Langmuir 1990,6, 953-957 and SH. This shows that the main alkyl chain of C20Et6 is partially like an oil drop. It is suggested that the increase in the hydrophobicity around the head group makes the C20 chain somewhat more mobile, compared to C20Me6. However, further increase in the hydrophobicity, Le., the large size of the head group, causes the steric hindrance among the three alkyl chains of the head group. The Serpvalue of CzoPr6 is comparable to Si, not to SH.Thus, the aggregation behavior of C20Pr6 is mainly attributed to the packing of the tripropylammonium head group. However, from the analysis of 13C and 14N spin-lattice relaxation data, it was reported15 that the hydrocarbon chains of CzoEts adopt a predominantly stretched form in micelles. To determine the conformation of a,w-type surfactant in micelles precisely, a more extensive work is needed.
Conclusion The effects of the head group size on cationic a,w-type (bolaform) surfactant, eicosane-1,20-bis(trialkylammo-
953
nium) dibromide (CzoMes, C20Ets, and CzoPrd, in aqueous solution have been investigated. The cmc decreases, the ionization degree increases, the aggregation number decreases, whereas the solubilizing power and the distribution coefficient of the hydrophobic compounds increase as the numbers of the carbon atoms of the head group alkyl chain increase. These tendencies are similar to those of the homologues of alkyltrialkylammonium bromides, i.e., the homologues of the conventional surfactant. From the surface area occupied per head group at the micelle surface, it is suggested that the main alkyl chain of is a predominantly stretched conformation and that of CzoEts is partially like an oil drop, whereas the conformation in the main alkyl chain of CzoPr6 is affected by the packing of the tripropylammonium head group. Registry No. C&%, 21949-01-9;C&k, 105476-67-3;C d r 6 , 126328-26-5;PyCHO, 3029-19-4;orange-OT, 2646-17-5; yellowOB, 131-79-3;pyrene, 129-00-0.
Differential Electrochemical Mass Spectrometry Using Smooth Electrodes: Adsorption and H/D-Exchange Reactions of Benzene on Pt Thomas Hartung and Helmut Baltruschat* Institute of Physical Chemistry, University of WittenlHerdecke, Stockumer Strasse 10, 5810 Witten, FRG Received May 19,1989. In Final Form: December 16, 1989 While previously a porous electrode fixed to a hydrophobic membrane had to be used as the inlet system for differential electrochemical mass spectrometry, a newly designed thin-layer cell allows for the first time processes to be studied that occur on a smooth electrode. The feasibility of the new cell is demonstrated for the cathodic desorption of benzene adsorbed on annealed Pt in 0.5 M H&Or. Partial desorption occurs in the H region. Complete desorption is only achieved under hydrogenation to cyclohexane at more negative potentials, where the hydrogen evolution reaction takes place. No partially hydrogenated products are formed. Experments in which C6D6 was used show that no C-D bond rupture occurs upon adsorption. Various degrees of HID exchange occur in the adsorbate.
Introduction Differential electrochemical mass spectrometry (DEMS) allows the straightforward on-line detection of volatile electrochemical reaction p r o d ~ c t s . l -It ~ has, e.g., helped in both understanding the nature of adsorbed intermediates4and in elucidating electrochemical reaction pathways by isotopic labeling.s In all these studies, a porous gas diffusion electrode was used which formed the interface to the mass spectrometer. Here we will describe how a smooth electrode (1) Wolter, 0.; Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 2, 6. (2) Willsau, 3.; Heitbaum, J. J. Electroanal. Chem. 1985, 194, 27. (3)Banach, B.; Baltruschat, H.; Heitbaum, J. Electrochim. Acta 1988, 33, 1479. (4) Willsau, J.; Wolter, 0.;Heitbaum, J. J. Electroanal. Chem. 1985, 185, 163. ( 5 ) Wohlfahrt-Mehrens, M.; Heitbaum,J. J.Electroanal. Chem. 1987, 237, 251.
