Molecular Adsorption at Well-Defined Platinum Surfaces - ACS

Chapter 2

Molecular Adsorption at Well-Defined Platinum Surfaces

Electrochemical Surface Science Downloaded from pubs.acs.org by YORK UNIV on 12/07/18. For personal use only.

Voltammetry Assisted by Auger Spectroscopy, Electron Energy-Loss Spectroscopy, and Low-Energy Electron Diffraction Arthur T. Hubbard, Donald A. Stern, Ghaleb N. Salaita, Douglas G. Frank, Frank Lu, Laarni Laguren-Davidson, Nikola Batina, and Donald C. Zapien Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172 Reviewed here are surface electrochemical studies of organic molecules adsorbed at well-defined Pt(111) electrode surfaces from aqueous solution. Emphasis is placed upon studies of n i c o t i n i c acid (NA), pyridine (PYR), and nine related pyridine carboxylic acids. Packing densities (moles per unit area) adsorbed from solution at controlled pH and electrode potential, and measured by means of Auger spectroscopy are discussed. V i b r a t i o n a l spectra of each adsorbed obtained by use of electron energy-loss spectroscopy (EELS) are compared with the IR spectra of the parent compounds. E l e c t r o chemical r e a c t i v i t y is studied by use of c y c l i c voltammetry. Monolayer structures observed by means of low-energy electron d i f f r a c t i o n (LEED) are described. Substances studied are as follows: 3-pyridine carboxylic acid ( n i c o t i n i c acid, NA, "niacin"); pyridine (PYR); 3-pyridylhydroquinone (3PHQ), synthesized here for the first time; 4-pyridine carboxylic acid ( i s o n i c o t i n i c acid, INA); 2-pyridine carboxylic acid ( p i c o l i n i c acid, PA); 3,4-pyridine dicarboxylic acid (3,4PDA); and the analogous other pyridine dicarboxylic acids 3,5PDA, 2,3PDA, 2,4PDA and 2,6PDA. Each of the pyridine derivatives is adsorbed at P t (111) i n a t i l t e d v e r t i c a l orientation; an angle of 70-75° between the p y r i d y l plane and the P t (111) surface is t y p i c a l . Pt-N bonding is evidently the predominant mode of surface attachment of these compounds, although coordination of carboxylate is an important mode of a d d i t i o n a l surface attachment at positive potentials. EELS spectra display strong O-H stretching vibrations near 3550 cm-1 due to carboxylic acids i n the meta and para-positions, and weak-to-moderate signals near 3350 cm due to ortho-carboxylates. -1

c

0097-6156/88/0378-0008$08.25/0 1988 American Chemical Society

2.

HUBBARD ET AL.

Molecular Adsorption at Weil-Defined Platinum Surfaces

N i c o t i n i c acid and related meta-carboxylic acids display the remarkable c h a r a c t e r i s t i c that coordination of the pendant carboxylic acid moieties to the Pt surface i s cont r o l l e d by electrode p o t e n t i a l . Oxidative coordination of the carboxylate pendant occurs at p o s i t i v e electrode potentials, r e s u l t i n g i n disappearance of the 0-H vibration and loss of surface a c i d i t y as judged by absence of r e a c t i v i t y towards KOH. Carboxylate i n the 4-position of pyridine (as i n INA) i s v i r t u a l l y independent of electrode potential, whereas strong coordination of ortho-carboxylates to the Pt surface is present at most electrode potentials. Adsorbed pyridine carboxylic acids are stable i n vacuum; when returned to solution the adsorbed material displays the same chemical and electrochemical properties as prior to evacuation. An era of remarkable progress i n the study of s o l i d electrode surfaces has begun (1-5). This a r t i c l e o f f e r s a b r i e f report of recent progress, arranged as f o l l o w s : rationales for multitechnique i n v e s t i g a t i o n of e l e c t r o d e s u r f a c e s ; well-defined e l e c t r o d e s u r f a c e s as a p a r t of e l e c t r o c h e m i c a l research; evidence f o r the s t a b i l i t y i n vacuum of l a y e r s adsorbed from solution; Auger spectroscopy as a t o o l f o r q u a n t i t a t i o n and c h a r a c t e r i z a t i o n of adsorbed l a y e r s ; e l e c t r o n energy-loss spectroscopy (EELS) f o r investigation of molecular structure and mode of surface attachment of substances adsorbed at electrode surfaces; low-energy electron d i f f r a c t i o n (LEED) as a probe of both s u b s t r a t e s u r f a c e s t r u c t u r e and adsorbed layer long-range order; and voltammetric experiments i l l u s t r a t i n g the influence of electrode surface crystallographic structure on the electrochemic a l r e a c t i v i t y of adsorbed molecules. A substantial added benefit of multi-technique investigations of electrode surfaces i s that the same experimental tools required to bring the electrode surface to a well-defined state happen also to be very p r o f i c i e n t at revealing the various important changes i n the nature of the surface which occur during subsequent chemic a l or electrochemical treatment or use of the electrode. Also, the d i a g n o s t i c power of the m u l t i - t e c h n i q u e approach i s part i c u l a r l y advantageous for i n v e s t i g a t i o n of complicated surfaces. Advances i n technique have brought many complicated surfaces to a well-defined state (3-6). Although i n p r i n c i p l e there are many techniques from which to choose, i n practice i t i s advisable to select techniques which work well together, answer the more important questions about the sample, and y i e l d the necessary informat i o n without appreciably a l t e r i n g the sample. For example, when the objective i s to determine the structure of the all-important f i r s t atomic/ionic/molecular layer at a s i n g l e - c r y s t a l surface, LEED i s the method'of c h o i c e ( 7 ) ; X-ray d i f f r a c t i o n , EXAFS, neutron d i f f r a c t i o n , and atomic/ion scattering, while excellent f o r certain other purposes, are disadvantageous for t h i s task due to l e s s e r s e n s i t i v i t y , g e n e r a l i t y and convenience. Auger spectroscopy i s preferable for measurement of packing density and elemental composition of surface molecular layers (8), although XPS (ESCA,9) or SIMS (10) might also be employed. Vibrational spectra of molecular or atomic layers are obtainable at high sen-

