J. Phys. Chem. 1995,99, 335-344
335
Photooxidation of CH3Cl on TiOz(110) Single Crystal and Powdered Ti02 Surfaces Jason C. S. Wong, Amy Linsebigler, Guangquan Lu, Jingfu Fan, and John T. Yates, Jr.* Su$ace Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: July 20, 1994; In Final Form: October 17, 1994@
The photooxidation of CH3C1 on Ti02 has been investigated using two approaches. In the fust, TiOz(l10) (rutile) containing oxygen anion vacancy defect sites has been studied using temperature-programmed desorption to follow the production of photooxidation products. In the second approach, powdered Ti02 (mainly anatase) has been studied using transmission infrared spectroscopy. On TiOz(110) the observed products are H2C0, CO, H20, and HC1. On powdered Ti02, the products are CO, COZ,H2O (as surface OH groups), HCl, and CH2C12. The dissimilarities are postulated to be due to two factors: (1) different surface site distributions for the two types of Ti02 surface; (2) higher pressure of reactants in the powdered Ti02 investigation. Both sets of experiments were done in the absence of water and demonstrate that water is unnecessary for the photooxidation reaction. However, 0 2 is crucial for the photooxidation of CH3C1. The formation of CHzCl2 as one of the photooxidation products over powdered Ti02 suggests that a free-radical-mediated mechanism exists in this case.
I. Introduction The use of semiconductor photocatalysts such as Ti02, ZnO, and CdS to destroy environmental contaminants such as halogenated hydrocarbons, herbicides, surfactants, and organophosphorus compounds is a topic of current interest.*-4 This is because (1) the semiconductor photocatalysts are generally nontoxic, inexpensive, and long-lived, (2) the energy source required for the degradation reaction is sunlight, and (3) the semiconductor photocatalysts are able to treat pollutants satisfactorily in both gaseous and aqueous It is known that Ti02 is one of the most active and stable photocatalysts. Therefore, most of the photocatalytic studies involve TiO2. 1-4 Chlorinated hydrocarbons, e.g., polychlorinated biphenyl (a pesticide) and trichloroethylene (a common industrial degreasing solvent), form one of the major classes of environmental pollutants. Extensive studies of the photocatalytic degradation of chlorinated hydrocarbons on Ti02 have been reported.' It has been demonstrated that the complete mineralization of chlorinated hydrocarbons to form C02, H20, and HCl can be achieved by using colloidal Ti02 in aqueous solution saturated with oxygen g a ~ . ~In- ~addition, both the kinetics and the identification of intermediates of the photodegradation of chlorinated hydrocarbons have been r e p ~ r t e d . ~ - ' ~ It is now generally accepted that the photocatalytic reaction is initiated by bandgap photoexcitation of the Ti02 semiconductor with UV light having energy equal to or larger than the band gap of Ti02 (3.2 eV). Electrons are photoexcited to the conduction band from the valence band producing electronhole pairs within the Ti02 solid. Some of the electrons and holes migrate to the Ti02 surface and initiate redox reactions with adsorbates through interfacial electron It is proposed that adsorbed oxygen molecules act as electron scavengers and combine with electrons to form superoxides ( 0 2 * - ) which initiate complete degradation of the chlorinated hydrocarbons. In addition, holes are believed to be trapped by adsorbed water molecules or hydroxyl (OH) groups on the oxide surface to produce OH' radicals6J1which are also able to attack chlorinated hydrocarbon^.^^^^^^^^ The above ideas are supported by the following experimental evidence: (1) incomplete min@
Abstract published in Advance ACS Abstracts, December 1, 1994.
eralization of organic compounds under the condition of insufficient supply of oxygen, (2) the detection of 'OH radicals as spin-trapped adducts during the UV irradiation of Ti02 in aqueous suspension, and (3) the isolation of hydroxylated intermediates in some photocatalytic degradation p r o c e ~ s e s . ~ ~ ~ In spite of the above supporting evidence, the exact role of 0 2 and Hz0 (OH) and the detailed mechanism of the photoreaction are still controversial. Okamoto et a1.I2 suggested that hydroxyl radicals were formed not only via holes and H20 but also via electrons and oxygen as shown by the following equations:
2H0,'
- + -'03
H202
+
H202 0,'-
H202
hv
0,
(3)
2'03
-I-OH-
(4)
+ 0,
(5)
Draper et a1.13 observed that direct electron transfer oxidation instead of 'OH radical mediated oxidation is responsible for the oxidation of a variety of electron transfer agents on Ti02 in aqueous suspension. Stafford et al.14 showed that 4-chloropheno1 can be photooxidized to hydroquinone on the Ti02 surface in the absence of water or oxygen. Most of the Ti02 photooxidation studies in the literature were carried out either in aqueous suspensions or under inert atmosphere without precise control of the nature of the adsorbed species (e.g., H20,C02, and 0 2 ) and their coverages on the Ti02 s ~ r f a c e . ~Therefore, .~ the active surface sites on the Ti02 and the role of oxygen and water (OH groups) cannot be clearly identified and studied. In order to get a correct and complete picture about the basic mechanism of the surface photooxidation reaction, here, we employ modern ultrahigh-vacuum (UHV) surface science techniques to study, at the Ti02 gastsolid interface, the photooxidation of chloromethane (CH3C1). The advantages of using UHV techniques is that one is able to have precise control of
0 1995 American Chemical Society QQ22-3654/95/2Q99-Q335$Q9.QQ/Q
Wong et al.
