4563
J. Phys. Chem. 1992,96, 4563-4567 reversible hydrogen electrode in a cell having zero ohmic resistance. With the cell employed, at least 50% of the maximum possible efficiency was exploited as seen from the ratio of ) I ~to, ~ L.G.The gap between these two efficiencies is smaller in strongly acid and alkaline solution where hydrogen overvoltage and ohmic resistance are smallest. In a comparison of the maximum values of & and qG,+ it should be recalled that in the whole pH range the anode potential corresponding to the maximum of qGa was more cathodic than that pertaining to the maximum of LG (Figure 4). This implies that at the optimum energy conversion point of the cell the internal quantum efficiency was smaller than under the condition of determining the maximum of LG. Nozik2' expressed the energy efficiency for photoelectrolysis of water by a relation which can be written as
AoHfF@is the molar heat of combustion of H2and the remaining quantities are as defined above. Values of qHp obtained by 20-mW broad-band (300-400-nm) photolysis employing a single-crystal TiOz anode and a platinized Pt counterelectrode were ca. ~
~
~~
(21) Nozik, A. J. Nature (London) 1975, 257, 383.
0.03-0.04, independent of pHa2' The efficiencies qH,oand qG,o, eq 24, are connected by qH,O
= %,o -
Iph 0
TAoSf,H20
2F
(26)
where AoSf+H20 is the standard entropy change for formation of H20(1),-163 J.mo1-'SK-'. Taking vG,o= 0.02 at pH 1.8 from the present study, qH,ois 0.03, close to Nozik's values.
Conclusion In situ photocalorimetry employing pyroelectric monitoring of heat evolved at a semiconductor photoelectrode is a technique useful for studying the energy efficiency of photoelectrolysis with respect to absorbed radiant power. As with other photothermal methods, no additional measurement of the light intensity is required. Both the maximum possible energy efficiency attainable with the photoelectrode in conjunction with a thermodynamically reversible counterelectrode and the efficiency for a cell with additional losses from overvoltage at the counterelectrode and from electrolyte resistance can be determined. Acknowledgment. Support of our work by Deutsche Forschungsgemeinschaft and, in part, Fonds der Chemischen Industrie is gratefully acknowledged.
Infrared Spectroscopy as a Probe of the Adsorption and Electrooxidation of a Cyanide Monolayer at Platinum under Aqueous Electrochemical Conditions Vicki Berger Paulissen and Carol Korzeniewski* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 481 09 (Received: November 5, 1991; In Final Form: January 28, 1992)
Adsorbed cyanide was observed on platinum electrodes using infrared spectroscopy. In situ spectral analyses were performed in a cyanide-free electrolyte solution following cyanide monolayer preparation according to an electrochemical procedure developed for earlier LEED and AES studies of cyanide adsorption on platinum electrodes. This approach minimized spectral interferences associated with reactive cyanide species in bulk solution and simplified the assignment of bands associated with surface species. Two forms of the adsorbed cyanide surface layer were identified. With the electrolyte solution at pH 12.5, a potential-dependent band appears at about 2058 cm-I and is assigned to the vibrations of a surface layer of CN-. With the electrolyte solution at pH 8, a potential-dependent band appears at about 2147 cm-I and is assigned to a surface cyanide layer in which CNH is the predominate species.
Introduction In recent years, considerable progress has been made in understanding molecular adsorption at the electrode/solution interface.' Electrochemical and spectroscopic methods have provided valuable insights into the effect of solvent, electrolyte, and electrode potential on adsorbate structure and bonding. Infrared spectroscopy is recognized as a valuable tool for examining these interfacial interactions, as recent in situ studies of molecular adsorption and electrooxidation on electrodes of well-defined surface structure will confirma2 The present work employs infrared spectroscopy to examine the adsorption of cyanide on platinum electrodes. The majority of vibrational spectroscopic studies concerning cyanide electrosorption have employed silver and gold electrodes. Early studies utilized surface-enhanced Raman spectroscopy (SERS)? and later work employed infrared ~pectroscopy.~These vibrational studies assign a potential-dependent band centered at about 2100 cm-'to the *N stretching mode of cyanide adsorbed Linearly through the carbon atom and report vibrational band shifts
* To whom correspondence should be addressed.