9

10

E L E C T R O C H E M I C A L SURFACE SCIENCE

s i t i v i t y and v i r t u a l l y unlimited frequency range by EELS (11). On the other hand, i n f r a r e d r e f l e c t i o n - a b s o r p t i o n spectroscopy (IRRAS, 12) o f f e r s higher resolution than EELS i n a few s p e c i a l cases, such as carbon monoxide, where the signal due to a c e r t a i n v i b r a t i o n i n a monolayer meets p r a c t i c a l l i m i t s of detection. Surface regular Raman spectroscopy i s showing signs of usefulness f o r molecular l a y e r s at smooth s u r f a c e s and promises to allow d i r e c t comparison between gas-solid and l i q u i d - s o l i d i n t e r f a c i a l Raman spectra at least i n favorable cases (13). C y c l i c voltammetry and chronocoulometry are r e l i a b l e methods for electrochemic a l characterization, along with variations based upon the use of thin-layer electrodes (14), although numerous a l t e r n a t i v e or more complicated methods are a v a i l a b l e (15). Examination of a surface i n the presence of a bulk f l u i d generally requires s a c r i f i c e s i n sensitivity, resolution, signal-to-noise ratio, interfering signals, and other aspects. Such s a c r i f i c e s are pointless i f not repaid by observation of some otherwise-unobservable phenomenon; repayment i s to be expected only i n s p e c i a l situations yet to be fully identified. In any event, no one technique measures enough of the p r o p e r t i e s of a s u r f a c e to b r i n g i t to a w e l l - d e f i n e d state, and accordingly there i s no substitute for a balanced approach to surface characterization based upon Auger, EELS, LEED, IRRAS, and related measurements. Procedures for preparation and c h a r a c t e r i z a t i o n of e l e c t r o d e s u r f a c e s are d i s c u s s e d i n more d e t a i l i n Reference 5 and references cited therein. Before proceeding to methods and applications of electrode surface characterization, perhaps we should remind ourselves of the m o t i v a t i o n s f o r i n v e s t i g a t i n g s u r f a c e s t r u c t u r e and composition. Chemical and e l e c t r o c h e m i c a l processes at s u r f a c e s depend upon surface structure, composition, mode of bonding and related c h a r a c t e r i s t i c s . P r a c t i c a l p r o p e r t i e s such as s u r f a c e s t a b i l i t y , e l e c t r i c a l behavior, photochemistry, c o r r o s i o n , passivation, h y d r o p h i l i c i t y , adhesion, mechanical d u r a b i l i t y and l u b r i c i t y , to name a few, often vary strongly with surface atomic structure and composition. Surfaces shown to be clean by Auger spectroscopy (or equivalent methods) and shown to have a s p e c i f i c atomic structure by LEED (or equivalent methods) are termed "welldefined surfaces". With apologies for stating the obvious, surfaces not d i r e c t l y characterized as to surface structure and composition are not "well-defined surfaces" even when prepared from an oriented single c r y s t a l . This i s because surface s c i e n t i s t s worldwide have observed that s p e c i f i c c r y s t a l planes do not form automatically, even on the surfaces or oriented s i n g l e - c r y s t a l s . Instead, contamination, s t r u c t u r a l reconstruction and disorder are the general r u l e . Only by repeated and persistent experimentat i o n , guided by d i r e c t s u r f a c e c h a r a c t e r i z a t i o n (LEED, Auger, EELS) i s the surface brought to a clean, ordered, "well-defined" state. R e v e r s i b l e e l e c t r o c h e m i c a l r e a c t i v i t y i s present i n the chemisorbed states of some compounds. C h a r a c t e r i s t i c s shared by each of these compounds reported to date are as follows: (a) r e v e r s i b l e e l e c t r o a c t i v i t y i n the unadsorbed state; (b) attachment to the surface by way of a functional group other than the electroactive center;