336 J. Phys. Chem., Vol. 99, No. 1, 1995
Apertured QMS
hv
To I R Detector
\
-
7KBr Window
,
To Vacuum System (MS+GC)
/ITPCIIP(
sapphire/ Window
' 4
(To Supporting Plate]
[Thermocouple]
T
Catalyst Supported on Tungsten Grid c KBr Window
IR Beam
--
c r7 >,u 111111
Figure 1. Design of crystal holder for photochemistry and TPD measurement from the TiOz(ll0) single crystal. the coverages and the kinds of surface sites and adsorbed species on the Ti02 surface. Chloromethane, a simple model chlorinated hydrocarbon, was chosen for these initial studies. In this study, the photooxidation of chloromethane was canied out in parallel experiments on both the single crystal Ti02 (1 10) surface and on a high-surface-area powdered TiOz. The welldefined surface geometry of the Ti02( 110) single crystal allows one to accurately identify and control the coverages of adsorbates and surface sites. This simple model catalyst enables one to observe the physical interaction and chemical reaction between the adsorbates and the surface sites. The high-surfacearea powdered Ti02 (Degussa P-25) is the one of the most commonly employed photocatalysts used in the destruction of pollutants.' This practical catalyst, possessing several different kinds of crystallographic planes and defect sites, provides additional important insight about the photooxidation of chlorinated hydrocarbons on a real photocatalyst. The results obtained from these two different surfaces are compared and contrasted.
11. Experimental Section The experiments were performed on two different vacuum systems to be described below. Also, two major measurement techniques, temperature-programmed desorption (TPD) for the single crystal and transmission infrared spectroscopy (TIR) for the powdered catalyst, were used to monitor the products as well as the extent of the photooxidation of CH3C1 on the Ti02 surfaces. A. Ti02 (110) Study. The photooxidation of CH3Cl on the TiOz(110) surface was carried out in an ultrahigh-vacuum system15 (with base pressure < 1 x Torr) equipped with (1) a differentially pumped and aperture-sampling UTI IOOC quadrupole mass spectrometer (QMS) multiplexed with a VTI computer interface for TPD measurement, (2) a Perkin-Elmer single pass cylindrical mirror electron analyzer (CMA) for Auger electron spectroscopy (AES), (3) a home-built low-energy electron diffraction (LEED) apparatus for surface structure characterization, (4) an Ar+ ion sputtering gun for cleaning the surface, and (5) a molecular beam doser, quantitatively calibrated to deliver a known uniform flux of adsorbate to the crystal. The TiOz( 110) single crystal was obtained from Commercial Crystal Laboratories Inc. and is mounted by four tantalum clips onto an inert tantalum supporting plate for heating and cooling as shown in Figure 1. A type K thermocouple was inserted into a precut slot at one of the comers of the crystal and was fixed by a high-temperature ceramic adhesive (Aremco 571), so that the TiOz( 110) temperature can be measured accurately.
Figure 2. Schematic diagram showing the optical design of the IR cell for the simultaneous photochemistry and IR spectroscopy on the powdered photocatalyst. With the aid of a temperature controller (Honeywell DC521), the crystal temperature can be linearly ramped between 100 and 1000 K by passing electrical current through the tungsten heating leads embedded into the tantalum supporting plate. The crystal was cleaned by Ar+ ion sputtering. The defectfree surface was prepared by annealing the clean surface at 900 K in an oxygen beam followed by continuous exposure to oxygen at 300 K for at least 1 h. The crystal cleanliness and surface order were checked by AES and LEED, respectively. A 350 W high-pressure mercury arc lamp (Oriel Corp.) was used as the UV light source. It is equipped with a two-element UV fused silica condensing lens, a water filter, an iris diaphragm, and a filter holder. The UV light beam was focused at normal incidence (through a UV grade sapphire window on the UHV chamber) onto the crystal surface. The full arc was used in all of the reported experiments except as otherwise specified, and the energy density of the full arc received by the crystal was 0.24 J s-l cm-2 as measured by a calibrated ph~todiode.'~Line-of-sight photodesorption measurements could also be carried out by positioning the crystal as shown in Figure 1 with UV light irradiating the Ti02 crystal at an angle of incidence of 60" to the surface normal. The defect-free TiOz(110) surface is not active for the photooxidation of CH3C1.l6 Therefore, prior to performing every single photoexperiment, the Ti02 crystal was activated by temperature-programmed annealing in vacuum to 900 K momentarily in order to create defect sites (oxygen vacancies) on the s ~ r f a c e . ~The ~ - ~crystal ~ was then cooled to 103 K followed by dosing appropriate reactants (e.g., CH3C1, 0 2 , and H2O) using a calibrated capillary array The photoreaction was then carried out by irradiating the surface with UV light for 10 min. Unreacted CH3C1 and the adsorbed photoreaction products on the surface were analyzed by TPD measurement with a heating rate of 0.5 K s-l. Control TPD measurements under the exact experimental conditions but without UV light irradiation were performed as reference experiments. B. Powdered Ti02 Study. The details of the bakeable stainless steel IR cell used in this work had been reported p r e v i o ~ s l yexcept , ~ ~ the following: (1)The CaFz windows were replaced by KBr windows which are sealed by Viton O-rings and differentially pumped. This allows IR transmission down to 400 cm-'. ( 2 ) A 2.4 cm diameter UV grade sapphire viewport was installed. This provides an optical window (1606000 nm) for the W light used in the photochemistry, as shown in Figure 2. The IR cell is connected to a stainless steel vacuum system pumped by a 60 L s-l turbomolecular pump and a 30 L s-l ion pump, and the base pressure is < 1 x IO-* Torr. The system
J. Phys. Chem., Vol. 99, No. 1, 1995 337
