Surface chemistry of five-membered heteroaromatics at Pt(III) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry: furan, pyrrole and thiophene

37

J. ElectroanaL Chem ., 305 (1991) 37-55 Elsevier Sequoia S .A ., Lausanne

Surface chemistry of five-membered heteroaromatics at Pt(111) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry : furan, pyrrole and thiophene John Y . Gui, Donald A . Stern, Frank Lu and Arthur T. Hubbard Department of Chemistry and Surface Center, University of Cincinnati, Cincinnati, OH 45221-0172 (USA) (Received 26 June 1990 ; in revised form 3 October 1990)

Abstract Reported here are studies of the chemisorption . surface vibrational spectroscopy, elemental and molecular packing densities, and electrochemical reactivity of three related five-membered heterocycles at Pt(111) : pyrrole (PRR), furan (FRN), and thiophene (TPE). Packing density and stoichiometry were investigated by the use of Auger spectroscopy . None of the adsorbed layers exhibited long-range order detectable by LEED . Vibrational spectra of the chemisorbed species were obtained by means of electron energy-loss spectroscopy (EELS), and were compared with infrared spectra of the unadsorbed compounds . Electrochemical oxidation of the adsorbed molecules was explored by the use of cyclic voltammetry (CV) . The results reveal that PRR adsorbs at Pt(11 1) mainly in a horizontal orientation with the ring intact ; electrochemical oxidation of adsorbed PRR proceeds completely to NO t and CO2 . However, FRN is converted by hydrolytic, ring-opening isomerization to adsorbed pi-bonded butenoic acid (BTA) ; electrochemical oxidation of adsorbed BTA proceeds to CO2 . TPE forms a mixed layer consisting of S atoms and adsorbed TPE molecules S-bonded and near-vertically oriented ; adsorbed TPE is oxidized to COt and SOO - .

INTRODUCTION

Studies have been reviewed recently [1,2] in which organic compounds adsorbed from solution at well-characterized Pt(111) surfaces were characterized as to vibrational spectra by electron energy-loss spectroscopy (EELS), as to surface stoichiometry and packing density by Auger spectroscopy, and as to surface layer electrochemical reactivity by cyclic voltammetry (CV). Results of this previous work demonstrate the stability of a wide range of adsorbed organic molecules at Pt, Ag, and other metal surfaces in solution and in vacuum . Interfacial potential sharply influences the nature and abundance of adsorbed species ; pH also exerts a strong effect. Intermolecular interactions between adsorbed molecules are strong, including 0022-0728/91/$03 .50

0 1991 - Elsevier Sequoia S.A .

38 hydrogen-bonding, dimerization, electrostatic induction, molecular re-orientation, and steric blocking of reactivity. Adsorbate molecular orientation or mode of surface attachment profoundly influences surface chemical and electrochemical reactivity, including energetics, kinetics, and nature of the reaction products. Electrochemical oxidation and reduction of adsorbates containing aliphatic chains are localized to points of attachment of the chains to the surface . Results to date also illustrate the effectiveness of techniques such as EELS and Auger spectroscopy when combined with CV for investigation of metal-liquid interfaces . In the present article we explore the structure and electrochemical reactivity of adsorbed species formed at Pt(111) in aqueous solutions of three related five-membered heterocycles : pyrrole (PRR), furan (FRN), and thiophene (TPE) . The results indicate that each adsorbate behaves differently : PRR is mainly adsorbed in a horizontal orientation ; FRN hydrolyzes, with ring-opening, to form adsorbed butenoic acid ; and TPE forms a mixed adsorbed layer consisting of S atoms and near-vertically oriented TPE molecules . The adsorbed layers lack long-range order as judged by LEED . Similarities in surface chemistry also exist among the three adsorbates : complete electro-oxidation of the adsorbed intermediates correlating with the nature of the adsorbed states, and the hydrolysis of adsorbed parent molecules although the products and extent of the hydrolysis differ . EXPERIMENTAL Reported here are experiments in which an electrode surface containing an irreversible adsorbed layer is investigated by means of specially constructed instrumentation [1-3] : surface structure was examined by means of low-energy electron diffraction (LEED) ; surface composition and molecular packing density were determined using Auger spectroscopy ; vibrational bands of the adsorbed layer were observed by means of electron energy-loss spectroscopy (EELS) ; and electrochemical properties of the surface were explored using voltammetry and coulometry . The Pt(111) surfaces employed for this work were oriented and polished such that all six faces were crystallographically equivalent . All faces were cleaned simultaneously by Ar' ion bombardment at 700 eV under 5 X 10-5 Ton Ar pressure, and were annealed at 1000 K by electrical heating in ultra-high vacuum . Cleaning and annealing of the Pt surface was continued until Auger spectroscopy and LEED showed that the surface was free from detectable impurities and disorder . However, the adsorbed layers did not display long-range order detectable by LEED . This well-defined Pt(111) electrode was then isolated in an At filled antechamber, followed by a 3 min immersion in the aqueous electrolyte (10-2 M KF + HF, pH 3) containing the subject compound at a specific electrode potential . After removal from the above adsorbate solution, the electrode was rinsed three times with 2 mM HF (pH 3) at the same electrode potential to remove excess solute . The Pt electrode with the adsorbed layer was then evacuated in the antechamber followed by transfer into the main UHV chamber where surface characterization (EELS, Auger, LEED) took place .



