5560
J. Phys. Chem. 1991,95, 5560-5567
F m OlMCthv I.ZSM-5 at 298 K AE Determi~~I by M D C . l d r t k r ud PIG NMR Experilcab lad C o m q d " with E.thrte o f t k R"Walk Model
TABLE U
method MDI2
PFG NMRl6 (powder samples) PFG NMR" (oriented crystals) random walk model
anisotropy factor, 1/2(Dx+ DJDZ 7.7 55
determination of the diffusivity in the direction of the longest of ~ J ~in) the the crystals axes (corresponding to the z d i r e c t i ~ n ~and plane perpendicular to it became possible. Table I1 provides a + Dy)/Dzob comparison between the anisotropy factor 1/2 (0% tained in these PFG NMR experiments with the results of MD calculations (Table I). Combining eqs 2 , 9 , and 11 for the random walk model yields an anisotropy factor
4.c
>4.4
this conclusion model calculations for nonrigid frameworks show that the diffusivities are essentially independent of the framework rigidity.I2 A direct measurement of the principal elements of the diffusion tensor is complicated by the small size of the zeolite crystallites. First attempts to measure orientation-dependent diffusivities in zeolite ZSM-5 have been made by means of pulsed field gradient (PFG) NMR s p e ~ t r o s c o p y . ~In~ *measurements ~~ with methane adsorbed on ZSM-5 in powder samples, no deviation from the pattern of isotropic diffusion could be observed. In this way, a factor of the order of 5 was estimated as an upper limit for the ratio between the diffusivities in different directions.I6 Recently, for the same system PFG NMR measurements have been carried out with oriented ZSM-5 crystals," so that a separate (14) Urger, J.; Pfeifer, H.;Heink, W. Adu. Mag. Reson. 1988, 12, 1. (1 5) KHrger, J.; Pfeifer, H. Zeolites 1987, 7,90. (16) Zibrowius, B.; Caro, J.; KHrger, J. 2.Phys. Chem. (Leipzig) 1988, 269, 1101.
With a = b (cf. Figure l), the anisotropy factor becomes a minimum for pI = p2 = 0.5, attaining a value of 4.4. This result is in complete agreement with the MD calculations, which have led to values above this lower limit. In the PFG NMR measurements with oriented crystallites the anisotropy factors are found to be of the order of this lower limit. A tendency toward anisotropy factors slightly below the values of the random walk model might be related to lattice imperfections in the real zeolite crystallites and/or to a less perfect adjustment of the zeolite crystallites than was assumed in the PFG NMR experiments.
Acknowledgment. Stimulating discussions with Prof. Harry Pfeifer are gratefully acknowledged. (17) Hong, U.; Kirger, J.; Kramer, R.; Pfeifer, H.; Milller, U.;Unger, K. K.;Lllck, H.-B.; Ito, T. Zeolites, in press. (18) Price, G. D.; Pluth, J.; Bennett, J. M.;Patton, R. L. J . Am. Chem.
Soc. 1982, 104, 597 1.
(19) Hayhunt, D. T.; Aiello, R.; Nagy, J. B.; Crea, F.; Giordano, G.; Nastro, A.; Lee,J. C. ACS Symp. Ser. 1988. No. 368, 277.
Dependence of the Electrooxldath Rates of Carbon Monoxide at Gold on the Surface Crystabgraphk Orlentation: A ComMned Kinetlc-Surface Infrared Spectroscopk SWY Si-Chung Chug, Antoinette Hamelin,+ and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: October 16, 1990) Rate parameters evaluated by linear sweep voltammetry are reported for the electrooxidation of carbon monoxide on six oriented monocrystalline gold surfaces in acidic aqueous perchlorate electrolytes. At a given electrode potential, the rate amstants depend markedly upon the crystallographic orientation in the sequence Au (1 11) < Au(533) Au(100) S Au(221) < Au(210) < Au( 110). up to 100-fold differencesin rate being observed. Marked inhibition of the notably facile electrode kinetics are observed if 0.1 M H m 4is substituted for 0.1 M HCIO, electrolyte. Parallel voltammetric measurements coupled with real-time surface infrared spectroscopy at four surfaces, Au( 1 1l), Au( loo), Au( 1IO), and Au(210), enable the role of CO reactant adsorption in the surface-dependent catalysis to be assessed. A low-coverage, yet reactive, form of adsorbed CO was detected on Au(ll0) and (210) from a C-O vibrational ( v a ) band at 2100-21 15 cm-' which disappears at the onset of the voltammetric wave. The sequence of CO surface concentr@ons as discerned from infrared spcstmcopy, Au( 111). Au(100) < Au(ll0) < Au(210), differs from the above reactivity sequence, signaling the presence of additional factors in the electrocatalysis. Relationships are explored between the surface-dependentrates and the potentials of zero charge or the density of "broken bonds" in the surface lattice (Le., the average surface coordination number). These correlations suggest that the rate-determining electrooxidation step between coadsorbed CO and H20(or OH) species is favored either at step sites or on rows of low-coordination metal atoms, such as on Au( 110). This is speculated to be due to the ability of such sites to engender CO adsorption and nearby H20 (or OH) coadsorption.
-
An emerging topic in surface electrochemistry involves examining the sensitivity of electrochemical reaction kinetics to the surface crystallographic orientation.' The practicality of such experiments has increased substantially in recent years with the advent of reliable procedures for preparing ordered singlecrystal e l e c t r ~ d e s . ~A * ~central motivation behind these studies is to understand the manner and extent to which electrocatalytic 'Permanent address: Labontoire d'Electroohimie Interfaciale du C.N.R.S., I , Place A. Briand, 92195 Meudon, France. OO22-3654/91/2095-5560$02.50/0
processes are influenced by the surface bonding and stereochemical factors. For this purpose, it is highly desirable to supplement macroscopic kinetic data with microscopic information on the adsorbed intermediates as can be furnished by surface spectra(1) For a recent miew, we: Adzic, R. In Modern Aspects of Electmchrmfstry; White, R.E., Bock&, J. O'M.,Conway, B. E.,EQ.;Plenum P r w
New York, 1990, Vol. 21, Chapter 5. (2) Hamelin, A. In Modem Aspects of Electrwhemistry; Conway, B. E., White, R. E., Bockris, J. O'M.,Eds.;Plenum Pruur: New York, 1986; Vol. 16, Chapter 1. (3) Clavilier, J. ACS Symp. Ser. 1988, 378, 202.
