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SNIFTIRS with a flow cell: the identification of the reaction intermediates in methanol oxidation at Pt anodes
Necrrockimica Aera, Vol. 33, No. I I, ~9. 1691-1694. Printed in Great Britain.
oou-4686188 53.oof0.00 Pergamon Prwspk.
1988.
SHORT
COMMUNICATION
SNIFTIRS with a flow cell: the intermediates in methanol Department
identification of the reaction oxidation at Pt anodes
R.J. Nichols and A. Bewick of Chemistry. University of Southampton. Southampton SO9 5NH. England (Received 28 July 1988)
A spectroelectrochemical flow cell was used with the SNIFTIRS method to obtain spectra from the adsorbed reaction intermediates involved in methanol oxidation The flow of solution prevented at a platinum electrode in acid solution. depletion of the reactant in the thin electrolyte layer and allowed active Adsorbed COH and CHxOH were conditions for oxidation to be maintained. Identified as the intermediates. An increasing number of electrochemical laboratories worldwide are now using in-situ infra red spectroscopy to identify both adsorbed and solution free species involved in electrode reactions. notably EMIRS Cl.21 and SNIFTIRS The external specular reflectance methods. These, together with IRRAS 14.51. need 12.31. are the techniques most widely employed. to use a thin layer spectroalectrochemical cell in which the IR radiation traverses only a thin layer of the electrolyte solution on its path between the window and the reflecting electrode. Typically, thicknesses in the range l-20um are sufficient to ensure adequate transmission of radiation in the presence of electrolyte components which are strong IR adsorbers. The presence of this thin electrolyte layer has major consequences for current distribution over the electrode surface, the response time of the electrode following a change in the applied potential and the possibilities for diffusive interchange of species with the bulk electrolyte outside the thin layer. The EMIRS and SNIFTIRS methods each achieve surface selectivity and submonolayer sensitivity by subtracting the spectra at two different potentials, one being a reference potential and the other the sample potential. The EMIRS method requires these two states The SNIFTIRS method has the possibility to be accessed at least several times par second. to increase this time scale substantially but dwell times at each potential in excess of a few minutes leads to inadequate cancellation of electrolyte absorbances due to the inability to maintain the thickness of the thin layer constant within molecular dimensions over such long periods. Diffusive interchange either to replace consumed reactant or remove reaction products is impossible on this timescale. The root mean s uara value for the distance moved by a molecular species with diffusion coefficient low5 cm9 s-1 in 100s is about 5 x 10-2 cm. It should be noted however that significant movement of species by migration can occur on the timescale of the experiment, i.e. the electrical double layer will be able to reach equilibrium on a timescale faster than 0.1s for concentrated electroand the rest of the electrolyte in the thin layer will maintain its lyte solutions. original base electrolyte composition. The consequences of reactant depletion are particularly severe for electrocatalytic reactions such as the oxidation of methanol at a Pt anode, e.g. it becomes impossible to maintain the system in the region of active oxidation where the surface coverage by poisoning species is low and adsorbed reaction intermediates are at high concentration. Thus one of the early successes of in situ IR spectroscopy was the identification of the adsorbed CO poison [61 but identification of the reaction intermediates has been very difficult. Recently we have had limited success in overcoming the latter problem by manual replenishment of the thin layer periodically during the period of spectral acquisition 171. We now report the use of a continuous flow thin layer cell which allows clear identification of the intermediates. The
flow
cell
The flow cell, fig. 1, resembles our normal SNIFTIRS cell but the working electrode disc has a small hole (approx. 0.5 mm diameter) drilled at its centre. Hydrostatic pressure applied from an elevated reservoir of electrolyte connected to the cell and suction of electrolyte up the tube sealed onto the working electrode combine to give continuous flow of the electrolyte from the bulk solution at the circumference of the disc to the outlet The balance between the suction and hole at its centre thus ensuring radial symmetry. the pressurisation is set to maintain a reduced pressure in the thin layer. thus holding A flow rate of 1 L in 5 hours is easily the electrode firmly against the window. obtained: sufficient to replenish the thin layer about ten times per second.
