Langmuir 1990, 6, 323-333
323
Influence of Adsorbed Carbon Monoxide on the Electrocatalytic Oxidation of Simple Organic Molecules at Platinum and Palladium Electrodes in Acidic Solution: A Survey Using Real-Time FTIR Spectroscopy Lam-Wing H. Leung and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received May 10, 1989. In Final Form: August 4, 1989 The influence of adsorbed CO formed by dissociativechemisorption on the electrooxidation pathways and kinetics of 22 alcohols and aldehydes on polycrystalline platinum and palladium electrodes in relation to the adsorbed CO formed by dissociative chemisorption has been examined systematically by means of real-time sequences of FTIR spectra obtained during slow (2 mV s-') voltammetric sweeps in 0.1 M HC10,. The overall objective isdo explore the reactant structure-dependent role that adsorbed CO (and other C0,-producing species) plays in the organic electrooxidation. The reactants examined include short-chain primary and secondary alcohols, aldehydes, and small bifunctional molecules (e.g., ethylene glycol) containing alcohol, aldehyde, and/or carboxylic acid units. For both primary alcohols and aldehydes, the initial coverage of adsorbed CO, e,, diminishes progressively with increasing size of the carbon backbone, from 0.9 (methanol) to 0.25 for benzyl alcohol. The Bc0 values are derived from the intensity of the infrared C-0 stretch at 2020-2070 and 1940-1980 cm-' on platinum and palladium, respectively. Adsorbed CO appears to act primarily as an inhibitor rather than a reaction intermediate for methanol and formic acid electrooxidation on platinum on the basis of 12C/13Cisotope substitution experiments. The occurrence of specific electrooxidation pathways yielding the corresponding aldehyde, carboxylic acid, and CO, was followed quantitatively under voltammetric conditions from the potential-dependent appearance of characteristic infrared bands. Generally, the onset of even partial electrooxidation of primary alcohols, forming successively aldehydes and carboxylic acids, requires electrooxidative removal of adsorbed CO. Electrooxidationof secondary alcohols to ketones proceeds readily, with no adsorbed CO being formed in most cases. The interconnection between these adsorptive and mechanistic properties and their dependence upon reactant structure are discussed. The catalytic electrooxidation of small organic molecules on platinum and other transition-metal surfaces is a topic of longstanding interest in electrochemistry, in part because of its potential importance in fuel cells.' The recent advent of in situ infrared spectroscopy' has contributed significantly toward an understanding of the surface species involved, initially by demonstrating that substantial quantities of adsorbed CO could be formed on platinum from methanol and formic acid, which may act as a surface poison toward their electro~xidation.~ More recently, evidence for the presence of other surface species has been obtained by using this t e ~ h n i q u e . ~ (1) For a recent review, see: Parsons, R.; Vandernoot, T. J. Electroanal. Chem. 1988,257, 9. (2) Bewick, A.; Pons, S. In Advances in Infrared and Raman Spectroscopy; Vol. 12, Clark, R. J. H., Hester, R. E., Eds.; Wiley: Heyden, New York, 1985; Chapter 1. (3) (a) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J.Electroanal. Chem. 1981,121,343. (b) Beden, B.; Bewick, A,; Lamy, C. J. Electroanal. Chem. 1987,148, 147. (c) Beden, B.; Bewick, A.; Lamy, C. J . Electroanal. Chem. 1983,150,505. (4) (a) Nichols, R. J.; Bewick, A. Electrochim. Acta 1988, 33, 1691. (b) Beden, B.; Hahn, F.; Leger, L.-M.; Lamy, C.; Lopes, M. T. D. S. J. Electroanal. Chem. 1989, 258, 463. (5) (a) Corrigan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (b) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143. (c) Corrigan, D. S.; Weaver, M. J. Langmuir 1988,4,599. (d) Leung, L.-W. H.; Weaver, M. J. J. Electroanul. Chem. 1988, 240, 341. (e) Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1988,92,4019. (6) (a) Leung, L.-W. H.; Wieckowski, A,; Weaver, M. J. J. Phys. Chem. 1988, 92, 6985. (b)Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Electroanal. Chem. 1989, 266, 317. (c) Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1989,93,7218-7226. (d) Chang, S.-C.; Leung, L.W. H.; Weaver, M. J. J. Phys. Chem. 1989,93,5341. (7) (a) Christensen, P. A,; Hamnett, A.; Trevellick, P. R. J. Electroanul. Chem. 1988,242,23. (b) Christensen, P. A,; Hamnett, A.; Weeks, S. A. J.Electroanal. Chem. 1988,250,127. (c) Christensen, P. A.; Hamnett, A. J. Electroanal. Chem. 1989,260, 347.