Q 1991 American Chemical Society
Electrooxidation of CO on Au Surfaces copies. While such information is often difficult to obtain, an enticing approach for this purpose involves real-time infrared spectroscopy. An especially suitable system to examine is carbon monoxide elcctrcaxidation. Carbon monoxide is of interest from more than one standpoint, including its oft-touted role as an adsorbed poison for the ektmoxidation of organic molecules on transition metals.' The adsorption and electrooxidation of CO on monocrystalline platinum surfaces studied by means of in situ infrared spectroscopy has received extensive recent attention in our laboratory? Perhaps surprisingly, polycrystalline gold is a superior catalyst for CO electrooxidation in acidic and alkaline aqueous media.610 The electrosorption of CO on polycrystalline gold, although weak, can be detected by infrared spectroscopy."JZ In a recent preliminary communication, we reported linear sweep voltammetric and concurrent surface infrared spectroscopic data for the electrooxidation of solution CO on Au(210) in acidic, neutral, and alkaline aqueous media." Two types of adsorbed CO were detected in this manner: a reactive form having a C-0 ) 2100-2115 cm-', and an instretching frequency ( ~ ~at0 ca. hibiting form with vm located at ca. 1900-2000 an-'. Increasing the surface exposure time to solution CO at negative potentials yielded the latter form at the expense of the former adsorbate. More recently, we have examined the kinetics of CO electrooxidation on several other monocrystalline gold surfaces, specifically the low-index faces Au( 11l), Au( 100) and Au( 1lo), and on two additional high-index surfaces: Au(221) and Au(533). [The last two are formally stepped surfaces, denoted as1) 4( 111)-( 111) and 4( 111)-( lOO)]. Interestingly, the electrooxidation kinetics are very sensitive to the crystallographic orientation, the rates varying by up to ca. 100-fold in a given electrolyte. Contained herein is a description of these electrochemicalkinetics, along with corresponding surface infrared spectral data for Au(1 l l ) , Au(ll0) and Au(210). The latter provide direct information on the nature and crystal-face-dependent concentration of the adsorbed CO intermediate. This combination of electrochemical kinetic and spectral data on ordered monocrystalline surfaces allows insight to be obtained into the microscopic factors controlling the surface specificity for this archetypical electrocatalytic reaction.
Experimental Section The preparation of the gold singlc-crystal faces was undertaken in LEI-CNRS as described in the Appendix of ref 15. The geometric area of the gold faces utilized for infrared spectroscopy is 30-35 mm2;the area is known within &5%. The greater surface area compared to those, 15-18 mm2, for most of the faces was found to be necessary in order to achieve satisfactory infrared light throughput, and hence good spectral signal-tmoise ratios. Crystal growth of the larger faces, however, was significantly more difficult. Six different crystallographic orientations were examined. These included two (1 1l), (loo), (210), and (1 10) samples; (533) (4) For example, we: Parsons, R.; Vandcmoot, T. J . E/ectroano/. Ch" 1988,257,9.
( 5 ) For example, see: (a) Leung, L.-W. H.; Wieckwski, A.; Weaver, M. J. J. Phys. Chem. 1988.92,6985. (b) Chang, S.-C.; Weaver, M. J. Surf.Sci., in m.(c) Chang, S.-C.; Weaver, M.J. J. Chem. Phys. 1990,92,4582. (d) Chang, S.-C.; Roth, J. D.; Weaver, M.J. Surf. Scl. 1991, 211, 113. (6) Kita, H.; Nakajima, H.; Hayashi, K. J. Electroonul.Chem. 1985,190,
141. (7) Farrugia, T. R.;Frcdlein, R.A. Ausr. J . Chem. 1984,37, 2415. ( 8 ) Roberts, J. L.; Sawyer, D. T. J. EiecrroaM/. Chem. 1965, 10, 989. (9) Gibbs, T. K.; McCallum, C.; Pletcher, D. Electrochim. Acru 1!377,22, 525. (IO) Gormer, K.; Mizera, E. J . ElectroanaI. Chem. 1979, 98, 37. (1 1) (a) Kunimrtru, K.; Aramata, A,; Nakajima, H.; Kita, H. J . Elmtroonul. Chem. 1966,207,293. (b) Nakajima, H.; Kita, H.; Kunimatsu, K.; Aramata, A. J. Electroanol. Chem. 1986,201, 175. (12) Corripn, D. S.; Gao, P.; bung, L.-W. H.; Weaver, M.J. Lungmulr 1984,2,744. (13) Chang, S.-C.;Hamelin, A.; Weaver, M. J. Surf.Scl. 1990,239, L543. (14) Lang, B.; Joyner, R. W.; Sqmorjai, G. A. Surf. Scl. 1972,30,440. (15) Hamelin, A.; Morin, S.; Richer, J.; Lipkowski. J. J. Electroonul. Chem. 1990. 285,249.
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5561
E / V vs SCE Figure 1. Voltammograms at 50 mV s-' for CO electrooxidation on Au(100) in 0.1 M HClO, (a', solid trace), 0.09 M LiClO, + 0.01 M HCIO, (b', dotted trace), and 0.1 M LiClO, + 2 mM HCIO4 (I?,dashed trace). Curves a, b, and c are voltammograms obtained in these electrolytes but in the absence of CO. Electrode area is 19.6 mm2.
