Decontamination of Gaseous Acetaldehyde over CoOx-Loaded SiO2

The formation of CO2 begins after an induction period. ... TiO2 hybrid particles for the decomposition of gaseous acetaldehyde in the dark and under v...
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Langmuir 2005, 21, 2273-2280

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Decontamination of Gaseous Acetaldehyde over CoOx-Loaded SiO2 Xerogels under Ambient, Dark Conditions I. N. Martyanov, S. Uma, S. Rodrigues, and K. J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received May 3, 2004. In Final Form: October 21, 2004 A series of CoOx-doped silica xerogels with various Co2+ loadings (Co/Si ) 0, 1, 2, 4, 6, and 10 mol %) has been prepared. All xerogels exhibit large (800-1050 m2/g) surface areas. Narrow pore size distributions with pore size maxima around 3 nm are characteristic for Co/Si ) 1, 2, 4, 6, 10 samples. As-prepared CoOx/SiO2 xerogels show high catalytic activity in the air oxidation of gaseous acetaldehyde at room temperature. Carbon dioxide and trace amounts of methane are the only products detected in the gas phase. Acetic acid, a less volatile product, resides on the surface of the xerogels but can slowly desorb. The formation of CO2 begins after an induction period. The beginning of CO2 production coincides with the conversion of Co2+ incorporated in the SiO2 framework into Co3+. Thermogravimetry/gas chromatography/ mass spectrometry analysis, UV-vis and FTIR spectroscopies, as well as kinetic measurements are employed for CoOx/SiO2 catalyst characterization. A possible mechanism of the reaction is discussed.

Introduction High catalytic activity of cobalt-containing compounds in various processes, including hydrodesulfurization1 and oxidation,2-6 has received much attention and is well documented in the literature. Homogeneous catalytic liquid phase oxidation of acetaldehyde is an industrial route for production of acetic and peracetic acids.7 With cobalt-exchanged resins, this homogeneously catalyzed process can be made heterogeneous, offering the advantage of catalyst/reaction mixture separation.8 The process of co-oxidation of aldehydes with other organic compounds is another area of significant interest.9-14 Here, intermediates, such as high oxidation state transition metals or acetylperoxy radicals, are thought to be responsible for highly selective conversion of olefins into epoxides and oxygenation of unactivated C-H bonds. The mixture of cobalt and manganese acetates with hydrogen bromide in acetic acid is highly selective in oxidation of methyl groups of polymethylbenzenes (the Amoco MC method).15 * To whom correspondence should be addressed. E-mail: kenjk@ ksu.edu. (1) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; Chapter 5. (2) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (3) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. Rev. 1995, 143, 407. (4) Jorgensen, K. A. Chem. Rev. 1989, 89, 431. (5) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879. (6) Spivey, J. J. Ind. Eng. Chem. Res. 1987, 26, 2165. (7) Hagemeyer, H. J. In Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Howe-Grant, M., Eds.; John Wiley & Sons: New York, 1991; Vol. 1, p 94. (8) Chou, T.-C.; Lee, C.-C. Ind. Eng. Chem. Fundam. 1985, 24, 32. (9) Johnson, B. J. S.; Stein, A. Inorg. Chem. 2001, 40, 801. (10) Nam, W.; Kim, H. J.; Kim, S. H.; Ho, R. Y. N.; Valentine, J. S. Inorg. Chem. 1996, 35, 1045. (11) Khenkin, A. M.; Rosenberger, A.; Neumann, R. J. Catal. 1999, 182, 82. (12) Kholdeeva, O. A.; Grigoriev, V. A.; Maksimov, G. M.; Fedotov, M. A.; Golovin, A. V.; Zamaraev, K. I. J. Mol. Catal. A: Chem. 1996, 114, 123. (13) Murahashi, S.-I.; Naota, T.; Hirai, N. J. Org. Chem. 1993, 58, 7318. (14) Murahashi, S.-I.; Oda, Y.; Naota, T. J. Am. Chem. Soc. 1992, 114, 7913. (15) Partenheimer, W. Catal. Today 1995, 23, 69.

