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YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy
Solid State Ionics 136–137 (2000) 863–872 www.elsevier.com / locate / ssi
Electrochemical promotion (NEMCA) of CH 4 and C 2 H 4 oxidation on Pd / YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy A.D. Frantzis, S. Bebelis, C.G. Vayenas* Department of Chemical Engineering, University of Patras, Patras GR-26500, Greece
Abstract The catalytic activity of Pd for the oxidation of methane to CO 2 can be markedly and reversibly affected using the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP), i.e. by interfacing polycrystalline Pd films with Y 2 O 3 -stabilized ZrO 2 (YSZ) and varying Pd catalyst-electrode potential in galvanic cells of the type: CH 4 , O 2 , CO 2 , PduYSZuAu, CH 4 , O 2 , CO 2 . It was found that by applying positive overpotentials or currents and thus, supplying O 22 onto the catalyst surface, up to 70-fold increase in the catalytic rate of CH 4 oxidation can be obtained, compared to the open circuit (unpromoted) catalytic rate. Electrochemical oxygen removal from the Pd catalyst-electrode surface, following negative overpotential or current application, also enhances the catalytic rate by up to a factor of 10. The induced changes in catalytic rate were typically two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. Faradaic efficiency L values of the order of 150 were attained. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. The existence of charged back-spillover species giving rise to catalyst work function change for positive overpotential application is manifest in this work for the first time using AC impedance spectroscopy. 2000 Elsevier Science B.V. All rights reserved. Keywords: NEMCA; Electrochemical promotion; Solid electrolytes; Methane oxidation; Ethylene oxidation; Palladium; AC impedance spectroscopy Materials: CH 4 ; C 2 H 4 ; Pd; ZrO 2 (Y 2 O 3 )
1. Introduction Palladium is the most attractive catalyst for complete oxidation of methane, aiming to thermal energy *Corresponding author. Tel.: 130-61-997-756; fax: 130-61997-269. E-mail address: [email protected] (C.G. Vayenas).
production or to emission control downstream of combustion units, as it exhibits high catalytic activity at temperatures above ¯ 2508C, lower than that needed with other noble metals, while its price is relatively low. The dependence of the catalytic activity for methane catalytic oxidation on the chemical state of palladium is quite complex [1,2]. The active phase is generally considered to be
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00528-2
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palladium oxide [2–4], which may exist in more than one forms depending on oxidized particles size and on the support [3–5]. In the present work we examine the possibility of using an oxygen ion conducting solid electrolyte (YSZ) as active catalyst support in order to increase the activity of Pd for catalytic combustion of methane. This can allow for lowering of the reaction temperature with obvious environmental and economic benefits. The use of solid electrolytes as active catalyst supports to induce the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) [6] or electrochemical promotion [7] has been reported for more than 50 catalytic reactions on Pt, Pd, Rh, Ag, Ni, Au, IrO 2 and RuO 2 surfaces. In addition to the group which first reported on this effect [6,8], several other research groups have made important contributions in this field [9–17]. Work in this area has been reviewed recently [8]. In brief it has been found that the catalytic and chemisorptive properties of polycrystalline metal films interfaced with solid electrolytes or mixed electronic–ionic conductors can be affected in a dramatic and reversible manner by electrically polarizing the catalystusolid electrolyte interface. Catalytic rate increases up to a factor of 100 [8] have been reported, corresponding to catalytic rate changes up to five orders of magnitude larger than the steady state rate of electrochemical supply of ionic species from the solid electrolyte onto the catalyst surface [8]. Significant selectivity changes are also observed [8]. The effect is quite reversible and does not appear to be limited to any type of reaction, metal catalyst or solid electrolyte, while it has also been demonstrated in aqueous electrolyte systems [8,18]. The catalytic and electrocatalytic oxidation of methane on palladium electrodes in an YSZ cell has been recently studied at temperatures 550–7508C and stoichiometric composition by Stoukides et al. [19] focusing mainly on monitoring via solid electrolyte potentiometry (SEP) the oxidative state of palladium during reaction. Under polarization conditions enhancement in the catalytic rate (up to 30%) was observed with increasing catalyst potential and the induced changes exceeded by up to four times the corresponding rate of oxygen ion transfer through the solid electrolyte. The present study is a more detailed
study of electrochemical promotion for this catalytic system, which focuses on a lower temperature (4008C), where the NEMCA effect is expected to be more significant [8]. In addition a simpler reactor design (single pellet), more suitable for technological applications, is utilized.
2. Experimental A ‘single pellet’ type [8] continuous flow atmospheric pressure reactor configuration was used, i.e. an YSZ disk (1 / 2–3 / 49 O.D., 2 mm thickness; Didier Werke AG) was suspended in the interior of a quartz tube of volume 40 cm 3 , surrounded by the gas mixture. A porous Pd catalyst-working electrode film, with a superficial surface area of 0.6–1.4 cm 2 , was deposited on the one side of the YSZ disk, using a Pd paste (A2895 Engelhard). Inert Au counter and reference electrodes were deposited on the other side of the YSZ pellet, opposite to the Pd electrode, using a Au paste (A1118 Engelhard). The superficial surface areas of the counter and reference electrodes were 0.6–1.4 cm 2 and 0.2–0.4 cm 2 , respectively. Their inertness under open and closed-circuit conditions was tested with blank experiments, as described elsewhere [20] along with details on electrode preparation procedure and on the gas analysis system, utilizing on-line gas chromatography and IR spectroscopy. The only products detected were CO 2 and H 2 O. Three catalyst samples, labeled R1, R2 and R3 were tested in this investigation and all three gave qualitatively similar results. The three catalysts were prepared under identical conditions but differed only in their surface area and total mass. The catalyst films were porous with thickness on the order of a few mm as shown using scanning electron microscopy. Their true surface areas expressed as surface Pd mol, N, (N 5 3 3 10 27 –2.4 3 10 26 mol Pd) were measured via isothermal surface titration of adsorbed CO with oxygen at 4008C as described in detail elsewhere [8]. The surface area can also be expressed in cm 2 (100–950 cm 2 ) using the surface density of the Pd(111) face, i.e. 1.53 3 10 15 cm 22 . The reactor was found to behave as CSTR in the flow-rate range of the present investigation, while the conversion of methane was kept below 10% both
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under open-circuit and closed circuit conditions, i.e. the reactor was operated as a differential one. The absence of internal and external mass transfer limitations under the conditions of the experiments was also checked. The ohmic-drop free catalyst potential VWR was calculated by subtracting from the measured catalyst 9 the parasitic working-reference ohmic potential V WR drop. The corresponding residual ohmic resistance was determined using AC impedance spectroscopy. AC impedance measurements were performed using a Solartron 1255 frequency response analyzer combined with a Solartron 1286 electrochemical interface. The EQUIVCRT software developed by B. Boukamp was used for the analysis of the impedance spectra.
