Surface characterization of molecules at pt(111) using leed, auger, hreels and electrochemistry in ultrahigh vacuum

465

CatalysisToday, 12 (1992) 465-479 Elsevier Science Publishers B.V., Amsterdam

SURFACE LEED,

CHARACTERIZATION

AUGER,

HREELS

OF MOLECULES

AND ELECTROCHEMISTRY

AT Pt(ll1)

USING

IN ULTRAHIGH

VACUUM

G.N. SALAITAl

and A.T. HUBBARD2

1Surface Science Center, Union Carbide Corporation, P.O. Box 8361, South Charleston, WV 25303, USA. 2Surface Center, Department of Chemistry, University of Cincinnati, Cincinnati, OH 452210172, USA.

SUMMARY Reviewed in this article is surface characterixation of organic molecules adsorbed at welldefined Pt( 111) electrode surfaces from aqueous solution. Among the metal electrodes and surface-structure sensitive properties studied in this work are those involving electrocatalytic activity, such as platinum metal. Platinum is one of the most interesting materials for study in view of its exceptional spectrum of catalytic properties and its good stability in various electrolytic media. The compounds investigated were: (I) 4-Phenylpyridine (4PPY), (II) 4,4’Bipyridyl (44BPY), (III) 2-Phenylpyridine (2PPY), (IV) 2-2’ Bibyridyl (22BPY), (V) Pyridazine (PD). and (VI) 4-Pyridazinecarboxylic acid (4PDCA). For (I) and (II), the pyridine ring binds to the Pt( 111) surface in a tilted, nearly vertical orientation having a pendant phenyl ring, and is virtually unreactive toward electrochemical oxidation. However, (IQ and (IV) attach to Pt( 111) with their pyridine ring vertically oriented and N-attached, while the other aromatic ring is oriented parallel to the electrode surface. Similarly, (V) and (VI) are attached to Pt( 111) through their ring nitrogen atom, with a tilted-vertical orientation and average ting-tosurface angles ranging from 730 to 860 . Compounds (I-VI) were studied in order to understand the influence of molecular structure on surface bonding, molecular orientation, electronic structure, and electrochemical reactivity at well-defined surfaces such as Pt( 111). as a function of pH, electrode potential and adsorbate concentration. INTRODUCTION Chemisorption at Pt from aqueous solutions of a variety of classes of organic compounds at controlled pH and electrode potential have been studied [ 11. A wide range of surface science techniques combined with cyclic voltammetry in uhrahigh vacuum have been employed to study the adsorption of bipyridyls and related compounds on the Pt( 111) surface in order to understand the oxidation/reduction species.

behaviour and mode of attachment of the chemisorbed

Surface structure was investigated by low-energy electron diffraction (LEED).

Surface elemental composition, packing density and surface cleanliness were monitored by Auger-electron spectroscopy (AES). Residual gas composition in the main vacuum chamber and the kinetics of desorption from electrode surfaces were probed by quadrupole mass spectroscopy (TDMS). Vibrational bands were established by high resolution electron energy loss spectroscopy (HREELS), and assigned with reference to the IR spectra of the parent compounds. Electrochemical reactivity is studied by use of cyclic voltammetry. Pyridyl rings

1992 Elsevier Science Publishers B.V.

unhindered at positions ortho to the ring nitrogen atom adopt tilted vertical orientations with surface attachment mainly through the nitrogen atom. However, pyridyl rings hindered by bulky groups ortho to nitrogen favour the horizontal orientation. Absolute packing densities obtained from Auger spectroscopy indicated that typical aromatic adsorbates form oriented overlayers at a well-defined Pt( 111) single-crystal surface [2]. These orientations exhibited by molecules adsorbed from solution onto a Pt( 111) surface have a significant practical influence on chemical reactivity and heterogeneous catalysis at electrode surfaces [3,4.5]. EXPERIMENTAL A diagram of the surface electrochemistry instrument employed for this work is shown in Figure 1. All components were constructed with attention to modern ultrahigh vacuum (UHV) practices. The electrode remained inside the UHV enclosure from start to finish. Cycling between atmospheric pressure (argon) and UHV required about five

Viewpart EELS

t

rl

Ion

Bombardment

Translolor Glancing Incidence EleCtrOll

Main Pumping LEED Optics

Ouodrupole Moss Analyzer

II Electrochemical Cell

Fig. 1. Schematic diagram of surface electrochemistry instrument_ minutes and the pt surface was free from contamination as tested by Auger, LEED, EELS, and cyclic voltammetry. Pyrolytically distilled water was used to prepare all solutions [6]. All six faces of the Pt(1 ll)-oriented

single crystal were cleaned simultaneously

by Art ion

bombardment, and annealed by resistance heating (lOOOK)in ultrahigh vacuum. The Pt surface was isolated in an argon-filled antechamber for immersion into buffered aqueous electrolytes which contained the adsorbates to be studied. The electrode potentials were measured and controlled with three-electrode electrochemical circuitry based on operational amplifiers. The electrochemical cell was made of Pyrex glass.

