8845
J. Phys. Chem. 1993,97,8845-8848
Origin of Non-Faradaic Electrochemical Modification of Catalytic Activity S. Ladas, S. Kenno&+S. Bebelis, and C. G. Vayenas’ Institute of Chemical Engineering and High Temperature Chemical Processes and Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece Received: May 26, 1993; In Final Form: July 16, I993
X-ray photoelectron spectroscopy (XPS) was used to investigate the effect of electrochemical oxygen pumping on Pt catalyst films interfaced with an 02--conducting yttria-stabilized zirconia solid electrolyte. It was found that electrochemical oxygen pumping to the catalyst causes spillover of significant amounts of anionic oxygen from the solid electrolyte onto the platinum film surface. The spillover oxygen species has an XPS binding energy 528.8 eV compared to 530.4 eV for chemisorbed oxygen, which is also observed on the surface, and is less reactive than chemisorbed oxygen with the reducing ultrahigh vacuum background. The detection of the anionic oxygen species upon electrochemical pumping confirms the previously proposed explanation of the non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion in catalysis.
Controlled modification of the rate and selectivityof catalytic reactions on metal and metal oxide surfaces has been a longsought goal in heterogeneous catalysi~,l-~ where dopants and metal-support interactions are frequentlyused to improvecatalyst perf~rmance.l-~It was recently found that the cata1ytic”ll and chemisorptiveI2properties of metals can be affected in a dramatic and reversible manner by using solid electrolytes, such as 8 mol % Y2O3-doped Zr02 (YSZ), an 02-conductor, or jY-A1203,a Na+ conductor, as active catalyst supports to induce the effect of non-Faradaic electrochemicalmodification of catalytic activity (NEMCA)&12or electrochemical promotion in catalysis:13The metal catalyst, usually in the form of a 1-40-pm thick porous film, is deposited on a solid electrolyte, such as YSZ, and also acts as the working electrode (WE) in a solid electrolyte cell of the type: gaseous reactants, metal catalyst IYSZIM, 0, (e+ CO 0,) (e.g. Pt, Pd, Ag)
+
where M stands for the metal counter electrode (CE) which catalyzes the reaction
0,+ 4e- * 20”
(1) and supplies 0 2 - to the catalyst through the gas-impervious YSZ solid electrolyte under the influence of an externally applied voltage or current (Figure 1). The induced increase Ar in catalytic reaction rate r, e.g. in the case of C2H4 oxidation on Pt,7 is up to 3 X 10s times higher than the rate Z/2F (where I is the applied current and F is the Faraday constant) of the supply of 02-to or from the catalystCl2and up to 70 times higher than the opencircuit ( Z = 0) catalytic rate r,. The NEMCA effect has been studied for over 20 catalytic reactions on Pt, Pd, Au, Ni, and Ag surfaces”’2J4 using 02-,@J1J2 Na+,&”J and H+l4 conducting solid electrolytes. The resulting dramatic modification in catalytic rates is accompanied by significant variation in activation energies7JJ1 and product selectivity.6.’ l A first step in elucidating the origin of NEMCA was the theoretical pr~position~.~ and experimental verification (via a Kelvin probes.9) that the work function e@ of the gas-exposed metal catalyst surface is significantlyand reversibly altered under NEMCA conditions and in fact that Ae@ = eAVwR where VWR is the ohmic-dropfree catalyst (working electrode: WE) potential with respect to a reference electrode (RE) (Figure 1). Thus a To whom correspondence should be addressed.
t Permanent address: Department of Physics, University of Ioannina, GR45 1 10, Ioannina, Greece.
OO22-3654/93/2097-8845$04.QOlO
X-Ray Source
Photoelectron Energy Analyzer
Figure 1. Schematic of the experimental setup. WE, RE, and CE are the working (Pt),reference (Pt),and counter (Ag)electrodes,respectively; G-P is galvanostat-ptentiotat.