39

Electrode potentials were measured and controlled by means of a three-electrode potentiostat . Solutions and At gas for solution de-aeration were transferred only through glass tubing and Teflon tubing which was jacketed with Tygon tubing and purged with Ar to minimize diffusion of air through the Teflon tubing walls . The electrochemical cell, constructed of Pyrex glass, contained the Ag/AgC1 reference electrode and a Pt auxiliary electrode . The cell was introduced into the antechamber by means of a bellows assembly and gate valve ; there are no sliding seals or other sources of contamination in the apparatus . All potentials were referred to a standard Ag/AgC1 (1 M KCI) reference half-cell ; however, to minimize contamination of adsorbate solutions with chloride, a dilute KCI solution (10 mM) was employed in the reference electrode, which was calibrated against the standard Ag/AgCI (1 M KCI) reference half-cell . Electrolytes employed for adsorption and electrochemical measurements contained 10 mM KF (adjusted with HF to pH 3) to provide adequate conductivity and buffer capacity. The rinsing solution was 2 mM HF (pH 3) or 10 -4 KOH (pH 10). Water used in the experiments was pyrolytically distilled in pure 0 2 through a hot (800 ° C) Pt gauze catalyst, and distilled again . All glassware was cleaned by heating to 425 ° C overnight before use . All adsorbates studied in the present work were obtained from Aldrich Chemical Company (Milwaukee, WI) and redistilled before use .

~N H O FRN PRR

S TPE

Electron energy-loss spectra (EELS) were obtained by means of an LK Technologies EELS spectrometer (Bloomington, IN) . Some modifications were made to improve the signal-to-noise ratio . The EELS spectrometer was operated at a resolution of about 10 meV (80 cm -1 ) and a scan rate of 1 mV/s in these experiments . Vapor-phase IR spectra of the above compounds were obtained from the Aldrich IR library [4] . Auger electron spectra were collected using a cylindrical mirror analyzer equipped with an integral electron gun (model 10-155, Perkin Elmer, Eden Prairie, MN) . A lock-in amplifier (model 128, Princeton Applied Research, Princeton, NJ) was used to acquire the first derivative of the spectrum . The equipment was interfaced to a computer (Hewlett-Packard model 3497A interface and model 9920 computer, Hewlett-Packard, Palo Alto, CA) so that the data could be collected and stored on disk for later manipulation. The incident beam current was only 0 .1 AA at 2000 eV to minimize the effect of beam damage, and was controlled to within 5% to limit scatter in the resulting data .



40

Packing densities, I'x (nmol of adsorbed X atoms/cm Z ) or r (nmol of adsorbed molecules/cmZ) were obtained as follows : Auger signals, Ix , due to each element X were measured (if X Auger signal overlapped with the Pt Auger signal, subtraction of the Pt contribution was made) and normalized by the Auger signal at 161 eV due to the clean Pt surface IA . Elemental packing densities were obtained from (Ix/IP,) by means of equations of the following form, as discussed in ref . 5 and applied to numerous adsorbates reviewed in refs. 1 and 2: N

rx = (IX/IPt) /Bx L. Li fx

(1)

