J. Phys. Chem. 1992,96, 5053-5059
or acetylene it is necessary to add hydrogen to the reactant mixture. As the fraction of hydrogen in the gas feed was increased there was a corresponding increase in both the yield of carbon filaments and the degree of their crystalline perfection. By careful selection of the hydrocarbon/hydrogen mixture it was possible to make carbon filaments which were entirely graphitic in nature at temperatures of the order of 550 OC. Product analysis indicated that in all cases ethylene was less reactive than acetylene. It is suggested that prior to carbon filament formation ethylene can undergo a structural rearrangement on the surface to form either an acetylenic intermediate which eventually leads to the growth of carbon filaments, or an ethylidyne intermediate which is responsible for methane production along with solid carbon formation. Acknowledgment. This work was supported by the Department of Energy, Basic Energy Sciences, Grant No. DE-FGO589ER14076. We would like to thank Engelhard Corp. for the gift of the platinum used in this work. Registry No. C, 7440-44-0; Pt, 7440-06-4; graphite, 7782-42-5; acetylene, 74-86-2; ethylene, 74-85-1; hydrogen, 1333-74-0.
References and Notes (1) Bacaud, R.; Charcosset, H.; Guenin, M.; Torrelas-Hidalgo, R.; Tournayan, L. Appl. Catal. 1981, 1, 81. (2) Barbier, J.; Marecot, P.; Martin, N.; Elassal,L.; Maurel, R. Srud. Surf Sci. Caral. 1982, 6, 53. 13) Parara. J. M.: Figoli. N. S.: Traffano. E. M. J. Catal. 1983. 79.481. (4) Barbier, J.; C0rro:G.f Zhang, Y.; Boumonville, J. P.; Frank, J: P. Appl. Caral. 1985, 13, 245. ( 5 ) Mieville, R. L. J . Catal. 1987, 105, 536. (6) Parera, J. M.; Betramini, J. N. J . Caral. 1988, 112, 357. (7) Cabral, R. A.; Oberlin, A. J. Caral. 1984, 89, 256. (8) Gallezot, P.; Lxqlerq, C.; Barbier, J.; Marecot, P. J. Caral. 1989, 116, 164. (9) Fryer, J. R.; Paal, Z. Carbon 1973, 11, 665. (10) Wu,N. L.; Phillips, J. J . Catal. 1988, 113, 383. (11) Murphy, D. B.; Carroll, R. W . Carbon 1990,28, 733. (12) Chang, T. S.;Rodriguez, N. M.; Baker, R. T. K. J . Catal. 1990,123, 486.
5053
(13) Baker, R. T. K. Carbon 1989, 27, 315. (14) Morgan, A. E.; Somorjai, G.A. Surf.Sci. 1968, 12, 405. (15) Smith, D. L.; Merrill, R. P. J. Chem. Phys. 1970,52, 5861. (16) Weinberg, W.H.; Deans, H. A.; Merrill, R. P. Surf.Sci. 1974,41, 312. (17) Lang, B. Surf.Sci. 1975, 53, 317. (18) Kesmodel, L. L.; Baetzold, R. C.; Somorjai, G.A. Surf.Sci. 1977, 66, 299. (19) Fischer, T. E.; Kelemen, S . R. Surf.Sci. 1977, 69, 485. (20) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G.A. J. Chem. Phys. 1979, 70, 2180. (21) Freyer, N.; Pirug, G.;Bonzel, H. P. Surf.Sci. 1983, 125, 327. (22) Avery, N. R. Langmuir 1988, 4, 445. (23) Oudar, J.; Pinol, S.;Berthier, Y. J. Caral. 