467

Solutions and gases were transferred through Teflon-jacketed tubing. The jacket was purged with argon to minimize diffusion of air into the tubes conveying the solutions and inert gases. The electrochemical cell containing the reference electrode (Ag/AgCl prepamd with 1 mM KCI) and Pt auxiliary electrode was introduced into the antechamber using a bellows and gate valve. All potentials am referred to Ag/AgCl (1 M KCI). Packing densities, Ix (moles of adsorbed atoms/cmz) or r (moles of absorbed molecules/cm*) were obtained as follows [Id]: Auger signals, Ix, due to each element x were measured, and normalized by the Auger signal at 161 eV due to the clean Pt surface, I$. Packing density was obtained from (I.&)

with equations of

the following form:

VI

Where Bx was calibrated using hydnquinone

(Bc, Bo) [la] or L-DOPA (BN) [ le]; BC = 0.314

cm*/nmol, Bg = 0.574 cm*/nmol, BN = 0.747 cm*/nmok Li 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; f, = 0.70 for x = C, N, or 0, based on the observed attenuation of Pt Auger electrons (235 eV) by a (3x3) layer of horizontally-oriented hydroquinone [la]; and Mi is the number of non-hydrogen atoms located on the average path from the emitting atom to the detector. RESULTS

AND DISCUSSION

We have recently deduced the molecular orientations of pyridine [If] and various of tits derivatives adsorbed from aqueous solutions at Pt( 111) based upon packing densities measured by use of Auger spectroscopy. It was found that the pyridine ring binds to the Pt( 111) surface through its nitrogen atom in a nearly vertical orientation with an average angle of I$ = 710 between the aromatic ring and the surface. (i) Cphenvlpvridine (4PPYl The molecular packing density of adsorbed 4PPY at Pt( 111) was calculated from the packing density equations given in Table 1. The observed packing density, based on Auger data (Table 2), was 0.45 nmol/cm*. This measured packing density is intermediate between theoretical packing densities based on equations 2 and 3. Molecular area, s (A*>= a (b coscp+ 3.4 sincp)

El

packing density, P (nmol/cm*) = 16.61/r

131

(where a = 6.72

A, b = 11.37A)

466 for horizontaLlyoriented 4PPY (0.22 nmol/c&) and vertically-oriented 4PPY (0.73 nmoVcm2). Thus, the experimentally inferred structure depicted in Figure 2A is similar to that observed for pyridine itself in which the angle between the plane of the ring and the Pt surface is 790, and the phenyl ring is pendant with respect to the surface.

A 0

8

/07 1, ,,,

E

-0.2 v

0.5 v

0.0 v

Fig.2. Structural model of adsorbed molecules at Pt(ll1). D, 22 BPY, E, PD, F, 4 PDCA.

A. 4 PPY, B. 44 BPY, C. 2 PPY,

Shown in Figure 3A is the voltammogram of adsorbed 4PPY. This voltammogram is similar to that observed for pyridine, showing passivation of the Pt( 111) surface with respect to oxidation and hydrogen adsorption. Figure 4A (upper curve) shows the electron energy-loss spectrum for adsorbed 4PPY at the Pt( 111) surface. All of the bands present in the EELS spectrum of 4PPY are also present in the vapour-phase IR spectrum of 4PPY (Figure 4A, lower curve). Assignments of the EELS vibrational bands based upon the accepted IB assignments are given in Table 3. . . (ii) 2-Phenvlpvndme (2PPY) The observed molecular packing density of adsorbed 2PPY at Pt( 11l), obtained from the Auger equations given in Table 1, was 0.23 nmol/cm 2. This result is consistent with either horizontally-oriented

adsorbed 22PPY, or a vertical orientation of the pyridine ring with the

phenyl ring being parallel to the surface, as shown in Figure 2B. Shown in Figure 3B is the voltammogram of adsorbed 2PPY. which demonstrates greater electrochemical reactivity of 2PPY than of 4PPY towards oxidation at positive potentials. Perhaps this is due to the phenyl ring being relatively

distant from the surface.

