Adsorption and Thermal Behavior of Benzotriazole Chemisorbed on γ

Adsorption experiments for benzotriazole, (BTAH) on the γ-Al2O3 surface were carried out at 150 and 293 K. The thermal stability of the adsorbed laye...
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Langmuir 1997, 13, 6169-6175

6169

Adsorption and Thermal Behavior of Benzotriazole Chemisorbed on γ-Al2O3 Irene Popova and John T. Yates, Jr.* Department of Chemistry, Surface Science Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received June 10, 1997. In Final Form: September 5, 1997X Adsorption experiments for benzotriazole, (BTAH) on the γ-Al2O3 surface were carried out at 150 and 293 K. The thermal stability of the adsorbed layer was investigated. At low temperatures, adsorption results in the production of a thick condensed layer of BTAH. Strong intermolecular hydrogen bonds are observed. With increasing sample temperature, the BTAH molecules diffuse into the Al2O3 pore structure. Hydrogen bonding of the BTAH molecule specifically to isolated Al-OH groups is observed. Deprotonation of the BTAH molecule begins to occur through interaction, with the surface oxide anions near 300 K. As a result of such deprotonation-type interaction the production of associated surface OH groups and of the BTA- anion are observed. The BTA- anion is bound to the surface Lewis acid site via a nitrogen atom lone pair. Extensive thermal treatment (T ) 573-623 K) causes decomposition of the adsorbed BTAspecies, and modes related to the nitrogen ring disappear. After thermal treatment up to 873 K, IR spectroscopy detects weak and broadened features of aromatic molecules on the surface. Auger spectroscopy of the sample after this thermal treatment shows that nitrogen containing species have disappeared whereas carbon remains. Infrared features due to aromatic residues persist to higher temperatures (T ) 873-1023 K).

1. Introduction Benzotriazole (BTAH) belongs to a wide class of nitrogen-containing heterocyclic compounds. The BTAH molecule is shown in the last two figures of the paper for those unfamiliar with its structure. This class of molecules exhibits a wide range of chemical properties, determining the range of their biological and industrial applications.1 As corrosion inhibitors, this type of compound was first applied in the 1930s, and by the early 1960s the use of benzotriazole-type compounds as corrosion inhibitors was widely practiced. The application of benzotriazole as a corrosion inhibitor was first seriously investigated in 1967.2 Benzotriazole, benzimidazole, pyridazole, tetrazole, other nitrogen-containing heterocycles, and their derivatives have been found to be highly effective in inhibiting corrosion for copper, nickel, iron and other metals.3-7 The high effectiveness of heterocyclic molecules as corrosion inhibitors is based on their chelating action and the formation of an insoluble physical diffusion barrier on the oxidized surface of the metal, preventing metal reaction and dissolution (corrosion).8-11 Such a barrier is readily formed by nitrogen-containing heterocyclic molecules. A high degree of self-association, due to the formation of strong intermolecular hydrogen bonds and π-interaction between the aromatic rings stabilizes surface layers X

Abstract published in Advance ACS Abstracts, October 15, 1997.

(1) Katritzky, A. R. (in collaboration with Bird, C. W., et al.) Handbook of Heterocyclic Chemistry, 1st ed.; Pergamon Press: Oxford, U.K., 1985. (2) Cotton, J. B.; Scholes, I. R. Br. Corros. J. 1967, 2, 1. (3) Poling, G. W. Corros. Sci. 1970, 10, 359. (4) Dugdale, I; Cotton, J. B. Corros. Sci. 1963, 3, 69. (5) Mansfield, F.; Smith, T.; Parry, E. P. Corrosion 1971, 27, 289. (6) Brown, G. P.; Aftergut, S. J. Polym. Sci., Part A 1964, 2, 1839. (7) Oertel, M.; Klu¨sener, P.; Kempen, M.; Benninghoven, A.; Rother, H. J.; Helm, R. Appl. Surf. Sci. 1989, 37, 135. (8) Uhlig, H. H. Corrosion and Corrosion Control; John Wiley & Sons, Inc.: New York, 1963. (9) Godart, H. P.; Jepson, W. B.; Bothwell, M. R.; Kane, R. L. The Corrosion of Light Metals; John Wiley & Sons, Inc.: New York, 1967. (10) Malachevsky P. A. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Eds.; Marcell Dekker: New York, 1976. (11) Samuels, B. W.; Sotoudeh K.; Foley, R. T. Corrosion 1981, 37, 92. (12) Xu, Z.; Lau, S.; Bohn, P. W. Surf. Sci. 1993, 296, 57.