1987, 107, 434. (24) Yagasaki, E.; Masel, R. I. Sur$ Sci. 1989, 222, 430. (25) Boronin, A. I.; Bukhtiyarov, V. I.; Kvon, R.; Chesnokov, V. V.; Buyanov, R. A. Surf.Sci. 1991, 258, 289. (26) Baker, R. T. K. Caral. Rev. Sci. Eng. 1979, 19, 161. (27) Hagstrem, S.;Lyon, H. B.; Somorjai, G.A. Phys. Rev. Lerr. 1965, 15, 491. (28) Bonzel, H. P.; Ku, R. Surf.Sci. 1972, 33, 91. (29) Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Biberlan, J. P.; Kesmodel, L. L.; Bartos, I.; Somorjai, G. A. Surf.Sci. 1981, 103, 189. (30) Barteau, M. A.; KO,E. J., Madix, R. J. Surf. Sci. 1981, 102, 99. (31) Shi, A. C.; Fung, K. K.; Welch, J. F.; Wortis, M.; Masel, R. I. Mar. Res. Soc. Symp. Proc.; Tracy, M. M. J., Thomas, J. M., White, J. M., Eds.; MRS: Pittsburgh, 1988; Vol. 111, p 59. (32) Yagasaki, E.; Masel, R. I. Surf.Sci. 1990, 226, 51. (33) Tomita, A.; Sato, N.; Tamai, Y. Carbon 1974, 12, 143. (34) Baker, R. T. K.; Shenvood, R. D.; Dumesic, J. A. J . Catal. 1980,66, 56. (35) Holstein, W. L.; Boudart, M. J. Caral. 1981, 72, 328. (36) Yang, R. T.; Gcethel, P. J.; Schwartz, J. M.; Lund, C. R. F. J . Caral. 1990, 122,206. (37) Yang, R. T.; Chen, J. P. J . Catal. 1989, 115, 5 2 . (38) Kim, M. S.;Rodriguez, N. M.; Baker, R. T. K. J . Caral. 1992, 134, 253. (39) Weisweiler, W.; Mahadevan, V. High Temp.-High Pressures 1972, 4, 27. (40) Naidich, Yu. V.; Kolesnichenko, G. A. Parosh Mer. 1961, 6, 55. (41) Humenik, M.; Hall, D. W.; van Alsten, R. L. Mer. Progr. 1962, 4, 101. (42) Naidich, Yu. V.; Petevertailo, V. M.; Nevodnik, G.M. Parosh Met. 1971, 11, 58. (43) Stull, D. R.; Westrum, E. F., Jr.; Sinke, G.C. The Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969; p 334.
Photochemistry on Surfaces. Photodegradation of 1,3-Diphenyiisobenzofuran over Metal Oxide Particles K. Vinodgopal* Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408
and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: December 13, 1991; In Final Form: March 9, 1992) Steady-state and diffuse reflectance laser flash photolyses have been camed out to elucidate the mechanism of photodegradation of 1,3-diphenylisobenzofuran (DPBF) on solid surfaces of AlzO3, TiOz, and ZnO. In the absence of oxygen, the semiconductor supports TiOz and ZnO catalyze the photodegradation by accepting electrons from excited DPBF. The fluorescence of degassed DPBF on Ti02 and ZnO is quenched relative to that on alumina, thereby offering independent c o d m a t i o n of charge transfer. In oxygenated samples, the primary mechanism of photodegradation involves reaction with singlet oxygen. This is confirmed by the studies on the insulator surface A1203,where significant degradation is observed only in the case of air-equilibrated samples. The dependence of the rate of DPBF photodegradation on the surface coverage indicates that only the molecules that are in direct contact with the surface undergo photodegradation. The results that highlight the role of the support material in guiding the course of a photochemical reaction are described here.