3. Results and discussion Fig. 1 shows for catalyst film R1 (true surface area 6.7 3 10 27 mol Pd) a typical NEMCA potentiostatic transient, i.e. the response of the rate r of CH 4 oxidation on Pd and current I to a step change in potential between the Pd catalyst and the Au reference electrode. Initially (t , 0) the circuit is open
Fig. 1. Reaction rate, r, and current, I, response to a step change 9 . Catalyst R1 (true surface area in applied positive potential, V WR N56.7310 27 mol Pd, geometric surface area A51.4 cm 2 ); see text for discussion.
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(I 5 0), the open-circuit catalytic rate, r o , is 5.9 3 29 21 10 mol O s and the open-circuit catalyst po9 of 1 V tential is 2 147 mV. At t 5 0, a potential V WR is imposed and the rate increases gradually upon pumping O 22 to the catalyst surface. At steady state the rate is 68 times larger than the open circuit rate ( r 5 r /r o 5 68), while the rate increase Dr54.03 10 27 mol O s 21 is 153 times larger than the rate of O 22 supply I / 2F. Thus the system exhibits NEMCA behavior with corresponding enhancement factor or Faradaic efficiency L5Dr /(I / 2F ) [8] equal to 153. The observed changes in catalytic rate are quite reversible. As shown in Fig. 1, upon current interruption the catalytic rate and potential tend to relax to their open circuit values. Fig. 2 shows for catalyst film R2 (true surface area 3.0310 27 mol Pd) the steady state effect of varying the ohmic-drop free catalyst potential VWR on the catalytic rate and corresponding work function change D(eF)5eDVWR [6,8] at 4008C, pO 2 51.9 kPa and pCH 4 52.6 kPa. As shown in the figure, methane oxidation on Pd is both an electrophobic [8] and an electrophilic [8] reaction, i.e. the catalytic rate
Fig. 2. Effect of applied ohmic-drop free catalyst potential, VWR , and corresponding work function change, D(eF), on the rate of CH 4 oxidation; Open circuit rate r o 52.7310 29 mol O s 21 ; Catalyst R2 (true surface area N53.0310 27 mol Pd, geometric surface area A50.6 cm 2 ).
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increases (by up to a factor of 13 in this case) both with positive current application (O 22 supply to the catalyst) and, to a lesser extent, with negative current application (O 22 removal from the catalyst), respectively. The maximum induced rate increase observed in this study reached 7000% (Fig. 1). Even larger rate increases (up to 9000%) have been reported for positive current application in a recent similar study [20]. Fig. 3 shows, for catalyst film R2, the effect of current on the induced catalytic rate change Dr, expressed in mol O s 21 . As shown in this figure, the absolute values of the Faradaic efficiency or enhancement factor L were typically between 30 and 150, although values up to 2700 were measured for low currents. In agreement with previous electrochemical promotion studies [8] the order of magnitude of the measured L values is in very good qualitative agreement with the parameter 2Fr o /I0 (equal to 65 in the present case), where r o is the open-circuit (unpromoted) catalytic rate and I0 is the exchange current [8] of the catalystusolid electrolyte interface. The latter, of the order of 8 mA under the present conditions, is extracted from current–overpotential plots [8]. Figs. 4 and 5 show the dependence of the steady-
Fig. 3. Effect of applied current on the induced change in the rate of CH 4 oxidation. Dashed lines are constant enhancement factor L lines. Catalyst R2.
Fig. 4. Reaction rate dependence on pCH 4 at constant pO 2 52 kPa for open circuit conditions (circles) and when the catalyst 9 , is maintained at 10.5 V (squares) and at 11 V potential, V WR (triangles), T54008C; Catalyst R3 (true surface area N52.43 10 26 mol Pd, geometric surface area A51.4 cm 2 ).
Fig. 5. Reaction rate dependence on pO 2 at constant pCH 4 52 kPa for open circuit conditions (circles) and when the catalyst potential, V 9WR , is maintained at 10.5 V (squares), T54008C; Catalyst R3.
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state rate on the partial pressures of methane pCH 4 (0.8–4.8 kPa) and oxygen pO 2 (0.6–9.3 kPa) at temperature 4008C and different catalyst potentials 9 , including open circuit conditions. As shown on V WR Fig. 4 the rate is almost first order in methane both under open and closed circuit conditions, which is indicative of weak bonding of methane on the catalyst surface. The apparent kinetic constant increases with increasing catalyst potential, probably indicating the creation of free sites for methane adsorption, due to the weakening of the metal– oxygen bond (electron acceptor). The strong bonding of oxygen on the Pd surface, well known from the literature [21,22], is reflected on the practically zero order kinetics with respect to oxygen partial pressure shown on Fig. 5, for oxygen to methane ratios higher than 1. The observed kinetic behavior is consistent with an Eley–Rideal-type mechanism involving gaseous or weakly bonded methane and atomically adsorbed oxygen in agreement with the literature [23]. Both Figs. 4 and 5 show that the kinetic effect of the reactants remains practically the same under NEMCA conditions, with significant increase in each
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case of the apparent kinetic constant. It should be also noted that the TOF (turnover frequency, s 21 ) values measured in the present study and in previous similar study of the same system [20] are in reasonable agreement with reported TOF values [20], in view of the fact that comparison is made between supported and unsupported catalysts and that methane oxidation on Pd is a structure sensitive reaction. Fig. 6 shows complex impedance spectra (Nyquist plots) corresponding to selected experimental points in Fig. 2. Focusing in the frequency region below |10 4 Hz (as all features above this frequency were found to correspond to the bulk electrolyte and the grain boundaries) three distinct features are observed: (i) in the range of highest frequencies (higher than |300 Hz) part of small depressed semicircle (semicircle 1) is resolved for absolute values of overpotential lower than 0.6 V. (ii) For frequencies lower than |300 Hz, the main feature under open circuit conditions is an inclined line, which can be considered as part of a depressed semicircle. Indeed, by absolutely increasing VWR to highly positive or negative values a depressed semicircle (semicircle 2)
Fig. 6. Complex impedance spectra (Nyquist plots) of the CH 4 , O 2 , PduYSZ system at different Pd catalyst potentials. Conditions as in Fig. o 2. Open circuit potential V WR 5 20.13 V.