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can be used for DEMS. The use of smooth electrodes is essential in fundamental studies because of the wellknown influence of surface roughness on various electrochemical parameters.'j To demonstrate the feasibility of the method, we investigated the desorption of benzene and its H/D exchange from annealed Pt electrodes. The adsorption of benzene on Pt electrodes has been studied in a number of papers.'-" Little attention, though, has been paid to processes in the hydrogen region. Only (6)White, J. H.; Soriaga, M. P.; Hubbard, A. T. J.Electroanal. Chem.
1984, 177, 89.
( 7 ) Gileadi, E.; Duic, L.; Bockris, J. O'M Electrochim. Acta 1968,13, 1915. (8) Kazarinov, V. E.; Frumkin, A. N.; Ponomarenko, E. A.; Andreev, V. N. Elektrokhimiya 1975,11,860. (9) Zelenay, P.; Sobkoweki, J. Electrochim. Acta 1984,29, 1715. (10)Vasilev, Yu. B.; Maksimov, Kh. A.; Gorohhova, L. T. Elektrokhimiva 1985.21. 186. (11)KLliev, S. A.; Vasilev, Yu. B.; Bagotskii, V. S. Elektrokhimiya 1986, 22, 7. I~
~~
0 1990 American Chemical Society
954 Langmuir, Vol. 6, No. 5, 1990
Hartung and Baltruschat
Figure 1. Schematic representation of the thin-layer cell: (a) electrode support; (b) platinum sheet (WE); (c) circular P T F E spacer; (d) cell body; (e) electrolyte outlet and connection to counter electrode; (f) electrolyte inlet; (9) porous P T F E membrane separating electrolyte and vaccum (supported by a porous steel frit); (h) connection to vaccum chamber. Not shown: reference electrode connected by a third capillary.
recently it was found, using long optical path length thinlayer spectroelectrochemistry,12 that adsorbate layers formed by various phenols might partially desorb in the hydrogen region and that hydrogenation to the respective saturated compound occurs at even more negative potentials. Adsorption isotherms for hydroquinone and its derivatives have been determined for smooth Pt electrodes in thin-layer electrochemical cells by Hubbard and c o - ~ o r k e r s . ~ 3AJt~low concentrations, the packing densities correspond to a flat (v6) orientation; a t higher concentrations, a transition occurs to the vertical (v2) orientation. From the amount of the charge necessary for the hydrogenation, it is concluded that cyclohexanediol is produced from the flat adsorbate and that only partial hydrogenation of the edgewise-oriented molecule (e.g., to cyclohexenediol) occurs. We recently studied the oxidation and hydrogenation of adsorbed toluene and acetone on porous Pt electrodes.l6J7 In both cases, part of the adsorbate is desorbed molecularly, whereas another part comes off as the completely hydrogenated product to methylcyclohexane or propane, respectively. In the case of acetone, a complete H/D exchange occurs in the a d ~ 0 r b a t e . lIt~ should be interesting to know whether such an H/D exchange is typical for organic electrosorbates. A study of these exchange reactions should provide fundamental information about the catalytic properties of the respective electrode surface.
Experimental Section The experimental setup for DEMS using porous electrodes has been described before.1.4 The newly designed thin-layer cell, which allows the use of smooth electrodes, is shown in Figure 1. The electrode is separated from the porous Teflon membrane (Gore Tex) by a 50-wm-thick Teflon spacer which gives a cell volume of about 1WL.The electrolyte is introduced through (12) Gui, Y.; Kuwana, T. Langmuir 1986,2, 471. (13)Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982, 104, 2735.
(14) Soriaga, M. P.; Hubbard, A. T. J. Phys. Chem. 1984,88, 1758. (15) Mebrahtu, T.;Berry, G. M.; Soriaga, M. P. J.Electroanal. Chem. 1988,239,375; 1988,247, 241. (16) Zhu, J.; Hartung, Th.; Tegtmeyer, D.; Baltruschat, H.; Heitbaum, J. J.Electroanal. Chem. 1988,244, 273. (17) Biinsch, B.; Hartung, Th.; Baltruschat, H.;Heitbaum, J. J.Electroanal. Chem. 1989, 259, 207.
