Auger Spectroscopy, and Elect - American Chemical Society

Spectroscopy, and Electrochemistry. Ping Gao, Chiu-Hsun Lin, Curtis Shannon, Ghaleb N. Salaita, James H. White,. Scott A. Chaffins, and Arthur T. Hubb...
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Langmuir 1991, 7, 1515-1524

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Studies of Adsorbed Saturated Alcohols at Pt(111) Electrodes by Vibrational Spectroscopy (EELS), Auger Spectroscopy, and Electrochemistry Ping Gao, Chiu-Hsun Lin, Curtis Shannon, Ghaleb N. Salaita, James H. White, Scott A. Chaffins, and Arthur T. Hubbard' Department of Chemistry, Surface Center, University of Cincinnati, Cincinnati, Ohio 45221 -01 72 Received October 1, 1990. I n Final Form: January 2, 1991 Adsorption of a representative series of saturated alcohols from aqueous fluoride solutions at a Pt(ll1) electrode surface has been studied by means of Auger spectroscopy, electron energy-loss spectroscopy (EELS),and cyclic voltammetry (CV). Alcohols studied are as follows: methanol (MeOH);ethanol (EtOH); propanol (PrOH);2-propanol(2PrOH); 1-butanol (BuOH);racemic 2-butanol (2BuOH);(R)-(-)-2-butanol (R2BuOH); (S)-(+)-2-butanol (S2BuOH); 2-methyl-2-propanol (tert-butyl alcohol, tBuOH); 1-hexanol (HxOH);cyclohexanol (CyOH);1-heptanol(HpOH);1,4-butanediol[Bu(OH)z];1,Shexanediol [Hx(OH)z]. Each alcohol studied chemisorbs at Pt(ll1) from aqueous fluoride electrolyte solution (pH = 4; electrode potential typically -0.1 V vs Ag/AgCl reference; alcohol concentration 0.1 M or saturated) to form a layer that is stable under vacuum and in solution. Surface attachment of saturated alcohols is through the oxygen atom and possibly the adjoining carbon atom. The aliphatic moieties of EtOH, PrOH, 2BuOH, R~BuOH,S~BUOH, BPtOH, BuOH, ~BuOH,R~BuOH,S~BUOH, HxOH, HpOH, CyOH, and Bu(OH)z are in contact with the Pt(ll1) surface. However, the alkyl chain and one OH group of Hx(OH)2 are pendant; tBuOH likewise has a pendant alkyl moiety in the adsorbed state. "Short-chain" alcohols such as MeOH,EtOH, PrOH, 2PrOH, BuOH, and Bu(0H)zundergo partial decompositionas a result of adsorption from aqueous solution at Pt(ll1) to form a mixed adsorbed layer consisting of CO and the appropriate alkoxide. Electrochemical oxidation of the adsorbed intermediate is limited primarily to the carbon atom adjacent to the attached oxygen atoms.

Introduction The electrochemical oxidation of saturated alcohols has been the subject of continuous interest for many years in connection with electrocatalytic processes, particularly energy conversion by means of alcohol-based fuel cells. The electrooxidation kinetics of short-chain saturated alcohols, particularly methanol, at metal single-crystal surfaces is found to be sensitive to the metal crystallographic orientation.'* This surface structure sensitivity is generally attributed to adsorption processes and the reactivity of the adsorbed intermediates.'+' CO-like adsorbed intermediates have been postulated or identified3-8 under electrochemical conditions. Identifying the surface species taking part in the oxidation process has been the motivation for a large number of studies of methanol adsorption/decomposition on various metal substrates by using ultrahigh vacuum (UHV) and surface analysis techniques such as EELS, TDMS, XPS, and UPS. To date, studies of methanol on single-crystal surfaces of Pt," Pd,12 Ni,13 Al," Cu,16Fe,16 Zn," and Ru18 have been reported. Chemisorbed methoxy species have been identified at transition-metal (1) Adzic, R. R.; Tripkovic, A. V.; OGrady, W. E. Nature (London) 1982.2M. 137. ~-(2) Clakier, J.;Lamy,C.;Leger,J. M. J . Electroanal. Chem.Interfacial Electrochem. 1981,125, 249. (3) Lamy, C.! Leger, J. M.; Clavilier, J.; Parsons,R. J . Electroanal. Chem. Interfacial Electrochem. 1983, 150, 71. (4) Sun.S. G.: Clavilier, J. J. Electroanal. Chem. Interfacial Electrochem. i987, 2;36,96. (5) Bittinr-Cattaneo, B.; Santoe, E.; Vielstich, W.; Linke, U. Electrochim. Acta 1988,33,1499. (6) Beden, B.;.Hahn, F.; Lamy, C.; Leger, J. M.; de Tacconi, N. r.; h a , A. J. J. Electroanal. Chem. Interfacial Electrochem. Lema, R. 0.; 1989,261,401. (7) Beden,B.; Juanto,S.;Leger, J. M.;Lamy, C. J.Electroanal. Chem. Interfacial Electrochem. 1987,238, 323. (8)Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J . Electroanal. Chem. Interfacial Electrochem. 1981,121, 343. (9) Takky, D.; Beden, B.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. Interfacial Electrochem. 1988,256, 127.

