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An in-situ ftir study of the electroreduction of Co2 by copc-coated edge graphite electrodes
361
J. Electroanal. Chem., 241 (1988) 361-371 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note AN IN-SITU FI’IR STUDY OF THE ELECTROREDUCTION CoPc-COATED EDGE GRAPHITE ELECTRODES
P.A. CHRISTENSEN,
A. HAMNE’M
OF CO, BY
and A.V.G. MUIR
Inorganic Chemrstty Laboratory, South Parks Road, Oxford OXI 3QR (Great Britain) (Received 23rd November 1987; in revised form 2nd December 1987)
INTRODUCTION
CoPc has been reported [l-3] to be one of the most active phthalocyanines for CO, reduction. The mechanism of the reduction in aqueous solution is thought to involve cobalt hydride intermediates [2,4,5], but little evidence exists for such species. There are differing reports concerning the reduction products [3,5], and it was hoped that the relatively new SNIFTIRS [6] technique would shed some light upon the problem. EXPERIMENTAL
Electrochemical methods Cobalt(I1) phthalocyanine (CoPc) was prepared in high yield by refluxing stoichiometric quantities of anhydrous cobalt(I1) acetate (East Anglia Chemicals) and phthalonitrile in quinoline for 4 h. The crude product was filtered off and purified by two sublimations at 450°C and lop3 Torr. Thin layers of CoPc were deposited onto the edge graphite electrodes by adsorption from its solution in pyridine (Aldrich Gold Label) or THF (M & B Laboratory Chemical). It has been reported [7,8] that the dipping time is relatively unimportant, and was about 2 min. Electrodes were washed with copious amounts of doubly distilled water prior to insertion into the cell. All electrochemical experiments were performed in 0.1 M potassium hydrogen phosphate + 0.1 M sodium hydroxide (pH 7.2) or sodium citrate + sodium hydroxide (pH 6) buffers made up with doubly distilled water that had been pyrolised in 0, at llOO”C, over a Pt/Rh catalyst. Solutions were purged for at least 20 min with either N, (cryogenic boil-off) or CO, (BOC CP grade). Cyclic voltammetry experiments were carried out in a three-electrode single-compartment cell with a calomel reference electrode and Pt gauze counter electrode. The purging gases were deoxygenated using a BASF catalyst. 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
362
Spectroelectrochemical methodr The working electrode was a 7 mm platinum disc, 2 mm thick, soldered onto a 3 mm dia. brass screw, sheathed in epoxy. The epoxy consisted of a 1: 10 mixture of Aralditee MY753 epoxy resin and HY951 hardener. The resin-coated electrode was screwed into a PTFE body into which the 0.0125 cm platinum wire counter electrode was countersunk. The disc was polished mechanically with 0.3 pm followed by 0.075 pm alumina, and hand polished with 0.015 pm alumina as required. The electrode was then sonicated in water for 30 min at 150 W. An elasticated loop was used to ensure good optical contact between the reflective working electrode and the cell window [9]. The spectroelectrochemical cell has been described elsewhere [9]. Briefly: the body of the cell was 30 mm o.d. Pyrex, glass blown to form a cylinder 15 mm long, closed at one end. Two 3 mm o.d. glass tubes 15 mm long were then fused to the glass face of the cell to form inlet and outlet ports for solvent or gas. A standard glass thermometer screw housing was fused to the top of the cell, to hold the Luggin capillary of the reference electrode. The working electrode guide was a 10 mm o.d. Pyrex cylinder fused to the glass face of the cell body. Two Viton “0” rings countersunk into the PTFE body ensured a solvent-tight seal, and aided the snug fit of the electrode into the cell. An IR transparent calcium fluoride window (30 mm dia., 3 mm thick, BDH Crystran) was glued to the remaining open face of the spectroelectrochemical cell using inert epoxy resin. An aluminium “G” clamp was used to grip the window, and thus hold the cell in the sample slit of a standard SpectraTech Series 500 VariableAngle Specular Reflectance unit incorporating a KRS-5 wire-grid polariser [lo]. This reflectance attachment was housed in the sample compartment of an unmodified Digilab Qualimatic QSlOO FTIR spectrometer fitted with a wide-range MCT detector and interfaced to a 68000 based microcomputer (OxSys Micros). The latter contained and controlled the potentiostat/galvanostat card used to provide the potential at the working electrode. The spectrometer was purged, and the air-bearing driven, by dry nitrogen from a cryogenic boil-off. The N,-feed was also used to purge solutions and drive an in-situ delivery device [9] which was connected to the inlet port of the cell by PVC tubing. The outlet port of the cell was connected to a waste receptacle. Spectra were collected at 8 cm-’ resolution. The single-step or “staircase” [6] spectra consisted of 69 scans at each potential. The results are presented as SJS,, where S, is the spectrum collected at the base potential E, vs. SCE and S,, is the spectrum collected at potential En. RESULTS AND DISCUSSION
Cyclic voltammograms of a CoPc-coated edge graphite electrode immersed in N,-purged KH,P04 + NaOH buffer at pH 8.4 are shown in Figs. 1A and B. A wave can be seen clearly at -0.5 V vs. SCE corresponding to the one-electron reduction of CoPc [5]; the second reduction step is observed at about - 1.0 V vs SCE, in
363
(A)
Fig. 1. Cychc voltammograms KH,PO, + M NaOH buffer.