Cobalt oxides such as Co3O4 were found to be active in the processes of heterogeneous gas/solid phase oxidation of carbon monoxide into carbon dioxide by molecular oxygen.16-18 This reaction can proceed at temperatures well below ambient but unfortunately is accompanied by rapid deactivation of the catalyst. The illumination of Co3O4 with the full arc of a Hg lamp sensitized the CO oxidation and decreased the activation energy of the process.19 Acetaldehyde, being a toxic compound, forms in considerable amounts inside of buildings upon oxidation by incoming ozone of organic compounds released by furniture and carpets.20 A number of remediation processes have been suggested. A deep photocatalytic oxidation of organic compounds and acetaldehyde in particular over TiO2 nanoparticles21-27 has received perhaps the greatest attention so far. Indeed, in this process the oxidation can proceed at atmospheric concentrations of oxygen at room temperatures. Despite some obvious advantages of this approach, the necessity for UV light is a problem. Recently cobalt oxide dispersed in mesoporous molecular sieves, Al-MCM-41, unlike bulk Co3O4 or CoO was found to be photoactive in the reaction of acetaldehyde oxidation under visible light.28 Adjusting the preparation procedure to (16) Lokhov, Y. A.; Tikhov, S. F.; Bredikhin, M. N.; Zhirnyagin, A. G.; Sadykov, V. A. Mendeleev Commun. 1992, 10. (17) Jansson, J. J. Catal. 2000, 194, 55. (18) Jansson, J.; Skoglundh, M.; Fridell, E.; Thormahlen, P. Top. Catal. 2001, 16/17, 385. (19) Steinbach, F. Nature 1967, 215, 152. (20) Reiss, R.; Ryan, P. B.; Tibettes, S. J.; Koutrakis, P. J. Air Waste Manage. Assoc. 1995, 45, 811. (21) Falconer, J. L.; Magrini-Bair, K. A. J. Catal. 1998, 179, 171. (22) Muggli, D. S.; Lowery, K. H.; Falconer, J. L. J. Catal. 1998, 180, 111. (23) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 2699. (24) Muggli, D. S.; McCue, J. T.; Falconer, J. L. J. Catal. 1998, 173, 470. (25) Sauer, M. L.; Ollis, D. F. J. Catal. 1996, 158, 570. (26) Sopyan, I.; Watanabe M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol. A: Chem. 1996, 98, 79. (27) Muggli, D. S.; Larson, S. A.; Falconer, J. L. J. Phys. Chem. 1996, 100, 15886. (28) Rodrigues, S.; Uma, S.; Martyanov, I.; Klabunde, K. J. J. Photochem. Photobiol. A: Chem. 2004, 165, 51.

10.1021/la040070+ CCC: $30.25 © 2005 American Chemical Society Published on Web 02/05/2005