2. HUBBARD E T A L .

Molecular Adsorption atWell-Defined Platinum Surfaces

(c) and none of the atoms i n the electroactive part of the molecular a r e i n v o l v e d i n chemical bonding d i r e c t l y with the surface. Current-potential behavior of two such compounds, 2,5dihydroxy-4-methylbenzylmercaptan (DMBM) and ( 3 - p y r i d y l ) hydroquinone (3PHQ), i s i l l u s t r a t e d by Figures 1-A and 1-B, i n which only an adsorbed layer i s present to participate i n the electrode reaction. The reactions are:

P r i o r to recording the current-potential curves i n Figure 1-A, the P t ( l l l ) surface was immersed into 0.7mM DMBM, followed by r i n s i n g with pure supporting e l e c t r o l y t e (lOmM t r i f l u o r o a c e t i c a c i d ) . Figure 1-B ( s o l i d curve) shows a similar experiment involving immersion into 0.5mM 3PHQ followed by r i n s i n g with lOmM KF (adjusted to pH 4 with HF). The dotted curves i n Figures 1-A and 1-B are obtained by a variation of the above procedures i n which the adsorbed layer of DMBM or 3PHQ i s subjected to one hour i n vacuum p r i o r to c y c l i c voltammetry. The voltammetric r e s u l t s before and a f t e r evacuation are v i r t u a l l y i d e n t i c a l , demonstrating that DMBM and 3PHQ are retained at the P t ( l l l ) surface i n vacuum and are not removed from t h e s u r f a c e by r i n s i n g with water. Adsorbed 2,2 ,5,5 -tetrahydroxybiphenyl (THBP), a l s o , i s s t a b l e toward evacuation and r i n s i n g , Figure 1-C. Adsorbed hydroquinone (HQ) i s t y p i c a l of numerous h o r i z o n t a l l y - o r i e n t e d , adsorbed simple aromatic compounds i n being stable toward solution and evacuation (3,4), Figure 1-D. Auger spectroscopy y i e l d s data f o r molecular l a y e r s from which r e l a t i v e l y a c c u r a t e and p r e c i s e packing d e n s i t i e s and elemental analyses are obtained. Two methods are available, a l lowing v e r i f i c a t i o n of precision and self-consistency. The f i r s t of these methods i s based upon measurement of elemental Auger s i g nals (IQ, 1^, IQ, etc.), while the other method measures the a t !

f

11

12


»

-0.2

I

1

1

I

0.0

0.2

0.4

0.6

POTENTIAL,

I

I

VOLT vs. Ag/AgCI

1

1

1

1

1

1

1

i

i

1

i

i

i

1

-0.1

0.0 0.1 0.2 0.3 0.4

E L E C T R O D E P O T E N T I A L ( V O L T vs. Ag/AgCI)

Figure 1. Cyclic voltammetry of adsorbed molecules at Pt(lll). Sweep rate, 5 mV/s. A. Solid curve (—): immersion into 0.7 mM DMBM followed by rinsing with 10 mM TFA. Dotted curve (....): as in solid curve, except 1 h in vacuum prior to voltammetry. B. Solid curve (—): immersion into 0.5 mM 3PHQ, followed by rinsing with 1 mM HF. Dotted curve (....): as in solid curve, except 1 h in vacuum prior to voltammetry. Continued on next page.

2.

HUBBARD ET AL.


13

c\J

£ o

< AFTER BEFORE^

/

UHV

UHV

CO

-z. UJ Q

START, \

UJ

rr O

i:

1/xA/cnV

0.0

0.2

0.4

0.6

POTENTIAL, VOLT vs. Ag/AgCI 1 C\J

E
H CO

OH

/

BEFORE-*

OH

1

AFTER J UHV |

" _

/ START

LJ

Q

LJ

(Z 01

:

V

~

6/i A/cm J

(J 1

1

1

0.0 0.2 0 . 4 POTENTIAL,

1

1

0.6 0.8

1

1

1.0

1.2

VOLT vs. A g / A g C I

Figure 1. Continued. C. Solid curve (—): immersion into 2 mM THBP followed by rinsing with 10 mM TFA. Dotted curve (....): as in solid curve, except 1 h in vacuum prior to voltammetry. (Reproduced with permission from ref. 3. Copyright 1984 Elsevier.) D. Solid curve (—): immersion into 0.03 mM HQ followed by rinsing with 10 mM TFA. Dotted curve (....): as in solid curve, except 1 h in vacuum prior to voltammetry.

