6115
J. Phys. Chem. 1993,97, 6115-6119
Electrochemical Modification of CH30H Oxidation Selectivity and Activity on a Pt Single-Pellet Catalytic Reactor C. A. Cavalca, G. Larsen, C. C. Vayenas,? and G. L. Haller' Department of Chemical Engineering, Mason Laboratory. Yale University, New Haven, Connecticut 06520, and Institute of Chemical Engineering & High Temperature Chemical Processes, Department of Chemical Engineering, University of Patras, GR-26110 Patras, Greece Received: December 18. 1992; In Final Form: March 19, I993
It was found that the catalytic activity and selectivity of Pt for the oxidation of methanol to formaldehyde and COz can be altered significantly and reversibly by depositing a Pt catalyst film on an yttria-stabilized zirconia (YSZ)disc and by applying current or potential between the catalyst film and a Ag film deposited on the other side of the 02--conducting YSZ disc. Both the catalyst film and the Ag counter and reference electrodes are exposed to the reacting CHsOH-02 mixture. The observed increase in the rate of HzCO production was typically a factor of 100 higher than the rate of 02-supply to the catalyst with a concomitant 2-fold increase in selectivity. This demonstration of the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) to reversibly modify catalyst activity and selectivity in a single-pellet flow reactor is a new result. It has considerable practical importance as it shows that the NEMCA effect can be utilized in conventional flow-type catalytic reactors. The present study has also shown the spontaneous generation of significant reactiondriven potential differences between the catalyst and the counter electrode. This observation is significant both for catalytic and also for sensor applications.
Introduction The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or, electrochemical promotion in catalysis, has been described for approximately 20 reactions in a number of recent papers,'-" and work prior to 1991 has been reviewed in a recent monograph.l2 In brief, it has been found that the catalytic activity and selectivity of metal surfaces can be modified in a reversible and very pronounced manner by interfacing the metal with a solid electrolyte, such as YzOp-doped ZrO2, an 02-conductor, and by supplying or removing ions to or from the catalyst surface by applying a current or voltage to cells of the type gaseous reactants, metal catalyst)ZrO, (8 mol % ' I Y203)IM,0, (1)
where the metal "counter electrode" M catalyzes the electrocatalytic reaction
0,+ 4e-
-
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(2) and serves as a means of supplying 02-to the catalyst through the gas-impervious solid electrolyte under the influence of an external current or voltage (Figure la). Previous studies have shown that the change induced in the rate of catalytic reactions can be up to 3 X lo5 times higher than the rate of supply or removal of 0 2 - 1 ~ ~ 9 1and 2 that the effect can also be induced using other types of solid electrolytes such as 8"-A1203, a Na+ conductor.ls9 There is compellingevidence that NEMCA, or electrochemical promotion in catalysis,13 is due to the change induced in the strength of chemisorptive bonds upon polarization of the metalsolid electrolyte interface,"JJ2i.e.,upon changing the Ohmic drop free catalyst potential V m (working electrode: W) with respect to a reference (R) electrode (Figure lb). When V m is varied
* To whom correspondence should be addressed.