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further enhance the degree of CoOx dispersity led to the somewhat unexpected result: the properties of CoOx in the SiO2 framework started to resemble the properties of cobalt salts and complexes. In particular it was found that thus-prepared CoOx/SiO2 xerogels are catalytically active in the reaction of acetaldehyde oxidation under ambient conditions in the dark. In this work, we focus our attention on the catalytic activity of CoOx/SiO2 xerogels in the gas/solid phase catalytic reaction and their possible applications for gaseous acetaldehyde remediation. The preliminary results are indeed promising: the ambient oxidation of acetaldehyde over CoOx/SiO2 xerogels proceeds fast with carbon dioxide as the only major product detected in the gas phase. Experimental Section Reagents and Materials. Tetraethyl orthosilicate (TEOS), cobalt(II) nitrate hexahydrate (Co(NO3)2‚6H2O), and acetaldehyde were bought from Aldrich. Ethyl alcohol, absolute (200 proof), was from AAPER. Distilled water was additionally purified with a water purification system from Millipore Corp. Catalyst Preparation. In a typical procedure, an ethanol solution of a predetermined amount of cobalt nitrate (ca. 18 mL) was quickly added under vigorous stirring to a mixture of 40 mL of ethanol and 20 mL of TEOS. Addition of 13.6 mL of water initiated TEOS hydrolysis and condensation, which was accelerated by keeping the mixture at 50 °C. Depending on the concentration of cobalt nitrate added, formation of a gel occurred within 3-5 h. No gelation was observed when no Co(NO3)2 was added. After 24 h of aging, the CoOx/SiO2 precursors were dried in air at 80 °C for 2 days. In the last preparation step, the powders were annealed at 450 °C for 1 h in air, giving the final product. By adjusting the concentration of cobalt nitrate, a series of the CoOx-loaded SiO2 xerogels with Co/Si ratios of 0, 1, 2, 4, 6, and 10 mol % were prepared. Kinetic Experiments. All kinetic experiments were carried out in a 305 mL glass reactor under vigorous stirring in order to continuously mix the gases. The temperature of the reaction mixture was maintained by passing water of a required temperature through the reactor’s water jacket. In a typical experiment, ca. 100 mg of the CoOx-loaded SiO2 xerogel was placed in the reactor filled with air followed by reactor sealing. The oxidation reaction was started with an introduction of 4 mL of air saturated with acetaldehyde vapor into the reactor containing CoOx/SiO2 xerogels. The course of the process was followed by periodically monitoring concentrations of acetaldehyde and carbon dioxide in the gas phase with a GCMS-QP5000 from Shimadzu equipped with a capillary column (phase XTI-5, Restek Corp.) maintained at 40 °C. The analytical procedure allowed complete separation of the major components of the gas phase mixture (acetaldehyde and carbon dioxide). The quantification of acetaldehyde and carbon dioxide was done from the peak areas under the m/z ) 44 fragment after comparison with corresponding calibration curves. The calibration curves for acetic acid and formaldehyde were also established and used later for determination of detection limits for these compounds. The composition of the gas phase after the reaction completion was additionally analyzed with infrared spectroscopy. For this purpose, the reaction mixture was passed through a standard rectangular IR cell with a 1 cm optical path (SG Precision Cells) and later analyzed with a FTIR spectrometer (Nexus 670 FTIR) from Nicolet Instrument Corp. Sample Characterization. A number of techniques were employed for characterization of the CoOx/SiO2 xerogels. The surface areas of the powders were calculated according to the Brunauer-Emmett-Teller (BET) model from N2 adsorption isotherms recorded at 77 K on a NOVA-1200 instrument from Quantachrome Corp. The pore size distributions were derived from desorption branches of corresponding isotherms according to the Barrett-Joyner-Halenda (BJH) method. Prior to surface area/pore size distribution measurements, the samples were subjected to evacuation at 300 °C for about 1 h. Thermogravimetric analysis (TGA) was performed on a TGA50 instrument from Shimadzu. The heating rate was set to 10

Figure 1. Surface areas of the CoOx-loaded SiO2 xerogels vs CoOx loadings. °C /min, and the flow of the gas was ca. 30 mL/min. Two series of experiments (in helium and air flow) were carried out. In the case of helium flow experiments, the products desorbing from the linearly heated sample were directed to the gas chromatograph/mass spectrometer (GCMS) and analyzed through an automatic periodic sampling with a valve from Valco Instruments Co., Inc. The analytical procedure allowed a clear separation of carbon dioxide, acetic acid, acetone, and acetaldehyde. A Cary 500 Scan UV-vis-NIR spectrophotometer from Varian Analytical Instruments equipped with a diffuse reflectance accessory was employed for measurement of UV-vis optical spectra of CoOx/SiO2 powders. Poly(tetrafluoroethylene) powder was used as a reference standard in each case. All spectra were recorded at a 600 nm/min scan rate with 5 nm slit resolution. Infrared spectra of the powders before and after the reaction were taken after their pelletizing with KBr powder in air. The typical amount of the CoOx/SiO2 in the KBr pellet was close to 10 wt %.

Results and Discussion The sol-gel procedure employed for preparation of CoOx/ SiO2 catalysts yields powders with very high surface areas. Although the trend in surface area vs CoOx loading remains not obvious (Figure 1), the surface areas of all the samples fall in the range of 800-1050 m2/g. The pore size distributions of the samples with various CoOx loadings are presented in Figure 2. Interestingly, for all samples having some CoOx loading a sharp maximum around 3 nm is observed. An increase in CoOx loading does not change the position of that maximum but leads to a consistent decrease in the number of pores and, thus, the pore volume. A similar pore size distribution trend could, in principle, be expected from an increase in the molar weight of the samples upon substitution of light silicon by heavier cobalt. However, since the cobalt concentration for all samples does not exceed 10 mol %, the contribution of this effect cannot result in more than a 3% decrease in the number of the pores per gram of the sample and cannot account for the observed trend. As seen from Figure 2, the pore size distribution of the pure silica powder is very different from what is observed for CoOx/SiO2 samples. This finding is not unusual in view that the gelation of samples with no Co(NO3)2 occurs under much different conditions, i.e., at 80 °C upon solution drying vs at 50 °C for the samples with Co(NO3)2. All CoOx-containing silica xerogels show remarkable activities in the reaction of acetaldehyde oxidation. A

Decontamination of Gaseous Acetaldehyde

Figure 2. Pore size distributions in the Co-loaded SiO2 xerogels at various CoOx loadings.

