19466
J. Phys. Chem. 1996, 100, 19466-19474
Role of Photoinduced Charge Carrier Separation Distance in Heterogeneous Photocatalysis: Oxidative Degradation of CH3OH Vapor in Contact with Pt/TiO2 and Cofumed TiO2-Fe2O3 M. Sadeghi, W. Liu, T-G. Zhang, P. Stavropoulos,* and B. Levy* Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215 ReceiVed: May 9, 1996; In Final Form: September 4, 1996X
As a means of testing the widely held premise of a positive correlation between photoinduced charge carrier separation distance (CCSD) and the photocatalytic activity of particulate photocatalysts, an investigation of the oxidative degradation of CH3OH vapor in contact with coatings of particulate Pt (0-8 wt %)/TiO2 is combined with time-resolved photocharge (TRPC) measurements. Preliminary studies were also performed on several commercially available cofumed Fe2O3-TiO2 (0, 2, and 6-8 wt % Fe) particulate photocatalysts. TRPC is capable of providing a noncontact electronically derived measure of CCSD. Contrary to common wisdom prevalent at the initiation of this study, Pt/TiO2 catalysts displaying the largest CCSD values do not correspond with highest photocatalytic efficiency for the oxidative degradation of CH3OH. Indeed the present investigation supports an extension of a “Russell-like” mechanism for oxidative degradation of organic molecules, where photoelectrons, e-, and photoholes, h+, each initiate the formation of interacting chemical intermediates, i.e. O2•- and HOCH2(OO)•, in close proximity on the catalyst surface, thereby reducing the need for surface diffusion over large distances for the reaction to proceed toward production of the observed HCOOCH3 product. This process competes with charge carrier recombination at small CCSD values, which yield the highest rates of CH3OH consumption and HCOOCH3 formation attained in this study. It is speculated that this extension of the Russell-like mechanism, as applied to the solid/vapor photocatalytic degradation of methanol, may also provide the basis for the maximum in the rate of degradation of aqueous solutions of dichloroacetic acid, using sol-gel prepared Fe2O3-TiO2 mixed oxide photocatalysts as a function of weight percent Fe.
Introduction An investigation by Heller et al.1 in support of commonly held views assuming the desirability of large charge carrier separation distance (CCSD) in particulate TiO2, as a means of reducing the efficiency of charge carrier recombination and improving quantum efficiencies of photocatalytic processes, focuses on the effects of catalyst pretreatment: (i) in a reducing environment, (ii) in an oxidizing environment, and (iii) by ball milling in creation of defect structure. Reduction of 0.2 µm diameter TiO2 was accomplished by thermal treatment with H2, forming Ti3+-containing particles for evaluation in oxidative degradation reactions. It is considered that the presence of Ti3+ raises the TiO2 Fermi level, leading to formation of a depletion layer in the TiO2. The resulting upward band bending of the TiO2 was further considered to reduce recombination at the surface, thereby enhancing photochemical quantum yields for the oxidation of 2-propanol. As plausible as this explanation may seem, it is difficult to completely reconcile it with the tendency for the system to move toward a flat band condition under constant intense illumination typical of many practical processes, since this removes or lowers the drift potential for photoelectron transport away from the surface, as discussed by Gra¨tzel.2 On the other hand, oxidative pretreatment of the TiO2 particles was found by Heller et al.1 to lower the quantum yield for oxidation of 2-propanol, presumably due to lowering of the TiO2 Fermi level, which decreases the drift potential for photohole transport to the surface and increases recombination. Introduction of bulk defects by ball milling was assumed to increase electron-hole recombination, thereby lowering the X
Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01335-4 CCC: $12.00
concentration of holes reaching the surface and reducing the quantum yield for photocatalyzed oxidation of 2-propanol. A more recent account by Schwitzgebel et al.3 of the photocatalyzed air oxidation of C8 organics in contact with nanocrystalline n-TiO2-coated glass microbubbles provides a different view. Here, the overall reaction path leading to desired products is initiated by photoelectrons and photoholes. Oxygen plays a dual role: (i) it reacts with a conduction band electron, e-, to form a superoxide radical ion O2•-, and (ii) it combines with an organic radical generated by a photohole, or an •OH radical reaction to produce an organoperoxy radical, RR′CH(OO)•, as shown in reactions 1-6.