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surfaces,lC'-lS particularly a t temperatures below 200 K. References 10-18 reflect some differences of opinion as to the nature of the products resulting from decomposition of chemisorbed methoxy species a t elevated temperatures. Several studies of the effects of alcohol molecular structure on the kinetics of electrocatalysis have also been reported; the significance of an "a-hydrogen effect" in promoting alcohol electrooxidation via adsorbed intermediates has been discussed.l*2l This interesting issue is related to the oxidation mechanisms of various alcohol structures, the nature of the intermediates, and their contributions to the electrocatalytic process. In contrast to methanol, the chemistry of ethanol and longer-chained alcohols on metal surfaces has not been fully explored. An ethoxy species was identified by EELS for ethanol adsorption on Cu(100) a t low temperature.16 Ethlidyne and propylidyne species were proposed as the major stable dissociation products for propanol and butanol on Pt(ll1) above 400 K, respectively, based on (10) Hegde, M. S. Proc. Indian Acad. Sci., Chem. Sci. 1984, 93,373. (11) Sexton, B. A. Surf. Sci. 1981, 102, 271. (12) (a) Gates, J. A.; Keemodel, L. L. J. Catal. 1983,83,437. (b) Bhattacharya, A. K.; Cheaters, M. A.; Pemble, M. E.; Sheppard, N. Surf. Sci. 1988,206,L845. (c)Chrirtmann, K.; Demuth, J. E. J. Chem. Phys. 1982, 76, 6318. (13) (a) Demuth, J. E.; Ibach, H. Chem. Phys. Lett. 1979,60,395. (b) Richter, L. J.; Gurney, B. A.; Villarrubia, J. S.;Ho, W. Chem. PhyS. Lett. 1984,111, 185. (14) (a) Chen, J. G.; Basu, P.; Hg, L.; Yates, J. T., Jr. Surf. Sci. 1988, 194,397. (b) Waddill, G. D.; Keemodel, L. L. Surf. Sci. 1987,182, L248. (15) Sexton, B. A. Surf. Sci. 1979,88, 299. (16) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1988, 133, L469. (17) Sen, P.; Rao, C. N. R. Surf. Sci. 1986, 172,269. (18) Paul, J.; Hoffman, F. M. Surf. Sei. 1986, 172, 151. (19) (a) Chrietov, M. V.; Sokolova, E. I. J . Electroanab Chem. Interfacial Electrochem. 1984, 175, 183. (b) Sokolva, E. Electrochim. Acta 1975, 20, 323. (c) Sokolva, E. Z.; Chrietov, M. V. J . Electroanal. Chem. Interfacial Electrochem. 1984,175, 195. (20)Takky,D.;Beden, B.; Leger, J.-M.; Lamy, C. J . Electrwnal. Chem. Interfacial Electrochem. 1985, 193, 159. (21) Leung,L.-W. H.; Weaver, M. J. J . Electroanal. Chem. Interfacial Electrochem. 1988, 240, 341.

0 1991 American Chemical Society

Gao et al.

1516 Langmuir, Vol. 7, No. 7, 1991 1

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Langmuir, Vol. 7,No. 7,1991 1517 11

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Figure 1. Cyclic voltammetryof adsorbed alcohols at Pt(ll1): solid curve, immersion into aicohol solution, followed by rinsing with pH 4 HF solution; dotted curve, as above, after 1-2 h under vacuum; (A) MeOH, (B) EtOH, (C) PrOH, (D) 2PrOH, (E) BuOH, (F) 2BuOH, (G) R2BuOH, (H) tBuOH, (I) HxOH, (J) CyOH, (K) HpOH, (L) Bu(OH)Z, (M) Hx(OH)*. Experimental conditions: scan rate, 5 mV/s; electrolyte, 10 mM KF adjusted to pH 4 with HF; temperature 23 f 1 OC; reference electrode, Ag/AgCl (1M KC1); adsorption from 8 mM HpOH or 100 mM other alkenols.

TDMS,XPS, and UPS data.22Several recent studieshave utilized reflection FTIR spectroscopy as an in situ probe of the electrooxidation pathways of ethano123-N and propanoln on p t ( l l 1 ) ~and on polycrystalline platinum (22) (a) Rendulic, K. D.; Sexton, B. A. J . Catal. 1982, 78, 126. (b) Sexton,B. A.; Rendulic, R. D.; Hughee, A. E. Surf. Sci. 1982,121,181.

(23) +eung,L.-W.H.;Chang,S. C.;Weaver,M. J. J.Electroam1. Chem. Interfacral Electrochem. 1989,266, 317. (24) h u n g , L.-W. H.; Weaver, M. J. J. Phys. Chem. 1988,93,4019. (26) Beden, B.;Morin,M. C.;Hahn, F.;Lamy,C. J.Electroanal. Chem. Interfacial Electrochem. 1987,229, 353. (26) Iwadta, T.;Vielstich, W. J. Electroaal. Chem. Interfacial Electrochem. 1988,257,319.

s u r f a ~ e s . ~These ~ - ~ preliminary ~ studies have shown that platinum is a good electrocatalytic substrate for alcohol oxidation. Although reaction mechanisms were postulated,23,26v27 the adsorbed intermediates, other than the partial decomposition product, CO, were difficult to identify due to experimental limitations. To our knowledge, no EELS data have been reported for alcohols other than methanol and ethanol. We have been utilizing a complementary set of techniques including electron energy loss spectroscopy (EELS), Auger spectroscopy, low-energy electron diffraction (27) Gao, P.; Chang, S.-C.; Zhou, 2.; Weaver, M. J. J. Electroaml. Chem. Interfacial Electrochem. 1989,272, 161.

