摘要:

Photocatalytic oxidation of trichloroethylene using TiO2 coated optical microfibers Charles V. Rice and Daniel Raftery* Department of Chemistry H.C. Brown Laboratory Purdue University West Lafayette Indiana 47907 USA. E-mail raftery@chem.purdue.edu Received (in Bloomington IN USA) 21st December 1998 Accepted 9th April 1999 TiO2 particles have been attached to optical microfibers to allow the irradiation of the interior of a powdered photocatalyst and the UV initiated photocatalytic oxidation of trichloroethylene was followed using 13C solid-state NMR. The photocatalytic activity and surface chemistry of TiO2 is currently of significant interest owing to its use in light harvesting1 and pollutant remediation applications.2 Much is known about TiO2 photocatalysis at the liquid/solid interface whereas the gas/solid interface is less well understood.3 A variety of experimental techniques have been applied to study the surface photochemistry of TiO2 including GC–MS,4 IR5 and XPS.6 We have shown that solid-state nuclear magnetic resonance (SSNMR) spectroscopy can provide information about the identity and nature of surface-bound and gas-phase species during in situ photocatalytic reactions.7 Owing to the light scattering characteristics of TiO2 it is best to disperse the catalyst as a thin film to expose the particles in a homogeneous fashion.However the outer portions of TiO2 powders packed inside the SSNMR rotor will either scatter or absorb most of the incoming photons and create dark regions within the interior of the sample. Reactants in these dark regions will not participate in the photochemical reaction producing long-lived species and complicating the interpretation of results.Our laboratory has previously circumvented this problem by supporting monolayer TiO2 films on porous Vycor glass. This reduces scattering and can be successfully used to follow the photoreactions with SSNMR.7 Here we report a different approach which allows UV light to penetrate the interior of a powdered sample and creates an opportunity to use SSNMR to study the photocatalytic surface chemistry on TiO2 powders. TiO2 powders can be supported on glass plates,8 rods,9 beads,10 meshes11 and large diameter optical fibers.12 However these supports are impractical for use in SSNMR experiments. Thus we utilized microscopic quartz optical fibers to deliver the UV photons to the interior of the sample.The optical microfibers can be tightly packed inside the SSNMR rotor and the incident photons can be directed towards either the ends of the fibers or perpendicular to their long axis. The photochemical behavior of the coated fibers was evaluated by monitoring the conversion of trichloroethylene (TCE) an important environmental contaminant into several intermediates and final products. The quartz microfibers (Quartzel Fiber® 9 mm diameter Quartz Products Company) were supplied as a roving with each strand composed of 4800 individual fibers. Approximately 20 sections of the roving (each 15 cm long) were placed in a test tube (2.5 cm OD 30 cm long) and calcined at 400 °C to remove the polyimide cladding. The fibers were allowed to cool bound together at one end with copper wire and dipped into an aqueous suspension of TiO2 similar to that used by Nazeeruddin et al.13 The suspension was prepared by adding 4 ml of water to 10 g of TiO2 powder (Degussa P-25 surface area 55 m2 g21 70% anatase) in a ceramic mortar.A thick paste was formed by grinding the mixture and adding 800 ml of pentane-2,4-dione to promote dispersion of the particles. The paste was diluted by the slow addition of 16 ml of water while grinding. To allow the suspension to cover the fibers evenly 200 ml of a detergent (Triton X-100 Aldrich) was added. After dipping the fibers they were heated at 500 °C for 30 min cooled to room temperature and rinsed under flowing water to remove excess TiO2. The dipcoating heating and rinsing procedure was repeated 2 additional times.It was found that the surface area of the coated fibers could be increased further by allowing the coated fibers to dry then dipping them into a 20% solution of titanium