Photooxidation of CH3Cl on Ti02 Surfaces also contains a differentially pumped quadrupole mass spectrometer (Dycor Electronics Inc. MlOOM, range 1-100 amu) for leak checking and gas analysis through a Granville-Phillips leak valve. The pressure of gaseous reactants is measured by an MKS Baratron (1 16A) capacitance manometer (range lo3 Torr). A Hewlett-Packard gas chromatograph (5890 Series II) equipped with a flame ionization detector is also connected to the vacuum system for gas analysis. Inside the IR cell, the Ti02 catalyst is supported on a 0.0025 cm thick tungsten grid which is held rigidly to a power/ thermocouple feedthrough via a pair of nickel clamps. The temperature of the sample is measured using a K-type thermocouple spot-welded on the top-central region of the tungsten grid. Using a temperature controller, the sample temperature can be maintained at any temperature within f l K from 150 to 1400 K by passing electrical current through the high-resistance W grid and using liquid nitrogen coolant.25 The Ti02 used was Degussa titanium dioxide P25 (50 m2/g, mainly anatase). One gram of the Ti02 was mixed with 10 mL of distilled water and then agitated in an ultrasonic bath for about 45 min to form a well-dispersed slurry. Acetone (Mallinckrodt, analytical reagent grade) (90 mL) was then added. The final mixture was then uniformly sprayed by a nitrogen gas pressurized atomizer onto the entire area (5.2 cm2) of the tungsten grid which was heated electrically at about 60 "C to flash evaporate the solvent.24 The net weight of samples sprayed on the tungsten grid was 49.3-66.5 mg (9.48-12.8 mg/cm2). The sprayed Ti02 sample was then mounted into the IR cell and outgassed in situ at 773 K for 36 h under vacuum. The sample was cooled in vacuum to room temperature before the experiment. Infrared spectra were obtained with a nitrogen gas purged Mattson Fourier transform infrared spectrometer (Research Series 1) equipped with a wide band HgCdTe detector. The spectra shown here were recorded with 4 cm-I resolution by averaging 100 to 2000 scans, which takes 0.38 to 7.66 min, depending on the relative signal-to-noise ratio. Difference spectra (Ti02 background subtracted) are shown except where otherwise specified. In addition to monitoring the surface species on the TiO2, IR spectra of gaseous species were also recorded by moving the IR cell to a reproducible preset position, so that only the gas phase with an identical optical path length was sampled by the IR beam. The UV light source is the same mercury arc lamp as the one used in the single crystal work, except a 325 Ifr 50 nm bandpass filter was added. The photoflux of the filtered UV light as measured by a calibrated EG&G (HUV-4OOOB) photodiode15 was 1.1 x 1017photons cm-2 s-l, with an absolute accuracy of &lo%. Figure 2 is a schematic diagram showing the optical alignment of the W lamp with the Ti02 photocatalyst in the IR cell for the simultaneous photochemistry and IR spectroscopic measurements. The Ti02 photocatalyst supported on the tungsten grid is positioned in such a way that both the IR beam from the FTIR spectrometer and the UV light from the mercury arc lamp are focused at an angle of incidence of about 45" to the normal of the grid. The orthogonal arrangement of W light with the IR beam ensures that a minimal amount of UV light can get into the IR detector except light scattered by the inner wall of the stainless steel IR cell. The interference of the UV light on the IR spectra collected is negligible and is proven by showing that identical IR spectra (in terms of absorbance, S/N ratio, and spectral peak position) were recorded no matter whether the UV light was on or off. The oxygen gas was obtained from Matheson in a breakseal glass bulb with 99.9999% purity. Chloromethane was also
;+
I
EXPOSURE 5x10"
CH3Cl/cmZ
'fi'."" z X 1015
150
200
250
300
TEMPERATURE ( K )
Figure 3. Temperature-programmeddesorption spectra of CH3Cl from a 900 K-annealed TiOz(110) surface showing a monolayer and multilayers of CH&I formed as a function of CHsC1 exposure. The temperature of CH3CI adsorption is 103 K. The heating rate during TPD measurement is 0.5 Ws.
purchased from Matheson with 99.5% purity and was transferred to a glass storage bulb using freeze-pump-thaw purification procedures.
III. Results A. Adsorption of CH3CI on TiOz. 1. On the Ti02(110) Surface. Figure 3 shows a series of temperature-programmed desorption spectra of CH3C1 from the TiOz( 1IO) surface which was activated by annealing at 900 K. They were obtained by dosing the Ti02 single crystal at 103 K with different CH3Cl exposures (number of CH3C1 molecules per unit surface area of TiOz) as shown in Figure 3. The heating rate during TPD measurements was 0.5 K s-l, and the parent ion (CH3C1+ ion, amu 50) signal was monitored by the QMS. At low exposures ( 5 2 x 1015 CH3Cl/cm2), one desorption peak at about 180195 K was observed, and it was attributed to the chemisorbed CH3C1 on the Ti02 below the monolayer coverage. As the CH3C1 exposure increased, a second desorption peak at a lower temperature (125-135 K) was found, and it was assigned to the multilayer CH3C1 which is condensed (physisorbed) on the top of the chemisorbed CH3C1. The desorption peak temperature of the chemisorbed C H F l is shifted to a lower temperature as the CH3Cl exposure increases. This could be explained by the fact that the repulsive intermolecular interaction between CH3C1 molecules at higher coverages decreases the chemisorptive strength of CH3Cl on the Ti02. A submonolayer coverage of CH3Cl was used in the photooxidation experiments. 2. On Powdered Ti02. a. Low-TemperatureAdsorption of CHjCl on Ti02. Figure 4 shows a series of FTIR spectra of CH3Cl adsorbed on the powdered Ti02 at 170 K as an increasing number of moles of C H F 1 was admitted into the IR cell. The powdered Ti02 was activated in situ under vacuum (< 1 x Torr) in the IR cell at 773 K for at least 36 h. (Therefore, nothing was chemisorbed on the Ti02 surface except a very tiny amount of OH groups as proven by the IR spectrum.26) Then the Ti02 was cooled under vacuum to 170 K before the CH3C1 admission. A band is observed at 2958 cm-* and is assigned to the symmetric CH3 stretching mode of the adsorbed CH3C1. Two IR bands at 1439 and 1351 cm-' are also found and are assigned respectively to the antisymmetric and symmetric CH3 deformation modes of the adsorbed CH3C1. All of the IR spectra in Figure 4 were recorded with pressure under 1 x Torr, so the contribution to the IR bands from any gaseous CH3Cl can be neglected. These three IR bands are the
338 J. Phys. Chem., Vol. 99, No. I , 1995
L CHICl
-
admitted
14.7
A 3000
2900
i b 0 0 1500 1400 1300 1200
WAVENU M BERS (cm-' )
Figure 4. Infrared spectra of CH3C1 adsorbed on a 773 K-activated powdered Ti02 at 170 K showing the development of the C-H stretching and deformation bands of CH3Cl(a) as a function of CH3Cl exposure. Resolution of the IR spectra is 4 cm-'.