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is clearly formed. This semicircle becomes smaller with absolutely increasing polarization overpotential, indicating that this semicircle corresponds to the PduYSZ interface, as the apparent polarization resistance R app of the latter is expected to become smaller with absolutely increasing overpotential. (iii) Interestingly, only at high positive overpotentials (higher than 0.6 V) a third depressed semicircle (semicircle 3) appears in the lowest frequencies range. The frequency corresponding to the onset of formation of this semicircle gradually increases from |0.5 to |15 Hz as the overpotential increases from 0.6 to 2 V. In order to simplify the analysis of the impedance spectra, each of the above semicircles were fitted to a single resistor–capacitor combination and the corresponding capacitance values were determined. Although this represents an approximation, as depressed semicircles correspond to resistor-constant phase element combination and there is also significant overlapping of the semicircles, the thus determined capacitance values C 1 , C 2 and C 3 reflect the inherent characteristics of the corresponding processes and, as shown below, give valuable information concerning the physical understanding of the system. Fig. 7a depicts the dependence of the individual capacitances on ohmic-drop-free catalyst potential VWR . The frequencies f1 , f2 and f3 corresponding to the maximum absolute value of the imaginary part of the impedance for each semicircle are also depicted on Fig. 7b. It should be noted that the inverse of the corresponding angular frequency represents the characteristic time constant ti related to the relevant process, i.e. ti 5 1 / 2p fi 5 R i C1 [24], where the resistance R i is the intersection of the semi-circle with the x-axis in the Nyquist plot. As shown in Fig. 7, increasing catalyst potential has a minor effect on C1 but causes C2 to pass through a maximum. Both C1 and C2 are quite small, i.e. less than 10 mF cm 22 of electrolyte. The main feature is the appearance of a large (|200 mF cm 22 ) capacitance, labeled C3 , at highly positive VWR values. Similar observations have been made by Kek et al. [25] in a very interesting recent work concerning the dependence of metal / YSZ single crystal interface capacitance on electrode potential for quasipoint contact Pt and Au electrodes. Comparison of the values of C3 and C2 (Table 1) shows that C3 is on the average 2500 times higher
Fig. 7. Dependence on catalyst potential of the individual capacitances, Ci (a) and of the corresponding frequencies, fi , at maximum absolute negative part of impedance (b) for the CH 4 , O 2 , PduYSZ system. See text for discussion. Conditions as in Fig. 2.
than C2 . Interestingly, the ratio C3 /C2 compares well with the ratio N /Ntpb , equal to 3570, of the gas exposed surface of the catalyst (measured via surface titration) N53310 27 mol Pd and the ‘surface’ corresponding to the three-phase (gas–metal–electrolyte) boundary (tpb) region Ntpb 58.4310 211 mol
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Table 1 Comparison of the ratio of C3 /C2 to the ratio N /Ntpb 5 3570 for methane and ethylene oxidation on PduYSZ Methane oxidation
Ethylene oxidation
VWR (V)
C3 /C2
VWR (V)
C3 /C2
1 1.1 1.23 1.37 1.5
2700 3190 2570 2060 1810
1.1 1.4 1.75
710 920 890
Pd. The latter is computed from the total Pd electrode mass (0.3 mg) and its surface area via Ntpb 5 ˚ is the atomic (l tpb / 2r Pd ) /NAV , where r Pd 51.8 A radius of Pd, NAV is Avogadro’s number and l tpb is the three-phase boundary length computed via l tpb 5 ] ] p A /d [26], where d is the average catalyst crystallite size and A the geometric surface of the catalystelectrode (A50.6 cm 2 ). This observation provides a strong indication that the appearance of the depressed semicircle in the low frequency range for highly positive overpotentials relates to charge separation extending all over the catalyst surface, which can be explained by a back spillover of charged species, in agreement with the model proposed to explain the origin of the effect of electrochemical promotion [6,8] and with the interpretation given by Kek et al. [25] to the very high capacitances they measured under similar anodic polarization conditions. On the other hand C2 , relates to the tpb region, where the electrocatalytic sites for charge transfer reaction are located. Interestingly, the dependence of f2 on catalyst potential (Fig. 7b) parallels the observed catalytic rate dependence (Fig. 2). The capacitance C1 , being on the order of 0.1 mF cm 22 , could relate to a charged surface layer, i.e. palladium oxide or even to the interface between the palladium catalyst and the solid electrolyte [24]. The observed behavior is not unique to the methane oxidation system. Fig. 8 shows impedance spectra for various positive overpotentials for the case of ethylene oxidation on Pd ( pC 2 H 4 52.6 kPa, pO 2 510.2 kPa, T53408C), a reaction which can also be electrochemically promoted [27]. Compared to the case of methane oxidation, practically the same features are observed. The only significant difference here is that the semicircle corresponding to the three-phase boundary (C2 ) is already formed
Fig. 8. Complex impedance spectra (Nyquist plots) of the C 2 H 4 , O 2 , PduYSZ system at different Pd catalyst potentials. Catalyst R2. pC 2 H 4 52.6 kPa, pO 2 510.2 kPa, T53408C, V oWR 5 20.11 V.