with applied potential in the range 30-35 cm-'/V. Additional bands are observed in the infrared spectra and have been assigned to solution-free cyanide and metal-cyanide complexes formed by dissolution of the electrode surface upon oxidation.s Fewer vibrational studies have been reported for cyanide adsorption on platinum electrodes. It appears that spectral features of the adsorbed ion are obscured by products of the cyanide oxidation reaction,6 and as a result, vibrational assignments of (1) (a) Hubbard, A. T. Chem. Reu. 1988, 88, 633. (b) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177. (2) Chang, S.;Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (3) (a) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon 11, J. G.; Philpott, M. R. SurJ. Sci. 1985, 158, 596. (b) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon 11, J. G.; Philpott, M. R. Lungmuir 1988, 4, 337. (c) Gao, P.; Weaver, M. J. J. Phys. Chem. 1989, 93, 6205. (4) (a) Kunimatsu, K.;Seki, H.; Golden, W. G. Chem. Phys. Len. 1984, 108, 195. (b) Corrigan, D. S.; Gao, P.; h u n g , L. H.; Weaver, M. J. Lungmuir 1986, 2, 744. (c) Seki, H. Electrochemical Surface Science; Soriaga, M . P., Ed.; American Chemical Society: Washington, DC, 1988; Vol. 378, Chapter 22. ( 5 ) Jones, L. H.; Penneman, R. A. J. Chem. Phys. 1954, 22, 965. (6) Hinman, A. S.; Kydd, R. A.; Cooney, R. P. J. Chem. SOL.,Faraday Trans I 1986, 82, 3525.
0022-365419212096-4563%03.00/0 0 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 1 1 , 1992
platinum-adsorbed cyanide appear to conflict. In early work, Kitamura and co-workers’ used polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to examine cyanide adsorption on platinum electrodes. Cyanide oxidation products were detected in solution, and a potential-dependent band at about 21 16 cm-l was assigned to adsorbed cyanide ion. PMIRRAS experiments undertaken by Ashley and -workerss assign a bipolar band centered at about 2160 cm-’ to the linearly bonded cyanide ion. Zhang et aL9 used potential difference infrared spectroscopy to examine cyanide adsorption on platinum and observed a potentialdependent band between 2087 and 2095 cm-’ which they attribute to the linearly bonded cyanide species. In similar potential difference experiments, Hinman and ceworkers6 observe two weak potential-dependent bands between 2200 and 2250 cm-’ and suggest their assignment to adsorbed CN- and CNH, respectively. Other vibrational spectroscopic studies of cyanide adsorption on platinum employed sum frequency generation (SFG) spectroscopy.I0 Two potential-dependent vibrational bands are observed in SFG spectra. They appear at about 2070 and 2150 cm-’ and have been assigned to nitrogen-bonded and carbon-bonded CN-, respectively. In related work, the structure and composition of cyanide overlayers formed on platinum under aqueous electrochemical conditions were examined using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES).” These studies suggest that a mixed monolayer containing carbon-bound CNH and CN- with associated countercation forms. Countercation exchange in aqueous solutions of different electrolyte composition was possible,II8 and the pH dependence of countercation association indicated that the surface behaves as a polyprotic acid.Ilb In other experiments,I2 Pt( 1 1 1) was dosed with HCN under ultrahigh-vacuum conditions and examined using temperatureprogrammed desorption (TPD), scanning kinetic spectroscopy, LEED, and work function measurements. Results suggest that HCN adsorbs molecularly and bonds to the platinum surface through the nitrogen atom. The present work examines cyanide adsorption on platinum electrodes in situ using infrared spectroscopy. In contrast to earlier infrared studies, spectral analyses were performed in a cyanide-free electrolyte solution; the platinum electrode was transferred into this solution following treatment under electrochemical conditions according to the cyanide monolayer preparation procedure used in earlier LEED and AES studies.II8 This approach eliminated many spectral interferences associated with reactive cyanide species in bulk solution and simplified the assignment of bands associated with surface species. Experimental Section Thin-layer spectroelectrochemical cells were constructed from Kel-F using a design similar to that previously reported.13 Two cell types were used: a stagnant thin-layer