Figure 3. The course of acetaldehyde oxidation over CoOxloaded SiO2 (Co/Si ) 2 mol %) at 25 °C in the presence of air.

typical example of the process on the SiO2 sample with 2 mol % of cobalt oxide loading (Co/Si ) 2 mol %) is shown in Figure 3. For the experiment presented in Figure 3, the turnover number calculated as a ratio of the carbon dioxide molecules formed to the number of cobalt ions present in the sample is about 0.25. At the same time, the reaction is catalytic in nature. Indeed, the turnover number with respect to CO2 can exceed 100 as was proved in a separate experiment. The experimental data for acetaldehyde consumption and carbon dioxide formation could be fitted reasonably well with first-order kinetics drawn in Figure 3 with solid lines. As will be shown later, the decrease of acetaldehyde concentration is caused by two processes: adsorption and reaction taking place simultaneously. At the same time, since the adsorption of CO2 on CoOx/SiO2 was determined to be negligible with respect to the amount of CO2 evolved, the evolution of carbon dioxide most likely reflects the true course of the chemical reaction with little disturbance from adsorption by CoOx/SiO2. The results of testing the various CoOx/SiOx samples in the reaction of acetaldehyde oxidation are presented in

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Figure 4. The first-order reaction constants of acetaldehyde disappearance and carbon dioxide formation vs CoOx load at 25 °C in air.

Figure 5. The FTIR spectra of the gas phase after oxidation of acetaldehyde over CoOx-loaded SiO2 (Co/Si ) 2 mol %) at 25 °C in air.

Figure 4. The increase in CoOx loading leads to a linear increase in activities. Since, however, the linear approximation shown by solid lines does not predict zero activity at zero concentration of CoOx (which is actually the case), significant diffusion limitations of the oxidation process are evident. Carbon dioxide and trace amounts of methane were the only products detected in the gas phase in the course of acetaldehyde oxidation over CoOx/SiO2 at room temperature. The gas phase concentrations of other organic compounds were below GCMS detection limits, which were approximated as 1 and 17 µM for acetic acid and formaldehyde, respectively. The analysis of the gas phase mixture with IR spectroscopy supports the results of the GCMS analysis (Figure 5). Carbon dioxide was the dominant gas phase product of the reaction of acetaldehyde oxidation. As seen in Figure 3, the acetaldehyde oxidation over CoOx/SiO2 samples leads only to partial conversion of acetaldehyde into carbon dioxide. Other products must form, which may remain adsorbed on the xerogel surface. To check this assumption, a series of TGA experiments

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CH3COOH(ad) f CH3COOH(g) CH3COO(ad) + OH(ad) f CH3COOH(g) + O(l) Lattice oxygen O(l) was proposed to be consumed in dissociative adsorption of acetic acid:29

CH3COOH(g) + O(l) f CH3COO(ad) + OH(ad)

Figure 6. The weight loss and variation of relative concentrations of the compounds desorbing from CoOx-loaded SiO2 (Co/ Si ) 2 mol %) right after participation in the reaction with acetaldehyde at 25 °C in air; He flow.

coupled with GCMS analysis of desorbing products was carried out. The results of TG analysis of the CoOx/SiO2 sample (Co/Si ) 2 mol %) right after its participation in oxidation of acetaldehyde are presented in Figure 6. The rapid weight loss can be associated with release of acetic acid for the 170 °C transition, acetone for 410 °C, and carbon dioxide for both of them. The growth of CO2 concentration at temperatures above 900 °C has been found irrespective of the presence of the sample and is attributed to desorption of CO2 from some parts of the TGA apparatus located in close proximity to the hot furnace. The low volatility of acetic acid causes some delay (ca. 40 °C) between its actual release and detection by gas chromatography/mass spectrometry. All the products, namely, acetic acid, acetone, and carbon dioxide, might originate solely from acetic acid formed during the roomtemperature oxidation of acetaldehyde. Indeed, the studies on adsorption and thermal decomposition of acetic acid on TiO2 and other oxides demonstrated that adsorption of acetic acid proceeds via two pathways leading to molecularly and dissociatively adsorbed species.29 The acetic acid adsorbed in the molecular manner desorbs reversibly at low temperatures, whereas surface carboxylates forming through dissociation may either recombine with hydroxyl species and desorb as acetic acid at 130 °C or decompose into acetone and carbon dioxide at ca. 340 °C. These data are quite consistent with the results in Figure 6. Indeed, the high-temperature peak for acetone and carbon dioxide release can be explained29 through the bimolecular carboxylate reaction:

2CH3COO(ad) f CH3C(O)CH3(g) + CO3(ad) CO3(ad) f CO2(g) + O(l) whereas the release of acetic acid in the temperature range of 100-480 °C might be attributed to desorption of molecularly adsorbed species as well as recombination of carboxylates with hydroxyl moieties: (29) Kim, K. S.; Barteau, M. A. Langmuir 1988, 4, 945.

