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Adsorption of Monosubstituted Benzenes on Low Surface Area Silica Studied by Temperature-Programmed Desorption and Laser-Induced Thermal Desorption Methods Pierre Voumard, Qiao Zhan, and Renato Zenobi" LPAS (bbtiment de chimie), Ecole Polytechnique Fhdhrale, 1015 Lausanne, Switzerland Received August 1, 1994. In Final Form: November 1, 1994@ The adsorptionof aniline,phenol, and toluene on low surfacearea silica has been studiedwith temperatureprogrammed desorption (TPD) and laser desorption methods. These adsorption systems can serve as models for studyingthe mechanismoflaser-inducedthermal desorption,where heating rates and desorption rates are orders of magnitude larger than in TPD. For this purpose, results obtained at low heating rates need to be extrapolated t o the regime of very high laser heating rates. This extrapolation relies on a model of the adsorbate-surface interaction that involves a distribution of binding sites. Calculations based on this model match experimental measurements on the depletion of adsorbed layers subjected to laserinduced desorption. The interaction energy of aniline and phenol was found to depend on the hydration of the surface, demonstrating the role of hydrogen bonding with silanol sites. 1. Introduction The adsorption of relatively complex organic molecules on dielectric material surfaces is a topic of considerable relevance for studying the behavior of inert material surfaces, e.g. in laser-induced desorption experiments or in the investigation of the support phase in model catalytic systems. However, this chemical inertness is relative, as specific surface-adsorbate interactions exist on an atomic level, with particular sites that may provide quite high binding energies, and may, for example, exhibit catalytic activity themse1ves.l The chemical inertness of dielectric materials is also employed for the production of intact gas phase molecules from polar, nonvolatile, high molecular weight, and thermally labile organic overlayers using laser-induced thermal desorption (LITD)methods2 LITD is different from laser desorption with visible or ultraviolet light, a process that often involves electronic transition of the adsorbate or creation of electron-hole pairs.3 LITD is typically carried out using far-infrared radiation and starts with rapid heating of the sample substrate. However, many questions about the exact mechanism of LITD are still unresolved. Interesting physical effects, such as nonequilibrium effects, may come into play for weakly bound surface adsorbate^.^ For studying the details of the desorption mechanism, it is not sufficient t o know the time evolution of the transient surface temperature jumps; a detailed knowledge about the interaction strength of the adsorbate with the surface is also needed to support or discount proposed mechanisms for LITD. In this context, we have started a research program investigating the adsorption and desorption behavior of monosubstituted benzenes on amorphous silica surfaces.
* To whom correspondence should be addressed: Prof. Renato Zenobi, LOC-Universitatsstr. 16, ETH Zentrum, CH-8092Zurich, Switzerland; Tel 01-632 4 3 76, Fax 01-632 12 92, e-mail
[email protected]. Abstract published in Advance ACS Abstracts, February 1 , 1995. (1)Haruta, M.;Kageyama,H.; Kamijo, N.; Kobayashi,T.;Delannay, F. Stud. Surf. Sci. Catal. 1989, 44, 33. (2) Lubman, D. M. Lasers and Mass Spectrometry, Oxford University Press: New York., 1990. ~ ~ (3) Cavanagh, R. R.; King, D. S.; Stephenson, J. C.; Heinz, T. F. J. Phys. Chem. 1993,97, 786. (4) Zare, R. N.; Levine, R. D. Chem. Phys. Lett. 1987, 136, 593. @