7 University
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0022-3654/93/2097-6115$04.00/0
by AVWR,the average catalyst surface work function e@changes by
Ae@ = eAVw, (3) as predicted theoretically435and verified by in situ measured Ae@ via a Kelvin probe.lJ1 This change in e@ is caused by spillover of ions from (or to) the solid electrolyte to (or from) the catalysts surface.1-13 There is in situ XPS evidence for ion spillover onto catalyst surfaces ( 0 2 - or 0- in the case of Ag/YSZ) under polarization c0nditions.1~ The spillover ions (02-or 0- in the case of YSZ, Na*+in the case of @"-A1203)are accompanied by their compensating charge in the metal, thus forming neutral spillover dipoles.12 These spillover dipoles establish an effectiveelectrochemicaldouble layer on the catalysts surface (we thank one of our reviewers for suggestingthe term effectiveelectrochemicaldouble layer), which strongly affects the local electric field and changes the work function e@ by eAVwR, according to eq 3. The effective electrochemical double layer thus established on the catalyst surface interacts electrostatically with covalently chemisorbed reactants and intermediates and alters their binding strength. Increasing e@,Le., supplying 02-onto the surface, weakens the chemisorptive bond of electron acceptor adsorbates (e.g., chemisorbedoxygen O(a))and strengthensthe chemisorptivebond of electron donor adsorbates. Exactly the opposite behavior is obtained when decreasing e@,e.g., by supplying Na*+ onto the catalyst surface.1~~Direct interaction of spillover ions with covalently bonded adsorbed species is also po~ib1e.l~ These trends, evidenced so far from kinetic studies of NEMCA10-12and the observed effect of e@on activation energies,', have been recently verified experimentally for the NEMCA effect of atomic 0 adsorption on Ag.lO A general rule emerging so far from the study of some 20 catalytic reactions under NEMCA conditions12is that over wide ranges of e+ (typically 0.3-1.0 eV) catalytic rates depend exponentially on e@ according to ln(r/ro) = a(e@- e @ * ) / k b T
(4)
where ro, a! (typically -1 < a! < l), and e@* are catalyst and reaction specific constants. 0 1993 American Chemical Society
Letters
6116 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
The NEMCA effect was originally demonstrated and studied in experimental setups where only the catalyst film was exposed to the reactants while the counter and reference electrodes were exposed to a reference gas, usually air.l-I This "fuel-cell type" configuration is useful for fundamental studies but not very well suited for practical applications where a conventional flow type reactor would certainly be preferable. Thus, if a single-pellet configuration, such as the one used in the present study (where the catalyst as well as the counter and reference electrodes are all exposed to the reacting gas mixture), can still be used to obtain NEMCA, then this would be very useful from a practical viewpoint. There have been three very recent studies involving singlepellet configurations to explore NEMCA1"18 during CH4 oxidative coupling on Ag,16 H2S decomposition on Pt," and CO oxidation on Pt.18 In the latter study enhancement factor A values up to 300 were measured.18 The parameter is defined by'-12
A = Arl(I12F') (5) where Ar is the induced change in catalytic rate r, I is the applied current defined positive when anions are supplied to the catalyst, and Fis Faraday's constant. A reaction exhibits NEMCA when lAl> 1. When A > 1, Le., when the reaction rate increases with catalyst work function e@,the reaction is termed electrophilic.12 The present study is the first one demonstrating significant selectivity modification via NEMCA in a single-pellet configuration. The oxidation of CH30H on Pt was chosen as a model reaction. The NEMCA behavior of this reaction has been investigated in a fuel-cell type configuration at temperatures from 350 to 500 OC.8 In addition to demonstrating NEMCA and selectivitymodification in a single-pelletconfiguration,the present study has also led to the observation of significantreaction-induced potential differences between the catalyst and the counter and reference electrodes. The appearance of this potential difference demonstrates the experimental feasibility of using NEMCA, without external potential application, as recently proposed.12 This finding is important both for potential practical applications of NEMCA, or electrochemical promotion, and also in the sensor technology. Experimental Section The single-pellet catalytic reactor is shown schematically in Figure la. The Pt catalyst film was deposited on one side of the ZrO2 (8 mol % Y2O3) disc, which was 2 mm thick and had a diameter of 17mm, as described pre~iously,2~~~8~9J2 i.e., by applying a thin coating of Engelhard A1 121 Pt paste, followed by calcining in air at 820 OC. The catalyst film superficial surface area was -1 cm2. The true surface area was of the order of 66 cm2 as estimated from the magnitude of the NEMCA-induced galvanostatic rate transients T and from the relationship T zs 2FN/I (6) where N is the equivalent of catalyst surface area expressed in g-atom of metal covered by transported 0". Equation 6 has been found to provide a semiquantitative fit to the T vs N data in previous NEMCA studies where Nwas measured with surface titration.'-12 Two porous Ag films were deposited on the opposite side of the Pt film on the YSZ disc using GC electronics Ag paint 22201 followed by drying and calcining at 500 OC, as described previously.5J0J2 One electrode with a surface area of 0.8 cm2 served as the counter electrode while the other with a surface area of 0.1 cm2 served as a reference electrode. Thin Ag wires were attached to the three electrodes and connected to the galvanostat-potentiostat (EG&G Versastat 273-8 1) through the top of the Pyrex continuous flow reactor. Since Ag is known not only to promote methanol dehydrogenation and partial oxidation