Despite the ultrahigh purity of the helium gas employed for conducting the experiment shown in Figure 6, trace concentrations of oxygen could always be detected in the helium flow. Thus partial (maybe even catalytic6) burning of adsorbed organics can be expected to happen to some extent. The prolonged (over a week) exposure of used CoOx/ SiO2 samples to open air resulted in acetic acid desorption. The TGA of those samples showed a less pronounced weight loss around 130 °C as well as the absence of weakly adsorbed acetic acid (Figure 7). However, the high temperature (ca. 410 °C) peak of acetone and carbon dioxide remained almost unchanged, indicating strong carboxylate adsorption. The TGA experiments with CoOx/ SiO2 (Co/Si ) 2 mol %) before the reaction with acetaldehyde showed a small weight loss in the 100-150 °C region due to adventitious water and the absence of such at ca. 410 °C. The analogous experiments conducted in air flow resulted in weight losses similar to those observed in helium. This supports the presumption that the major portion of adsorbed organic compounds, indeed, leave the surface of the xerogels during helium flow experiments rather than convert into nonvolatile carbonaceous species. Interestingly, the weight loss at ca. 410 °C increased with CoOx loading but was insensitive to the amount of acetaldehyde reacted. This finding implies that the tightly bound acetate forming in the reaction is attached to the CoOx moiety of CoOx/SiO2 xerogels. Introduction of gaseous acetaldehyde to the CoOx/SiO2 samples in the presence of air causes the color of the samples to change from blue to olive green (Figure 8). At 25 °C, such color changes occur in 1-3 min and happen before recording the kinetic curves in Figure 3. Continuation of the reaction does not influence the catalyst color. Moreover, the color of the used catalysts remains the same after removing gaseous acetaldehyde by exposure of the CoOx/SiO2 xerogels to the atmosphere. UV-vis spectra of the CoOx/SiO2 samples with (Co/Si ) 2, 10 mol %) before and after reaction are shown in Figure 9. The triplet with maxima at 520, 590, and 643 nm is clearly observed. Similar UV-vis spectra were consistently observed with Co2+-incorporated silica xerogels,30 highly dispersed CoO over SiO2,31 and Co2+-exchanged zeolites32,33 and are attributed to 4 A2 f 2T1, 4T1, and 2T2 transitions of Co2+ ions in a tetrahedral environment.31 It is important to note that in our case the degree of Co2+ loading does not influence the position of the peaks but rather leads to an increase in their intensity. This observation favors the assumption that variation of Co(NO3)2 loading in the range of Co/Si (30) Kojima, K.; Taguchi, H.; Matsuda, J. J. Phys. Chem. 1991, 95, 7595. (31) Okamoto, Y.; Nagata, K.; Adachi, T.; Imanaka, T.; Inamura, K.; Takyu, T. J. Phys. Chem. 1991, 95, 310. (32) El-Malki, E.; Werst, D.; Doan, P. E.; Sachtler, W. M. H. J. Phys. Chem. B 2000, 104, 5924. (33) Fierro, G.; Eberhardt, M. A.; Houalla, M.; Hercules, D. M.; Hall, W. K. J. Phys. Chem. 1996, 100, 8468.

Decontamination of Gaseous Acetaldehyde

Figure 7. The weight loss and variation of relative concentrations of the compounds desorbing from CoOx-loaded SiO2 (Co/ Si ) 2 mol %) in He flow. After participation in the reaction with acetaldehyde at 25 °C in air, the catalyst was left in open air for ca. 7 days.

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Figure 9. Diffuse reflectance UV-vis spectra of (a,b) CoOxloaded SiO2 (Co/Si ) 2 mol %) before and after reaction with acetaldehyde; (c,d) the same for CoOx-loaded SiO2 (Co/Si ) 10 mol %).

