Effect of the bulk concentration of formic acid on the distribution of the adsorbates at a smooth rhodium electrode in acid medium : an EMIRS investiga

Journal of Electron Spectroscopy and Related Pherumena,

Elsevier Science Publishers B.V., Amsterdam-Printed

45 (1987) 153-160

153

in The Netherlands

EFFECT OF THE BULK CONCENTRATION OF FORMIC ACID ON THE DISTRIBUTION OF THE ADSORBATES AT A SMOOTH RHODIUM ELECTRODE IN ACID MEDIUM : AN EMIRS INVESTIGATION. M. CHOY de MARTINEZ, B. BEDEN, F. HAHN and C. LAMY

Laboratoire Universite

de Chimie 1,Electrochimie et Interactions,U.A.auCNRS no 350, de Poitiers, 40 avenue du Recteur Pineau,86022Poitiers(France)

SUMMARY The dependence on the bulk concentration of formic acid of the nature of the adsorbed species formed at a rhodium electrode was investigated, using Electrochemically Modulated Infrared Spectroscopy (EMIRS]5.C0 species are formed at all HCOOH bulk concentrations in the range 10 toads 10 M. Above low2 M the linearly bonded CO is predominent while the bridge-bonded CO species becomes the major species at the lowest concentrations.Furthermore the minor contribution of a third species, presumably a COads species engaged in a higher coordination site, is also detected. INTRODUCTION Numerous studies during the last few years have aimed to investigate the adsorption and oxidation processes of formic acid at a rhodium electrode, in aqueous media, using purely electrochemicalmethods (ref. I), radiochemical (refs. 2-3) or spectroelectrochemicaltechniques, either in the UV-visible range (ref. 4), or in the infrared range (ref. 5). This latter technique, developed by BEWICK et al. (ref. 6), has been demonstrated to be particularly suitable for the in situ identificationof the adsorbed species resulting from the chemisorption of small organic molecules at electrode surfaces. For instance, using ElectrochemicallyModulated Infrared Reflectance Spectroscopy (EMIRS), it was shown that the electraoxidationof CH30H (refs.7-IO),HCOOH (refs. ll12), (CH20H)2 (ref.l3), and C2H50H (ref.l4), at Pt electrodes leads always to a fast poisoning of the surface by adsorbed CO species, thus explaining the decrease of the Pt catalytic properties. In a preliminary study of the HCOOH adsorption on rhodium (ref.5), similar conclusions were reached, but with a different distribution of the CO adsorbed species. From this point of view, the system is veng interesting and is worth studying in detail, particularly with respect to the dependence of the composition of the adsorbed layer on the bulk concentration of the electroactive species. EXPERIMENTAL EMIRS is used in this work. The technique is widely described in refs. 6 and 15-17 and is not discussed further.

036%2048/67/$03.50

0 1987 Elsevier Science Publishers B.V.

154 The experimental truments,

set-up

England)

detected

an EMIRS

III spectrometer

microcomputer.

by a liquid N2 cooled Hg Cd Te detector

USA), then demodulated stored and processed

by a PAR-5204

The working

(high purity

spectroelectrochemical

electrode

tial measurements The solutions

are quoted

are prepared

HC104 and "pro analysi"

(Infrared

Associates

The output

Inc.,

signal

is

cell is fitted with a CaF2

polished

on the Reversible

with Millipore

InSbeam is

ratio AR/R.

is a 10 mm diameter

99.995 %, Johnson-Matthey)

(Hi-Tek

The infrared

lock-in amplifier.

to the final dimensionless

The three-electrode window.

includes

driven by a Commodore

polycrystalline

rhodium disc

with fine alumina.

Hydrogen

Electrode

Super Q water,

The poten-

(RHE) scale.

"Suprapur"

Merck

Merck HCOOH.

RESULTS All the EMIRS spectra were taken cover widely

the domain

corresponding

of the CO vibrational

to the adsorbed

in electrocatalysis. all experiments,

in the range 1600-2350

species

The supporting

cm-' in order to

bands, especially

the bands

which are known to play an important

electrolyte

while the concentration

was fixed at 0.25 M HC104

of HCOOH

was varied from

role in

1 x 10-5 M

to 5 M. Prior to the spectroscopic rhodium

electrode

investigations,

were taken either

indispensable,

for understanding

the behaviour

of the potential

i) Cyclic

voltammetry

Several rhodium

voltammograms

are given

for the electrochemical

electrolyte

sweep. At x = 10s3 M, however,

peak appears,

of the cell, but also system,

oxidation

with hydrogen

the negative region,

concentration

adsorption

is clearly

any oxidation

of

sweep a second oxidation

formic

decrease.

is nearly complete

potential

seen and gives a peak

near 0.25 V/RHE. These two peaks

acid,

while the currents

It is estimated,

region, that the coverage

from HCOOH chemisorption

by adsorbed

at x&IO-'

from the evo-

species

resulting

M.