1518 Langmuir, Vol. 7,No.7,1991

Gao et

Table I. Formulas for Obtaining Packing Densities from Auger Swctra

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Y

Q)

0

EtOH

Ec

Y

r = 1/gC

PrOH

r=

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r = rc/4

tBuOH

r = rc/4

HxOH

r = r,/6

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r = rc/7

CyOH

r = rC/6

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r = rc/4

WOHh

r = rC/6

(LEED), and electrochemistry to investigate electrochemical processes on well-defined metallic surfaces:28EELS provides direct information as to the nature of molecular species present on the surface;Auger spectroscopy provides quantitative information of elemental compositions and molecular packing densities of the adsorbed layer; LEED indicates the long-range order of the surface configuration of the adsorbed layer; cyclic voltammetry reveals the electrochemical behavior of the surface species. Previous systematic studies28 reveal that for a variety of organic molecules at Pt surfaces, the electrochemical properties of the adsorbed layer are not altered by lengthy exposure to vacuum. Accordingly, surface spectroscopic techniques in ultrahigh vacuum are suitable for the characterization of the solid/liquid electrochemical interface. Recently we have reported studies of adsorption and electrochemical oxidation of unsaturated alcoholsm and terminal alkenolsw on a Pt(111)electrode. Attachment to the surface is primarily through the unsaturated moieties (such as C-C, C=C, phenyl, or pyridyl ring). In parallel, the present work attempts to systematically explore various molecular structural effects of saturated alcohols upon adsorption, orientation, and electrocatalyticoxidation. For this purpose, 14 saturated alcohols were chosen based on the following considerations: (A) straight-chain alcohols from methanol up to heptanol, to explore the influence of the carbon-chain length; (B)branched alcohols to explore (28) (a) Hubbard, A. T. Chem. Reu. 1988,88,633. (b) Hubbard, A. T. Langmuir 1990,6, 97. (29) Gui, J. Y.; Kahn, B. E.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. J. Electroanal. Chem. Interfacial Electrochem. 1988,252,169. (30) Chaffins, S. A.; Gui, J. Y.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Kahn, B. E.; Hubbard, A. T. J. Electroanul. Chem. Interfacial Electrochem. 1990, 284, 67.

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-LOG C (MI Figure 2. Packing density of 2BuOH at Pt(ll1) vs concentration. Experimental conditions: electrode potential, -0.1 V vs Ag/AgCl reference (1M KCl); electrolyte 10mK KF adjusted to pH 4 with HF; rinsing with 0.1 mM HF; temperature, 23 & 1 "C.

the influence of the substitution of the 2-carbon and steric effects; (C) branched alcohols with nonsymmetric center to explore the influence of chirality; and (D) straightchain dialcohols to reveal the surface attachment and the influence of carbon-chain length. Reported here are EELS, Auger, and electrochemical data for chemisorbed alcohol layers on Pt(ll1) surfaces, with emphasis on identification of the adsorbed species and exploration of their electrochemical behavior. Comparison of voltammetric data obtained before and after evacuation indicates that adsorbed long-chain saturated alcohols (C4-C,) are stable under vacuum a t room temperature. However, short-chain alcohols (CI-C~)and their decomposition products formed during adsorption are removed to some extent by evacuation. Adsorbed saturated alcohols at Pt(ll1) showed only diffuse LEED intensity, indicating a lack of long-range order in the alcohol layer. Attachment of the saturated alcohols to the Pt surface is predominantly through the oxygen atom, perhaps with contributions from an adjoining carbon atom. CO is the main dissociation product for short-chain alcohols including MeOH, EtOH, PrOH, 2PrOH, and BuOH. The surface orientation of each saturated alcohol studied is proposed. Electrochemical oxidation of the adsorbed intermediates is limited primarily to the carbon atom adjacent to the surface attached oxygen atom.

Experimental Section Experimental procedures employed in this work were as described p r e v i ~ u s l y . ~The ~ Pt(ll1) single-crystal surfaces employed for this work were oriented and polished such that all faces were crystallographicallyequivalent to (111).All faces were cleaned simultaneously by bombardment with Ar+ ions at 500 eV and annealed by reaistance heating (lo00 K) in UHV. Cleaning and annealing were continued until LEED showed an ordered surface and Auger spectroscopy demonstrated that the surfaces were free of detectable impurities. The clean, ordered Pt(ll1) surface was isolated in an argon-filled antechamber prior to immersion into buffered aqueous electrolyte solutions containing the subject alcohol adsorbates. Electrode potentials were measured and controlled by means of three-electrode electrochemical circuitry based upon operational amplifiers. The reference and auxiliary Pt electrodes were contained in a Pyrex glass electrochemical cell. The cell was introduced into the antechamber by means of a bellows assembly and gate valves. Solutions employed for adsorption procedures and voltammetry contained 10 mM KF (Puratonic Grade, Johnson-Mat(31) Hubbard, A. T. In Comprehensive Chemical Kinetics; B d o r d , C. H., Tipper, D. F. H., Eds.; Elsevier: Amsterdam, 1988, Vol. 28, Chapter 1.

Langmuir, Vol. 7, No. 7,1991 1519

Adsorbed Saturated Alcohols at Pt(ll1) Electrodes

electrode potential -log v vs C" Ag/AgCl 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.1 1 -0.28 1 -0.1 2.1 -0.1 1 0.081 1 -0.34