under open circuit conditions due to the lower interphase resistance (higher exchange current density) for this system. Thus C1 cannot be separated from C2 . The individual capacitance values as well as the frequency at the maximum absolute value of the imaginary part are somewhat lower but on the same order of magnitude with the ones in the case of methane oxidation (Fig. 9a and b) and show practically the same dependence on catalyst potential. The ratio C3 /C2 in this case is |4 times lower than the ratio N /Ntpb (Table1). This can be explained by a partial blocking of the gas exposed catalyst surface by carbonaceous species originating from ethylene adsorption and / or the higher reactivity of ethylene, compared to methane, with the back spillover oxygen species. Concerning the characteristic time constants t of the phenomena underlying the appearance of the depressed semicircles (equal to 1 / 2p fi , where f is the frequency corresponding to the absolutely maximum value of the imaginary part of the impedance for each semicircle), they show the opposite behavior to the corresponding capacitance value (Fig. 7). Furthermore, the relaxation time constants corresponding to the third semicircle (capacitance C3 )
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Fig. 9. Dependence on catalyst potential of the individual capacitances, Ci (a) and of the corresponding frequencies, fi , at maximum absolute negative part of impedance (b) for the C 2 H 4 , O 2 , PduYSZ system. See text for discussion; conditions as in Fig. 8.
vary between 0.5 and 1.2 s, in reasonable agreement with the relaxation time constants tR 5 (1 /TOF ) corresponding to the catalytic reaction of methane oxidation in the region of high positive overpotentials (on the order of 9s), where TOF is the turnover frequency, that is the rate expressed in mol per
surface palladium site per s. This further corroborates the fact that the appearance of the third semicircle in high positive overpotentials relates to catalytically important species. The present results establish that the catalytic properties of Pd for methane oxidation can be markedly affected via the NEMCA effect by using YSZ as an active catalyst support and promoter donor. Catalytic activity changes up to 7000% were observed by varying catalyst potential or equivalently catalyst work function, while the measured Faradaic efficiency, L, values are typically of the order of 150. These changes are much larger than the ones observed by Athanasiou et al. [19] in the same system at much higher temperatures (650–8508C). At these high temperatures the appearance of strong non-Faradaic rate enhancement is not expected due to the lower polarizability of the catalyst-solid electrolyte interface, as explained in detail elsewhere [8]. Besides in addition to the surface catalytic steps the oxidation of methane involves also non-catalytic gas-phase reactions [28] which cannot be affected by electrochemical promotion of the catalytic properties of Pd. As in previous electrochemical promotion studies the present results can be rationalized on the basis of the theoretical considerations invoked to explain electrochemical promotion behavior, i.e. the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential [8]. The back-spillover oxide ions are less reactive with CH 4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and methane [8]. The appearance of these species, which has already been proved using XPS [29] and TPD [30] under electrochemical promotion conditions, is also manifest in this study for the first time, using AC impedance spectroscopy. Under the conditions of the present study ( pO 2 5 1.9 kPa, pCH 4 52.6 kPa) the rate is nearly first order in CH 4 and nearly zero order in oxygen (Figs. 4 and 5). The same kinetics were also observed under closed circuit conditions. These observations imply that the Pd surface is here predominantly covered with oxygen and with very little or no adsorbed methane. Oxygen is strongly bonded onto the Pd
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surface, forming a stable surface or subsurface (incorporated) metal oxide layer [21,22]. Increasing catalyst potential and work function eF causes a weakening in the binding strength of electron acceptor adsorbates, such as dissociatively chemisorbed oxygen. The latter is the only kinetically important species which is strongly bonded to the catalyst surface. The NEMCA-induced increase in catalyst work function at positive overpotentials results in a decrease in the strength of palladium–oxygen bond. Chemisorbed oxygen becomes progressively more loosely bound to the catalyst surface as the catalyst potential increases and thus much more susceptible to reaction with gaseous methane, as is experimentally observed. At high positive catalyst potentials, methane adsorption on the catalyst surface becomes significant, as shown from the deviation from linearity of the rate vs. methane partial pressure curve corresponding to 11000 mV in Fig. 4. The enhanced adsorption of methane at very high positive catalyst potentials, most probable on the same sites with oxygen, could explain the tendency of the catalytic rate to reach a constant value with increasing catalyst potential or current (Figs. 4 and 5), as the ratio of the surface coverages of oxygen and methane (in the form of CH x ) tends to become comparable. Direct electrocatalytic reaction of methane with O 22 is not expected to play a significant role due to the low reaction temperature and thus low current values. The observed increase in catalytic rate with decreasing negative overpotential (Fig. 2) can be explained by the strengthening of the oxygen–palladium bond and the concomitant increase in methane coverage, due to the decrease in catalyst work function.
4. Conclusions The catalytic activity of Pd for the complete oxidation of methane can be reversibly affected via the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion by interfacing polycrystalline Pd films with 8 mol% Y 2 O 3 -stabilized–ZrO 2 (YSZ), an O 22 conductor, and varying the potential of the Pd catalyst film. In the temperature range 350–4508C the reaction exhibits electrophobic NEMCA behaviour and the rate of CO 2 formation increases (by up
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to 7000%) with increasing catalyst potential or work function. The steady-state rate change is typically 150 times larger than the steady-state rate I / 2F of electrochemical supply of O 22 to the catalyst. As in previous electrochemical promotion studies the observed behavior is due to the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential. The back-spillover oxide ions are less reactive with CH 4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and methane. The appearance of these back spillover species which cause NEMCA is manifest in this study using in situ AC impedance spectroscopy by the appearance of a high capacitance (.200 mF cm 22 ) which reflects the creation of an effective double layer over the entire gas exposed electrode surface. The conclusions drawn from the AC impedance analysis are well supported by those obtained recently by Nicole using cyclic voltammetry [31].