and a flow-through design. The latter design incorporates two ports which permit the bulk and thin-layer solution composition to be changed without movement of the electrode. The solution in the stagnant thin-layer design remains constant throughout the experiment. A trapezoidal calcium fluoride window (Harrick) was employed to maximize surface selectivity. The incident light was polarized using a KRS-5 wire-grid polarizer (Specac) and was incident at about 60° at the metal/solution interface with respect to the electrode surface normal. The working electrode was constructed from a disk of (7) Kitmura, F.; Takahashi, M.; Ito, M. Chem. Phys. Left. 1986, 130, 181. (8) Ashley, K.; Lazaga, M. Surf. Sci. 1989, 219, L590. (9) Zhang, J.; Lu, J.; Cha, C.; Feng, Z. Wuli Huaxue Xuebao 1989, 5 , 409. (10) Guyot-Sionnest,P.;Tadjeddine, A. Chem. Phys. Letf. 1990,172,341. (11) (a) Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. Y.; Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. J . Elecfroanal.Chem. 1985, 188,95. (b) Schardt, B. C.; Stickney, J. L.; Stern, D. A.; Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita. G. N.; Soriaga, M. P.; Hubbard, A. T. Inorg. Chem. 1985, 24, 1419. (12) Hagans, P. L.; Guo, X.; Chorkendorff, I.; Winkler, A,; Siddiqui, H.; Yates, Jr., J. T. Surf. Sci. 1988, 203, 1 . (13) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Specfrosc. 1985, 39, 437.
Paulissen and Korzeniewski
I
.10 mA
0.1
I 1.o
0.0
E
VI.
SCE
Figure 1. Cyclic voltammograms obtained using 0.1 M NaC10, with ( - - - ) and without (-) 25 mM NaCN. Sweep rate is 100 mV/s.
polycrystalline platinum (8-mm diameter). The disk was sealed in the end of a glass tube and polished to a mirror finish using alumina polishing powder of 1 .O, 0.3, and 0.5 pm. The electrode was electrochemically polished in 1 M H2S04and stored in 0.1 M perchlorate solution prior to each experiment. A saturated calomel reference electrode was located in a compartment which was separated from the main sample chamber by a glass stopcock and connected via a Luggin capillary. All potentials are reported with respect to the saturated calomel electrode (SCE). Infrared spectroscopic measurements were performed using a Digilab FTS-40 Fourier transform infrared (FTIR) spectrometer equipped with a liquid nitrogen cooled MCT detector. A resolution of 4 cm-l was utilized. The spectroelectrochemical cell was positioned in the sample compartment such that infrared radiation was specularly reflected from the surface of the working electrode. The potential of the working electrode was controlled by a JAS Instrument Systems potentiostat. Spectral data were collected at s t e p of 100 mV (negative to positive) over the potential range indicated by the corresponding cyclic voltammograms. A total of 1024 interferograms were collected at each potential and ratioed to a reference spectrum selected to “ize spectral interferences. Spectra are displayed in units of ARIR. All chemicals were purchased from Aldrich. Electrolyte solutions were prepared using Nanopure water (Barnsted Nanopure system) at a concentration of 0.1 M. Solutions were degassed by bubbling with argon prior to each experiment.
Results and Discussion Figure 1 shows cyclic voltammograms of polycrystalline platinum in 0.1 M NaClO, with and without 25 mM NaCN. The sharp features in the hydrogen region observed with the 0.1 M NaClO, solution are absent with the cyanidecontaining solution, presumably due to adsorption of cyanide on the electrode surface.I4 Infrared reflectance spectra of a platinum electrode immersed in the solution of Figure 1 with 25 mM NaCN were obtained using the single-potential alteration methodIs and are shown in Figure 2. The strong band extending downward at 2170 cm-I and the weaker band at 2094 cm-I have been assigned to cyanate’ and solution HCN: respectively. The band extending upward at 2079 cm-’ has been assigned to solution CN-.@ These spectra indicate that protonation and oxidation of CN- occur to form HCN and OCN-, respectively, as the electrode potential is increased to (14) Tamura, H.; Arikado, T.; Yoneyama, H.; Matsuda, Y. Elecfrochim. Acfa 1974, 19, 273. (15) (a) Corrigan, D. S.;Weaver, M. J. J . Phys. Chem. 1986, 90, 5300. (b) Corrigan, D. S.;Leung, L. H.; Weaver, M. J. Anal. Chem. 1987,59,2252. (c) Pons, S.;Datta, M.; McAleer, J. F.; Hinman, S. J . Elecfroanal. Chem. 1984, 160, 369.