1691
R.
1692
The
oxidation
of
The
measurements
and
A.
BEWICK
methanol were
Fig. 0.5M HCl04. potentials PA and hydrogen adsorption
carried
2 shows used PB. region
clean, by of
J. NICHOLS
active electrode holding the electrode the figure.
a
To
free at
suction
out
at
a
polycrystalline
Pt
electrode
the linear sweep voltammogram to obtain the difference spectra and the other in the region of of poison l.OV for
was produced a short time;
with
O.OlM
for this system are marked: active oxidation
at the start of this is illustrated
each in
methanol
in
and the two one is in the of methanol. acquisition the lower
period part
A
pump
From
reservoir
T
Cleaning
mspectral m
acquisition
Cleaning
period
time
IR Figure
Fig. 3 bands,
1.
beam
The
shows mean
bands.
Figure at 100 CH30H.
thin
layer
flow
absorption
at
PB.
respectively the formation of at the more positive potential. are produced 1440 cm-l at
U per: linear sweep voltammogram s.- Y for Pt in 0.5M HCIO + O.OlM Lower: sketch of pulse se!uence.
cell.
a difference spectrum obtained more absorption of radiation at
more
2. mV
Large CO2
positive
and the The large
of
about
5
x 10-4 in an
10s2
absorbance, active state.
the
way: PA
bands
increasing negative
disappearance
at
downward and upward 2345 cm-l
amount band at
showing A fully
that the poisoned
of
CH30H
of ClO41040 cm-1
going going and
bands,
negative
bands. 1110 cm-l
positive show
in the double layer and the smaller ones
oxidation at potential Pg. Bands at 2070 cm-1 and 1850 is from water in the layer. Th_qvery large band at 1640 cm -1 the former being the linearly bound respectively are from the adsorbed CO poison, These bands are relatively weak. f”,“,m and the latter the more highly coordinated form. approx. maintained
by
in this potential
flow system electrode
by
enables the gives bands
electrode with an
to be amplitude
absorbance.
These are positive bands are seen at 1425 cm-l, 1320 cm-l and 1215 cm-l. not observed when the more positive They are shown in more detail in figs. 4 and 5. to +O.ZV where oxidation is very is lowered from the active region, +0.45V. potential. Pg. We assign these bands to the reactive intermediates ZCOH and ZCHOH (or xCH2OHl: slow. at present our data do not allow us to distinguish between the latter two possibilities. Our detailed assignments are as follows:
Additional
small
1320
cm-l
vC0
of
fCOH)ads
1215
cm-l
vC0
of
(CH,OH),ds:
1425
cm-’
dCOH
of
(COH),ds
x
=
1 or
2
SNIFTIRS
with a flow cell
1693
These are based upon data from a range of organic alcohols in which the carbon atom to which the OH group is attached has a variety of bonded neighbours and a variety of The It is also supported by ab-initio MO calculation [8]. hybridisation states. assignments will be fully discussed in a full paper to follow.
Figure
3.
Difference and
spectrum
for
PD 500 mV l SCEl
PA0
mV
.
9
8T lb1
kR/lo - 4
+Rllo-4
6
2
h_
4
2
I
yF$y&
0 c 12.fLO
cm
1240
1200
cm
*,
1
0. 1500
.A
1160
Figure 4. Difference spectra for PA OmV and (a) PD ZOOmV. lb) PS 450mV.
Figure and
PS
1400
5.
Difference
-1
1300 spectrum
for
1 PA OmV
5OOmV ISCE).
Our Finally. fig. 6 shows how the various species observed fit into the reaction schame. earlier measurements I91 without a flow cell had indicated the presence of some of the new In addition, a carbonyl bands reported here but they were difficult to identify properly. stretch possibly corresponding to (CHD)ads has also been detected by IMIRS and by SNIFTIRS [9.101.