Acknowledgements The authors gratefully acknowledge the financial support of the PENED Programme.
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Electrochemical promotion (NEMCA) of CH 4 and C 2 H 4 oxidation on Pd / YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy A.D. Frantzis, S. Bebelis, C.G. Vayenas* Department of Chemical Engineering, University of Patras, Patras GR-26500, Greece
Abstract The catalytic activity of Pd for the oxidation of methane to CO 2 can be markedly and reversibly affected using the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP), i.e. by interfacing polycrystalline Pd films with Y 2 O 3 -stabilized ZrO 2 (YSZ) and varying Pd catalyst-electrode potential in galvanic cells of the type: CH 4 , O 2 , CO 2 , PduYSZuAu, CH 4 , O 2 , CO 2 . It was found that by applying positive overpotentials or currents and thus, supplying O 22 onto the catalyst surface, up to 70-fold increase in the catalytic rate of CH 4 oxidation can be obtained, compared to the open circuit (unpromoted) catalytic rate. Electrochemical oxygen removal from the Pd catalyst-electrode surface, following negative overpotential or current application, also enhances the catalytic rate by up to a factor of 10. The induced changes in catalytic rate were typically two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. Faradaic efficiency L values of the order of 150 were attained. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. The existence of charged back-spillover species giving rise to catalyst work function change for positive overpotential application is manifest in this work for the first time using AC impedance spectroscopy. 2000 Elsevier Science B.V. All rights reserved. Keywords: NEMCA; Electrochemical promotion; Solid electrolytes; Methane oxidation; Ethylene oxidation; Palladium; AC impedance spectroscopy Materials: CH 4 ; C 2 H 4 ; Pd; ZrO 2 (Y 2 O 3 )
1. Introduction Palladium is the most attractive catalyst for complete oxidation of methane, aiming to thermal energy *Corresponding author. Tel.: 130-61-997-756; fax: 130-61997-269. E-mail address: [email protected] (C.G. Vayenas).
production or to emission control downstream of combustion units, as it exhibits high catalytic activity at temperatures above ¯ 2508C, lower than that needed with other noble metals, while its price is relatively low. The dependence of the catalytic activity for methane catalytic oxidation on the chemical state of palladium is quite complex [1,2]. The active phase is generally considered to be
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00528-2
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palladium oxide [2–4], which may exist in more than one forms depending on oxidized particles size and on the support [3–5]. In the present work we examine the possibility of using an oxygen ion conducting solid electrolyte (YSZ) as active catalyst support in order to increase the activity of Pd for catalytic combustion of methane. This can allow for lowering of the reaction temperature with obvious environmental and economic benefits. The use of solid electrolytes as active catalyst supports to induce the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) [6] or electrochemical promotion [7] has been reported for more than 50 catalytic reactions on Pt, Pd, Rh, Ag, Ni, Au, IrO 2 and RuO 2 surfaces. In addition to the group which first reported on this effect [6,8], several other research groups have made important contributions in this field [9–17]. Work in this area has been reviewed recently [8]. In brief it has been found that the catalytic and chemisorptive properties of polycrystalline metal films interfaced with solid electrolytes or mixed electronic–ionic conductors can be affected in a dramatic and reversible manner by electrically polarizing the catalystusolid electrolyte interface. Catalytic rate increases up to a factor of 100 [8] have been reported, corresponding to catalytic rate changes up to five orders of magnitude larger than the steady state rate of electrochemical supply of ionic species from the solid electrolyte onto the catalyst surface [8]. Significant selectivity changes are also observed [8]. The effect is quite reversible and does not appear to be limited to any type of reaction, metal catalyst or solid electrolyte, while it has also been demonstrated in aqueous electrolyte systems [8,18]. The catalytic and electrocatalytic oxidation of methane on palladium electrodes in an YSZ cell has been recently studied at temperatures 550–7508C and stoichiometric composition by Stoukides et al. [19] focusing mainly on monitoring via solid electrolyte potentiometry (SEP) the oxidative state of palladium during reaction. Under polarization conditions enhancement in the catalytic rate (up to 30%) was observed with increasing catalyst potential and the induced changes exceeded by up to four times the corresponding rate of oxygen ion transfer through the solid electrolyte. The present study is a more detailed
study of electrochemical promotion for this catalytic system, which focuses on a lower temperature (4008C), where the NEMCA effect is expected to be more significant [8]. In addition a simpler reactor design (single pellet), more suitable for technological applications, is utilized.
2. Experimental A ‘single pellet’ type [8] continuous flow atmospheric pressure reactor configuration was used, i.e. an YSZ disk (1 / 2–3 / 49 O.D., 2 mm thickness; Didier Werke AG) was suspended in the interior of a quartz tube of volume 40 cm 3 , surrounded by the gas mixture. A porous Pd catalyst-working electrode film, with a superficial surface area of 0.6–1.4 cm 2 , was deposited on the one side of the YSZ disk, using a Pd paste (A2895 Engelhard). Inert Au counter and reference electrodes were deposited on the other side of the YSZ pellet, opposite to the Pd electrode, using a Au paste (A1118 Engelhard). The superficial surface areas of the counter and reference electrodes were 0.6–1.4 cm 2 and 0.2–0.4 cm 2 , respectively. Their inertness under open and closed-circuit conditions was tested with blank experiments, as described elsewhere [20] along with details on electrode preparation procedure and on the gas analysis system, utilizing on-line gas chromatography and IR spectroscopy. The only products detected were CO 2 and H 2 O. Three catalyst samples, labeled R1, R2 and R3 were tested in this investigation and all three gave qualitatively similar results. The three catalysts were prepared under identical conditions but differed only in their surface area and total mass. The catalyst films were porous with thickness on the order of a few mm as shown using scanning electron microscopy. Their true surface areas expressed as surface Pd mol, N, (N 5 3 3 10 27 –2.4 3 10 26 mol Pd) were measured via isothermal surface titration of adsorbed CO with oxygen at 4008C as described in detail elsewhere [8]. The surface area can also be expressed in cm 2 (100–950 cm 2 ) using the surface density of the Pd(111) face, i.e. 1.53 3 10 15 cm 22 . The reactor was found to behave as CSTR in the flow-rate range of the present investigation, while the conversion of methane was kept below 10% both
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under open-circuit and closed circuit conditions, i.e. the reactor was operated as a differential one. The absence of internal and external mass transfer limitations under the conditions of the experiments was also checked. The ohmic-drop free catalyst potential VWR was calculated by subtracting from the measured catalyst 9 the parasitic working-reference ohmic potential V WR drop. The corresponding residual ohmic resistance was determined using AC impedance spectroscopy. AC impedance measurements were performed using a Solartron 1255 frequency response analyzer combined with a Solartron 1286 electrochemical interface. The EQUIVCRT software developed by B. Boukamp was used for the analysis of the impedance spectra.
