Intense raman spectra of surface carbon and hydrocarbons on silver electrodes

Volume 76. number 3

INTENSE

CHEhlICAL

RAMAN

SPECTRA

PHYSICS

OF SURFACE

15 December 1980

LE-l-l-ERS

CARBON

AND

HYDROCARBONS

ON SILVER ELECTRODES Ralph P. COONEY, Merrlck

R. hIAHONEY

Chemrstr)

of Nebrcastle,

Recelled

7 August

Raman specoa

Department.

and Martm W. HOWARD

New South

R’ales 2308,

Amtraha

1980

spectra habe been recorded

IS anrerpreted

dlffuslon

Unrcersttv

m terms

of the hydrocarbons

of

from sll\er electrodes

red-light

away

from

rransm~rfance

the electrode

of

m salt solutions high

IS shown

surface

area

fo be much

at -0

2 V (WE).

carbons slower

Enhancement

on the electrodes

than

of the

The Llnetlcs of

the rate of formanon

of

hydrocarbons

1. Introduction

A recent Raman study [ll has shown that III the electrochemical system Ag/O.l hi KF (HZO). pH = 5.5-I 1. at -0.2 V (SCE) carbon dewed from CO- IS formed on the surface and at -1.2 V (SCE) this carbon IS reduced to hydrocarbons A subsequent study [2] has revealed that the so-called SERS (sur-

face enhanced Raman spectra) for pyrldme m related systems arlse prlmarlly from adsorption on/m these carbra overlayered surfaces. In the present paper, the earher study [l] has been extended to examine, among other features, the excltatlon curve for the carbon overlayers at -0 2 V (SCE) and the kinetics of formatlon and dlffuslon of the hydrocarbon products The latter aspect of the study provides both a basis for generation of clean sdver surfaces and a framework for optlmizmg yield of hydrocarbon products

2. Experimental The spectroscopic and electrochemxal equipment and materials has been described previously [ 1.21 All potentials m this study are measured and quoted relative to a saturated calomel reference electrode (SCE). The approaches described previously [l-3] have been used m electrode pretreatment with the recent approach [2] preferred for most experiments 448

The pretreatment approaches generated carbon spectra of similar appearance but with greater hne mtensmes observed after the former method [l] This method was employed m obtaining data for the eucitation

tune

of carbon.

The

spectra of the carbon

phase m the exatatlon study were deconvoluted using the method previously described [4] The excltatlon curve was based on the mtensltles of the deconvoluted G bands (=1580 cm-‘) measured relative to the mtenslty of rpl(ClOJ as the G band was the best defined feature in the carbon spectrum. All calculations were based on the same input constraints and were performed on the ICL 1903A dtgltal computer of the Umverslty of Newcastle. In attempts to improve the hydrocarbon yield and to Identify the rate determmmg steps In the CO2 to hydrocarbon conversion, experiments were carried out m which CO-, was passed through the solution firstly at a point well removed from the electrode surface and secondly, over the electrode surface prior to recordmg spectra Both approaches (and especially the latter) lead to an Increase m carbon spectral mtenslty

at -0

2 V whde

the latter

approach

produces a less slgmficant Increase m hydrocarbon bne Intensities at -1 2 V Presumably the passage of gas over the electrode surface physlcally increases the rate of hydrocarbon diffusion from the Interface at the same time as It increases the rate of mass transport of reactant COz species to the Interface.

CHEhfICAL

Volume 76. number 3

PHYSICS

3.1. 77le spectmm of dze carbolt orerlayers (-0 2 IT) The appearance of the surface carbon spectrum in the regions of the disorder or D band (= 1360 cm-‘) and the graphite or G band (=I580 cm-‘) has been observed to be sensltlve to the choice of supporting electrolyte (fig. 1). The intensity ratio I(D)/I(G), may be correiated [5] with the _eraphltlc lamcllar dlmenslon, La. This ratio IS 0.38 (correspondmg to an L, of 8 6 nm) for KCI/H,O solutions The order of magnitude of the L, estimation is supported by calculations based on Clar’s empIrIca equation [6] and assummg that the electronic band at 750 nm reported for the electrode surface [7] is associated with the carbon overlayers. Evtrapolatlon procedures, assuming that the separation of opposite bonds m a six membered fused rmg is 0 245 nm [S] leads to a conjugation length of 4 9 nm i e. in fair accord with the L, estimated from f(D)/I(G) ratios. 1711s calculation was performed assuming that the 750 nm band was the p band (usually the most Intense) of a cata-condensed polyacene structure.