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to H2CO but also to demonstrate NEMCA for these reactions at temperatures above 450 OC,5 care had to be taken in the present study for its potential catalytic activity and NEMCA behavior, despite the low (250-320 "C) operating temperatures. It was verified that the Ag counter and reference electrodes did not promote, to any detectable extent, the partial or complete oxidation of methanol by using the Pt film as a reference electrode and applying various positive and negative currents between the two Ag films without observing any measurable change ( 1 values. As shown in the figure, methanol dehydrogenation to formaldehyde exhibits electrophobic behavior (A > 1). The measured A values are typically of order 100, in rough agreement with the expression
1A1= 2Fr0/Z0 (9) which has been shown in previous NEMCA studies to allow for a prediction of the order of magnitude of the absolute value of A in terms of the open-circuit catalytic rate ro and of the exchange current Io of the catalyst-solid electrolyte interface. The latter was measured using standard Tafel (current vs overpotential) p10tsI.~and was found to be 10 PA at 305 OC.
Letters
6118 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
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0, Le., with increasing VWRand catalyst work function e@,12can be explained within the framework of previous NEMCA studies.l-12 Positive currents, i.e., 02-supply to the catalysts, creates spillover ions which, together with their compensating charge in the metal, spread over the entire catalyst surface. This species is distinct from atomically adsorbed oxygen O(a)and is less reactive than O(p)as established in several previous NEMCA studies.4~6JOJ2 It acts as a promoter by establishing an effective electrochemicaldouble layer on the gas-exposed catalyst surface. It increases the work function e@of the surface by Ae@= eAVwR (typically up to 1 eV) as measured in situ via a Kelvin probe.lJ1 The effective electrochemical double layer thus established and concomitant change in work function e@also affects the strength of chemisorptive bonds of covalently bonded reactants and intermediates.4~12 Thus, for electron acceptor adsorbates, such as atomically adsorbed oxygen O(,), increasing coverage of 02and thus increasing e@ causes a decrease in the strength of the chemisorptive bond.'2 Thus, in order to explain the observed increase in the rate of H2CO production with positive currents, one must examine the effect of increasing e@and surfacecoverageof 02-on the adsorbed methoxy group which is known to form on Pt surfaces upon methanol adsorption under ultrahigh vacuummJ1and atmospheric pressureconditions.*J9 Increasinge@ strengthensthe local electric field which orients the adsorbed methoxy intermediatewith oxygen end down. Thus, methanol binding via the C atom, which is known to lead to complete oxidation, e.g., as in aqueous environments?' is not favored, and this can explain the increase in selectivity to formaldehyde with increasing e@. Furthermore,
Letters since the adsorbed methoxy group is expected to retain overall an electron acceptor character? increasing e@ reduces backdonation of binding electrons from the metal, thus weakening the metal-methoxy group bond in the same way that the metal+) bond is weakened. Consequently, the adsorbed methoxy group and adsorbed oxygen O(*) become more weakly bound and thus more reactive. This explains the observed enhanced reactivity with increasing e@. An additional factor enhancing selectivity to formaldehyde is that increasing e@ is expected to strengthen H binding to the surface.12 Consequently, abstraction of H from adsorbed methoxy groups to produce adsorbed H and formaldehyde is favored. The abstraction of H may also be favored by the enhanced reactivity of adsorbed O(a)to form adsorbed OH. In the same way one can explain the observed decrease in the rate of formaldehydeproduction and in selectivity to formaldehyde with Z < 0 (Figure 2), i.e., with decreasing VWRand e@. This trend is consistent with the observation that on Pt electrodes near the point of zero charge in aqueous environments, where VwR is estimated to be 1-1.5 V lower than under ultrahigh vacuum,21.22 there is no significant formaldehyde production as the electric field which orients the adsorbed methanol with oxygen end down is weakened; thus no adsorbed methoxy intermediate is formed, and rupture of the CH bond, instead of the OH bond, takes place to form adsorbed CH2OH and complete oxidation products.21 The present results show conclusively that CH30H oxidation on Pt exhibits the NEMCA effect even at low (240-320 "C) temperatures and that single-pelletcatalytic reactors can be used to study NEMCA and to influence significantly catalyst selectivity. The latter is quite important from a practical viewpoint, as it shows that the effect can be utilized in conventional flow: type catalytic reactors.