most intense IR bands recorded. The assignment of these IR bands is based on the IR studies of the gaseous and crystalline CH3C1.27,28A comparison of IR bands frequencies and intensities of CH3C1 in different states is given in Table 1. b. Room Temperature Adsorption of CHjCl on the TiOz. Figure 5 shows a series of FTIR spectra, in selected IR regions, of CH3Cl(g) and adsorbed CH3C1 on the 773 K-activated Ti02 at 315 K with an increasing equilibrium pressure of CH3Cl in the IR cell as shown. The spectra on the left panel show the IR features of both the adsorbed CH3C1 surface species on the Ti02 and the gaseous CH3C1. Three IR bands at 2964, 1444, and 1346 cm-' were observed, and they are assigned respectively to v,(C-H), d,(C-H), and d,(C-H) modes of the adsorbed CH3C1 with the same argument as discussed before. These frequencies are also tabulated in Table 1 for comparison. The corresponding IR spectra of the gaseous CH3C1 only are shown on the right panel of Figure 5 . The IR feature shown is the PQR contour of the symmetric CH3 stretching mode of CH3Cl(g),29 and it is the strongest mode observed for CH3Cl(g) above 1000 cm-'. Obviously, the intensity of the IR band of CH3Cl(g) (note: the absorbance scale of the gas phase IR spectra is magnified by 10 times) is much smaller than that of the adsorbed CH3C1. Therefore, the IR bands observed through the powdered Ti02 (shown in the left panel) are mainly derived from the adsorbed CH3C1 surface species. After the evacuation of the CH3Cl in the IR cell to vacuum (< 1 x Torr), IR spectra were taken again through the Ti02 and the blank and are shown in the top part of Figure 5 . All of the adsorbed CH3C1 IR bands disappeared after the evacuation. This proves that CH3C1 is not irreversibly adsorbed at 315 K on the 773 K-activated TiOz. B. Photooxidation of CH3Cl on TiOz. 1. On the TiOz(110) Surface at 130 K . At 103 K, the 900 K annealed TiO2(110) was first dosed with 4.56 x 1013/cm2exposure (-0.01 monolayer) of lsOz and then with 3.85 x 1013/cm2exposure (-0.01 monolayer) of CH3C1. The surface was then exposed to the focused full arc UV light at normal incidence for 10 min. During the UV illumination of the crystal, the temperature of the crystal never exceeded 130 K. After 10 min of the UV irradiation, the TPD measurement was performed. Figure 6 shows the TPD spectra of products formed from the photooxidation of CH3C1 with 1 8 0 2 . The desorption peak at amu 30
Wong et al. (Cl80+) was detected at 150 K, and it is due to Cl8O formed from the photooxidation reaction. Isotopic experiments with 180-labeled Ti02( 110) indicate that oxygen from the Ti02 does not participate in the CO product channel. At a higher desorption temperature, 335 K, three desorption peaks with the same desorption pattern were detected at amu 30 (Cl8O+), 31 (HCl8O+), and 32 (HzC180+). They are assigned to the parent ion and the cracking fragments of formaldehyde, H2C180, which is also a product of the photooxidation reaction. This is c o n f i i e d by the fact that the relative intensities of these three peaks correspond to the mass spectral cracking pattern for pure H2CI80(g). In addition to Cl8O and H@O, a peak with amu 20 (H2180+) was detected also at about 335 K, and this is assigned to the H2180 formed from the photoreaction. The TPD spectrum of CH3C1 after the photooxidation reaction was also recorded (not shown)16 and shows from the integrated peak area that about 50% of the CH3C1 was consumed after 10 min of the photoreaction. The TPD spectrum of a control experiment performed under the same conditions but without UV irradiation is also shown in Figure 6. The control experiment clearly shows that none of the Cl8O, H@O, and H2180 products were produced without UV light illumination, and there is no thermal reaction between CH3Cl and l 8 0 2 . It is important to point out that l6O (from the lattice oxygen of the TiOz) labeled products (H2O and H2CO) were also detected by the TPD measurement; however, at least 70% of the products are labeled by l8O isotope.16 Since molecular oxygen does not exchange with the lattice oxygen of the Ti02 at such a low temperature (103 K),30this suggests that the lattice oxygen of the Ti02 is also involved in minor reaction channel(s) in the photooxidation of CH3Cl on the Ti02. 2. On the Powdered Ti02 Surface at 315 K. The photooxidation of CH3C1 with 0 2 was carried out by first introducing 109 mTorr of CH3Cl and 453 mTorr of oxygen into the IR cell at 300 K containing the 773 K-activated Ti02. Before the irradiation of the Ti02 with UV light, IR spectra through the gas phase only and through the Ti02 were taken, and then the Ti02 was exposed to the focused and filtered UV (325 f 50 nm) light. During the W irradiation of the powdered Ti02, the temperature of the Ti02 never exceeded 3 15 K. The course of the photooxidation of CH3C1 was continuously measured by in situ IR measurements as described below. An IR spectrum through the Ti02 was taken first, and it was immediately followed by taking the second IR spectrum through the gas phase only. Each IR spectrum was obtained by signalaveraging 200 scans which took 0.76 min; therefore, a time lag of about 1 min exists between the first (through TiOz) and the second (through gas phase only) IR spectrum. If the photoreaction rate of CH3Cl(g) is slow enough that the concentrations of gaseous species do not change much within the 1 min time lag, the gas phase contribution in the first spectrum can be removed by subtracting out the second spectrum. These two IR spectra form a pair of IR spectra. A pair of IR spectra was recorded every 5 min in the first 30 min of the photooxidation reaction, then every 10-15 min in the next 3 h of the reaction, and finally every 30 min in the last 5 h of the reaction. a. Consumption of CH3Cl. Figure 7 shows a series of IR spectra, in selected IR regions, recorded with increasing UV irradiation time. These IR spectra represent only the IR features of surface species produced or consumed on the powdered Ti02 surface. Before the irradiation of the Ti02 with UV light (Le., W time = 0 min), the major surface species found on the Ti02 is the adsorbed CH3C1. This is characterized by a sharp IR peak in the v(C-H) region at 2964 cm-I and two medium