of a CoPc-coated edge graphite Scan rate: (A) 200, (B) 250 mV/s.
agreement with some reports in the both these reduction processes remain observed at about -1.25 V, probably None of these waves are observed in In the absence of COz, at potentials observed in the reflectance spectrum
electrode
immersed
in N2-purged
M
literature [5]. The potentials associated with controversial [3,11]. A third reduction wave is corresponding to the reduction of protons. the absence of CoPc. E, -0.9-cE -c0 V vs. SCE, no features are of the modified electrode, when stepped from
364
(A)
2 Trans
4
35
3 lO-3
1500
18 kn c
IOMm b
,04
1300
vc/cm'
1100
Zb
32 Km d
z
G/cm-’
15
I
9c!o
Fig. 2. (A) Reflectance spectra of a CoPc-coated edge graphite electrode immersed in N,-purged 0.1 M KH,PO, +O.l M NaOH buffer at - 1.2 V vs. SCE. The spectra (69 scans, 8 cm-’ resolution) were colIected at the indicated times after stepping from the base potential of 0 V, and were all ratioed to the reference spectrum taken at this base potential. One of the scans comprising the spectrum recorded at 32 mm has picked up a mains spike on the interferogram; this transforms as the observed sinusoidal baseline variation [9]. (B) Reflectance spectra of an uncoated edge graphite electrode immersed in KH,PO, +NaOH at pH 10 or pH 6 (HPO:and H,POi, respectively) at open circmt, 20 minutes after the addition of the KH,PO, solution to the spectroelectrochemical cell. Spectra (69 scans, 8 cm-’ resolution) were ratioed to the electrode in water at the same pH (pure water for pH 6 and aqueous NaOH for pH 10).
365 TABLE
1
Features
in phosphate
spectra
and their assignments Assignment
v/cm-’ KH,PO,
K,HPO,
K3PO4
a
b
a
b
a
b
2800-3OOO(s,br) 2380(m,br) 1810(w)
3100(?) _ _
2900-3OOO(s,br) 2380(w) 183O(vw)
2950(m)
29OO-3OOO(s,br) 228O(vw)
2800(s) _
1230(s)
1221(s,sh)
1230(w)
1150(W)
1159(vs)
1072(vs)
1078(vs)
1076(vs) 988(m)
2400(w) _
1088-1064(s) 990(m)
947(vs) 878(s) 862(w)
_
O-H stretch O-H stretch combination band P-O-H in-plane defmn. PO* as str. PO, deg str. PO, sym str. PO, sym str. POH as P-O str. P(OH), sym P-O str. P-O(H) str.
a From ref. 12. b Our results.
the base potential of 0 V. However, at E c -0.95V vs. SCE, the CoPc sustains its second reduction and concomitantly deprotonates the H,PO; buffer. At potentials < - 1.2 V and in the absence of CO,, the buffer sustains a second deprotonation resulting in the formation of PO:- _ This latter process is the result of the CoPc catalysing the reduction of protons; however H, does not appear to be an important product. Figure 2A shows reflectance spectra of a CoPc-coated edge graphite electrode immersed in N,-purged KH,PO, + NaOH buffer at - 1.2 V vs. SCE, taken over a period of time. The spectra (8 cm-‘, 69 scans) were collected at the indicated times after stepping from the base potential of 0 V, and were all ratioed to this previously-stored spectrum. Figure 2B shows reflectance spectra of KH,PO, + OHin water at pH 10, and 6 (where it is in the form of HPOiand H,POy, respectively), obtained after introducing these species into the thin layer and taking spectra which are then normalised to a previously-stored reference spectrum of Pt in water at the same pH. Table 1 gives the assignments of the features in the phosphate spectra taken from ref. 12. As can be seen from Fig. 2B and Table 1, Fig. 2A shows the loss of H,PO; [upward-pointing features at 1221 cm-’ (s,sh), 1159 cm-’ (vs) and 1078 cm-’ (vs)] at -1.2 V vs. SCE, and the initial formation of HPOi[downward-pointing peaks at 2950 cm-’ (m), 2400 cm-’ (w), 1076 cm-’ (vs) and 990 cm-’ (m)], followed by the formation of PO:- [downward-pointing features at 2800 cm-’ (s) and 1011 cm-i (vs)]. If the potential step-sense is reversed, (i.e. - 1.2
366