0743-7463/90/2406-0323$02.50/0
Most infrared spectroscopic studies of this type utilize potential modulation to achieve the necessary subtraction of solvent and other interference^.^'^ Recently, our lab~ratory'.~and others7i8 have begun to utilize potential difference FTIR spectroscopy with spectra being acquired during a single potential excursion. This approach, which we have dubbed "single potential alteration infrared spectroscopy" (SPAIRS),'" is of particular value for the examination of catalytic multistep processes since in favorable cases irreversible potentialinduced changes in both the surface and thin-layer solution composition can be followed quantitatively.' Besides the examination of electroorganic reactions on polycrystalline ~ u r f a c e swe , ~ have recently initiated studies on oriented single-crystal platinum and rhodium.6 While a number of electrooxidation processes have been examined so far, including reactions involving alcohols, aldehydes, and amides,68 most reports have been somewhat limited in scope; detailed examinations of the comparative kinetics of homologous reaction series are lacking. In a recent preliminary note,5d we reported SPAIR spectra for a number of such reactions on platinum which indicate a wide-ranging influence of adsorbed CO formed by dissociative chemisorption on their electrooxidation kinetics. Contained herein is an expanded account of the application of SPAIRS under potential sweep conditions to the examination of electrooxidation pathways and kinetics for 23 aliphatic and aromatic alcohols and aldehydes and related systems on polycrystalline platinum and palladium in 0.1 M HCIO,. Attention is focused on the manner and extent to which adsorbed CO forma(8) (a) Holtz, R.; Vielstich, W. Electrochim. Acta 1988, 33, 1629. (b) Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1988, 250, 451.
0 1990 American Chemical Society
324 Langmuir, Vol. 6, No. 2, 1990
tion, and its consequent influence upon the electrooxidation kinetics, depend upon the reactant structure. Experimental Section Most experimental details concerning the infrared instrumentation and procedures are given in ref 5a and 5b. The infrared spectrometer was a Bruker-IBM IR 98-4A Fourier transform instrument,with a globar light source and either an MCT narrow-band or InSb detector (Infrared Associates). The spectral resolution was f4 cm-'. The combined electrochemicalinfrared measurements were performed after pushing the disk electrode up to either a CaF, or ZnSe optical window to form a thin (ca. 5-pm) solution layer. The polycrystalline platinum and palladium electrodes were 9-mm disks mounted on a glass plunger. The platinum surface was pretreated immediately prior to use by mechanical polishing and dipping in hot chromic acid followed by rinsing and potential cycling in 0.1 M HClO, at 0.1 V s-' between -0.25 and 1.4 V vs saturated calomel electrode (SCE) for about 10 min. The palladium electrode was pretreated similarly, except that the potential was cycled between 0 and 1.2 V. All the electrochemical measurements employed a PAR Model 173/179 potentiostat driven by a PAR Model 175 potential programmer, the voltammograms being recorded on a HP 7045 X-Y recorder. Bulk-phase transmittance infrared spectra for reactants and anticipated (or suspected) products were obtained, usually in 0.1-0.25 M concentrations in 0.1 M HClO,, by using a conventional cell (path length 13 pm) with either CaF, or ZnSe windows. The organic reactants were obtained ("high-purity" grades) from Aldrich or Fluka, and perchloric acid (doubledistilled from Vycor) was from G. F. Smith. All solutions for electrochemical measurements were purged previously with nitrogen. Electroda potentials are quoted versus SCE, and all measurements were made at room temperature, 23 f 1 "C. Results and Discussion Electrooxidation of Alcohols on Platinum. Besides the electrooxidation of methanol, which has been investigated extensively with electrochemical and infrared technique^,^ only ethanol electrooxidation has received any concerted a t t e n t i ~ n . ~ ~A? ~major ~ , " behavioral difference between the electrooxidation of methanol as compared with higher primary alcohols on platinum in aqueous acidic media is that only the former yields predominantly CO,, t h e l a t t e r reactants producing t h e corresponding carboxylic acid with intermediate aldehyde f ~ r m a t i o n . ~ ~Nevertheless, ,'~ we have shown by SPAIRS that primary alcohols usually yield substantial quantities of adsorbed CO under these conditions from the characteristic infrared band for C-0 stretching (vco) a t ca. 2060 ~ m - ' , ' ~its electrooxidation to CO, presaging the overall voltammetric electrooxidation of the solution al~ohol.~"*~~ For the electrooxidation of methanol, then, it is conceivable that adsorbed CO may act as a significant reaction intermediate responsible for most of the C 0 2 produced rather than a poison where its electrooxidative removal is required in order to allow the reaction to proceed via other adsorbed species.5b The observation from infrared spectroscopy that the production of CO, commences under voltammetric sweep conditions at potentials corresponding to the electrooxidative removal of adsorbed C05b*'1is by itself consistent with either type of mechanism. A distinction, however, can readily be (9) (a) Kunimatsu, K.; Kita, K. J . Electround. Chem. 1987, 218, (b) McNichol, B. D. J . Electround. Chem. 1981, 118, 71. (10) (a) Beden, B.; Morin, M.-C.; Hahn, F.; Lamy, C. J. EEectroanal. Chem. 1987,229,353. (b)Iwasita, T.;Vielstich, W. J. Electround. Chem. 1988,257, 319. (c) Bittans-Cattaneo, B.; Wilhem, S.; Cattaneo, E.; Buschmann, H. W.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1988,92, 1210. (11) Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275. 155.
