FTIR Study of the Interaction of Ethyl Iodide with Different Oxides and

Langmuir 2002, 18, 8829-8835

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FTIR Study of the Interaction of Ethyl Iodide with Different Oxides and Rh/SiO2 Catalysts L. O Ä va´ri and F. Solymosi*,† Institute of Solid State and Radiochemistry, University of Szeged, Reaction Kinetics Research Group of the Hungarian Academy of Sciences, P.O. Box 168, H-6701 Szeged, Hungary Received April 15, 2002. In Final Form: July 31, 2002 The adsorption and surface reactions of C2H5I on SiO2, Al2O3, MgO, TiO2, and Rh/SiO2 were monitored with Fourier transform infrared spectroscopy. Molecular adsorption was found on all the oxides examined at 206 K. Only reversible adsorption was observed on SiO2, while on the other oxides an ethoxide group was detected from ∼263 K. Surface OH groups and possibly Lewis acidic and/or basic centers are involved in the process. In the case of alumina, one part of C2H5O(a) was formed on a special configuration of active sites (ensemble effect). Spectroscopic identification of C2H5O(a) was confirmed by ethanol adsorption. Ethoxide formation was rather limited on MgO probably due to the lack of strong Lewis acid sites. Dissociative adsorption of C2H5I occurred on Rh/SiO2 at 206 K. Ethylidyne (CCH3(a)) was formed, very probably through an ethyl intermediate. CCH3(a) was stable up to 373 K. In addition, the formation of ethoxide species bonded to the SiO2 support was observed, which was explained with a spillover process of ethyl species from the Rh onto silica.

1. Introduction Contamination of ground water and soil by various alkyl halides is a major environmental concern. In recent years, there have been numerous studies motivated by the need for finding efficient means for the destruction of these compounds. Semiconducting oxides and sulfides proved to be efficient photocatalysts for this process.1 One of the most suitable catalysts, among them, is TiO2 because it is cheap, resistant, and biologically inactive. Wu et al.2 have recently examined the thermal decomposition and photooxidation of C2H5I on TiO2 and observed the formation of ethoxide groups below 473 K. In the absence of oxygen, ethylene was the only gas-phase product. However, in the presence of oxygen H2O, CO, and CO2 were also generated. The adsorption and reaction of CH3I on powdered TiO2 and TiO2(110) single crystals has been the subject of several studies, which are well documented in the paper of Su et al.3 In our laboratory, we found that Cr2O3-doped SnO2 is an active catalyst for the oxidative and nonoxidative decomposition of CH3Cl.4 Supported Pt metals were also applied to catalyze the destruction of these compounds.5,6 McGee et al.6 studied the decomposition of ethyl chloride on supported Pt catalysts. They detected two reaction pathways: the first one involves R, β-elimination of HCl to give adsorbed ethylene. The second one gives adsorbed ethyl groups and chlorine atoms. Another motivation of the studies related to alkyl halide adsorption is its frequent use in generating alkyl frag* To whom correspondence should be addressed. Fax: + 36 62 424 997. E-mail: [email protected]. † This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged. (1) Hoffman, M. R.; Martin, T. S.; Choi, W.; Bademann, D. W. Chem. Rev. 1995, 95, 69. (2) Wu, W. C.; Liao, L. F.; Shiu, J. S.; Lin, J. L. Phys. Chem. Chem. Phys. 2000, 2, 4441. (3) Su, C.; Yeh, J.-C.; Chen, C.-C.; Lin, J.-C.; Jin, J.-L. J. Catal. 2000, 194, 45. (4) Solymosi, F.; Rasko´, J.; Papp, E.; Oszko´, A.; Bańsa´gi, T. Appl. Catal. 1995, 131, 55. (5) Muller, H.; Deller, K.; Despreyroux, B.; Peldszuc, E.; Kammerhofer, P.; Kuhn, W.; Spielmannleither, R.; Stoger, M. Catal. Today 1993, 17, 383. (6) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal. 1995, 157, 730.