V base potential), then these features also reverse in sign, indicating that (as would be expected) the deprotonations are reversible. The transfer of the first electron onto the CoPc would be expected to cause major changes in the double layer as the system attempted to reestablish charge neutrality. In addition, if the electron were placed on the central cobalt ion forming a metal hydride species, deprotonation of the buffer should occur. However no changes are observed in the IR after the first reduction step, and only the second reduction process results in the deprotonation of the buffer. Thus it does not seem unreasonable to suppose that the first electron is placed on the a*-system of the phthalocyanine ring and the second on the cobalt ion; i.e. the first electron transfer results in Co”Pc-, the second in Co’Pc-, the latter deprotonating the buffer to give CotPc-. Several reports in the literature have assigned the first reduction process to I!I the metal ion [10.13,14]. However, the reduction mechanism is reported [15] to depend strongly on the nature of any axially coordinated species. The studies described in refs. 10, 13 and 14 were performed on soluble derivatives of the phthalocyanines, or in organic solvents such as DMF and DMSO. Given the fact that in our work the phthalocyanine is adsorbed onto the electrode surface, it is no surprise that a different reduction sequence should obtain. We conclude, therefore, that CoPc is first reduced to Co"PC-, followed by a second reduction to Co’Pc- at E < - 0.95 V vs. SCE. The latter species then abstracts a proton from any phosphate in the double layer to give Co’Pcand I
k
HPO,LIn the presence of CO,, the CoPc undergoes the first reduction, again with no change in the IR. However, the second reduction coincides with the onset of the electroreduction of the CO, [increase in intensity of the upward-pointing (loss) feature at 2340 cm-‘, see Figs. 3A and B]; as well as the first deprotonation of the < -0.95 V vs. SCE. Figure 3A shows H,PO;. CO, is reduced at potentials reflectance spectra of a CoPc-coated edge graphite electrode immersed in CO,saturated KH,PO, + NaOH buffer at -0.7 V, before onset of CO, reduction, and at - 1.2 V vs. SCE. A typical potential profile of the intensities of the phosphate and CO, features is shown in Fig. 3B, showing the onset of CO, reduction at E < - 0.95 V. At potentials < - 1.2 V vs. SCE the CO, in solution starts to dissolve in the form of CO:-, being shown by the appearance of a strong feature in the SNIFTIRS spectrum at 1400 cm-‘, see Fig. 4A, indicating that the pH in the thin layer is greater than 10 (for carbonic acid pK, = 6.37, pK, = 10.25 [16]). Any HCO, initially produced is rapidly deprotonated by the CoPc. Once all of the dissolved CO, has been reduced or converted to CO,‘-, the HPOiis then further deprotonated to give PO:- ; the pH in the thin layer at this point may have risen as high as 13 (pK, of H,PO, = 12.67). Figure 4B shows the various peak intensities at 1400 cm-‘) plotted as (-OH at 2800 cm-‘, PO:at 1011 cm-’ and CO:functions of potential. From Figs. 4A and B it can be seen clearly that, once the
367 (Ai %
Tram
-12Vb
-07Va
tlo
i
(6)
7
50
t 45 40 35 30 i a E zzo % 15 10 5 01 -I I
-_I
_A
-I
-0.9
-0.7
-08 E/V
&Y5
-0.4
-03
vs SCE
Fig. 3. (A) Reflectance spectra of a CoPc-coated edge graphte electrode immersed m COz-purged KH2P0., +NaOH buffer at (A) -0.7 V and (B) - 1.2 V vs. WE. Both spectra (69 scans, 8 cm-’ resolution) ratioed to the reference taken at 0 V. (B) Plot of peak intensity vs. potential for the C02-loss feature m (A).
368 (A)
V vs SCE
t
b t
-1.3
d
1
LOO0
3cm
loo0
2000
vml~
(B)
A R/R 4 35 3 25 2 1.5 1 0.5 0 -1.4
-13
-I_2 E/mV
-1.1 vs.SCE
Fig. 4. (A) Conditions as for Fig. 3A, except spectra collected at several potentials negative of CO, reduction. (B) Plot of peak intensity vs. potential for the features in Fig. 4A.
of the onset
intensity of the CO, loss feature reaches a plateau (i.e. when all the CO* in the thin layer has been converted), the intensity of the CO:gain feature also attains a constant value, indicating that CoPc is inactive to the reduction of CO: -. CO, is reduced at potentials, E, - 1.2 V < E < - 0.95 V vs. SCE, by the Co’Pch species to give a product with features at 1294 cm-’ and 1359 cm-’ (see Fig. 5A). That the CO, is electroreduced at these potentials is evidenced by the observed loss of CO* from solution and lack of any strong bicarbonate or carbonate gain features.