O2 + e- f O2•-
(1)
and under acidic conditions
O2•- + H+ f •OOH
(2)
RR′CH2 + h+ f RR′CH• + H+
(3)
H2O + h+ f •OH + H+
(4)
RR′CH2 + •OH f RR′CH• + H2O
(5)
RR′CH• + O2 f RR′CH(OO)•
(6)
or
It is further suggested that the superoxide diffuses on the surface, or desorbs and diffuses in solutionsin either case eventually finding and reacting with the organoperoxide radical on the © 1996 American Chemical Society
Charge Carrier Separation Distance in Photocatalysis
J. Phys. Chem., Vol. 100, No. 50, 1996 19467
surface to form an organotetroxide by reactions 7 or 8. The
RR′CH(OO)• + •OOH f RR′CHOOOOH
(7)
RR′CH(OO)• + O2•- f RR′CHOOOO-
(8)
organotetroxide decomposes in a reaction sequence similar to that occurring in Russell reactions, as discussed by Schwitzgebel et al.,3 with product formation analogous to that shown in (9).
RR′CHOOOOH f RR′CO + H2O + O2
(9)
As stated above, one of the key steps proposed is the surface diffusion, or desorption plus solution diffusion, of O2•- or •OOH to a surface site occupied by RR′CH(OO)•. It is now proposed that overall reaction efficiency might be further enhanced if the radical intermediates leading to the formation of the organotetroxide, as shown in reaction steps 7 and 8, were formed in close proximity on the catalyst surface.4 The implication is that since these species are the products of reactions of photoelectrons, in (1) and (2), and photoholes, (3)-(6), the photoelectrons and photoholes should themselves be separated by a distance that achieves a balance between minimization of recombination and maximization of the rate of formation of the organotetroxide by reactions 7 or 8. Factors capable of controlling CCSD, such as variation in wt % Pt deposited on Pt/TiO2 catalysts, may also be capable of trapping photoelectrons at energy levels which are determined by Pt cluster size, thereby influencing the dynamics of the competing processes, as is discussed below in the Results and Discussion section. Upon introduction of the electron acceptor FeCl3 to the reaction solution, a marked decrease in the yields of all of the final products of oxidation was observed and attributed by Schwitzgebel et al.3 to competition by Fe3+ and O2 for photoelectrons by reactions 1 and 10. This was regarded as
e- + Fe3+ f Fe3+e-
(10)
strong support for the direct participation of both photoelectrons and photoholes in a Russell-type oxidative degradation. The role of Fe3+ has also been studied by Bahnemann5 in the photocatalyzed oxidation of dichloroacetic acid (DCA) using sol-gel prepared nanosized mixed Ti and Fe oxides containing 0-50% Fe. In flash spectroscopic investigations, the immediate formation of a transient at 600 nm attributable to Ti3+, simultaneous with the appearance of adsorption at 350 nm and concomitant decay of the 600 nm adsorption, was attributed to initial trapping of a conduction band electron at a Ti4+ site followed by e- trapping at the energetically more favored Fe3+ site. The yields for H+ formation based on number of incident photons, for reaction mixtures at pH 2.6 and 11.3, as a function of % Fe content of the Ti/Fe mixed oxides go through maxima at 2.5% Fe and 0.2% Fe, respectively. Bahnemann attributes the increased efficiency in degradation of DCA with increase in % Fe, up to the observed maxima, to a decrease in recombination brought about by increased e- trapping by Fe3+ and Ti surface states. Further increase in % Fe3+ beyond the observed maxima for DCA degradation is stated to increase recombination due to the decreasing number of available Tisurface states. An alternative explanation is offered in the Results and Discussion subsection dealing with Ti/Fe Mixed Oxide Photocatalysts. Experimental Section Reagents. H2PtCl6‚6H2O and 2-propanol used in preparation of Pt/TiO2 catalysts were supplied by Fisher Scientific as
Figure 1. Electron micrograph, using STEM facility at MIT, of photolytically deposited platinum cluster on TiO2 (Degussa P-25). Weight % Pt is 0.09%, as determined by X-ray fluorescence microprobe analysis. The small ∼1 nm area and the larger particle with which it is in contact were identified by microprobe X-ray fluorescent analysis to be Pt and TiO2, respectively.