Table 11. Auger and Electrochemical Data for Alcohols at Pt(ll1). elemental packing molecular packing density, nmol/cm* density, nmol/cm2 from from IclZpt" ZolIpt" ZpIIIpt" carbon oxygen Zc/Zpto Zpt/Zpto pC/cmZ 0.162 0.113 0.801 0.516 0.239 0.52 0.64 190 0.151 0.616 0.928 0.481 0.107 0.24 0.11 123 0.395 0.123 0.694 1.257 0.214 0.42 0.46 677 0.199 0.083 0.857 0.634 0.145 0.21 0.15 169 0.389 0.170 0.756 1.239 0.295 0.30 0.31 279 0.434 0.244 0.676 1.556 0.39 0.42 0.424 444 0.383 0.315 0.703 1.374 0.549 0.34 0.39 302 0.378 0.157 0.731 1.356 0.274 0.34 0.35 319 0.380 0.158 0.731 1.422 0.324 0.36 0.34 303 0.642 0.221 0.458 2.046 0.385 0.34 0.47 523 0.588 0.093 0.640 2.202 0.191 0.37 0.35 598 0.362 0.189 0.453 2.016 0.329 0.29 0.41 537 0.429 0.278 0.671 1.366 0.484 0.34 0.33 270 0.860 0.171 0.443 3.222 0.322 0.54 0.56 493

n"ox (oxidation of n', C-atom (complete compd n, adjacent to 0) oxidation) MeOH 4 5 EtOH 5 11 PrOH 17 17 2PrOH 8 23 BuOH 9 23 9 2BuOH 12 23 9 R2BuOH 9 23 9 S2BuOH 10 23 9 tBuOH 9 23 7 HxOH 16 35 15 CyOH 17 33 19 HpOH 19 41 15 Bu(OH)z 8 20 8,16 Hx(OH)z 9 33 9 a Experimental conditions: C" = molar concentration of alcohol adsorbate; supporting electrolyte, 10 mM HF (pH 4, HF); rinsing solution, HF (pH 4). Auger spectroscopy: incident beam, 100 nA at 2000 eV, normal incidence; modulation, 5 V (p-p). Reference electrode, Ag/AgCl (1 M KCl); temperature, 23 1 "C.

Q-a,

they, Inc., Seabrook, NH) and were adjusted to pH 4 with HF solution (Fisher Scientific, Pittsburgh, PA). Solutions were prepared from water distilled pyrolytically in pure 02 through a Pt gauze catalyst (800"C). During the immersion procedure, solutions were transferred through jacketed Teflon tubing; the jacket was continuously purged with argon to minimize diffusion of air through tubing walls into the solution. Adsorption of each alcohol on Pt(ll1) was carried out for 3 min at a controlled potential of -0.1 V vs an Ag/AgCl reference electrode prepared with 1M KC1, followed by rinsing the surface 3 times with dilute HF solution (pH 4). After evacuation of the antechamber, the crystal was transferred into the main chamber for surface characterization by EELS, LEED, and Auger spectroscopy. The subject alcohols were obtained from Aldrich Chemical Co. (Milwaukee,WI) and except for PrOH, Bu(OH)z,and CyOH were used as received. OH

I

CHJOH

CHJCH~OH

CH3CH2CH20H

CHjCHCH3

MeOH

EtOH

PrOH

2PrOH

OH

I

CH3(CH2)2CH20H

CH3CHCH2CH3

BuOH

PBuOH

CH3CHz \ CH3 ,C-OH H SPBuOH

(CHs)&-OH tBuOH

CH3CH2

C$ ,C-OH

.I4.

R3BuOH CH,(CH2)&H20H HxOH

Auger spectroscopy. The incident beam was 100 nA at 2000 eV, incident normal to the surface. (i) The Auger signal, IC,due to carbon (272 eV) was measured and normalized to the Pt Auger signal (161 eV) from the clean Pt(ll1)surface.= The elemental packing density was obtained from (Zx/Zpto)by means of eq 1

where Bc is calibrated with reference to hydroquinone present as an ordered chemisorbed layer at Pt(111)" (Bc= 0.314 cmz/ nmol), Li is the fraction of carbon atoms located at level i of the surfacelayer (i = 1isadjacenttothesurfaceandNistheoutermoet layer), jx is the attenuation factor for Auger electrons at 235 eV by the chemisorbed HQ layer,apa and Mi is the number of carbon or oxygen atoms located on the average path from the emitting atom to the detector. The specific forms of eq 1appropriate to the adsorbates involved in this study are given in Table I. Contributions of LifxMiterms are typically modest, less than 25 % for small molecular adsorbates; self-consistencybetween the two methods of measurement provides an immediate check of the calculations. (ii) The Auger signal at 161 eV due to the Pt substrate was measured for the clean (Zpt") and coated (Zpt) surface. Molecular packing density, r, was obtained from the ratio (Zpt/Zpto) by use of eq 2

UoH CyOH

CH3(CH2)5CH20H

HOCH2(CH2)2CH20H

HOCH2(CH2)4CH20H

HpoH

BU(W2

Hx(OH12

PrOH was purified as follows: 1 mL of Br2 was added to 50 mL of PrOH to convert traces of allyl alcohol to the dibromo alcohol, after which pure PrOH was isolated from the mixture by fractional distillation.92 Bu(0H)z was purified by the same procedure as for PrOH. CyOH was washed with a 1M aqueous ferrous sulfate solution to remove peroxides/aldehydes; solid 2,4-dinitrophenylhydrazonewas added, and CyOH was recovered by distillation under nitrogen.82 EELS spectra were obtained by use of an LK Technologies spectrometer (Bloomington IN). Beam current was approximately 120 PA; beam energy was 7 eV; spectrometer resolution was about 12 meV (100 cm-1). Packing densities (nmol/cm2) of adsorbed alcohols were measured by use of two recently developed methods based upon (32) Perin, D. D.; Armarego, W. L.F.; Perrin, D.R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980.

n(1-J$,r) N

(Ipt/Zpto)=

(2)

ill

where Ji is the number of carbon atoms in the ith level of the adsorbed molecule and K , is equal to 0.165 cmz/nmolbased upon a Pt(ll1)-hydroquinone adsorbed layer at saturation."