1_1

where Bx was calibrated by the adsorbed hydroquinone (Be, Bo), sulfur (B.) or L-DOPA, 3-(3,4-dihydroxyphenyl)-L-alanine (B„) : BB = 0.314 cmZ/nmol, Bp = 0.574 cmZ/nmol, B, = 3 .85 cmZ/nmol, B„ = 0 .633 cmZ/nmol, L; is the fraction of element X located in level i (i = 1 is adjacent to the Pt surface and N is the outermost layer) ; fx is the attenuation factor for Auger electrons of element X by light atoms such as C, N, or 0 ; fx = 0 .70 for X = C, N, or 0, based upon the observed attenuation of Pt Auger signals (235 eV) by a (3 X 3) layer of horizontally oriented hydroquinone and Mi is the number of nonhydrogen atoms located on the average path from the emitting atom to the detector . Molecular packing density is related to the elemental packing density, rx, of each element X by eqn . (2):

r = Ix/n

(2)

where n is the total number of atoms of element X in the molecule (X = carbon in the present article). Cyclic voltammograms were obtained by means of a three-electrode potentiostat and Hewlett-Packard 7015B X-Y recorder . Integration of the broad voltammetric peak due to irreversible oxidation of an adsorbed layer at the Pt surface yields a total oxidation charge, Q ox . Similar voltammetric integration for surface oxidation of the bare Pt electrode during the second scan gives the background, Q 6. Upper limits of integration were chosen such that the oxygen evolution rates for the initially coated (first scan) and bare (second scan) Pt surfaces are equal (near 1 .2 V in each case) ; the extent of oxidation of the Pt surface during each pair of scans was equal within the reproducibility of the measurements (a few percent), as judged from the peak near 0 .4 V for reduction of surface oxide . The lower limit was 0 .1 V . Based upon the Faraday law, the number of electrons n .., to desorb an adsorbed molecule oxidatively can be calculated : no. = ( Qox - Qe)/FAI (3) where F is the Faraday constant, A is the geometric surface area of the electrode (cm') and r is the molecular packing density obtained from Auger data . Each adsorbate studied in this work coats the Pt(111) surface fully and uniformly as evidenced by the virtual absence of voltammetric current for hydrogen adsorption and desorption.



41 RESULTS AND DISCUSSION

Furan (FRN)

The EELS spectrum (Fig . 1A, solid curve) of the adsorbed layer at Pt(111) formed in an aqueous solution (2 mM FRN in 10 mM KF/HF, pH 3, -0 .2 V) exhibits unexpected characteristics : strong carboxylic acid vibrational bands (O-H stretching at 3578 cm - ', C=0 stretching at 1713 cm - ' and O-H bending at 1375 cm - '), and a stretching band at 2075 cm - ' due to adsorbed CO . Rinsing the above acidic adsorbed layer with 10 -° M KOH (pH 10) produces a neutral surface layer of potassium carboxylate, as evidenced by the strong carboxylate (OCO) stretching band at 1603 cm - ', disappearance of the O-H and C~O stretching bands (Fig . 1B), and occurrence of a large potassium Auger signal (Fig . 2B) . Adsorption of FRN at a more positive potential (0 .4 V) onto Pt(111) results in an EELS spectrum (Fig . 1A, dotted curve) containing a prominent stretching band due to surface coordinated carboxylate at 1534 cm - ', a characteristic feature of carboxylic acid adsorption at positive potentials as reported earlier [6,7] . These results suggest that as a result of contact with the Pt(111) surface in aqueous solution at -0 .1 V, FRN undergoes hydrolysis with ring opening to form adsorbed pendant butenoic acid (BTA), while adsorption at 0 .4 V forms an adsorbed layer having carboxylate bonding to the surface:

HO + H2 O

0

`~ _ /C H 2

Pt(III)

// J//// +0.4 V

O 0= C' CH

(4)

/////// However, acid catalyzed hydrolysis with ring-opening of unadsorbed FRN has been reported [8] to form succinaldehyde (OHC-CH Z CH2-CHO) at elevated temperatures. That is, interaction of the FRN molecule with Pt(111) evidently stabilizes one of the C=C double bonds, thus favoring formation of an alkenoic acid rather than a di-aldehyde . Further evidence for eqn . 4 is the similarity both of frequency and of intensity of the EELS spectra of adsorbed FRN (Fig. 1A) and the corresponding EELS spectra of adsorbed BTA (Fig . 1C), at negative and at positive potentials [9] . Small additional peaks in the EELS spectra of adsorbed FRN are probably due to traces of intact FRN, CO and other adsorbates derived from FRN, see Table 1 . It is noteworthy that intramolecular hydrogen-pi bonding [10] of adsorbed BTA (derived from FRN) may occur as illustrated in Fig . 3A . This unique mode of intramolecular hydrogen bonding may be the factor which stabilizes the free carboxylic acid pendant of BTA at the surface and prevents formation of intermolecular hydrogen bonds between adsorbed BTA molecules. This behavior of BTA is in contrast to that of alkenoic acids of other structural types for which strong



42

14

11 w

r

. . .