Cyanide Adsorption on Pt Electrodes
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4565 2093
b+.80v
+.40 V
+.20
v
-.20
v
-.50 V
1.1 x
:170 2400
2200
2000
Wavrnumbrrr (em-1)
2400
I--2080 2200
2000
Wavrnumbrn (em-1)
Figure 2. In situ infrared spectrum collected at +0.80 V using a solution containing 0.1 M NaCIO, with 25 mM NaCN. A flow-throughcell was used. The spectrum was obtained using p-polarized radiation. The background was measured in the same solution with the electrode held at a potential of -0.80 V.
positive values. Spectra shown in Figure 2 were obtained using p-polarized light, and similar features are observed using s-pol a w light. This result indicates that the CN-, HCN, and OCNspecies are present in the thin-layer cavity and not confined to the electrode surface. To gain insight into the nature of the surface-adsorbed species, spectra were obtained using a NaCN-free background. This procedure has been suggested as a way to simplify the in situ spectra by eliminating interferences from solution CN- species in the backgr~und.~ Representative spectra are shown in Figure 3. These spectra were obtained using a thin-layer flow-through cell. The background spectrum was collected with the electrode poised at -0.8 V in a solution containing 0.1 M NaClO,. Subsequently, the solution was exchanged with one containing 0.1 M NaC104 with 25 mM NaCN. Potential-dependent sample spectra were collected and ratioed to the perchlorate background. As a consequence, the bipolar character of the 2080- and 2093-cm-l bands is replaced with two distinct bands. The 2080-cm-' band appears at negative potentials and becomes weaker as the potential is increased to positive values. The 2093-cm-' band appears at more positive potentials and becomes more intense as the potential increases. A shoulder at about 2120 cm-' and a weak band at 2133 cm-I also appear. Previous studies have assigned the 2133-cm-l band to a [R(CN),I2- surface film.6 Although using a CN--free electrolyte solution background simplifies the appearance of the spectra and enhances the detail, similar spectral features are observed using both p- and s-polarizations (Figure 3A,B), and therefore little information regarding the nature of the surface species can be derived. In order to further eliminate,solution interferences, a procedure devised for electrochemically dming CN- onto platinum electrodes for study under ultrahigh-vacuum conditions was adapted.Ila To adsorb CN-, the clean platinum electrode was immersed into a solution containing 0.1 M KCN and held at a potential of +0.2 V for 2 min. Following CN- adsorption, the electrode was rinsed with water and then transferred to a solution containing 0.1 M HClO, for 2 min. This step exchanges potassium ions associated with the charged CN- surface layer for protons and leaves the surface in a highly protonated state. Following a distilled water rinse, the electrode was transferred to the thin-layer cell containing a lithium perchlorate solution. Lithium perchlorate was used as supporting electrolyte to minimize the strength of the interaction between the surface-bound CN- and countercations in solution, as the Li+ ion is highly solvated.lia The importance of the countercation on the spectral features will be discussed below. Shown in Figure 4 are reflectance infrared spectra obtained subsequent to this CN- monolayer preparation procedure. Spectra
2ioo
2200 2000 Wavrnumbrn (em-1)
Figure 3. Infrared spectra collected in situ as a function of potential for the same solution and electrochemical cell as in Figure 2. The background was obtained using a NaCN-free electrolyte solution containing 0.1 M NaCIO4with the electrode held at a potential of -0.80V. Spectra were obtained using (A) p- or (B) s-polarized radiation.