ments with the thermal- or photo-induced dissociation of the carbon-halogen bond. These studies are mainly concerned with single-crystal surfaces,7-11 but some works were also published on oxide-supported metal catalysts. Grassian et al. investigated the adsorption of methyl halides12,13 and ethyl halides5,14 on supported Pt and Cu catalysts, while our group examined the adsorption of methyl halides15 and methylene halides16 on Pd/SiO2. The interaction of ethyl halides with supported Rh catalysts, to our knowledge, has not been investigated yet. The subject of the present work is to examine the adsorption of ethyl iodide on several oxides used as support materials and to see how the chemistry of this compound is changed when Rh is deposited on the SiO2 surface. 2. Experimental Section The following oxides were used: Al2O3 (Degussa P110Cl) 100 m2/g, SiO2 (Cab-O-Sil) 200 m2/g, MgO (DAB6) 170 m2/g, and TiO2 (Degussa P25) 150 m2/g. Rh/SiO2 samples were prepared by incipient wetting of silica with an aqueous solution of rhodium chloride. For preparation, triply distilled water was used. The Rh content was 2 and 10 wt %. After impregnation, the samples were dried in air at 373 K. For IR studies, self-supporting wafers (30 × 10 mm, 10 mg/cm2) were used. The pretreatment of samples was performed in a vacuum IR cell: the samples were (a) heated (20 K/min) to 573 K under continuous evacuation, (b) oxidized with 100 Torr (13.3 kPa) of O2 for 30 min at 573 K, (c) evacuated for 15 min at 573 K, (d) heated to 773 K (20 K/min), and (e) reduced in 100 Torr of H2 for 60 min at 773 K. This temperature is sufficient to achieve a complete reduction of Rh.8 This was followed by degassing at the same temperature for 30 min and by cooling the sample to the temperature of the experiment. The dispersion of the reduced Rh samples has been determined by (7) Berko´, A.; Solymosi, F. J. Phys. Chem. 1989, 93, 12. (8) Solymosi, F.; Berko´, A.; Re´ve´sz, K. Surf. Sci. 1990, 240, 50. (9) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (10) Bent, B. E. Chem. Rev. 1996, 96, 1361. (11) Solymosi, F. Catalytic Activation and Functiionalisation of Light Alkanes; Kluwer Academic: Dordrecht, The Netherlands, 1998; p 369. (12) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal. 1996, 159, 69. (13) Driessen, M. D.; Grassian, V. H. J. Catal. 1996, 161, 810. (14) Driessen, M. D.; Grassian, V. H. Langmuir 1998, 14, 1411. (15) Rasko´, J.; Bontovics, J.; Solymosi, F. J. Catal. 1993, 143, 138. (16) Solymosi, F.; Rasko´, J. J. Catal. 1995, 155, 74.

10.1021/la0203638 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/17/2002

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Langmuir, Vol. 18, No. 23, 2002

Figure 1. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.1 Torr) on SiO2 at 206 K (a), 225 K (b), 253 K (c), and 293 K (d). Adsorption and evacuation of C2H5I (2 Torr) at 300 K (e). H2 adsorption. We obtained the following values: 18.0% for 2% Rh/SiO2 and 4.5% for 10% Rh/SiO2. C2H5I (Merck) was purified by fractional distillation and stored in a glass bulb. It was protected against light. Infrared spectra were recorded with a Biorad (Digilab. Div.) Fourier transform IR spectrometer (FTS 155) with a wavenumber accuracy of (4 cm-1. Typically 128 scans were collected. All subtractions of the spectra were taken without the use of a scaling factor (f ) 1.0).

3. Results 3.1. Adsorption of Ethyl Iodide on Oxides. The interaction between ethyl iodide (0.1 Torr) and different oxides (SiO2, Al2O3, MgO, and TiO2) was first investigated at low temperatures (206-293 K). The IR spectrum was recorded after the 5 min adsorption of ethyl iodide and evacuation at the given temperature. This was followed by heating the sample under continuous evacuation up to the subsequent temperature. In the high-temperature measurements, after the adsorption of 1 Torr (2 Torr on SiO2) of ethyl iodide and evacuation for 15 min at 300 K, the sample was heated progressively to 673 K during continuous evacuation. After the sample was kept at the appropriate temperature for 15 min, the sample was cooled to room temperature and the spectrum was recorded at 300 K. SiO2. Infrared spectra of C2H5I adsorbed on silica at 206 K show absorption bands at 3023, 2985, 2970, 2927, 2871, 1457, 1445, and 1381 cm-1 (Figure 1). Heating the adsorbed layer to different temperatures under continuous evacuation caused the attenuation of these spectral features and their disappearance at about 253 K. The adsorption of 2 Torr of C2H5I at 300 K produced no IR bands stable after evacuation at room temperature. Al2O3. Adsorption of C2H5I on Al2O3 (which was in a γ form) at 206 K led to similar spectral features as observed on SiO2 (Figure 2). As Al2O3 is transparent to IR light down to 1000 cm-1, we could detect the intense 1213 cm-1 band which was not seen on silica. When the temperature was raised, the intensity of these bands attenuated; the bands at 2927, 2870, and 1444 cm-1 gradually shifted to higher wavenumbers and remained detectable even after