and (221) faces were also prepared. The definition of each face was checked in 10 mM perchloric acid solutions by means of cyclic voltammetry; the profiles of current for the formation of a monolayer oxide as well as the potential of the capacitive current minimum in the double-layer region (potential of zero charge) were found to be in good agreement with literature voltammogramsS2Each electrode surface was cleaned immediately prior to use by heating in an oxy-gas flame, cooled in ultrapure water and transferred to the electrochemical cell, avoiding any contact with the laboratory atmosphere. The counter electrode was made of gold, and the reference electrode was an external saturated calomel electrode (SCE). Supporting electrolytes were prepared from HCIO,, HNO,, H$04, LiCIO, (GFS Chemicals), or KOH (Alfa ultrapure). Carbon monoxide (99.8%) was from Matheson. Water was purifiad by means of a Milli-Q system (Millipore, Inc.). The solutions were thoroughly deaerated with argon. The electrochemicalsurface infrared measurements were largely as described in refs 5c and 16. The spectrometer was an IBM (Bruker) IR-98-4A Fourier transform instrument. The electrochemical thin-layer cell utilized a CaFz window bevelled at 60° to the surface normal. The kinetic parameters for solution CO electrooxidation were obtained by using linear sweep voltammetry as described in the Results section. This utilized a PAR 273 potentiostat (EGBrG) with a HP 7045B X-Y recorder (Hewlett Packard). Differential capacitance measurements, with which values of the potential of zero charge were determined, employed a PAR 173/175 potentiostat, a HP 3314A function generator, and a PAR 5204 lock-in amplifier. All electrode potentials are reported versus the SCE, and all measurements were made at 23 1 oc.
Results ElectrocbemicPIKinetics. The primary supporting electrolytes chosen for this study are 0.1 M HClO,, 0.09 M LiClO, 10 mM HCIO,, and 0.1 M LiCIO, 2 mM HCIO,. Acidic perchlorate electrolytes were selected since this anion displays only weak specific adsorption; the pH range examined is sufficient to judge
+
+
(16) (a) Corrigan, D. S.; bung, L.-W. H.; Weaver, M. J. A w l . Chem. 1987, 59, 2252. (b) Comgan, D. S.; Weaver, M.J. J. E/ectmaM/. Chem. 1988, 241, 143.
5562 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
Chang et al.
TABLE I: Ekctrocbemical Kinetic Parawten at 23 'C for the Electraoxidatioa of CO in Acidic Perchlorate Ekctrolyte# at V a h Cold Surface CnstauOarphic 0rk.btioar
[H,O+] = 10 mM'
[H30+] = 100 mM*
surface
EmE
E d
Au( 1 1 1) Au( 100)
0.24 0.09 0.04 -0.02
0.45 0.34 0.365 0.35 0.305 0.25
Au(533) Au(221) Au(210) Au(ll0)
-0.09 -0.02
IO'&,,,,/ 0.08 0.6 0.30 0.55 1.6 8
a,w#
Ep/2r
0.5 0.7 0.7 0.5 -0.7 0.75
0.41 0.265 0.265 0.235 0.255 0.18
10'k8pP'
0.20 4.2 3.5 6.5 7.0 62
[H30+]= 2 mMd a8pP1
Ep/2r
0.45 0.65 0.6 0.55 0.75 0.75
0.25 0.205 0.19 0.16 0.16 0.13
103k,,f 2.8 12 22 30 60 190
0.4 0.55 0.6 0.55 0.65 0.7
'Values refer largely to 0.1 M perchlorate electrolytes (V vs SCE) (see text for details). bElectrolyte is 0.1 M HCI04, with saturated CO. cElectrolyteis 0.09 M LiCIO, + 10 mM HCI04,with saturated CO. dElectrolyteis 0.1 M LiClO, + 2 mM HCI04,with saturated CO. cHalf-peak ytcntial (V vs SCE) (i.e., potential at which voltammetric current equals half the peak value); voltammetric sweep rate = 50 mV 6' (see text). Apparent (Le., observed) rate constant (cm s-I) for CO electrooxidation at 0.30 V vs SCE, extracted from voltammetric data by using eqs 1 and 2 (see text). #Apparent anodic transfer coefficient, obtained from voltammetric data by using eq 2 (see text). the sensitivity of the reaction rate to the hydronium ion concentration. The cyclic voltammetry of each gold face was first examined in the supporting electrolyte alone, cycling the potential at 50 mV s-I so to examine anodic oxide film formation and removal, and to the onset of proton reduction. Typical voltammograms obtained in this manner for Au( 100) in 0.1 M HClO,, 0.09 M LiCIO, 10 mM HClO,, and 0.1 M LiC104 + 2 mM HClO, are shown as curves a, b, and c, respectively, in Figure 1. The contact between the electrode and the solution involved touching the electrolyte with the surface and then pulling the electrode up slightly to yield a raised meniscus.2 Stable voltammograms could be obtained within 1-2 potential cycles. Following this procedure, CO was bubbled into the electrolyte for several minutes to yield near-saturated (about 1.O mM) solutions with the potential held at a suitably negative value, ca. -0.3 V. The solution was blanketed subsequently with argon. The resulting anodic voltammograms for CO electrooxidation on Au(100) in these three electrolytes,obtained at 50 mV s-' starting from -0.2 V, are shown as curves a', b', and c' in Figure 1. These data display a totally irreversible anodic wave, corresponding to the two-electron oxidation to yield COP Subsequent voltammograms were found to be entirely reproducible (e.g., peak potentials within 5 mV) if the solution was stirred briefly between potential cycles. Although voltammetry was typically performed at 50 mV s-', the sweep rate was varied between 20 and 500 mV s-I. The observed linear dependence of the peak current on the square root of the sweep rate confirmed the occurrence of diffusion control. Although the CO solution concentration could not be conveniently controlled in a precise manner, this is of little consequence since the kinetic analysis employed here does not require direct knowledge of the reactant concentration. As emphasized for Au(210) in ref 13, exposing the surface to the CO-saturated solution for longer times (10-15 min), especially at more negative initial potentials, yields voltammograms that are shifted significantly (by up to 200 mV) to higher overpotentials, resulting in part from the formation of an inhibiting form of adsorbed CO. This effect is less prevalent with all the other faces studied here, the wave being shifted positive under these conditions only by 50 mV or less. Most importantly for the present purpose, it could be avoided entirely by exposing the surface only for relatively short times at potentials negative of the voltammetric wave. The kinetic analysis of the anodic voltammograms followed essentially the procedure outlined in ref 17. Values of the apparent (Le., double-layer uncorrected) rate constants, klpp(cm s-'), were evaluated at the voltammetric peak potential at a given sweep rate, Y (V &), as well as at lower overpotentials corresponding to one-half and one-quarter of the peak current, Ep12 and Epj4,respectively, by using the relation