S0743-7463(97)00618-5 CCC: $14.00

produced by the nitrogen heterocyclic molecules.13 The high electron density on the ring system electrostatically hinders the interaction of the metal surface with chloride, bromide and other anions, which are the most common corrosion-inducing agents. Additional stabilization of the protective surface layer formed may be gained by electrochemical polymerization of heterocyclic molecules on the metal surface.14,15 Benzotriazole (BTAH) was found to be one of the most effective corrosion inhibitors for copper. The first serious attempts to investigate it were made by Cotton and Scholes.2 It was established that the protective action of BTAH is based on the formation of a polymeric surface layer, which was water-insoluble. The formation of the polymeric surface complex involving Cu+ or Cu2+ cations and the BTA- anion and the self-assembly of the hydrophobic organic layer were postulated to be responsible for the exceptional corrosion resistivity originating from BTAH interaction with copper surfaces. Despite the long history of BTAH use as a corrosion inhibitor for copper, only recently have the molecular details of formation of the surface layer been studied by means of SERS, XPS, FTIR, UV transmission spectroscopy, and SIMS/TPD methods.7,16,17 The work reported here involves a study of the surface chemistry of BTAH on the aluminum oxide surface, with the goal of extending BTAH corrosion inhibition to aluminum and its alloys. Aluminum surfaces are covered with an oxide film having a “duplex” structure of an inner dense and an outer permeable layer. The inhibition of corrosion on aluminum surfaces is often carried out by the use of inorganic acids with complex anions or their salts such as chromates, phosphates, polyphosphates, and silicates.18 Stable complexes between aluminum and nitrogen-containing heterocyclic molecules have been reported.19 These molecules are electron-rich nucleo(13) Bougeard, D.; Le Calve´, N.; Saint Rouch, B.; Novak, A. J. Chem. Phys. 1976, 64, 5152. (14) Hu¨sler, P.; Beck, F. J. Appl. Electrochem. 1990, 20, 596. (15) Scholl, H.; Jimenez, M. M. D. Corros. Sci. 1992, 33, 12, 1967. (16) Xu, Z.; Lau, S.; Ding, J.; Lu, P.; Dong, J. Phys. Chem. 1991, 95, 7380. (17) To¨rnkvist, C. J. Electrochem. Soc. 1989, 136, 58. (18) Mears, R.; Elderedge, G. Trans. Electrochem. Soc. 1943, 83, 403.

© 1997 American Chemical Society

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Popova and Yates

philes, able to interact with the surface through the nitrogen atom lone-pair electrons. Coordination of the basic nitrogen sites to Al3+ Lewis acid sites present on Al2O3 films on aluminum surfaces is expected to occur. The presence of labile N-H hydrogen in the molecule also makes possible the interaction with electronegative proton-accepting surface oxide anions (Lewis base sites). The alumina surface is regarded as weakly acidic, having both Lewis and Bronsted acid sites present, depending on the pretreatment temperature. Models of the alumina surface have been developed by Peri20 and later by Kno¨zinger and Ratnasamy.21 Five types of surface hydroxyl groups are reported with IR frequencies of 3800, 3780, 3744, 3733, and 3700 cm-1.20 The three lower frequencies correspond to the hydrogen-bonded or associated hydroxyl groups, and the two higher frequencies, to the isolated hydroxyl groups. Thermal treatment of the Al2O3 surface causes its dehydroxylation. Consumption of the associated hydroxyl groups upon thermal treatment (they convert into isolated ones, producing water) increases the number of surface oxide anions (Lewis base sites). The vacancies in the oxide anion lattice create the Lewis acid sites (Al3+ cations).21 The ratio of Lewis acid to Bronsted acid sites thus grows upon the thermal pretreatment. Lewis and Bronsted acid sites, as well as Lewis base sites on Al2O3 would be expected to react in different manners with the BTAH molecule. In this work, we have employed transmission FTIR spectroscopy to monitor the mechanism of adsorption and decomposition of the BTAH molecule on high surface area alumina surfaces, having different proportions of Lewis acid and Bronsted sites. 2. Experimental Section The IR cell used for these experiments, operable over a sample temperature range of 150-1500 K, was discussed in detail previously22,23 and is shown in Figure 1a,b. Two of the six cell flanges are equipped with differentially pumped double-O-ring sealed 6 mm thick KBr windows for IR measurements. The Al2O3 samples are spray-deposited onto a tungsten support grid (0.0254 mm thick with uniform 0.22 mm square-shape openings). The grid, giving approximately 70% optical transparency, is held rigidly by nickel clamps attached to an electrical power/thermocouple feedthrough at the center of the cube-shaped cell. The sample can be cooled by liquid N2 in the reentrant Dewar, which holds the feedthrough. The sample may be heated electrically by an electronic temperature controller/programmer. The temperature of the sample is measured by means of a K-type thermocouple, welded to the top-central region of the grid. The grid and sample temperature may be maintained at a fixed setpoint within (2 K by the electronic controller. The cell is connected to a stainless steel bakeable vacuum system. A base pressure of 1 × 10-8 Torr is maintained by turbomolecular (60 L/s) and ion (30 L/s) pumps. The pressure was measured by the ion-pump current (10-4-10-8 Torr pressure range) and by an MKS Baratron capacitance manometer (10-3103 Torr pressure range). The system is also equipped with a Dycor M100 quadrupole mass spectrometer for product gas analysis and leak checking. Emulsions of Al2O3 samples were prepared by ultrasonication of 1 g of Al2O3 (Degussa aluminum oxide C (101 m2/g)) which is slurried in a 100 mL water-acetone mixture (1:9 volume ratio). The slurry was then sprayed onto the grid with a N2 pressurized atomizer. Only one-third of the total area of the grid (∼1.5 cm2) (19) Garnovsky, A. D.; Ochlobystin, O. Y.; Osipov, O. Y.; Yanusov, K. M.; Kolodiazhny, Y. V.; Golubinskaya, L. M.; Svergun, V. I. J. Obsh. Chim. 1970, 42, 920 (in Russian). (20) Peri, J. B. J. Phys. Chem. 1965, 69, 220. (21) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev. 1978, 17, 31. (22) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321. (23) Wong, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 335.