. . . .

.

1000 2000 ENERGY LOSS / cm'

3000

4000

2000 1000 ENERGY LOSS /cm - '

3000

4000

C

u

2

0



43

2 H

D

1000 2000 ENERGY LOSS /cm

3000

4000

'

Fig. 1 . EELS vibrational spectra of surface monolayers adsorbed at Pt(111) from aqueous solution containing 2 mM subject adsorbent at both negative and positive potentials, compared with the IR spectrum of corresponding unadsorbed species (lower curve) . (A) FRN adsorbed at -0 .1 V (---) and 0.4 V ( ) vs . the IR spectrum of FRN vapor [4]. (B) KOH rinsed (10 -4 M, pH 10, open-circuit) FRN surface layer adsorbed at -0-1 V vs . the IR spectrum of the potassium salt of 3-butene-l-oic acid (K*BTA - ) . (C) BTA adsorbed at -0.1 V ( ) and 0 .35 V ( ) vs . the IR spectrum of BTA vapor [4]. (D) PRR adsorbed at -0 .1 V (-) and 0.4 V ( ) vs. the IR spectrum of PRR vapor [4] . (E) TPE adsorbed at -0 .1 V ( ) and 0 .4 V ( ) vs . the IR spectrum of TPE vapor [4] . Experimental conditions : EELS beam 100 pA at 4 eV, resolution 10 meV (80 cm - '), adsorbed layers were obtained by immersing Pt(1l1) into 10 mM KF/HF electrolyte (pH 3) containing about 2 mM subject compound, then rinsing with 2 mM HE (pH 3) at a specified potential .

44 TABLE 1 Assignments of EELS bands for FRN, BTA, PRR and TPE adsorbed at Pt(111) from aqueous solutions i/cm - '

Description

FRN -0 .1 V/pH

3578 3015 2075 1713 1563 1375 1161 889 758 587 475

BTA 3

-0.1 V/pH

3

3571 2960 1734 1574 1370 1174 895 661 556

FRN 0 .4 V/pH 3

BTA 0.35 V/pH 3

3541 3004 2038

3580 3026 2075 1681 1485

1534 1222 860 632

Description

v/cm ' FRN ads . rinse

-0.1 V/pH

1185 895 689 558

O-H stretch C-H stretch CO stretch C=O stretch C=C stretch C-H, O-H bend C-C, C-O stretch Vinyl C-H, OCO bend C=C, OCO bend CCO bend Skeletal def., Pt-C stretch

BTA 3

0 V/pH 10 3010

3 -0 .1 V/pH 10 -D .1 V/pH

1603

2954 1739 1574

1359

1399

1169

1156

907

914 745 578

596 e/cm -t

stretch C=O stretch Cam, CO, Asym . stretch O-H, C-H bend, CO, sym . stretch . C-C, C-O stretch, C-H bend . Vinyl C-H bend OCO bend, C-H def. CCO bend C-H

Description

PAR -0.1 V/pH 3

PRR 0 .4 V/pH 3

3423 3088 1470 1242 1023

3400 3137 1544 1261 1035

910 615

790

N-H stretch

C-H stretch CC, CN stretch C-H bend . C-O stretch C-H bend ; C-C, C-N stretch C-H def., N-H bend Ring, skeletal deL Skeletal def .

45 TABLE 1 (continued) s/cm - '

Description

TPE -0 .1 V/pH 3

TPE 0 .4 V/pH 3

3597

3597 3375 2985 1645 1411 1213 988

2985 1645 1411 1228 1042 729

O-H stretch O-H stretch C-H stretch C=C stretch C-H bend C-H bend, C-O stretch C- H def. Ring def.