Figure 8. Influence of the reaction of acetaldehyde in air on Co2+/SiO2 color (Co/Si ) 2 mol %).

) 1-10 mol % does not lead to formation and growth of CoOx particles but rather increases the density of Co2+ centers. Regarding the blue to green color change upon exposure to acetaldehyde in air, a similar color change can be observed when liquid H2O2 (30 wt %) or peracetic acid is directly applied to CoOx/SiO2. No pronounced color change, however, is observed upon addition of acetic acid. The change of the color to dark green is consistent with literature data and can be attributed to the oxidation of Co2+ into Co3+ (ref 2, p 44; ref 33). The UV-vis spectra of the CoOx/SiO2 samples with the two maxima around 350 and 610 nm observed after the reaction qualitatively resemble the spectra of cobalt oxides with cobalt ions in a high oxidation state. Indeed, two broad peaks at 410 and 710 nm were observed31 and ascribed to Co3O4. A single broad maximum at 690 nm was observed for CoZSM-5 samples when treated in O2 at 500 °C overnight.33 This and other literature data34 imply that UVvis spectra of cobalt ions in the 3+ oxidation state are sensitive to the degree of cobalt dispersion. In this regard, (34) Chin, R. L.; Hercules, D. M. J. Phys. Chem. 1982, 86, 3079.

Figure 10. FTIR spectra of (a) SiO2 xerogel (Co/Si ) 0 mol %) and (b,c) CoOx-loaded SiO2 (Co/Si ) 10 mol %) before and after reaction with acetaldehyde.

we may tentatively suggest that for our samples the interaction of highly dispersed Co2+ with acetaldehyde in air results in formation of isolated Co3+ incorporated in the SiO2 framework. The IR spectra of CoOx/SiO2 (10 mol %) xerogels taken before and after the reaction as well as the IR spectrum of bare SiO2 are shown in Figure 10. The absorption bands in the range 3000-3800 cm-1 can be attributed to the stretching vibrations of surface hydroxide groups and water molecules. The presence of water molecules at the surfaces of all samples is evidenced from the absorption band around 1640 cm-1. The SiO2 bands near 1200 cm-1 can be assigned to internal symmetric modes, and the bands near 800 cm-1 to internal and external symmetric vibrations.32 Introduction of cobalt ions and change of its oxidation state result in alteration of the IR spectrum of bare SiO2 near 900 cm-1 (Figure 10). As shown by Sachtler and co-

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Figure 11. FTIR spectra of CoOx-loaded SiO2 xerogels before and after reaction with acetaldehyde: (a,b) CoOx-loaded SiO2 (Co/Si ) 2 mol %) before and after reaction with acetaldehyde; (c,d) the same for CoOx-loaded SiO2 (Co/Si ) 10 mol %).

workers, introduction of transition metal ions causes the perturbation of the lattice vibrations in zeolites emerging as a disturbance of the IR spectrum and appearance of new absorption bands in the 800-1200 cm-1 region.35,36 According to Ricchiardi and co-workers,37 the 960 cm-1 IR band in titanium silicate catalyst should be attributed to out-of-phase antisymmetric stretching of the four connected Ti-O-Si oscillators. A close inspection of the IR spectra of CoOx/SiO2 xerogels before and after the reaction with acetaldehyde reveals another interesting feature, which is the appearance of absorption bands at 1560 and 1430 cm-1 (Figure 11). Since the intensity of these bands increases with CoOx loading but is independent of the amount of acetaldehyde subjected to oxidation, it is reasonable to suggest that their appearance is related to the CoOx moiety in the CoOx/ SiO2 xerogel. Since the position of the bands is consistent with IR adsorption expected from carboxylic salts,38 the adsorption at 1560 and 1430 cm-1 can be attributed to acetates attached to Co3+ ions forming during the reaction of acetaldehyde with CoOx/SiO2 samples. As mentioned earlier, the change of color of the CoOx/ SiO2 powders from blue to green at 25 °C occurs quite fast, in the first few minutes of the reaction. To study this conversion in greater detail, the reaction was carried out at lower temperature, i.e., at 10 °C (Figure 12). The course of the reaction after the first introduction of acetaldehyde is shown by filled circles and squares. It was unexpected to observe rapid consumption of acetaldehyde with no evolution of carbon dioxide in the first 6 min of the reaction during which the CoOx/SiO2 remained blue in color. Later, however, as the color of the catalyst turned green, the formation of CO2 became observable. Carrying out the reaction to completion and flushing the reaction container with air did not change the green color of CoOx/SiO2 samples. Upon introduction of a new portion of acetaldehyde, the formation of CO2 started right away with no (35) El-Malki, E.; Werst, D.; Doan, P. E.; Sachtler, W. M. H. J. Phys. Chem. B 2000, 104, 5924. (36) Blasco, T.; Camblor, M. A.; Corma, A.; Esteve, P.; Guil, J. M.; Martı´nez, A.; Perdigo´n-Melo´n, J. A.; Valencia, S. J. Phys. Chem. B 1998, 102, 75. (37) Ricchiardi, G.; Damin, A.; Bordiga, S.; Lamberti, C.; Spano, G.; Rivetti, F.; Zecchina, A. J. Am. Chem. Soc. 2001, 123, 11409. (38) Handbook of Chemistry and Physics, 54th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, 1973; p F-216.