14


tenuation of substrate Auger signal by the adsorbed layer. Packi n g d e n s i t y measurements by these methods are i n e x c e l l e n t agreement. For example, molecular packing d e n s i t y of 3PHQ a t P t ( l l l ) (-0.1V, pH4, 0.5mM) i s 0.28 nmol/cra based upon I /Ip£, 0.27 nmol/cnr based upon Ip /Ip°> or 0.27 nmol/cm based upon coulometric electrochemical data (16). Shown i n Figure 2-A are the experimental Auger signal r a t i o s IQ/IPJ (carbon signal, 272eV/clean surface Pt signal, 161eV), and I p A p ° (Pt s i g n a l , 161eV, from coated surface/clean surface) graphed versus n i c o t i n i c acid (NA) concentration ( C ^ ) at P t ( l l l ) . Note that Iq/I^ increases with Cma due to increases i n T ^ , while Ip^/IpJ. decreases accordingly. Molecular packing densities obtained from such data using Equations 3 and 4 are shown i n Figure 2-B ( B = 0.377 cm /nmol; f=0.70; and K= 0.16 cm /nmol) for 2

c

t

t

2

2

c

r

=

I

( C

/

I

Pt

)

/

t

6

B

C

(

1

/

3

+

2

T = (1-I /I °)/(9K) p t

p

f

/3)l

( ) 3

(4)

adsorption of NA at -0.2V (vs. Ag/AgCI reference). Packing dens i t i e s of NA at +0.3V are shown i n Fieure 2-C. The saturation NA packing density i s near 0.38 nmol/cm (43.7 A/molecule), which f a l l s between the theoretical packing densities of 0.290 nmol/cm^ (57.2A/molecule) f o r a horizontal orientation, and 0.579 nmol/cm (28.7A/molecule) f o r the N-attached v e r t i c a l orientation, based upon c o v a l e n t and van der Waals r a d i i (17). T h i s i s one of several types of evidence pointing to a f i l l e d - v e r t i c a l orientat i o n of adsorbed NA i n which the pyridine ring forms an angle of 75° with the P t ( l l l ) surface. A model i l l u s t r a t i n g the method of c a l c u l a t i o n of t h i s angle i s shown i n Figure 3. Pyridine and a l l of the pyridine carboxylic acids studied to date are oriented near v e r t i c a l l y at P t ( l l l ) with most or a l l of the angles of t i l t i n the range from 70 to 76°. A brief l i s t i n g of data w i l l be given i n Reference 16. Underlying causes of t h i s recurring orientation w i l l be discussed below a f t e r consideration of further types of evidence. The precision, simplicity and self-consistency of NA packing density data based upon Auger spectroscopy i s characterist i c of data obtained for a wide variety of adsorbed compounds. Auger measurements of t h i s type are a highly recommended s t a r t i n g point f o r studies of molecular substances at electrode surfaces. (See Table I.) A s t r i k i n g feature of the EELS spectra of NA adsorbed from a c i d i c solutions (HF/KF) at negative potentials i s the pressure of a strong 0-H stretching band at 3566 cm , Figure 4-A. This band vanishes when the NA layer i s rinsed with base, Figure 4-C, and also when adsorption i s carried out at positive potentials, Figure 4-B. The EELS spectra are e s s e n t i a l l y the envelope of the gasphase or s o l i d IR spectra (lower curves i n Figures 4-A through 4C). Assignments of the EELS s p e c t r a of analogy with the IR spectra are given i n Table I I . The intensity of the 0-H peak varies smoothly from maximum to minimum as the electrode potential during adsorption i s varied from negative to positive, Figure 5-A. This variation of 0-H signal i s not due to variation i n packing

2.

HUBBARD E T A L .


LOG E o o E 3

cn CO LJ Q

0.5

CNAGTOI/L)

(O) FROM I /I* c

B -

}

(O)FR0M I p / I p

t

0.4 0.3 C0 H 2

02

N

CD

52

15

pH3.3, -0.2V,Pt(lll)

0.1

-5

-4

-3

LOG

C (M)