Introduction Polychlorinated and polybrominated dibenzofurans (PCDF, PBDF) are a group of extremely hazardous chemicals whose toxicity has been well established.'-$ The hazardous nature of these chemicals is rendered more acute by their widespread presence. In addition to being formed as byproducts in the in-
0022-3654/92/2096-5053$03.00/0
dustrial manufacture of polychlorinated biphenyls and chlorophenols, etc.,6 these furans are also formed following the thermal combustion of these above compounds. ~OnSequentlY,their presence has been detected as byproducts from the incineration Of both "kip1 and industrial waste? More threatening their presence in aquatic sediments, in marine and freshwater fish, and
0 1992 American Chemical Society
5054 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Vinodgopal and Kamat
SCHEME I: Sensitization of a Semiconductor Material as a Means of Degrading Organic Substrates
Xenon lamp
' -
w
s+
Products
in human fat, blood, and milk.s The ubiquitous and hazardous nature of these furans has given rise to several methods to degrade them to less environmentally harmful products, including a dechlorination process involving treatment with potassium polyethylene g l ~ c o l a t e . Naturally ~ occurring colored compounds such as humic acids can also act as photosensitizers in the degradation of trace organic contaminants.I0 Further, naturally occurring silica, alumina, titania, and clays provide ordered, two-dimensional environments for effecting and controlling photochemical processes more efficiently than can be attained in homogeneous solutions. However, little effort has been made to study how the intrinsic properties of the support material can influence the photochemical degradation of organic compounds. In recent years, semiconductor particulate systems have been used to degrade organic pollutants (e&, degradation of chlorophenols on Ti02 In such systems, the semiconductor particles are excited with ultraviolet or visible light to induce charge separation. The photogenerated holes would then oxidize the pollutant. An alternative approach is to excite the adsorbed organic material and then inject charge from the excited organics into the semiconductor particle. The oxidized form of the organic material can then undergo further degradation. This process which is commonly referred as photosensitization is extensively used in photoelectrochemistry and imaging science. The principle of such a process is illustrated in Scheme I. It has been recently shown by usI6 that the dye rose bengal upon photoexcitation undergoes complete degradation on the TiOz surface as the excited dye participates in the charge injection process. The feasibility of such a process in which a colored substrate itself acts as a sensitizer and initiates its own degradation has not yet been explored in greater detail. The advantage of this process is the utilization of visible light for degrading colored pollutants. In the present study, we have used 1,3-diphenylisobenzofuran (DPBF) as a model compound since it strongly absorbs ySH5
CSH5 DPBF
in the visible and is photochemically reactive in solution^^^-'^ and on surfaces.20 Although DPBF itself is not a major chemical pollutant, its photochemical study on oxide surfaces can yield valuable information regarding the degradation of other colored pollutants with visible light. Steady-state photolysis and diffuse reflectance laser flash photolysis experiments which elucidate the mechanistic and kinetic aspects of DPBF photodegradation on A1203,Ti02, and ZnO surfaces are described here.
Experimental Section Materials. 1,3-Diphenylisobenzofuran(DPBF) was obtained from Aldrich. Ti02 and A1203powders were gift samples from Degussa Corp. TiOz has a particle diameter of 30 nm and BET surface area of 50 m2/g. A203has a particle diameter of 20 nm
U aamplr
Figure 1. Schematic diagram of the diffuse reflectance laser flash pho-
tolysis setup. and BET surface area 100 m2/g-'. ZnO powder was obtained from Johnson Mathey Chemicals Ltd. (Puratronic grade). The surface area of the ZnO powder was 1 m2/g as determined by BET method (Quantrachrome Quantasorb). DPBF was coated onto the surface of oxides by dispersing the individual oxide in acetonitrile and adding a known amount of the furan solution. The suspension was stirred for 2-4 h in the dark and then subjected to vacuum by evaporating the solvent. Samples of DPBF coated Al2O3, Ti02, and ZnO were prepared corresponding to quartermonolayer, monolayer, and four-monolayer coverages. The amount of DPBF required to achieve necessary coverage was determined from the surface area of the particle and an assumed area occupied by a single DPBF molecule. For example, if we assume the area occupied by each DPBF molecule on the A1203 surfaces is 1 nm2, the amount required to achieve a monolayer coverage corresponds to 0.16 mmol DPBF/g of A 1 2 0 3 . Similarly for Ti02,0.08 "01 DPBF/g of support corresponds to monolayer coverage. ZnO samples were also prepared with a similar coverage (Le., 0.08 mmol of DPBF/g of ZnO). To prevent degradation during