b00 -Al2O3 (a Na+ or K+ conductor), or mixed ionic–electronic conductors such as TiO2 or CeO2 via application of an electric potential (±2 V) between the catalyst film and an auxiliary electrode. Electrochemical promotion allows for continuous in situ control of the coverage of promoting species (Nad+, Kd+, Od
) on the catalyst surface. EPOC has been investigated extensively during the last 30 years for more than 100 catalytic reaction systems using a variety of metal catalysts (or conductive metal oxides), solid electrolytes and catalytic reactions. Work in this area has been reviewed several times in recent years [23–28].

99

Numerous surface science and electrochemical techniques have shown that EPOC is due to an electrochemically controlled migration (spillover or more commonly reverse-spillover or backspillover) of promoting ionic species (e.g. O2
in the case of YSZ, Na+ or K+ in the case of b00 -Al2O3), from the ionic or mixed ionic– electronic conductor-support and the gas exposed catalyst surface, through the catalyst–gas–electrolyte three phase boundaries (tpb) [23–28]. Thus, both catalytic activity and selectivity are affected in a pronounced, reversible, and, to some extent, predictable manner [23,26]. The close connection between EPOC, classical chemical promotion and metal–support interaction (MSI) with ionically conducting supports has been established by a variety of techniques [23,24,28,32,33]. Two parameters that are commonly used to quantify the magnitude of EPOC [23] are as follows: 1. The rate enhancement ratio, q, defined by Eq. (9) is

q ¼ r=ro

ð9Þ

in which r is the electropromoted catalytic rate and ro is the unpromoted rate (i.e. the open-circuit catalytic rate), and 2. The apparent Faradaic efficiency, K, defined by Eq. (10) is

Ki ¼ Dr catalytic =ðI=FÞ

ð10Þ

where Drcatalytic is the current- or potential-induced observed change in catalytic rate (in g-eq/s), and I is the applied current. In the present case this implies

KCH4 ¼ 8Dr CH4 =ðI=FÞ

ð11Þ

KCO ¼ 2Dr CO =ðI=FÞ

ð12Þ

where r CH4 is in mol CH4/s and r CO is in mol CO/s. A reaction is termed electrophobic (or nucleophilic) when the rate increases with increasing catalyst potential (or/oUWR > 0), electrophilic when the rate decreases with increasing catalyst potential (or/oUWR < 0), volcano-type when the reaction rate exhibits a maximum with varying potential, and inverted volcano when the rate goes through a minimum with varying potential [23]. The catalyst potential UWR is an increasing function of the work function, U, of the catalyst surface and over wide temperature ranges the two are related via DU = eDUWR [23,28]. The range of validity of this equation, which has been extracted both from Kelvin probe measurements [34] and from the cutoff electron energy of UPS spectra [35], has been investigated both experimentally, using Pt, Au, Ag [36,37], Ir–IrO2 [38,39] and Ru– RuO2 electrodes [40] and theoretically [41,42]. It has been found that the equation is valid as long as a sufficiently dense adsorption layer, formed by adsorbed species and spillover ions and their compensating charge in the metal, is present at the metal–gas interface. When the temperature is too low, e.g. below 300 °C for Pt, the ion mobility is too low and thus this effective double layer cannot form [36,41] and significant deviations from equality are observed [34–37]. When the temperature is too high, e.g. above 450 °C for Pt, this effective double layer desorbs and thus again significant deviations from the above equality are observed. It has been found that simple rules exist, valid both for classical promotion and for electrochemical promotion [43–45], which allow for the prediction of the promotional behavior on the basis of the open-circuit reaction kinetics with respect to the electron donor (D) and electron acceptor (A) reactant species. The electrochemical promotion of CO2 hydrogenation has been studied in the past over Cu [46,47], Rh [46,48], Pt [46,49–51], Pd [52] and Ni [53] catalyst film electrodes deposited on YSZ [46,48,49,52,53], and K-b00 -Al2O3 [47,50,51] ceramic supports. In general, methanation was found to exhibit electrophobic behavior, whereas the RWGS follows electrophilic behavior. The same type