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Our particular goal is to provide a sound base for extrapolating TPD data to the regime of heating rates reached in laser desorption experiments (108-1010 Ws). Simulations of LITD have been carried out for metal5-' and semiconductor surfaces.8 They showed the sensitivity of the process to the spatial and temporal pulse profile as well as to the desorption kinetic parameters. The organic molecules studied (aniline, phenol, toluene) were chosen because they are volatile enough to be introduced into a clean vacuum system as gases but already exhibit some of the multifunctional properties expected for much more complex adsorbates, such as n electron systems, apolar or polar substituents, nonbonding electron pairs, hydrogen bonds, or the possibility to undergo acid-base interactions with the surface. A few surface science studies on these molecules on metalsg-13 and on iron and zinc oxides14 exist and can be used for comparison. Amorphous silica was chosen in this study not only because it is often used in practical laser desorption experiments but also because it is a material of great technological importance. Its physical and chemical properties are well-known. Not nearly as much information on the chemistry of low surface area dielectric materials exists compared to the large data base on high surface area oxides or single crystal metal surfaces; ~O nevertheless the situation is beginning to ~ h a n g e . l ~ -The limited amount of information is primarily due to the (5) Hall, R. B.;Bares, S. J. In Chemistry and Structure at Interfaces; Hall, R. B., Ellis, A. B., Eds.; VCH Publishers: Deerfield Beach, FL, 1986; p 85. (6) Brand, J. L.;George, S. M. Su$. Sci. 1986, 167, 341. (7) George,S. M. InInvestigatwnsofInteTfacesand Surfuces;Rossiter, B. W., Baetzold, R. C., Eds.; Wiley Interscience: New York, 1990. (8) Koehler, B. G.; George, S. M. Su$. Sci. 1991, 248, 158. (9) Gland, J. L.; Somorjai, G. A. Adv. Colloid Interface Sci. 1976,5, 205. (10) Schoofs, G. R.; Benziger, J. B. J. Phys. Chem. 1988, 92, 741. (11)Myers, A. K.; Benziger, J. B. Langmuir 1989,5, 1270. (12) Solomon, J. L.; Madix, R. J.; Stohr, J. Surf. Sci. 1991,255, 12. (13)Huang, S.X.; Fischer, D. A.; Gland, J. L. J. Vac. Sei. Technol. A 1994,12, 2164. (14) Nakazawa, M.; Somorjai, G. A. Appl. Surf. Sci. 1993, 68, 517. (15)Tro, N. J.; Haynes, D. R.; Nishimura, A. M.; George, S. M. J . Chem. Phys. 1989, 91, 5778. (16)Arthur, D. A.; Meixner, D. L.; Boudart, M.; George, S. M. J . Chem. Phys. 1991, 95, 8521. (17) Meixner, D. L.;Arthur, D. A.; George, S.M. Surf. Sci. 1992,261, 141.
0743-7463/95/2411-0842$09.00/0 0 1995 American Chemical Society
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Figure 1. Schematic representation of a fully hydrated silica surface, of different kinds of silanol groups, and of possible binding interactions of aniline with surface OH groups on silica. formidable experimental challenges posed by mounting, heating, cooling, preparing, and characterizing insulator surfaces in ultrahigh vacuum (UHV). A large number of studies have dealt with the adsorption of molecules on high surface area silica. For many organic molecules the role of silanols as adsorbing sites have been recognized, mostly through infrared spectroscopy.21-26 Several different possible configurations exist for the silanol groups, as shown in Figure 1. The interaction of adsorbates with the surface is expected to depend on the type of silanols present on the surfaces. At extremely low density of OH groups, unsaturated Lewis acid sites (Si+d) dominate on Si02. On partially hydroxylated surfaces, aniline and phenol are known to bind mostly by their substituent group to the surface silanols that represent weak Bronstead acid sites.27 In the case of aniline, hydrogen bonding involving the nitrogen lone pair as well as interaction of the n system of the aromatic ring with OH groups was observed2* (see Figure 1). Similar interactions are likely to exist for phenol. The density of the hydroxyl groups on the surface can be changed in a fairly reproducible way.23,24A fully hydrated silica surface can lose 75%of its silanol groups if heated to 1300 K in vacuum, where neighboring OH groups (geminal, vicinal) are lost most easily. This allows "tuning" the specificsurface sites available to an adsorbate. In a broader sense, adsorption interactions with silica surfaces can be controlled by introducing a variety of surface functionalities using silylation reaction^.^^,^^ (18)Wu, M.-C.; Truong, C. M.; Goodman, W. J . Phys. Chem. 1993, 97,4182. (19)Le Grange, J . D.; Markham, J . L.; Kurkjian, C. R. Langmuir 1993,9,1749. (20)Nakazawa, M.; Somojai, G. A. Appl. Surf. Sci. 1993,68, 539. (21)Hair, M. L.Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967. (22)Cusumano, J . A,; Low, M. J. D. J. Catal. 1971,23,214. (23)Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (24)Knozinger, H. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North Holland Publishing Co.: Amsterdam, 1976. (25)Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988,110,2074. (26)Ballinger, T. H.; Basu, P.; Yates, J. T., Jr. J.Phys. Chem. 1989, 93,6758. (27)Kiselev, A. V.;Lygin, V. I. Surf. Sci. 1964,2,236. (28)Low, M. J. D.; Hasegawa, M. J. Colloid Interface Sci. 1968,26, 95. (29)Erard, J.-F.; Nagy, L.; Kovats, E. sz. Colloid Surf. 1984,9,109. (30)Szab6, K.;Ha, N. L.; Schneider, P.; Zeltner, P.; Kovats, E. sz. Helv. Chim. Acta 1984,67,2128.