general observationextracted from previous NEMCA studiesG12 is that over wide (0.3-1 .O-eV) ranges of e@the rates rof catalytic reactions per unit catalyst surface area, or turnover frequencies1 (TOF), depend exponentially on e@:
ln(r/rJ = a(e@- e@*)/k,T
(2)
wherea (typically-1 < a < 1) ande@*arereaction- and catalystspecific constants and kb is the Boltzmann constant. It has been proposed”l2 that the NEMCA effect is due to an electrochemically induced and controlled spillover of ions from the solid electrolyte onto the catalyst surface. The spillover ions are accompanied by their compensating charge in the metal, thus forming spillover dipoles. The presence of spillover dipoles and the concomitant change in work function e@alter the strength of the chemisorptive bond of covalently bonded reactants and intermediates, thus changing activation energies and reaction rates.” Spillover of hydrogen and oxygen1JsJ6is known to play an important role in several catalytic systems such as remote control phenomena.16 In the case of NEMCA the distances to be covered by spillover ions are surprisingly long (1-40 pm), yet the spillover ion hypothesis was consistent with the measured work function change#-9 and also with the observation that the catalytic rate relaxation time constant T upon constant current application 0 1993 American Chemical Society
8846 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993
(galvanostatic transient) is of the order of 2FN/I, where N (in moles of metal) is the total metal catalyst surface area.-J1 We now report direct in situ XPS evidence that anion spillover from the solid electrolyte to the catalyst surface is indeed the origin of NEMCA. A 2-mm thick YSZ slab (10 mm X 13 mm) with a Pt catalyst film (working electrode), a Pt reference electrode, and a Ag counter electrode (Figure 1) was mounted on a resistively heated Mo holder in an ultrahighvacuum (UHV) chamber (base pressure 5 X IO-lOTorr)and thecatalyst film(9mm X 9") wasexamined at temperatures of 300-800 K by X-ray photoelectron spectroscopy (XPS) using a Leybold HS- 12analyzer operated at constant AE mode with 100-eV pass energy and a sampling area of 5 mm X 3 mm. Electron binding energies have been referenced to the metallicPt 4f7/2 peak of the grounded catalyst at 7 1.1 eV,17which always remains unchanged with no trace of nonmetallic components. Catalyst preparation and characterization details have been presented el~ewhere.~JlJ2Two samples were investigated, and both gave the same results. Results presented here were obtained with a catalyst film with a reactive oxygen uptake of the order of 3 X le7mol of 0 as measured via titration with C2H4 at atmospheric pressure and T = 705 K as described elsewhere." Parts A and B of Figure 2 show the 0 1sphotoelectron spectrum at T = 673 K under open-circuit conditions (I = 0, Figure 2A) and when a constant overpotential AVWR= 1.2 V is imposed by the potentiostat, corresponding to a steady-state current I = +40 pA (Figure 2B). Electrochemical oxide ion pumping to the catalyst for 15min causes a 58% increase in the total 0 1s spectrum area and a shift of the peak maximum to lower binding energy by 1.8 eV. The broad 0 1s spectrum at Z = 0 was deconvoluted (Figure 2A) in states a and j3, each with a full width at half-maximum (fwhm) of 2.1 eV.17 Both states a and j3 were also observed on the uncovered YSZ surface. The a state, centered at 532.4 eV, has been previously attributed to adsorbed oxygen and/or hydroxylic species on the YSZ.18 We found that its size can be reduced by prolonged heating at 800 K and Ar+ sputtering. The j3 state centered at 530.4 eV is the oxidic contribution from the YSZ which is visible through microcracks in the Pt film," as confirmed by the simultaneous presence of Zr 3d5/2 at 182.6 f 0.2 eV, typical of Zr02,18 From the intensity ratio of the Zr 3d5p and Pt 4f7/2 peaks and their relative sensitivities,onecan estimatelg that microcracks accountfor 11%of the superficial Pt film surface area in agreement with values estimated from SEM. The appearance of the 0 (YSZ) peaks a and j3 in the XPS spectrum is in general not desirable as they would obscure any 0 1s signal from surface oxygen on Pt which is expected to appear at 530.2 f 0.2 eV.17 However as shown below the YSZ-derived peaks shift around upon changing VWR,thus providing some unique electrochemical information and also uncovering the 0 1s spectrum of oxygen species chemisorbed on Pt. In order to locate the additionaloxygen state upon polarization, Le. NEMCA, conditions (AVWR = 1.2 eV), we have used the following procedure (Figure 2B,C): The 0 Is spectrum at Z = 0 is first shifted to the right by 0.9 eV until its leading edge (state a) coincides with that for the AVWR= 1.2 eV spectrum. Then state j3 is shifted further until its total displacement equals that (1.2 eV) of the Zr 3d5/2 peak upon polarization (Figure 2D) and thus the difference spectrum is obtained (Figure 2C). This procedure is based on the fact that species associatedwith the grounded Pt catalyst electrode have, to a first approximation, i.e., neglecting chemical shifts, a fixed binding energy upon polarization, whereas species associated with the YSZ below the Pt catalyst electrode WE shift by eAVwR due to the applied overpotential AVWR. Figure 2E indeed shows that the Zr 3ds/2 binding energy shift is exactly equal to - ~ A V W Rwhen the catalyst electrode is grounded. As expected -~AVWR also equals
Letters ,eV
I
718
720
722
724
726
728
I
I
I
I
I
1
a A
538
536
534
532 530 €6 .el/
528
526
524
Ek .eV
1066
1068
1070
1072
1074
1078
-
e A VWR,eV
Figure 2. Effect of electrochemical0'-pumping on the 0 1s (A-C) and Zr 3dy2 (D,E) XPS spectra at 673 K (A) AV- = 0, I = 0; (B) AV= 1.2V, I = 40 PA,(C) 0 1s difference spectrum; (D) Zr 3dsp spectrum shift; (E) effect of overpotential AV- on the binding energy shifts of Zr 3d5/2 (circlts, working electrode grounded) and of Pt 4f7/2 (triangle, reference electrode grounded). See text for discussion.