369
(0)
Fig. 5. (A) Conditions as for Fig. 3(A) except spectrum was collected at -0.95 V vs. SCE; phosphate absorption region. (B) (a) Spectrum of “product” features from (A). (b) Spectrum of HCO;
If the potential step sense is reversed (i.e., - 0.95 V base potential), these “product” features are not observed, indicating that they are not the product of a reversible reaction, unlike the deprotonated phosphate species. The features at 1294 cm-’ and 1359 cm-’ are soon obscured by the COf- band, and appear to be a combination of a product band overlaid by weak HCO; bands [1360 cm-’ and 1290 cm-’ (sh)], see Fig. 5B, caused by a small amount of CO, dissolution. Further product features
370
may be hidden under the strong phosphate bands. The band at 1294 cm-’ suggests that the surface CoPc turns the CO, over only as far as a species of the form:
which would result in an electron-rich Co-CO; fragment. The absorptions due to this species would be expected to be lower in fi than those for formate, e.g. 1640 cm -* (HCO; asymmetric stretch) and 1340 cm-’ (HCO; symmetric stretch) [17]. Protons would be attracted to such an electron-rich species; but any perturbation of the C-O stretches by associated Hf would be expected to be negligible compared to that exerted by the cobalt ion. The absence of any clearly resolved asymmetric stretch suggests that the CoPc (CO;)H+ species lies such that the phthalocyanine ring is flat on the surface of the electrode, as shown above, this absorption being forbidden for such an orientation by the surface selection rule [lo]. However, because the region of the spectrum where the asymmetric stretch may be expected to absorb is masked by a band due to the carbon substrate, we cannot be certain of the orientation of the CoPc on the surface of the electrode. At no time was CO, either free or adsorbed, observed as a product of the electroreduction reaction. However, long-term studies on CoPc-modified edge-graphite electrodes using gas chromatography have shown CO as the major product, and both CO (51 and HCOO[3] have been reported as products. This is not surprising since the local pH environment of the electrodes in the relatively short term spectroelectrochemical experiments would be expected to be very different from that in the long-term studies. Hence, it is not unreasonable to suppose that the long-term runs result in the dehydration of the CoPc(CO,)H+ species initially formed to give CO and H,O. In order to facilitate diffusion into the thin layer, an experiment was run in which the electrode was held back from the window by spacers [9] such that the align function of the spectrometer was 40% of that at optimum alignment [9] of the electrode and window (1 pm). Although the expected electroreflectivity effects were observed [9], both CO, reduction and deprotonation of the buffer to HPOiwere still observed, and followed the same potential dependence as in the thin layer experiments. This shows, as expected, that the primary source of protons is the H,PO,-. Gas evolution in a thin layer spectroelectrochemical cell causes dramatic baseline effects in the observed reflectance spectrum, and such gross baseline deviations were absent from all the spectra reported above (and, indeed, gas evolution was not observed with the naked eye). Hence, hydrogen gas evolution appears not to be an important product under the conditions of the above experiments (i.e. less than about lo-” moles in the thin layer), and the CoPc cannot be turning over H’ to H, rapidly. However, if the CoPc is adsorbed from its solution in THF instead of pyridine, a gas that is transparent to IR is evolved, and is probably H,. Again the
371
H,PO; buffer is deprotonated, and the CO2 is electroreduced at E < -0.95V vs. SCE, but these effects are now superimposed on gross baseline changes. These take the form of a Y-dependent increase in reflectivity and concomitant loss of water < - 1.05 V vs. SCE (in features at 1620 cm-’ and 2700-3600 cm- ’ [9] at potentials CO,-saturated solution) or < - 1.1 V (in N,-saturated solution). CONCLUSIONS
(1) The first reduction of CoPc occurs on the ring r* system. (2) The second reduction gives ~‘Pcwhich, in the presence
of CO,, reacts
to
give an insertion product [2,5,18,19y (3) Long-term studies, where the pH is maintained constant at the electrode surface, have shown that the insertion product reacts via dehydration [19]. (4) A CoPc-coated edge graphite electrode does not reduce CO:-. (5) At potentials < - 1.2 V vs. SCE, Co’Pcundergoes a further reduction B process, again involving protons. As gas evolution is not observed in the thin layer, hydrogen is not an important product of this process. (6) The electrochemical behaviour of the CoPc is critically dependent upon the solvent used in the solution from which it was deposited on the electrode. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
S. Meshitsuka, M. Ichikawa and K. Tamaru, J. Chem. Sot., Chem. Commun., (1974) 158. H. Tanabe and K. Ohno, Electrochim. Acta, 32 (1987) 1121. S. Kapusta and N. Hackerman, J. Electrochem. Sot., 131 (1984) 1511. K. Kusuda, R. Ishihara, H. Yamaguchr and I. Izumt, Electrochim. Acta, 31 (1986) 657. CM. Lieber and N.S. Lewis, J. Am. Chem. Sot., 106 (1984), 5033. A. Bewick and S. Pons in R.J.H. Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. 12, Wiley Heyden, London, 1985, p. 1. A. Ebing, A. Van der Putten, W. Visscher and E. Barendrecht, J. Electroanal. Chem., 200 (1986) 313. A. Elzing, A. Van der Putten, W. Visscher and E. Barendrecht, J. Electroanal. Chem., 233 (1987) 99. P.A. Christensen, A. Hamnett and P.R. Trevellick, J. Electroanal. Chem., 242 (1988) 23. R.G. Greenler, J. Chem. Phys., 44 (1966) 310. S. Zecevtc, B. Simic-Glavaski, E. Yeager, A.B.P. Lever and P.C. Mmor, J. Electroanal. Chem.. 196 (1985) 339. A.C. Chapman and L.E. Thirwell, Spectrochim. Acta, 20 (1964) 937. L.D. Rollmann and R.T. Iwamoto, J. Am. Chem. Sot., 90 (1968) 1455. D.W. Clack and J.R. Yandle, Inorg. Chem., 11 (1972) 1738. A.B.P. Lever, M.R. Hempstead, CC. Leznoff, W. Lm, M. Melruk, W.A. Nevm and P. Seymour, Pure Appl. Chem., 58 (1986) 1467. R.C. Weast (Ed.), CRC Handbook of Chemistry and Phystcs, 66th ed., CRC Press, Boca Raton, FL, 1985. G. Socrates, Infrared Characteristic Group Frequencies, Wiley-Interscience, New York, 1980. R. Eisenberg and B. Fisher, J. Am. Chem. Sot., 102 (1980) 7361. J.L. Grant, K. Goswarm, L.O. Spreer, J.W. Otvos and M. Calvin, J. Chem. Sot., Dalton Trans., (1987) 2105.