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behavior has been also observed in recent studies of electropromotion of Ru/Na-b00 -Al2O3 [54], Ru/K-b00 -Al2O3 [55] and Ru/YSZ catalysts [54,56], i.e. electrophobic behavior for methanation and electrophilic behavior for the production of CO via the RWGS reaction. Studies of CO2 hydrogenation with proton conductive materials are rather rare in the literature. This is most likely due to the lack of commercially available solid electrolytes with sufficient proton conductivity, especially in the range of temperatures in which this reaction can be studied. During the last two decades new materials with proton conductivity have been developed and proposed mainly for fuel cell applications [57–59]. Proton conducting solids are either water containing (e.g. PEMs such as Nafion 117), oxo acids and their acidic salts (e.g. hydrogen sulfates such as CsHSO4 and corresponding selenates, phosphates, and arsenates), or high temperature proton conductors, in which protons dissolve through partial hydration in the presence of water vapor. In the latter class, the most promising materials are based on barium cerates (BaCeO3) and barium zirconates (BaZrO3) which have a perovskite structure. Unlike barium cerate, barium zirconate displays sufficient chemical stability in CO2 [60,61], making it attractive for many practical applications and aggressive environments. Several groups have investigated BZY solid electrolytes with different compositions [57,60,62–64]. The reported conductivities vary from 10
5 to 10
1 S cm
1 depending on temperature (300–1000 K), material composition and microstructure. Both cerates and zirconates have been used already to electropromote some catalytic reactions, i.e. ethylene oxidation [65,66], NH3 synthesis [67] and NH3 decomposition [68]. In general, previous studies utilizing ceramic proton conductors have led to large (>3) q values and large (>10) K values only in oxidizing environments and for oxidation reactions [65,66] where proton supply may lead to the formation of promoting hydroxyl species on the catalyst surface [65,66]. No pronounced electrochemical promotion behavior (e.g. q > 3, and simultaneously |K| >10) has been observed so far with any hydrogenation reaction via proton supply to the catalyst [23,28]. An explanation for this is reached by the present study. In this study we have investigated the effect of catalyst potential and catalyst loading on the catalytic activity and selectivity of CO2 methanation and CO production via the RWGS reaction over Ru catalyst films deposited over yttria doped barium zirconate (BZY) proton conductor of composition BaZr0.85Y0.15O3
a + 1 wt% NiO with a  0.075 at temperatures 300–450 °C and ambient pressure. The yttrium acts as an acceptor dopant increasing the concentration of protons dissolved from water vapor, and the NiO is added as a common sintering aid for BaZrO3-based materials. It should be noted that NiO during use under reducing conditions may be reduced to Ni particles on the BZY surface without any observed adverse effect on electrolyte performance. According to our knowledge this is the first time that such a proton conductor is used at such low temperatures for a hydrogenation reaction. This material was found to exhibit excellent mechanical, chemical and electrochemical properties for carrying out electrochemical promotion studies. The main novel aspects of the present work are the following: a. It is the first electrochemical promotion study of a hydrogenation reaction using a proton conductor and leading to very significant K (up to 500 vs 10 in previous studies) and q (up to 4.5 vs 3) values. Thus the measured q and K values are the highest reported in the catalytic literature for a hydrogenation reaction. b. It is one of the very few electrochemical promotion studies where the rates of two parallel reactions increase and decrease simultaneously upon catalyst polarization.

c. It is also one of the first two or three EPOC studies where the effect of catalyst film thickness has been studied in detail.

2. Experimental 2.1. Catalyst preparation The solid electrolyte supports were disks of BZY (BaZr0.85Y0.15O3
a + 1 wt% NiO), a  0.075, with a diameter of 18 mm and a thickness of 2 mm. They had been produced by reaction sintering of a ball-milled mixture of the nominal amounts of dried analytical grade BaCO3, 8 mol% Y2O3 stabilized ZrO2 (TOSOH 8YSZ), and NiO at 1500 °C in air, and were used as delivered (NorECs AS, Norway). Gold organometallic paste (Metalor, A1118) was used for the deposition of the Au counter and reference electrodes on one side of the disk, followed by calcination in air at 650 °C for 1 h. Blank experiments utilizing a BZY disk with three Au electrodes showed that gold was catalytically inactive both for the methanation and for the RWGS reaction both under open-circuit and under polarization. The Ru catalyst films were deposited on the other side of the disks via impregnation of a 150 mM RuCl3 solution in isopropanol at 50 °C, followed by calcination in air at 500 °C for 1 h. Four different metal loadings were tested, varying from 1 mg/cm2 to 6.1 mg/cm2. The superficial catalyst electrode surface area, opposite to the counter electrode, was 1 cm2. The catalyst, support and counter and reference electrode geometry is shown in Fig. 1. SEM images from the top view of a used Ru catalyst electrode film are shown in Fig. 2. Catalyst characterization information using X-ray diffraction characterization and surface area estimation is given in the Supporting Material. Prior to any hydrogenation activity measurements, a reduction pretreatment in 5% H2/He was performed at 450 °C for 1 h. The experiments were carried out in a continuous flow single chamber reactor, which has been thoroughly discussed previously [54,56]. The feed gas composition and total gas flow rate, Fv (200– 400 cm3 STP/min) were controlled by a set of flow meters (Brooks smart mass flow and controller B5878). Reactants were certified

Fig. 1. Schematic of the BZY solid electrolyte support disk showing the location of the Ru catalyst-electrode (working electrodes, WE) and of the Au counter electrode (CE) and reference electrodes (RE).

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3. Results 3.1. Open-circuit and promotional kinetics Fig. 3 shows the transient effect of positive (+0.5 V) and negative (
0.5 V) potential applications on the cell current and on the rates of CH4 and CO formation over the Ru catalyst (sample 1, mRu = 1 mg/cm2) at 350 °C. As already noted, positive potential or positive current application corresponds to proton removal from the catalyst surface via the overall reaction:

Hþ ðRuÞ þ e
! HðBZYÞ

Fig. 2. SEM images (top views) of the surface of the Ru/BZY sample.

standards of 4.80% CO2 in He and 30.14% H2 in He. Pure (99.999%) He was fed to further adjust the total flow rate and the inlet gas composition at desired levels. All the experiments were performed at ambient pressure. Feed reactant partial pressures were varied between 0.25 and 2.3 kPa for CO2 and 1–15 kPa for H2. In general, reducing conditions were maintained in the reactor throughout the runs in order to avoid surface or even bulk oxidation of the Ru catalyst film. Reactants and products were analyzed by online gas chromatography (Shimadzu GC-2014) in conjunction with an IR CO2–CO–CH4 gas analyzer (Futzi Electric ZRE) and a Quadrupole Mass Spectrometer (Pfeiffer Omnistar). Constant currents and potentials were applied using an AMEL 2053 galvanostat– potentiostat.