2. Methods
The surface used was that of a synthetic amorphous fused silica disk, 19 mm in diameter and 2 mm thickness, polished to optical quality. In order to eliminate all metallic contaminants from the polishing agents, the disk was cleaned in concentrated boiling nitric acid for 60 h and then fully hydrated by keeping it in boiling bidistilled water for the same time.31 It was then coated on the back side only (covering the front surface) with a 300-nm film of evaporated gold for resistively heating the sample. The disk was mounted in the vacuum chamber onto a copper sample holder which was in good thermal contact with a liquid nitrogen Dewar. The temperature was measured by a type K thermocouple cemented (type 9400 ceramic adhesive, Kager GmbH, Frankfurt, Germany) into a tiny hole that was laser-drilled into the edge of the sample disk. To ensure good temperature homogeneity across the sample, very slow heating rates, typically about 1/3 Ws, were employed in all TPD measurements. The base pressure of the UHV chamber was in the mid to low 10-lo mbarrange. The chemicals were 99.5%pure and degassed by several freeze-pump-thaw cycles. Exposures were controlled by a leak valve and measured by a nude ion gauge. The pressure read was multiplied by 6.2 for phenol, by 6.5 for aniline, and by 6.8 for toluene to correct for the ionization probability of these subs t a n c e ~ . ~The ~ - desorbing ~~ species were detected by a quadrupole mass spectrometer (QMS) (Multiquad, mass range m / z = 0-300, Leda Mass, U.K.), surrounded by a Feulner cap.35,36The latter is pumped through three holes with a total area of 0.2 cm2, resulting in a calculated characteristic pressure decay time of 0.13 s. Positioning the surface very close to the entrance opening of the Feulner cap ensures that predominantly molecules desorbing from the surface are detected. This arrangement also provides a near constant pump speed of the interior volume of the Feulner cap, a n enhanced concentration of the desorbing species around the QMS ion source, and therefore greater sensitivity. The possible contribution of electron-stimulated desorption (ESD)by stray electrons from the QMS ion source was checked by positioning a silica surface covered with ca. 0.3 monolayer (ML)aniline in front of the hole of the Feulner cap for 60 min. A TPD spectrum was recorded subsequently. Compared t o previous TPD spectra, no change was detected, indicating that ESD was absent or negligible. An X-ray photoelectron spectroscopy(XF'S)analysis was done on a silica sample that had been used repeatedly for TPD and laser desorption of aniline films. The only surface contaminant detected was carbon. In the 400-eV region, no signal corresponding to the N 1s peak of aniline was detected.37 We can therefore exclude the formation of a polyaniline layer. The carbon contamination was due either to graphite formed by decomposition of a small fraction ofthe organic adsorbates over a n extended period of time or, more likely, to a hydrocarbon film originating from the ambient air (the sample had to be transported to another chamber where XPS spectra were recorded a t ambient temperature). Using XPS analysis of high surface (31)Kovlts, E. sz. Personal communication. (32)Holanda, R. J. Vac. Sci. Technol. 1973,19, 1133. (33)Summers, R.L. Ionization gauge sensitivities as reported in the literature; NASA Technical Note TND 5285. (34)Leybold. Korrektionsfaktoren fiir Ionisations-Messsysteme, Leybold AG technical note no. 9,1988. (35)Feulner, P.; Menzel, D. J.Vac. Sci. Technol. 1980,17, 662. (36)Schlichting, H.; Menzel, D. Surf. Scz. 1993,285,209. (37)Kishi, K.; Chinmoi, K.; Inoue, Y.; Ikeda, S. J. Catal. 1979,60, 228.