O Ä va´ ri and Solymosi

Figure 2. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.1 Torr) on Al2O3 at 206 K (a), 253 K (b), and 293 K (c). Adsorption of C2H5I (1 Torr) at 300 K and evacuation at 300 K (d), 373 K (e), 473 K (f), and 523 K (g).

evacuation at 293 K. In the CH deformation region, new spectral features appeared from 253 K with increasing intensity at 1395 and 1092 cm-1. In the OH stretching region, the freshly pretreated alumina sample exhibited absorption bands at 3792, 3776, 3728, and 3678 cm-1. Adsorption of ethyl iodide at 206 K resulted in the development of negative features in the difference spectra at 3797, 3733, and 3689 cm-1. At the same time, a broad band appeared at 3565 cm-1. All the bands attenuated in the OH stretching region of the difference spectra at 253-293 K, but weak negative features at 3800, 3757, 3696, 3673, and 3624 cm-1 were still present. After adsorption and evacuation at 300 K, bands were observed at 2983, 2938, 2883, 1473, 1452, 1395, 1213, 1195, 1167, 1153, 1094, and 1079 cm-1. When the degassing temperature was increased, these bands attenuated with the exception of that at 1079 cm-1. At 473 K, a new band appeared at 1108 cm-1. All these bands attenuated or almost disappeared at 523 K. MgO. In the low-temperature adsorption measurements, spectral features observed on magnesia were similar to those on alumina (Figure 3). From 263 K, new bands developed at 1393 and 1095 cm-1, in this case too, while in the CH stretching region the 2972 cm-1 band gradually shifted to higher wavenumbers. A diffuse feature was observed from 2950 to 2850 cm-1 with maxima at ∼2935, ∼2901, and ∼2881 cm-1 at 293 K. In the OH stretching region, a sharp band at 3751 cm-1 and a broader one at 3562 cm-1 were detected in the spectrum before C2H5I adsorption. A negative feature at 3754 cm-1 and broad bands at 3713 and 3579 cm-1 appeared in the difference spectra upon adsorption of ethyl iodide at 206 K. At higher temperatures, the intensities of the 3713 and 3754 cm-1 bands were weaker, but some negative bands remained even at 293 K. When a larger amount (1 Torr) of C2H5I was contacted with the sample at 300 K, the new bands at 1391-1393 and 1094-1095 cm-1 observed at 263 K were more intense and the other bands in the CH stretching and deformation regions appeared at somewhat different positions compared to those observed at 206 K. Degassing the sample at 373 K resulted in significant attenuation of all the bands occurring in the CH stretching and deformation regions.

Interaction of Ethyl Iodine with Different Oxides

Langmuir, Vol. 18, No. 23, 2002 8831

Figure 3. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.1 Torr) on MgO at 206 K (a), 263 K (b), and 293 K (c). Adsorption of C2H5I (1 Torr) at 300 K and evacuation at 300 K (d), 373 K (e), 473 K (f), 573 K (g), and 673 K (h). Figure 5. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.1 Torr) on 2% Rh/SiO2 at 206 K (a) and after heating to 263 K (b) and 293 K (c). Adsorption of C2H5I (1 Torr) at 300 K and evacuation at 300 K (d), 373 K (e), 473 K (f), 573 K (g), and 673 K (h).

Figure 4. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.1 Torr) on TiO2 at 206 K (a), 263 K (b), and 293 K (c). Adsorption of C2H5I (1 Torr) at 300 K and evacuation at 300 K (d), 373 K (e), 473 K (f), 573 K (g), and 673 K (h).

From 373 to 473 K, new bands at 2960, 2855, 1282, 1118, and 1047 cm-1 appeared and were stable up to 673 K. After adsorption of 1 Torr of C2H5I and subsequent evacuation at 300 K, negative features were observed at 3754 and 3741 cm-1 with a broad band at 3536 cm-1. At higher temperatures, the 3536 cm-1 band attenuated and new bands could be seen at 3712, 3595, 3497, and 3414 cm-1. At 673 K, only negative bands were observed at 3765, 3752, and 3742 cm-1 indicating the net consumption of OH groups in the overall process. TiO2. Spectra obtained after adsorption and evacuation of 0.1 Torr of C2H5I on TiO2 at 206 K agreed well with those registered for Al2O3 and MgO (Figure 4). Annealing the adsorbed layer at 206-293 K caused practically the same spectral changes. At 293 K, the remaining bands were at 2981, 2940, 2930, 2879, 1444, 1394, 1385, 1212, 1090, and 1026 cm-1. Adsorption of 1 Torr of C2H5I produced absorption bands at 1392, 1090, and 1026 cm-1. In addition, a new band, not observed before, developed at 1472 cm-1 (and very small ones at 1148 and 1074 cm-1). The bands in the CH stretching regions appeared at higher positions than those observed at 206 K.