ð13Þ

where H(BZY) denotes hydrogen stored in the bulk of BZY in the form of protons plus compensating charge in the BZY. Negative potential or negative current application corresponds to proton supply to the catalyst surface according to the reverse of Eq. (13). As shown in Fig. 3, proton removal from the Ru catalyst causes a pronounced threefold increase in the rate of methanation ðqCH4  3Þ and a 50% decrease in the rate of CO formation ðqCO  0:5Þ. Both effects are reversible, i.e. the rates return to their initial values upon interruption of the applied potential. The corresponding Faradaic efficiencies, computed from Eqs. (11) and (12), are KCH4 ¼ 360 and KCO ¼
43. Consequently the effect is strongly non-Faradaic and, similar to recent studies of CO2 hydrogenation on Ru using O2
, Na+ and K+ solid electrolytes [54–56], the electropromotion effect is electrophobic (nucleophilic) for the methanation reaction ð@r CH4 =@U WR > 0Þ and electrophilic ð@r CO =@U WR < 0Þ for the reverse water gas shift (RWGS) reaction. As also shown in Fig. 3, the same trends are observed via negative current application, but the effect is much smaller, as q is near unity and K is also of order unity. As already noted in the catalyst characterization section, the time constant, s, of the rate transients shown in Fig. 3 can be used to estimate, the rather low, active catalyst surface area NG, i.e. the area of the metal–gas interface, (expressed in mol metal) from the expression N G ¼ Is=F [23] where F is Faraday’s constant. The main assumptions involved in deriving this useful approximate expression are adsorption site uniformity, one Ru atom per site, and fast surface migration of the electroactive species, as discussed in detail elsewhere [23,28]. Thus, one computes N G ¼ 1:1  10
7 mol which, as noted in the Supporting Information, is consistent with an average catalyst particle radius of the order of 30–40 nm. This value has

2.2. Catalytic rate measurements As in all previous atmospheric pressure electrochemical promotion studies [23,28], the rates of CO2 consumption and CH4 and CO formation were computed from

r CO2 ¼ NðyoCO2
yCO2 Þ r CH4 ¼ NyCH4 r CO ¼ NyCO where N (mol/s) is the total molar flow rate, computed from the total volumetric flow rate Fv (cm3 STP/min) via N = Fv/[(22400) (60)] mol/s, yoCO2 is the mol fraction of CO2 in the feed and yCO2 , yCH4 and yCO are the mol fractions of CO2, CH4 and CO in the exit of the reactor. Although the single chamber reactor used has been shown to behave as a CSTR [16,21], the total conversion of CH4 was kept below 20% to maintain nearly-differential conditions. Carbon mass balance closure was better than 1%.

Fig. 3. Transient effect of constant applied positive (+0.5 V) and negative (
0.5 V) potential on the current and on the catalytic rates rCH4 and rCO and corresponding turnover frequencies TOFCH4 and TOFCO of CH4 and CO formation on Ru/BZY; mRu = 1 mg/cm2, PH2 = 7 kPa, PCO2 = 1 kPa, T = 350 °C, FT = 400 cm3 STP/min. Total molar flow rate N = 2.98  10
4 mol/s.

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Fig. 4. Steady-state effect of catalyst potential UWR on current, on the rates of CH4 and CO formation and on the corresponding q and K values at 400 °C; PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/min, N = 1.49  10
4 mol/s.

shows that, in general, negative potential (
0.5 V) application, i. e. proton supply to the catalyst, has a much smaller effect than positive potential application (+0.5 V), i.e. than proton removal from the catalyst, as already shown in Figs. 4 and 5. 3.2. Promotional rules Fig. 6 provides a concise example of the rules of electrochemical and chemical promotion [43–45], i.e. electrophobic behavior ð@r=@U WR > 0Þ is obtained when the rate is positive order in the electron donor reactant (H2, Fig. 6 top left) and zero or negative order in the electron acceptor reactant (CO2, Fig. 6 top right) while electrophilic behavior ð@r=@U WR < 0Þ is obtained when the rate is zeroth or negative order in the electron donor reactant (H2, Fig. 6 bottom left) and positive order in the electron acceptor reactant (CO2, Fig. 9 bottom right). This behavior can be described by the inequalities ð@r=@U WR Þð@r=@PD Þ > 0 or ð@r=@U WR Þð@r=@PA Þ < 0 which provide a mathematical description of the promotional rules [43–45]. Consequently the rate vs potential (or work function) dependence traces the rate vs electron donor dependence [43–45]. Fig. 5. Steady-state effect of catalyst potential on the selectivities to CH4 and to CO at 400 °C and 450 °C; PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/min, N = 1.49  10
4 mol/s.

been used in subsequent Figures to compute turnover frequencies (TOF). The transient trends of Fig. 3 can be observed more clearly in Fig. 4, which shows the steady-state effect of catalyst potential on the cell current, on the rates of CH4 and CO production, and on the measured q and K values. Fig. 5 shows the effect of catalyst potential on the selectivity to CO and to CH4. At 400 °C the latter increases from 15% to 65% as the catalyst potential, U WR , is increased from
0.3 V to 0.9 V and thus the catalyst work function, U, is increased by 1.2 eV. Fig. 6 presents the open-circuit (o.c.) and electropromoted (UWR = ± 0.5 V) kinetics. One observes that the rate of methanation, rCH4 , is positive order in H2 and negative order in CO2 both under open-circuit (o.c.) and under electropromotion conditions. At the same time the rate of CO production via the RWGS reaction is near-zero order in H2, becoming negative order upon positive potential application, and is positive order in CO2. Fig. 6 also