+
the constant K equals 0.339, -0.469, or -0.851 at potentials corresponding to E,, Ep12,and EpI4,respectively. The required values of the apparent transfer coefficient, (rap,, were obtained from (cf. ref 18): (rap,
= 47.7 mV/(E,
- Epj2)
(2a)
or
Here D is the CO diffusion coefficient, 2.0 X lo-' cm s-l,* and
= 22.6 mV/(Ep/2 - E,/,) (2b) (Although the overall electrooxidation process here is formally a two-electron transfer, we use the quantity alPpin the same manner as for one-electron reactions.) In most cases, reasonable consistency between a,* values obtained by using eqs 2a and 2b was obtained ((rr differed by SlWo), although the latter relation was usually pregred here since it corresponds to a region close to the 'foot" of the wave where diffusion polarization is less extensive. This analysis presumes that the electrochemical reaction order in solution CO is unity, since eq 1 in essence accounts for the diffusion polarization effects encountered when approaching the voltammetric peak. A detailed quantitative examination of this reaction order was not undertaken, due to difficulties in controUing precisely the solution CO concentration. However, values close to unity (within 10-20%~)were consistently extracted by examining the variations in the currents close to the foot of the voltammetric wave obtained by 3-4-fold alterations in the CO concentration. A summary of the resulting kW and aW values for the six gold faces examined here in the three perchlorate electrolytes is given in Table I. For ease of comparison, the klPpvalues are quoted at a common potential, 0.30 V vs SCE;this value was chosen so to minimize the extrapolation in the log k,pp- E data that was required. The measured ,k values were reproducible generally to within ca. 20% and the alPpvalues to within 0.05. Inspection of Table I shows that the rate constants at a given electrode potential and pH are strikingly sensitive to the crystallographic orientation, k varying by up to 100-fold under these conditions. Generally, tx"e order of k,, values is Au( 111) < Au(533) Au(100) 5 Au(221) < Au(210) < Au(ll0). A significant, although not great, variation in the al values is also observed, a*, tending to be larger on the more eTectrocatalytic surfaces. The klppvalues at a fiied potential for a given gold face also increase systematically as the hydrogen ion concentration, [H30+],is decreased (Table I). From these data, the reaction order in [H30+] is seen to lie between about -0.5 and -1.0 over the pH range ca. 1-3. Rate parameters were also obtained for a polycrystalline gold sample that was subjected to the same pretreatment as the single cm s-l) on polycryscrystals at -0.3 V; the k, value (8 X talline gold is intermediate between that for Au(210) and (1 10) (Table I). However, the alp value, O S , is significantly smaller than observed on these monocrystalline faces. For convenience, values of the potential of zero charge, denoted E, for each gold face are also included in Table I. These were
Hamelin, A.; Weaver, M.J. J . Eleclrwnul. Chrm. 1986, 209, 109.
(18) For example, see: Bard, A. J.; Faulkner, L. R. Elecrrochemicol Merhods; Wiley: New York, 1980; p 223.
log klpp K
(17)
+ log (al#DF/RT)'I2
(1)
aapp
-
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5563
Electrooxidation of CO on Au Surfaces
A
i,
6
Y
I
I
I
-*
).4
0.4
0
0.8
1.2
E / V vs SCE
Figure 2. Voltammograms at 50 mV s-’ for CO elcctrooxidation on Au(210) in (A) 0.1 M HC104and (B) 0.1 M H$O+ Electrode area is 35 mm*. Dashed traces in both (A) and (B) are voltammograms obtained in the absence of CO.
taken in part from ref 19; additional pzc values were obtained here for Au(221) and Au(533). As before,19 the values were obtained from appropriate differential capacitance-electrode potential (Cd-E) minima. For consistency and simplicity, the pzc values refer to 0.1 M perchlorate electrolytes containing dilute (1-10mM) acid. Insomecases [Au(lll) andAu(100)], theuse of more dilute electrolytes is necessary to obtain clearcut Cd-E minima and hence reliable pzc values; appropriate data extrapolation to 0.1 M was then utilized. The reliability of the pzc values in Table I is no better than f0.02 V for any face. In the case of Au( loo), there is significant hysteresis in the Cd-E plots o b tained during negative- and positive-going potential sweeps2 If the latter condition is chosen instead, significantly (ca. 0.06 V) more positive pzc values can be obtained. Kinetic data for CO electrooxidation were also obtained in 0.1 M H N 0 3 and 0.1 M H#O, in order to investigate the effect of altering the supporting electrolyte anion, and consequently the extent of specific anion adsorption. The kinetic behavior in 0.1 M H N 0 3 is for the most part not greatly different from that in 0.1 M HCIO,. On the most electrocatalytic faces, however, the former electrolyte yielded voltammograms shifted by ca. 2&100 mV to lower overpotentials, and a, values that are typically 0.1-0.15 smaller than those obtain3 in 0.1 M HCIO,. The voltammetric behavior for each gold face in 0.1 M H#O, was observed to be substantially different than in 0.1 M HClO,. Figure 2, A and B, shows anodic voltammograms (50 mV s-I) for the electrooxidation of near-saturated CO solutions on Au(210) in 0.1 M HCIO4 and 0.1 M H2S04, respectively. (As before, conditions were chosen to avoid the Occurrence of significant reaction retardation associated with buildup of the inhibiting form of adsorbed CO.) Although the onset of anodic current is seen to Occur at comparable potentials in 0.1 M HCIO, and 0.1 M H W , ,the vdtammetric wave is elongated in the latter electrolyte so that E, is shifted to a substantially (ca. 0.6 V) more positive potential than in 0.1 M HCIO,. Roughly comparable effects were observed in these electrolytes for the other gold f a a s studied here. The a, v a l w in 0.1 M H W 4 are ca. 2- to 2.5-fold smaller than in 0.1 HCIO,. Interestingly, the rate retardation effects with increasing surface exposure time in 0.1 M HCIO,, noted above, are essentially absent for all gold faces in 0.1 M H2S04.
k
(19) Hamelin, A.; Weaver,
M.J. J . Elccfroanal. Chrm. 1987, 223, 171.