1
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1.0
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15 E I V
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" U 00 05 10 15 I I V
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Figure 2. Cathodic desorption of benzene from smooth Pt in 0.5 M after adsorption from the same solution saturated with benzene (&d = 0.5 V, u = 12 mV/s): (a) CV (broken
line: supporting electrolyte); (b) MSCV for m/z = 78 C6H6 (dotted: 2nd cycle); (c) MSCV for m/z = 84 C6H12 (dotted: 2nd cycle). one of the capillaries; the other one serves as the outlet in electrolyte-exchange experiments and a t the same time provides the connection to the counter electrode. A third bore acts as the Luggin capillary for the reference electrode. Adsorption is performed a t controlled potential by forcing fresh electrolyte, saturated with benzene, through the thinlayer cell (typically 0.5 mL in 1 min), followed by a thorough exchange with the pure supporting electrolyte. During this procedure, the thin-layer cell has always to be filled with electrolyte to avoid any gas contact of the porous membrane. Volatile species generated at the electrode diffuse through the electrolyte, evaporate a t the pores of the Teflon membrane into the vacuum, and are detected by the mass spectrometer with a time constant of 2-3 s. This time constant is small enough to allow so-called mass spectrometric cyclic voltammogramms (MSCV) to be recorded in parallel to cyclic voltammogramms. Up to five m/z values can be recorded simultaneously with the computerized quadrupole mass spectrometer (Balzen QMG 511). Prior to use, the polycrystalline Pt sheet (99.98% purity Degussa) was annealed in a hydrogen flame and quenched in Millipore water. All solutions were prepared from HzS04, suprapure
Langmuir, Vol. 6, No. 5, 1990 955
Benzene Adsorption and HID Exchange on Pt IlpA c
' I
0.0 0.5 1.0 1.5 EIV Figure 4. MSCV showing the desorption of cyclohexane in pure HzO + 0.5 M after adsorption of CeDe from the same solution saturated with benzene ( E d = 0.5 V, u = 12 mV/s): m f z = 88, cyclohexane-dr;m / z = 90, cyclohexane-de.
, I 19
0.0
0.5
1,o
1.5 E N
Figure 3. Desorption of benzene in pure DzO + 0.5 M after adsorption of CeHe from the same solution saturated with benzene (&d = 0.5 v, u = 12 mV/s): (a) CV; (b) MSCV for
benzene-d2 and benzene-d3.
(Merck),and Millipore water and deaerated with 99.9997% Ar. Benzene was introduced into the electrolyte by bubbling with Ar saturated with benzene p.a. 99.7% (Carl Roth GmbH) at room temperature. Since this benzene is in equilibrium with pure liquid benzene, the solution itself is saturated with benzene, which gives a concentration of 0.02 M.18 Benzene-de (99.5atom % D) and DzO (99.8 atom % D; Sigma Chemie)were used as received. Potentials are given vs a reversible hydrogen electrode in the same solution.
Results Benzene was adsorbed at E = 0.5 V by passing 0.5 mL of the saturated solution through the thin-layer cell. After the thorough electrolyte exchange, the potential was still kept at the adsorption potential for 5-10 min to allow the partial pressure of benzene in the mass spectrometer to decrease. The potential was then swept negative into the hydrogen evolution region (Figure 2a). The corresponding MSCV for m / z = 78 and 84 (Figure 2b and 2c) show that benzene is desorbed in the potential region of the hydrogen adsorption. At more negative potentials, however, where hydrogen evolution occurs, the adsorbate is desorbed under hydrogenation to cyclohexane. By introduction of a benzene-loaded hydrogen gas stream with varying benzene concentrations into the mass spectrometer, it was confirmed that cycylohexane is not produced by a reaction of benzene with hydrogen (evolved negative of 0.0 V) in the vacuum chamber. When extending the sweep to -0.02 V, the desorption is (nearly) complete, as revealed by the H adsorption peaks in the voltammogramm recorded after the desorption. No partially hydrogenated products, e.g., cyclohexene, were detected. (18) LZmdolt-B8mstein,Zahlenwerte und Funktionen; 6. Aufl.,Bd. 11/2b, Berlin, 1962; p 3-395.