R. J. NICHOLS
1694
CO2
1
( CH30H
lads
T
CO2
+ H20
+2’OHjads
Iads T
‘CHojads
I A reaction Figure 6. showing the adsorbed
CO2
+ 2H20
+3(OH
weakly adsorbed
and A. BEWICK
>
I.“+
+n’OH
“Ojads
jade
+ H20
scheme for oxidation of methanol at a Pt electrode in acid poisons and major products. reactive intermediates,
solution
Acknowledgements The generous ledged.
support
of the
SERC
for
equipment
and
a studentship
is gratefully
acknow-
References 1. 2. 3. 4. 5. 6. 7. a. 9. 10.
Pons and J.W. Russell, J. Electroanal. Chem.. B.S. A. Bewick, K, Kunimatsu. -160 47 ’1984). A. Bewick and B.S. Pons in “Advances in Infrared and Raman Spectroscopy”. vol. 12. ed. R.J.H. Clark and R.E. Hester, 1985 Wiley Heyden. p.1. S. Pans, T. Davidson and A. Bewick, J. Electroanal. Chem., 1160, 63 ‘1984). J.W. Russell, J. Overend. K. Scanlon, M. Sever-son and A. Bewick. J. Phys. Chem., 86. 3066 ‘1982). J.C. Gordon and M.R. Philpott, Langmuir, 2, U64 K. Kunimatsu. H. Seki, W.G. Golden, 11986). B. Beden. C. Lamv. Chem., 112. 343 , A. Bewick and K. Kunimatsu. J. ‘Electroanal. (1981). R.J. Nichols and A. Bewick. ISE meeting, Maastricht, Shi-Gang Sun, J. Clavilier. abstract No. 1 .l . September 1987. extended J.M. Bowman, J.S. Bittmann and L-B. Harding, J. Chem. Phys., 85. 911 ‘1986). Shi-Gang Sun, J. Clavilier, R. J. Nichols and A. Bewick, in preparation. A. Bewick in “Trends in Interfacial Electrochemistry”. ed. A. Fernando Silva, Nato ASI Series C 179, 1986 Reidel, Dordrecht.
oou-4686188 53.oof0.00 Pergamon Prwspk.
1988.
SHORT
COMMUNICATION
SNIFTIRS with a flow cell: the intermediates in methanol Department
identification of the reaction oxidation at Pt anodes
R.J. Nichols and A. Bewick of Chemistry. University of Southampton. Southampton SO9 5NH. England (Received 28 July 1988)
A spectroelectrochemical flow cell was used with the SNIFTIRS method to obtain spectra from the adsorbed reaction intermediates involved in methanol oxidation The flow of solution prevented at a platinum electrode in acid solution. depletion of the reactant in the thin electrolyte layer and allowed active Adsorbed COH and CHxOH were conditions for oxidation to be maintained. Identified as the intermediates. An increasing number of electrochemical laboratories worldwide are now using in-situ infra red spectroscopy to identify both adsorbed and solution free species involved in electrode reactions. notably EMIRS Cl.21 and SNIFTIRS The external specular reflectance methods. These, together with IRRAS 14.51. need 12.31. are the techniques most widely employed. to use a thin layer spectroalectrochemical cell in which the IR radiation traverses only a thin layer of the electrolyte solution on its path between the window and the reflecting electrode. Typically, thicknesses in the range l-20um are sufficient to ensure adequate transmission of radiation in the presence of electrolyte components which are strong IR adsorbers. The presence of this thin electrolyte layer has major consequences for current distribution over the electrode surface, the response time of the electrode following a change in the applied potential and the possibilities for diffusive interchange of species with the bulk electrolyte outside the thin layer. The EMIRS and SNIFTIRS methods each achieve surface selectivity and submonolayer sensitivity by subtracting the spectra at two different potentials, one being a reference potential and the other the sample potential. The EMIRS method requires these two states The SNIFTIRS method has the possibility to be accessed at least several times par second. to increase this time scale substantially but dwell times at each potential in excess of a few minutes leads to inadequate cancellation of electrolyte absorbances due to the inability to maintain the thickness of the thin layer constant within molecular dimensions over such long periods. Diffusive interchange either to replace consumed reactant or remove reaction products is impossible on this timescale. The