Results and Discussion 1. CyclicVoltammetry. ThePt(ll1)electrodesurface was immersed at controlled electrode potential into an aqueous electrolyte containing the alcohol, followed by rinsing with pure electrolyte. The cyclic voltammogram of the surface layer was recorded, first positive- then negative-going, giving the solid curves shown in Figure 1. The controlled electrode potential during adsorption was (33)Batina, N.; Frank, D. G.; Gui, J. Y.;Kahn,B. E.; Lm,C.-H.; Lu, F.; McCargar,J. W.; Salaita, G. N.; Stern,D. A.; Zapien, D. C.; Hubbard, A. T. Proceedings of the 4th International Fiecher Symposium, Electrochim. Acta 1989,34, 1031. (34) Lu,F.; Salaita, G.N.; Laguren-Davidson, L.;Stem, D. A.; Wellner, E.; Frank, D. G.; Batina, N.; Zapien, D. C.; Walton, N.; Hubbard, A. T. Langmuir 1988,4,637.

1520 Langmuir, Vol. 7,No. 7, 1991

Gao et al.

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3 z

3 z

V 0

W 0

0

2000

ENERGY LOSS ( c m - 1 )

ENERGY LOSS (cm-1)

ENERGY LOSS ( c m - 1 )

G

H

0

n V \ U " c W

tW

(I:

(I:

LL

D:

t-

z 3 U 0

0

1000 2000 ENERGY LOSS ( c m - 1 )

3000

4000

ENERGY LOSS ( c m - 1 )

Langmuir, Vol. 7, No. 7, 1991 1521

Adsorbed Saturated Alcohols at P t ( l l 1 ) Electrodes

J

0

u

n

\

U

u

W IU

w Iz

3 0 V

0

1000 2000 ENERGY LOSS ( c m - 1 )

ENERGY LOSS ( c m - 1 )

K

3000

4000

HOCH2CHrCH2CH20H

u U

\ U v

W I-

U

rr: I-

3

0 U

W

0

1000

2000

3000

IO00

0

4000

ENERGY LOSS ( c m - 1 )

0

2000

3000

4000

ENERGY LOSS ( c m - 1 )

1000

2000 ENERGY LOSS ( c m - 1 )

3000

4 000

Figure 3. Vibrational s ectra of alcohols: (A) (upper curve) EELS spectrumof MeOH adsorbed at Pt(lll), (lower curve) IR spectrum of MeOH vapor at 150 O 8 ; = (B) (upper curve) EELS spectrum of EtOH adsorbed at Pt(lll), (lower curve) IR spectrum of EtOH vapor at 150 OC;= (C) (upper curve) EELS spectrum of PrOH adsorbed at Pt(lll), (lower curve) IR spectrum of PrOH vapor at 150 OC;= (D) (upper curve) EELS spectrum of 2PrOH adsorbed at Pt(lll),(lower curve) IR spectrum of 2PrOH vapor at 150 "C;=(E) (upper curve) EELS spectrum of BuOH adsorbed at Pt(lll), (lower curve) IR spectrum of BuOH vapor at 150 "C;= (F) (upper curve) EELS spectrum of 2BuOH adsorbed at Pt(lll), (lower curve) IR spectrum of 2BuOH vapor at 150 OC;= (G)(upper curve) EELS spectrum of R2BuOH adsorbed at Pt(lll),(lower curve) IR spectrum of R2BuOHvapor at 150OC;= (H) (upper curve) EELS spectrumof tBuOH adsorbed at Pt(lll), (lower curve) IR spectrum of tBuOH vapor at 150 OC;= (I) (upper curve) EELS spectrum of HxOH adsorbed at Pt(lll), (lower curve) IR spectrum of HxOH vapor at 150 OC;= (J) (upper curve) EELS spectrum of CyOH adsorbed at Pt(lll), (lower curve) IR spectrum of CyOH vapor at 150 OC;= (K) (upper curve) EELS spectrum of HpOH adsorbed at Pt(lll), (lower curve) IR spectrum of EtOH vapor at 225 OC;= (L) (upper, solid curve) EELS spectrum as received Bu(OH)2 adsorbed at Pt(lll), (upper, dotted curve) EELS spectrum of twice-distilled BU(OH)~ adsorbed at Pt(lll),(lower curve) IR spectrum of Bu(OH)r vapor at 225 OC;S (M) (upper curve) EELS spectrum of Hx(0H)z adsorbed at Pt(lll), (lower curve) IR spectrum of Hx(0H)z vapor at 290 O C . S Experimental conditions: adsorption from 8 mM HpOH or 100 mM other alcohols in 10 mM KF adjusted to pH 4 with HF, followed by rinsing with HF solution (pH 4); EELS angles of incidence and detection 6 2 O from the surface normal; beam energy, 7 eV; beam current, 120 PA; EELS resolution 10 meV (80 cm-l) fwhm, FTIR resolution about 4 cm-'.

chosen to be in the range where Faradaic current was minimal as indicated in Table 11. Alcohol concentration was 0.1 M in all cases except for HpOH for which a saturated solution was employed (about 8 mM). The electrolyte was 10 mM KF, adjusted to pH 4 with HF.