Reagent Grade. Methanol was Baker Analyzed Reagent. Compressed O2 was WESCO Grade 4.4. Catalyst Preparation. The nonderivatized TiO2 was Degussa P-25. Adapting the procedure of Fernandez et al.6 for the photodeposition of Rh clusters on TiO2, the Pt (0-8 wt %)/ TiO2 samples of the present investigation were prepared from suspensions of 10 g of TiO2 (P-25) in 500 mL of a solution consisting of 450 mL of 2-propanol, 50 mL of water, plus appropriate volumes of 1.9 × 10-2 M H2PtCl6. These slurries were magnetically stirred for 3.5 h during illumination while held at a distance of ∼10 cm from a 150 W Xe lamp (Varian Catalog No. R 150-7a). The suspensions were then filtered and the solid catalysts dried overnight in an oven maintained at 60 °C. The dried samples were lightly ground with a mortar and pestle to break up large aggregates and stored in capped bottles. Values of wt % Pt reported below are upper limits, based on the supposition of complete reduction on the TiO2 surface of all of the H2PtCl6‚6H2O available to the suspension. Detailed information concerning the number and size distributions of the photodeposited Pt clusters on the Degussa P-25 TiO2 is not available. The importance of this type of information is recognized and indeed TEM and STEM images of Pt/ TiO2 samples prepared according to the procedure described above, but using shorter illumination time for photoreduction of the Pt complex, i.e. 1 h, have been obtained at the MIT Center for Materials Science and Engineering. With the aid of Dr. T. Garret-Reed, STEM images revealed Pt particles only in the nominal 2 wt % Pt sample in which elemental analysis by microprobe X-ray fluorescence indicated an actual % Pt of 0.09%. As seen in the micrograph, the size of the TiO2 particles appear to be about 30 nm and the Pt deposit about 2 nm. TEM procedures were less successful in identifying Pt deposits in any of the other samples, where the nominal wt % Pt ranged from 0 to 2%. The cofumed Ti-Fe oxide samples were prepared by Degussa using a proprietary process and were available only with 2 and 6-8 wt % Fe. The process has been described as a simultaneous flame oxidation of flowing streams of gas mixtures consisting of TiCl4 plus O2, and of FeCl3 plus O2, upon exiting separate nozzles into a common chamber. This allows mixing of the primary TiO2 and Fe2O3 particles as aerosols prior to formation of large aggregates, the intent being the attainment of uniform composition in the aggregated particulate powder.
19468 J. Phys. Chem., Vol. 100, No. 50, 1996
Sadeghi et al.
Figure 2. Schematic diagram of the time-resolved photocharge apparatus (TRPC).