root mean s uara value for the distance moved by a molecular species with diffusion coefficient low5 cm9 s-1 in 100s is about 5 x 10-2 cm. It should be noted however that significant movement of species by migration can occur on the timescale of the experiment, i.e. the electrical double layer will be able to reach equilibrium on a timescale faster than 0.1s for concentrated electroand the rest of the electrolyte in the thin layer will maintain its lyte solutions. original base electrolyte composition. The consequences of reactant depletion are particularly severe for electrocatalytic reactions such as the oxidation of methanol at a Pt anode, e.g. it becomes impossible to maintain the system in the region of active oxidation where the surface coverage by poisoning species is low and adsorbed reaction intermediates are at high concentration. Thus one of the early successes of in situ IR spectroscopy was the identification of the adsorbed CO poison [61 but identification of the reaction intermediates has been very difficult. Recently we have had limited success in overcoming the latter problem by manual replenishment of the thin layer periodically during the period of spectral acquisition 171. We now report the use of a continuous flow thin layer cell which allows clear identification of the intermediates. The
flow
cell
The flow cell, fig. 1, resembles our normal SNIFTIRS cell but the working electrode disc has a small hole (approx. 0.5 mm diameter) drilled at its centre. Hydrostatic pressure applied from an elevated reservoir of electrolyte connected to the cell and suction of electrolyte up the tube sealed onto the working electrode combine to give continuous flow of the electrolyte from the bulk solution at the circumference of the disc to the outlet The balance between the suction and hole at its centre thus ensuring radial symmetry. the pressurisation is set to maintain a reduced pressure in the thin layer. thus holding A flow rate of 1 L in 5 hours is easily the electrode firmly against the window. obtained: sufficient to replenish the thin layer about ten times per second.
1691
R.
1692
The
oxidation
of
The
measurements
and
A.
BEWICK
methanol were
Fig. 0.5M HCl04. potentials PA and hydrogen adsorption
carried
2 shows used PB. region
clean, by of
J. NICHOLS
active electrode holding the electrode the figure.
a
To
free at
suction
out
at
a
polycrystalline
Pt
electrode
the linear sweep voltammogram to obtain the difference spectra and the other in the region of of poison l.OV for
was produced a short time;
with
O.OlM
for this system are marked: active oxidation
at the start of this is illustrated
each in
methanol
in
and the two one is in the of methanol. acquisition the lower
period part
A
pump
From
reservoir
T
Cleaning
mspectral m
acquisition
Cleaning
period
time
IR Figure
Fig. 3 bands,
1.
beam
The
shows mean
bands.
Figure at 100 CH30H.
thin
layer
flow
absorption
at
PB.
respectively the formation of at the more positive potential. are produced 1440 cm-l at
U per: linear sweep voltammogram s.- Y for Pt in 0.5M HCIO + O.OlM Lower: sketch of pulse se!uence.
cell.
a difference spectrum obtained more absorption of radiation at
more
2. mV
Large CO2
positive
and the The large
of
about
5
x 10-4 in an
10s2
absorbance, active state.
the
way: PA
bands
increasing negative
disappearance
at
downward and upward 2345 cm-l
amount band at
showing A fully
that the poisoned
of
CH30H
of ClO41040 cm-1
going going and
bands,
negative
bands. 1110 cm-l
positive show
in the double layer and the smaller ones
oxidation at potential Pg. Bands at 2070 cm-1 and 1850 is from water in the layer. Th_qvery large band at 1640 cm -1 the former being the linearly bound respectively are from the adsorbed CO poison, These bands are relatively weak. f”,“,m and the latter the more highly coordinated form. approx. maintained
by
in this potential
flow system electrode
by
enables the gives bands
electrode with an
to be amplitude
absorbance.