the shift h E b of the Pt 4f,/2 state in the catalyst electrode when the reference electrode is grounded, in which case h E b = 0 for Zr 3d512. Although intuitively obvious from an electrochemical viewpoint, these observations (Figure 2E) (which are consistent withanegligible (30) hours. In the process of doing this the y- and 6-state spectrum gets entirely disentangled from the YSZ-derived a-and 8-state spectrum by taking advantage of the significant (up to -12-V) charging of the reference electrode and of the frozen (nonconducting due to the low temperature) YSZ solid electrolyte. Thus after the spectrum shown in Figure 2B was taken at 673 K, the sample was cooled to 400 Kin 50 min, with the potentiostat turned off at T = 500 K (where the YSZ became essentially nonconducting),and the new spectrum shown on Figure 3A was obtained: Due to charging of the YSZ electrolyte and of the reference electrode, VWR has already shifted to -7 V with a concomitant negative shift of the a and 8 states. At this point the y and 6 states have been clearly separated from the a and 8 states. They- and &state spectrum has practically not changed from the one shown on Figure 2C. This shows both the validity of the above deconvolution procedure and also that the amount of chemisorbed and spillover oxygen did not change appreciably during the cooling of the sample. By further cooling the sample to 380 K in 1 h and to 320 K in 4 h, the spectra shown in Figure 3B, C, respectively, were obtained. Peaks a and 8 shift more to the left, state 6 stays constant while state y gradually decreaseswith the simultaneous appearance of a new state e at 531.7 eV (OH or H2O adsorbed on Pt17). It is remarkable that state 6 (spillover oxygen) is less reactive than state y (chemisorbed oxygen) with the reducing background gas ( P 1.5 X 10-9Torr). The system remained at 300 K in the state shown on Figure 3C for 12 h without any measurablechange. The temperature then was increased by steps over a period of 2 h as shown in Figure 3D (353 K), Figure 3E (420 K), and Figure 3F (573 K). Peak e decreases drastically around 400 K, whereas peak 6 has been completely removed at 573 K. Note that in Figure 3D-3F there is a gradual positive shift of the a and 8 states, which again masks the e- and y-state region, due to the activation of ion conduction in the YSZ and reduced charging. At 573 K the initial spectrum, of Figure 2A i.e., only a and 8 peaks, is obtained, which underlines the remarkable reversibility of the system. Figure 4 shows the time evolution of the XPS signal at Eb = 528.8 eV (location of &oxygen) and Eb = 181.7 eV (final shifted position of the Zr 3 d ~ peak) p upon imposing I = 15 PA at T = 690 K, which causes a steady-state overpotential AVWR= 0.9 V. The figure also shows the time evolution of VWR. Initially both the 0 1s and the Zr 3ds/2 signals rise sharply with the same time constant, following the sharp increase in VWR. In less than 1.5 min the Zr 3d~/zsignal and AVWRhave approached within 10% their steady-state values, while the observed slow subsequent increase in the 0 1s signal is due to the growth of the 6 state of oxygen on the Pt surface, Le. due to the increase in coverage of the spillover oxide species. This slow growth of the 6 state of oxygen parallels the slow increase in the catalytic rate, or turnover frequency, observed upon positive current application in atmospheric pressure NEMCA studies (e.g. during C2H4 oxidation on Pt, Figure 6 in ref 7). For the transient shown in Figure 4 the amoung of &state oxygen, as measured by the peak height, reached steady-state after approximately 150 min with a time constant of the order of 50 min. This time constant is in good qualitative agreement with r = 2FN/I = 64 min (with N = 3 X lP7mol, I = 15 PA) which is a measure of the time required to form a monolayer of spillover oxide ions on the catalyst surfaceand has been shown in all previous atmospheric pressure NEMCA studies612 to provide a good estimate of the observed catalytic rate relaxation time constants during galvanostatic transients. This good agreement corroborates the proposition that the 6 state of oxygen is the promoter
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8848 The Journal of Physical Chemistry, Vol. 97, No. 35, I993
Letters moment measurements,9J0one can conclude that in general solid electrolytescanact as dopant donor phases tosignificantlyenhance metal catalyst performance. In the case of YSZ,the resulting catalyst material, i.e. Pt "decorated" with 0s(or 0.)is unique in that gas-phase supplied 02cannot produce thesamesurfacespecie. This then can explain its remarkable catalytic properties.uJ1 A side conclusion of the present investigation is that XPS can beused for in situ monitoring overpotentials in solid electrolyte electrochemistry.