Figure 1. (a) Schematic of the IR cell with the BTAH doser affixed to one of its ports. The doser is heated to 353-373 K for BTAH transport to the sample. (b) Cross section geometry of the cell with the sample. was sprayed, leaving the other two thirds (∼3.5 cm2) clean. Infrared transmission through the unsprayed part of the grid may then be used for measuring background spectra. During spraying, the grid was heated to 313-333 K to flash-evaporate the solvent mixture. Spraying was stopped when the openings of the grid were completely covered with powder. The area density of material sprayed is 7-10 mg/cm2. After spraying, the grid was transferred into the cell and heated in vacuum at 473 K for 8-12 h. Different degrees of dehydroxylation of the sample were then achieved by further thermal treatment in vacuum at 573-1073 K. The transmission spectra were recorded using a Mattson Research Series I Fourier transform single beam infrared spectrometer. The spectrometer, equipped with a liquid N2 cooled HgCdTe detector, was constantly purged with pure N2 gas. Spectra were measured with either 1000 or 200 averaged scans at 4 cm-1 spectral resolution. The absorbance spectra of the sample were then obtained by ratioing the sample and background scans. Benzotriazole C6H5N3 (BTAH, 99%), obtained from Aldrich Chemical Company, was used without further purification. Spectra of the gas phase benzotriazole (BTAH) were recorded in a small metal cell (equipped with CaF2 windows) at temperatures up to 573 K. Solid BTAH was transported into the cell under dry nitrogen conditions and pumped to 10-4 Torr on the gas line. The cell was closed for recording spectra of the gas phase. The CaF2 windows were overheated by 5-15 K compared to the cell body to prevent condensation of the compound. Spectra of the solid phase BTAH were also recorded at 293 K. BTAH was transported into the cell under dry nitrogen conditions,

Benzotriazole Chemisorbed on γ-Al2O3

Langmuir, Vol. 13, No. 23, 1997 6171 Table 1. Comparison of the IR Frequencies Observed for BTAH Adsorbed on Al2O3 at 150 K and Spectral Features of Solid BTAH at 293 Ka obsd frequency Al2O3/BTAH (T ) 150 K) (cm-1)