J. Electroanal. Chem., 241 (1988) 361-371 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note AN IN-SITU FI’IR STUDY OF THE ELECTROREDUCTION CoPc-COATED EDGE GRAPHITE ELECTRODES
P.A. CHRISTENSEN,
A. HAMNE’M
OF CO, BY
and A.V.G. MUIR
Inorganic Chemrstty Laboratory, South Parks Road, Oxford OXI 3QR (Great Britain) (Received 23rd November 1987; in revised form 2nd December 1987)
INTRODUCTION
CoPc has been reported [l-3] to be one of the most active phthalocyanines for CO, reduction. The mechanism of the reduction in aqueous solution is thought to involve cobalt hydride intermediates [2,4,5], but little evidence exists for such species. There are differing reports concerning the reduction products [3,5], and it was hoped that the relatively new SNIFTIRS [6] technique would shed some light upon the problem. EXPERIMENTAL
Electrochemical methods Cobalt(I1) phthalocyanine (CoPc) was prepared in high yield by refluxing stoichiometric quantities of anhydrous cobalt(I1) acetate (East Anglia Chemicals) and phthalonitrile in quinoline for 4 h. The crude product was filtered off and purified by two sublimations at 450°C and lop3 Torr. Thin layers of CoPc were deposited onto the edge graphite electrodes by adsorption from its solution in pyridine (Aldrich Gold Label) or THF (M & B Laboratory Chemical). It has been reported [7,8] that the dipping time is relatively unimportant, and was about 2 min. Electrodes were washed with copious amounts of doubly distilled water prior to insertion into the cell. All electrochemical experiments were performed in 0.1 M potassium hydrogen phosphate + 0.1 M sodium hydroxide (pH 7.2) or sodium citrate + sodium hydroxide (pH 6) buffers made up with doubly distilled water that had been pyrolised in 0, at llOO”C, over a Pt/Rh catalyst. Solutions were purged for at least 20 min with either N, (cryogenic boil-off) or CO, (BOC CP grade). Cyclic voltammetry experiments were carried out in a three-electrode single-compartment cell with a calomel reference electrode and Pt gauze counter electrode. The purging gases were deoxygenated using a BASF catalyst. 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
362
Spectroelectrochemical methodr The working electrode was a 7 mm platinum disc, 2 mm thick, soldered onto a 3 mm dia. brass screw, sheathed in epoxy. The epoxy consisted of a 1: 10 mixture of Aralditee MY753 epoxy resin and HY951 hardener. The resin-coated electrode was screwed into a PTFE body into which the 0.0125 cm platinum wire counter electrode was countersunk. The disc was polished mechanically with 0.3 pm followed by 0.075 pm alumina, and hand polished with 0.015 pm alumina as required. The electrode was then sonicated in water for 30 min at 150 W. An elasticated loop was used to ensure good optical contact between the reflective working electrode and the cell window [9]. The spectroelectrochemical cell has been described elsewhere [9]. Briefly: the body of the cell was 30 mm o.d. Pyrex, glass blown to form a cylinder 15 mm long, closed at one end. Two 3 mm o.d. glass tubes 15 mm long were then fused to the glass face of the cell to form inlet and outlet ports for solvent or gas. A standard glass thermometer screw housing was fused to the top of the cell, to hold the Luggin capillary of the reference electrode. The working electrode guide was a 10 mm o.d. Pyrex cylinder fused to the glass face of the cell body. Two Viton “0” rings countersunk into the PTFE body ensured a solvent-tight seal, and aided the snug fit of the electrode into the cell. An IR transparent calcium fluoride window (30 mm dia., 3 mm thick, BDH Crystran) was glued to the remaining open face of the spectroelectrochemical cell using inert epoxy