The faradaic charge, Q,due to oxidation of the Pt(ll1) surface and adsorbed alcohol layer was measured by digital integration of the voltammetric current; the background charge, &b, due to oxidation of the Pt(ll1) surface was determined by performing a second positive-going vol-

1522 Langmuir, Vol. 7, No. 7, 1991

Gao et al.

tammetric scan in the pure electrolyte. The resulting charge, Q - Qb, for oxidation of the adsorbed alcohol layer is given in Table I. Separate voltammetric experiments were carried out in which the surface containing the alcohol adsorbed layer was rinsed with dilute electrolyte (HF, pH 4) and transferred to ultrahigh vacuum for a period of about 1 h, followed by cyclic voltammetry (dotted curves in Figure 1). Voltammograms obtained before and after evacuation are essentially identical (solid curve vs dotted curve), indicating that the adsorbed layer formed at Pt(ll1) from each of these alcohols is stable under vacuum, permitting quantitation by Auger spectroscopy and further characterization with EELS. The agreement between the two sets of data is less strong for short-chain alcohols (CrC3) indicating that some loss of adsorbed material occurs upon exposure to vacuum. Potential scans to fairly positive (ca. 0.9 V) limits can cause significant disordering of a single-crystal electrode surface. Therefore, Auger and EELS analysis of adsorbate-covered surfaces were always performed on freshly prepared samples (ion-bombarded, annealed, and coated with adsorbate), followed by electrochemical characterization. Each positive-goingvoltammogram displayed two stages of oxidation of the adsorbed layer: the first occurred near 0.2 V, and the second near 0.9 V vs the Ag/AgCl reference electrode. The 0.2-V feature ismore distinct for the smaller alcohols, such as MeOH, EtOH, PrOH, and 2PrOH, than for the larger alcohols, although each of the alcohols gave detectable current in that potential region. This first oxidation process appears to be associated with adsorbed CO produced in significant amounts during adsorption of various short-chain alcohols, as discussed below. Accordingly, the oxidation processes occurring at more positive potentials, near 0.9 V, are associated with the adsorbed “intact” alcohols or alkoxides. PrOH displays an oxidation peak near 0.38 V, in addition to smaller peaks near 0.2 and 0.9 V due to the side-products of its adsorption (CO and propene). Evidently, adsorbed PrOH undergoes efficient oxidation to COz at potentials near 0.38 V. 2. Auger Spectroscopy. Molecular packing densities of adsorbed alcohols were obtained from Auger spectra as described in the Experimental Section and in ref 33. The results are given in Table 11. An isotherm for adsorption of racemic 2BuOH is shown in Figure 2. Auger data for each of the alcohols studied indicate that the adsorption behavior of 2BuOH as a function of alcohol concentration is typical of that of the other alcohols: adsorption begins to be appreciable at about 1mM and approaches a plateau near 0.1 M. Accordingly, 0.1 M solutions were employed to prepare adsorbed layers at Pt(ll1) for EELS, Auger (packing density), and voltammetric measurements wherever permitted by the stability of the alcohol. 3. Electron Energy Loss Spectroscopy (EELS). EELS spectra of adsorbed saturated alcohols were obtained by immersing the Pt(ll1) electrode at controlled electrode potential into an aqueous solution of the alcohol containing 10 mM KF adjusted to pH 4 with HF, Figure 3. The upper curve in each instance is the EELS spectrum and the lower curve is the reference infrared spectrum of the unadsorbed alcohol.3s Assignments of the EELS bands based upon comparison with the standard IR spectra are given in Table 111. EELS spectra of the Pt(ll1) surface after immersion into solutions of MeOH, EtOH, PrOH, 2PrOH, and BuOH displayed bands near 1800 and 2060 cm-1 assignable to

adsorbed CO from decomposition of the alcohols.3s To demonstrate that adsorbed CO is indeed the product of the dissociative chemisorption and not a contaminant from the vacuum system, we performed an experiment in which a freshly argon ion bombarded and annealed Pt(ll1) surface was exposed to vacuum for 1 h, after which an EELS spectrum (0-4000 cm-l) was recorded. The essentially featureless spectrum displayed no peaks due to adsorbed carbon containing impurities. Although in practice it is not possible to completely eliminate all of the CO from a UHV system, we conclude that the Pt(ll1) sample can be kept free of such impurities for periods of 1-2 hand that the EELS spectra reflect changes in surface composition resulting from adsorption of alcohols from solution. Using absolute EELS intensities alone, it is difficult to quantify the amount of dissociative chemisorption occurring on these surfaces. EELS intensities are extremely sensitive to the position of the sample; therefore, comparisons of intensity data are most useful in identifying of qualitative trends. Surface coulometry provides a more accurate measure of the true CO surface concentration; the results of these measurements are presented in the following section. Decomposition to CO was much less noticeable for the other, larger alcohols (2BuOH, R2BuOH, S ~ B U O H+BuOH, , HxOH, CyOH, HpOH, Bu(OH)~, and Hx(OH)2,Figure 3F-M). Evidently, the tendency toward dissociation of these alcohols to form adsorbed CO decreases with increasing chain length. The dividing line occurs at about Cq: adsorbed BuOH and tBuOH show detectable CO bands while adsorbed 2BuOH, R2BuOH, and S2BuOH do not. Qualitatively, the EELS spectra of adsorbed alcohols display the followingcharacteristics: the dominant feature above 2000 cm-l is the C-H stretching mode, while the C-H stretching and C-0 stretching modes are the most prominent bands in the fingerprint region. The surface EELS spectra closely resemble the IR spectra of the corresponding undissociated parent alcohd except for the absence of OH stretching bands. The absence of these losses from the EELS spectra is a strong indication that adsorption results in loss of the hydroxyl hydrogen and formation of an alkoxide adsorbed layer (containing, in the case of the C1 to C3 alcohols, an admixture of adsorbed CO). Consideration of the CH stretching region of the spectrum provides further evidence to support this conclusion. If the primary mode of bonding to the surface were through the aliphatic portion of the alcohol,one would expect the CH stretching frequencies to be red-shifted from their gas phase values, which clearly is not the case for the spectra reported here. It should be noted that although chemisorption through the aliphatic portion of the molecule has been ruled out by EELS, the packing density measurements mentioned previously indicate that the aliphatic moieties are weakly surface active. The CH stretching and fingerprint regions of the EELS spectra are similar for both straight-chain and branched, secondary alcohols. This is as expected for molecules oriented roughly parallel to the surface of the electrode, since in this orientation the conformations of the primary and secondary carbon chains on the surface are such that the in-plane and out-of-plane molecular motions are equivalent in the surface frame of reference for both classes of molecule. On the other hand, had the vertical orientation been favored, in-plane motions in the straight-chain alcohols would correlate with out-of-plane motions in branched alcohols, resulting in dissimilar EELS patterns for the two cases, in contrast to our findings. One