5-
AcLnowkdgment. We thank the EEC SCIENCEand JOULE programmes for financial support and Prof. R. M. Lambert for helpful discussions.
0-
R e f e w and Notes (1) Boudart, M.;DjCgo-Mariadamu, G.
-0.2l
l
l
l
l
l
l
l
l
a
l
l
Kinetics of Heterogeneous Cutulytic Reuctions; Princeton University P r e s Princeton, NJ, 1984. (2) Campbell, I. M.Cutulysfs ut Surfuces; Chapman and Hall: New York, 1988. (3) Hcgedus, L. L.;Bell, A. T.;Chen, N. Y.; Haag, W. 0.;Wei, J.; Ark, R.;Boudart, M.; Gates, B. C.; Somorjai, G. A. Cutufystdesign: Progress und perspectives; Wiley: New York, 1987. (4) Haller, G. T.; Ressllco, D. E. Adv. Cutul. 1989, 36, 173. (5) Grant, R.B.; Harbach, C. A. J.; Lambert, R.M.;Tan, S . A. J . Chem. Soc., Furuduy Truns. 1 1981,83,2035. (6) Vayenas, C. G.; Bebelis, S.;Ncophytidts, S . J. Phys. Chem. 1988, 92. - -,-5013. -- - . (7) Bcbelis, S.;Vayenas, C. G. J. Coral. 1989, 118, 125. (8) Vayenas, C. G.; Bcbelis, S.;Ladas,S. Nuture (London) 1990,343, 625. (9) Ladas, S.; Bebelis, S.;Vayenas, C. G. Sur/. Sei. 1991, 251/252, 1062. (IO) Vaycnas, C. G.; Bebelis, S.;Dcapotopoulou, M.J. Cutul. 1991,128,
415.
(11) Vayenas, C. G.; Bebelis, S.;Yentckakh, I. V.; Lintz, H.-G. NonFaradaicElectrochemicalModificationof CatalyticActivity: A Statureport. Cutulysis Today; Elsevier: Amsterdam, 1992; Vol. 11, pp 303442. (12) Bebelis, S.; Vayenas, C. G. J . Curd 1992, 138, 570. (13) Pritchard, J. Nurum (London) 1990, 343, 592. (14) Politova, T. I.; Sobyanin, V. A.; Belyaev, V. D. React. Kfnet. Curd
Lett. 1990, 41, 321.
(15) Conner, W. C., Jr.; Pajonk, G. M.;Teichner, S.J. Ado. Cutul. 1987, 34, 1. (16) Delmon, B. J. Mol. Cutal. 1990,59, 179. (17) Peuckert, M.; Bonzel, H. P. Surf. Sci. 1984,145,239, and references
cited therein. (18) Ingo, G. M.;Del Machio, R.;Scoppio, L. Surf. Interfuce Anal. 1992, 18, 661.
(19) &ah, M.P. In PrucriculSurfuceAnalysis, 2nd ed.;Brigga, D., &ah, M.Po,Eds.; Wiley: New York, 1990, Vol. 1. (20) Lang, J. F.; Masel, R. 1. Surf. Sci. 1986, 167, 261. (21) Marbrow, R. A.; Lambert, R. M. Surf. Scf. 1976, 61, 329. (22) Arakawa, T.;Saito, A.; Shiokawa, J. Chem. Phys. Lett. 1983, 94, 250. (23) Vohrer, U. Ph.D. Thesis, University of TObingen, Germany, 1992. (24) Jiang,Y.; Kaloyannis, A.; Vayenas, C. G. Electrochim.Acru,in p w . (25) Cavalca, C. A.; Latsen, G.; Vayenas, C. G.; Haller, G. L. J . Phys. Chem. 1993,97,6119. (26) Block, J. H.; Kreuzer, H. J.; Wang, L. C. Surf. Sci. 1991,246,125.