3.3. Magnitude of q and K Fig. 7 shows the effect of gaseous composition on the magnitude of the rate enhancement ratio, q, and of the Faradaic efficiency, K, for the rates of formation of CH4 and CO. One observes that negative potential application, i.e. H+ supply to the catalyst surface, leads to q and K values near unity, while positive potential application, i.e. proton removal from the catalyst surface, leads to pronounced electropromotion with qCH4 values up to 4 for low

pH2 and high pCO2 values and q
1 CO values up to 2 for high pH2 and low pCO2 values. The apparent Faradaic efficiency values for CH4 formation, KCH4 , increase with increasing pH2 and decreasing pCO2 , reaching values above 500. The corresponding apparent Faradaic efficiency KCO decreases with both pCO2 and pH2 reaching values near
50 for T = 350cC. 3.4. Effect of catalyst loading Quite interestingly the magnitudes of qCH4 , KCH4 , qCO , and KCO were found to depend, rather strongly, on the thickness of the Ru

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103

Fig. 6. Steady-state effect of PH2 (left) and PCO2 (right) on the rate of CO2 methanation and RWGS reaction on Ru/BZY under open circuit-, positive-, and negative-potential application; mRu = 1 mg/cm2, PH2 = 7 kPa, PCO2 = 1 kPa, T = 350 °C, FT = 400 cm3 STP/min N = 2.98  10
4 mol/s.

Fig. 7. Steady-state effect of PH2 (left) and PCO2 (right) on the faradaic efficiency, K and rate enhancement ratio, q for the CO2 methanation and RWGS reaction on Ru/BZY under positive and negative potential application; mRu = 1 mg/cm2, PH2 = 7 kPa, PCO2 = 1 kPa, T = 350 °C, FT = 400 cm3 STP/min, N = 2.98  10
4 mol/s.

catalyst film, as shown in Fig. 8. Both qCH4 and q
1 CO (Fig. 8a, left and 8b, left) decrease from values near 3 to values near 1, as the catalyst loading, mRu, increases from 1 mg/cm2 to 6.1 mg/cm2. Such a behavior might at first be considered to suggest some gas phase internal or external diffusional limitations. This, however, is not the case, as shown by Fig. 9a, which presents the effect of the catalyst loading, mRu, on the total rate of CO2 hydrogenation at T = 400 and 450 °C. The rate levels with mRu values above 3 mg/cm2, but even in the flat region the activation energy is around 92 kJ/mol, i.e. quite high for a gas-diffusion controlled process. Consequently the effect of mRu must be a kinetic rather than a gas-diffusion effect.

Interestingly, as shown in Fig. 9b, increasing metal loading, mRu, causes a decrease in the exchange current, I0, which suggests that the length of the three-phase boundary, ‘tpb , may also play an important role. This length is proportional to the exchange current density, I0 , of the catalyst/support i.e. Ru/YSZ, interface [23,28]. The value of I0 is obtained from I
U data such as those in Fig. 4 top left, via the Butler–Volmer equation (Eq. (14)), i.e.

I=I0 ¼ expðaa FðU
U 0 Þ=RTÞ
expð
ac FðU
U 0 Þ=RTÞ

ð14Þ

where aa and ac are the anodic and cathodic transfer coefficients [23]. It should be noted that maximization of I0 which is very important in fuel cells is not in general desirable in EPOC studies,

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Fig. 8. Effect of Ru loading on the faradaic efficiency, K and rate enhancement ratio, q for the CO2 methanation and RWGS reaction on Ru/BZY under positive-, and negativepotential application at (a) 400 °C and (b) 450 °C; PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/min, N = 1.49  10
4 mol/s.

since K is known to be of the order of 2Fro/I0 where ro is the opencircuit catalytic rate [23,28]. As shown in Fig. 9b the magnitude of I0 decreases by more than a factor of 2 upon increasing the catalyst loading mRu. This decrease in I0 , thus in ‘tpb , appears to be closely related to the observed pronounced decrease in q with increasing mRu (Fig. 8), as also analyzed in Section 4. Figs. 10 and 11 show, for two different temperatures, the combined effect of catalyst loading and catalyst potential on the rate enhancement ratios, qCH4 and qCO , and on the corresponding Faradaic efficiencies, KCH4 and KCO , respectively. One observes in Fig. 10 that qCH4 values up to 4.5 are obtained for the lowest loading of 1 mg/cm2 but q diminishes rapidly with increasing loading. Similar is the behavior of q
1 CO which drops from 2 to 1 upon increasing the metal catalyst loading from 1 to 6.1 mg/cm2.

Fig. 11 shows a similar behavior regarding KCH4 and jKCO j which reach values of 220 and 45, respectively, for the lowest loading where interestingly both KCH4 and jKCO j exhibit a pronounced maximum with varying anodic potential. The same figure shows that the absolute values of KCH4 and KCO decrease significantly as temperature is increased from 400° to 450 °C. 3.5. Selectivity dependence on temperature, catalyst loading and potential Fig. 12 shows the effect of temperature and catalyst loading on product selectivity under open-circuit conditions. The selectivity to CH4, SCH4 , decreases from 50% to 5% upon decreasing the catalyst loading. It also exhibits a shallow maximum with temperature and decreases significantly above 360 °C.