J. Phys. Chem., Vol. 99, No. 1, 1995 339
Photooxidation of CH3C1 on Ti02 Surfaces
TABLE 1: Comparison of IR Vibrational Frequencies and Intensities of Different States of CHJCl CHsCl(g) at 300 K, ref 28 CHsCl(s) at 180 K, ref 27 CH3Cl(a)at 170 K, this work CH&l(a) at 315 K, this work assignment freq (cm-l) int freq (cm-l) int freq (cm-I) int freq (cm-l) int vl (sym C-H str) 2966.2 vs 295 1.2 S 2958 vs 2964 vs 2958.2 S (doublet) v5 (antisym C-H def) 1454.6 m 1437.1 vs 1439 m 1444 m 1441.3 vs vs 1444.7 (triplet) vZ(symC-H def) 1354.9 S 1335.7 S 1351 m 1346 m 1346.1 S (doublet) Gas
+
S u r f a c e Species
la(c-H)]
'
G a s Only
I
I.S(C-H)I
I w
x10
-
x10
-
0
-
0 Z
I
U
148
-1 ' 66-1
3000
zwo
/-/
iaoo
ism i+oo
1300
A
izoo w o o
x
1
zoo0
WAVENUMBERS ( c m - ' )
Figure 5. Left panel showing infrared spectra of CHjCl(g) and the adsorbed CH3Cl on a 773 K-activated powdered Ti02 at 315 K with an increasing equilibrium partial pressure of CH3C1 in the IR cell. Right panel showing infrared spectra of CH&l(g) only, with identical pressures. The contribution of CH3Cl(g) to the IR spectra of CH3Cl(a) is insignificant. Evacuation of the IR cell to vacuum desorbs all of the CH3Cl(a). This demonstrates the reversible chemisorption of CH3C1 on the Ti02 at 315 K. absorbance in the ~(c-H) regionat 1444 and 1346 cm-i as discussed previously. Shortly after the exposure of the Ti02 to the W light, dramatic changes of the IR spectra of the surface species were observed. In the v(C-H) region, the intensity of the peak derived from the adsorbed CH3Cl at 2964 cm-I gradually decreased at a function of irradiation time and eventually disappeared after 200 min of the UV light exposure. This indicates all of the adsorbed CH3C1 was photooxidized on the Ti02 surface. The changes of the d(C-H) modes of adsorbed CH3Cl are difficult to observe clearly, because they overlap with other developing IR features which are derived from the products of the photoreaction as will be described below. b. Formation of Water (OH Groups). In the v(0H) region, IR features at 3663, 3500, 3480, and 3281 cm-' were found to develop and increase in intensity as the photooxidation reaction of CH3C1 proceeded. These IR features are assigned to the stretching modes of OH groups and chemisorbed water on the Ti02 surface. Accompanying these developing v(0H) IR features, an IR band at 1617 cm-' in the &OH) region was also observed to develop and is assigned to the corresponding deformation mode of the OH groups and chemisorbed water on the Ti02 surface. All of these IR band assignments for OH groups and chemisorbed water are in agreement with the earlier detailed studies reported in the l i t e r a t ~ r e . ~ l -This ~ ~ shows clearly that water is one of the products formed during the photooxidation of CH3Cl on the Ti02 surface.
too
200
300
400
100
TE M P ERATU RE( K )
Figure 6. Temperature-programmed desorption spectra of products formed (CI8O,HzCL80,and Hzl*O)from the photooxidation of CH3Cl with 1 8 0 z on a 900 K-annealed TiOz(110) surface at 135 K. The W exposure time is 10 min. A control experiment performed without W light shows no reaction between CH3Cl
,So2.
c. Production of CO. In the v(C0) region, a sharp IR band at 2127 cm-' was observed to develop and increase in intensity as the adsorbed CH3Cl was decomposed. This band is assigned to the CO chemisorbed on the Ti02 surface. The assignment of this band is based on a control CO adsorption experiment, involving only CO and a 773 K-activated Ti02 sample at 315 K, which also yielded a band at 2125 cm-' as well as two bands at 2208 and 2188 cm-'. The adsorption of CO on Ti02 (both anatase and rutile) under different experimental conditions has been previously studied by White,36 D. J. C. Y a t e ~ , ~and l M~rterra.~'The results obtained here are in good agreement with those reported in the l i t e r a t ~ r e , ~except ~ . ~ ~ the . ~ ~v(C0) absorbance at 2125-2127 cm-I which has not been previously reported. The surface site which is responsible for giving the v ( C 0 ) absorbance at 2125-2127 cm-' is still subject to further study. This shows clearly that CO is formed during the photooxidation of CH3C1 on the powdered Ti02 surface. d. Formation of C02 and Carbonate Sulface Species. In the v,(C02) region of Figure 7, an IR absorbance at 2346 cm-' was found to develop during the photoreaction. This feature is assigned to adsorbed C02 on the Ti02 surface. Accompanying the development of this COZ surface species, peaks at 1575, 1438, and 1359 cm-' in the Y ( C O ~ - region ~) were also found to develop during the photoreaction, and they are assigned to
Wong et al.
