Langmuir 1989,5, 849-853
or amorphous22*23 in structure and with various degrees of hydration. Our measurements suggest that the picture may be more complex. First, there may be a gradient of oxidation states of iron: Fe(0) (at the electrode surface), Fe(II), Fe(II,III), and Fe(II1) (at the surface of the outer coating). Second, these oxides may be obtained in various degrees of hydration; as a rule, the more aged the oxide, the less hydrated it would be. Third, for a semi-immersed electrode there should be changes in composition as we move along the electrode surface from the gaseous phase to the liquid phase. Finally, even within the immersed oxide layer there should be considerable fluctuations in chemical composition, due to the immiscibility of the various oxides in the solid phase. In this context, what would be the mechanism of iron passivation by immersion in nitric acid? We suggest that this treatment generates a layer of iron(II1) oxide, poorly hydrated. This is a uniform coating, which cannot undergo the many transformations suffered by other oxide coatings and is thus free from mechanical tensions associated with interfacial tension gradients. Aluminum Electrode Coatings. The weight-potential curve for aluminum electrodes in H2S04solutions is not substantially modified by the presence of oxygen in the solution; i.e., at an oxidizing potential the electrode is always more wettable than the electrode at reducing potentials. However, the base line for the weight-potential curve was substantially increased for oxygen-bubbled solutions, indicating that in this case the wettability was substantially increased. From these results, we conclude that anodic oxide layers on aluminum 'are more wettable than adjacent-to-the-metal surface oxide coatings. They are thus adherent to it, beneath aqueous solutions, affording a stable protection to the metallic surface. (22) Okamoto, G.; Shibata, T. Corrosion Sci. 1970,10, 371. (23) O'Grady, W. E.; Bockris, J. OM. Surf. Sci. 1973, 10, 249.
849
Coating Flaws. Almost every metallic anodic film coating presents flaws. The behavior of metal carrying an anodic film, when fully immersed in electrolyte solutions, has been studied intensively in order to determine local processes occuring at flaws which are responsible for the passivation breakdown.24 For aluminum, in 1 M H2SO4 solutions, since the anodic oxide layer wettability is larger than that of the AMOC, the liquid will penetrate the flaws but will not contact the AMOC since the interfacial tension between electrolyte and this layer is larger than the anodic oxideelectrolyte tension. The opposite happens for iron in oxygen-free solutions. Stabilizing a metal surface is then possible if it is coated with a more wettable surface than the initial one. The flaws which are present on every surface will be stabilized due to the lower surface energy of the oxide coating. Conclusion The Wilhelmy pull technique associated with periodic variations of electrode potential may be used to evaluate wettability characteristics of anodic oxide coatings on both iron and aluminum electrodes. For iron electrodes, anodic oxide passivating layers generated in an anoxic medium are less wettable than the metal and are thus poorly adherent, in the presence of water. On the other hand, oxide layers generated in the presence of chemical oxidants are more wettable than metal and are thus adherent to it, even in the presence of water. These findings can explain the effectiveness of strongly oxidizing iron passivating procedures. For aluminum electrodes, the anodic oxide layer is more wettable than the adjacent-to-the-metal surface oxide coating (AMOC) and is thus adherent to it. Registry No. Al, 7429-90-5; Fe, 7439-89-6;AlzOs, 1344-28-1; HzS04, 7664-93-9; H2, 1333-74-0; 02,7782-44-7; iron oxide, 1332-37-2. (24) Thompson, G. E.; Wood, G. C. Treatise on Material Science and Technology; Scully, J. C. Ed.; Academic Press: London, 1983; Vol. 23.
Tetracyanoethylene Adsorption on Magnesia Activated below 410 "C: An IR Spectroscopy Study J. J. Hoagland and K. W. Hipps* Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 Received December 7,1988. In Final Form: February 14, 1989
The saturation vapor-phase adsorption (at 100 "C) of tetracyanoethylene (TCNE) onto magnesia activated at 200 and 400 "C is studied. Fourier transform infrared and electron spin resonance spectroscopy are used to characterize the chemisorbed material. FT-IR spectra of authentic samples of TCNE, TCNEl-, TCNE", and the tricyanovinyl alcoholate anion (TVA-) are also reported for comparison purposes. We find that the uptake of TCNE as a function of activation temperature is strongly related to the surface hydroxyl coverage and more weakly correlated with the BET surface area. The primary chemisorption product is a tricyanovinyl alcohol (TVAH) like species (perhaps polymeric). Radical signals are detected, presumably from TCNEl-, but correspond to less than 5% of the chemisorbed TCNE. Introduction Over the years, there has been a continuing interest in the oxidation-reduction (redox) chemistry of TCNE with various Most of the early work was directed (1) Che, M.; Dyrek, K.; Louis, C. J. Phys. Chem. 1985,89, 4531. Faraday (2) Flockhart, B. D.; Liew, K. Y.; Pink, R. C. J.Chem. SOC., Trans. 1 1980, 76, 2026. (3) Marczewki, M.; Malinowski, S. Bull. Acad. Polonaise Sci. 1976, X X V , 187.
toward the detection and identification of radicals formed in the chemisorptionprocess.1-'2 Only recently has surface (4) Davidson, R. S.; Slater, R. J. Chem. SOC., Farady Trans. I 1976, 72, 2416. (5) Kijenski, J.; Malinowski, S.; Zielinski, B. React. Kinet.Catal. Lett. 1976, 4, 251. (6) Fiedorow, R.; Wieckowski, A. React. Kinet. Catal. Lett. 1976,2, 355. (7) Che, M.; Naccache, C.; Imelik, B. J. Catal. 1972, 24, 328.