Leung and Weaver drawn in principle: in order for adsorbed CO to act as an adsorbed intermediate, it is necessary for this species to be replenished upon CO electrooxidation by further dissociative chemisorption of the reactant at a rate sufficient to account for the overall reaction kinetics.5b Some evidence supporting the feasibility of such a mechanism for methanol electrooxidation on polycrystalline platinum was obtained by means of double potential step SPAIRS, in which the readsorption rates following partial electrooxidation removal of the adsorbed CO were monitored from the reappearance of the vco band.5b These measurements indicate that the adsorption rates can indeed be sufficient to account for the observed kinetics, although the readsorption kinetics are monitored necessarily a t potentials below where facile methanol electrooxidation proceeds. Another instructive tactic for probing the extent to which CO may act as a reaction intermediate rather than a surface inhibitor involves the use of 13C/12Cisotopic mixtures. Two types of such experiments were performed in the present study. In the first type, a layer of 13C0 was formed on platinum in 0.1 M HC10, a t a relatively negative potential, 0 to -0.25 V, by addition of a low concentration of 13CH30H,ca. 10-20 mM, and an excess (typically 5-10 fold) of 12CH30Hadded immediately prior to the initiation of a positive-going potential sweep (at 1-10 mV s-l). The second type of experiment involves forming an irreversibly adsorbed l2C0 layer by bubbling in CO, followed by nitrogen purging and adding 13CH30Himmediately prior to the potential sweep. In either case, any significant replenishment of the adsorbed CO layer during electrooxidation should be signaled by a change in the uco frequencies, reflecting an alteration in the surface isotopic composition. The simultaneous acquisition of SPAIR spectra during the voltammetric sweep enables the adsorbed CO infrared spectra to be monitored at potentials where methanol electrooxidation proceeds. A typical set of up? SPAIR spectra gathered under the latter type of conditions is shown in Figure 1. The lefthand spectral sequence (Figure 1A) was obtained after adding 50 mM 12CH30Hjust prior to forming the thin layer and initiating the voltammetric sweep at 1 mV s-l from 0 V, whereas the right-hand sequence (Figure 1B) was obtained similarly but by adding 50 mM 13CH30H instead. In both cases, each spectrum shown involved acquiring 10 interferometer scans (consuming ca. 7 s) during the voltammetric sweep, the solvent interference being removed by subtracting a similar spectrum taken subsequently at a sufficiently positive potential (ca. 0.6 V) to electrooxidativelyremove the CO. While the bottom spectrum in A and B refers to a potential, 0.1 V, prior to the initiation of electrooxidation, the upper four spectra refer to potentials (0.3-0.4 V as indicated) where methanol electrooxidation proceeds, and the CO layer is progressively being removed as indicated by the decreasing intensity of the vco band. Close comparison of A and B of Figure 1 reveals that the addition of 13CH30H rather than 12CH30Hexerts little or no influence on the vco frequencies during electrooxidation. Thus, in both cases the uco band frequency decreases only slightly, from about 2070 to 2055 cm-', as the CO layer (and hence the vco intensity) decreases during electrooxidative removal. Much larger (up to ca. 60 cm-') frequency decreases are expected in Figure 1B if the l2CO layer undergoes replenishment during electrooxidation by means of dissociative I3CH3OH chemisorption. This is illustrated by the pair of dotted
Organic Electrocatalytic Oxidations at Pt and Pd
2 0,w
2150
v / cm-'
1950
Figure 1. Single-potential alteration infrared (SPAIR) sgectra obtained for electrooxidation of irreversibly adsorbed CO on polycr stalline platinum electrodes after the addition of (A) 50 mM YCHa30Hand (B) 50 mM "CH,OH during a positivegoing potential sweep at 1 mV s-l from 0 V vs SCE in 0.1 M HClO,. Each spectrum, displayed in absorbance units, was acquired from 10 interferometer scans at the average potentials indicated,the solvent and other interferencesbeing removed by subtracting a spectrum obtained subsequently at ca. 0.6 V. The dotted spectra shown were obtained without prior dosage of irreversibly adsorbed "CO under otherwise identical conditions (see text for details).
'
s ectra in Figure lB, which were obtained during BCH30H electrooxidation in the absence of the l2C0layer; the markedly lower vco frequencies are consistent with the isotopic mass effect. For comparison, the dotted spectra in Figure 1A are obtained in the same manner, with 12CH30H. Nevertheless, the CO layer may be replenished to a small extent during methanol electrooxidation; mixed 12CO/13C0 layers display single vco peaks at intermediate frequencies as a result of dipole-dipole coupling.''J2 Qualitatively similar findings were also obtained by using the first approach described above, i.e., dosing the surface first with 13CH30H and then with an excess of 12CH30H. In this case, however, significant replacement of adsorbed 13C0 is observed prior to the initiation of the voltammetric sweep. While these results do not eliminate adsorbed CO as a possible reaction intermediate for methanol electrooxidation, they indicate that if such a mechanism occurs it only utilizes a minority of the total CO adsorption sites. We have obtained qualitatively similar findings for the electrooxidation of formic acid on polycrystalline platinum (vide infra) and on Rh( 111).6c Given that extensive adsorbed CO formation is observed for a range of alcohols as well as aldehydes and amides on polycrystalline platinum,5d it is of central interest to ascertain its effects upon the electrooxidation kinetics. Table I includes values of the fractional CO coverage, Oco, for the 10 alcohol reactants studied here. These are ~
~~~~~
(12) Severson, M. W.; Russell, A.; Campbell, D.;Russell, J. W. Langmuir 1987, 3, 202. (13) Corrigan, D. S.; Weaver, M. J. J . Electround Chem. 1988, 239, 55.