Figure 3. Voltammograms at 50 mV s-I on Au( 111 ) in 0.1 M HClO,. Solid and dashed traces represent the voltammograms obtained in the conventional electrochemical cell and following transfer to the infrared spectrochemical cell, respectively. Elcctrode area is 33 mm2.
Kinetic measurements for CO electrooxidationwere also undertaken in alkaline electrolytes, primarily in 0.1 M KOH. Some difficulties, however, were experienced in obtaining reproducible voltammograms. This can be traced in part to a typically rapid buildup of the inhibiting CO adsorbate under these conditi011s.l~ In addition, the voltammetric waves tend to display irregular oscillations at potentials close to and beyond E,, often vitiating the extraction reliability of kinetic parameters. Nevertheless, reproducible current-potential segments close to the foot of the voltammetric wave were normally obtained. The potential shifts observed between the voltammogram in acidic and alkaline media close to (within are consistent with a reaction order in [H,O+] ca. 10% of) -1.0. Surface Mrrwd Spedmmpy. The observed marked sensitivity of the rate parameters for CO electrooxidation to the gold crystallographic orientation raises the question of the extent to which these rate variations arise from differences in the surface concentrationand binding site of the adsorbed CO intermediate. As noted above, valuable information of this type can be obtained by means of surface infrared spectroscopy performed under conditions relevant to the kinetic measurements. Real-time surface infrared spectra cannot be obtained with sufficient sensitivity on a time scale commensurate with the voltammetric measuremenu described above. Nevertheless, suitable spectral sequences can be acquired during positive-going potential excursions by using slower (ca. 1-5 mV s-’) sweep rates.sJ3J6 This method, which we have dubbed “single potential alteration infrared spectroscopy’’ (SPAIRS),l6 involves acquiring sets of interferograms during a given potential sweep. In order to remove solution spectral interferences, subtracted from each group is a set obtained either prior to, or following completion of, the faradaic process within the spectral thin layer. Unlike potential-difference methods that rely on repeated modulation, the SPAIRS approach enables irreversible potential-induced changes in the infrared spectra to readily be discerned, such as those involving CO electrooxidation. The electrochemical behavior of the larger area gold surfacea required for the infrared measurements was essentially identical with that of the smaller electrodes that were utilized primarily for the electrode kinetic experiments. In addition, the electrochemical responses of a given surface were virtually identical in the infrared and conventional electrochemical cells, even though the ‘raised meniscus’’ procedure could only be used in the latter environment. In the former case, the periphery of the circular oriented face was wrapped with Teflon tape to expose only the desired surface region. As a comparison, typical cyclic voltammograms obtained for Au( 1 1 1) in 0.1 M HClO, in the infrared
5564 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
P -0.15
U
-0.19
Chang et al.
I
-0.25
-
-0.37 .
I
2200
Figure 4. Potential-dependent surface infrared spectra obtained during 1 mV s-' sweep in CO-saturated 0.1 M HCIO, following dosing for 3 min on (A) Au(210) and (B)Au(ll0). Each spectrum involved the acquisition of 100 interferometer scans from which were subtracted a similar set obtained subsequently at 0.2 V after CO oxidation was complete. and conventional electrochemical cells are shown as solid and dashed traces, respectively, in Figure 3. Reprmntative SPAIR spectra obtained in the above manner during CO electrooxidation in CO-saturated 0.1 M HCIO, on Au(210) and Au( 1 10) are shown in Figure 4, A and B, respectively. The pmitivegoing sweep rate was 1 mV e'. Each spectrum involved the acquisition of 100 interferometer scans (consuming ca. 60 s), subtracted from which was another set obtained after CO electmxidation was complete. The potentials indicated beside each spectrum are average values during the appropriate data acquisition. The electrode exposure times at the initial potentials (-0.25 to -0.35 V) prior to the voltammetric sweep were chosen to be short ( 4 - 2 min) to eliminate the occurrence of reaction inhibition effects (vide supra). The potentialdependent spectra on Au(210) (Figure 4A) are essentially as described p r e v i ~ u s l y . ~In~ the frequency region (1900-2250 an-')shown, a single weak uco band is obtained under these conditions. The uco frequency, 2105-21 15 cm-'(increasing, as usual, toward more positive potentials), is consistent with the presence of terminally bound CO (i.e., ooordinated to a single gold atom).13 The uco band disappears upon reaching a potential, ca. 0.1 V, at the onset of solution CO electrooxidation under these conditions. As noted previ~usly,~~ this identifies the adsorbed species as the likely intermediate for the overall electrooxidation of solution CO. In addition to this uco feature, a band at 2343 cm-' appears at the onset of CO electmxidation, identified with the asymmetric 0-C-O stretch of CO,. (The C 0 2 remains effectively trapped within the thin layer on the measurement timescale, 5-10 min.I6) Since the 2343cm-I band arises from a bulk-phasc species, it can be utilized to determine the quantity of C 0 2 produced, Q(C0,) (cf. ref 16b). By correcting for the contribution to Q(CO3 arising from CO in the thin-layer solution rather than adsorbed CO, we can obtain a rough estimate of the surface CO concentration, rcol of 1.5 (f0.5)X mol cm-2.13 While the inhibiting form of adsorbed CO, yielding a uc0 band at 1900-2000 cm-', develops on Au(210) at the expense of the reactive uco form for longer solution exposure times,') this condition is avoided here. Inspection of the corresponding potential-dependent spectra obtained on Au( 1 10) (Figure 4B) shows a uco band similar to that observed on Au(210); as before, this band disappears upon sweeping to potentials, ca. -0.05 V, at the onset of solution CO
I
I
2000
I
I
2200
I
V/C" Wcm-' Figwe 5. As for Figure 4, but in 0.1 M H2S01.