Figure 2. FTIR spectra of (a) gas phase BTAH (T ) 543 K), (b) solid phase BTAH (T ) 293 K), (c) BTAH condensed on Al2O3 (T ) 150 K). and the cell was pumped to 10-4 Torr. The cell was heated and cooled again. The thin film of solid BTAH crystallized on the windows (mp 98-99 °C) was studied by transmission FTIR spectroscopy. For infrared studies of BTAH adsorption on Al2O3, the solid compound was mechanically transferred into a storage reservoir under dry N2 conditions. The low vapor pressure (5 × 10-6 Torr at 300 K) of BTAH made it impossible to vapor transport the compound to the Al2O3 through a gas line path at room temperature. Instead, the reservoir with its valve were connected to one of the IR cell ports, with a copper tube doser being attached to the valve. The copper tube doser and the BTAH reservoir and valve were heated externally, using heating tape, causing heating of the entire reservoir and doser tube and allowing the wellcontrolled transfer of gaseous BTAH to the Al2O3 sample through the heated assembly. The doser components were heated to 353373 K (with a BTAH vapor pressure of 6.6 × 10-2-6.7 × 10-2 Torr in this range of temperatures).24 The grid with its Al2O3 sample was oriented 45° to both the incident IR beam and the dosing beam to allow deposition and recording of transmission FTIR spectra to be carried out without changing the sample position (Figure 1b). The thermal behavior of the adsorbed layer was probed by stepwise electrical heating of the sample in 10 K increments, followed by cooling to the reference temperature (150 or 293 K) for FTIR measurements. Auger spectroscopic studies of the thermally decomposed BTAH on the Al2O3 sample were carried out by placing the support grid with the sample into an ultrahigh vacuum chamber. Auger measurements were carried out with an electron current of 1 µA at 2 keV. Charging effects were not observed during these measurements.

3. Results A. IR Spectra of Gas and Solid Phase BTAH. The gas phase spectrum of BTAH was obtained at approximately 543 K (Figure 2a). The spectrum is nearly identical to that reported previously for gas phase BTAH and is partially rotationally resolved.25 All characteristic features are present, indicating the stability of the gas phase molecule at these temperatures.26-29 In the skeletal vibration region (“ring frequency”) (1000-1700 cm-1) characteristic ring stretching and bending modes of BTAH are observed. The high-frequency region (2000-4000 cm-1) contains only C-H and N-H stretching modes (υ(C-H) and υ(N-H), respectively). The υ(C-H) stretch(24) Jime´nez, P.; Roux, M. V.; Turrio´n, C. J. Chem. Thermodyn. 1989, 21, 759. (25) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, 1st ed.; Milwaukee, WI, 1985. (26) Morgan, K. J. J. Chem. Soc. 1961, 2343. (27) O’Sullivan, D. G. J. Chem. Soc. 1960, 3653. (28) Rubim, J.; Gutz, I. G. R.; Sala,O.; Orville-Thomas, W. T. J. Molec. Struct, 1983, 100, 571. (29) Mohan, S.; Settu, K. Ind. J. Pure Appl. Phys. 1993, 31, 850.

reported frequency solid BTAH (T ) 293 K) cm-1)

int

1006 1021 1095 1128 1145 1211

1097 1140 1146 1210

m m mw w mw s

1266

1268

m

1280 1304 1384 1398 1432 1465

1280 1310 1383 1420 1468

m m w mw ms ms

1495 1512 1596

1500 1510 1596

w ms

1625 2525 2767 2804 3039 3082 3112 3145

1622 2800 3040 3080 3095

ms w mw mw s mw m m

assignmentb benzene ipb δ(CdC) benzene ipb δ(CdC) δ(NH) (ipb) combination δ(CH) (ipb) triazole ring breathing δ(N-N-N) (s) combination of ring stretching δ(CH) (ipb) triazole ring stretching (as.) triazole ring stretching (as.) triazole ring stretching (as.) benzene ring stretching combination benzene stretching υ(CdC) benzene stretching or υ(NdN) benzene stretching υ(CdC) combination υ(NH) associated υ(CH) (stretching) υ(CH) (stretching) υ(CH) (stretching) υ(CH) (stretching)

a Intensities: w, weak; mw, medium-weak; m, medium; ms, medium-strong; s, strong. Vibrations: υ, stretching; δ, in-plane bending; γ, out-of-plane bending. b This work and ref 28.