shown in Figure 4A,B were obtained using a lithium perchlorate electrolyte solution adjusted to pH 12.5. Spectra obtained under these conditions using ppolarized light (Figure 4A) show a band at about 2058 cm-'which decreases in intensity and shifts toward higher energy as the potential is increased to positive values. A weak band appears at 2170 cm-' and can be assigned to solution cyanate, a CN- oxidation product. Spectra shown in Figure 4B were obtained using s-polarized infrared radiation. The 2058-cm-' band is absent which supports its assignment to a surface-adsorbed species. Appearance of a band at 2170 cm-l indicates the formation of solution cyanate upon oxidation. Spectra in Figure 4C,D were obtained using a lithium perchlorate solution at pH 8.0. A band appearing at 2147 cm-'using p-polarized light (Figure 4C) is absent in the spectrum obtained using s-polarized light (Figure 4D) and is therefore assigned to a surface species. A strong band appears a t 2343 cm-' in spectra obtained using either s- or ppolarized light and indicates the presence of the carbon dioxide oxidation product in the thin-layer cavity. Experiments were also performed in which the acid treatment step in the CN- monolayer preparation procedure was eliminated; the electrode was transferred from the KCN adsorption solution, rinsed with water, and then transferred directly to the lithium perchlorate solution for spectral analysis. The resulting spectral features were essentially identical to those presented in Figure 4, where the acid treatment step was utilized. Hence, exchange of the K+ countercation for H+ prior to immersion into the lithium perchlorate solution has little effect on the spectral properties.
Paulissen and Korzeniewski
4566 The Journal of Physical Chemistry, Vol. 96, No, 1 1 , 1992 +.20
v
+.lo
v
-,lo
v
t.80 v +.40 V
-.so v .oo v -.50 V
-.so v -.70 V -.60 V
2ioo
2200
2ioo
2000
2200
2000
Wavrnumbrrr (cm-1)
Wavrnumbrn (cm-1)
0
B
2170
+.a0
v
+.40 V -,lo
v
.oo v
-.so v
-.so v
-.50 V
-.70 V
2400 2200 2000 Wavrnumbrrr (cm-1)
2ioo
2200
2000
-
Wavrnumbrrr (cm 1 )
Figure 4. Infrared spectra collected in situ as a function of potential for a platinum electrode containing a cyanide monolayer in an electrolyte solution containing 0.1 M LiC104 at pH 12.5 (A, 9)or pH 8 (C, D). See text for cyanide monolayer preparation. The background spectrum was obtained at 0.50 V in (A) and (9)and at 1.0 V in (C) and (D). Spectra were collected using p- (A, C) or s-polarized (B, D) radiation.
This result suggests that protonation and deprotonation of the charged CN- adlayer are reversible and rapid. Assignment of the vibrational bands to adsorbed species can be aided by findings reported from LEED and AES studies of prepared CN- monolayers, from previous in situ infrared experiments and from recent sum frequency generation (SFG) experiments. The LEED and AES studiesIlb which examined the structure and composition of Pt( 111) following electrochemical treatment in CN- solutions identified the highly ordered CN-/ C N H mixed monolayer and demonstrated the pH dependence of countercation association. In experiments using Cs+ countercations, it was shown that between pH 4 and pH 9 the Cs+ to CNratio was about 0.25; the remaining CN- sites were associated with the proton. Above pH 9, the Cs+ surface coverage increase rapidly owing to deprotonation of the surface layer. Assignment of carbon-surface coordination for CN- and CNH in these experiments was supported by the observed attenuation of the carbon AES signal relative to the nitrogen signal." Studies in which H C N adlayers were formed on platinum by dosing from gaseous H C N suggested that HCN bonds to the surface via the nitrogen atom.12 A dramatic decrease in work function upon H C N adsorption a t 87 K followed by molecular desorption at higher temperatures was used as evidence to support the bonding assignment to nitrogen-platinum surface coordination. The different adlayer preparation conditions likely affect the differences in the observed adsorption geometries, although more experiments are needed to address this issue. Consistent with the LEED and AES results obtained using electrochemically prepared cyanide monolayers is the assignment of the potential-dependent infrared bands at 2058 and 2147 cm-' to two different protonated states of the cyanide surface layer. The former band, observed under high-pH conditions, can be assigned to a surface layer in which CN- is the dominate species.