Figure 2. Auger signal ratios and packing densities of NA adsorbed at Pt(lll). Experimental conditions: adsorption from 10 mM KF adjusted to pH 7 with HF, followed by rinsing with 1 mM HF (pH 3.3); temperature, 23±1 ° C; electron beam at normal incidence, 100 nA, 2000 eV. A. Auger signal ratios: (I(/Ip) and (I /I ) for NA adsorbed at -0.2 V vs. Ag/AgCI reference. B. Packing density of NA adsorbed at -0.2 V. Continued on next page. t

pt

pt

16

ELECTROCHEMICAL SURFACE SCIENCE

W

0.6

E

(0) FROM I /I* c

o o |

05

(0) FROM I

p t

c

f

/Ip

f

tz

CO

0.3

UJ Q

C0 H 2

0.2 O

pH 3.3, +0.3V, Pt(lll)

0.1

_L

0.0

-5

-4 LOG

-3

-2'

-I

C

Figure 2. Continued. C. Packing density of NA adsorbed at +0.3 V.

|4

h

3.28 5

4#-2.03-»| 3

1

I+L20-J09 |^-2.42 - * | l . 2 5 | 1.25 H '

H

MOLECULAR AREA, X « 8.46 2

8.46

4 0

*! •)

(7.33cos 74% 3 . 4 s i n 7 4 ° )

Figure 3. Model illustrating the method of calculation of angle from packing density data.

2.

HUBBARD ET AL.


ENERGY LOSS

17

(cm-1)

Figure 4. Vibrational spectra of NA. Experimental conditions: adsorption from 1 mM NA in 10 mM KF, pH 3 (A and B) or pH 7 (C), followed by rinsing with 2 mM HF (A and B, pH 3) or 0.1 mM KOH (pH 10 for C); EELS incidence and detection angle 62° from the surface normal; beam energy, 4 eV; beam current about 120 pA; EELS resolution, 10 meV (80 cm ) F.W.H.M.; IR resolution, 4 cm . A. Upper curve: EELS spectrum of NA adsorbed at Pt(lll) [pH 3; electrode potential, -0.3 V]. Lower curve in A and B: mid-IR spectrum of NA -1

-1

vapor (18). Continued on next page.

18


ENERGY LOSS

(cm-1)

Figure 4. Continued. B. Upper curve: EELS spectrum of NA adsorbed at Pt(lll). [pH 3; electrode potential, +0.6 V], Continued on next page.



ENERGY LOSS

19

Ccm-1)

Figure 4. Continued. C. Upper curve: EELS spectrum of K NA" adsorbed at Pt(lll). [pH 10; electrode potential, -0.3 V]. +

20


T O

-i

r

r

1-2

A

fee K

0.6

0.5

1.0 h

8 3 CD

6

0.4

0 8

m

CO X

O

0.6

0.3

0.4

0.2

0.2

0.1

CO —I

O X

o CO _l

UJ LJ

-< 3 3

0.0

0.0 -0.8

-0.6

-0.4

-0.2

POTENTIAL,

0.0

0.2

0.6

0.4

0.8

VOLTS vs. Ag/AgCI

o E > CO

z UJ

o o

2 O 2 -0.4

-0.2

0.0

0.2

0.4

0.6

ELECTRODE POTENTIAL (volt vs. Ag/AgCI)

Figure 5. OH-CH signal ratio (EELS) and packing density (Auger) of NA adsorbed at Pt(lll) vs. electrode potential. A. Ratio of EELS O-H (3566 cm" ) to C-H (3068 cm" ) peak height. B. Packing density of K ions. Experimental conditions: (A) adsorption from 1 mM NA in 10 mM KF at pH 7, followed by rinsing in 2 mM HF (pH 3); (B) adsorption from 1 mM NA in 10 mM KF at pH 3, followed by rinsing with 0.1 mM KOH (pH 10). EELS conditions as in Figure 4; Auger conditions as in Figure 2. 1

+

1


Molecular Adsorption at Well-Defined Platinum Surf aces

Table I. Packing Density, r(namomol/cnr) Ring-to-Surface Angle, Jd (degree)

Compound 3PHQ PYR NA INA PA BA 3,5PDA 3,4PDA 2,3PDA 2,4PDA 2,5PDA 2,6PDA

Electrode Potential Volt -0.1 -0.1 -0.2 -0.3 -0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.4

r

r

from I /Ip?

from

W t

P t

0.27 0.45 0.36 0.35 0.29 0.35 0.29 0.26 0.26 0.27 0.29 0.27

0.28 0.46 0.36 0.42 0.28 0.32 0.31 0.32 0.28 0.30 0.35 0.31

from Coulometry 0.27

0 74° 71° 75° 71° 76° 0° 74° 69° ca 60° ca 65° 76° 75°

Experimental conditions: ImM adsorbate i n lOmM KF (pH3), followed by r i n s i n g with 2mM HF (pH3).