I
I
2000
electrooxidation. The uco band intensity on Au( 1 lo), however, is about 3-fold weaker than on Au(210), indicating a significantly lower CO concentration, ca. 5 X lo-" mol cm-,, on the former surface under these conditions. Corresponding spectra obtained on Au(ll1) and (100) showed no detectable u a band, even utilizing potential-modulationtechniques to optimize the signal to noise. The corresponding rc0values are estimated in this manner to be below ca. 1.5 X lo-'' mol cm-2 under these conditions. Potential-dependent infrared spectra were also obtained in a similar fashion for Au(210), (1 10) and (1 11) in 0.1 M H2S04. Typical ua spectra observed on the first two surfaces are shown in Figure 5 , A and B, respectively. Comparison of these results with the corresponding data obtained in 0.1 M HCIO, (Figure 4A,B) shows that closely similar spectra are obtained in t h m two electrolytes. The presence of H#04 rather than HClO, therefore exerts little influence on the degree and nature of CO adsorption on Au(210) and Au( 110) surfaces under these conditions. In addition, the adsorbed CO is electrooxidized at similar potentials on a given surface in these two electrolytes. This finding is in harmony with the similarity in the potentials at which the onset of solution CO electrooxidation occurs in 0.1 M HCIO, and 0.1 M H$04 (Figure 2). Note that the large differen- in the anodic voltammograms in these electrolytes are characterized chiefly by the potentials where the current peaks are obtained, not the current onset (Figure 2). As in 0.1 M HClO,, the infrared spectra ob tained for Au( 111) in 0.1 M H#04 show no detectable uco bands.
Discuesion Two overall features of the present results are of particular interest. First, the electrochemical rate parameters for CO electrooxidation are strongly dependent upon the gold surface crystallographic orientation. This finding indicates that the electrocatalytic pathway(s) is sensitive to energetic and/or stereochemical factors affecting the adsorbed intermediates. Second, while the extent of CO adsorption varies substantially between the three gold faces examined additionally by infrared spectroscopy, the order of increasing CO surface concentration, Au( 111) C Au(ll0) C Au(210), differs from the corresponding sequence of electrocatalytic activity, Au(ll1) C Au(210) C Au(l10). This disparity indicates that the dependence of the CO electmxidation rates on the crystallographic orientation is not determined merely by the surface concentration of the chemisorbed CO. This point is underscored by the large alterations in the electrochemical
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5565
Electrooxidation of CO on Au Surfaces
I
-I.
0.4
-2!
B
r
-E?
k
-3!
W
0.2-
-4! 0.1, 0.lL
'
I
I
1
0
0.I
0.2
Ewe, v EpzctV
VS
SCE
Figure 6. Logarithm of the apparent rate constant, k, (cm s-'), for solution CO electrooxidation measured at various gold faces plotted against corresponding pzc values, Em Electrolytes are 0.1 M LiCI04 + 2 mM HC104 (squares) and 0.1 M HCIO, (circles);data taken from Table I. The continuous and dashed straight lines depict rough corre-
I
vs SCE
Figure 7. Half-wave peak potentials, EPlz,for electrooxidation of CO at various gold faces plotted against corresponding pzc values, EW Other details as in caption to Figure 6.
lations encompassing the low- and high-index faces, respectively. kinetics induced by substituting 0.1 M HzS04for 0.1 M HC104, even though similar Tc0 values are observed in these two electrolytes. As a first step in unraveling the mechanistic factors that are involved, it is useful to examine correlations between the rate parameters and surface crystallographic properties. Of the latter, the potential of zero charge is a useful experimental quantity which can be related in turn to the metal surface structure, most specifically the relative surface energy or the density of "broken (or "dangling") bonds",z.20 Such rate-pzc correlations have been established for other electrocatalyticsystems, for example, proton electroreduction on monocrystalline gold faces.19 Displayed in Figure 6 is a plot of log k,, measured at 0.30 V for each gold face in 0.1 M HClO, (circles) versus Epanthe values being extracted from Table I. Figure 7 shows a related plot, that The y axis in the latter figure reflects the of E d 2 versus E., overpotential at essentially a constant reaction rate, whereas the former figure involves the rate evaluated at a fixed overpotential. Inspection of Figure 6 shows that a rough correlation between log k,, and E,, is obtained at both pH values. There is some justification for the presence of separate, crudely linear, correlations involving the low-index and the higher index faces; these are depicted as continuous and dashed lines in Figure 6. The corresponding E - E, plots in Figure 7 exhibit comparable correlations, aItR6lugh the distinction between the low- and high-index faces is less clearcut for the 2 mM HC104 electrolyte in this case. A related, yet distinct, correlation of interat involves the density of "broken" bonds, dbb, for the surface metal atoms in each crystallographic orientation. This quantity is defined for facecentered-cubic metals, such as gold, byz* dbb
= (8h + 4 k ) / ( h 2 + kz + p)'Iz
(3)
where h, k, and 1 are the Miller indices; here dbb is expressed in units of l/a2, where o is the lattice parameter (4.08 A for gold). delevie, R. J. Electroanal. Chem. 1990, 280, 179. (21) (a) Mackcnzie, J. K.;Moore, A. J. W.; NichoLu, J. F.J. Phys. Chem. Solids 1962, 23, 185. (b) Nicholas, J. F. An A t l w of Models of Crystal Surfoecz Gordon and Breach: New York, 1965. (20)
-3D-
-4.0
80
9.0
7.0
dbb
Flgm,8. Logarithm of the apparent rate constant, &., measured at 0.30 V at various gold faces for electrooxidation of CO plotted against the corresponding density of broken bonds, dw, as estimated from cq 3. Symbols as in Figure 6.
Of course, this estimation of dbbignores all surface reconstruction that may occur. Figure 8 shows log k, at 0.30 V, again in 0.1 M HC104 (circles) and 0.1 M LiC184 2 mM HC104 (squares), plotted against dw for the various gold crystallographic orientations. In some respects, this plot has a similar form to the corresponding log ,k - EF figure (Figure 6), as expected since a relationship between E p and dw has been established.z*MClose inspection of Fwre 8, however, reveals one sisnificant differencewith Fiure 6 in that the points for the Au(221) face now fall on a line bisecting roughly the (1 11) and (1 10) extremities. This difference reflects the distinctly different location of the dbb and E p values for Au(221) in relation to the respective values for the other faces. The corresponding plot of E versus dw is shown in Figure 9 (cf. Figure 7). Similarly to F!&e 8, this plot displays a roughly
+
Chang et al.