Evacuation at 373 K resulted in the disappearance of the 1213 cm-1 band and the appearance of a new band at 1118 cm-1 and an increase of the 1026 cm-1 band. Evacuation at higher temperatures caused the increase of the 1118 and 1074 cm-1 bands and the decrease of the 1023 cm-1 band together with the diminution of the bands in the CH stretching and deformation regions. All these spectral features disappeared at 673 K. In the OH stretching region, a sharp band at 3718 cm-1 and a broader one at 3667 cm-1 were found on the freshly pretreated sample. Adsorption of ethyl iodide at 206 K resulted in the appearance of negative bands in the difference spectra at 3717, 3688, 3662, and 3645 cm-1 and positive bands at 3518 and ∼3450 cm-1. These OH stretching bands were attenuated at higher temperatures up to 373 K. At 473 K, however, new OH groups were formed indicated by the positive OH bands at 3666 and 3642 cm-1. At 673 K, only negative bands were observed in the OH stretching regions indicating the net consumption of OH groups in the overall process. 3.2. Adsorption of Ethyl Iodide on Rh/SiO2. As no signs of the chemisorption of ethyl iodide were observed on SiO2 at and above 300 K, this oxide seemed to be the most convenient support to examine the surface forms produced in the adsorption of C2H5I on Rh crystallites. Figure 5 shows the Fourier transform infrared (FTIR) spectrum of Rh/SiO2 following adsorption and subsequent evacuation of C2H5I (0.1 Torr) at 206 K. Absorption bands appeared at 3025, 2982, 2925, 2870, 1457, 1444, 1379, and 1334 cm-1. These bands, with the exception of the 1334 cm-1 feature, were also registered on the Rh-free sample (Figure 1). Although all these bands gradually decreased in intensity with heating, they remained detectable even at 293 K. This behavior is in contrast to that observed for pure silica. When the Rh/SiO2 was exposed to C2H5I (1 Torr) at 300 K and degassed for 15 min at the same temperature, vibration bands at 2983, 2938, 2881, 2795, 1401, and 1338 cm-1 were identified. Interestingly, the most intense one was the 1338 cm-1 band. An attenuation of these bands started above 373 K

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Figure 6. FTIR difference spectra following the adsorption and evacuation of C2H5I (0.02 Torr) on 10% Rh/SiO2 at 206 K (a). Adsorption of C2H5I (0.1 Torr) at 206 K and subsequent evacuation at 206 K (b), 225 K (c), 253 K (d), and 293 K (e).

and disappeared at 473 K. An exception was the band at 2983 cm-1 which was detectable even at 673 K. To increase the surface concentration of species bonded to the metal, the content of the Rh has been increased to 10%. Spectra are displayed in Figure 6. In this case, a new band appeared at 2962 cm-1, when a small amount of C2H5I (0.02 Torr) was adsorbed on the sample and subsequently evacuated at 206 K. After adsorption of 0.1 Torr of C2H5I and evacuation at 206 K, this band was probably masked by the intense 2980 cm-1 band and caused its asymmetry. A strong shoulder appeared at 2879 cm-1. The 2961 cm-1 was, however, clearly detectable after evacuation at 225 K. When the evacuation temperature was increased to 253 K, the band slightly shifted to 2968 cm-1. At that temperature, the 2871 cm-1 band disappeared, and the previous shoulder at 2879 cm-1 became dominant. The 2968 cm-1 spectral feature was clearly identified even at 293 K, when we obtained a broad absorption between 2920 and 2950 cm-1. Another difference between the spectra for samples containing 2 and 10% Rh is an enhancement of the 1336 cm-1 band, which was stable between 206 and 293 K; moreover, its intensity slightly increased at 293 K. 4. Discussion 4.1. Adsorption of Ethyl Iodide on Oxides. SiO2. For the interpretation of the results, we collected the characteristic absorption bands for gaseous and molecularly adsorbed C2H5I in Table 1. Adsorption of ethyl iodide at 206 K produced IR bands very close to the bands of gas-phase ethyl iodide,17 clearly indicating molecular adsorption at 206 K (Figure 1 and Table 1). Heating the sample to higher temperatures caused the attenuation of these bands and their final disappearance at 253 K. No sign of the transformation of C2H5I was found even after 300 K adsorption of a larger amount (2 Torr) of C2H5I. The probable reason for the low reactivity of silica towards ethyl iodide is the lack of active sites such as acidic or basic centers. (17) Durig, J. R.; Thompson, J. W.; Thyagesan, V. W.; Witt, J. D. J. Mol. Struct. 1975, 24, 41.