Langmuir, Vol. 6, No. 2, 1990 325 obtained from the integrated intensity, Ai, of the vco feature at -0.25 V (for 50 mM reactant in 0.1 M HC10,) in ~ a) saturated ~ ~ proportion to that (Ai = 0.045 ~ m - for CO layer formed from solution CO under the same conditions. In most cases, the vco bands were obtained from SPAIR spectra under voltammetric potential sweep or step conditions, although for systems where Oco is small (Oca d 0.2) the spectra were extracted by means of repeated potential modulation between -0.25 and 0.2 V so as to enhance the sensitivity (see ref 5b-e for details.) These Oco values are probably reliable to f0.05; the required assumption that Ai a Oc0 is known to be valid at least for adsorption from solution C0.5b Very similar Oc0 values are extracted from infrared spectra over the potential range -0.25 to 0.1 V and for reactant concentrations up to at least 0.25 M. Listed alongside the Oco values for each reactant in Table I are corresponding fractional coverages of adsorbed hydrogen atoms, OH, at -0.25 V. These values were determined from the faradaic charge required for voltammetric oxidation of the adsorbed hydrogen layer (from -0.25 to ca. 0.1 V) in the presence of the reactant in relation to that in its absence. Inspection of these Oco and By values shows that while Oco for the series of aliphatic primary alcohols decreases in the sequence methanol > ethanol > 1-propanol > 1butanol, the sum Oco OH is close to unity, 20.8,in each case. This indicates that although the extent of adsorbed CO formation declines monotonically as the alkyl chain lengthens, there is no extensive coadsorption of other alcohol fragments preventing hydrogen adsorption in sites unoccupied by CO. As we have noted p r e v i o u ~ l ydis,~~ sociative chemisorption to form adsorbed CO appears to require the presence of at least one hydrogen bound to the CY carbon.5d This "a-hydrogen effect" suggests that dissociative chemisorption to form CO involves initial replacement of at least one LY hydrogen by formation of a platinum-carbon bond. Examination of Table I shows that the secondary alcohols 2-propanol and 2-butanol do not yield detectable amounts of adsorbed CO (Oc0 < 0.05), even though these species each contain one LY hydrogen. Of the cyclic aliphatic secondary alcohols in Table I, neither cyclopentanol nor cyclohexanol forms detectable amounts of adsorbed CO; however, cyclobutanol yields Oco = 0.2. This latter finding is consistent with the lower stability of the four-membered carbon ring aiding the C-C bond cleavage necessary for dissociative chemisorption of cyclobutanol. The greater quantity of adsorbed CO formed for the corresponding primary alcohol, 1-butanol (Table I), can be rationalized by the need to cleave only one C-C bond in this case. The evaluation of 8, for the secondary alcohols is hampered by the onset of reactant electrooxidation at potentials that overlap with those for removal of adsorbed hydrogen. The resulting estimates of OH are therefore only approximate, being given in parentheses in Table I. Nevertheless, for these systems typically OH + Oco d 0.3, suggesting that there is significant coadsorption of the reactant and/or other molecular fragments. Of central interest here is the reactivity of the adsorbed CO, as well as the extent of its formation, in relation to the overall electrooxidation kinetics of the solution reactants. Figure 2A-H depicts typical anodic-cathodic cyclic voltammograms at 0.1 V s-l from -0.25 to 1.3 V for a selected sequence of simple alcohols (50 mM in 0.1 M HC10,) at platinum as follows: A, methanol; B, ethanol; C, 1-propanol; D, 2-propanol; E, 1-butanol; F, 2-butanol;
+
326 Langmuir, Vol. 6, No. 2, 1990
Leung and Weaver
Table I. Extent of Adsorbed Carbon Monoxide Formation and Relationship to Electrooxidation of Simple Organic Molecules in Platinum in 0.1 M HClO, reactant’
OHb
co
0
formic acid methanol ethanol 1-propanol 2-propanol 1-butanol 2-butanol cyclobutanol cyclopentanol cyclohexanol benzyl alcohol acetaldehyde propionaldehyde benzaldehyde ethylene glycol glycoaldehyde glyoxal glycolic acid glyoxylic acid oxalic acid
0.05 0.1 0.3 0.4
1,2-propanediol 1,3-propanediol glycerol
(0.2)
0.45 (0.2)
(0.25) (0.2) (0.2) (0.3) 0.1
0.4 0.25 0.1 0.2 0.2 0.4 0.2 0.7 0.2 0.25 0.25
Oca" 1.0 0.9 0.9 0.5 0.45 50
E(e,0/2),d V vs SCE 0.30 0.32 0.26 0.38 0.43 0.40 0.37 0.60 0.40 0.38 0.70 0.38 0.38 0.38 0.40 0.38 0.35 0.34 0.38 0.39
----
Ei,f V vs SCE CO, 0.35 0.20 0.27 0.55 0.67 0.70
RCO,H
RCHO
0.50 -0.55
0.35 -0.40
-0.608
-0.45
0.05
0.80 0.65 0.80 0.78 0.54 0.85 0.50 0.50 0.49 0.64 0.55 0.34 0.27 0.34 0.30
R,CO
-0.158 -0.208 -0.158 -0.358 -1.20 0.50 -0.55 1.108 0.55h 0.55h 0.60h
-0.908
-
0.55h -0.758 -0.658 -0.558
-0.568 -0.588