3,5 PDA

3,4PDA

2,6PDA

2,3 PDA

2.4POA

21

22


Table II - Assignments of EELS Bands of Adsorbed N i c o t i n i c Acid and Pyridine

Compound NA

NA

pH/ Electrode Potential 3/-0.2

3/+0.6

Peak Frequency cm

Primary Symmetry Species f

3566 3068 1748 1566 1368 1132

C: A A* A A A A ,A"

784 652 465

A ,A" A\A" A", A'

3/-0.1

f

1

f

1

f

3071 1733 1592 1388

C : A* A A A\A"

1192 1117 1007 824 673 463

A A A ,A" A" A" A",A

3055 1537 1471 1250 1134 1000 719 416

0-H stretch C-H stretch C=0 stretch CC stretch CC, CN stretch C-0 stretch; C-H, 0-H bend 0C0, C-H bend CC, 0C0 bend ring; Pt-N stretch

s

C-H stretch C=0 stretch CC stretch CC,CN stretch, C-H bend C-H bend C-0 stretch ring, C-H bend 0C0, r i n g bend 0C0, ring bend ring bend; Pt-N stretch Pt-0 stretch

g

1

T

1

1

1

!

203 PYR

Description

A

f

C : A ,B C-H stretch i» 2 stretch i» ? > stretch B-^ C-H bend A ;B C-H bend; X-sens. A ,A ,B ring; C-H bend; X-sens. B ring, C-H bend 1? 1 8 J ~N stretch 2 V

1

2

A

B

C

A

B

cc

1

C

C

N

2

1

2

2

1

B

A

r i n

b e n d

Pt

2.

HUBBARD E T A L .


density, Figures 4 and 5. The disappearance of the 0-H v i b r a t i o n with increasing positive potential i s due to coordinate covalent bonding of the carboxylic acid moiety with the Pt surface, Equat i o n 5.

At negative potentials i n a l k a l i n e solutions, adsorbed NA retains K ions, as demonstrated by Auger spectroscopy, Figure 5-B. This retention of K ions i s due to i n t e r a c t i o n of K with the pendant carboxylate moiety and greatly exceeds the amounts expected simply from d i f f u s e double-layer interactions. Potential-dependence of K retention i s e s s e n t i a l l y absent f o r compounds i n c a p a b l e of p o t e n t i a l - d e p e n d e n t c a r b o x y l a t e pendancy ( p y r i d i n e , p i c o l i n i c acid, i s o n i c o t i n i c acid and 2,6-pyridine dicarboxylic a c i d ) . Shown i n Figure 6-A are EELS spectra of the entire series of pyridine carboxylic acids and diacids adsorbed at P t ( l l l ) from a c i d i c solutions at negative electrode p o t e n t i a l . Under these conditions a l l of the meta and para pyridine carboxylic acids and diacids exhibit prominent 0-H vibrations (OH/CH peak r a t i o near unity). In c o n t r a s t , at p o s i t i v e p o t e n t i a l s only the paracarboxylic acids display pronounced 0-H vibrations, Figure 6-B. A l l of the 0-H vibrations are absent under a l k a l i n e conditions, Figure 6-C. This s i t u a t i o n i s i l l u s t r a t e d by the reactions of adsorbed 3,4-pyridine dicarboxylic acid: +

+

+

+

23

24


ENERGY LOSS (cm-1) Figure 6. EELS spectra of pyridine carboxylic acids adsorbed at Pt(lll). Experimental conditions: (A and B) adsorption from 1 mM NA in 10 mM KF at pH 3, followed by rinsing with 2 mM HF (pH 3); (C) adsorption from 10 mM KF (pH 3), followed by rinsing with 0.1 mM KOH (pH 10); other conditions as in Figure 4. A. Adsorption at -0.2 V vs. Ag/AgCI (pH 3). Continued on next page.

2.

HUBBARD E T A L .


ENERGY LOSS Ccm-1) Figure 6. Continued B. Adsorption at +0.6 V (pH 3). Continued on next page.

25


i

0

1

M'

1

i

1

'

1

1

i

1

1

1

1

1000

i

1

'

1

1

i ' ' '

1

2000

ENERGY LOSS

i ' * '

1

i

1

1

'

1

3000

i

4000

Ccm-i)

Figure 6. Continued. C. Adsorption at -0.2 V (pH 10).

2.

HUBBARD ET AL.