intermolecular hydrogen bonding at the surface lowers the frequency and intensity of O-H stretching . Therefore, BTA is the only terminal alkenoic acid among those studied [9] which displays a strong O-H stretching band in its EELS spectrum . The packing density of the surface layer adsorbed from 2 mM FRN solution at -0.1 V is 0 .53 nmol/cm2 (Tables 2 and 3) which is essentially identical to the packing density (0 .52 nmol/cm2 ) of adsorbed BTA [9] under the same experimental conditions . A packing density of 0 .49 to 0 .52 nmol/cmz is expected based upon the surface structural model shown in Fig . 3A, depending upon the assumed conformation, and covalent and van der Waals radii [11,12] . Vertically or horizontally oriented FRN adsorbed intact at Pt(111) would result in a much higher (0 .725 nmol/em2 ) or lower (0 .458 nmol/cm2 ) molecular packing density . Evidently, the conversion of dissolved FRN to adsorbed BTA is essentially complete . Shown in Fig . 4A is a cyclic voltammogram of adsorbed FRN in the electrolyte solution (10 mM KF/HF, pH 3) which closely resembles that of adsorbed BTA . The broad anodic peak at about 0 .85 V during the first potential scan is due to oxidative desorption of adsorbed FRN and formation of surface oxide at Pt(111) . The cathodic peak at 0 .34 V is the result of electrochemical reduction of Pt surface oxide . After this surface oxidation and reduction process, the Pt(111) is virtually free of adsorbate, but its surface structure is disordered, leading to behavior resembling a polycrystalline Pt surface [13] . The cathodic and corresponding anodic peaks between 0.0 V and -0 .3 V are due to hydrogen adsorption and desorption at the Pt surface . The close similarity of these hydrogen adsorption and desorption peaks during the first and second potential cycles confirms the completeness of oxidative desorption of adsorbed FRN from Pt(111) surface during the first anodic scan . Accordingly, integration of voltammograms for the "FRN-coated" (first scan) and the clean Pt (second scan) surfaces were employed to determine the charge Q ox - Q h required to oxidize the FRN adsorbed layer . Combining (Q ox - Qb) with the molecular packing density, F, from Auger spectroscopy, Table 2, by use of eqn . 3 yields n„x , the average number of electrons transferred in oxidation of an adsorbed FRN molecule . The result, n ox = 16, is close to that expected for complete

IPA TA

0 .625 1 .107 0.631 0.659 0 .662 0 .462 1 .68 0.686 1 .68 0.683

-

Is Ix IPtoIR0 2.11 2.23 1 .63 1 .66 2.27 1 .44 1 .62

0 .462 0.549 0 .594 0 .192 0 .329 0 .447

0.240 0.370 -

0 .365

-

0 .888 0 .888

0.526 0 .557 0 .409 0.416 0 .567 0.359 0.405

1226 -

819 967

-

16 24

Elemental packing density/nmol cm-2 Molecular F [(Q,.-Qs)/A1 n,, /nmol cm -r /µC cm-' r rN rO r5

Experimental conditions: Adsorption from 10 mM KF/HF solution (pH 3) containing 10-1 M subject adsorbate at the electrode potential indicated above ; the same electrode potential was applied during the rinse except when rinsing with pH 10(10-4 M KOH) with open circuit . Auger spectroscopy, incident beam 100 nA, 2000 eV at normal incidence, voltammetric conditions as in Fig . 4.

TABLE 2 Auger spectroscopic and electrochemical data IN Io Com- Electrode Rinse Ic a IY~ pound potential pH Ip IPi o /V 0562 0 .265 FRN -0.1 3 -0.1 10 0 .595 0.315 0 .341 0.4 3 0 .512 -0.1 0 .522 0.152 0 .110 PRR 3 0 .712 0 .234 0 .189 0.4 3 -0.1 3 0.417 TPE 0.4 3 0 .471 0.257

47

LL)

n w z 9

100

200

300

400

500

600

Kinetic Energy/eV

Fig. 2 . Auger spectra of (A) clean Pt(111) ; (B) FRN adsorbed layer (pH 10, open circuit) ; (C) PRR adsorbed layer (pH 3, -0 .1 V) ; (D) PRR polymer (pH 3, -0 .1 V) and (E) TPE adsorbed layer (pH 3, -0 .1 V) . Experimental conditions: adsorption from 10 mM KF/HF (pH 3) solution at -0 .1 V, rinsing pH and potentials were as indicated above. PRR polymer formation as described in text ; excitation electron beam 100 nA at 2000 eV at normal incidence .

C .TPE

Fig. 3. Model structures of (A) FRN, (B) PRR and (C) TPE adsorbed at Pt(111) (pH 3, -0 .1 V). Other experimental data as in Fig . 1 .