’Reactant concentration was 50 mM in each case, except for CO, which refers to an irreversibly adsorbed layer (2 X lo4 mol cm-,) obtained by CO solution dosage followed by nitrogen purging. Fractional coverage of adsorbed hydrogen at -0.25 V vs SCE, obtained from ratio of the voltammetric charge required to oxidize layer in presence of 50 mM organic reactant in comparison to that (350 pC cm-2) measured in 0.1 M HC10, alone. Values given within parentheses are less reliable since reactant electrooxidation commences at potentials prior to complete electrooxidation of adsorbed hydrogen. Fractional coverage of terminally adsorbed CO determined from integrated intensity of vco band at ca. 2020-2070 cm-‘ at -0.25 to 0.2 V relative to that (ca. 0.045 cm-l)& for a saturated CO layer (2 X loes mol cm-,) formed from solution CO (see text). Values probably reliable to f0.05. Electrode potential (V vs SCE) at which Oco declined to half its initial value, as obtained from the potential-dependent SPAIRS vco band intensity during a positive-going potential sweep at 2 mV s-l from -0.25 V (see text for more details). ‘Total quantity (nmol cm-’) of CO, product formed by the end of a voltammetric sweep from -0.25 to 1.2 V at 2 mV s-l, as determined from the integrated intensity of the 2343-cm-’ band (see text and ref 5). ’Electrode potential (V vs SCE) by which 2 x mol cm-* of specific product indicated (CO,, carboxylic acid, aldehyde, or ketone) is formed during 2 mV s-l voltammetric sweep from -0.25 to 1.2 V. Values obtained in most cases (except for CO,) from intensities of specific SPAIRS bands together with tb values given in Table 11, using eq 1 as outlined in the text and in ref 5e. Values estimated by using molar absorptivities for related product species, since actual products are insufficiently soluble to enable direct determination of eb. Values determined from intensity of ca. 1235-1240-cm-’ feature assuming that major species formed is oxalic acid (see text).
*
G, cyclobutanol, and H, cyclohexanol. Comparison of Figure 2 with the 8c0 values in Table I shows that secondary alcohols that yield little or no adsorbed C0-2propanol, 2-butanol, and cyclohexanol-undergo facile electrooxidation by ca. 0.1 V. In contrast, the three primary alcohols (and cyclobutanol) that yield moderate or substantial 8c0 values do not commence electrooxidation until about 0.4 V. As we have demonstrated previously for as well as for methanolsb (vide supra) on the basis of SPAIR spectra, the onset of reactant electrooxidation corresponds closely with the potential at which the CO layer is largely removed. This comparison is made more quantitatively in Table I, on the basis of parameters extracted from sequences of SPAIR spectra. These results were obtained during anodic voltammograms at 2 mV s-l from -0.25 to 1.2 V, again for 50 mM reactant in 0.1 M HC10,. The first parameter, labeled E(8,,/2), denotes the potential at which CO electrooxidation has caused 8c0 to decline to half its initial value, as deduced from the point where the vco band intensity is diminished twofold. (Generally, CO electrooxidation under these conditions proceeds to completion over a relatively narrow potential interval, ca. 0.150.2 V.5bpd*e96b)The second parameter, QXCO,), denotes the total quantity (nmol cm-,) of C02 produced during the voltammetric sweep, as determined from the absorbance of the 0-C-0 asymmetric stretch feature at 2343
cm-1.5*6(Note that most of the CO, remains trapped within the thin-layer cavity on the time scale of the voltammetric sweep, so Q(C0,) provides an approximate representation of the total amount of CO, formed under these conditions.) The last parameter, Ei, is the potential at which a given amount, 2 X lo* mol cm-’ (“=one Langmuir”), of each electrooxidation product is formed. (This quantity is chosen to be sufficiently small so as to yield an Ei value reflecting the “initial” significant formation of each product.) The first three Ei entries, for CO, and the appropriate carboxylic acid and aldehydeEi(C02),Ei(RC02H),and E,(RCHO), respectively- refer to species formed from primary alcohols, whereas the fourth entry, Ei(R,CO), refers to production of the appropriate ketone from the secondary alcohols. An illustration of typical SPAIR spectra used to extract these parameters is given in A and B of Figure 3 for the electrooxidation of 50 mM cyclobutanol and 2-propanol, respectively, in 0.1 M HClO, at 2 mV s-’. Each spectrum shown involved acquiring 25 interferometer scans (consuming ca. 17 s) and ratioing these to a corresponding “reference” spectrum obtained at the initial potential (-0.25 V) just prior to the voltammetric sweep. The potential values given in Figure 3 refer to the average values during each spectral acquisition period; for clarity, only selected members of the spectral sequence are shown. Even though the voltammetric sweep rate is nec-