usmg standard thlocyanate solutlon. The sohd appeared

3. Results and discussion As mentloned

m the previous

paper [l] analyses

15 December 1980

this system can be based on two potentials, viz. -0 2 V and -1.2 V. The spectrum at -0.2 V is dominated by features assrgned to surface carbon [I]. while the spectrum at -1 2 V is dommated by lines assigned to hydrocarbons.

The hydrocarbon spectrum at -1.2 V could only arIse (in a CO? reactant-based system) from alkanes or long-cham carbouylates. The posslbllity of the latter product type was ehmmated m two ways Firstly, unsuccessful attempts were made to detect various long-chain carboxylates at the electrode surface after they had been added to the electrolyte_ Secondly, after extended reduction at -1 2 V an extractIon procedure was followed, which would transfer any carboxylates (but not alkanes) mto D20 solution. In this procedure aqueous AgNOJ was added to the solution untd preclpltation, If any, was complete The dried preclpltate was treated with KBr/D20 solution. so that any carboxylate species would be solubihzed III D20. No trace of alkll proton resonances were detected in the proton magnetic resonance spectrum of the D,O so!utlon, mdlcating the absence of long-chain carbouylates. Further the preclpltate from the pH = 11, W solution, after extended reduction at -1 2 V, was analyzed for sliver contamed 78% Ag by weight and therefore to be pure Ag,COA 178.2% Ag by weight).

LEII-ERS

of

I

10K

1

I

1000

1200

IL00

1605

1800

2000

Raman

shrfl

2200

ZLO’)

2600

2aoo

I

3000

I cm-’

Fig. 1. Raman spectra at -0 2 V (SCE) for the systems. (a) Ag/O 1 hi N&F cHzO). (b) Ag/O.l hf KF (HzO). KC1 (H20) (all solutions at pH= 11 0) Exciting lme 5 14 5 nm. 200 mW and 8 1 cm-’ bandpass

(cl Ag/O 1 hf

449

Volume

CHEMIC4L

76, number 3

PHYSICS

LETTERS

15 December

1980

The data of Voet [9] Indicate that a decrease III particle diameter correlates, as expected, with erternal surface area for high surface area carbons GraphIcal evtrapolatlon based on the carbon spectrum in the Ag/CI- system leads to an errenzaisurface area of =I890 m2 g-’ If It IS assumed that

tra of the carbon overlayers, the mtenslty enhancement towards the red IS presumably the result of red transmlsslon of high surface area carbons [9] As the SERS for pyridme at sliver electrodes has been shown to arise pnmanly from adsorptlon on/m such

lamellar dlmenslon can be equated to particle diameter UltrahIgh external surface areas specllied

enhancement of the pyndme spectrum towards the

carbon o\erlayers,

It also appears probable

that the

m progress at present.

red IS also associated with the increase m transmlttance of the surface carbon at longer wavelengths Such an effect provides a simple evplanatlon for the enhancement of O-H vIbratIonal frequencies (VIZ , 3068 cm-’ of pyrldme m the Ag/pyridme systems [2] and the IJ(C-H) of the product alkanes in the present study) which IS unexpected on the basis of resonance based theories [I l] Xs pre\lously described [2] for the qstem. Ag/O.l h15K~$!~!;_q.01hi pyrldme, a single anodlc cycle

The excltatlon curve for the carbon overlayers has been determmed as described in sectIon 2 for the system Ag/O 1 hl KCIOJ (H,O) The curve (fig 2) shows an mcreasr rn Intensity of the carbon spectrum as longer vlslble excltatlon wavelengths are employed This trend IS slmllar to that reported for pyrldme m related systems [IO] In the present case of the spec-