ing feature (3072 cm-1) exhibits a lower frequency than the υ(N-H) stretching mode (3499 cm1). The spectrum of the solid phase BTAH, obtained at 293 K, is shown in Figure 2b. The υ(N-H) stretching mode in the high-frequency region is strongly reduced in frequency for the solid compared to the gas phase spectrum. This mode is transformed into a multipeaked and broadened feature due to self-association in the solid layer as a result of hydrogen bonding.30,31 The υ(N-H) stretching mode shifts significantly to lower frequencies, indicating the presence of the strong intermolecular hydrogen bonds (of N-H‚‚‚N type).12 The most intense mode (2084 cm-1) in the group of the υ(N-H) modes is assigned in the literature to the hydrogen-bonded N-Hassoc. stretching mode (υ(N-H‚‚‚N).28 The perturbation of the C-H stretching mode (transformed for solid BTAH into the group of overlapping bands centered at ∼3075 cm-1) by the intermolecular association is very much less, moving the mode to a slightly higher frequency compared to the gas phase molecule. This indicates the presence of much weaker bonds possibly of the C-H‚‚‚N type. The origin of these weak C-H‚‚‚N type modes is still a matter of discussion in the literature.26-29 The observation of the small perturbation of the C-H modes upon formation of the solid phase confirms that weak interactional effects involving C-H bonds are present. The assignment of the major vibrational modes for solid BTAH is given in the Table 1. B. BTAH/Al2O3 Adsorption Experiments at 150 K. Different stages of alumina surface dehydroxylation were (30) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman and Co.: San Fransisco, 1960. (31) Parry, E. P. J. Catal. 1963, 2, 371.

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Figure 3. FTIR spectra for the adsorption of BTAH on Al2O3 at 150 K with increasing adsorption times. Skeletal vibration (1100-1700 cm-1) and high-frequency υ(C-H) and υ(N-H) stretching (2000-3500 cm-1) regions are shown. The υ(N-H‚‚‚X) stretching mode (X ) N, O) corresponds to hydrogen bonding of BTAH molecules to themselves and to the surface (the latter makes a minor contribution at 150 K).

achieved by pretreatment in vacuum at 573, 823, and 1073 K. The spectral features observed for the surface hydroxyl groups are in agreement with the ones assigned in the literature.21,22 Associated hydroxyl groups evolve H2O, being converted into the isolated hydroxyls as the temperature is increased. The ratio of infrared signal intensity of the isolated hydroxyl groups to the associated hydroxyl groups increases with increasing pretreatment temperature. Partially dehydroxylated alumina samples (573-1073 K pretreatment) were exposed to BTAH gas at 150 K. Adsorption resulted in the deposition of a thick layer of solid BTAH on the outer surface of the Al2O3 powders (Figure 2c). The spectrum obtained strongly resembles the spectrum of solid BTAH. The similarity can be seen by comparing the spectra of Figure 2c and Figure 2b. The infrared modes in the skeletal vibration region were only weakly perturbed by BTAH condensation on Al2O3 at 150 K. Comparison and assignment of major IR frequencies for BTAH/Al2O3 and solid BTAH are given in Table 1. The development of the condensed layer at 150 K is shown in Figure 3 for increasing times of exposure to BTAH(g). Spectral features at constant frequencies develop with increasing adsorption time. The strongest mode observed in the skeletal vibration region, at 1211 cm-1 (triazole ring breathing mode), can be used to judge the thickness of the layer condensed on the surface. In the high-frequency region, the frequencies and broadened shapes of both groups of the υ(C-H) and υ(N-H) stretching modes indicate their involvement in intermolecular hydrogen bonding in the condensed layer at 150 K, as in the case of solid BTAH. The broadened υ(N-H···X) (X ) N, O) mode in the 2000-3600 cm-1 frequency range is due primarily to (N-H‚‚‚N)-type intermolecular hydrogen bonding in the condensed layer. This type of interaction is limited only to the outer surface of the Al2O3 particles, since diffusion of BTAH molecules into the pore structure of Al2O3 does not occur at 150 K. This is verified by our observation that the degree of surface dehydroxylation of the Al2O3 prior to BTAH adsorption does not change the spectral pattern dramatically for BTAH films condensed at 150 K (not shown). Ring stretching and bending modes in the skeletal vibration region (1000-1700 cm-1) are only slightly influenced by BTAH condensation on the Al2O3 surface when condensation occurs on Al2O3 with various levels of hydroxyl coverage.

Popova and Yates

Figure 4. Physical effect of the BTAH condensed at 150 K on the surface hydroxyl bands. The Al-OH stretching modes for high and low surface OH coverage are suppressed in intensity with increasing BTAH coverage (3400-4000 cm-1 region). The sample with high hydroxyl coverage has been heated in vacuum initially up to 473 K; the sample with low OH coverage has been heated in vacuum initially up to 873 K.

Figure 5. FTIR spectra for the adsorption of BTAH on Al2O3 at 293 K, with increasing adsorption times. Skeletal vibration (1100-1700 cm-1) and high-frequency υ(C-H) and υ(N-H) stretching (2000-3500 cm-1) regions are shown. As isolated Al-OH groups are converted into associated groups, an isosbestic point at ∼3644 cm-1 is observed.