Also, the observed no, value,

21

electrons/molecule, an indication that oxidation of the phenyl ring of 2PPY occurs, as desired by Equation 4: CllHg+12Hfl

-

C5HsN + 6C@+UH++27e-

[41

469

TABLE 1 Formulas for Obtaining Packing Density from Auger Spectra.

Canpound Formula

PPY

r, = (IJ$)/[BE(3f2/11+

6fIll + Ul l)]

I- = I-J11 ~N=(INi$)bb@ 4BPY

r, = &/$)I[B,(3f2/10

+ 6f/lO + l/10)]

r = rdio rN=w$)/[BN(fLbm PPY

r, = wQr~~(2~11

+ 9/l 1)]

r = rdll rN=uNi$)/BNf 2PPY

r, =w#tm,~2f/5 +3/5)1 r = rylo

‘D

TN= WJ$MBN/~) op%,, = (1-3Kw

r, = (v@rB,(3/10

+ 7~1011

r = rJs rN=oN$$(BN/f) r. = (wQr(~~(3/4 &I/$)

= (1-3Io

+ f/lr)i (1-6Iol

aConstants: B, = 0.314 cm2/nmol; BN = 0.633 cm%m~~ cm%mok K = 0.165 cm%mok f = 0.70.

B. = 0.574 cm%uno~ BK = 3.03

470

-0.4

0.4

00

POTENTIAL.

0.8 VOLT

1.2

vs. IO/A&I

Fig. 3A,B. Cyclic voltammetry of adsotkd pyrkline type compounds at Pt( 111). A. Solid Curve U): immersion into 1 mM 4 PPY at -O.lV, pH = 3; followed by rinsing with 0.1 mM HF, first scan Dotted Curve (...): second scan. B. Solid Curve 0; immersion into 1 mM 2 PPY at -O.lV, pH = 3; followed by rinsing with 0.1 mM HF. first scan Dotted Curve(...): second scan.

.

.

,

,

, ,

Fig. 3C,D. Cyclic voltammetry of adsabed pyridine type. compounds at Pt( 111). C. Solid Curve (: immersion into 1 mM 44 BPY at -0.2V, pH = 3; followed by rinsing with 0.1 mh4 HF, first scan. Dotted Curve (...): second scan. D. Solid curve (A: immersion into 1 mM 22 BPY at -0.2V, pH = 3; followed by rinsing with 0.1 mM HF. first scan. Dotted Curve (...); third scan.

471 TABLE 2 Auger datafoa Molaxles Adsorbed at Pt(l11) Surface Compound

-Loge

4PPY 44BPY 44BPY 44BPY 44BPY 44 BPY 44 BPY 2PPY 2:: :::

FE

Z:8

iii

%

PD FE PD 4PDCA 4PDCA 4 PDCA 4PDCA 4PDCA

Rin=

Pot: V

pH

-0.15 -0.2

3 3

-0.15

22 BPY 22 22BPY BPY 22BPY 22BPY 22BPY

z

Ekc1:

;: 1:5

Zf - .b2 -.o.z -0.2 % -0;

iii

1.00

0:Oo

:::

8:E

4.00 5.00

0.00

!:E 33:

E -012 0.0

3.0 3:o 3.0

83 d.5

IPt$

;

0.410 0.694 0.598 0.455 0.391

: 3

0.346 0.331 0.570

; 3 :

0.641 0.530 0.508 0.541 0.554

3

0.616

i:8 3.0

0.78 0.60 0.56

:

0.53 0.52

::

0.54 0.53

i

: 7 3.0 10.0 3.0 10.0 3.0 10.0

0.57 0.52 0.62 0.56 0.40 0.49 0.51 0.49 0.59

w$

wit

0.84

1.089 0.374 0.458 0.762 0.911 1.056 1.034 0.744 0.439 0.478 0.584 0.672 0.628 0.702 0.43 0.46 0.50 0.49 0.45 0.41 0.39 0.38 0.37 0.23 0.5 1 0.52 0.64 0.66

0.38

@.