0
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A crude estimate of the relative amounts of desorbed benzene and cyclohexane can be made taking into account their different degress of fragmentation: only 10-20% of the adsorbate is desorbed as benzene, the rest as cyclohexane. To get more insight into the desorption process, DzO was used as a solvent in an adsorption/desorption experiment (Figure 3a). Here, the negative potential limit was chosen such that hydrogenation does not yet occur. The MSCV for the different deuterobenzenes (two of which are presented in Figure 3b as an example) show that during the anodic sweep benzene is still desorbed. Obviously this desorption is a relatively slow process. Notice that the desorption of a part of the adsorbate layer during the first sweep does not lead to an increased hydrogen adsorption in subsequent sweeps. Instead, there is an extra cathodic charge in the first sweep, presumably due to double-layer effects. If in the same experiment, i.e., after the benzene desorption described above, the cathodic potential limit is made more negative, hydrogenation and desorption of the remaining adsorbate take place, yielding isotopic contents of cyclohexane from C6H12 to C6D12. The same experiments were done using C6D6 in HzO/ HzS04 electrolyte. An example for the corresponding MSCV for cyclohexane is given in Figure 4. The results are qualitatively identical with those obtained with C6H6 in D20 but for a quantitative analysis more reliable (because of small amounts of HzO in the D2O-electro-
956 Langmuir, Vol. 6, No. 5, 1990 lyte due to normal impurity and addition of HzS04). The distribution of the mass intensities for benzene and cyclohexane isomers is shown in Figure 5 for the case of c&3 adsorbed from HZO-electrolyte. These data were corrected for the occurrence of the M-1 (and/or M-2 in the case of the deuterated compound) fragment of benzene. The following features are typical for all the experiments done: high abundancies of the species in which either all or no hydrogens have been exchanged (both for benzene and cyclohexane) and the relatively even distribution among the species which underwent partial H/D exchange. Although no cyclohexane with more than six deuterons is expected in this kind of experiment, the respective mass intensity ( n / z = 91) in Figure 5 is higher than expected from the C-13 abundance. The excessive D atom probably originates from neighboring CsDe molecules. In all these experiments, no potential dependence of the isotope abundancies was found.
Discussion Our results show that a part of the adsorbed benzene is cathodically desorbed in the hydrogen region. The remaining adsorbate can be completely desorbed under hydrogenation. This behavior resembles that of adsorbed toluene, where, however, a porous electrode was used and where the cathodic potential limit was not negative enough to achieve a complete desorption.16 In this work, we used a smooth, annealed Pt electrode and high benzene concentrations. One therefore might expect an edgewise (q2) orientation of the adsorbed benzene molecule analogous to the work of Hubbard and co-workers on adsorbed hydroquinone and its deriva t i ~ e s . ~ JSoriaga ~ J ~ et al. report that hydrogenation is complete to cycl~hexanediol~~ for flatly adsorbed hydroquinone (in accordance with Kuwana9, but a partial hydrogenation (to cyclohexenediol)was suggested for the edgewise orientation. This was based on the measured number of electrons for reduction as well as on the argument that the double bond which is most distant from the Pt surface is not accessible for hydrogenation. We did not find any partially hydrogenated product. Our findings of both molecular desorption and complete hydrogenation, however, could suggest that the value n H in ref 15 is the average for desorption and complete hydrogenation. On the other hand, it is not astonishing that no partially hydrogenated products are formed. Thermodynamically, they should be hydrogenated even more readily than benzene. Of course, it cannot be excluded that they are formed as short-lived intermediates, but on the time scale of our experiment and especially that of ref 15 there is plenty of time for them to readsorb in a different orientation and to undergo further and complete hydrogenation. This view is sustained by the fact that benzene derivatives, which are desorbed at intermediate potentials from the flat orientation into solution, are subsequently hydrogenated to the respective cyclohexane at more negative potentials, as spectroeledrochemicdy shown by the decrease in aromate concentration in the thinlayer ce11.12 Further reduction of possibly formed cyclohexene derivatives in the case of vertical adsorption should even be more facile. The above discussion therefore still holds, if one assumes that benzene behaves quite differently from hydroquinone and is adsorbed horizontally even at high concentrations. The fact that in our case no hydrogen bond is broken upon adsorption (see below) suggests such a flat orientation indeed.