340 J. Phys. Chem., Vol. 99, No. 1, 1995
--w
I l
4000
3500
3000
2900 2400
2300 2200
1444
13.40
2100 1800 l e 0 0 1400 1200
WAVENUMBERS (em-')
Figure 7. Infrared spectra of the surface species formed during the photooxidation of CH3Cl with 0 2 on a 773 K-activated powdered Ti02 as a function of UV exposure time at 3 15 K. The consumption of CHsCl is identified by the decreasing IR peak at 2964 cm-'. Chemisorbed water (OH groups), CO, COz, and carbonate species are the products of the photoreaction. This is stage one of the photooxidation of CH3C1, in which all of the CH3Cl is decomposed.
the surface carbonate or bicarbonate species formed from the reaction of the Ti02 with C02 (produced from the photooxidation of CH3Cl). The adsorption of COz on Ti02 and the formation of the carbonate and bicarbonate surface species had been previously studied in detail by D. J. C. Yates?l P~-imet?~ White,36Parfitt,38and M ~ r t e r r a .Although ~~ some disagreements regarding the assignment of the IR bands to a specific geometry (Le., bicarbonate, monodentate carbonate, bidentate carbonate, and carboxylate species) are found among these authors, the results obtained here are generally in agreement with those reported.31,35,36,38.39 This clearly proves that C02 and carbonate surface species are produced when CH3C1 is photooxidized on the Ti02 surface. e. Changes of Gas Composition in the IR Cell. In addition to monitoring the surface species on the TiO2, as mentioned earlier, the composition of the gases inside the IR cell was also followed by IR measurements through the blank side. In the same experiment shown in Figure 7, IR spectra of gases, in selected IR regions, with the corresponding UV light exposures are shown in Figure 8. Before the UV irradiation (UV time = 0 min), the only gaseous species in the IR cell was CH3Cl(g), which is characterized by the IR spectrum showing the distinctive PQR contour of the symmetric CH3 stretching mode at 2981, 2964, and 2952 cm-I. Neither CO(g) nor COz(g) was detected before the onset of the photooxidation. Soon after the UV light was turned on, decomposition of CH3Cl(g) took place as indicated by the gradually decreasing absorbance of the v,(CH3) peak of CH3Cl(g) in the IR spectra. All of the CH3Cl(g) was consumed after 200 min of the photoreaction as shown by the total disappearance of the characteristic vs(CH3) peak in the IR spectrum (Figure 8). The corresponding surface species IR spectrum (Figure 7) also shows no CH3Cl adsorbed on the Ti02 at this moment. Accompanying the consumption of CH3Cl(g), the formation of CO(g) and COz(g) is also observed as shown in Figure 8. COz(g) is characterized by the asymmetric stretching IR bands centered at about 2346 cm-1,29and CO(g) is identified by the
w
0
z < m
I 0 Ul
m
CHz rocking mode, of the CHzClz molecule.29 The qualitative identification of the produced CH2C12(g) is further confirmed by the mass spectroscopic analysis of the gases in the IR cell after 5 h of the photoreaction. Mass spectra of the gases from the IR cell were obtained by the QMS in the vacuum system. The fragmentation pattern of the CH2C12(g) observed in the mass spectrum is in excellent agreement with that reported in the literat~re.~' 4. Control Experiments. We have previously reported that C-C1 bonds in CC4 break on Fe(l10) surface and the reactive :CClZ(g) as well as 'CCl3(g) species are It is important to determine whether the CH2Clz(g) is formed from the photooxidation of CH3Cl with 0 2 on the Ti02 surface or from the thermal reaction or photoreaction of C H F l with the stainless steel walls of the IR cell. A control experiment shows that no CHzClz could be produced when 2.2 Torr of C H F l and 3.3 Torr of 0 2 in the empty IR cell were irradiated with the UV light for 3 h at 315 K. This shows that CHzC12(g) is formed during the photooxidation of CH3C1 with 0 2 on the powdered Ti02 surface.
N. Discussion A. Adsorption of CH3CI on TiO2. On the Ti02(110) surface, both a chemisorbed monolayer and physisorbed multilayers of CH3Cl were detected by TPD measurement when CH3C1 was adsorbed at 103 K. The physisorbed CH3C1 desorbs at a maximum rate at 125-135 K; the chemisorbed CH3Cl
Wong et al.
342 J. Phys. Chem., Vol. 99, No. 1, 1995 I
I
1
I
I
0
0
10
200 300 400 UV IRRADIATION TIME (MIN.)