48 TABLE 3 Auger equations for obtaining packing densities a

rc = ( Ic/Ir,° )/Bc( 2 + z ) ro = ( Io /r,° )/Bo

FRN

FK -

PRR

UK/IPt°)/BK

• = (UC/I r,° )/Be FN = (IN /IA ° )/BN

F0 - (Ia/In°)/Bo TPE

1'C= (Ic/Ia,°)/BC(

4 + f))

• = (Is/Ia,°)/Bsf ` •

'Auger constants : Bc =0 .314 cmt/nmol ; Bo =0.574 cmc/nmol ; BK =3 .03 cm2/nmol ; BN =0.633 cm2/nmol: Bs =3 .85 cm r/nmol ; f = 0.70.

oxidation of adsorbed FRN (adsorbed BTA) to CO, : C02 H

H2 / H Z C=CH + 6H 20

~

4C0 2 +'aH'+ 18e

(5)

1 7 1

Pyrrole (PRR)

EELS spectra of PRR adsorbed at Pt(111) from aqueous solution (1 mM PRR in 10 mM KF/HF, pH 3), Fig . 1D, indicate that at least some of the adsorbed molecules have retained the PRR molecular structure without hydrolysis and/or ring opening : a C-H stretching band is seen near 3100 cm -1 ; there is a broad N-H stretching band at about 3410 cm - ' ; and C=C stretching band at about 1500 cm - ' . The assignments of other EELS vibration bands based on accepted interpretations of PRR IR spectra [8,10,14,151 are given in Table 1 . Comparison of the molecular packing density, 0 .416 nmol/cm2, calculated from Auger data (Figure 2C and Table 2) for PRR adsorbed from aqueous solution (pH 3, -0 .1 V) with the limiting Fig. 4 . Cyclic voltammograms of (A) adsorbed FRN ; (B) adsorbed PRR and (C) adsorbed TPE at Pt(111) in the 10 mM KF/HF (pH 3) electrolyte . The solid and dashed curves represent the first and second scans of an adsorbed layer which has not been exposed to vacuum . The dotted curves represent the first scan of an adsorbed layer which had been exposed to vacuum for one hour prior to voltammetry . Experimental conditions : adsorbed layers were obtained at -0 .1 V ; other conditions were as in Fig . 1 . Scan rate 5 mV/s ; temperature 23±1°C .



49

- 0.2

0.0

Q2

0.4

POTENTIAL

-0 .2

0.0

0.2

0.4

Q6 OB

1 .0

1 .2

/ V c vs. AQiAQCu

0.6

POTENTIAL/v

QB

1 .0

1 .2

c rs. AQ/AQ01 )

POTENTIAL / V (vs. Ag/AgCU

50

theoretical molecular packing densities for PRR surface layers in the horizontal (0.393 nmol/cm2) or vertical (0 .725 nmol/cm2 ) orientation through either N or C=C, respectively, leads to the conclusion that PRR adsorbs at Pt(111) predominantly in a horizontal orientation :

Pt (III)

The above conclusions were also supported by the cyclic voltammograms shown in Fig. 4B, where complete oxidative desorption occurs in the potential range from 0.60 V to 1 .24 V. If PRR adsorbs vertically at Pt(111) through the nitrogen lone-pair of the pyrrolenine tautomer [16] H

then complete oxidation to CO 2 and NO2 would not be expected by analogy with the voltammetric behavior of adsorbed pyridine [6] . The magnitude of n 0„ obtained from the voltammetric electrooxidative charge (Q 0 - Q,,) and the molecular packing density (I') from Auger data is 24 electrons/molecule, compared with a theoretical n 0„ of 25 electrons/molecule, assuming complete oxidation of horizontally adsorbed PRR to NO 2 and C02 :

~t

N-H +

I

)OH I O~N01 +4COp+25N+25e

(7)

f f f

As can be seen from Fig . 4B, the cyclic voltanunograrns obtained for the PRR adsorbed layer before and after evacuation for one hour are essentially identical . This suggests that PRR adsorbed at Pt(111) is stable in vacuum, as has been observed for other similar adsorbates [1-3,6,7,13] . It is of interest that adsorbed PRR has an oxygen Auger signal, see Fig . 2C . The oxygen packing density increases with increasing electrode potential and equals about one-half of the corresponding molecular packing density, see Table 2 . By analogy with the FRN results described in the previous section, the oxygen present in the PRR layer probably originates from hydrolysis of adsorbed PRR, catalyzed by H + and the Pt(111) surface . Although several hydrolysis products are possible,



the pyrrolidinone (PRD) and the butenamide (BAM), 0 11

0 H (PRD)

CH2 =CHCH 2CNH2

(BAM)

are ruled out by the absence of an EELS C=O stretching band (near 1750 cm -'), Fig. 1D . Ring opening of the PRR to form BAM should be more difficult than for FRN because the C-N bonds of PRR are expected to be stronger than the corresponding C-O bonds of FRN . Furthermore, the EELS band at about 1250 cm- ' which is absent from the IR spectrum (Fig. ID) of unadsorbed gaseous PRR is assignable to a C-0 stretching vibration . Accordingly, the following two tentative surface structures of adsorbed hydrolyzed PRR are proposed :