0.65

w$

0.122 0.100 0.153 0.250 0.293 0.326 0.354 0.093 0.141 0.159 0.153 0.175 0.181 0.222 0.38 0.37 0.36 0.28 0.3 1 0.28 0.26 0.24 0.27 0.18 0.42 0.43 0.45 0.5 1 0.37 0.51

idt

rc

rc

4.98 1.79 2.19

0.38 0.42 0.48 0.48 0.40 0.42

Z 5.04 4.94 2.51 1.59 1.73 2.11 2.43 2.27 2.54 1.59 1.71 1.87 1.82 1.69 1.55 1.46 1.43 1.37 0.86 2.05 0.72 2.10 0.79 2.58 0.90 2.66 0.90 2.18 0.75 2.10 0.79

rN

.334 ,180 ,275 .449 ,521 .585

.635 .178 .269 ,303 ,292 .334 ,346 ,424 .85 .84 .80 .34 .38 .34 .32 .29 .33 .22 .95 .91 1.20 1.15 .84 1.15

rK

r

0.364 0.435 0.504 0.494 0.228 0.159 0.173 0.211 0.243 0.227 0.254 .40 .43 .47 .45 .42 .39 .36 .36 .34 -22 .41 .22 .42 .52 .28 .53 44 .I3 .42

%qximental Conditions: beamcurrent.100 r.A, 200 eV, at normalincidence; modulation.5 V pp; referenceelectrode, AgIAgCl (1 mM KCI); adsorptionfrom 10 mM KP adjustedto the pH indicated; rinsingwith HP (pH 3); 0.1 mM KF (PH 7) or 0.1 mM KOH (pH 10). Indicaks potassiumsignaladjustedfor contributionsdue to immersion.

Also shown in Figure 4B (upper curve) is the EELS spectrum of ZPPY. All of the bands present for 2PPY are also present in the vapour phase IR spectrum Figure 4B (lower curve). EELS vibrational bands are summarized in Table 3. The out-of-plane C-H bending (750 cm-t) is weak for 2PPY, compared with 4PPY, as expected in view of the closeness of the phenyl ringof2PPYtothePt(lll)surface. (iii) 4,4’-Binvridvl(44BPYl The molecular packing density observed for adsorbed 44BPY at 1OmM concentration is O.Snmol/cm*, as depicted in Figure 2C, whereby the pyridine ring is attached to the surface with a tilt angle of 810. This experimental packing density, O.Snmol/cm*, is intermediate between the theoretical packing densities based on Equations 2 and 3 for horizontally-oriented (0.23 nmol/cm*) and vertically-oriented 44BPY (0.73nmol/cm*). Depicted in Figure 5 is the adsorption isotherm of 44BPY at -0.2V. The limiting packing density of 44BPY O.Snmol/cm*, was obtained at a concentration of lOmM, while pyridine reached saturation at 0.1 mM concentration [If]. The higher concentration to reach a limiting packing density of 44BPY may be due to the energy barrier to twisting of the two aromatic rings towards a coplanar arrangement, implied in formation of a vertical orientation.

412

-0.4

0.0

0.4

0.8

1.2

POTENTIAL. VOLTvs. WA~CI

Fig. 3E.F. Cyclic voltammetry of adsorbed pyridine type compounds at Pt( 111). E. Solid Curve (: immersion into 1 mM PD at O.OOV,pH = 3 KF/HF electrolyte, first scan. F. Solid Curve (_): immersion into mM 4 PDCA at O.OOV,pH = 3 KF/HF electrolyte, first scan. As shown in the cyclic voltammogram of Figure 3C, adsorbed 44BPY passivates the Pt( 111) surface. Shown in Figure 4C, is the EELS spectrum of adsorbed 44BPY (upper curve). Also shown in Figure 4C is the vapour phase IR Spectrum of 44BPY (lower curve). The intensities of the out-of-plane C-H bending vibrations (653 and 792 cm-t) are substantial, due to the presence of a pendant pyridine group which is virtually unaffected by the Pt( 111) surface. (iv) 2.2’-Biovridvl(22BPY) Shown in Figure 5 is the adsorption isotherm of 22BPY at Pt(ll1).

The limiting packing

density observed for 22BPY is 0.25 nmol/cm 2. From the adsorption isotherm it is evident that the adsorption of 22BPY varies less sharply with on concentration than does 44BPY. This is indicative of the fixed non-coplanar conformation as shown in Figure 2D. This quantitative model shows the non-coplanar conformation of 22BPY which miniizes

steric repulsions and

allows the attachment of both pyridine rings to the Pt( 111) surface. Shown in Figure 3D is the voltammogram of adsorbed 2BPY. From the voltammogram it is evident that the Pt(ll1) surface was passivated to a substantial extent by adsorption of 22BPY. thus providing evidence for the adsorbate molecular orientation shown in Figure 2D. The EELS spectrum of adsorbed 22BPY. Figure 4D (upper curve), is similar to the vapourphase IR spectrum (lower curve).