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Fig. 13 shows the effect of catalyst loading and catalyst potential on product selectivity. Increasing loading and increasing potential enhance SCH4 , which at 400 °C reaches values above 65%, significantly higher than those obtained at any temperature and catalyst loading under open-circuit conditions (Figs. 12 and 13). The magnitude of the selectivity enhancement diminishes upon increasing the temperature above 400 °C as shown in Fig. 13.

4. Discussion 4.1. General features and promotional rules The observed kinetic and electropromoted kinetic behavior is consistent with the rules of chemical and electrochemical promotion [43–45] which suggest that positive potential application, i.e. proton removal from the catalyst causes enhanced chemisorption of gas supplied hydrogen and weakens the chemisorption of CO2 on the catalyst surface. Thus all the experimental observations regarding the rate and selectivity dependence on catalyst potential can be directly rationalized on the basis of the rules of promotion [43–45]. These, constant catalyst loading, observations involve the following: 1. The observed kinetic and electropromotion dependence on pH2 and pCO2 , including the electrophobic/electrophilic dependence of methanation and RWGS, respectively (Figs. 3–6). 2. The magnitude of q and K and their dependence on pH2 and pCO2 (Fig. 7). 3. The magnitude of the galvanostatic or potentiostatic time constants of the catalytic transients (Fig. 3).

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Positive potential application also weakens the adsorption of electron acceptor adsorbates, such as CO2. Consequently, when the rate of methanation is positive order with respect to H2, and negative order with respect to CO2, as is the case here (Fig. 6, top left and top right) an increase in the coverage of H on the catalyst surface via positive potential application will lead to enhanced rate of methanation as observed (Fig. 6, top left). When the rate of the RWGS reaction is zero or negative order with respect to H2 (Fig. 6, bottom left) and positive order with respect to CO2 (Fig. 9, bottom right), such an increase in H coverage will decrease the coverage of CO2 and will thus suppress the CO production rate, as experimentally observed (Figs. 3 and 6, bottom left and right). These considerations then directly explain the observed pronounced increase in the selectivity to CH4 with increasing potential (Figs. 5 and 13) or decreasing temperature (Figs. 5 and 12), as experimentally observed. On the other hand, negative potential application, i.e. proton supply to the catalyst can only cause Faradaic or nearly Faradaic (K  1) increases in catalytic rates as experimentally observed. The size of a proton by itself is extremely small for forming a double layer of a thickness in the 0.1–1 nm range as is the case with O2
or Na+ ions and thus causing EPOC behavior. A proton can cause promotional effects only when it is attached to a coadsorbed species, e.g. an O atom forming adsorbed OH species [66]. This can explain why all attempts to electropromote catalytic hydrogenations via H+ supply to the catalyst under reducing conditions have led to small (K  1) rate enhancements. Significant (K > 10) values with proton conductors have only been obtained in oxidizing environments and thus in the presence of adsorbed O species.

4.2. Promotional and poisoning species Thus, potential-controlled proton removal from the metal catalyst into the BZY support via positive potential application causes an increase in the catalyst work function [23,24,28] and this enhances the chemisorption of electron donor adsorbates, such as atomic H originating from gaseous H2 adsorption. The latter is known to lead to two TPD peaks [69]. We have carried out some preliminary experiments confirming that indeed, similar to the case of O2 TPD on Pt/YSZ, an O2
conductor [70], the H2 TPD spectrum changes substantially with applied potential and in fact, indeed, positive potential enhances hydrogen chemisorption.

Little is known about the promoting spillover or backspillover species which cause the change in catalyst work function in electrochemical promotion studies utilizing proton conductors [65– 67]. In the present case there are at least two possible molecular mechanisms which can induce the well established via Kelvin probe and UPS measurements [23,28,34–42] change, DU(=eDUWR) in the work function, U, of the gas-exposed, i.e. catalytically active, surface of the catalyst-electrode upon application of a potential change DUWR.

Fig. 9. Effect of Ru loading on the CO2 consumption rate (a) and exchange current, Io (b) at 400 °C and 450 °C; PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/min, N = 1.49  10
4 mol/s.

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Fig. 10. Effect of catalyst potential and Ru loading on the methane and CO rate enhancement ratios at 400 °C (left) and 450 °C (right); PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/min, N = 1.49  10
4 mol/s.

The first mechanism simply invokes the creation of vacant, via proton removal, hydrogen chemisorption sites on the Ru catalyst surface via the reaction

Hþ ðRuÞ þ e
! HðBZYÞ

ð13Þ

Since electropositive species are being removed from the catalyst surface, this is indeed expected to increase the catalyst work function, and to enhance gas supplied hydrogen chemisorption. It is worth noting that in the present catalyst system protons act as promoters for CO production and as poisons for the methanation reaction, e.g. Fig. 3.

The second mechanism is based on IRS observations [20] and invokes the formation of anionic formate species generated at the metal/support/gas i.e. Ru/BZY/gas three phase boundaries (tpb) via deprotonation of formic acid adsorbed at the tpb, i.e. via the reaction

HCOOHðtpbÞ þ e
! HCOO
ðRuÞ þ HðBZYÞ

ð15Þ

These formate species may then migrate (backspillover) over the entire metal/gas interface, causing again an increase in catalyst work function, which as already discussed in Section 4.1 can rationalize all the experimental observations discussed there and

Fig. 11. Effect of catalyst potential and Ru loading on the methane and CO faradaic efficiencies at 400 °C (left) and 450 °C (right); PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/ min, N = 1.49  10
4 mol/s.