C N )+H2O

Pt(III) e

I

H (I) (31) where oxygen attachment to the surface with loss of hydroxyl hydrogen is based upon the absence of an EELS O-H stretching band, Fig . ID, in line with results reported earlier for adsorbed alcohols, carboxylic acids and phenols [6,17-191 . EELS and Auger spectroscopic results for polypyrrole films electrogenerated from aqueous solution may also be of interest . Studies of poly-PRR and related materials by means of a variety of methods have been reported for metal and semiconductor surfaces, including studies involving electrochemistry, UPS and XPS spectroscopy, UV-Vis and IR spectroscopy, NMR and EELS [20-25] . Reported here is surface characterization of poly-PRR films at Pt(111) by use of LEED, Auger, and EELS . The polymer was obtained by immersing a well-defined Pt(111) electrode into 2 mM PRR solution (10 mM KF/HF, pH 3) at -0 .1 V where a chemisorbed PRR monolayer was formed as described above, followed by scanning of the electrode potential from -0 .1 V to 1 .24 V and back to -0 .1 V in I mM PRR solution [26] . Based upon the cyclic voltammogram, Fig . 5A, the polymerization process began near 0 .4 V, which is consistent with the relatively high molecular packing density of PRR at 0 .4 V, see Table 2 . The poly-PRR film thus obtained was rinsed with 2 mM HF solution at -0 .1 V, followed by evacuation . Based upon the Auger spectrum, Fig . 2D, the ratio of nitrogen to carbon signals, L N/L o , for poly-PRR is 0 .277, which is very close to that of adsorbed PRR monolayer, 0.291, while the ratio of oxygen to carbon Auger signals, Io/Ic = 0.072 for the poly-PRR film is much smaller than that for the PRR monolayer, 0 .211 . Evidently, poly-PRR retains the carbon and nitrogen stoichiometry of PRR monomer . The EELS spectrum of poly-PRR exhibits a broad band near 3100 cm -1 ,

E

(

LO 1 .2 14

vs . Ag/AQCI)

0.6 0,e

POTENTIAL/v

-02 0 .0 0 .2 0.4

Fig . 5 . (A) Cyclic voltammogram of PRR polymerization at PI(Ill) in the 10 mM KF/HF (pH 3) electrolyte containing 2 mM PRR . Scan rate 5 mV/s ; temperature 23±1 ° C. (B) LEED pattern of pyrolyzed poly-pyrrolc film at Pt(111), 66 eV .

U U

w

Z

0

w

Z Z

a r t-

U

N

53

assignable to C-H and N-H stretching . Another band near 1300 cm - ' is attributable to overlapping contributions from C-H bending and C-C stretching modes . When the poly-PRR film was heated to 700°C in vacuum for 10 min, the oxygen and nitrogen Auger signals decreased by about two-fold, while the carbon Auger signal increased by about 50% . The EELS spectrum showed only an elastic peak, with no distinguishable energy-loss vibrational bands . Perhaps the most surprising characteristic of the surface film formed by heating poly-PRR is its sharply hexagonal LEED pattern, Fig . 5B . Platinum Auger peaks were undetectable, demonstrating that the Pt surface was fully covered by the carbonaceous layer. Taken together, these results indicate that heating converts poly-PRR to a highly-ordered graphitic or similar carbon film . Thiophene (TPE) EELS spectra of TPE adsorbed at Pt(111) exhibit virtually the same frequencies as the IR spectrum of TPE vapor, Fig. 1E. Amplitudes are also similar except for attenuation of the C-H out-of-plane deformation modes (below 900 cm - ') in the EELS spectra . While this attenuation could be related to the probable vertical orientation of the adsorbed TPE molecule, one cannot rule out the possibility that those deformation modes are simply weaker in the EELS spectrum than in the gas-phase IR spectrum or are weakened by the interaction of TPE with Pt(111) . Close correspondence between EELS and near-IR spectra is indicative of an adsorbed state in which there is relatively little perturbation of molecular structure and electronic properties by the adsorption process [1,2,6,27] . Previous studies of aromatic sulfur compounds [7,13] have indicated that surface attachment occurs primarily through the sulfur atom . Accordingly, a sulfur-bonded, near-vertically oriented state for TPE adsorbed at Pt(11l), Fig . 3C, is most probable . The EELS vibrational bands at 3597 cm - ' and 3375 cm', due to free and hydrogen-bonded 0-H stretches, are barely perceptible for TPE adsorbed at -0 .1 V, but are prominent at 0 .4 V . An oxygen Auger signal is obtained for TPE adsorbed at 0 .4 V but not at -0 .1 V, see Table 2 . Evidently, adsorption of TPE at sufficiently positive potentials involves hydrolysis processes analogous to those for FRN and PRR . Sulfur Auger signal for TPE amounts to a packing density ratio I' s/1'c = 0.62, Fig . 2E and Table 2, which is much larger than that expected from the molecular formula of TPE, 0 .25 . Apparently, TPE molecules hydrolyze and dissociate in contact with Pt(111) in aqueous electrolyte to form adsorbed sulfur atoms and products, such as terminal butenaldehyde or butenoic acid which do not adsorb in competition with S and TPE : + H20 + 2 H ,O