(35) Sadtler Standard Infrared Vapor-Phase Spectra; Sadtler Research Laboratories: Philadelphia, 1984.

(36) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations;Academic Press: New York, 1982.

Langmuir, Vol. 7,No. 7, 1991 1523

Adsorbed Saturated Alcohols at P t ( l 1 l ) Electrodes

MeOH 2928 2062 1818 1357

EtOH 2945 2055 ls00

1040

1080 800 450

Table 111. Assignments of EELS Bands for Adsorbed Alcohol Layers PrOH 2PrOH BuOH 2BuOH R2BuOH 2937 2940 2940 2937 2950 2028 2038 2026 2020 1351

1348 1213 1100 967

1114

1100 780

tBuOH

HxOH

2956

2924

2938

2928

1417 1180 1033 860 630

1360

1434 1264 1150 8601947 510

1446 1305 1080

1112 863

CyOH

133511438

HpOH

1368 1244 1062 850 Bu(OH)z 3266 2957 205011806 1537

1371 1220 1074 875 Hx(OH)z 2932 1360

1070 8301996

1080

description

C-H stretch linear C-O stretch bridged C-O stretch C-H bend C-H bend C-O stretch CH rock P t C O stretch description 0-H stretch C-H stretch linear C-O stretch C = C stretch C-H bend C-H bend C-O stretch CH rock C C stretch

I

Figure 4. Structural models of adsorbed alcohols at Pt(ll1): (A) MeOH; (B) EtOH;(C)PrOH; (D) 2PrOH; (E)BuOH; (F) 2BuOH; (G)R2BuOH; (H) tBuOH; (I) HxOH; (J) CyOH; (K)HpOH; (L) Bu(OH)z; (M) Hx(0H)z.

important difference in adsorption behavior between straight-chain and branched-chain alcohols was revealed by EELS spectroscopy: secondary and tertiary alcohols are less likely to form CO upon adsorption than their straight-chain analogues. For example, the branched secondary butanols and tBuOH all adsorb as the alkoxide in contrast to BuOH, which forms a significant amount of CO. 2PrOH, which also dissociatively chemisorbs, is an exception to this general trend. The adsorption of diols at Pt(ll1) is a more complex phenomenon than that of similar alcohols as evidenced by both Bu(OH)2 and Hx(OH)2. Bu(OH)2 offered some special problems of purification: Aqueous solutions prepared from the alcohol as received from the supplier reacted with the Pt(ll1)surface to form an adsorbed layer having a prominent, broad band near 1675 cm-l, Figure 3L (upper solid curve), suggesting the presence of traces of aldehydes such as from partial oxidation of the alcohol by atmospheric oxygen. Double-distillation of Bu(OH)2 prior to use (as described in the Experimental Section) led to an adsorbed layer for which the EELS spectrum

closely resembled that of 3-butene-l-o1,3OFigure 3L. This spectrum bears no resemblance to the IR spectrum of the unadsorbed parent molecule in contrast to our findings for all other alcohols studied. In particular, the peak a t 1537 cm-l is the characteristic stretching frequency of a C-C double bond. Thus, either a trace of the alkenol is present in the purified material which adsorbs in preference to the intended diol, or the alkenol forms at the Pt(ll1) surface as a result of the adsorption process. The EELS spectrum of adsorbed Hx(OH)2 is shown in Figure 3M. If Hx(OH)2is oriented with its molecular axis nearly normal to the surface as the Auger packing density data indicate, we would expect to observe a loss at ca. 3200 cm-l due to the pendant OH groups, contrary to our results. There are a number of reasons for such a mode to be absent from the EELS spectrum. First, if the average OH orientation is parallel to the electrode surface, the contribution to the EELS intensity from dipole-typescattering is expected to be a minimum due to screening of the dynamic dipole by the Pt conduction electrons. Second, the remaining intensity contributed by impact scattering

Gao et al.