100
I
20
50
40
500
50
800
Figure 10. Gas phase kinetics plot of the photooxidation of CH3C1 with 02 on powdered Ti02 at 315 K. The insert spreads out the data points during the fist 50 min of the reaction.
w
0
z
c K
0 u1
m
c
J
100
1
I
1250
120(
'
WAVEN U M B E RS (c m - ) Figure 11. Infrared spectra of CH*C12(g) formed during the photooxidation of CH3C1 with 0 2 on the powdered Ti02 as a function of W exposure time at 315 K.
desorbs with a maximum rate at 180-195 K, depending on surface coverage. On the powdered TiOl, infrared bands observed in Figure 4 at 170 K (lower than the melting point of CH3C1, 176 K) are derived from the adsorbed CH3C1 on Ti02 instead of the condensed crystalline or liquid CH3C1. This conclusion is supported by the following facts: (1) the band splitting of the vS(C-H) [doublet], d,(C-H) [triplet], and &(C-H) [doublet] bands (which is due to site symmetry and intermolecular coupling) found in crystalline CH3ClZ7is absent in Figure 4; (2) the relative intensities of the IR bands in Figure 4 are different from those reported in crystalline CH3Ctz7 and (3) IR bands of adsorbed CH3C1 with comparable peak frequencies, line shape, fwhm (full width at half-maximum), and relative intensities were also found on the Ti02 surface when CH3C1
was admitted at 315 K (Figure 5 ) where condensation would be impossible. The clean IR spectrum of powdered Ti02 after the evacuation of the adsorbed CH3C1 at room temperature (Figure 5 ) shows clearly that the adsorption of CH3C1 on the powdered Ti02 at 315 K is reversible, and there is no thermal reaction between the Ti02 and CH3C1 at 315 K. The thermal reaction of CH3Cl with A1203 has been studied previ0usly.4~ Methoxy surface species are produced on the A1203 surface at about 400 K;43 this reaction does not occur with Ti02 at 315 K. B. Photooxidation of CH3CI on TiO2. I . Product Distribution. It is expected that the final photooxidation products of CH3Cl with 0 2 are C02, H20, and HCl. In this study, however, the photooxidation of CH3Cl on the TiO2(110) surface yields only CO, HzCO, and H20 on the surface as detected by TPD measurement. Unexpectedly, neither COz nor HC1 could be detected from the photodesorption measurement during UV illumination or from the TPD measurement after 10 min of the photoreaction. However, the formation of HC1 was observed indirectly from the increase in the background partial pressure of HC1 after the photooxidation of CH3C1 experiments were performed (not shown). The C02 formation is observed from a separate photoexperiment at 130 K involving only CO and 0 2 on the TiOz( 110) surface; the produced C02 was immediately photodesorbed from the TiOz(110) surface after it was formed. Therefore, CO2 may be formed also during the photooxidation of CH3C1 in quantities too small to be detected by QMS measurements during UV irradiation. On the powdered TiOz, the photooxidation of CH3C1 produces CO, C02, and HzO as shown by IR measurements. The formation of HCl cannot be identified by the IR measurement because the vibrational frequency of adsorbed HC1 on the powdered Ti02 overlaps with spectral features of OH groups and chemisorbed water on the surface. The production of HC1 was only found indirectly from the increase of the background partial pressure of HC1, as monitored by the QMS, after the CH3C1 photoexperiments. It is interesting to point out that CO is a common product observed from the photooxidation of CH3C1 on both TiOz( 110) and powdered Ti02 samples. This implies that a key surface
Photooxidation of CH3C1 on Ti02 Surfaces
J. Phys. Chem., Vol. 99, No.
site, which is responsible the production of CO, exists both on TiO2( 110) and on powdered Ti02 surfaces. None of the experimental evidence shows that H2CO is produced as a product of the photooxidation of CH3Cl on the powdered Ti02 surface even though effort was devoted to look for it. (For example, photooxidation of CH3Cl on the powdered Ti02 was performed at low temperature and at low oxygen partial pressures; however, CO was the only product found, and no H2CO was detected by IR measurements.) One possible explanation for this difference found on these two surfaces is that the H2CO does form on the powdered Ti02 surface; however, it is kinetically very unstable and is readily oxidized to either CO or C02 after it is formed. Therefore, the steady state concentration of H2CO on the powdered Ti02 surface was so low that it cannot be detected by JR measurements. The extra surface reactivity of H2CO on the powdered Ti02 may be due to the presence of a specific surface defect site on the powdered TiO2; this surface defect site may be absent on the TiO2( 110) single crystal surface. In addition to this proposed Ti02 structural effect, the high partial pressure of 02(g) in a closed system used in the powdered Ti02 study will also enhance the production of COz(g). This is because the reacted or photodesorbed 0 2 from the powdered Ti02 surface can be replenished from the 02(g) in the IR cell, giving a continuous supply of 0 2 for the photoreaction. On the other hand, under the UHV conditions of the TiOz(110) study, the unreacted chemisorbed 0 2 is photode~orbed~~ and pumped immediately out of the system. Hence, oxygen may not be available for the further oxidation of CO to C02 on the Ti02(110) surface under the conditions of the experiment performed here. 2. Role of 0 2 and Adsorbed Water (OH Groups). Control photoexperiments involving CH3C1 on both Ti02( 110) and powdered Ti02 surfaces under the same conditions but without 0 2 have been performed. These experiments show clearly that CH3C1 cannot be photooxidized in the absence of 0 2 . This proves definitely that added 02(g) is a crucial reactant for the photooxidation of CH3Cl on TiO2. Adsorbed water (OH groups) is unnecessary for the onset of the photooxidation of CH3Cl on the Ti02 surfaces. This view is supported by the following experimental evidence: (1) CH3C1 can be photooxidized on the TiOA 110) surface in the absence of water. (2) The powdered Ti02 used in this work is highly dehydroxylated after the 773 K activation for 36 h; only a very small coverage of OH groups is initially present on the surface. CH3C1 can also be photooxidized on such a highly dehydroxylated Ti02 surface without added water, and in fact adsorbed water and associated surface hydroxyl groups are produced during photooxidation (Figure 7). (3) A similar result had been reported for the photooxidation of 4-chlorophenol on powdered Ti02 in the absence of water by Stafford et al.14 Since water is produced during the photooxidation of CH3C1, it is hard to argue that water is not important for the progress of the photoreaction. Further photooxidation experiments with CCl4, which does not produce H20 during photooxidation, will be performed to clarify this argument. 3. Formation of CH2Cl2. The formation of CHzCl2 from CH3Cl suggests a free-radical-mediated mechanism as one reaction process involved in the photooxidation of CH3Cl with 0 2 . A proposed mechanism for the CH2C12 formation is shown by the following equations:
0, 2CH3C1
+ 20,'-
+ e- - 0;-
-
CH300CH,
(1)
+ 2C1- + 0,
(2)
CH,00CH3 CH,O'
+ HC1-
hv
2CH30'
CH30H
+ C1'
+ CH3C1- HC1+ 'CH2C1 'CH2C1 4- C1' + M - CH2C1, + M* C1'
(6)
The first step is the formation of superoxide, 02'-, from the photoexcited electrons and adsorbed molecular oxygen on the Ti02 surface. The superoxide reacts with CH3C1 to produce the dimethyl peroxide, CH300CH3; this reaction has been observed in aprotic solvents by electrochemical methods.44The dimethyl peroxide can be photolyzed by UV light to form the methoxy radical; this reaction has been studied in gas-phase photo~hemistry.4~The methoxy radical removes a hydrogen atom from a HC1 molecule (a product of the photooxidation of CH3Cl) to form a 'C1 radical. This reaction is an analogy reaction of tert-butoxy radical with HCl observed in gas phase.& Further, the formation of chlorine atoms is supported by the detection of the chlorine gas as one of the byproducts of the heterogeneous photooxidation of gaseous chlorinated organics on a powdered Ti02 surface.47 The 'C1 radical then abstracts a hydrogen atom from a CH3C1 molecules to form a 'CH2C1 radical; this step is well-known in organic radical chemistry. The 'CH2Cl radical combines with a chlorine atom in the presence of a third body (M) to form dichloromethane as detected by JR measurements. Steps 2-6 shown above have been observed by others in solution electrochemistry or in gasphase photochemistry experiment^^-^^ and are proposed also as heterogeneous steps on the TiO2. However, none of the intermediate species and byproducts postulated above are seen in the IR measurements which only detect CH2C12(g) as a product. C. Mechanism for the Photooxidation of CHJCI. The results presented here do not allow us to postulate a complete mechanism for the heterogeneous photooxidation of CH3Cl on the Ti02 surface. Some important insights about the photoreaction are indicated below: (1) On the powdered Ti02, our experiments indicate that there are two parallel separate reaction routes which are responsible for the formation of CO and C02. This view is supported by the observation that both CO and C02 are formed with similar kinetics in the first 200 min of the photoreaction as shown in Figure 10. If C02 was solely formed from the photooxidation of CO, the rate of formation of C02 would be proportional to the concentration of CO only; this rate would be highest when the concentration of CO reached the maximum48(Le., UV time = 200 min in Figure 10). However, the slope of C02 formation curve (Figure 10) is definitely not at the maximum when UV time = 200 min. Therefore, a reaction channel which does not produce CO as the intermediate must be involved in the formation of C02 from CH3C1. In addition, there is the third reaction route which is responsible for the conversion of CO to C02 during stage two of the photooxidation of CH3C1 with 02, as indicated by the decline in the CO partial pressure during stage two photooxidation. (2) Similarly, different routes are responsible for the formation of H2CO and CO on the TiO2( 110) surface as clearly indicated by oxygen isotope experiment^.^^ The results show clearly that the oxygen in H20 is definitely involved in the formation of H2CO but not in the CO formation. This implies that two different reaction channels are involved in the photooxidation of CH3C1 on Ti02(110), and they lead to the formation of CO and H2CO.
Wong et al.
344 J. Phys. Chem., Vol. 99, No. 1, 1995 It is very difficult to conclude whether there is a separate channel to convert H2CO photochemically to CO on the Ti02( 110) surface during the photooxidation of CH3Cl with 0 2 . A separate control photoexperiment involving only H2CO and 0 2 on the Ti02(110) surface has demonstrated that CO is a product of the photoreaction. However, in a photooxidation experiment of CH3Cl with D2180 and 1602, there was no l 8 0 isotope labeled Cl8O formed even though l8O isotope labeled H2C180 was detected.49 This is likely due to the limited number of empty surface defect sites (oxygen vacancies) on the TiO2( 110) surface which are consumed by adsorbed CH3Cl and 02, so that defect sites are unavailable under these conditions for the photochemically produced H2C180 to undergo further oxidation to CO.
V. Conclusions By employing temperature-programmed desorption and transmission infrared spectroscopy respectively on a TiOz( 110) single crystal and on a powdered Ti02 surface, the heterogeneous (gas/ solid interface) photooxidation of CH3C1 on these two surfaces was investigated. The following conclusions can be made: 1. CH3C1 is partially photooxidized on the TiOd 110) surface to HzCO, CO, HC1, and H20 at 135 K, under conditions of limited oxygen supply. 2. CH3C1 is completely photooxidized on the 773 K activated powdered Ti02 to C02, HCl, and H20 at 3 15 K under conditions of abundant oxygen supply. CO was also observed as a primary product of the reaction; it can be further photooxidized to C02 with prolonged UV irradiation. 3. The photooxidation of CH3C1 on the Ti02 can take place in the absence of water or OH groups on the surface but not in the absence of added oxygen. 4. On the powdered TiOz, formaldehyde was not observed as a product of the photoreaction. This can be rationalized by postulating the presence of a specific defect site on the surface of powdered Ti02 on which H2CO is kinetically unstable and rapidly converted to CO or C02. 5 . Several parallel reaction routes are believed to be responsible for the formation of H2C0, CO, and COZ during the photooxidation of CH3Cl with 0 2 on the Ti02 surfaces. 6. Production of CH2C12 accompanying the photooxidation of CH3C1 with 0 2 suggests that a free-radical-mediated mechanism also exists for the photooxidation of CH3C1 on the powdered Ti02 surface.
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