5566 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
While the mechanism infers that OH species are formed prior to the rds (c), it also can encompass pathways where (b) is closely coupled to (c), such as proton loss from water requiring some incipient bond formation with adsorbed CO. An argument against the former possibility is that the onset of adsorbed OH formation on monocrystalline gold apparently does not occur until more positive potentials (ca. 0.546 V vs SCE) than those corresponding to the region of kinetic interest here (ca. 0.1-0.5 V) are r e a ~ h e d . ~Applying ~ , ~ ~ the steady-state approximation, at a given electrode potential: d [C02l Kakl k2 [CO(soln) 1 [H,OadI rate = (4) dt k-,[H+] + k2Ka[CO(soln)]
4
0.4
w UI
>
0.3t
1
/
Ode
--
In the likely case where k l , k-, a quasi-equilibrium, so that
>> k2, step b can be expressed as
rate = d[COz]/dt = KaKIk2[CO(soln)][H+]-'[H20ad] ,.OlOI
0.1
1
I
I
9.0
8.0
I
7.0
dbb
Figure 9. Half-wave peak potentials, EPl2,for elcctrooxidation of CO at various gold faces against corresponding densities of broken bonds, dw, as estimated from eq 3. Symbols as in Figure 6.
common correlation involving the (1 1 l), (221), and (1 10) faces, with a marked deviation seen only for the (210) surface. (Note that the straight lines drawn in Figure 6-9 are merely suggested "guides to the eye" and have no statistical significance.) While mechanistic interpretation of these empirical trends is somewhat speculative in the absence of data for a more systematic set of crystallographic orientations, some insight can nonetheless be gleaned into the likely influence of surface stereochemistryon the electrocatalytic pathway. The progressive increases in the CO electrooxidation rates observed as the density of "dangling bonds" increases, or as E, decreases, can be understood partly in terms of the increasing extent of CO adsorption anticipated under these conditions. This expectation is at least qualitatively borne out by the infrared measurements which show that rc0on the (210) and (1 10) faces is markedly greater (by at least 10- and 3.5-fold, respectively) than on Au( 11 1). A straightforward relation between the reaction rate and the ability of the surface to bind the reactant might be expected in the present systems, where the adsorbed reactant coverages are sufficiently low so that the supply of surface sites for the incoming reactant (or coreactant) remains plentiful. As already noted, however, the lack of a consistent correlation between the face-dependent reaction rate and rc0 clearly implicates the importance of additional factors in the observed electrocatalysis. Since CO electrooxidation to C 0 2requires oxygen atom transfer from the solvent (or related species), a likely candidate for the ratedetermining step (rds) involves reaction between an adsorbed CO molecule (present at low coverage) and an adjacent adsorbed H 2 0 or OH ~ p e c i e s . ~The ~ * observation ~~ of an approximately inverse proton dependence of the electrooxidation rate suggests that either adsorbed OH species are involved, or adsorbed water reacts with adsorbed CO with concurrent proton loss. A plausible formal mechanistic pathway is CO(soln)
H20.d
toad
(a)
k
d= 0H.d + H+ + et-l
toad + OHad -% c02 + H++ e-
where K I = k l / k l . The latter simplified expression is largely consistent with the present, albeit somewhat cursory, experimental examination of the rate law. Some mechanistic information can in principle also be discerned from the transfer coefficient values. The present experimental values (aapp= 0.5-0.7) are consistent, although not exclusively so,with a smgle electron-transfer step being rate determining. (In the simplest case of an energetically symmetrical one-electron process, aaPp = 0.5.) If step c in the above mechanism is rate determining, one might anticipate that larger cr, values (ca. 1.5) would be obtained since the preceding step &) also involves electron transfer, yielding a substantially potentialdependent0% concentration. Indeed, the present observation of smaller a, values might be construed as favoring the occurrence of a concert3 two-electron, two-proton reaction with the first electron/protontransfer step being rate determining. The interpretation of such pawdata,however, is complicated by the potential-induced changes in adsorbate coverages and other factors,?6 so that a more quantitative interpretation is unwarranted. Of greater concern here are the consequences of the likely ratedetermining reaction between CO and H 2 0 (or OH) present at adjacent adsorption sites to the interpretation of the observed surface crystallographic effects. One factor that can account in global terms for the observed EPl2- E, correlation (Figure 7) is that the "oxygen down" orientation of adsorbed water, necessary for oxygen transfer to adsorbed CO, should be favored increasingly at positive electrode charges, i.e., at electrode potentials positive of the This circumstance should also favor OH adsorption. Indeed, CO electrooxidation in 0.1 M HClO, commences on most gold faces in the vicinity of the pzc. The substantial inhibition of CO electrooxidation seen toward more positive potentials in 0.1 M H$04 may also originate in part from the effects of sulfate adsorption on the orientation of wadsorbed water or the adsorption of OH species. The significantly different rate-E, correlations displayed by the low-index and higher index faces (Figures 6 and 7) can also be rationalized in part by the need to coadsorb CO and H20/OH at suitable adjacent sites. The Au(210) face, in particular, is an atomically "rough" surface, the top layer of gold atoms having a low surface coordination number (Le,, a greater number of dangling Although CO can be envisaged to bind relatively strongly to such sites, their "isolated nature" may well hamper the coadsorption of H20/OH into adjacent sites in suitably close proximity to yield efficient oxygen transfer. These considerations can therefore account for the relatively low reactivity of the (210) surface in comparison with the low-index faces given
(b) (c)
(22) Gilman, S.J . Phys. Chem. 1964,68, 70. (23) A useful discussion M also contained in: Conway, B. E. In Electrodes of Conductlor Metal Oxides; Trasaati, S., Ed.; Elscvier: New York, 1981; Part B, Chapter 9.