Organic Electrocatalytic Oxidations at Pt and Pd I
0
0
0
I
I
I
I
I
I
I
L4.L 0
h
0
&
Langmuir, Vol. 6, No. 2, 1990 327 idation, besides the negative-goingband centered at 1100 cm-' which is due to potential-induced migration of perchlorate into the thin-layer cavity.13 As already noted, the negative-going 2343-cm-' band signals the production of CO, in the thin-layer solution. From the integrated absorbance of this band (given that Ai = 0.069 mol cm-,), the quantity of cm-l for Q(C0,) = 2 X CO, formed by the end of the voltammetric sweep is determined to be about 3 X mol cm-'. Part of this CO, arises from the electrooxidation of adsorbed CO present at the initial potential. This species is detected in Figure 3A as the bipolar band at 2030-2040 cm-' for potentials below ca. 0.3 V, which becomes a positive-going unipolar band by 0.5 V where CO electrooxidation is complete. (The latter unipolar feature arises from the utilization of a spectrum at the initial potential for reference purpose^.^,^) The total quantity of CO, produced is substantially greater than that, ca. 4 X lo-'' mol cm-' (i.e., dcp 0.2, Table I), attributable to electrooxidation of initially adsorbed CO. This finding, together with the observation that a substantial fraction of the CO, is formed at potentials beyond ca. 0.5 V (Figure 3), shows that the CO, arises predominantly from another source (vide infra). The other two negative-going features in Figure 3A, a strong band at 1765 cm-' and a weaker feature at 1388 cm-', indicate the formation of the corresponding ketone;14in particular, the former band is characteristic of a ketone C=O stretch. (Note that the increased "noise" and apparent additional small features obtained in the ca. 1500-1700-cm-' region are due to strong infrared absorption by the aqueous solvent.) Inspection of the corresponding SPAIR spectra for 2propanol electrooxidation (Figure 3B) shows several similarities, in particular, the appearance of a comparable quantity of CO, product (2 X lo-' mol cm-') by the end of the voltammetric sweep. However, a difference compared with spectra for cyclobutanol electrooxidation (Figure 3A) is that no vco bands are detected (eco < 0.05, vide supra), implying that the CO, is formed entirely from a source other than initially adsorbed CO. Further, the production of acetone, as signaled by characteristic features at 1238, 1370, and especially 1700 cm-' (as determined from infrared transmittance measurements), is seen to commence at about 0.05 V, in accordance with the onset of anodic voltammetric current (Figure 2D). The relatively facile electrooxidation kinetics for this species can be attributed in part to the absence of an inhibiting adsorbed CO layer. Indeed, forming such a layer by bubbling in solution CO inhibits entirely the onset of acetone formation until 0.25 V, whereupon the adsorbed CO commences electrooxidation, again deduced from the SPAIR spectra. Examination of the E(8,,/2), Qf(CO,), and E, values listed in Table I for these and the other alcohol reactants enables more general conclusionsto be reached concerning the extent to which the formation and electrooxidation characteristics of adsorbed CO influence the overall kinetics. The E(8,,/2), Qf(CO,), and Ei(CO,) values were determined from SPAIR spectra as noted above. The other Ei values were obtained from plots of the quantity of product formed, Q, against electrode potential obtained from the corresponding integrated absorbance, Ai, of a suitable SPAIRS band by usingb
-
1 1 1 1 1 1 1 11 1 1 1 1 1 1 1 0
0.4
0.8
E
1.2
0
0.4
0.8
1.2
vs SCE
/ \
Figure 2. Cyclic voltammograms obtained at 0.1 V 8-l for the electrooxidation of (A) methanol, (B) ethanol, (C) 1-propanol, (D)2-propanol, (E) 1-butanol,(F)2-butanol, (G)cyclobutanol, and (H) cyclohexanol on platinum (electrode area = 0.70 cm2). Solution contained 50 mM reactant in 0.1 M HC10,.
A
0'54T I*=
0.13
. 2oi4
I
-
-0.05.-*
I
I
I
Figure 3. SPAIR spectra obtained during the voltammetric oxidation of (A) cyclobutanol and (B) 2-propanol at the platinum-aqueous interface. Solution contained 50 mM reactant in 0.1 M HCIO,. Potential sweep rate was 2 mV s-l, positive going from -0.25 V. Each spectrum, displayed as relative reflec-
(ARIR),was acquired from 25 interferometer scans at the average potentials indicated, ratioed to the corresponding spectrum obtained at the initial potential. tance
essarily slower than in D and G of Figure 2 and a thinlayer configuration is employed, the voltammetric features obtained under the SPAIRS and these "more typical" electrochemical conditions are very similar. Several spectral features in Figure 3A provide mechanistically useful information for cyclobutanol electroox-
Q Ai/2.5~b (1) where tb (M-' cm-') is the corresponding bulk solution (14) Sokolova, E.Electrochim. Acta 1975, 20, 323.