A physical effect of the condensation of a heavy layer of BTAH onto the Al2O3 surface at 150 K may be seen from the behavior of the infrared bands of the Al-OH groups. Figure 4 shows the IR spectral behavior for BTAH deposition on the surface for both high and low Al-OH coverages. The major effect in both cases is an overall attenuation of the υ(Al-OH) spectral intensity without evidence (based on Al-OH frequency shifts) of chemical interaction of BTAH molecules with individual Al-OH groups. In addition, no significant variation in relative intensity of the different Al-OH groups is seen, as would be expected if there were hydrogen bonding of BTAH molecules to the Al-OH groups. This overall attenuation of the intensity of the Al-OH modes is thought to be caused by the dielectric screening of the electric vector of the infrared radiation by the condensed dielectric layer of BTAH on the outer surface of the Al2O3 powder particles. C. BTAH/Al2O3 Adsorption Experiments at 293 K. Partially dehydroxylated alumina samples were exposed to BTAH at 293 K as shown in Figure 5. Adsorption resulted in the production of thinner layers of BTAH, compared to BTAH layers condensed at 150 K (judging by the intensity of 1211 cm-1 triazole ring breathing mode at equivalent deposition times). This is due to the

Benzotriazole Chemisorbed on γ-Al2O3

Figure 6. Disappearance of N-H stretching υ(N-H) and N-H in-plane bending δ(N-H) modes as a result of thermal treatment in the temperature range 150-523 K. Spectra were recorded after sample treatment at 150, 295, 340, 368, 425, and 523 K.

reduction in the net adsorption rate of BTAH at higher surface temperatures. The development of the adsorbed layer is shown in Figure 5. Spectral features of BTAH develop with constant frequency with increasing deposition time. A dramatic effect of BTAH adsorption on the Al2O3 hydroxyl groups is observed for adsorption at 293 K. BTAH diffusion into the pore structure of Al2O3 perturbs the surface hydroxyl groups, as may be seen in the Al-OH region of the spectra in Figure 5. As the BTAH adsorption time increases, the isolated Al-OH species are quantitatively converted to associated Al-OH groups (having their absorbance at lower frequencies). An isosbestic point at ∼3644 cm-1 is observed. This effect is absent for BTAH deposition at 150 K, as may be seen in the spectra in Figure 4. The adsorption pattern shown in Figure 5 does not change dramatically as the initial surface OH coverage is changed in additional experiments (not shown). D. Thermal Behavior of Adsorbed BTAH. The thermal behavior of a thick layer of BTAH adsorbed at 150 K was studied. Heating to selected temperatures, followed by cooling to 150 K for recording spectra, resulted in the spectra shown in Figure 6. The thermal effects observed may be categorized into the characteristic temperature regions, as shown below. 1. Temperature Region 150-295 K. Stepwise heating in this region results in the desorption of excess of BTAH. The loss of the thick condensed layer of BTAH may be monitored by the overall proportional decrease of the BTAH spectral intensities at constant frequency, including the intensity decrease of the triazole ring breathing mode at 1211 cm-1 (not shown). In the high-frequency region the υ(N-H‚‚‚N) stretching mode near 2804 cm-1, corresponding to the self-associated hydrogen-bonded molecules, decreases in intensity more rapidly than other υ(N-H) stretching modes that are present in the broad band from about 2700 to 3300 cm-1. This intensity decrease may be attributed to the breaking of strong intermolecular hydrogen bonds, as the condensed layer of BTAH on the surface disappears as a result of heating. In addition, the in-plane δ(N-H) bending mode (1102 cm-1) shifts to higher frequency. 2. Temperature Region 295-573 K. When the sample containing adsorbed BTAH is heated, both υ(NH) stretching and δ(N-H) bending components of the N-H species gradually disappeared from the spectrum (Figure 6). BTAH deprotonation on the surface led to the loss of all N-H stretching and bending mode intensity (N-H bond dissociation) in this temperature region. The intensity of the υ(C-H) stretching modes increased slightly upon heating in this temperature region.

Langmuir, Vol. 13, No. 23, 1997 6173

Figure 7. Growth of spectral features assigned to BTA- with thermal treatment in the range 150-623 K. Spectra were recorded after sample treatment at 150, 273, 368, 453, 523, and 623 K.

Figure 8. Integrated intensity of Al-OH stretching modes υ(Al-OH) in (A cm-1) units (3000-4000 cm-1) after heating of the BTHH-covered sample in the temperature range 150-623 K followed by cooling. At temperatures higher than ∼273 K, the integrated intensity increases significantly due to hydrogen bonding and to the creation of new surface OH groups by BTAH deprotonation. Above ∼500 K, the integrated intensity decreases due to dehydroxylation of the surface.