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Figure 12. The course of the reaction of acetaldehyde oxidation at 10 °C in air over CoOx-loaded SiO2 (Co/Si ) 2 mol %) after first and second injection of acetaldehyde.

Figure 13. The course of the acetaldehyde interaction with CoOx-loaded SiO2 (Co/Si ) 2 mol %) at 10 °C. The reactor was flushed with argon before acetaldehyde introduction.

induction period (Figure 12). As concluded, the presence of Co3+ is crucial for oxidation of acetaldehyde into CO2 at a detectable rate, whereas it has little effect on consumption of acetaldehyde by CoOx/SiO2 powders at least in the beginning of the process. In this regard, the initial consumption of acetaldehyde can occur due to its simple adsorption by the CoOx/SiO2 xerogel. A substitution of atmospheric oxygen in the reactor by argon suppresses formation of CO2 (Figure 13). The catalyst remains blue (in Co2+ form) all the time. A consistent decrease in concentration of gaseous acetaldehyde in this case was attributed simply to its adsorption by the high surface area xerogels, since a similar trend was observed with bare SiO2 (Figure 14). Strong adsorption capabilities of the prepared xerogels even with respect to highly volatile acetaldehyde (bp 22 °C38) largely explain the difficulties with detecting acetic acid in the gas phase reaction mixture. Indeed, acetic acid, being much less volatile (bp 118 °C38), would be expected to stay adsorbed on the CoOx/SiO2 surface with only a very small fraction in the gas phase.

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Certain organo-metallic compounds of Co2+ are known to readily form superoxo complexes by reaction with the oxygen molecule (ref 2, p 74). The LxCo3+-O- O•- structure is often suggested to be accountable for the reaction initiation.10 By analogy, the possible initiation step may proceed in the following way:

Co3+-O-O•- + CH3CHO f Co3+-O-OH + CH3•CO (r2) Although the starting cobalt compound has only cobalt ion in the +2 oxidation state, the formation of some small amounts of Co3+ in SiO2 during the heat treatment step looks feasible. In this case, the formation of the acetyl radical may proceed in a straightforward manner:

Co3+ + CH3CHO f Co2+ + CH3•CO + H+ (r3)

Figure 14. Two blank experiments: (a) evolution of acetaldehyde concentration when no catalyst is present at 10 °C; (b) adsorption of acetaldehyde on SiO2 xerogel (Co/Si ) 0 mol %) at 10 °C.

Possible Mechanism of the Reaction. The acetaldehyde autoxidation has been studied in great detail.39,40 The mechanism for the liquid phase processes catalyzed by cobalt-containing compounds was also thoroughly investigated and includes initiation, chain propagation, and chain termination steps.8,10,12,41 By analogy with these studies, a sequence of events, including chain initiation, propagation, and chain termination, is expected to occur during oxidation of acetaldehyde over CoOx/SiO2 xerogels. A few routes can be considered with respect to their possible contribution to the reaction initiation. One possibility suggested in the literature9 is a reduction of Co2+ to Co+ with simultaneous creation of the acetyl radical:

CH3CHO + Co2+ f CH3•CO + Co+ + H+

(r1)