pt(III) mlxi'>

S( ,as) + CH 2=CHCH 2CHO S(-&, + CH 2=CHCH zCOOH + 2 H'+ 2 e -

(9) (10)



54 The above desulfurization processes are also suggested by the measured charges for electrooxidation of the adsorbed layer, Fig . 4C . Complete oxidation of adsorbed TPE, as illustrated by eqn . 11 :

4 S ) (ads .) + 12 H 2O

-y

SOa - + 4 COz + 28 H' + 26 e

(11)

would have required only 901 iC/cm Z based on a TPE molecular packing density of 0 .359 nmol/cmZ , Table 2 and eqn . 3, while that observed was 1226 gC/cm Z . The excess oxidative charges are attributable to oxidation of adsorbed sulfur atoms, eqn . 12 : St, ds _ i + 4 H20-> S042- + 8 W+6 e -

(12)

ACKNOWLEDGEMENTS This research is supported by the National Institutes of Health . Equipment was provided by the National Science Foundation, the Air Force Office of Scientific Research, and the University of Cincinnati . The technical assistance of Arthur Case, Frank Douglas, Douglas Hurd, Richard Shaw and Vickie Townsend is gratefully acknowledged . REFERENCES 1 A .T. Hubbard, Chem . Rev ., 88 (1988) 633 . 2 A .T. Hubbard, Langmuir, 6 (1990) 97 . 3 AT. Hubbard, in C .H . Bamford, D.F .H. Tipper and R.G . Compton (Eds.), Comprehensive Chemical Kinetics, Vol . 28, Elsevier, Amsterdam, 1988 . Ch . 1 . 4 E.T . Pouchert, The Aldrich Library of FTIR Spectra, Aldrich Chemical Co ., Inc ., Milwaukee, WI, 1985 . 5 N . Batina, D .G. Frank, J .Y . Gui, B.E . Kahn, C.H . Lin. F . Lu, J.W. McCargar, G .N . Salaita, D .A. Stem, D.C . Zapien and A.T . Hubbard, Electrochim. Acta ., 34 (1989) 1031 . 6 D .A . Stem, L. Laguren-Davidson, D .G . Frank, J .Y . Gui, C.H. Lin, F . Lu, G .N . Salaita, N . Walton, D .C. Zapien and A .T. Hubbard, J . Am . Chem . Soc., 111 (1989) 877 . 7 N . Batina, J .Y . Gui, B.E . Kahn, C .H . Lin, F. Lu, J .W . McCargar, G .N . Salaita. D .A . Stern, A .T. Hubbard and H .B . Mark, Langmuir, 5 (1989) 588 . 8 A .P. Dunlop and F.N . Peters, The Furans, Reinhold, New York, 1953 . 9 N . Batina, S.A. Chaffins, B.E. Kahn, F . Lu, J.W . McCargar, J. Rovang, D.A. Stem and A .T. Hubbard, Catal . Lett ., 3 (1989) 275 . 10 RA- Nyquist, The Interpretation of Vapor-Phase Infrared Spectra, Sadder Research Laboratories, Philadelphia, 1984 . 11 L.C. Pauling, The Nature of the Chemical Bond, 3rd ed ., Cornel University Press, Ithaca, New York, 196012 V . Schomaker and L .C. Pauling, J . Am . Chem . Soc., 61 (1939) 1769 . 13 J .Y . Gui, B.E . Kahn, L . Laguren-Davidson, C.H . Lin, F . Lu, G.N . Salaita, D .A. Stem and A.T . Hubbard, Langmuir, 5 (1989) 819 . 14 G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1980 .