(-0.2 v . .+0.120 V) Is sufficient to ger;erate detectable surface carbon and also to generare the SERS effect for pyrldine adsorbed on/m the carbon overlayers. The intensity of the G band (=1580 cm-‘) of the carbon overlayers was momtored as a function of the number of such cycles TypIcally, I(G) was observed to Increase linearly for

for commercial carbons approach but do not exceed this figure (e.g. Plttsburg VPL, 1150 m’ g-‘) The imprecise estimate of external surface area at =189Om’g-’ IS certain to be much too low If mtercalatlon (mtraiayzr adsorption) occurs as has been suggested for pyrldme [I]. It IS hoped that a more precise quantlficatlon of surface area and ths mass of carbon

in

the o\erlayers

of expenments

W-III be achle\ed

as a result

FIN 2 Excuarlon curve of the G band at AC = 1595 cm-’ relative to r~(Ci0J peaks, 0 Non-decomoluted peabs mdlcated by T 450

at 930 cm-‘. Curve drawn through deconvoluted

Volume

76. number 3

CHEMICAL

PHYSICS

a few cycles and then plateau.

Such results, predtctably, were not reproductble as they depend on the mitral concentratron of oxrdtzed carbon (fuel) m the system Further, If the pretreatment of the srlver electrode at -1.4 V IS unusually long and thorough, subsequent anodic cycles may generate lrttle or no

carbon as evidenced by the Raman spectrum. This observation,

IS possibly associated with either hydrocarbon dtffuston “out of ’ the electrochemtcal system or deoxygenatton of the mterfactal region A feature of the study of the carbon surface phase was the appearance of consrderable “structure” m the D and G band profiles recorded usmg 647.1 nm Kr‘. Some of these mmor features appeared m all red-line spectra whrle others were observed less consistently

Two

meak

consrstently

features

observed

at z-1 170

and

and assigned

=1535

cm-’

were

to polyenes

[12]

Polyenes are mtermedtates in the surface carbonization of hydrocarbons [13]

LETTERS

1.5December

1980

the time spent at -1.2. V (see below) To further complicate any attempt to quantify an average molecular weight, there appear to be adsorption induced shifts m the product hydrocarbon frequencies (fig. 3) comparable to those observed for hydrocarbons physlsorbed on ovrde surfaces [l-l]_ In the present system, the hydrocarbons formed appear to diffuse away from the eiectrode surface at -1.2 V. The rate of this dtffusion IS increased if the potential IS stepped out to -1.4 V where incipient Ha evoiutron asststs the removal of hydrocarbons from the surface. As part of an attempt to define the opttmum condtttons for hydrocarbon formation, the kinetics of the followmg consecutne process was

followed [surface carbon] - -’ ’ X [surface hydrocarbons] -

[sohrtron hydrocarbons].

3.2 7&e specmrrn of the hydrocarbon prodrrcrs (-I 2 If)

rJ\

On steppmg the electrode potential from -0.2 to -1 2 V, the carbon spectrum decreases m mtenstty and a spectrum appears which has been assigned to saturated hydrocarbons (see sectton 2 for the evrdence against carboxylate products). The spectrum from the system, Ag/O.l M KF (HzO), pH = 5, IS as follows strong lines at 2915, 2855 and 2820 cm-’ (assrgned to u(C-H)) and weaker features at 2715 cm-’ (non-fundamental), 1440 cm-’ (CHa or CHS asymmetrrc deformatron) and 830 cm-’ (CH-. or CHj skeletal vrbratrons). Residual surface carbon IS represented by diminished G (=1580 cm-‘) and D (-1360 cm-‘) lures of similar half-band width and Z(D)/Z(G) ratro as the orrgmal carbon spectrum at -02v. Hydrogenatron of a graphite layer would be predrcted to produce saturated hydrocarbons mvolvmg extenswe branching. As with other polymer s) ntheses, the saturated product matenal would

mcorporate a dMrtbution of molecular weights. It IS therefore not surprtstng that the overall C-H stretchmg profile appears to involve at least four components which change their relative mtenstty depending on the history of the electrode surface

and

I

2600

1

2100

2800 2900 Raman snmtl I cm-’