isopropyl alcohol (Aldrich) in anhydrous isopropyl alcohol (Aldrich) and calcining at 500 °C for 3 h. After cooling the fibers were rinsed a final time and dried. Assuming even coverage of the fibers gravimetric analysis indicated that the TiO2 layer is ca. 0.5 mm thick. In a typical experiment ca. 40 000 coated microfibers (estimated by weight) were gathered into a bundle and inserted into a Teflon tube (4.5 mm ID Small Products Company). A 1 cm section of the fiber-filled tube was cut held at the opening of a standard 5 mm NMR tube (Norell) and a glass rod used to push the fiber bundle into the bottom of the NMR tube.After attachment to a vacuum manifold the fibers were calcined at 400 °C to remove any remaining organic material cooled to room temperature and exposed to humid air for 12 h to rehydrate the surface. The NMR tube was reattached to the manifold and heated at 150 °C under vacuum (1024 Torr) for 2 h to remove excess water. The tube was then cooled to 77 K for the addition of per-13C trichloroethylene (Cambridge Isotopes) and oxygen. The sample was isolated from the manifold by flame sealing. The gas pressure of the reagents in the tube at room temperature is estimated to be 1–2 atm. NMR analysis was performed on a 300 MHz Varian Unity Plus NMR spectrometer with a home built double resonance optical magic angle spinning (MAS) probe which has been described previously.7 A 300 W Xe arc lamp (ILC Technology) was used as the light source and a dichroic mirror (Oriel Corp.) was used to limit the light to 350–450 nm.Irradiation of the sample in the end-on configuration was performed outside the magnet while the side irradiation experiments were performed in situ as described previously.7 The proton decoupled 13C MAS NMR spectra collected during the photocatalysis of 48 mmol of TCE in the presence of 96 mmol of O2 are shown in Fig. 1. The sample was irradiated in the end-on configuration. Each spectrum represents an average of 256 transients obtained with a recycle time of 4 s. Spectra taken after various irradiation times show the formation and decomposition of reaction intermediates which have been identified by comparison with reported 13C chemical shift data.As can be seen in the Fig. 1 dichloroacetyl chloride (DCAC Cl2CHCOCl doublets at d 70 and 167) dichloroacetic acid (Cl2CHCO2H doublets at d 63 and 167) oxalyl chloride (ClCOCOCl d 159) phosgene (CCl2O adsorbed d 143 and gas phase d 139) trichloroacetaldehyde (CCl3CHO doublets at d 93 and 177) and carbon dioxide (d 123) are formed during the reaction. The major products are phosgene and CO2 with small quantities dichloroacetic acid and trichloroacetaldehyde remaining at the end of the reaction. Although these results demonstrate the photocatalytic behavior of the coated optical fibers the question as to whether the Chem. Commun. 1999 895–896 895 light is penetrating to the interior of the sample remained. This issue was explored by monitoring the formation and degradation of DCAC which is known to chemisorb to the TiO2 surface by reaction with hydroxyl groups to form dichloroacetate.The degradation of dichloroacetate requires additional UV photons and thus its persistance under continued irradiation indicates the presence of dark regions within the sample.7 In the case of the TiO2-coated optical fibers the concentration of dichloroacetate was maximized by stopping the photocatalysis of TCE after 30 min (when the DCAC concentration was highest) and storing the sample in the dark for several days to allow the DCAC to react with the surface hydroxyl groups. The sample was then broken open and evacuated to remove all non-surface bound species. Oxygen was reintroduced and the sample flame sealed. Fig. 2 shows the 13C NMR spectrum taken using cross polarization (CP) and MAS conditions.Surface bound dichloroacetate has two isotropic peaks (d 64 and 177) as well as a number of spinning sidebands. Subsequent irradiation of this sample in the end-on configuration shows the loss of the acetate signal [Fig. 2(b)] indicating that light is indeed reaching the interior of the sample. The experiment was repeated with the light