The main objection to this route is the highly negative redox potential of the Co+/Co2+ couple. The reduction of Co2+ to Co+ could be achieved in aqueous media by solvated electrons generated in pulse radiolysis experiments but not CO2•-.42,43 After formation in water, Co+ can quickly disproportionate into Co2+ and Co0 with the latter acting as a catalyst of the Co+ degradation. As observed for CoZSM-5 zeolites, reduction/oxidation cycles correspond to O/Co ) 0.5 alteration of oxygen content.33 Though such a process is formally consistent with reduction of Co2+ to Co+, this possibility was rejected on the basis of instability of the 1+ oxidation state of cobalt ions. Although the initiation step (r1) by aqueous Co2+ looks unlikely,10 some stable organic complexes of cobalt formally in the +1 oxidation state have been prepared and characterized.44 (39) Clinton, N. A.; Kenley, R. A.; Traylor, T. G. J. Am. Chem. Soc. 1975, 97, 3746. (40) Clinton, N. A.; Kenley, R. A.; Traylor, T. G. J. Am. Chem. Soc. 1975, 97, 3752. (41) Hendriks, C. F.; van Beek, H. C. A.; Heertjes, P. M. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 260. (42) Buxton, G. V.; Sellers, R. M. J. Chem. Soc., Faraday Trans. 1 1975, 71, 558. (43) Ershov, B. G.; Sukhov, N. L.; Janata, E. J. Phys. Chem. B 2000, 104, 6138. (44) Bosnich, B.; Jackson, W. G.; Lo, S. T. D.; McLaren, J. W. Inorg. Chem. 1974, 13, 2605.

The reaction of Co2+ with peroxide compounds possibly present in trace amounts in acetaldehyde cannot be excluded either. Regardless of the relative contribution of each initiation step, further processes are expected to include8 formation of the acetylperoxy radical:

CH3•CO + O2 f CH3CO3•

(r4)

followed by formation of peracetic acid:

CH3CO3• + CH3CHO f CH3C(O)OOH + CH3•CO (r5) The reaction of peracetic acid with Co2+ is anticipated to yield Co3+ ion and acetoxyl radical:

CH3C(O)OOH + Co2+ f CH3CO2• + Co3+ + HO- (r6) Due to its high oxidation potential, Co3+ is expected to be easily reduced by acetaldehyde present in high amounts in the reaction mixture (r3). Thus formation of Co3+ should drastically accelerate the sequence of reactions 3-6 thus leading to formation of considerable quantities of carbon dioxide:

CH3CO2• f CO2 + •CH3

(r7)

The methyl radical formed in reaction 7 is expected to yield methane after interaction with acetaldehyde:

CH3 + CH3CHO f CH4 + CH3•CO

•

(r8)

Additionally peracetic acid is known to be able to react with acetaldehyde resulting in formation of acetic acid:

CH3C(O)OOH + CH3CHO f 2CH3CO2H

(r9)

Numerous chain termination reactions can occur at the same time.41 The induction period is not unknown for the autoxidation process (ref 2, p 45; ref 45). Interestingly, this phenomenon occurs when oxidation is carried out in nonpolar liquids, such as neat hydrocarbons, but is not characteristic for polar solvents, such as acetic acid. It was argued that the reason for such unusual behavior (45) Black, J. F. J. Am. Chem. Soc. 1978, 100, 527.

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seemingly lies in the formation of a complex of cobalt ions with hydroperoxides. By analogy, in our case we may suggest the formation of the relatively stable Co2+CH3C(O)OOH complex that forms at initial reaction stages:

Co2+ + CH3C(O)OOH T Co2+CH3C(O)OOH Accumulation of acetic acid that is expected to coordinate to cobalt ions would suppress that complex formation and accelerate the step generally depicted by reaction 6. In summary, high surface area CoOx/SiO2 xerogels were prepared via a sol-gel route. As evident from UV-vis spectra in the as-prepared samples, cobalt exists as Co2+ in a tetrahedral environment. The CoOx/SiO2 xerogels showed remarkable activities in the oxidation of acetaldehyde under ambient conditions. Desorbing carbon

Martyanov et al.

dioxide and acetic acid residing on the surface of CoOx/ SiO2 are the major products of the reaction. Formation of Co3+ from Co2+ in the presence of an acetaldehyde/air mixture appears to be essential for the oxidation reaction to proceed at a high speed. The developed high surface area CoOx/SiO2 xerogels are expected to serve as roomtemperature catalysts for remediation of toxic acetaldehyde and as cocatalysts in the photooxidation processes. They also can be interesting for heterogenizing of established and emerging processes involving the oxidation of aldehydes under ambient conditions. Acknowledgment. The support of the Army Research Office through a DARPA funded MURI grant (DAAD 1901-10619) is acknowledged with gratitude. LA040070+