Voumard et al.
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Figure 2. Temperature programmed desorption from low surface area silica of (a) aniline, (b) phenol, and (c) toluene at various coverages up t o about one monolayer. The heating rate was 1/3 Ws, and the partial pressure of the intact molecular ion signals was measured. Inserts show the linear dependence of the integrated peak areas on exposure. area silica samples, Kallury et aL3*found that a heat treatment at 470 K on a vacuum line left only 3-5% of carbon contamination mostly in the form of hydrocarbons, which turned to graphite when heated to 670 K. Our goal is to extrapolate desorption data obtained a t low heating rates to the regime of laser heating rates. For this purpose, a fairly accurate description of the desorption kinetics is sought, and a n estimated error for the extrapolated temperatures should be given. Various methods exist for interpreting TPD data, ranging from Redhead's classical analytical approach,39over various integral,40differential,41direct numerical,42 and Monte Carlo methods.43 For reasons discussed below, our desorption kinetics did not follow a clear order law when analyzed with some of the common methods, using constant desorption activation energies and preexponential factors. Although it is known that both preexponential factors and activation energies can be coverage depende n t t 4 we chose a numerical method based on first-order desorption kinetics to simulate the TPD traces. The activation energy was either kept constant, taken as a function of coverage, or expressed as a distribution based on different surface sites. The actual functional form for the binding energy is given below. Laser desorption experiments were done with a linetunable pulsed COZlaser (Lumonics Model TEA 820 HP). Laser pulses consist of a 100-ns spike followed by a 2-ps tail. The 3 cm diameter beam was apertured and directed to the sample without focusing. If necessary it could be attenuated further by neutral density filters. Very high (38)Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994,10,492. (39)Redhead, P. A. Vacuum 1962,12,203. (40) Chan, C. M.; A r i s , R.; Weiberg, W. H. Appl. SUI$ Sci. 1978,I, 360. (41)Miller, J. B.;Siddiqui, H. R.; Gates, S. M.; Russell, J. N.; Yates, J. T., Jr.; Tully, J. C.; Cardillo, M. J. J . Chem. Phys. 1987,87, 6725. (42)Froitzheim, H.; Schenk, P.;Wedler, G . J .Vac. Sci. Technol. 1993, A 11, 345. (43)Meng, B.;Weinberg, W. H. J . Chem. Phys. 1994,100,5280. (44) Seebauer, E.G.; Kong, A. C. F.; Scmidt, L. D. Surf. Sci. 1988, 193,417.
heating rates are obtained by irradiation of the silica surface with a pulsed COZlaser. The resulting temperature transients can be measured45 or computed.46 For computations, the real laser pulse profile as well as the temperature dependence of the thermal parameters of silica have to be taken into account explicitly. The optical properties also play a crucial role for the calculations. We determined the complex index of refraction for our silica sample with a n FTIR reflectivity measurement and found it to be very close to tabulated values.47 The program we used to calculate surface temperature transients was a modified version of the one-dimensional finite difference code developed by Philippoz et al.;46our adaptation also computes the desorption rate with a forth-order RungeKutta method using parameters from TPD simulations.