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107

4.3. Effects of metal loading In addition to the previous observations, the present study has also revealed several interesting additional features which refer to the effect of catalyst loading for this system. These observations involve the following: 1. The observed decrease of qCH4 and q
1 CO with increasing metal loading under anodic (+1 V) polarization (Fig. 8). 2. The appearance of a maximum in KCH4 and
KCO upon varying the metal loading (Fig. 8). 3. The pronounced electrochemical promotion for low catalyst loading and the decreasing effect of potential with increasing metal loading (Figs. 8, 10, 11 and 13). 4. The appearance of a maximum in K upon increasing U for low metal loadings (Fig. 11). 5. The pronounced increase in selectivity to CH4 upon increasing potential and metal loading (Figs. 12 and 13).

Fig. 12. Effect of temperature and Ru loading on the selectivity for CH4 and CO on Ru/BZY under open-circuit application; PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/ min, N = 1.49  10
4 mol/s.

presented in Figs. 3–7. On the basis of the available experimental information it is not possible to conclude which of the two molecular mechanisms is dominant and it is likely that they both act in parallel. Since these both can affect the catalyst work function, they can both cause non-Faradaic catalytic rate modification as experimentally observed. Nevertheless, as discussed below, they can both also rationalize the observed effects of metal loading.

A first step in rationalizing these observations can be made via Fig. 9 which shows that increasing metal loading, mRu, increases the catalytic rate, and thus the metal surface area, NG, but at the same time decreases the exchange current, I0, and thus the length, ‘tpb , of the three-phase-boundaries metal–support–gas. The ‘tpb value plays an important role on the transport of protons from the Ru metal catalyst to the BZY support (spillover), which occurs upon potential application, via reactions (13) and (15). There is also the reverse process of proton transport from the BZY support to the Ru surface (reverse spillover or backspillover)

1=2H2 ðgÞ ! HðBZYÞ ! Hþ ðRuÞ þ e


ð16Þ

which can occur spontaneously in the reducing environment of the reacting mixture. Furthermore, as already discussed in the Introduction (Eq. (6)), protons on the Ru surface are involved in the equilibrium

Fig. 13. Effect of catalyst potential and Ru loading on the selectivity for CH4 and CO on Ru/BZY at 400 °C (left) and 450 °C (right); PH2 = 7 kPa, PCO2 = 1 kPa, FT = 200 cm3 STP/ min, N = 1.49  10
4 mol/s.

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HCOOH Hþ þ HCOO


ð17Þ

and therefore a decrease in surface proton concentration can cause an increase in HCOO- concentration, thus an increase in methanation rate and selectivity. As already noted, increasing metal loading mRu causes a decrease in ‘tpb . Consequently, in the absence of applied current, the rate of proton transport to the catalyst via proton backspillover (Eq. (16)) is diminished and the proton activity and concentration on the catalyst surface are also decreased. Therefore the concentration of vacant H adsorption sites is increased and, due to the equilibrium of Eq. (17), the concentration of promoting HCOO- species is also increased, thus causing a high open-circuit methanation rate and selectivity for high metal loading, as experimentally observed. This explains observation 5, i.e. Figs. 12 and 13. Because surface concentration of formate ions or of the free H adsorption sites and thus also the selectivity to CH4 on such high metal loading catalysts is already high and ‘tpb are low, there is little space for improvement by further decreasing the proton concentration via positive potential application; consequently, the electropromotion effect is diminished, as experimentally observed (observation 1, i.e. Fig. 8). Exactly the opposite holds for low mRu catalysts: In this case ‘tpb is high and thus the proton concentration on the Ru surface is high; consequently, in view of the equilibrium of Eq. (17), the promoting formate coverage, or of the free H adsorption site coverage, is low, and therefore the rate and selectivity of CH4 is low, as experimentally observed. In this case, electrochemical proton removal from the catalyst is expected to have a pronounced effect on rate and selectivity, as experimentally observed (observation 1 and Fig. 8). The appearance of the maxima in K with increasing UWR can be rationalized as follows. Upon closing the electrical circuit, there is a spontaneous migration of protons from the Ru surface into the BZY according to the reverse of Eq. (16). This migration is accompanied by a very small current, so that KCH4 , which is computed from Eq. (11), i.e. from

KCH4 ¼

8Dr CH4 ; ðI=FÞ

ð11Þ

increases in a pronounced manner since Dr CH4 is increasing exponentially with UWR [23]. When the UWR value is reached where the electrochemical potentials of protons on the Ru surface and in the BZY become equal, then a significant current is needed to deprotonate the adsorbed formic acid or to cause proton spillover into the BZY support; thus, current increases exponentially with UWR according to the Butler–Volmer equation, and therefore KCH4 starts decreasing sharply (Figs. 4 and 11). The KCH4 maxima, upon increasing the metal loading mRu at fixed applied potential (Figs. 8) can be explained in a similar manner: When mRu is small then, as already noted, ‘tpb is large and a large current is needed to electropromote the rate; thus, KCH4 is small. Upon increasing mRu, ‘tpb decreases, thus the current decreases and K increases until the point is reached at large mRu where rCH4 cannot be electropromoted further (qCH4  1, Fig. 8), and thus K diminishes approaching zero, as experimentally observed (Fig. 8a and b). 5. Conclusions The use of a mechanically robust and sufficiently conductive in the range 300o-450 °C proton conductor, such as BZY-NiO, allows for the efficient electrochemical promotion of porous supported Ru catalyst films and to the very significant enhancement of the rate of methanation with concomitant suppression in the competing rate of CO production. There is an up to fourfold enhancement