Al2O3. In order to understand the surface processes on alumina, it is useful to summarize briefly what active centers should be considered on its surface. Al2O3 has a defect spinel structure. Al3+ ions can be in either the tetrahedral or octahedral holes of the oxygen lattice. Various aluminas, as classified by the ratio of octahedral and tetrahedral Al3+ coordinated ions and the oxygen lattice packing density, exist. Kno¨zinger and Ratnasamy22 reviewed previous results and proposed a model of active sites on alumina. For energetic reasons, its surface is terminated by anions that are preferably hydroxyl groups, according to Pauling’s electrostatic valence rule23 to neutralize the surface charge. Five types of isolated OH groups were observed by IR spectroscopy24 that were assigned by Kno¨zinger and Ratnasamy22 according to the number and coordination of the neighboring Al3+ ions. The OH configuration of type III is the most acidic, while those of type I are the most basic. Pyridine adsorption studies25 showed that alumina is a weak Brønsted acid unable to protonate pyridine while the protonation of ammonia was observed on γ-Al2O3.26 Evacuation at high temperature results in progressive dehydroxylation of the surface and in the formation of Lewis acidic and basic sites (cus Al3+ and O2- sites). Dehydroxylation at T g 673 K leads to the formation of even stronger active sites characterized by a change in the electron densities of Al and O ions and supposed to be an ensemble of sites.22,27 Adsorption of different compounds on these special sites caused preferential perturbation of type Ia OH groups.22 As our pretreatment temperature reaches 773 K, these special sites are probably formed also under our conditions. Ethyl iodide adsorbs molecularly at 206 K, as suggested by the IR spectrum characteristic for gas-phase ethyl iodide (Figure 2 and Table 1). C2H5I is bonded to the surface, at least partly, through hydrogen bonds, as shown by the negative bands in the OH stretching region and the appearance of the 3565 cm-1 band. We assume that another part of C2H5I is bonded to coordinatively unsaturated aluminum and oxygen sites. This could not be confirmed experimentally as the oxides are not transparent in the low-frequency region. This prevented us from detecting vibrations of ν(C-I) and ν(M-I) modes at 511 and 258 cm-1, respectively, well below the cutoff of the oxides we used. The amount of C2H5I adsorbed gradually decreased as temperature was raised. At the same time, from 253 K, ethyl iodide remaining on the surface is converted to a new surface compound which gave stable bands at 1395 and 1092 cm-1. We attribute these new bands to the vibrations δs(CH3) and νa(CCO) of ethoxide species. With the increase of the temperature, these bands became stronger, corresponding to the enhanced extent of the surface process. It is very likely that the gradual increase in the amount of the new form with respect to molecularly bonded ethyl iodide causes an upward shift of the bands of CH stretchings. The spectral feature arising after adsorption and evacuation of C2H5I at 300 K (spectrum (18) Zaera, F.; Hoffmann, H.; Griffiths, P. R. Vacuum 1990, 41, 735. (19) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (20) Bol, C. W. J.; Friend, C. M. J. Phys. Chem. 1995, 99, 11930. (21) Solymosi, F.; Bugyi, L.; Oszko´, A. Langmuir 1996, 12, 4145. (22) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev.sSci. Eng. 1978, 17, 31 and references therein. (23) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960; Chapter 13. (24) Peri, J. B. J. Phys. Chem. 1965, 69, 211. (25) Parry, E. P. J. Catal. 1963, 2, 371. (26) Gordymove, T. A.; Davydov, A. A. Zh. Prikl. Spektrosk. 1983, 39, 621. (27) Datta, A. J. Phys. Chem. 1989, 93, 7053.


Langmuir, Vol. 18, No. 23, 2002 8833

Table 1. Characteristic Vibrational Frequencies of Gaseous and Adsorbed C2H5I

a

assignmenta

C2H5I(g)a,b

SiO2c

νa(CH2) νa(CH3) νa(CH3) νs(CH2) νs(CH3) 2δa(CH3) δa(CH3) δa(CH3) δs(CH3) ν(CC) + δ (C-C-I) ω(CH2) γ(CH2) ν(CC) F(CH2) ν(C-I) δ(C-C-I)

3025 2989 2986 2973 (R) 2932 2877 1462 1447 1382 1215 1207 1201 954 741 511 258 (R)

3023 2985

Pt(111)e

Rh(111)f monolayer on p(2×1)-18O

Rh(111)g

2950

2950

1445

1435

1205

1220

975 740

1000 760 516 260

2969

2970 2927 2871 1457 1445 1381

1211 1203

2914 2864 1454 1438 1377 1212 1203

951

952

1454 1440

260

Reference 17. R ) Raman active. Present work. b

Pt(111)d

c

d

e

f

g

Reference 18. Reference 19. Reference 20. Reference 21.