The voltatmnograms (fig. 1) were recorded in the spectroelectrochemical but with the electrode against

distortions

the window,

pulled back from the window.

which

the

is similar to

at least during the positive

the oxidation

at 0.65 V/RHE. During

lution of the hydrogen

principally

of x M HCOOH on

alone and does not display

species,

close to the hydrogen

develop with the increasing associated

of the electrochemical

or in

studies

limits.

from the electroactive

with a maximum

the cleanliness

in fig. 1 . At x = 10-5 M, the voltammogram

that for the supporting current

Such purely electrochemical

not only for testing

influence

of the

in the 0.25 M HC104 base electrolyte

the 0.25 M HC104 t x M HCOOH solutions. were

cyclic voltammograms

is the position

When the electrode

for the spectroscopic

cell, is pushed

studies,severe

of the i(E) profile occur, at low x values, because of mass trans-

155 fer limitations in the thin layer of electrolyte. However such limitations are not critical in the present spectroscopic study, since it is the adsorption which is of interest and not the oxidation process itself.

.lOi/mA cti*-. l.l-

O-. l.2O-.2-

.2O-.2t

,

,

,

0

.5

1

1

EIWRHE) Fig. 1. Voltammograms for a rhodQm electrode Q-I0.25MHC10t3 + x M HCOO_H,

solution at 25'C and v = 50 mV s

; a) x = 10 ; b) x = 10

;c)x=lO

;

d)x=l.

ii) EMIRS spectra of HCOOH adsorbates at a rhodium electrode Whatever the concentration of formic acid in the bulk, and at variance with the results on platinum (ref.9), two main EMIRS bands near 1900 and 2020 cm" are always seen in the spectrum (fig.2). However their position (wavenumber center) and intensity (peak to peak intensity) depend markedly on the HCOOH bulk concentrations. In addition, a shoulder is always seen at slightly lower -1

wavenumbers i.e. near l&l0 cm

.

156

species resulting from the chemisorption Fig.2. EMIRS spectrum of the CO of 0.1 M HCOOH at a rhodium ele@!>ode. Limits of potential :Ec=O, E, = 0.4 V/RHE ; f = 13.5 Hz.

All of the EMIRS spectra obtained during this investigationexhibit the same characteristics as the one given in fig.2. The band near 2020 cm-' is -1 attributed to linearly bonded CO (on one Rh site) and the one at 1900 cm corresponds to bridge-bonded CO, i.e. bonded to two Rh sites. By analogy with the solid-gas interface (ref.l8), the shoulder at ca. 1840 cm-' might be related to COads species engaged in a higher coordination site, such as a three fold (or more) bridge-bonded CO species. It has to be emphasized that in contrast to platinum (ref.9), the three bands are always present, even at the lowest concentrations of HCOOH in the bulk. The evolution of the wavenumber centers with respect to the log of the HCOOH concentrations,x,is given in fig.3. It is interesting to note that the three bands shift positively with increasing x, but that saturation is observed at lower concentrations for the bridge-bondedspecies(x>5.10W3 M), than for the linearly bonded one (x > 0.5 M). Confirmation is given by the dependence of the intensity of the bands (Ipp , peak to peak intensity for the bipolar bands and Ip

, absolute intensity for

the single-sided band), vs log x, shown in fig.4 . The two types of bridgebonded CO species predominate at low HCOOH concentrations,thenthere is a transition near x= tom2 M which leads to more linearly bonded species when x iS increased further. The intensity of the COL band reaches 0.75 10W3, in absorbance units, (i.e. slightly more than a full covered Pt surface by the same COL species under the same experimental conditions), then decreases at x > 0.5 M, presumably because of HCOOH molecular adsorption.

2000

Fig.3. Dependence of the band center wavenumber,;, of the EMIRS bands detected in the range 1600-2200 cm-', vs. the concentration of HCOOH in the bulk.

0 J

1950

___~o___-o---a~--o--o 1900

.

.

I

I)---q--_-o

op__-a---5

l I

1850

.

.

0

OX’

c

qy-p-_

.* X X

X

X

X

1800

d -5

I

-4

I

I

-3

-2

I

I

0

-1

1

logkhiol ilr

Fig.4. Peak to peak intensity, I of the CO and CO bands (@ ibsolute intensity ! for the shoulder) as a fu&tPoA of the HCOOH concentration in the bulk. AE = 0.4 V ; E = 0.2 V/RHE

.8-

(-_) COL ; (---I

,6-

COB;

I

1

I -5 -4

-3

I

I

I

0

1

(-.-.-)shoulder. .4-

.2-

O-

1 -2 -1

log(c/m01+)

158 iii) Dependence

of the intensity

If the mean potential intensity

of the bipolar

for the bridge-bonded pulses there positive

of the bands,

vs. AE, the height of the pulse

E of the pulse is fixed at 0.2 V/RHE, the peak to peak bands

increases

linearly with

AE, until

AE = 0.4 V

CO and 0.5 V for the linearly bonded CO (fig.5).Atlonger

is a drastic

decrease,

due to oxidation

limit of the pulse and depletion

calculated

slopes

comparable

to the value for platinum

of CO into CO2 at the

of the adsorbed

layer (fig.6). The

give 1.4 10S3 a.w.V -' for CO, and 1.8 10m3 a.u.V -' for Cot (fig.5).