328 Langmuir, Vol. 6, No. 2, 1990
Leung and Weaver
Table 11. Frequencies (em-') and Integrated Absorptivities, eb (M-' cm-'), of Major Infrared Bands of Various Products Formed in Organic Electrooxidations. As Determined by Solution Transmittance Measurements moleculeso acetaldehyde acetic acid propionaldehyde propionic acid acetone butyraldehyde butyric acid
1724 (3.1 x 103) 1724 (6.1 x 103) 1725 (5.5 x 103) 1725 (1.6 x 104) 1700 (4 x 103) 1718 (-8 x 103) 1718
2-butanone cyclobutanone cyclopentanone cyclohexanone benzaldehyde
1697 1765 1720 1693 1693
benzoic acid
1693
glycolaldehyde glyoxal
--
1370 1280 (5.8 x 103)
1370 1380
1227 (6.3 x 103)
1385 1370 (1.3 x 104) 1390
1205 (6.7 x 103)
1240
1390 1369
1184 1388 1404 1427, 1315, 1232 1211 (2.0 x 103)
1398 1280 (2.0 x 104)
1398
1650 1650
glycolic acid
1733
glyoxylic acid
1733
oxalic acid
1735
1107, 1045 1074 (2.2 x 103) 1093
1242 (1.0 x 103) 1245 (1.8 x 103) 1235 (2.5 x 103)
Concentrations were typically 0.1-0.25 M in 0.1 M HC10,. (eb,
other bandsb 933 (2.2 x 103)
8CHlb.e
YC4b'd
VC4b'C
1100,1060
Infrared band frequencies (cm-') with integrated molar absorptivities
M-' cm-*) given within parentheses. C=O stretching mode. C-0 stretching mode. e CH3 bending mode.
molar absorptivity extracted from transmittance spectra. (The factor 2.5 in eq 1accounts for the optical properties of the thin-layer reflectance arrangement used here.5e) Determination of Ei(R,CO) and Ei(RCO,H) utilized in most cases the C=O and C-0 stretching bands, vc4 and vc-o, at 1700-1750 and 1200-1300 cm-l, respectively. Detection of the formation of the corresponding aldehyde intermediate faces the difficulty that suitably intense infrared bands that are uniquely characteristic of this functional group are lacking in some cases. While the v(C=O) feature is suitably intense, its fequency tends to be virtually coincident with that for the corresponding carboxylic acid. Fortunately, however, the initial formation of aldehyde tends to occur prior to the significant production of the carboxylic acid. For acetaldehyde, formed from ethanol electrooxidation, a feature at 935 cm-' which arises from the hydrated from was employed as detailed in ref 6b. A summary of the major SPAIR spectral frequencies seen for the reacting systems considered in Table I, together with tentative spectral assignments and cb values extracted from infrared transmittance measurements, is provided in Table 11. A comparative examination of the parameters in Table I uncovers several interesting features. With the exception of methanol, each alcohol forms only a small quantity of CO,, QXCO,) S 3 X mol cm-' during the anodic voltammetric sweep. (For methanol and a few other reactants, only a lower limit for Qf(CO,) is given since complete exhaustion of the thin-layer reactant occurs during the sweep.5b) These Qf(CO,) values were generally found to be virtually independent of the reactant concentration, from 0.025 to 0.25 M, suggesting that the CO, arises from electrooxidation of adsorbed species. That Qf(CO,) is in most cases significantly larger than that attributable to electrooxidation of adsorbed CO on the
basis of the corresponding 8c0 values, as for cyclobutan01 and 2-propanolelectrooxidation (vide supra), implies that significant amounts of dissociatively chemisorbed species are present in addition to CO. This appears most likely to be the case for the secondary alcohols, for which (0, + Oca) 5 0.4, indicating that likely presence of substantial amounts of other adsorbates at the initial potential. A number of systems, however, yield Qf(CO,) values that are significantly larger than that corresponding to ,,8 even though (eco ),8 1.0; i.e., no substantial quantities of other adsorbates are present at the initial potential. In most cases, production of this "additional" CO, occurs at potentials 0.2-0.4 V more positive than where adsorbed CO electrooxidation takes place. This suggests that the additional CO, is produced from adsorbate formed at surface sites made available upon CO electrooxidation. Comparison between the potentials characterizing removal of the adsorbed CO and the significant appearance of the various electrooxidation products, denoted E(B,,/2) and Ei,respectively (Table I), is particularly instructive. The onset of aldehyde formation from the aliphatic primary alcohols corresponds closely in each case to the potential region where CO removal occurs, as deduced from the proximity of E,(RCHO) and E(BC0/2). An interesting exception is provided b benzyl alcohol electrooxidation. As for benzaldehydeJ (vide infra), the electrooxidative removal of adsorbed CO occurs only at substantially (ca. 0.2-0.3 V) more positive potentials than for the aliphatic reactants, the formation of benzoic acid commencing only at about 0.8 V. This striking inhibition is most simply attributed to coadsorption of benzyl fragments." An illustration of these differences is provided in more detail in Figure 4, which consists of lots of the surface CO concentration, I'(C0) (= 2 X 10- Oc0
+
-
B
Organic Electrocatalytic Oxidations ut Pt and Pd
Langmuir, Vol. 6, No. 2, 1990 329 I
1-"
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E / V vs S C E Figure 4. Plots of quantity, Q, of carboxylic acid (solid trace), aldehyde (dotted trace), CO, (dashed trace) and adsorbed CO (dashed-dottedtrace) present as a function of potential during the voltammetric oxidation at 2 mV s-l of (A) 50 mM ethanol and (B)50 mM benzyl alcohol in 0.1 M HClO, on platinum, obtained from the correspondingspectra as outlined in the text. The Q(C0,) and Q(C0) values in A are magnified fivefold for
clarity.