Table 2. Characteristic Vibrational Frequencies and Their Assignments for Adsorbed Ethoxide, OCH2CH3 assignmenta Al2O3b (C2H5I, 473 K) TiO2b (C2H5I, 473 K) MgOb (C2H5I, 473 K) Al2O3c (C2H5OH) TiO2b (C2H5OH) MgOd (C2H5OH) νa(CH3) νs(CH3) νs(CH2) δa(CH3) δa(CH3) δs(CH3) δ(CH2) γ(CH2) F(CH3) νa(CCO) νa(CCO) νa(CCO) a

2982 (s) 2938 (m) 2912 (w) 2883 (w) 1472 (w) 1452 (m) 1395 (s) 1289 (vw) 1171 (w) 1108 (s) 1078 (s)

2982 (s) 2941 (m)

2960 (m)

2891 (m) 1475 (w) 1449 (m) 1388 (s) 1360 (w)

2904 (vw) 2855 (m)

2970 (s) 2930 (m) 2900 (m) 2870 (m)

1450 (w) 1388 (w)

1450 (m) 1390 (s)

2973 (s) 2934 (m)

2967

2877 (m) 1467 (w) 1446 (w) 1382 (m) 1358 (w)

2834 1437 1383

1282 (m) 1118 (s) 1074 (s) 1023 (s)

Reference 5. b Present work. c Reference 28.

d

1118 (s) 1047 (m)

1170 (m) 1115 (s) 1070 (s)

1143 (sh) 1120 (s) 1074 (s) 1042 (s)

∼1115 1063

Reference 29.

d in Figure 2) is originated mainly from this surface species as the strongest band of molecularly adsorbed ethyl iodide (1213 cm-1) is very weak. Heating the sample at temperatures higher than 300 K caused the complete disappearance of absorption bands of C2H5I, and the absorption bands of ethoxide became dominant (Table 2). Other bands at 1108 and 1079 cm-1 belonging to νa(CCO) also developed slowly, very likely due to different coordination or environment. Analogously, McGee et al.6 observed ethoxide formation from ethyl chloride on alumina, while Beebe et al.30 reported methoxide formation from methyl chloride. Considering the active sites of Al2O3, we may assume that Lewis acidbase sites and OH groups are involved in the ethoxide formation. Note that OH consumption was detected in the spectra. Reaction with 3776 cm-1 type Ia OH groups is preferred because it was the largest negative band in the difference spectra at 523 K, though it was the smallest one for the freshly pretreated sample. This suggests that the above-mentioned special sites are also involved in the surface process. Ethoxide groups are stable up to 473 K and almost completely decomposed at 523 K. At that temperature, however, besides the negative bands in the OH stretching region, a broad positive band was found at ∼3630 cm-1 which is probably originated from the dehydrogenation of ethoxide groups. The formation of ethoxide species from C2H5I can be described by the following equations:

C2H5I(a) + O(s) ) C2H5O(a) + I(a)

(1)

(28) Golay, S.; Doepper, R.; Renken, A. Appl. Catal., A 1998, 172, 97.

or

C2H5I(a) + OH(s) ) C2H5O(a) + HI(a)

(2)

The O(s) and I(a) mean surface oxygen of Al2O3 and adsorbed iodine, respectively. We could also assume the reaction

C2H5I(a) ) C2H5(a) + I(a)

(3)

and the bonding of C2H5 to the uncoordinated Al sites. The attachment of the alkyl species to the cation of the support is not totally impossible as adsorbed CH3 has been detected over highly dehydroxylated silica in the stream of CH3 radicals produced by pyrolysis of azomethane.31 To facilitate the assignment, in Table 3 we collected the characteristic vibration of adsorbed ethyl species produced by the dissociation of various ethyl-containing compounds. We have no spectroscopic evidence of that kind of species in the present case. TiO2. Titania is similar to alumina in the sense that it also contains Lewis acidic and basic centers. Two types of Lewis acidic centers were found by ammonia and pyridine adsorption studies both on rutile and on anatase samples.37 On the other hand, no signs of Brønsted acidity (29) Spitz, R. N.; Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A. W. J. Phys. Chem. 1986, 90, 4067. (30) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 1296. (31) Rasko´, J.; Solymosi, F. Catal. Lett. 1997, 46, 153. (32) Lloyd, K. G.; Roop, B.; Campion, A.; White, J. M. Surf. Sci. 1989, 214, 227. (33) Kliveńyi, G.; Kovaćs, I.; Solymosi, F. Surf. Sci. 1999, 442, 115.