Assignments of the EELS vibrational bands based on

literature IR assignments [7,8,9] are given in Table 3.

473 TABLB 3 Assignments of EELS Bands 4PPY

2PPY

AdsoW

3082 1599 1437 1178 1018 738 391

3071 1601 1441 1245 1097 900 750

C-H stretch asym ring stretch sym. ring satch C-H bend (in plane), ring-ring sm%ch C-H bend (in plane) C-H bend ring breattb, C-H bend

I

44BPY

22BPY

Adsorbed -O.ZV, pH 3; rinse pH 3 Description

3062 1575 1436 1270 1095 792 653

3062 1580 1442

C-H smztch asym. ring stretch sym. ring stretch C-H bend (ii plane), ring-ring stretch C-H bend (ii plane) C-H bend (out of plane) C-H bend (out of plane) rinn bend lingbend

1172 958 818

1

475

I PZatpH 3/o.OOv

3071 1409 1127

-O.lV. pH 3; IiIISePH 3 DesCIiPdon

PM at pH 3/0.OOv

PD at pH 3/O.wv

3080 1545 1397 1186 1078

3080 1561 1363 1159

descriptioa

CH saxtch CC, CN suetch CC, CN satch CH bend, ring breathing ring breathing CH band CH bend, ring bend ring bend ring bend, Pt-N stretch

907 776

798 693 450

442

448

4PDCA pH 3/-0.2OV pH 3/O.OOV pH 3/0.5OV pH 10/0.2OV pH lO/O.OOV pH 10/0.5OV description 3561 3072 1728 1571 1336

3579 3087 1739

3589 3071 17M)

3074

3110

3083

1609

1653

1645

1358

1358

1370

825 740

814

1373

1395

1204 998

1166

1170 1001

1142

686 516

729

786 504

832 719 512

OH stretch CH stretch C=O snetch OCQCC stretch CC, CN stretch CC, CN, stretch; CH bend CC, CN, OCO s&etch; CH bend CH, OH bend; ring breathing CH bend; ring breathing CH bend; C-O stretch U-I, ring OCO bend CH, ring bend ring bend, Pt-N stretch

~Experimntal conditions: adsorption from 1 mh4 subject compound in 10 mM KF/HP (pH 3) solution followed by nnsmg with either HP @H 32 mM) or KOH @H 10.0.1 mM) sohttion as indicated in the table.. Electmdc potential applied during adsorption and rinsing processes is the same band frequencies are expressed in cm-t.

414

A

0

1000 ENERGY

2000 LOSS

3000

4000

(cm-11

Fig. 4A. Vi&ational spectraufphenyl pyridines. UpperCurve: EZLSspcctrumof4PPYatPt(lll)adsorbedffom1mMPPYin1OmMKF (adjusted to pH 3 with I-IF) at -0.1 V (vs. Ag/AgCl), followed by rinsing with FH (pH3). Lower&e: VapourphaseIRspectmmof4PPY.

0

1000 ENERGY

3000

2000 LOSS

4000

(cm-l)

Fig. 4B. Vibrational spectraof phenyl pyxidines. UpperCurve: EEESspcctnmIof2PPYatPt(lll)adso&edf?om1mM2PPYin1OmhIKP (adjusted to pH3 with HF) at -O.lV (vs. AgIAgCl), followed by rinsing with HP (pH=3).

476

C

0

1000 ENERGY

3000

2000 LOSS

4000

(cm-l)

Fig. 4C. Vibrational spectra uf phenyl pyridks. UpperCurve: EEXSspecmmof44BPYatPt(lll)ahxbedfmm1mM44BPYin1OmM KF (adjusted to pH3 with HP) at -O.lV (vs. Ag/AgCl), followed by rinsing with HF (pH3). Lower Curve: Vapourphasc IR spectmm of 44 BPY.

ENERGY

LOSS

(cm-l)

Fig. 4D. Vibmtional qmtra af phenyl pyridhs. UppcrCum EELs~af22BpYatpt(lll)dsorbedfnnn1mM22BPYia1OmM KF (adjusted to pH3 with HP) at -O.lV (vs. A@AgCl), followed by rinsing witb HF (pH3). LowerCume: VapourphaselRspecmunof22BPY.