The latter band, observed under low pH conditions, can be assigned to a cyanide surface layer in which C N H is the dominate species. SFG spectral featuresl0 are similar to those observed in the infrared spectra reported here, although the SFG spectra have been assigned somewhat differently. Comparable to infrared spectra shown in Figure 4,SFG spectra obtained using aqueous solutions containing 25 mM KCN in 0.1 M sodium perchlorate (the pH of this solution is about 10) show a broad band at about 2070 cm-' which decreases in intensity and shifts toward higher energy with increasing positive potential. The shift in vibrational frequency with applied potential was determined to be 65 cm-'/V, which compares well to the value of 59 cm-'/V for the 2058-cm-' band observed in the infrared spectra in Figure 4. As the intensity of the 2070-cm-I band decreases, a potential-dependent band centered at 2150 cm-' begins to appear, and in SFG spectra collected following a series of potential cycles to positive values, this 2150-cm-I band becomes more prominent. The tuning rate of this band is 10 cm-'/V. The tuning rate of the 2147-cm-l band observed in the infrared spectra shown in Figure 4 is 7 cm-I/V. SFG studies have assigned the 2070- and 2150-cm-' bands to adsorbed CN- bound to the platinum surface through the nitrogen atom and the carbon atom, respectively. Coincidence between the spectral band positions and the tuning rates observed using infrared and SFG spectroscopy suggests that these spectral features are assignable to similar molecular structures. The infrared studies show that the appearance of the two forms of surface CN- is dependent upon the pH of the electrolyte solution and that interconversion between the two forms is rapid and reversible, The appearance of the SFG spectra can also be explained using pH arguments by considering potentialdependent changes in the pH of the thin-layer cavity, a problem which is associated with using a thin-layer electrochemical cell arrangement.16 Previous infrared studies have shown that oxi-
J. Phys. Chem. 1992, 96,4567-4571 dation products can accumulate in the thin-layer cavity owing to slow exchange with bulk solution. The pH becomes lower on account of platinum surface oxidation. Hence, appearance of the 2150-cm-l band in SFG spectra at potentials which coincide with surface oxidation supports assignment of this band to a protonated form of the surface species. Early infrared studies attribute bands between 2070 and 2160 cm-l to vibrations of adsorbed CN- bonded to the surface through the carbon a t ~ m . ~While - ~ the present infrared studies cannot rule out the possibility that the 2058- and 2147-cm-’ bands are associated with nitrogen- and carbon-bound CN-, as the SFG studies predict, in view of the LEED and AES results, it is reasonable to attribute these bands to two different protonated states of the CN- surface layer. Further, that the transformation between the various forms of the charged surface CN- layer is rapid and reversible favors a protonation mechanism rather than the bond breaking, adsorbate reorientation, and subsequent bonding processes required for conversion of the adsorbate between carbonand nitrogen-bonded forms. The importance of the Li+ and K+ countercations in influencing adsorbate structure and chemical environment should also be considered, as previous studies have shown that countercation association is highly specific, favoring cations which are highly charged, followed by those of relatively small size and those which are least strongly hydrated.’ Platinum-adsorbed SCN- layers are expected to behave similarly, and recent infrared studies of this system attribute spectral shifts on the order of a few wavenumbers to countercation interaction^.'^ It is therefore unlikely that different levels of potassium ions in the aqueous electrolyte solutions used in the present study can account for the observed (16) Bae, I. T.; Scherson, D. A.; Yeager, E. B. Anal. Chem. 1990,62,45. (17) Ashley, K.; Samant, M. G.; Seki H.; Philpott, M. R. J . Electroanal. Chem. 1989, 270, 349.