.6

0

Fig.5.

Dependence

iv) Influence

of Ipp vs

the characteristic

amplitude,

AE = 0.4 V

concentrations

there

of HCOOH

.

, is illustrated

of the

in fig.6 for

in the bulk. At x = 10m3 M , COD When the concentration

is more COL than COD at low E values.

process of CO into CO2 starts, before COD

E

at all E until its oxidation.

sed, however,

of the pulse. o = COL ; x = COD

of the COD and COL bands vs. the mean potential

at constant

predominates

AE, the height

of the mean potential

The dependence pulse,E,

-4 .3 AEN(RHEI

it is obvious

is increa-

When the oxidation

from fig.6 that COL is oxidized

,

IO3x Ipp , I .6-

I

- --*

*_-

I \

.4-

I

\

.2 O.6.4.2O-

.4

.2

0

.6

.4

:.2

0

.6

"E/v(R~-~EI

i3vI~i-i~)

Fig.6. Evolution of the intensity of the COL and COB bands, with E, the mean potential of the pulse at AE = 0.4 V , and various x.M HCOOH. a) dependence of I vs. E (-- COL ; --- COB) ; b) dependence of the ratio. Ipp co9 / Ipp COL pp vs. E .

CONCLUSION From the above spectroscopic drawn

several

conclusions

can be

: - the dissociative

surface

produces

platinum,

chemisorption

CO adsorbed

- bridge-bonded ted unambiguously

species

of formic acid at a rhodium

electrode

;

CO (COB) and linearly bonded CO species

at 1900 and 2020 cm-' respectively,

(COL) are detec-

and in contrast

at the rhodium surface whatever -5 to 5 M; in the bulk over the range IO

to

are both present

the HCOOH concen-

- beside CO9 and COL

, a third type of COads species is also detected,

tration

giving

investigation,

rise to a weak absorption

band at ca. 1840 cm-',most

probably

a multi-

160 bonded CO species

engaged

in a higher coordination

- the ratio of the intensity bulk concentration Furthermore present

activity confirm

of HCOOH and decreases

when the HCOOH concentration

the fact that several CO adsorbed

at the rhodium

a small organic

electrode

molecule

surface,

species

on the increases.

are simultaneously

as a result of the chemisorption

like HCOOH, may account

of Rh comparatively

site (three fold or more);

of the COB to the COL bands depends

to Pt. Experiments

for the lack of catalytic are still in progress

to

this point of view.

ACKNOWLEDGEMENTS The authors Southampton,

are very grateful

for reading

to Prof. A. BEWICK,

University

of

the manuscript.

REFERENCES

7 8 9 IO 11 12 13 14 15 16 17 18

A. Capon and R. Parsons, J. Electroanal. Chem., 44 (1973) 231. A. Wieckowski, J. Sobkowski and P. Zelenay, J. Electroanal. Chem., 84 (1977) 109. J. Sobkowski and P. Zelenay, J. Electraanal. Chem., 91 (1978) 309. R.R.. Adzic and A.V. Tripkovic, J. Electroanal. Chem., 99 (1979) 43. F. Hahn, B. Beden and C. Lamy, J. Electroanal. Chem., 204 (1986) 315. A. Bewick, K. Kunimatsu, B.S. Pons and J.W. Russell, J. Electroanal. Chem., 160 (1984) 47. B. Beden, A. Bewick, K. Kunimatsu and C. Lamy, J. Electroanal. Chem., 121 (1981) 343. K. Kunimatsu, J. Electroanal. Chem., 145 (1983) 219 ; J. Electron. Spectroscopy, 30 (1983) 215. B. Beden, F, Hahn, S. Juanto, C. Lamy and J.M. Leger, J. Electroanal. Chem., 225 (1987) 215. S. Juanto, B. Beden, F. Hahn, C. Lamy and J.M. Leger, J. Electroanal. Chem., in press. B. Beden, A. Bewick and C. Lamy, J. Electroanal. Chem., 148 (1983) 147. B. Beden, A. Bewick and C. Lamy, J. Electroanal. Chem., 150 (1983) 505. F. Hahn, B. Beden, F. Kadirgan and C. Lamy, J. Electroanal. Chem., 216 (1987) 169. B. Beden, M.C. Morin, F. Hahn and C. Lamy, J. Electroanal. Chem., 229 (1987) 353. A. Bewick and B.S. Pons, Advances in Infrared and Raman Spectroscopy, R.J.H. Clarke and R.E. Hester (Eds), chap.1, vol.12 (1985). B. Beden, Spectra 2000,13 (95) (1984) 19 ; 13 (96) (1984) 31. B. Beden and C. Lamy, in R.J. Gale (Ed), Spectroelectrochemistry, Theory and Practice, Plenum Press, New York, in press. N. Sheppard and T.T. Nguyen, in R.J.H. Clarke and R.E. Hester (Ed), Advances in Infrared and Raman Spectroscopy, vol.5 Heyden and Son Ltd, London,1978, 67-148.

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