Significant changes that occur in the skeletal vibration region are shown in Figure 7. In this region the new features (1448, 1483, 1575 cm-1) develop within the temperature range ∼273-453 K (Figure 7). These features are assigned to BTA- (benzotriazole anion) υ(C-C) ring stretching modes.28 The growth of the modes assigned to BTA- corresponds to the deprotonation of the BTAH molecules as they diffuse into the pore structure of Al2O3 and interact with the internal surface. A further increase of the temperature (453-623 K) did not cause any additional intensity growth for the BTA- features. During heating, the integrated intensity and band shape of the Al-OH groups changed dramatically. Extensive Al-OH infrared intensity growth and a shift to lower frequencies was observed. The spectra observed for the υ(Al-OH) stretch modes after thermal treatment corresponded to the production of an increased number of associated hydroxyl groups on the surface by BTAH deprotonation. The integrated intensity of the hydroxyl groups in the 3200-3800 cm-1 range is shown in Figure 8. The increase in the integrated area cannot be interpreted quantitatively, since associated OH groups have absorptivities

6174 Langmuir, Vol. 13, No. 23, 1997

Figure 9. Auger spectrum of the sample treated in vacuum at 873 K. Only the C(KLL) feature is detected (271 eV) in addition to Al3+ (KLL) (54 eV) and O2- (KLL) (510 eV).

higher than isolated hydroxyl groups. However, much of the Al-OH intensity increase seen in Figure 8 is due to BTAH deprotonation, which causes additional Al-OH production. Thermal treatment of the layer adsorbed at 293 K leads to essentially the same results (not shown). Partial deprotonation of the molecules takes place at the adsorption temperature. 3. Temperature Region 573-623 K. In this relatively short temperature range, significant changes are observed in the spectra. Surface dehydroxylation takes place, beginning near 500 K. In addition, the triazole ring, being the more temperature sensitive and reactive part of the molecule,1 decomposes first. 4. Temperature Region 623-1023 K. In this temperature region, IR modes characteristic of the remnants of aromatic species32,33 are still observed as weak broadened infrared features near 1300, 1450, 1500, and 1600 cm-1. Auger spectroscopic measurements of the sample after heating to 873 K were carried out and are shown in Figure 9. The sample was transferred in air to the UHV chamber, equipped with an Auger spectrometer. Within the sensitivity limits of Auger spectroscopy, only a C(KLL) feature is observed, along with Al3+ (KLL) and O(KLL) features from the Al2O3 substrate. The sample was visibly carbonized at 873 K, exhibiting a black color, which persisted to 1073 K. 4. Discussion A. Adsorption and Hydrogen-Bonding Effects. The properties of the condensed BTAH layer deposited at 150 K are determined mostly by self-association [13]. A considerable downshift (600 cm-1) of the υ(N-H) frequency, compared to the gas phase and the broadening of the υ(N-H) mode serve as strong evidence for intermolecular hydrogen bonding in the condensed layer at 150 K. The dominance of self-association over interaction with the surface in the condensed layer is indicated by the absence of hydrogen bonding for the surface Al-OH groups and also the close resemblance of the spectra obtained to the spectra of solid BTAH. The thick condensed layer of BTAH molecules causes a general decrease of the spectral intensity of the surface hydroxyl groups due to dielectric screening of the incident IR radiation. At temperatures above ∼273 K, interaction of the BTAH molecules with the surface starts to take place, as indicated (32) Bellamy, L. J. The IR Spectra of Complex Molecules, Vol. 2: Advances in Infrared Group Frequencies, 2nd ed.; Chapman and Hall Co.: London, New York, 1980. (33) Randall, H. M.; et al. The IR Determination of Organic Structures; Van Nostrand Co.: New York, 1949.