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Table 3. Characteristic Vibrational Frequencies and Their Assignments for Adsorbed Ethyl and Ethylidyne Groups C2H5 assignment νa(CH3) νa(CH2) νs(CH3) νs(CH2) 2δ(CH3) 2δa(CH3) δ(CH2) δa(CH3) δs(CH3) ω(CH2) ν(CC) F(CH3) a

CCH3

Pt(111)a

Rh(111)b

Rh(111)c

Rh(111)d

Cu(100)e

Rh(111)f

(C2H5Cl)

(C2H5I)

((C2H5)2Zn )

(C2H5I)

(C2H5Br)

(C2H4)

Rh/Al2O3g (C2H4)

2910 (s)

2900 (s)

2945 (s)

2900 (s) 2730 (sh)

2920 (w)

2939 (w)

2880 (m)

2885 (m)

2918 (s)

2797 (w) 1450 (sh) 1430 (s) 1376 (w) 1173 (m) 1022 (s) 941 (s)

1420 (s)

1430 (s)

1425 (s)

1420 (m)

1150 (m) 940 (w) 850 (w)

1140 (w)

1180 (m) 935 (m)

1140 (m)

Reference 32. b Reference 21. c Reference 33.

860 (m) d

1420 (w) 1337 (s)

1408 (w) 1342 (s)

1121 (m)

1110 (s)

855 (s)

Reference 20. e Reference 34. f Reference 35. g Reference 36.

were detected. High-temperature reduction of titania results in the formation of oxygen vacancies and in the reduction of Ti4+ to Ti3+. As no reductive treatment was performed in the present case, the shifts of the bands in the CH stretching region clearly indicate the occurrence of the surface reaction and the formation of adsorbed ethoxide. Surface processes during ethyl iodide adsorption on titania seem similar to those on alumina. Spectral features characteristic for the ethoxide were less evident in the low-temperature measurement, but its absorption bands were more pronounced when a larger amount (1 Torr) of ethyl iodide was adsorbed on the sample at 300 K. The strong bands at 1392, 1090, and 1026 cm-1 and shifts of the bands in the CH stretching region clearly indicate the occurrence of the surface reaction and the formation of adsorbed ethoxide. This assignment is corroborated by our control measurements of ethanol adsorption on TiO2 (see data in Table 2). Ethoxide groups of different coordination may be formed leading to the appearance of more than one νas(CCO) mode. The formation of new OH groups at 473 K (giving bands at 3666 and 3642 cm-1) is an indication that a dehydrogenation process also commenced at 473 K. Only negative bands were found at 673 K in the OH stretching region, indicating the net consumption of the OH groups in the overall process. MgO. The basic character of magnesia is well-known. The electron-rich oxygen ions on its surface act as strongly basic, electron-donating sites, while the electron-deficient magnesium cations act as weak electron-accepting sites. On our freshly pretreated MgO samples, a sharp band was found at 3751 cm-1 and a broad band at 3562 cm-1 in the OH stretching region, which were assigned to OH groups on the edges and corners and to OH groups located on the terraces, respectively.38,39 Following the adsorption of ethyl iodide on MgO at 206 K, we obtained the characteristic bands for intact C2H5I. Spectral changes occurred even at 263 K, when weak absorption bands at 1393 and 1095 cm-1 attributable to ethoxide developed. In this case, the concentration of molecularly adsorbed C2H5I is still high as indicated by (34) Lin, J.-L.; Chiang, Ch.-M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 147, 250. (35) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (36) Beebe, T. P., Jr.; Yates, J. T., Jr. J. Phys. Chem. 1987, 91, 254. (37) Kno¨zinger, H. Adv. Catal. 1976, 25, 184 and references therein. (38) Coluccia, S.; Marchese, L.; Lavagnino, S.; Anpo, M. Spectrochim. Acta 1987, 43A, 1573. (39) Coluccia, S.; Lavagnino, S.; Marchese, L. Mater. Chem. Phys. 1988, 18, 445.