in catalytic rate of CH4 formation with concomitant 50% suppression of the CO formation rate which proceeds in a parallel route. This is a rare case where the same electrochemically generated species act simultaneously both as a strong promoter for one catalytic reaction (methanation) and as a poison for a competing parallel reaction (RWGS). It is also the first time that K values as high as 500 have been obtained for a hydrogenation reaction. The results may be of practical usefulness and may suggest the use of BZY-NiO as a promising conventional support for some hydrogenation catalysts, in the same way that YSZ, an O2
conductor, is known [28] to be a very effective conventional support for several oxidation catalysts. Acknowledgments We thank our reviewers for some very helpful comments and suggestions. Work is supported by the ‘‘ARISTEIA” Action of the ‘‘Operational programme of education and lifelong learning” which is co-funded by the European Social Fund (ESF) and National Resources. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.08.023. References [1] W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev. 40 (2011) 3703–3727. [2] J. Ma, N. Sun, X. Zhang, N. Zhao, F. Xiao, W. Wei, Y. Sun, Catal. Today 148 (2009) 221–231. [3] Convert CO2 back for fuel, in: J.A. Bueno (Ed.), EUR 22984 – Research for Europe – A Selection of EU Success Stories, European Commission – Directorate General for Research, Luxembourg, 2008, pp. 28–29. [4] H.-R.M. Jhong, S. Ma, P.J. Kenis, Curr. Opin. Chem. Eng. 2 (2013) 191–199. ˇ a, E. Wagner, Q. Smejkal, A. Barkschat, M. Baerns, [5] U. Rodemerck, M. Holen ChemCatChem 5 (2013) 1948–1955. [6] A.L. Lapidus, N.A. Gaidai, N.V. Nekrasov, L.A. Tishkova, Y.A. Agafonov, T.N. Myshenkova, Pet. Chem. 47 (2007) 75–82. [7] R. Reske, M. Duca, M. Oezaslan, K.J.P. Schouten, M.T.M. Koper, P. Strasser, J. Phys. Chem. Lett. 4 (2013) 2410–2413. [8] D. Li, N. Ichikuni, S. Shimazu, T. Uematsu, Appl. Catal. A 172 (1998) 351–358. [9] M.R. Prairie, A. Renken, J.G. Highfield, K. Ravindranathan Thampi, M. Grätzel, J. Catal. 129 (1991) 130–144. [10] B. Hu, C. Guild, S.L. Suib, J. CO2 Utilization 1 (2013) 18–27. [11] J.A. Schwarz, Surf. Sci. 87 (1979) 525–538. [12] H. Shimizu, K. Christmann, G. Ertl, J. Catal. 61 (1980) 412–429. [13] C.Y. Fan, K. Jacobi, Surf. Sci. 482–485 (2001) 21–25. [14] P. Feulner, D. Menzel, Surf. Sci. 154 (1985) 465–488. [15] I.M.N. Groot, H. Ueta, M.J.T.C. Van Der Niet, A.W. Kleyn, L.B.F. Juurlink, J. Chem. Phys. 127 (2007). [16] K.L. Kostov, W. Widdra, D. Menzel, Surf. Sci. 560 (2004) 130–144. [17] N.M. Gupta, V.S. Kamble, R.M. Iyer, K.R. Thampi, M. Grätzel, Catal. Lett. 21 (1993) 245–255. [18] C. Janke, M.S. Duyar, M. Hoskins, R. Farrauto, Appl. Catal. B 152–153 (2014) 184–191. [19] S. Tada, O.J. Ochieng, R. Kikuchi, T. Haneda, H. Kameyama, Int. J. Hydrogen Energy 39 (2014) 10090–10100. [20] M. Marwood, R. Doepper, A. Renken, Appl. Catal. A 151 (1997) 223–246. [21] P. Panagiotopoulou, D.I. Kondarides, X.E. Verykios, J. Phys. Chem. C 115 (2011) 1220–1230. [22] P. Panagiotopoulou, D.I. Kondarides, X.E. Verykios, Catal. Today 181 (2012) 138–147. [23] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal– Support Interactions, Kluwer Academic/Plenum Publishers, New York, 2001. [24] C.G. Vayenas, C.G. Koutsodontis, J. Chem. Phys. 128 (2008) 182506. [25] A. Katsaounis, J. Appl. Electrochem. 40 (2010) 885–902. [26] C.G. Vayenas, J. Solid State Electrochem. 15 (2011) 1425–1435. [27] D. Tsiplakides, S. Balomenou, Catal. Today 146 (2009) 312–318. [28] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.-L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Chem. Rev. 113 (2013) 8192–8260. [29] M.N. Tsampas, F.M. Sapountzi, A. Boréave, P. Vernoux, Electrochem. Commun. 26 (2013) 13–16. [30] S. Souentie, L. Lizarraga, A. Kambolis, M. Alves-Fortunato, J.L. Valverde, P. Vernoux, J. Catal. 283 (2011) 124–132.