(5)
(24) Angerstein-Kodowska,H.; Conway, B. E.; Hamelin, A.; Stoiwiciu, L. Electrochim. Acta 1986, 31, 1051. (25) Also see: Peuckcct, M.; Coenon, F.P.; Bonzel, H. P. Surf. Scl. 1984, I l l , 515. (26) For example, we: GUeedi, E.; Conway, B. E. In Modern Aspects of Electrochemistry; Bockfie, J. O M . , Conway, B. E., Eds.; Buttenvorths: London, 1964; Vol. 3, Chapter 5. (27) Trasatti, S. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. OM.,Eds.; Plenum: New York, 1979; Vol. 13, Chapter 2.
Electrooxidation of CO on Au Surfaces the propensity of the former for binding CO. More detailed scrutiny of the rated,,,, correlations (Figures 8 and 9) reveals one additional feature of interest in this regard. The straight lines drawn in Figures 8 and 9 are rough guides to the eye that encompass the (1 11) and (1 10) faces. Unlike the corresponding rate-+ plots, the points for the (221) face fall close to these correlation lines. A common feature of the (221) and (1 10) faces is that they both can be regarded as stepped surfaces, formally 4( 1 1 1)-( 11 1) and 2( 111)-( 1 1l), respectively. This correlation therefore suggests that the progressively greater electrocatalytic activity observed for the (221) and (1 10) surfaces relative to the (1 11) terrace is due to preferential reaction at the (1 11) step sites. The comparable reactivities observed for the (221) and the (533) faces are also understandable on this basis given that the latter is a closely related stepped surface, 4( 1 11)-( 100). A simple rationalization for the catalytic nature of such sites is that they feature a low surface coordination number, favoring CO binding, along with the presence of adjacent sites located on the adjoining terrace at which H 2 0 or OH can bind in an appropriate configuration to achieve subsequent oxygen transfer to the CO. An additional comment can be made in this context concerning the relative reactivities of the Au(210) and Au( 110) faces. Inspection of atomic models2Ibshows that the latter surface contains close-packed rows of gold atoms. While the former surface also features the same distance between gold rows in one direction, t h m are significant gaps between all toplayer atoms. The greater catalysis observed on Au( 110) may therefore arise from adsorption of CO and H20/OH on adjacent sites along the rows. This particular coadsorbate configuration cannot a u r on Au(210). The densely stepped and kinked nature of Au(ll0) and Au(210), respectively, accounts for the prevalence of CO binding on both faces. However, the lower catalytic activity of Au(210) in spite of its greater facility for CO binding obliges consideration of additional factors, such as coadsorbate stereochemistry. One feature that restricts somewhat the molecular-level understanding of these systems at present is the likely Occurrence of surface reconstruction. This phenomenon for gold surfaces has been addressed several times in the recent literature.*” Recent in-situ studies of Au( 11 1) and Au( 100) in aqueous acidic media using second harmonic generation29and Au( 100) using X-ray reflectivity and diffraction” indicate that potential-dependent reconstruction does indeed occur. This reconstruction, however, (28) (a) Kolb, D. M.; Schneidcr,J. Electrochlm.Ada 1986,31,929. But ace: Hamclin, A. Electroehlm. Acra 1986, 31, 937; J . E / e c t m M / . Chem. I-, 2S5,281. (b) Zei, M. S.;Lehmpfuhl, 0 . ; Kolb, D. M. Surf. Scf. 1989, 221, 23. (29) Friedrich, A,; Pettinger, B.; Kolb, D. M.; LOpke, G.; Steinhoff, R.; Marowrkv. 0 . Chem. Phvs. Lett. 1989. 163. 123. (30) dcko, B. M.; Wing, J.; Davenport, A.;Isaacs, H. Phys. Rev. Lert. 1990,6S, 1466.
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5567 tends to be lifted at relatively positive potentials. Very recently, we have been able to obtain in-situ scanning tunneling microscopy (STM)images with high-quality atomic resolution in 0.1 M HClO, for a number of the gold faces examined in the present study. These findings will be presented elsewhere; they provide a detailed picture of the potential-dependent reconstructions that occur on gold electrodes.31 Broadly speaking, the results are consistent with the earlier in-situ measurements on Au( 111) and Au(100), showing that the reconstructions are lifted at potentials close to, and more positive than, the pzc. Similar findings apply to Au( 110). Consequently, then, surface reconstruction appears unlikely to play a predominant role in influencing the CO electrooxidation kinetics on low-index gold surfaces. The situation is presently less clearcut for the high-index surfaces, although the (221) face indeed appears to consist largely of equally spaced monoatomic steps.
Concluding Remarks The present study demonstrates the value of linear sweep voltammetry coupled with real-time surface infrared spectroscopy to delineate the role of reactant binding and other factors in electrocatalysis. The level of interpretation here is limited by several factors, including possible complications wrought by surface reconstruction. However, the substantial sensitivity of the electrooxidation rates to the surface crystallographic orientation earmarks the present reaction system as one worthy of more extensive examination in the future. In particular, a systematic kinetic study of sequences of crystal faces with varying densities and structures of step and terrace sites within crystallographic zones would be extremely worthwhile. While studies of this type are rare, an intriguingly detailed examination along these lines has been reported for formic acid electrooxidation on platinum The present CO/gold system offers an interesting contrast to such platinum electrocatalyses in that the former involves only weak reactant adsorption, so that complications from limited site availability are largely absent. In any case, the coupling of such kinetic studies with surface spectroscopic methods should pave the way for a much-needed understanding of the molecular-level factors responsible for electrocatalysis. Acknowledgment. We are grateful to Dr.R. Adzic for alerting us to the kinetic differences seen for some related electmxidations on gold in perchloric and sulfuric acids. S.C.C. thanks the W. R. Grace Foundation for fellowship support. This work is s u p ported by the National Science Foundation. Registr). NO. CO, 630-08-0; Au, 7440-57-5; HClOI, 7601-90-3; H2SO4, 7664-93-9; LiClO,, 7791-03-9; H20, 7732-18-5. (31) Gao, X.; Hamclin, A.; Weaver, M. J. Submitted for publication.
(32) Motm, S.;Furuya, N. Ber. Bumen-Ges. Phys. Chem. 1987,91,457.