Hartung and Baltruschat The extra cathodic charge connected with the partial desorption of benzene (cf. the first and subsequent sweeps in Figure 3a) can only be due to double-layer charging. The desorption itself is not connected with a faradaic charge, and a replacement by adsorption of hydrogen should show up in the following sweeps as well. The charging of the double layer can be easily explained by the fact that the DL capacity is larger the smaller the adsorbate coverage is (different thicknesses of the adsorbate layers) and by the fact that the desorption occurs negative of the pzc, which is believed to be at 0.2 V (NHE) at the clean surface.lg Of special interest is the question, if benzene is adsorbed intact or under dehydrogenation, as known, for instance, for CH30H at Pt electrodes or CzH4at the Pt-gas phase interface. If this was the case (e.g., adsorption as C6H4), the extra cathodic charge in the first sweep of Figure 3a would be due to hydrogenation of this adsorbate to benzene. The H/D-exchange experiments, however, exclude this possibility. Benzene molecules, in which none or only one H atom has been exchanged, have even more or about the same abundance, respectively, as those with two or more exchanged hydrogens. If the benzene molecules were adsorbed under dehydrogenation of the two C atoms vicinal to the surface, their abundance should be zero. The occurrence of a wide distribution of all the possible H/D-exchange products differs from the case of adsorbed acetone, where complete H/D exchange occurred in the a d ~ 0 r b a t e . lThis ~ might partially be due to the fact that in the case of acetone a porous electrode was used. The main reason for the fast H/D exchange in the case of adsorbed acetone, though, seems to be the acidity of the CH3 groups, which is due to the stabilization of the corresponding base as the enolate. This enolate anion, in turn, is more stable in the adsorbed state than in solution, resulting in a much higher H/Dexchange rate than in solution. Such an argument, of course, does not hold for benzene. On the Pt-vacuum interface, the exchange rate has also been found to be very small (time constant of hours at 320 KZO). In this case, a reaction mechanism was proposed, which involves the addition of one or two adsorbed hydrogens or deuterons, respectively, as the first step. It is astonishing that in Figure 5 maxima occur for those species (benzene and cyclohexane) in which either no or all hydrogens have been exchanged. At this point, we can only speculate about the reasons. It could be that once a molecule is in the activated state for the exchange reaction, all hydrogens are exchanged before the activation energy is relaxed. Another origin for this behavior could be the influence of the different crystallographic orientations of the polycrystalline surface. (One orientation might, the other might not, catalyze the H / D exchange.) More work to this end is necessary and in progress.
Conclusions By use of a thin-layer-type interface to the mass spectrometer, it is possible to use smooth electrodes with DEMS. This is not only of importance for fundamental research (because of the different catalytic properties of rough and smooth electrodes) but also for more applied (19)Trasatti, S.; In Aduances in Electrochemistry and Electrochemical Engineering; Gerischer, H.; Tobias, C. W.; Eds.;New York, 1977; Vol. 10,p 213. (20) Surmann, M.; Bare, S. R.; Hofmann, P.; King, D. A. Surf. Sci. 1983,126, 349.
Langmuir 1990,6, 957-970 work, since basically every type of technical electrode can now be used. We achieved also a good sensitivity: although the real surface area of the electrode was only 0.3 cm2, it was possible to detect 14 different m/z values for the adsorbate (corresponding to 15 isomers of both benzene and cyclohexane), which means that the sensitivity is at least in the range of a few percent of a monolayer. We studied the adsorption of benzene, because it is the parent compound of aromatic molecules, and found that (1) adsorbed benzene is partially desorbed in the
957
hydrogen region, (2) all remaining adsorbate can be desorbed after hydrogenation a t potentials where the hydrogen evolution takes Place, and (3) the degree of H / D exchange suggests that no C-H bond is broken upon adsorption.