I. Kalaitzidou et al. / Journal of Catalysis 331 (2015) 98–109 [31] A. De Lucas-Consuegra, J. González-Cobos, Y. García-Rodríguez, A. Mosquera, J. L. Endrino, J.L. Valverde, J. Catal. 293 (2012) 149–157. [32] A. de Lucas-Consuegra, J. González-Cobos, V. Carcelén, C. Magén, J.L. Endrino, J. L. Valverde, J. Catal. 307 (2013) 18–26. [33] M.A. Fortunato, A. Princivalle, C. Capdeillayre, N. Petigny, C. Tardivat, C. Guizard, M.N. Tsampas, F.M. Sapountzi, P. Vernoux, Top. Catal. 57 (2014) 1277–1286. [34] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627. [35] W. Zipprich, H.-D. Wiemhöfer, U. Vöhrer, W. Göpel, Ber. Buns. Phys. Chem. 99 (1995) 1406–1413. [36] C.G. Vayenas, D. Tsiplakides, Surf. Sci. 467 (2000) 23–34. [37] D. Tsiplakides, C.G. Vayenas, J. Electrochem. Soc. 148 (2001) E189–E202. [38] D. Tsiplakides, J. Nicole, C.G. Vayenas, C. Comninellis, J. Electrochem. Soc. 145 (1998) 905–908. [39] J. Nicole, D. Tsiplakides, S. Wodiunig, C. Comninellis, J. Electrochem. Soc. 144 (1997) L312–L314. [40] S. Wodiunig, V. Patsis, C. Comninellis, Solid State Ionics 136–137 (2000) 813– 817 (references therein). [41] I. Riess, C.G. Vayenas, Solid State Ionics 159 (2003) 313–329. [42] J. Fleig, J. Jamnik, J. Electrochem. Soc. 152 (2005) E138–E145. [43] S. Brosda, C.G. Vayenas, J. Wei, Appl. Catal. B 68 (2006) 109–124. [44] C.G. Vayenas, S. Brosda, C. Pliangos, J. Catal. 203 (2001) 329–350. [45] C. Vayenas, S. Brosda, Top. Catal. 57 (2014) 1287–1301. [46] E.I. Papaioannou, S. Souentie, A. Hammad, C.G. Vayenas, Catal. Today 146 (2009) 336–344. [47] E. Ruiz, D. Cillero, P.J. Martínez, A. Morales, G.S. Vicente, G. de Diego, J.M. Sánchez, Catal. Today 236 (2014) 108–120. [48] S. Bebelis, H. Karasali, C.G. Vayenas, J. Appl. Electrochem. 38 (2008) 1127– 1133. [49] G. Pekridis, K. Kalimeri, N. Kaklidis, E. Vakouftsi, E.F. Iliopoulou, C. Athanasiou, G.E. Marnellos, Catal. Today 127 (2007) 337–346. [50] E. Ruiz, D. Cillero, P.J. Martínez, Á. Morales, G.S. Vicente, G. De Diego, J.M. Sánchez, Catal. Today 210 (2013) 55–66.

109

[51] E. Ruiz, D. Cillero, Á. Morales, G.S. Vicente, G. de Diego, P.J. Martínez, J.M. Sánchez, Electrochim. Acta 112 (2013) 967–975. [52] S. Bebelis, H. Karasali, C.G. Vayenas, Solid State Ionics 179 (2008) 1391–1395. [53] V. Jiménez, C. Jiménez-Borja, P. Sánchez, A. Romero, E.I. Papaioannou, D. Theleritis, S. Souentie, S. Brosda, J.L. Valverde, Appl. Catal. B 107 (2011) 210– 220. [54] D. Theleritis, M. Makri, S. Souentie, A. Caravaca, A. Katsaounis, C.G. Vayenas, ChemElectroChem 1 (2014) 254–262. [55] M. Makri, A. Katsaounis, C. Vayenas, Electrochim. Acta (2015), http://dx.doi. org/10.1016/j.electata.2015.08.023. [56] D. Theleritis, S. Souentie, A. Siokou, A. Katsaounis, C.G. Vayenas, ACS Catal. 2 (2012) 770–780. [57] T. Norby, Solid State Ionics 125 (1999) 1–11. [58] K.D. Kreuer, Chem. Mater. 8 (1996) 610–641. [59] A. Goñi-Urtiaga, D. Presvytes, K. Scott, Int. J. Hydrogen Energy 37 (2012) 3358– 3372. [60] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65–69. [61] T. Yajima, H. Suzuki, T. Yogo, H. Iwahara, Solid State Ionics 51 (1992) 101–107. [62] T. Hibino, K. Mizutani, T. Yajima, H. Iwahara, Solid State Ionics 57 (1992) 303– 306. [63] Y. Yamazaki, C.K. Yang, S.M. Haile, Scripta Mater. 65 (2011) 102–107. [64] D. Pergolesi, E. Fabbri, A. D’Epifanio, E. Di Bartolomeo, A. Tebano, S. Sanna, S. Licoccia, G. Balestrino, E. Traversa, Nat. Mater. 9 (2010) 846–852. [65] M. Makri, A. Buekenhoudt, J. Luyten, C.G. Vayenas, Ionics 2 (1996) 282–288. [66] A. Thursfield, S. Brosda, C. Pliangos, T. Schober, C.G. Vayenas, Electrochim. Acta 48 (2003) 3779–3788. [67] C.G. Yiokari, G.E. Pitselis, D.G. Polydoros, A.D. Katsaounis, C.G. Vayenas, J. Phys. Chem. A 104 (2000) 10600–10602. [68] G.E. Pitselis, P.D. Petrolekas, C.G. Vayenas, Ionics 3 (1997) 110–116. [69] B. Lin, R. Wang, X. Yu, J. Lin, F. Xie, K. Wei, Catal. Lett. 124 (2008) 178–184. [70] A. Katsaounis, Z. Nikopoulou, X.E. Verykios, C.G. Vayenas, J. Catal. 222 (2004) 192–206.