3. Results and discussion Fig. 1 shows for catalyst film R1 (true surface area 6.7 3 10 27 mol Pd) a typical NEMCA potentiostatic transient, i.e. the response of the rate r of CH 4 oxidation on Pd and current I to a step change in potential between the Pd catalyst and the Au reference electrode. Initially (t , 0) the circuit is open
Fig. 1. Reaction rate, r, and current, I, response to a step change 9 . Catalyst R1 (true surface area in applied positive potential, V WR N56.7310 27 mol Pd, geometric surface area A51.4 cm 2 ); see text for discussion.
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(I 5 0), the open-circuit catalytic rate, r o , is 5.9 3 29 21 10 mol O s and the open-circuit catalyst po9 of 1 V tential is 2 147 mV. At t 5 0, a potential V WR is imposed and the rate increases gradually upon pumping O 22 to the catalyst surface. At steady state the rate is 68 times larger than the open circuit rate ( r 5 r /r o 5 68), while the rate increase Dr54.03 10 27 mol O s 21 is 153 times larger than the rate of O 22 supply I / 2F. Thus the system exhibits NEMCA behavior with corresponding enhancement factor or Faradaic efficiency L5Dr /(I / 2F ) [8] equal to 153. The observed changes in catalytic rate are quite reversible. As shown in Fig. 1, upon current interruption the catalytic rate and potential tend to relax to their open circuit values. Fig. 2 shows for catalyst film R2 (true surface area 3.0310 27 mol Pd) the steady state effect of varying the ohmic-drop free catalyst potential VWR on the catalytic rate and corresponding work function change D(eF)5eDVWR [6,8] at 4008C, pO 2 51.9 kPa and pCH 4 52.6 kPa. As shown in the figure, methane oxidation on Pd is both an electrophobic [8] and an electrophilic [8] reaction, i.e. the catalytic rate
Fig. 2. Effect of applied ohmic-drop free catalyst potential, VWR , and corresponding work function change, D(eF), on the rate of CH 4 oxidation; Open circuit rate r o 52.7310 29 mol O s 21 ; Catalyst R2 (true surface area N53.0310 27 mol Pd, geometric surface area A50.6 cm 2 ).
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increases (by up to a factor of 13 in this case) both with positive current application (O 22 supply to the catalyst) and, to a lesser extent, with negative current application (O 22 removal from the catalyst), respectively. The maximum induced rate increase observed in this study reached 7000% (Fig. 1). Even larger rate increases (up to 9000%) have been reported for positive current application in a recent similar study [20]. Fig. 3 shows, for catalyst film R2, the effect of current on the induced catalytic rate change Dr, expressed in mol O s 21 . As shown in this figure, the absolute values of the Faradaic efficiency or enhancement factor L were typically between 30 and 150, although values up to 2700 were measured for low currents. In agreement with previous electrochemical promotion studies [8] the order of magnitude of the measured L values is in very good qualitative agreement with the parameter 2Fr o /I0 (equal to 65 in the present case), where r o is the open-circuit (unpromoted) catalytic rate and I0 is the exchange current [8] of the catalystusolid electrolyte interface. The latter, of the order of 8 mA under the present conditions, is extracted from current–overpotential plots [8]. Figs. 4 and 5 show the dependence of the steady-
Fig. 3. Effect of applied current on the induced change in the rate of CH 4 oxidation. Dashed lines are constant enhancement factor L lines. Catalyst R2.
Fig. 4. Reaction rate dependence on pCH 4 at constant pO 2 52 kPa for open circuit conditions (circles) and when the catalyst 9 , is maintained at 10.5 V (squares) and at 11 V potential, V WR (triangles), T54008C; Catalyst R3 (true surface area N52.43 10 26 mol Pd, geometric surface area A51.4 cm 2 ).
Fig. 5. Reaction rate dependence on pO 2 at constant pCH 4 52 kPa for open circuit conditions (circles) and when the catalyst potential, V 9WR , is maintained at 10.5 V (squares), T54008C; Catalyst R3.
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state rate on the partial pressures of methane pCH 4 (0.8–4.8 kPa) and oxygen pO 2 (0.6–9.3 kPa) at temperature 4008C and different catalyst potentials 9 , including open circuit conditions. As shown on V WR Fig. 4 the rate is almost first order in methane both under open and closed circuit conditions, which is indicative of weak bonding of methane on the catalyst surface. The apparent kinetic constant increases with increasing catalyst potential, probably indicating the creation of free sites for methane adsorption, due to the weakening of the metal– oxygen bond (electron acceptor). The strong bonding of oxygen on the Pd surface, well known from the literature [21,22], is reflected on the practically zero order kinetics with respect to oxygen partial pressure shown on Fig. 5, for oxygen to methane ratios higher than 1. The observed kinetic behavior is consistent with an Eley–Rideal-type mechanism involving gaseous or weakly bonded methane and atomically adsorbed oxygen in agreement with the literature [23]. Both Figs. 4 and 5 show that the kinetic effect of the reactants remains practically the same under NEMCA conditions, with significant increase in each
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case of the apparent kinetic constant. It should be also noted that the TOF (turnover frequency, s 21 ) values measured in the present study and in previous similar study of the same system [20] are in reasonable agreement with reported TOF values [20], in view of the fact that comparison is made between supported and unsupported catalysts and that methane oxidation on Pd is a structure sensitive reaction. Fig. 6 shows complex impedance spectra (Nyquist plots) corresponding to selected experimental points in Fig. 2. Focusing in the frequency region below |10 4 Hz (as all features above this frequency were found to correspond to the bulk electrolyte and the grain boundaries) three distinct features are observed: (i) in the range of highest frequencies (higher than |300 Hz) part of small depressed semicircle (semicircle 1) is resolved for absolute values of overpotential lower than 0.6 V. (ii) For frequencies lower than |300 Hz, the main feature under open circuit conditions is an inclined line, which can be considered as part of a depressed semicircle. Indeed, by absolutely increasing VWR to highly positive or negative values a depressed semicircle (semicircle 2)
Fig. 6. Complex impedance spectra (Nyquist plots) of the CH 4 , O 2 , PduYSZ system at different Pd catalyst potentials. Conditions as in Fig. o 2. Open circuit potential V WR 5 20.13 V.