3000

3100

hg 3 Raman spectra of the 1 (CH) region for ta) n-octane; (b) Ag/O.l hf KF (H,O), pH =5 0 at -1-Z V (SCE) Excltmg lme 5 14 5 nm. ZOO mW and 8.4 cm-’ bandpass

Volume

76. number

3

CHEMICAL

PHYSICS

LE-i-I-ERS

15

December 1980

observation of surface enhanced Raman scattering from pyrldine [2], benzene [15], carbon dloxlde [2] and alkanes (this study)

Acknowledgement

I 0

r 5

I

10

1

15

I

1

20 i5 lrm4 linm

1

30

;5

I

1

LO

LS

;o

Fig 1 Intensity-time cur\es for the formatlon (Inset) and dlffuston of hydrocarbons at the slher electrode In the system Ag/O 1 bI KF (H=O), pH= 1 I 0 (The formatlon cutle was recorded usmg the N~colet 1074 computer and the dlffuslon cur\e recorded usmg a chart recorder )

As fig 4 reveals the first step 1s rapld. with mtenstty m the r*(C-H) region peakmg at less than 1 s (after steppmg from -0 2 V to -1 2 V) and the second step IS slow (half-hfe =18 mm) The hghter hydrocarbons would selectlveiy d:sorb and diffuse away from the surface leavmg the heavier molecular weight (or more --wax-Ike”)

hydrocarbons

to accumulate

on the sur-

face_ These “wax-hke” molecules would partially seal the surface and slow the overall dtffuslon process The surface hydrocarbons are detected only while residual surface carbon IS also detectable The Intense hydrocarbon spectra at -1 2 V are therefore a further example of SERS arlsmg from adsorptlon on/m the carbon overlayers. High surface area carbons are Ideal adsorbents for hydrocarbons

1. Conclusion UltrahIgh surface area carbon overlayers of the type characterized m this study have been recently shown by the present authors to be essential to the

4.52

The authors are grateful to the Australian Research Grants CommIttee for a post-doctoral fellowshtp for M.W H. and for the equipment used m this study. They are grateful to the Australian Department of Education for a postgraduate award for M.R M.

References [l]

hl R hlahon?y. h1.W. Howard and R P Coonry. Chem Phys Letters 71 (19801 59 [?-I hl W Howard, R P. Cooney and A J McQulllan, J Raman Spcctry 9 (1980) 273 [3] R P Cooney. E S Retd. hI Flelschmann and P J Hendra J Chem Sot Faraday I 73 (1977) 1691 [a] P Tsat and R P Cooney. Chem Geol 18 (1976) 187 [5] F Tumstra and J L Koemg. J Chem Phls 53 (1970) 1116

[6] H H JafiZ and hf Orchm. Theory and apphcattons ulrrarmler specrroscopy (Why New York, 1966)

of

p 287 [7] B Pettmger. U Wentung and D hf Kolb, Ber Bunsenges Physlk Chem 82 (1978) 1325 [8] P N Cheremlsmoff and F Ellerbusch. Carbon adsorptlon handbook (Ann Arbor Science, Ann Arbor. 19781 p 213 [9] A Poet. Rubber Age 82 (1958) 657 [IO] J A Creighton, M G Albrecht. R E Hester and J A D Matthew. Chem Phys Letters 55 (1978) 55 [I 11 T E Furtak and J. Reyes. Surface SCI 93 (1980) 351 [12] J Behrmger. m Raman spectroscopy, Vol I. ed H A Szymanskt (Plenum Press, New York. 1967) p 186 [13] P E. Eberley. J Phys Chem 71 (1967) 1717. [ 141 L H Little, infrared spectra of adsorbed species (Academtc Press, New York, 1966) p 302 [IS] hI W Howard and R P Cooney. to be pubhshed