C0 H 2

Q

0.6 V

/ / / / / / / / / / C0 H 2

-0.3 V

(6)

0.6 V

C0 K

C0 H

2

2

C0 H 2

-0.3 V

2 KOH

C0 K 2

52 / / / / \ / / / /

In summary, para c a r b o x y l i c a c i d groups remain a c t i v e at a l l potentials i n the useful range, while meta carboxylates are complexed with the Pt surface at p o s i t i v e potentials unless sheltered from the surface by ortho-substituents. Ortho-carboxylates are complexed to the Pt surface over most or a l l of the useful potent i a l ranges. LEED patterns of pyridine adsorbed at P t ( l l l ) are shown i n Figure 7-A. Best c l a r i t y of the pattern occurs when the PYR concentration i s lmM. Measurement of the lengths and d i r e c t i o n s of LEED vectors i n Figure 7-A, followed by conversion of the LEED vectors to r e a l space by means of standard formulas, reveals that the structure i s approximately (3.3x4.7) with an included angle of 77°. D i g i t a l simulation of the LEED pattern demonstrates that the best f i t between theory and experiment occurs when the model structure i s Pt(lll)(3.324x4. 738, 77.1°)R34.0°-PYR. In matrix notation t h i s i s : a

l

a

2

b-, b

9

1.684

2.145

-4.255

5.106

The simulated LEED pattern i s shown i n Figure 7-B. Comparison of the simulated LEED pattern with the observed LEED pattern (lower camera shutter speed) i s shown i n Figure 7-C. As can be seen, t h e r e i s good a g r e e m e n t . T h i s PYR l a y e r s t r u c t u r e i s incommensurate; that i s , the mesh vectors of the layer are not exact multiples of the substrate mesh. The nearest commensurate structure would have been (2/3x/21, 79°)R30°, Figure 7-D. A l though t h i s commensurate a l t e r n a t i v e i s numerically quite s i m i l a r

27

28


Figure 7. L E E D pattern and structure of pyridine at P t ( l l l ) . adsorbed at P t ( l l l ) , 51 eV. Continued on next page.

A . L E E D pattern of P Y R

HUBBARD ET AL.


Figure 7. Continued. B. Diagram of the LEED pattern in A. Continued on next page.

30


Figure 7. Continued. C. Comparison of calculated and observed LEED patterns. Continued on next page.


Molecular Adsorption atWell-Defined Platinum Surj'aces

32


Figure 7. Continued. E. Diagram of calculated LEED pattern corresponding to nearest commensurate structure, (2^/3x^/21, 79.1 ° )R30 °. Continued on next page.

2. HUBBARD ETAL.

Molecular Adsorption at Well-Defined Platinum Surf aces

33

Figure 7. Continued F. Structure of the PYR adsorbed layer: Pt(lll)(3.324 x 4.738, 77°)R34°-PYR. I 1.684 2.145 1 In matrix notation: | -4.255 5.106 | The PYR packing density in this structure is 0.421 nmol/cm . 2

34


i i | i i i i \i i i i i i i i i |i i i i i i i »»| • »'

1

l '

6kHz

ENERGY LOSS (cm-1) Figure 8. Vibrational spectra of pyridine. Upper curve: EELS spectrum of PYR adsorbed at Pt(lll) (pH 3); lower curve: mid-IR spectrum of liquid PYR (18). Experimental conditions: adsorption at -0.1 V from 1 mM PYR in 10 mM KF (pH 3), followed by rinsing with 2 mM HF (pH 3); other conditions as in Figure 4.

2.

HUBBARD E T A L .


to the proposed incommensurate structure, i t would have resulted i n a very d i f f e r e n t LEED p a t t e r n , F i g u r e 7-E (3.464x4.583, 79°)R30° or: 2 2 -4 5 Auger spectra f o r PYR indicate that the packing density at the s a t u r a t i o n l i m i t i s 0.45 nmol/cm^ based upon (Ig/Ip°) or 0.46 nmol/cm from (Ip^/Ip^)s t r i k i n g c h a r a c t e r i s t i c of the PYR isotherms (16) i s the v i r t u a l constancy of packing density over f i v e order of magnitude i n PYR concentration (10 M to neat PYR, 12M). These data point to a t i l t e d structure with a ring-tosurface angle, J0=71°, Figure 7-F. Based upon three PYR molecules per uniJ: c e l l , the packing density f o r t h i s structure i s 0.421 nmol/cm , i n good agreement with experiments. The r i g i d constancy of PYR packing density and the incommensuracy of the PYR adsorbed layer are indications that the 71° angle of adsorbed PYR r e s u l t s from a b a l a n c i n g of f o r c e s between sigma-donor bonding of the nitrogen atom to P t ( l l l ) and p i back-donation from P t ( l l l ) into the aromatic r i n g . More extensive C-Pt interaction would have resulted i n energy setbacks due to decreased aromatic character and decreased nitrogen sigma-donation. An EELS spectrum of PYR adsorbed a t P t ( l l l ) from aqueous solution i s shown i n Figure 8. Also shown i s the mid-IR spectrum of l i q u i d PYR (18). The EELS spectrum of adsorbed PYR i s essent i a l l y the envelope of the IR spectrum of l i q u i d PYR, with the exception of a peak at 416 cm" attributable at least i n part to the Pt-N bond. Assignments of the EELS peaks based upon accepted IR assignments (19) are given i n Table 2. There i s also a close correspondence between the EELS spectrum of PYR adsorbed from aqueous solution and the EELS spectra reported f o r PYR adsorbed at Pt s i n g l e - c r y s t a l surfaces from vacuum (20). A