resin. An aluminium “G” clamp was used to grip the window, and thus hold the cell in the sample slit of a standard SpectraTech Series 500 VariableAngle Specular Reflectance unit incorporating a KRS-5 wire-grid polariser [lo]. This reflectance attachment was housed in the sample compartment of an unmodified Digilab Qualimatic QSlOO FTIR spectrometer fitted with a wide-range MCT detector and interfaced to a 68000 based microcomputer (OxSys Micros). The latter contained and controlled the potentiostat/galvanostat card used to provide the potential at the working electrode. The spectrometer was purged, and the air-bearing driven, by dry nitrogen from a cryogenic boil-off. The N,-feed was also used to purge solutions and drive an in-situ delivery device [9] which was connected to the inlet port of the cell by PVC tubing. The outlet port of the cell was connected to a waste receptacle. Spectra were collected at 8 cm-’ resolution. The single-step or “staircase” [6] spectra consisted of 69 scans at each potential. The results are presented as SJS,, where S, is the spectrum collected at the base potential E, vs. SCE and S,, is the spectrum collected at potential En. RESULTS AND DISCUSSION
Cyclic voltammograms of a CoPc-coated edge graphite electrode immersed in N,-purged KH,P04 + NaOH buffer at pH 8.4 are shown in Figs. 1A and B. A wave can be seen clearly at -0.5 V vs. SCE corresponding to the one-electron reduction of CoPc [5]; the second reduction step is observed at about - 1.0 V vs SCE, in
363
(A)
Fig. 1. Cychc voltammograms KH,PO, + M NaOH buffer.
of a CoPc-coated edge graphite Scan rate: (A) 200, (B) 250 mV/s.
agreement with some reports in the both these reduction processes remain observed at about -1.25 V, probably None of these waves are observed in In the absence of COz, at potentials observed in the reflectance spectrum
electrode
immersed
in N2-purged
M
literature [5]. The potentials associated with controversial [3,11]. A third reduction wave is corresponding to the reduction of protons. the absence of CoPc. E, -0.9-cE -c0 V vs. SCE, no features are of the modified electrode, when stepped from
364
(A)
2 Trans
4
35
3 lO-3
1500
18 kn c
IOMm b
,04
1300
vc/cm'
1100
Zb
32 Km d
z
G/cm-’
15
I
9c!o
Fig. 2. (A) Reflectance spectra of a CoPc-coated edge graphite electrode immersed in N,-purged 0.1 M KH,PO, +O.l M NaOH buffer at - 1.2 V vs. SCE. The spectra (69 scans, 8 cm-’ resolution) were colIected at the indicated times after stepping from the base potential of 0 V, and were all ratioed to the reference spectrum taken at this base potential. One of the scans comprising the spectrum recorded at 32 mm has picked up a mains spike on the interferogram; this transforms as the observed sinusoidal baseline variation [9]. (B) Reflectance spectra of an uncoated edge graphite electrode immersed in KH,PO, +NaOH at pH 10 or pH 6 (HPO:and H,POi, respectively) at open circmt, 20 minutes after the addition of the KH,PO, solution to the spectroelectrochemical cell. Spectra (69 scans, 8 cm-’ resolution) were ratioed to the electrode in water at the same pH (pure water for pH 6 and aqueous NaOH for pH 10).
365 TABLE
1
Features
in phosphate
spectra
and their assignments Assignment
v/cm-’ KH,PO,
K,HPO,
K3PO4
a
b
a
b
a
b
2800-3OOO(s,br) 2380(m,br) 1810(w)
3100(?) _ _
2900-3OOO(s,br) 2380(w) 183O(vw)
2950(m)
29OO-3OOO(s,br) 228O(vw)
2800(s) _
1230(s)
1221(s,sh)
1230(w)
1150(W)
1159(vs)
1072(vs)
1078(vs)
1076(vs) 988(m)
2400(w) _
1088-1064(s) 990(m)
947(vs) 878(s) 862(w)
_
O-H stretch O-H stretch combination band P-O-H in-plane defmn. PO* as str. PO, deg str. PO, sym str. PO, sym str. POH as P-O str. P(OH), sym P-O str. P-O(H) str.
a From ref. 12. b Our results.