storage, samples were prepared on the day previous to the measurements and stored in the dark in a vacuum desiccator. The samples prepared by this method yield DPBF in both the adsorbed and crystalline state over the surface of these particles. It is to be noted that the DPBF concentrations are only estimates based upon an assumed area for a single DPBF molecule and that there is no loss of DPBF during the degassing procedure. Optical Measurements. The diffuse reflectance absorption spectra of DPBF-mated oxide particles were recorded with a Cary 2 19 spectrophotometer with a diffuse reflectance attachment (Harrick Scientific). Corrected emission and excitation spectra of the solid samples were measured with an SLM photon-counting spectrofluorometer in a front face configuration. The measurements on evacuated samples were carried out in a vacuum-tight 10 X 10 X 40 mm3rectangular quartz cell. Steady-state photolysis was carried out with a collimated light beam from a 250-W halogen lamp. Electrochemical experiments were performed with BAS100 electrochemical analyzer. Laser Flash Photolysis Experiments. Time-resolved diffuse reflectance laser flash photolysis experiments were carried out in a vacuum-tight 10 X 10 X 40 mm3 rectangular quartz cell. The dried samples were degassed by subjecting them to vacuum for 3-5 h. The evacuated sample cell containing DPBF on an oxide support was closed with a vacuum-tight Ace stopcock. Before each laser pulse was triggered, the cell was shaken to expose a fresh surface for excitation. Air-saturated samples were prepared by exposing the sample to air for an extended period of time. The 532-nmlaser pulse (10mJ, pulse width 6 ns) from a Quanta Ray DCR-I Nd:YAG laser system was used for the excitation of the sample. A 1 m W xenon lamp was used as the monitoring source. The diffusely reflected monitoring light from the sample was collected and focused onto a monochromator which was fitted to a photomultiplier tube, and the photomultiplier output was input to a Tektronix 7912A digitizer. The schematic diagram of the diffuse reflectance laser flash photolysis setup is shown in Figure
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5055
Photochemistry on Surfaces 0.6
a
0.5
-
0.4
-
0.3
-
A
(u
\
% I
-
0. O aI2
t
\
.-_ _ ---_ - b ----_ --1
1
I
n na
"*""I
440
400
520
560
600
640
Wavelength, nm Figure 3. Emission spectra of degassed DPBF (monolayer coverage) coated on (a) A1203(0.16 mmol/g of AZO3),(b) TiOz (0.08 mmol/g of TiO,), and (c) ZnO (0.08mmol/g of ZnO). The excitation wavelength was 380 nm.
- 1
\
0.02
I
b
\
\
I
TABLE I: Photodegradation of DPBF over Oxide Surfaces ECB?
I
Wavelang t h , nm Figure 2. Diffuse reflectance spectra of degassed DPBF coated on (A) Ti0, and (B) ZnO at coverages correspondingto 0.08 mmol of DPBF/g of support. The spectra were recorded (a) before and (b) after irradiation with visible light for 21 min.
1. A typical experiment consisted of 1-10 replicate shots/ measurement, and the average signal was processed with an LSI-11 microprocessor interfaced to a VAX 11/70 computer.
ReSdts Absorption Characteristics of DPBF on Oxide Surfaces. The diffuse reflectance spectra of DPBF on Ti02 and ZnO surface are described in Figure 2. DPBF/Ti02 and DPBF/ZnO samples absorb strongly in the visible with maxima at 425 and 435 nm, respectively. The absorption maximum of the DPBF/A1203 sample was also around 425 nm. Compared to the solution spectrum, the absorption band on the oxide surface is broad and the absorption tail extends into the region 490-500 nm. The broadening of the DPBF absorption band is attributed to its interaction with the surface of the support material. It has been shown earlie?' that the interaction between the adsorbed dye and the support material can alter the energetics of the electronically excited molecule. When air-equilibratedsamples were irradiated with visible light, DPBF readily underwent degradation over the oxide surfaces. This is evident from the diffw reflectance spectra recorded after steady state photolysis (spectra b in Figure 2). The photodegradation of DPBF over Ti02 and ZnO surface occurred in both airequilibrated and degassed samples. However, on A120, surface the degradation was seen only in air-equilibrated samples. The products formed during photolysis exhibited weak absorption in the 420-460-nm region. This weak absorption persisted even when the photolysis was extended for longer duration. Emission Characteristics of DPBF on Oxide Surfaces. The emission spectra of degassed samples of DPBF on A1203,Ti02, and ZnO are shown in Figure 3. The emission characteristics
support V vs ZnO TiOz TiO, TiO,
NHE
DPBFb coverage, M
-0.5
1 1 1 0.25
-0.5
4.0
-0.2 -0.5
~ ~ / min 2 , ~ deair(rel) gassed equilibrated &f
1.0 0.35 0.23
>18" 3.5 1.0 0.6 >18'
1.3 0.35
0.4
*
"Conduction band energy of the semiconductor. 1 M corresponds to a coverage of 83 pmol of DPBF/g of TiO,, 83 pmol of DPBF/g of ZnO, 166 pmol of DPBF/g of A1203. 'Time required for the initial (1 - R)'/2R value to be decreased by half. "Only 25% decrease was observed during 18-min irraaiation.
