Langmuir 1985, 1, 697-701
697
Fourier Transform Infrared Spectroelectrochemical Studies of Anodic Processes in Thiocyanate Solutions John K. Foley and Stanley Pons* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
J. J. Smith Office of Naval Research, Arlington, Virginia 2221 7 Received April 16, 1985 FTIR spectroelectrochemistrywas used to study the anodic reactions that take place on platinum and silver electrodes in aqueous and acetonitrile solutions of thiocyanate ion. At platinum electrodes,the SCNion is oxidized primarily to thiocyanogen (SCN)2in acetonitrile solution and to a mixture of products in aqueous solution. At silver electrodes, anodic dissolution of the metal takes place. In aqueous solution a surface film of AgSCN is formed. In acetonitrile, there are two anodic waves, the complex Ag(SCN)*being formed in the first and a surface film of AgSCN in the second.
Introduction In this paper we report in-situ infrared spectra taken in the course of electrooxidation reactions in aqueous and acetonitrile solutions of thiocyanate ion, SCN-, at platinum and silver electrodes. The electrochemical oxidation of thiocyanate has been studied since 1924 using a variety of solvents and electrode materials.’-12 At electrodes such as platinum and carbon, which are inert in the potential region of SCN- oxidation, it is generally accepted that the initial product of oxidation is thiocyanogen, formed by the reaction 2SCN- s NCS-SCN
+ 2e-
Thiocyanogen is stable in nonaqueous solutions but is hydrolyzed by water. Analyses of the products from oxidation of SCN- at platinum in aqueous solution5J1 have shown the presence of sulfate ion, SO:-, cyanide ion, CN-, and a red polymeric material, “parathiocyanogen”, of uncertain composition. At gold electrodes in acetonitrile, on the other hand, it was found that the electrode underwent anodic dissolution a t potentials less positive than those required for thiocyanate oxidation, and the products were the complex ions AU(SCN)~-and Au(SCN)~-,with formation of a surface oxide taking place a t more positive potentia1ss6 (Such reactions are reminiscent of the extraction of gold metal from native ore by cyanide.) Similar behavior is to be expected for silver electrodes: surface-enhanced Raman spectroscopy has shown the presence of AgSCN on a silver electrode surface a t high positive potentials in slightly acidic KSCN solution^.^ In addition, in-situ infrared spectra obtained by the FT-IRRAS method have shown Ag(CN),- to be the oxidation product in aqueous cyanide solutions, with a film of AgCN being formed on the electrode a t more positive p0tentia1s.l~ In this work in situ FTIR spectra were obtained by the SNIFTIRS method.14 The technique is based on external reflectance; a beam of infrared light passes through an infrared-transparent window then through a thin layer of solution sandwiched between this window and a polished metal electrode, is specularly reflected from the electrode, and passes back through the solution and window to a detector. Spectra are taken a t two (or more) potentials; a t each potential enough interferograms are collected to give the desired signal-to-noiseratio, and the two resulting single beam spectra are then ratioed. This procedure yields
* To whom correspondence should be addressed. 0743-7463/85/2401-0697$01.50/0
a difference spectrum which shows only changes in reflectance caused by changes in potential. Such changes may be due, for example, to changes in the number and/or bonding of species adsorbed on the electrode or to chemical changes in the solution or on the electrode surface caused by electrochemical reactions. A 50-pm solution thin layer (as determined from the charge under thin-layer voltammetric peaks) was formed between the electrode and window. This means that adsorbed species were not observed, since a t solution thicknesses greater than a few microns the intensity of the electric field of the light close to the electrode is very low.l5 Since the electrochemical cell used in SNIFTIRS is basically a thin-layer cell, bulk electrolyses may be effected quickly and conveniently and the infrared spectra of reactants and products obtained in a nearly ideal double-beam experiment. Species on the electrode surface are distinguished from species in solution by varying the polarization state of the incident light. Species very close to a reflecting metal surface are invisible to s-polarized light but can absorb p-polarized light (provided there is a component of the transition dipole moment normal to the surface16). This work is confined to the analysis of the reaction species in solution or in thick films present at the interface.