the base potential of 0 V. However, at E c -0.95V vs. SCE, the CoPc sustains its second reduction and concomitantly deprotonates the H,PO; buffer. At potentials < - 1.2 V and in the absence of CO,, the buffer sustains a second deprotonation resulting in the formation of PO:- _ This latter process is the result of the CoPc catalysing the reduction of protons; however H, does not appear to be an important product. Figure 2A shows reflectance spectra of a CoPc-coated edge graphite electrode immersed in N,-purged KH,PO, + NaOH buffer at - 1.2 V vs. SCE, taken over a period of time. The spectra (8 cm-‘, 69 scans) were collected at the indicated times after stepping from the base potential of 0 V, and were all ratioed to this previously-stored spectrum. Figure 2B shows reflectance spectra of KH,PO, + OHin water at pH 10, and 6 (where it is in the form of HPOiand H,POy, respectively), obtained after introducing these species into the thin layer and taking spectra which are then normalised to a previously-stored reference spectrum of Pt in water at the same pH. Table 1 gives the assignments of the features in the phosphate spectra taken from ref. 12. As can be seen from Fig. 2B and Table 1, Fig. 2A shows the loss of H,PO; [upward-pointing features at 1221 cm-’ (s,sh), 1159 cm-’ (vs) and 1078 cm-’ (vs)] at -1.2 V vs. SCE, and the initial formation of HPOi[downward-pointing peaks at 2950 cm-’ (m), 2400 cm-’ (w), 1076 cm-’ (vs) and 990 cm-’ (m)], followed by the formation of PO:- [downward-pointing features at 2800 cm-’ (s) and 1011 cm-i (vs)]. If the potential step-sense is reversed, (i.e. - 1.2
366
V base potential), then these features also reverse in sign, indicating that (as would be expected) the deprotonations are reversible. The transfer of the first electron onto the CoPc would be expected to cause major changes in the double layer as the system attempted to reestablish charge neutrality. In addition, if the electron were placed on the central cobalt ion forming a metal hydride species, deprotonation of the buffer should occur. However no changes are observed in the IR after the first reduction step, and only the second reduction process results in the deprotonation of the buffer. Thus it does not seem unreasonable to suppose that the first electron is placed on the a*-system of the phthalocyanine ring and the second on the cobalt ion; i.e. the first electron transfer results in Co”Pc-, the second in Co’Pc-, the latter deprotonating the buffer to give CotPc-. Several reports in the literature have assigned the first reduction process to I!I the metal ion [10.13,14]. However, the reduction mechanism is reported [15] to depend strongly on the nature of any axially coordinated species. The studies described in refs. 10, 13 and 14 were performed on soluble derivatives of the phthalocyanines, or in organic solvents such as DMF and DMSO. Given the fact that in our work the phthalocyanine is adsorbed onto the electrode surface, it is no surprise that a different reduction sequence should obtain. We conclude, therefore, that CoPc is first reduced to Co"PC-, followed by a second reduction to Co’Pc- at E < - 0.95 V vs. SCE. The latter species then abstracts a proton from any phosphate in the double layer to give Co’Pcand I
k
HPO,LIn the presence of CO,, the CoPc undergoes the first reduction, again with no change in the IR. However, the second reduction coincides with the onset of the electroreduction of the CO, [increase in intensity of the upward-pointing (loss) feature at 2340 cm-‘, see Figs. 3A and B]; as well as the first deprotonation of the < -0.95 V vs. SCE. Figure 3A shows H,PO;. CO, is reduced at potentials reflectance spectra of a CoPc-coated edge graphite electrode immersed in CO,saturated KH,PO, + NaOH buffer at -0.7 V, before onset of CO, reduction, and at - 1.2 V vs. SCE. A typical potential profile of the intensities of the phosphate and CO, features is shown in Fig. 3B, showing the onset of CO, reduction at E < - 0.95 V. At potentials < - 1.2 V vs. SCE the CO, in solution starts to dissolve in the form of CO:-, being shown by the appearance of a strong feature in the SNIFTIRS spectrum at 1400 cm-‘, see Fig. 4A, indicating that the pH in the thin layer is greater than 10 (for carbonic acid pK, = 6.37, pK, = 10.25 [16]). Any HCO, initially produced is rapidly deprotonated by the CoPc. Once all of the dissolved CO, has been reduced or converted to CO,‘-, the HPOiis then further deprotonated to give PO:- ; the pH in the thin layer at this point may have risen as high as 13 (pK, of H,PO, = 12.67). Figure 4B shows the various peak intensities at 1400 cm-‘) plotted as (-OH at 2800 cm-‘, PO:at 1011 cm-’ and CO:functions of potential. From Figs. 4A and B it can be seen clearly that, once the
367 (Ai %
Tram
-12Vb
-07Va
tlo
i
(6)
7
50
t 45 40 35 30 i a E zzo % 15 10 5 01 -I I
-_I
_A
-I
-0.9
-0.7
-08 E/V
&Y5
-0.4
-03
vs SCE
Fig. 3. (A) Reflectance spectra of a CoPc-coated edge graphte electrode immersed m COz-purged KH2P0., +NaOH buffer at (A) -0.7 V and (B) - 1.2 V vs. WE. Both spectra (69 scans, 8 cm-’ resolution) ratioed to the reference taken at 0 V. (B) Plot of peak intensity vs. potential for the C02-loss feature m (A).