4561
spectral alterations. The spectral features assigned to cyanate and carbon dioxide in Figure 4 merit further discussion. Electrochemical and spectroscopic experiments indicate that cyanate is a product of cyanide electrooxidation under basic aqueous solution conditions,’ Cyanate ion can decompose further to carbon dioxide or carbonate and nitrogen gas or ammonium ion,l4 depending upon solution pH. Alternatively, the appearance of carbon dioxide spectral features could be a consequence of pH lowering which occurs in the thin-layer cavity during electrooxidation. Although solutions were degassed by bubbling with argon just prior to electrochemical experiments, no attempt was made to eliminate dissolved carbon dioxide from the water used in the preparation of the basic perchlorate electrolyte solutions. Carbonate present in the basic electrolyte solutions shifts in favor of dissolved carbon dioxide as the solution p H is lowered. Control experiments with the basic perchlorate electrolyte solutions validate this concern. In conclusion, infrared spectroscopy has been used to identify the spectral features associated with cyanide adsorbed on polycrystalline platinum electrodes under aqueous solution conditions. Two forms of adsorbed CN- surface layers are identified. At pH 12.5, a potential-dependent band appears at about 2058 cm-I and is assigned to the vibrations of a surface layer of CN-. At pH 8, a potential-dependent band appears at about 2147 cm-l and is assigned to a surface CN- layer in which CNH is the predominate species. Future infrared experiments will examine the spectral features assigned to adsorbed cyanide over a wider range of pH and countercation type. Experiments will be extended to platinum surfaces of well-defined crystallographic orientation and will examine the importance of adsorbed intermediates in the electrocatalytic oxidation pathways. Acknowledgment. This work was supported in part by funds provided to C.K.from a Dow Corning Assistant Professorship.
A Solid-state 13C NMR Study of TetraalkylammoniumKlay Complexes T.K. Pratum Department of Chemistry, University of Washington, Seattle, Washington 98195 (Received: November 5, 1991; In Final Form: February 4, 1992)
Carbon-13 magic angle spinning NMR spectra of small organic cations complexed with the phyllosilicate minerals vermiculite and montmorillonite have been taken at 4.7, 7.0, and 11.7 T. These spectra show the effects of magnetic susceptibility,rapid motion, and efficient longitudinal ( T I )relaxation. Comparison of interrupted decoupling experiments of the complexed with those of the pure salts give an indication of the types of motion which may occur in these complexes.
Introduction Phyllosilicate minerals include clays and micas and consist of aluminosilicate-layered lattice structures. In the “2:l” structures, which include montmorillonite and vermiculite, each layer consists of an octahedral sheet existing between, and sharing oxygens with two tetrahedral sheets. Substitution of aluminum for silicon in the tetrahedral layers, or magnesium for aluminium in the octahedral layer, produces excess negative charge which is neutralized by exchangable cati0ns.l Additional substitution of iron for aluminum or magnesium can give these minerals paramagnetic properties. Clay minerals have often been noted, and used with regard to their affinity for organic molecules. Recently, this affinity has been shown to have been altered by complexation with organic ( I ) Schulze, D. G. Minerals in Soil Environments; Soil Science Society of America: Madison, WI, 1989; Chapter 1 .
cation^,^-^ these cations usually being derived from quaternary ammonium salts. While several recent NMR studies have focused upon exchangeable inorganic cations,b* we wished to investigate the environment of these complexed organic cations, as well as obtain an idea as to the potential 13CNMR behavior of naturally occurring clay-organic comple~es.~ Toward this end, a 13Cmagic (2) Boyd, S.A.; Lee, J. F.; Mortland, M. M. Nature 1988, 333, 345. (3) Boyd, S . A.; Shaobai, S.; Lee, J. F.; Mortland, M. M. Clays Clay Miner. 1988. 36. 125. (4) Lee, J. F.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Clays Clay Miner. 1990, 38, 1 1 3. (5) Srinivasan, K. R.; Fogler, H. S . Clays Clay Miner. 1990, 38, 277. (6) Bank, S . ; Bank, J. F.; Ellis, P. D. J . Phys. Chem. 1989, 93, 4847. (7) Weiss, C. A.; Kirkpatrick, R. J.; Altaner, S . P. Am. Miner. 1990, 75, 970. . . (8) Laperche, V.; Lambert, J . F.; Prost, R.; Fripiat, J. J. J . Phys. Chem. 1990, 94, 8821.
0022-365419212096-4567%03.00/0 0 1992 American Chemical Society