0743-7463189j2405-0849$01.50/0 0 1989 American Chemical Society
850 Langrnuir, Vol. 5, No. 3, 1989
vibrational spectroscopy been applied extensively to this area of r e s e a r ~ h . ' ~ In - ~ the ~ case of TCNE adsorption on alumina, it was shown that the radical anion of TCNE, TCNEl-, was not the major chemisorption product.13-17 The non-radical dianion, TCNE2-, and the tricyanovinyl alcoholate anion, TVAl-, could also be formed in significant quantities with their relative amounts determined by the activation history of the oxide and the adsorption conditions. Under less than saturation conditions, no trace of the parent TCNE was observed on the surface. FT-IR, Raman, and EELS studies of TCNE adsorbed on single-crystal metals and on electrode surfaces have yielded results that are surface dependent.'&% On carbon electrodes, for example, both the dianion and TVAl- are observed.% In contrast, the radical anion is the principal species reported on platinum e l e c t r o d e ~ . ~ lBoth - ~ ~ the dianion and the radical anion are reported to be formed on C~(lll),'*3'~ but only the radical anion is reported on Ni.25 This surface dependence of the mechanism and extent of TCNE chemisorption on various surfaces is interesting but not well understood. We have begun, therefore, a general study of the chemisorption of TCNE on a number of surfaces. In the present paper, we are concerned with the saturation adsorption (at 100 "C) of TCNE from the vapor on magnesia activated at 200 and 400 "C. These temperatures were chosen for the following reason. When activated below ca.300 "C, our magnesia sample is formally magnesium hydroxide. Activation of magnesia above ca. 360 "C results in the production of MgO having a rather high density of surface-bound hydroxyl groups. During the decomposition of Mg(OH)2,the surface area of the sample rises markedly. Thus, adsorption at these two temperatures should provide insight into the role of hydroxylation and surface area on the chemisorption of TCNE.
Experimental Section Materials. TCNE was purchased from Aldrich Chemical Co. and purified by sublimation through Alpha Products 28-mesh activated charcoal at 100 "C. The Mg(OH), was used as purchased from Alpha Products. Authentic samples of KTCNE, CsTVA, and Na,TCNE were prepared by previously described method~.'~-'~ Sample Preparation and Handling. Oxide activation and
TCNE chemisorption were carried out under vacuum on a standard grease-free double (UHP N2/LN2trapped vacuum) manifold in a demountable two-chamber cell. Each chamber of (8) Wong, P. K.; Allen, A. 0. J. Phys. Chem. 1970, 74, 774. (9) Flockhart, B. D.; Leith, I. R.; Pink, R. C. Trans.Faraday SOC.1968, 65, 542. (10) Terlecki, A.; Nicolau, C.; Cristea, A.; Angelescu, E.; Nicolescu, I. Reu. Roum. Chim. 1968,13, 997. (11) Naccache, C.; Kodratoff, Y.; Pink, R.; Imelik, B. J . Chim.Phys. 1966, 63, 341.
(12) Flockhart, B. D.; Naccache, C.; Scott, J. A,; Pink, R. C. Chem. Commun. (London) 1986,238. (13) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1982,86,5105. (14) Hipps, K. W. J. Electron Spectrosc. Relot. Phenom. 1983,30,175. (15) Mazur, U.; Hipps, K. W. J. Phys. Chem. 1984,88,1555. (16) Hipps, K. W.; Mazur, U. Reo. Sci. Instrum. 1984,55, 1120. (17) Mazur, U.; Hoagland, J. J.; Hipps, K. W., work in progress. (18) Erley, W. J. Phys. Chem 1987, 91, 6092. (19) Erley, W.; Ibach, H. J . Phys. Chem. 1987, 91, 2947. (20) Datta, M.; Jansson, R. E.; Freeman, J. J. Spectrosc. Lett. 1986, 19, 129. (21) Daschbach, J.; Heisler, D.; Pons, S. Appl. Spectrosc. 1986,40,489. (22) Pons, S.; Khoo, S.; Bewick, A.; Datta, M.; Smith, J.; Hinman, A.; Zachmann, G. J. Phys. Chem. 1984,88,3575. (23) Pons, S. J. Electroanal. Chem. 1983, 150, 495. (24) Chou, Y.C.; Liang, N. T. Chem. Phys. Lett. 1984,106,472. (25) Pan, F.; Hemminger, J.; Ushioda, S. J. Phys. Chem. 1986,89,862.
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TEMPERATURE
Figure 1. Percentage weight loss with heating of a Mg(OH), sample heated at 5 OC/min.
the glass cell was isolated by a greaseless and bakeable Teflon valve. First the TCNE (ca. 0.5 g) was loaded into the smaller chamber, pump-purged, and then sealed under vacuum. The amount of TCNE always exceeded the maximum possible uptake by the magneaia sample. Then, the Mg(OH), (ca.5.0 g) was loaded in the second chamber, and the adsorption cell was again attached to the manifold. Typically, four activations (two samples and two references) were carried out simultaneouslyby heating four reaction cells to the desired temperatures for >12 h under vacuum (