368 (A)
V vs SCE
t
b t
-1.3
d
1
LOO0
3cm
loo0
2000
vml~
(B)
A R/R 4 35 3 25 2 1.5 1 0.5 0 -1.4
-13
-I_2 E/mV
-1.1 vs.SCE
Fig. 4. (A) Conditions as for Fig. 3A, except spectra collected at several potentials negative of CO, reduction. (B) Plot of peak intensity vs. potential for the features in Fig. 4A.
of the onset
intensity of the CO, loss feature reaches a plateau (i.e. when all the CO* in the thin layer has been converted), the intensity of the CO:gain feature also attains a constant value, indicating that CoPc is inactive to the reduction of CO: -. CO, is reduced at potentials, E, - 1.2 V < E < - 0.95 V vs. SCE, by the Co’Pch species to give a product with features at 1294 cm-’ and 1359 cm-’ (see Fig. 5A). That the CO, is electroreduced at these potentials is evidenced by the observed loss of CO* from solution and lack of any strong bicarbonate or carbonate gain features.
369
(0)
Fig. 5. (A) Conditions as for Fig. 3(A) except spectrum was collected at -0.95 V vs. SCE; phosphate absorption region. (B) (a) Spectrum of “product” features from (A). (b) Spectrum of HCO;
If the potential step sense is reversed (i.e., - 0.95 V base potential), these “product” features are not observed, indicating that they are not the product of a reversible reaction, unlike the deprotonated phosphate species. The features at 1294 cm-’ and 1359 cm-’ are soon obscured by the COf- band, and appear to be a combination of a product band overlaid by weak HCO; bands [1360 cm-’ and 1290 cm-’ (sh)], see Fig. 5B, caused by a small amount of CO, dissolution. Further product features
370
may be hidden under the strong phosphate bands. The band at 1294 cm-’ suggests that the surface CoPc turns the CO, over only as far as a species of the form:
which would result in an electron-rich Co-CO; fragment. The absorptions due to this species would be expected to be lower in fi than those for formate, e.g. 1640 cm -* (HCO; asymmetric stretch) and 1340 cm-’ (HCO; symmetric stretch) [17]. Protons would be attracted to such an electron-rich species; but any perturbation of the C-O stretches by associated Hf would be expected to be negligible compared to that exerted by the cobalt ion. The absence of any clearly resolved asymmetric stretch suggests that the CoPc (CO;)H+ species lies such that the phthalocyanine ring is flat on the surface of the electrode, as shown above, this absorption being forbidden for such an orientation by the surface selection rule [lo]. However, because the region of the spectrum where the asymmetric stretch may be expected to absorb is masked by a band due to the carbon substrate, we cannot be certain of the orientation of the CoPc on the surface of the electrode. At no time was CO, either free or adsorbed, observed as a product of the electroreduction reaction. However, long-term studies on CoPc-modified edge-graphite electrodes using gas chromatography have shown CO as the major product, and both CO (51 and HCOO[3] have been reported as products. This is not surprising since the local pH environment of the electrodes in the relatively short term spectroelectrochemical experiments would be expected to be very different from that in the long-term studies. Hence, it is not unreasonable to suppose that the long-term runs result in the dehydration of the CoPc(CO,)H+ species initially formed to give CO and H,O. In order to facilitate diffusion into the thin layer, an experiment was run in which the electrode was held back from the window by spacers [9] such that the align function of the spectrometer was 40% of that at optimum alignment [9] of the electrode and window (1 pm). Although the expected electroreflectivity effects were observed [9], both CO, reduction and deprotonation of the buffer to HPOiwere still observed, and followed the same potential dependence as in the thin layer experiments. This shows, as expected, that the primary source of protons is the H,PO,-. Gas evolution in a thin layer spectroelectrochemical cell causes dramatic baseline effects in the observed reflectance spectrum, and such gross baseline deviations were absent from all the spectra reported above (and, indeed, gas evolution was not observed with the naked eye). Hence, hydrogen gas evolution appears not to be an important product under the conditions of the above experiments (i.e. less than about lo-” moles in the thin layer), and the CoPc cannot be turning over H’ to H, rapidly. However, if the CoPc is adsorbed from its solution in THF instead of pyridine, a gas that is transparent to IR is evolved, and is probably H,. Again the
371
H,PO; buffer is deprotonated, and the CO2 is electroreduced at E < -0.95V vs. SCE, but these effects are now superimposed on gross baseline changes. These take the form of a Y-dependent increase in reflectivity and concomitant loss of water < - 1.05 V vs. SCE (in features at 1620 cm-’ and 2700-3600 cm- ’ [9] at potentials CO,-saturated solution) or < - 1.1 V (in N,-saturated solution). CONCLUSIONS
(1) The first reduction of CoPc occurs on the ring r* system. (2) The second reduction gives ~‘Pcwhich, in the presence
of CO,, reacts
to
give an insertion product [2,5,18,19y (3) Long-term studies, where the pH is maintained constant at the electrode surface, have shown that the insertion product reacts via dehydration [19]. (4) A CoPc-coated edge graphite electrode does not reduce CO:-. (5) At potentials < - 1.2 V vs. SCE, Co’Pcundergoes a further reduction B process, again involving protons. As gas evolution is not observed in the thin layer, hydrogen is not an important product of this process. (6) The electrochemical behaviour of the CoPc is critically dependent upon the solvent used in the solution from which it was deposited on the electrode. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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