Acknowledgment. Thanks are due to Prof. Heitbaum for stimulating discussions. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Registry No. CeHs, 71-43-2;HO,1333-74-0;Pt, 7440-06-4.
Adsorption of Bipyridyls and Structurally Related Compounds at Pt( 11 1) Electrodes: Studies by Vibrational Spectroscopy (EELS),Auger Spectroscopy, and Electrochemistry Scott A. Chaffins, John Y. Gui, Bruce E. Kahn,? Chiu-Hsun Lin, Frank Lu, Ghaleb N. Salaita,l Donald A. Stern, Donald C. Zapien, and Arthur T. Hubbard* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72
C. Michael Elliott Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523 Received June 1,1989. I n Final Form: November 7, 1989 The adsorption behavior of bipyridyls and structurally related compounds from solution at well-defined Pt(ll1) surfaces is examined in this study to explore the influence of adsorbate molecular structure on surface bonding, molecular orientation, vibrational spectroscopy,acid-base reactivity, and electrochemical behavior of adsorbed aromatic molecules. Molecules were selected to represent various degrees of steric hindrance at the ring nitrogens: 2,2’-bipyridyl(22BPY), 2,4-bipyridyl (24BPY), 3,3’-bipyridyl (33BPY), 4,4’-bipyridyl (44BPY), 2-phenylpyridine (2PPY), 3-phenylpyridine (3PPY), 4-phenylpyridine (4PPY), 2,6-diphenylpyridine (26DPPY), 2-methylpyridine (PMPY), and 2,6-dimethylpyridine (26DMPY). A series of carboxylic acids was also studied in order to explore the interactions between acidic moieties and the Pt(ll1) surface: 2,2’-biphenyldicarboxylicacid (22BPDC), 4,4’-biphenyldicarboxylic acid (44BPDC), 2,2’-bipyridyl-4,4’-dicarboxylic acid (44DC), 2,2’-bipyridyl-4-methyl-4’-carboxylic acid (4M4C), 2,2’-bipyridyl-5,5’-dicarboxylic acid (55DC), and 2,2’-bipyridyL4,4’,5,5’-tetracarboxylic acid (4455TC). Packing densities (moles adsorbed per unit area) were measured by means of Auger spectroscopy. Linear potential scan voltammetry was used to determine the reactivity of the adsorbed layers toward electrochemical oxidation; electrochemicalreactivity of these compounds provides important clues to their mode of bonding to the surface. Surface vibrational spectra were obtained by electron energy-loss spectroscopy (EELS) and are assigned by comparison with the IR spectra of the pure compounds. The Pt(ll1) surfaces used in this study were characterized by LEED. Most of the subject compounds are adsorbed with the ring plane nearly perpendicular to the platinum surface; the exceptions are those prevented from doing so by steric constraints such as bulky substituents adjacent to the aromatic nitrogen atoms. All of the bipyridyl carboxylic acids studied adsorb strongly and have pendant carboxylic acid moieties that give vibrational spectra that are noticeably dependant on the electrode potential and that react readily with KOH. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1V vs AgJAgCl), as evidenced by diminution of the intensities of bands due to 0-H and C=O stretching, as well as shifts in the frequency and intensity of aromatic CC modes. The biphenyl carboxylic acids 22BPDC and 44BPDC allow the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity 0-H and C=O stretching vibrations. Introduction Adsorption of aromatic compounds from solution onto annealed Pt surfaces has been shown to produce orit Present address: Eastman Kodak Company, Ultra Technologies, Route 88 South, PO Box 267, Newark, New York 14513. Present address:Union Carbide Corporation, PO BOX 8361, South Charleston, West Virginia 25303.
*
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ented adsorbed 1ayers.l In particular, pyridine adsorbed at Pt(111) from aqueous solutions (10-6-1 M) or liquid pyridine forms an oriented layerlf having an average ringto-surface angle of 71°, based upon average molecular Packing densities obtained from Auger spectroscopic data. The pyridine adsorbed layer has a long-range ordered structure as judged by LEED, Pt(lll)(3.3224x4.738, 77.1°)R340-PYR, in which the ordered molecular layer 0 1990 American Chemical Society