Popova and Yates

Figure 10. Schematic of possible protonation-deprotonation reaction of the BTAH molecule. Electronic charge can be uniformly distributed among the nitrogen atoms of the triazole ring in each case (resonance stabilization). On Al2O3, the deprotonation reaction is favored.

by the spectral changes. Near 273 K, surface diffusion of the BTAH molecules into the pore structure of Al2O3 begins to occur, causing extensive hydrogen-bonding effects with the isolated Al-OH groups and resulting in intensification and downshifting of isolated Al-OH bands. The observation of an isosbestic point between the isolated and the associated Al-OH groups during BTAH adsorption at 293 K indicates that a stoichiometric interconversion is taking place as BTAH molecules enter into the Al2O3 pores and interact specifically with the isolated hydroxyl groups. B. Deprotonation of BTAH. BTAH is regarded to be a weak aromatic base, having a basicity intermediate between ammonia and pyridine. Due to the presence of the mobile proton in the molecule, it can take part in acidbase interactions, being protonated (acting as a base) or becoming deprotonated (acting as an acid) (Figure 10). Both protonation and deprotonation lead to resonancestabilized ions, having charge uniformly distributed among the nitrogen atoms of the triazole ring [13,16, 28]. Our experiments indicate that BTAH interaction with the surface is governed by its acidic properties, as shown in the left-hand reaction scheme in Figure 10. As a result of interaction with the surface oxide anions, deprotonation of BTAH molecules on the Al2O3 surface takes place in the 273-573 K temperature range and surface hydroxyl groups are formed by interaction of BTAH derived protons with O2- Lewis base sites in Al2O3. The deprotonation of BTAH molecules is postulated on the basis of the following observations: (a) All υ(N-H) stretching and δ(N-H) bending modes in the BTAH molecule disappear from the spectra. (b) New υ(C-C) stretching modes ascribed to BTA- species16,28 are observed as a result of the deprotonation reaction. (c) New hydroxyl groups in the low-frequency υ(Al-OH) range are formed from the BTAH deprotonation interaction with the surface oxide anions. These groups are associated, as indicated by their low-frequency, compared to isolated Al-OH groups. The BTA- anion formed as a result of the deprotonation reaction exhibits stronger basic properties, compared to the initial molecule and can interact with Lewis acid sites (Al3+ ions). The BTA- anion is postulated to bond to the Lewis acid sites through the lone electron pairs of the nitrogen atoms in the triazole ring. Polymeric layer formation is not observed in this case, as the IR modes of the triazole ring are not significantly influenced by adsorption. C. Thermal Behavior at Elevated Temperatures. At the higher temperatures (above ∼573 K) thermal decomposition of the surface species takes place, as indicated by the disappearance of triazole ring related vibrational modes. Ring vibrational modes having partial C-N character also decrease in intensity. The preferential destruction of the triazole ring is in accord with its higher reactivity and decreased thermal stability, compared to

Benzotriazole Chemisorbed on γ-Al2O3

Langmuir, Vol. 13, No. 23, 1997 6175

species on the surface. Heating to 873 K causes carbonization of the surface, as indicated by the black color of the samples, and the total decomposition of the adsorption products. Auger spectroscopy verified that only carbon remained on the surface.

Figure 11. Proposed schematic of BTAH interaction with the Al2O3 surface. Interaction of the molecule with the surface oxide anion leads to BTAH deprotonation and to the formation of a new hydroxyl group and bonding of the BTA- created to Al3+ (Lewis Acid site) through one of the nitrogen atoms’ lone pair electrons.

the aromatic ring. Modes, involving C-C and C-H modes are retained in the spectra, indicating the stability of the aromatic fragment of the molecule. Above 873 K, the final thermal decomposition of the surface species occurs. The thermal product exhibits IR modes characteristic of the remnants of the aromatic

5. Summary The interaction of benzotriazole (BTAH) with γ-Al2O3 was investigated by means of transmission FTIR spectroscopy and Auger spectroscopy. The following observations are made. (1) Adsorption of BTAH on Al2O3 at 150 K leads to the formation of a thick condensed layer with strong intermolecular hydrogen bonds being observed, as in solid BTAH. (2) BTAH molecules interact via hydrogen bonding with isolated Al-OH groups at ∼273 K, where BTAH diffusive mobility throughout the Al2O3 pore structure occurs. (3) At temperatures above ∼273 K, deprotonation of BTAH occurs, on Lewis base sites (O2-) producing additional Al-OH groups, as well as BTA- anions, both of which are spectroscopically detected. (4) The BTA- anions are probably adsorbed on Lewis acid sites (Al3+) through the triazole ring system. (5) Above ∼573 K, thermal decomposition of adsorbed BTA- species occurs through fragmentation of the triazole ring. A residue yielding infrared spectra characteristic of an aromatic ring system is retained. (6) At 873 K, only carbon remains on the surface. The sequential steps in the adsorption and decomposition of BTAH on γ-Al2O3 are summarized in Figure 11. Acknowledgment. We acknowledge, with thanks, the support of this work by the Air Force Office of Scientific Research. LA970618V