These are positive bands are seen at 1425 cm-l, 1320 cm-l and 1215 cm-l. not observed when the more positive They are shown in more detail in figs. 4 and 5. to +O.ZV where oxidation is very is lowered from the active region, +0.45V. potential. Pg. We assign these bands to the reactive intermediates ZCOH and ZCHOH (or xCH2OHl: slow. at present our data do not allow us to distinguish between the latter two possibilities. Our detailed assignments are as follows:
Additional
small
1320
cm-l
vC0
of
fCOH)ads
1215
cm-l
vC0
of
(CH,OH),ds:
1425
cm-’
dCOH
of
(COH),ds
x
=
1 or
2
SNIFTIRS
with a flow cell
1693
These are based upon data from a range of organic alcohols in which the carbon atom to which the OH group is attached has a variety of bonded neighbours and a variety of The It is also supported by ab-initio MO calculation [8]. hybridisation states. assignments will be fully discussed in a full paper to follow.
Figure
3.
Difference and
spectrum
for
PD 500 mV l SCEl
PA0
mV
.
9
8T lb1
kR/lo - 4
+Rllo-4
6
2
h_
4
2
I
yF$y&
0 c 12.fLO
cm
1240
1200
cm
*,
1
0. 1500
.A
1160
Figure 4. Difference spectra for PA OmV and (a) PD ZOOmV. lb) PS 450mV.
Figure and
PS
1400
5.
Difference
-1
1300 spectrum
for
1 PA OmV
5OOmV ISCE).
Our Finally. fig. 6 shows how the various species observed fit into the reaction schame. earlier measurements I91 without a flow cell had indicated the presence of some of the new In addition, a carbonyl bands reported here but they were difficult to identify properly. stretch possibly corresponding to (CHD)ads has also been detected by IMIRS and by SNIFTIRS [9.101.
R. J. NICHOLS
1694
CO2
1
( CH30H
lads
T
CO2
+ H20
+2’OHjads
Iads T
‘CHojads
I A reaction Figure 6. showing the adsorbed
CO2
+ 2H20
+3(OH
weakly adsorbed
and A. BEWICK
>
I.“+
+n’OH
“Ojads
jade
+ H20
scheme for oxidation of methanol at a Pt electrode in acid poisons and major products. reactive intermediates,
solution
Acknowledgements The generous ledged.
support
of the
SERC
for
equipment
and
a studentship
is gratefully
acknow-
References 1. 2. 3. 4. 5. 6. 7. a. 9. 10.
Pons and J.W. Russell, J. Electroanal. Chem.. B.S. A. Bewick, K, Kunimatsu. -160 47 ’1984). A. Bewick and B.S. Pons in “Advances in Infrared and Raman Spectroscopy”. vol. 12. ed. R.J.H. Clark and R.E. Hester, 1985 Wiley Heyden. p.1. S. Pans, T. Davidson and A. Bewick, J. Electroanal. Chem., 1160, 63 ‘1984). J.W. Russell, J. Overend. K. Scanlon, M. Sever-son and A. Bewick. J. Phys. Chem., 86. 3066 ‘1982). J.C. Gordon and M.R. Philpott, Langmuir, 2, U64 K. Kunimatsu. H. Seki, W.G. Golden, 11986). B. Beden. C. Lamv. Chem., 112. 343 , A. Bewick and K. Kunimatsu. J. ‘Electroanal. (1981). R.J. Nichols and A. Bewick. ISE meeting, Maastricht, Shi-Gang Sun, J. Clavilier. abstract No. 1 .l . September 1987. extended J.M. Bowman, J.S. Bittmann and L-B. Harding, J. Chem. Phys., 85. 911 ‘1986). Shi-Gang Sun, J. Clavilier, R. J. Nichols and A. Bewick, in preparation. A. Bewick in “Trends in Interfacial Electrochemistry”. ed. A. Fernando Silva, Nato ASI Series C 179, 1986 Reidel, Dordrecht.