1524 Langmuir, Vol. 7, No. 7, 1991 is expected to be significantly broadened due to intramolecular H bonding within the adsorbed layer resulting in an overall decrease in the peak intensity. Finally, the intrinsic impact scattering cross section for this mode may be quite small. In summary, consideration of both the EELS spectra and the Auger packing density measurements leads us to propose the structures shown in Figure 4 for the adsorbed molecular layers. The behavior of MeOH, EtOH, PrOH, 2PrOH, BuOH, HxOH, CyOH, HpOH, and Bu(0H)z is indicative of conformation in which the alkyl moieties are able to interact with the Pt(ll1) surface. Only two of the alcohols studied appear to adopt conformations having a pendant alkylgroup (tBuOH and Hx(OH)2). The pendant conformation of tBuOH is of course dictated by its highly branched and bulky structure. On the other hand, the pendant conformation of adsorbed Hx(OH)2 may be due to the hydrophilic nature of the OH-containing pendant chain. These models are not intended to imply that other, closely related molecular conformations do not exist at the surface, but rather they suggest probable structures that are consistent with EELS, Auger, and electrochemical data. 4. Surface Coulometry. Independent measurement of the molecular packing density, I', of the alcohol and the faradaic charge, Q - Qb, to oxidatively desorb the alcohol permits calculation of the average number of electrons, no,, per alcohol molecule no, = (8- Qb)/(FAr) where F is the Faraday constant and A is the true area of the Pt(ll1)electrode surface. Measured values of no, for saturated alcohols adsorbed a t Pt(ll1)are given in Table 11. The tabulated values include contributions due to oxidation of adsorbed CO produced during adsorption of MeOH, EtOH, PrOH, 2PrOH, and BuOH. For example, oxidation of the adsorbed layer formed from MeOH requires nox= 4 electrons per molecule, compared with a limiting value of five electrons for oxidation of adsorbed methoxy species to CO2

CH30(ads)+ H,O

-

CO,

+ 3H,O C,H,O(ads) + 5H,O C,H,O(ads) + 7H,O

-

CH,(CH,),O(ads)

+ 11H' + lle3C0, + 17H' + 17e4c0, + 23H' + 23e2C0,

However, PrOH (no, = 17) approaches the limit for complete oxidation to COZ(17 electrons). Surface coulometry can be used to estimate the amount of CO formed as a product of the dissociative chemisorption of any given alcohol. As noted previously, the voltammetric peak occurring at approximately 0.2 V is due to oxidation of adsorbed CO. By comparing the charge under this peak in the voltammogram of an adsorbed alcohol to the charge under the corresponding peak in the voltammogram of a surface that was saturated with CO, one can calculate the amount of CO coadsorbed with each alcohol fragment. To this end, we performed a reference experiment in which the Pt(ll1) electrode surface was coated with 1.35 nmol/cm2 CO (0 = 0.78, based on the theoretical limiting packing density). The amount of charge under the 0.2-V peak was found to be 167 CcC/cm2; the corresponding charge in Figure 1A is 25 pC/cmz. Thus, we estimate the coverage of CO formed as a result of MeOH dissociation to be 0.26 nmol/cm2 (8 0.15). This is in

-

+ 3H,O

-

CO,

+ C,H,CO,H + 9H'

CHI

I

CH3CH2CHO(ads) + 3Hz0

-

COz

+

CH3CH2C02H + 9H'

+ 9e+

9e-

0

+

(CH3)3CO(ads)

2H20

I1

* COP + CH3CCH3 +

CH,(CH,),O(ads)

+ 5H,O

-

CH,(CH,),O(ads)

+ 5H,O

-

O(CH,),O(ads)

+ 3H,O

-

CH,=CHCH,CH,OH(ads)

+ 5H' + 5e-

EtOH (no, = 5),2PrOH ( n , = 81,and BuOH (nox= 9) also fall short of the limiting values for complete oxidation C,H,O(ads)

close agreement with the value of 0.2 found by Chang et al. in studies of MeOH electrooxidation in solution.37 The longer-chainalcohols,which did not form detectable amounts of adsorbed CO, also stopped far short of complete oxidation: The adsorbed C4 to C7 alicyclicalcohols exhibit noxin the range from 8to 19,compared with limiting values of 23 to 41. Evidently, only the carbon atoms closest to the point of attachment to the surface undergo oxidation. Although the oxidation products of these reactions have not been analyzed, the no, values are suggestiveof oxidation of the carbon atom nearest the surface to COz and the remainder of the molecule to the corresponding carboxylic acid

HO(CH,),O(ads) m O ( a d s )

+ 3H,O

+ 7H20

-

7H'

+

7e-

+ C3H7C0,H + 15H' + 15e2C0, + C4HgC0,H + 15H' + 15eCO, + HO(CH,),CO,H + 8H' + 8e+ 6H,O 2C0, + HOCH,CO,H + 16H' + 1 6 6 2C0,

-

-

2C02

CO,

+ HO(CH,),CO,H + 9H' + 9e-

+ H02C(CH2)2C02H+ 19H' + 19e-

Measured values of n , for adsorbed saturated alcohols, Table 11, are consistent with previous studies of related compounds, including alkenoic acids,w alkenols,gO and alke n e ~ . ~Also @ included in Table I1 are the number of electrons required for complete oxidation of the adsorbed alcohols (n'ox)and the number of electrons required for partial oxidation as discussed above (n",). The proposed mechanism of partial oxidation agrees quite well with the measured n , values.

Acknowledgment. Acknowledgment is made to the Air Force Office of Scientific Rasearch, the Department of Energy, and the Gas Research Institute for support of this research. The technical assistance of Arthur Case, Frank Douglas, Douglas Hurd, Richard Shaw, and Vickie Townsend is gratefully acknowledged. (37) Chang, S.-C.;Leung, L.-W. H.; Weaver, M.J. J. Phy.9. Chem. 1990,94,6013. (38) Batina, N.;Chaffii, S. A.;Kahn, B. E.; Lu, F.; McCargar, J. W.; Rovang, J.; Stem,D. A; Hubbard, A. T. Catal. Lett. 1989,3, 276. (39) Batina, N.;Chaffii, S. k ;Gui, J. Y.;Lu, F.; McC ar, J. W.;

Rovang, J.; Stern,D. A.; Hubbard, A. T. J. Electroanal. Chemynterfacial Electrochem. 1990,2.84, 81.