1

References: 1. Hubbard, A.T. Accounts of Chemical Research 1980, 13, 177. 2. Hubbard, A.T. J . Vac. S c i . Technol. 1980, 17, 49. 3. Hubbard, A.T.; Stickney, J.L.; Soriaga, M.P.; Chia, V.K.F.; Rosasco, S.D.; Schardt, B.C.; Solomun, T.; Song, D.; White, J.H.; Wieckowski, A. J . Electroanal. Chem. 1984, 168, 43. 4. Hubbard, A.T. Chemical Reviews 1988, in press. 5. Hubbard, A.T. in "Comprehensive Chemical Kinetics", Bambord, C.H., Tipper, D.F.H., Compton, R.G., eds., Vol. 28, Chapter 1, (Elsevier, Amsterdam, 1988). 6. Somorjai, G.A. "Chemistry i n Two Dimensions: Surfaces", (Cornell University Press, Ithaca, NY, 1981). 7. (a) Duke, C.B. Adv. Chem. Phys 1974, 27, 215; (b) Estrup, P.J. i n "Characterization of Metal and Polymer Surfaces", L.H. Lee, ed., (Academic Press, NY, 1977), Vol. 1, pp 187ff; (c) Somorjai, G.A.; F a r r e l l , H.H. Adv. Chem. Phys. 1971, 20, 215. 8. (a) Chang, C . C . Surface S c i . 1971, 25, 53; (b) Hawkins, D. J . , "Auger Electron Spectroscopy, A B i b l i o graphy, 1927-1975" (Plenum, NY, 1977); (c) Somorjai, G.A.; Szalkowski, F.J. Adv. High Temp. Chem. 1971, 4, 137;

35

36

9.

10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20.


(d) Taylor, New Jersey, in "Techniques of Metal Research", R. F. Bunshah, ed. (Wiley-Interscience, NY, 1971), Vol. 7, pp 117ff; (e) Thompson, M.; Baker, M. D.; Christie, Α.; Tyson, J.F. "Auger Electron Spectroscopy" (Wiley, NY, 1985). (a) Betteridge, D. "Photoelectron Spectroscopy: Chemical and Analytical Aspects" (pergamon, NY, 1972); (b) Carlson, T.A. "Photoelectron and Auger Spectroscopy", (Plenum, NY, 1975); (c) Delgas, W.N.; Hughes, T.R.; Fadley, C.S. Catalysis Revs. 1970 4, 179; (d) Herglotz, H.K.; Suchan, H.L. Adv. Coll. Interf. Sci. 1975, 5, 79. Hoenig, R.E. Adv. Mass Spectrom. 1974, 6, 337. (a) Sexton, B.A. Applied Phys. 1981, 126, 1; (b) Kesmodel, L.L. J. Vacs. Sci. Technol. 1983, A1, 1456. Golden, W.G., in "Fourier Transform Infrared Spectroscopy", Ferraro, J.R. and Bastille, L.J., eds., (Academic Press, Orlando, Florida, 1985), Vol. 4, pp. 315ff. Campion, A. Chem. Phys. Lett. 1987, 135, 501. Hubbard, A.T. Critical Revs. Analytical Chem. 1973, 3, 201. Delahay, P., "Double and Electrode Kinetics" (Wiley, NY, 1965). Stern, D.A.; Laguren-Davidson, L.; Frank, D.G.; Gui, J.Y.; Lin,C.H.; Lu, F.; Salaita, G.N.; Walton, Ν.; Zapien, D.C.; Hubbard, A.T. J. Amer.Chem.Soc. Pauling, L.C. "The Nature of the Chemical Bond" (Cornell Univ. Press, Ithaca, NY, 1960), 3rd edition. (a) Pouchert, C.J."The Aldrich Library of FTIR Spectra" (Aldrich Chemical Co., Inc., Milwaukee, Wis., 1985). (b) "The Interpretation of Vapor Phase Spectra" (Sadtler Research Labs., Philadelphia, 1984), Volume 2. Green, J.H.S.; Dynaston, W.; Paisley, H.M. Spectrochim. Acta 1963, 9, 549. (a) Grassian, V.H.; Muetterties, E.L. J. Phys. Chem. 1986, 90, 5600; (b) Surman, M.; Bare, S.R.; Hoffman, P.; King, D.A. Surface Sci. 1987, 179, 243.

R E C E I V E D June 29, 1988

Molecular Adsorption at Well-Defined Platinum Surfaces - ACS

Recommend Documents