476

0 ENERGY

LOSS

(cm-l)

Fig. 4E. Vibmtiunal spectra of phenyl pyridines. of adsxbed PI) at Pt(ll1) from 1 mM solution of adsorbate in Upper Curve: EELS S10 mM KF/HF (pH3); followed by tinsing with 2 mM HF @H3) at O.OOV. LowerCurve: vapourphaseJRspectmmofadso&edPD.

Fig. 4F. Vibrational spectra ofphenyl pyridines. (A-C) EELS spectra of 4 PDCA at Pt( 111) from 1 mM solutions (10 mM KF/HF, pH3), followed by rinsing with 2 MM HF @H=3) and rinsing with 2 mM HF (pH3) at -0.2 V (A), o.OOv (B), and 0.5 v (C). TheLowcstc!uWe: rsthemid-IRspcctmmofsolid4PDCA. (D-F)EIEuspcctniof4PDcx at Pt( 111) from 1 mM solution (10 t&I KF/HF, pH3) followed by rinsing with 0.1 mM KOH (pH10) at -0.2V (D), O.OOV0. and09 (F). Thelowest curve is the mid - IR spectmm of potassiumsaltof4PDCA.

477

6

5

4

3

-LOG

2

1

0

C (Ml

Fig. 5. Adsorption isotherms of 44 BPY and 22 BPY. (v) Pvridazine (PDI Based upon the Auger equations and data given in Tables 1 and 2, packing densities for adsorbed PD at Pt( 111) were obtained as a function of adsorbate concentration, Figure 6. The packing density of adsorbed PD increases with concentration above 0.1 mM. This behaviour of adsorbed PD is different from that exhibited by pyrazine and pyrimidine, which levelled off at a concentration of O.lmM. The observed differences in these isotherms indicate that PD coordinates to the Pt( 111) surface primarily through two nitrogen atoms at low packing densities, and through a single nitrogen atom at the higher concentrations, as illustrated in Figure 2E. Theoretical limiting packing densities of PD are 0.383 nmoVcm2 for the horizontal orientation, 0.664 nmol/cmz for the N-N edgewise orientation, and 0.735 nmol/cm2 for the Nendwise vertical orientation (bonded to the surface via one nitrogen only). Depicted in Figure 4E in the EELS spectrum of PD (upper curve) which resembles the vapour-phase IR spectrum (lower curve). Assignments are given in Table 3. Voltammetry, cyclic Figure 3E, demonstrates that adsorbed PD passivates the Pt(ll1) surface analogously to pyridine. pyrizine and pyrimidine [ 11. (vi) 4-Pvridazinecarboxvlic acid (4PDCA) The molecular packing density of 4PDCA at 0.00~ is 0.52 nmol/cmz. The theoretical limiting packing density is 0.638 nmol/cm2 for the $-orientation, 0.591 nmol/cmz for the N2qI orientation, or 0.741 nmol/cm2 for the NIqt

orientation, Figure 4F. From the packing

density alone, it is difficult to differentiate between the vertical-edgewise

(112)and tilted-

edgewise (Bl) orientation. That is, three different modes of attachment of 4PDCA to the surface are possible:

478

Shown in Figure 4F, is the EELS spectrum of adsorbed 4PDCA at negative potentials. The spectrum contains an O-H stretching peak at 3561 cm- 1. men

the potential was changed from

negative (-02Ov) to positive (+O.Sv), a profound decrease in the intensity of the peak at 3651

r

pHp

j;:

r-4

0.1 -

: 0

PYRIMIDINE

0

PYRAZINE

0

PYRIDAZINE

6543210

-LOG C (Ml Fig. 6. Adsorption isotherms of PA, PM and PD at Pt(lll)

versus concentration.

cm-l was observed. As a result of rinsing of the adsorbed layer with KOH solution (pH=lO) exchange of H by K+ took place, Figure 4F and Table 1. EELS peaks at 814.1370 and 1645 cm-l, characteristic of carboxylate anions are a strong indication that a vertical orientation having a pendant carboxylate moiety is the dominant adsorbed state. As shown in Figure 3F, the cyclii voltammogram of adsorbed 4PDCA at Pt(ll1) surface.

surface indicates that 4PDCA passivates the

479

ACKNOWLEDGEMENT

We are grateful to the Air Force Office of Scientific Research, the Gas Research Institute, and the National Science Foundation for support of this rematch.

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