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is clearly formed. This semicircle becomes smaller with absolutely increasing polarization overpotential, indicating that this semicircle corresponds to the PduYSZ interface, as the apparent polarization resistance R app of the latter is expected to become smaller with absolutely increasing overpotential. (iii) Interestingly, only at high positive overpotentials (higher than 0.6 V) a third depressed semicircle (semicircle 3) appears in the lowest frequencies range. The frequency corresponding to the onset of formation of this semicircle gradually increases from |0.5 to |15 Hz as the overpotential increases from 0.6 to 2 V. In order to simplify the analysis of the impedance spectra, each of the above semicircles were fitted to a single resistor–capacitor combination and the corresponding capacitance values were determined. Although this represents an approximation, as depressed semicircles correspond to resistor-constant phase element combination and there is also significant overlapping of the semicircles, the thus determined capacitance values C 1 , C 2 and C 3 reflect the inherent characteristics of the corresponding processes and, as shown below, give valuable information concerning the physical understanding of the system. Fig. 7a depicts the dependence of the individual capacitances on ohmic-drop-free catalyst potential VWR . The frequencies f1 , f2 and f3 corresponding to the maximum absolute value of the imaginary part of the impedance for each semicircle are also depicted on Fig. 7b. It should be noted that the inverse of the corresponding angular frequency represents the characteristic time constant ti related to the relevant process, i.e. ti 5 1 / 2p fi 5 R i C1 [24], where the resistance R i is the intersection of the semi-circle with the x-axis in the Nyquist plot. As shown in Fig. 7, increasing catalyst potential has a minor effect on C1 but causes C2 to pass through a maximum. Both C1 and C2 are quite small, i.e. less than 10 mF cm 22 of electrolyte. The main feature is the appearance of a large (|200 mF cm 22 ) capacitance, labeled C3 , at highly positive VWR values. Similar observations have been made by Kek et al. [25] in a very interesting recent work concerning the dependence of metal / YSZ single crystal interface capacitance on electrode potential for quasipoint contact Pt and Au electrodes. Comparison of the values of C3 and C2 (Table 1) shows that C3 is on the average 2500 times higher
Fig. 7. Dependence on catalyst potential of the individual capacitances, Ci (a) and of the corresponding frequencies, fi , at maximum absolute negative part of impedance (b) for the CH 4 , O 2 , PduYSZ system. See text for discussion. Conditions as in Fig. 2.
than C2 . Interestingly, the ratio C3 /C2 compares well with the ratio N /Ntpb , equal to 3570, of the gas exposed surface of the catalyst (measured via surface titration) N53310 27 mol Pd and the ‘surface’ corresponding to the three-phase (gas–metal–electrolyte) boundary (tpb) region Ntpb 58.4310 211 mol
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Table 1 Comparison of the ratio of C3 /C2 to the ratio N /Ntpb 5 3570 for methane and ethylene oxidation on PduYSZ Methane oxidation
Ethylene oxidation
VWR (V)
C3 /C2
VWR (V)
C3 /C2
1 1.1 1.23 1.37 1.5
2700 3190 2570 2060 1810
1.1 1.4 1.75
710 920 890
Pd. The latter is computed from the total Pd electrode mass (0.3 mg) and its surface area via Ntpb 5 ˚ is the atomic (l tpb / 2r Pd ) /NAV , where r Pd 51.8 A radius of Pd, NAV is Avogadro’s number and l tpb is the three-phase boundary length computed via l tpb 5 ] ] p A /d [26], where d is the average catalyst crystallite size and A the geometric surface of the catalystelectrode (A50.6 cm 2 ). This observation provides a strong indication that the appearance of the depressed semicircle in the low frequency range for highly positive overpotentials relates to charge separation extending all over the catalyst surface, which can be explained by a back spillover of charged species, in agreement with the model proposed to explain the origin of the effect of electrochemical promotion [6,8] and with the interpretation given by Kek et al. [25] to the very high capacitances they measured under similar anodic polarization conditions. On the other hand C2 , relates to the tpb region, where the electrocatalytic sites for charge transfer reaction are located. Interestingly, the dependence of f2 on catalyst potential (Fig. 7b) parallels the observed catalytic rate dependence (Fig. 2). The capacitance C1 , being on the order of 0.1 mF cm 22 , could relate to a charged surface layer, i.e. palladium oxide or even to the interface between the palladium catalyst and the solid electrolyte [24]. The observed behavior is not unique to the methane oxidation system. Fig. 8 shows impedance spectra for various positive overpotentials for the case of ethylene oxidation on Pd ( pC 2 H 4 52.6 kPa, pO 2 510.2 kPa, T53408C), a reaction which can also be electrochemically promoted [27]. Compared to the case of methane oxidation, practically the same features are observed. The only significant difference here is that the semicircle corresponding to the three-phase boundary (C2 ) is already formed
Fig. 8. Complex impedance spectra (Nyquist plots) of the C 2 H 4 , O 2 , PduYSZ system at different Pd catalyst potentials. Catalyst R2. pC 2 H 4 52.6 kPa, pO 2 510.2 kPa, T53408C, V oWR 5 20.11 V.