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6119 Acknowledgment. Partial support by the National Science Foundation and by the EEC Joule Program is gratefully acknowledged. Special thanks is expressed toDr. Symeon Bebelis for preparing the catalysts. References and Notes 625.
(1) Vayenas, C. G.; Bebelis, S.;Ladas, S.Nature (London) 1990,313,
(2) Yentekakis, I. V.; Vayenas, C. G. J. Catal. 1988,111, 170. (3) Vayenas, C. G.; Bebelis, S.;Neophytides, S.J. Phys. Chem. 1988, 92,5083. (4) Bebelis, S.;Vayenas, C. G. J . Cafal. 1989,118,125. (5) Neophytides, S.; Vayenas, C. G. J. Coral. 1989,118,147. (6) Vayenas, C. G.; Bebelis, S.;Neophytides, S.;Yentekakis, I. V. Appl. Phys. 1989,A49, 95. (7) Vayenas, C. G.; Bebeli, S.; Neophytides, S.;Yentekakis, I. V.; Tsiakara, P.; Karasali, H.Platinum Met. Rev. 1990,34 (3), 122. (8) Vayenas, C. G.; Neophytides, S. J. Card. 1991,127,645. (9) Vayenas, C. G.; Bebeli, S.; Despotopoulou, M. J. Catal. 1991,128, 415. (10) Bebelis, S.;Vayenas, C. G. J . Catal. 1992,137,278. (11) Ladas, S.;Bebelis, S.;Vayenas, C. G. Surf. Sci. 1991,251/252, 1062. (12) Vayenas, C. G.; Bebelis, S.;Yentekakis, I. V.; Lintz, H.-G. Catal. Today 1992,1 I (3), 303-442. (13) Pritchard, J. Nature (London) 1990,313,592. (14) Arakawa, T.;Saito,A.;Shiokawa, J. AppLSurJ Sei. 1983,14365. (15) Tsiakaras. P.: Vavenas. C. G. J. Catal. 1993. 140. 53. (16) Chiang, P.-H-.;Eig, D.; Stoukides, M. J. Electrochem. SOC.1991, 138. L11. (17) Alqahtany, H.; Chiang, P.-H.; Eng, D.; Stodrides, M. Carol. Lcrt. 1992- 289. (18) Yentekakis, I. V.; Bebelis, S. J. Carol. 1992,137,278. (19) McCabe, R. W.; McCready, D. F. J . Phys. Chem. 1986,90,1428. (20) Abbas, N. M.; Madix, R. J. Appl. Su@ Sci. 1981, 7, 241. (21) Franaszczuk, K.; Herrero, E.; Zelenay, P.; Wickowski, A,; Wan, J.; Masel, P. I. J. Phys. Chem. 1992,96,8509. (22) Chang, S.-C.;hung, L.-W.; Weaver,M. J. J . Phys. Chem. 1989,93, 5341. 133