3. Results and Discussion Figure 2 shows TPD spectra of the three compounds studied: aniline (a), phenol (b), and toluene (c). The exposures range from 0.02 to 1langmuir (1langmuir = Torrs), corresponding to coverages up to xl ML. The surface was dosed a t 160 K for aniline and phenol and a t 140 K for toluene with a constant substance pressure for 100 s. TPD experiments typically covered a range of 160500 K. Runs to higher temperatures did not reveal additional desorption features. All three compounds were found to adsorb and desorb molecularly. Aniline (Figure 2a). At low coverages the TPD peak is asymmetric and broad. Unlike a typical first-order desorption feature, there is a steep rise followedby a slowly decaying tail. The presence of this nearly exponentially decaying tail was not due to any artifact of pumping or sticking to the walls of the Feulner cap. By variation of the heating rate by a factor of 10, this was confirmed by (45) Zenobi, R.; Hahn, J. H.; Zare, R. N. Chem. Phys. Lett. 1988,125, 1. (46)Philippoz, J.-M.; Zenobi, R.; Zare, R. N. Chem. Phys. Lett. 1989, 158,225. (47)Palik, E.D.Handbook of optical constants of solids; Academic Press: New York, 1985.
Monosubstituted Benzenes on Silica the observation that the shape of the TPD curve was strictly temperature dependent but did not vary with the timescale of the experiment. With increasing coverage, the TPD maximum shifts to lower temperature, from 294 K for 0.02 langmuir to 227 K for 0.5 langmuir. For the latter exposure, a shoulder appears a t the low temperature side and develops as a new peak located a t 197 K for 1 langmuir. As this peak does not saturate a t higher coverage, it is interpreted as the formation of multilayers, in agreement with observations by Huang et al. on Pt-
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Phenol (Figure 2b). The spectra of phenol closely resemble those of aniline except that the multilayer coverage is not reached, even a t high exposures. It develops clearly for higher exposures (data not shown) and lies around 210 K, in good agreement with the observation of Nakazawa and S0m0rjai.l~Its absence a t langmuir may be due to a lower sticking a n exposure of %l coefficient of phenol. The TPD maximum shifts to lower temperatures with increasing coverages as it does for aniline, but the peaks are narrower. Toluene (Figure2c). The maximum ofthe desorption rate for toluene occurs below 200 K a t all coverages, a t significantly lower temperature than for the two other compounds. The peaks are much more symmetric and narrow. The peak maximum shifts from 197 K for 0.02 langmuir to 174 K for 0.5 langmuir exposure. At higher coverages, a multilayer peak dominates, with the monolayer peak forming a shoulder on the high temperature side. Its location a t 165 K is in excellent agreement with the value reported by Nakazawa and Somorjai.14 Assuming a sticking coeficient equal to unity during adsorption, it is possible to evaluate the surface density of the molecules a t saturation of the monolayer. The monolayer density is calculated to be ~ 1 .molecules/nm2 4 (for aniline), >2.1 molecules/nm2 (for phenol), and el molecule/nm2(for toluene), respectively, with an estimated error of *30%. This is below the closest packing density estimated using van der Waals radii12 and below the density of the surface silanol groups, too. However, as pointed out above, the sticking coefficients may be less than 1,thus leading to an overestimation of the monolayer packing density. Sticking coefficients can in principle be measured by using an absolute calibration for the QMS sensitivity. The displacement of the TPD maxima to lower temperature with rising initial coverage indicates either a repulsive interaction of the molecules on the surface or a stronger interaction with specific surface sites that are available at low concentration only. The shift as well as the tailing is most pronounced for aniline, somewhat less pronounced for pheno1,and almost absent for toluene. This behavior parallels that of the acid-base properties of the adsorbates compared with the substrate: pKa (hydrated amorphous silica) z 6.5 < PKa (phenol) % 9.9