directed towards the side of the sample and the degradation of the acetate [Fig. 2(c) (d)] was also observed although the rate was slower. At this time the mechanism for light transmission to the interior of the sample is not known. It is likely that the photons propagate down the microfibers and that side irradiation is successful because the TiO2 layer is not thick enough to absorb all of the photons.14 Alternatively it may be that the coated microfibers are able to create void spaces within the powder allowing the light to propagate between the particles which may also explain the success of side irradiation.Future experiments are planned to explore this issue further. In summary we have demonstrated that supporting TiO2 powders on optical microfibers decreases light scattering permitting study of the surface photochemistry with SSNMR. Efforts are currently underway to expand this methodology to other photochemical systems where light scattering is problematic such as zeolite photochemistry.15 This work was supported by the National Science Foundation (CHE 97-33188 CAREER Award) the Petroleum Research Fund administered by the American Chemical Society and the AT&T/Lucent Technologies Industrial Ecology Program.C. V. R. thanks the Purdue Research Foundation for a fellowship. Notes and references 1 U. Bach D. Lupo P. Comte J. E. Moser F. Weissortel J. Salbeck H. Spreitzer and M. Gratzel Nature 1998 395 583; J. Hagen W. Schaffrath P. Otschik R. Fink A. Bacher H. W. Schmidt and D. Haarer Synth. Met. 1997 89 215; B. O’Regan and M. Gratzel Nature 1991 353 737. 2 D. F. Ollis E. Pelizzeti and N. Serpone Photocatalysis Fundamentals and Applications John Wiley and Sons New York 1989. 3 M. A. Fox and M. T. Dulay Chem. Rev. 1993 93 341; A. L. Linsebigler G. Lu and J. T. Yates Jr. Chem. Rev. 1995 95 735; A. Hagfeldt and M. Gratzel Chem. Rev. 1995 95 49; M. R. Hoffmann S. T. Martin W. Choi and D. W. Bahnemann Chem. Rev. 1995 95 69. 4 M. R. Nimlos W. A. Jacoby D. M. Blake and T.A. Milne Environ. Sci. Technol. 1993 27 732; Y. Luo and D. F. Ollis J. Catal. 1996 163 1. 5 J. Fan and J. T. Yates Jr. J. Am. Chem. Soc. 1996 118 4686; M. D. Driessen A. L. Goodman T. M. Miller G. A. Zaharias and V. H. Grassian J. Phys. Chem. B. 1998 102 549. 6 S. A. Larson and J. L. Falconer Appl. Catal. B Environ. 1994 4 325. 7 S.-J. Hwang C. Petucci and D. Raftery J. Am. Chem. Soc. 1997 119 7877; 1998 120 4388; S.-J. Hwang and D. Raftery Catal. Today 1999 49 353. 8 H. Tada and M. Tanaka Langmuir 1997 13 360; P. Wyness J. F. Klausner D. Y. Goswami and K. S. Schanze J. Sol. Energy Eng. 1994 116 2. 9 K. Hofstadler R. Bauer S. Novalic and G. Heisler Environ. Sci. Technol. 1994 28 670. 10 H. Al-Ekabi and N. Serpone J. Phys. Chem. 1988 92 5726. 11 C. Shifu Z. Mengyue and T. Yaowu Microchem.J. 1996 54 54; V. Brezova A. Blazkova L. Karpinsky J. Groskova B. Havlinova V. Jorik and M. Ceppan J. Photochem. Photobiol. A 1997 109 177. 12 N. J. Peill and M. R. Hoffmann Environ. Sci. Technol. 1995 29 2974; N. J. Peill and M. R. Hoffmann Environ. Sci. Technol. 1996 30 2806; N. J. Peill and M. R. Hoffman J. Sol. Energy Eng. 1997 119 229. 13 M. K. Nazeeruddin A. Kay I. Rodicio R. Humphry-Baker E. Muller P. Liska N. Vlachopoulos and M. Gratzel J. Am. Chem. Soc. 1993 115 6382. 14 H. Tada and H. Honda J. Electrochem. Soc. 1995 142 3438. 15 N. J. Turro A. L. Buchachenko and V. F. Tarasov Acc. Chem. Res. 1995 28 69. Communication 8/09770G Fig. 1 Proton-decoupled 13C MAS NMR spectra obtained during the photooxidation of 48 mmol of TCE and 96 mmol of O2 on the TiO2 coated optical fibers irradiated in the end-on configuration for (a) 0 (b) 30 and (c) 150 min. The spinning rate was 3 kHz. Fig. 2 13C proton-decoupled CP MAS spectra of chemisorbed dichloroacetate collected with 1H–13C cross polarization (contact time = 3 ms) (a) before end-on irradiation; (b) after end-on irradiation for 120 min; (c) before side irradiation; (d) after side irradiation for 150 min. The spinning rate was 3.0 kHz and asterisks denote spinning sidebands. 896 Chem. Commun. 1999 895–896

ISSN:1359-7345    

DOI:10.1039/a809970g

出版商:RSC    

年代:1999

数据来源: RSC