its strong absorption bands. Ethoxide will not become a predominant surface form, as it coexists with ethyl iodide even at 300 K. The concentration of both compounds is very low above this temperature. The extension of ethoxide formation on MgO seems to be smaller than that on Al2O3 or TiO2. The lack of Lewis acidic sites that were strong enough was assumed to be the reason for the low tendency for ethoxide formation from ethanol on MgO.40 4.2. Adsorption of Ethyl Iodide on Rh/SiO2. Several works have been published on the adsorption of C2H5I on metal single-crystal surfaces including Rh(111).20,21 A particularly great effort was made to establish the dissociation of the adsorbed molecule. For this purpose, the most convenient method was found to be X-ray photoelectron spectroscopy. The binding energy of I(3d5/2) was 0.1-0.2 eV higher for adsorbed C2H5I compared to atomically adsorbed I.9-11 It was more difficult to identify the nature of the other primary product of the dissociation. In this case, high-resolution electron energy loss spectroscopy was applied. From the comparison of the data in Tables 1 and 3, it appears clearly that there are very little differences between the FTIR spectra of adsorbed C2H5I and C2H5. On single-crystal surfaces, the losses or shoulders at ∼2960, ∼2925, ∼2875, ∼1450, and ∼1380 cm-1 are the features which help us to establish the presence of ethyl species. An interesting feature of the FTIR spectra obtained for 2% Rh/SiO2 is that the absorption bands attributed to the molecularly adsorbed C2H5I were present even above 263 K, in contrast to the spectrum for pure SiO2, where these bands were eliminated by degassing the sample at 253 K. We may conclude that these bands belong to the vibrations of C2H5I bonded to the Rh crystallites. We have no reason to assume that the presence of Rh could stabilize the weakly adsorbed C2H5I on silica. The intensities of the absorption bands registered were even higher following the adsorption and evacuation at 300 K. Taking into account the results obtained for Rh(111) under ultrahigh vacuum conditions,20,21 we can count on the dissociation of adsorbed C2H5I at this temperature and even on the further reactions of ethyl species formed. On the basis of the data presented in Table 3, we have no clear evidence for the presence of ethyl species, which is not surprising in the light of the reactivity of the ethyl species on Pt metals.9-11 If it exists transitorily in a small concentration, its characteristic bands may be masked by the vibration of other adsorbed compounds. However, when the Rh content has been increased to 10%, a new band was (40) Di Cosimo, J. I.; Dıéz, V. K.; Xu, M.; Iglesia, E.; Apesteguıá, C. R. J. Catal. 1998, 178, 499.


identified at 2961-2968 cm-1, which was present at 206 K, and even at higher temperatures, when no detectable adsorbed species exist on pure silica. Considering that the position of νas(CH3) for C2H5I is always higher than that for C2H5 (see data for Tables 1 and 3), we feel safe to conclude that this band belongs to the vibration of C2H5 formed in the dissociation of C2H5I and bonded to the Rh. The data in Table 3 suggest that the spectral features at 2938, 2881, 2795, 1401, and 1338 cm-1 observed on 2% Rh/SiO2 can be mainly attributed to ethylidyne species. The relative intensities of these bands also support this assignment. From the above features, the 1334 cm-1 band, which cannot be attributed to any other adsorbed CHx fragments, was detected even after adsorption at 206 K. It became dominant after adsorption at 300 K. This band was even stronger at a higher concentration of Rh, when the 2879 cm-1 band attributed to the νas(CH3) of ethylidyne was clearly identified even at 206 K. This suggests that the dehydrogenation process of the ethyl group already started at such a low temperature. From this picture, we can infer that the C2H5, the primary dissociation product of C2H5I, has been dehydrogenated further through the transient formation of ethylene leading to the most stable CCH3. Another interesting feature of the spectra for Rh/SiO2 is the strong band at 2983 cm-1 and its high thermal stability. It was observed even at 673 K. Its position matches that of characteristic bands of molecular ethyl iodide and ethyl groups. These compounds, however, are not stable on Rh at such a high temperature, and therefore

Langmuir, Vol. 18, No. 23, 2002 8835

this band is more likely originated from ethoxide groups. As the ethoxide species is also unstable over Rh, we assume that it is located on silica. As it was not detected following the adsorption of C2H5I on Rh-free silica, its formation may be explained by a spillover process of ethyl species produced in the activation of ethyl iodide at the boundary of Rh crystallites. Adsorption of ethanol on Rh/SiO2 and SiO2 confirm this consideration as it gave a strong band at 2986 cm-1 due to the Si-OCH2CH3 group. 5. Conclusion The adsorption and surface reactions of ethyl iodide were investigated on SiO2, Al2O3, TiO2, and MgO and on the Rh/SiO2 catalyst. At 206 K, molecular adsorption was found on all oxides examined. C2H5I(a) was converted to ethoxide groups on Al2O3, TiO2, and MgO below 473 K, while SiO2 was inactive. The dissociation of ethyl iodide occurred on Rh/SiO2 from 206 K giving adsorbed ethyl species, which partially dehydrogenated even slightly above 200 K resulting in a stable ethylidyne. Another fraction of ethyl species spilt over from the Rh onto the silica forming a surface ethoxide compound. Acknowledgment. A loan of rhodium chloride from Johnson-Matthey PLC and the financial support of OTKA (Contract Numbers T S040877 and D38489) are greatly acknowledged. LA0203638

FTIR Study of the Interaction of Ethyl Iodide with Different Oxides and

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