Experimental Section Experimental details of the SNIFTIRS technique have been (1) Kerstein, H.;Hoffman, R. Ber. 1924, 57, 491. (2) Cauqius, G.; Gerard, P. Bull. SOC.Chim. Fr. 1972, 2244. (3) Pereiro, R.; Arvia, A. J.; Calandra, A. J. Electrochim. Acta 1972, 17,1723. (4) Martinez, C.; Calandra, A. J.; Arvia, A. J. Electrochim. Acta 1972, 17, 2153. (5) Holtzen, D.A.; Allen, A. S. Anal. Chim. Acta 1974, 69, 153. (6) Martins, M. E.;Castellano, C.; Calandra, A. J.; Arvia, A. J. J. Electroanal. Chem. 1977. 81. 291. (7) Martins, M. E.;Ck&ano, C.; Calandra, A. J.; Arvia, A. J. J. Electroanal. Chem. 1978, 92, 45. (8) Shimizu, K.; Osteryoung, R. A. Anal. Chem. 1981,53, 2351. (9) Cooney, R. P.; Reid, E. S.; Fleischmann, M.; Hendra, P. J. J. Chem. Soc.. Faraday Trans. 1 1977, 73. 1691. (10) Nicholson, M. M. Anal. Chem. 1959, 31, 128. (11) Ruis, A.; Terol, S. An. R . SOC.Esp. Fis. Quim.,Ser. B 1948, 44 1234; 1949, 45, 359. (12) Gauguin, R. J. Chem. Phys. 1945, 42, 136; Ann. Chim. (Paris) 1949,4, 12th Series, 832; Anal. Chim. Acta 1951, 5 , 200. (13) Kunimatau, K.; Seki, H.; Golden, W. G. Chem. Phys. Lett. 1984, 108, 195; Proceedings, Spectroscopy of Adsorbed Species, Osaka, Sept 1984.
(14) Bewick, A.;.Kunimatau,K.;Pons, B. S.; Russell, J. W. J. Etectroanal. Chem. 1984, 160,47. (15) Seki, H.; Kunimatau, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437. (16) Greenler, R. G. J. Chem. Phys. 1966, 44, 310.
0 1985 American Chemical Society
698 Langmuir, Vol. 1, No. 6, 1985
Foley et al. t
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21DO 20DO 19 dQJENUM3EPS CN-.
c
2s 0
t
2050
1550 dRVENUNBER5 C M - 1
i oba
Figure 1. Difference spectra for the oxidation of 10 mM SCNin acetonitrile (0.1 M TBAF) at a platinum electrode. Potential modulation -1.0 to +0.5 V vs. Ag/Ag+ in the same electrolyte. Radiation is (a) p polarized or (b) s polarized.
Figure 2. Difference spectrum for the oxidation of 10 mM SCNin water (1.0 M KF) at a platinum electrode. Potential modulation 0.0 to + L O V vs. SCE.
given e1~ewhere.l~ The FTIR spectrometer was an IBM 98 instrument equipped with a cooled 77K HgCdTe solid-state detedor. A specular reflectance attachment supplied by IBM was used to control the angle of incidence of the radiation on the cell window. The cell itself was situated outside the spectrometerin a N2-purged box, and the spectrometer was evacuated to minimize the amount of C02and water vapor in the optical path. The window between the spectrometer and cell assembly was sodium chloride. The electrode potential was controlled with a Hi Tek DT2101 potentiostat and a Hi Tek PPRl waveform generator. Cyclic voltammograms were recorded on a Houston Instruments Omnigraphic 2000 X-Y recorder. The cell (JASInstrument Systems)was constructed from Pyrex. A 4-mm-thick 25-mm-diameter CaF2 window which had been beveled to give it a trapezoidal cross section was affixed to one end. The acute angle of 75' was selected 80 that when the incident light was normal to the beveled edge the angle of incidence on the electrode was about 70'. Under these conditions both s- and p-polarized light are transmitted equally through the air-window interface. The working electrode was a 7-mm-diameter Pt or Ag disk mounted on the end of a brass shaft which was tightly housed in a 9-mm-diameter Kel-F tube. Before each experiment the electrode was polished to a mirror finish with alumina of successively smaller particle size, starting with 1pm and ending with 0.05 pm. The electrode face was parallel to the window and was pushed up against the window to form a thin-layer cell of about 50-pm solution thickness. The secondary electrode was a platinum wire loop. The reference electrode was a SCE for aqueous solutions and a Ag/Ag+ (0.01 M in acetonitrile with 0.1 M tetra-nbutylammonium fluoroborate (TBAF))electrode for acetonitrile solutions. Water was triply distilled. Acetonitrile (Burdick and Jackson, nominally