mol cm-,), QCCO,), Q(RCHO), and Q(RCO0H) (dasheddotted, dashed, dotted, and solid traces, respectively) versus the electrode potential for 50 mM ethanol (Figure 4A) and 50 mM benzyl alcohol (Figure 4B) in 0.1 M HClO,. In both cases, aldehyde formation proceeds once the removal of the CO layer commences. Although significant formation of the carboxylic acid occurs only at more positive potentials, this species becomes the major product by the end of the voltammetric sweep, at 1.2 V. In contrast, the electrooxidation of the secondary alcohols to form ketones occurs at uniformly less positive potentials, as deduced from the Ei(R2CO)values (Table I), indicating the absence of inhibiting chemisorbed fragments. Generally, in contrast to C02 production the quantity of aldehyde, carboxylic acid, and ketone partial oxidation products formed increases approximately linearly with increasing reactant concentration.
Electrooxidation of Aldehydes and Formic Acid on Platinum and Palladium. Given the importance of aldehydes as intermediates in the electrooxidation of primary alcohols on platinum, it is of interest to examine the electrooxidation of these aldehydes as solution reactants. Figure 5 shows illustrative anodic-cathodic cyclic voltammograms for the electrooxidation of 50 mM acetaldehyde and benzaldehyde (solid and dashed traces, respectively) in 0.1 M HClO, at 0.1 V s-l, the dotted trace denoting the voltammogram in 0.1 M HClO, alone. The former shows a strong resemblance to the corresponding voltammogram for ethanol electrooxidation in Figure 2B, the major difference being the smaller amplitude of the feature at ca. 0.6 V for aldehyde with respect to alcohol electrooxidation. As detailed in ref 6b, these differences are due to the predominant formation of the intermediate aldehyde during the voltammetric feature around 0.6 V (also see Figure 4A). Nevertheless, the SPAIR spectra indicate that significant quantities of acetic acid are
0.4
0.0
I
1.2
E / V vs S C E Figure 5. Cyclic voltammograms obtained at 0.1 V s-l for the electrooxidation of 50 mM acetaldehyde (solid trace) and 50 mM benzaldehyde (dashed trace) in 0.1 M HClO, on platinum (electrodearea = 0.70 cm2). Dotted trace is corresponding voltammogram in 0.1 M HClO, alone.
also formed within this potential region on polycrystalline platinum (see Figure 5 of ref 6b). This point is also indicated in the data in Table I for the electrooxidation of propionaldehyde and benzaldehyde as well as acetaldehyde. The onset of carboxylic acid formation occurs a t potentials close to those for adsorbed CO electrooxidation and at Ei values comparable to those observed when the corresponding alcohols rather than aldehydes are used as solution reactants. The corresponding E(Oc0/2) values are also closely similar for these two reactant classes. Similarly to benzyl alcohol, the electrooxidation of adsorbed CO formed from benzaldehyde only occurs a t surprisingly positive potentials, ca. 0.7 V, followed by further oxidation of the electrogenerated solution aldehyde.5d A significant behavioral difference between the three aldehydes examined here-acetaldehyde, propionaldehyde,and benzaldehydeand the corresponding alcohols on platinum is that the former yield significantly greater Oco values. This greater extent of dissociative chemisorption is presumably also responsible for the larger Qf(C02)values obtained from aldehyde electrooxidation (Table I). Corresponding results were also obtained during the electrooxidation of acetaldehyde and benzaldehyde on polycrystalline palladium in 0.1 M HClO . Cyclic voltammograms for these reactions at 0.1 V s- f (again 50 mM reactant in 0.1 M HClO,) are shown as solid and dashed traces, respectively, in Figure 6; the dotted trace was obtained in 0.1 M HC10, alone (cf. Figure 5). These voltammograms, especially in comparison with the corresponding traces on platinum (Figure 5), show that the extent of electrooxidation on palladium is relatively small. Moreover, primary alcohols, even methanol, undergo no discernable electrooxidation on palladium;" corresponding SPAIR spectra for these systems yield no detectable adsorbed CO or CO, product formation, indicating the inability of palladium to engender dissociative chemisorption of primary alcohols. ~~~
~
~
(15) Capon, A.; Parsons, R. J. Electroanal. Chem. old, 44, 239.
Leung and Weaver
330 Langmuir, Vol. 6, No. 2, 1990 1
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E / V vs SCE
Figure 6. As for Figure 5 but on palladium (electrode area = 0.70 cm2).
Table 111. Extent of Adsorbed Carbon Monoxide Formation and Relatiopehip to Electrooxidation of Simple Organic Molecules on Palladium. ~(e/2):
reactant'
co
OcOC 1.0
formic acid