under open circuit conditions due to the lower interphase resistance (higher exchange current density) for this system. Thus C1 cannot be separated from C2 . The individual capacitance values as well as the frequency at the maximum absolute value of the imaginary part are somewhat lower but on the same order of magnitude with the ones in the case of methane oxidation (Fig. 9a and b) and show practically the same dependence on catalyst potential. The ratio C3 /C2 in this case is |4 times lower than the ratio N /Ntpb (Table1). This can be explained by a partial blocking of the gas exposed catalyst surface by carbonaceous species originating from ethylene adsorption and / or the higher reactivity of ethylene, compared to methane, with the back spillover oxygen species. Concerning the characteristic time constants t of the phenomena underlying the appearance of the depressed semicircles (equal to 1 / 2p fi , where f is the frequency corresponding to the absolutely maximum value of the imaginary part of the impedance for each semicircle), they show the opposite behavior to the corresponding capacitance value (Fig. 7). Furthermore, the relaxation time constants corresponding to the third semicircle (capacitance C3 )
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Fig. 9. Dependence on catalyst potential of the individual capacitances, Ci (a) and of the corresponding frequencies, fi , at maximum absolute negative part of impedance (b) for the C 2 H 4 , O 2 , PduYSZ system. See text for discussion; conditions as in Fig. 8.
vary between 0.5 and 1.2 s, in reasonable agreement with the relaxation time constants tR 5 (1 /TOF ) corresponding to the catalytic reaction of methane oxidation in the region of high positive overpotentials (on the order of 9s), where TOF is the turnover frequency, that is the rate expressed in mol per
surface palladium site per s. This further corroborates the fact that the appearance of the third semicircle in high positive overpotentials relates to catalytically important species. The present results establish that the catalytic properties of Pd for methane oxidation can be markedly affected via the NEMCA effect by using YSZ as an active catalyst support and promoter donor. Catalytic activity changes up to 7000% were observed by varying catalyst potential or equivalently catalyst work function, while the measured Faradaic efficiency, L, values are typically of the order of 150. These changes are much larger than the ones observed by Athanasiou et al. [19] in the same system at much higher temperatures (650–8508C). At these high temperatures the appearance of strong non-Faradaic rate enhancement is not expected due to the lower polarizability of the catalyst-solid electrolyte interface, as explained in detail elsewhere [8]. Besides in addition to the surface catalytic steps the oxidation of methane involves also non-catalytic gas-phase reactions [28] which cannot be affected by electrochemical promotion of the catalytic properties of Pd. As in previous electrochemical promotion studies the present results can be rationalized on the basis of the theoretical considerations invoked to explain electrochemical promotion behavior, i.e. the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential [8]. The back-spillover oxide ions are less reactive with CH 4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and methane [8]. The appearance of these species, which has already been proved using XPS [29] and TPD [30] under electrochemical promotion conditions, is also manifest in this study for the first time, using AC impedance spectroscopy. Under the conditions of the present study ( pO 2 5 1.9 kPa, pCH 4 52.6 kPa) the rate is nearly first order in CH 4 and nearly zero order in oxygen (Figs. 4 and 5). The same kinetics were also observed under closed circuit conditions. These observations imply that the Pd surface is here predominantly covered with oxygen and with very little or no adsorbed methane. Oxygen is strongly bonded onto the Pd
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surface, forming a stable surface or subsurface (incorporated) metal oxide layer [21,22]. Increasing catalyst potential and work function eF causes a weakening in the binding strength of electron acceptor adsorbates, such as dissociatively chemisorbed oxygen. The latter is the only kinetically important species which is strongly bonded to the catalyst surface. The NEMCA-induced increase in catalyst work function at positive overpotentials results in a decrease in the strength of palladium–oxygen bond. Chemisorbed oxygen becomes progressively more loosely bound to the catalyst surface as the catalyst potential increases and thus much more susceptible to reaction with gaseous methane, as is experimentally observed. At high positive catalyst potentials, methane adsorption on the catalyst surface becomes significant, as shown from the deviation from linearity of the rate vs. methane partial pressure curve corresponding to 11000 mV in Fig. 4. The enhanced adsorption of methane at very high positive catalyst potentials, most probable on the same sites with oxygen, could explain the tendency of the catalytic rate to reach a constant value with increasing catalyst potential or current (Figs. 4 and 5), as the ratio of the surface coverages of oxygen and methane (in the form of CH x ) tends to become comparable. Direct electrocatalytic reaction of methane with O 22 is not expected to play a significant role due to the low reaction temperature and thus low current values. The observed increase in catalytic rate with decreasing negative overpotential (Fig. 2) can be explained by the strengthening of the oxygen–palladium bond and the concomitant increase in methane coverage, due to the decrease in catalyst work function.
4. Conclusions The catalytic activity of Pd for the complete oxidation of methane can be reversibly affected via the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion by interfacing polycrystalline Pd films with 8 mol% Y 2 O 3 -stabilized–ZrO 2 (YSZ), an O 22 conductor, and varying the potential of the Pd catalyst film. In the temperature range 350–4508C the reaction exhibits electrophobic NEMCA behaviour and the rate of CO 2 formation increases (by up
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to 7000%) with increasing catalyst potential or work function. The steady-state rate change is typically 150 times larger than the steady-state rate I / 2F of electrochemical supply of O 22 to the catalyst. As in previous electrochemical promotion studies the observed behavior is due to the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential. The back-spillover oxide ions are less reactive with CH 4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and methane. The appearance of these back spillover species which cause NEMCA is manifest in this study using in situ AC impedance spectroscopy by the appearance of a high capacitance (.200 mF cm 22 ) which reflects the creation of an effective double layer over the entire gas exposed electrode surface. The conclusions drawn from the AC impedance analysis are well supported by those obtained recently by Nicole using cyclic voltammetry [31].
Acknowledgements The authors gratefully acknowledge the financial support of the PENED Programme.
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