Photocatalytic Reactor Description. The photocatalytic reactor and procedures were similar to those described by Lichtin et al.,7 consisting of a cylindrical 1 L glass water jacketed static reactor, supplied by Ace Glass. The light source, a 6 W VilburLourmat T-6L fluorescent lamp with output centered at 360 nm, was mounted coaxially inside the well of the water-cooled jacket of the reactor. The three reactor ports provided were utilized as follows: (i) for placement of a thermometer within the reaction volume, thermostatically maintained at 25 °C; (ii) connection to a vacuum system and introduction of O2; and (iii) for syringe injection of liquid methanol through a septum and sampling of the gas phase reaction mixture to monitor its composition prior to, and during, the onset of photocatalysis. Procedures Used To Coat Reactor with Catalyst and Monitor Progress of Reaction. Sonified slurries, consisting 0.9 g of catalyst powder in 10 mL of 2-propanol, were coated by manually rotating the outer chamber of the reactor, to achieve a thin uniform coating on the inside vertical walls. Following evaporation of the 2-propanol, the coated vessel was placed in an oven at 70 °C for a minimum of 45 min. After assembly the reactor was evacuated. The coating was preconditioned by purging the reactor with 90 mL/min of O2 under UV illumination for approximately 15 h. The system was evacuated prior to charging with O2 to a pressure of 760 Torr. Following injection of 16.6 µL of liquid CH3OH into the O2-charged vessel, the system was allowed to equilibrate, as verified by gas chromatographic analysis of 0.5 mL syringe aliquots of the reactor volume at frequent intervals over a period of an hour or more. When the CH3OH partial pressure reached a steady level, the UV lamp was turned on and the gas phase reaction mixture monitored during the course of the reaction. After completion of reaction, the reactor coating was again preconditioned by overnight O2 flow and UV exposure. This procedure was repeated prior to each additional trial in which the same catalyst coating was employed. Each catalyst coating was used in several trials. In addition, for a number of the catalyst samples, multiple coatings of the same sample were tested. Chromatographic Analysis. Monitoring the gas mixture was by gas chromatography, using a Hewlett-Packard 5890 HP GC with a FID detector and PORAPAK Q 80/100 mesh packed column. Calibration of elution times and integrated detector response vs partial pressure of methanol and methyl formate was performed using reagent grade chemicals. Time-Resolved Photocharge (TRPC) Measurements. The basis for the TRPC measurement, including a description of the calibration of the apparatus for purposes of evaluation of the mean ambipolar charge carrier separation distance, CCSD, has been presented previously8 and is repeated here. Schematic diagrams of the apparatus, calibration and measurement circuits are given in Figures 2 and 3.
Figure 3. Equivalent circuit diagrams for TRPC: (a, upper) measurement mode; (b, lower) calibration mode.
With reference to Figures 2 and 3, CCSD may be evaluated from the following expression,
( ) (
)
C Ct + C (d) I %A hν 100
( )( ) (
Va s CCSD ) |Vm| Vt ins
)( )
For fixed incident light energy, I, and constant sample thickness, the percentage of the incident light absorbed, % A, is to a first approximation assumed constant and the term (I/hν)(% A/100) is approximately equal to the number of absorbed quanta. Justification for the use of this assumption in the present investigation is presented in the Results and Discussion subsection dealing with Pt/TiO2 Photocatalysts. Under these circumstances,
CCSD = K |Vm| where |Vm| is the magnitude of the TRPC signal amplitude and K contains experimentally accessible terms, or terms that can be reasonably estimated, such as the dielectric constants of the sample, s, and the insulating components of the sample cell, ins; the cell capacitance, C; capacitance of test capacitor, Ct; calibration voltages, Va and Vt; and the electrode spacing of the sample cell, d. Or from a pragmatic standpoint, the relative values of CCSD or |Vm| may be used to evaluate trends resulting from variations in material parameters. This can also be seen intuitively by consideration of a basic principle of measurement of charge transport in time-of-flight measurements, as delineated by Tiedje,9 “... an injected charge inside the (photoconducting) material contributes to the integrated charge flowing in the external circuit in proportion to how far it moves through the sample. That is, if one electron moves halfway across the sample, one-half an electron charge will flow through the external circuit”. This same analysis is applicable to TRPC measurements, implying that the magnitude of the TRPC signal amplitude is proportional to the mean charge carrier separation distance, CCSD. In the apparatus shown schematically in Figure 2, as distinct from time-of-flight measurements, an externally applied field
Charge Carrier Separation Distance in Photocatalysis is not required, nor is there a requirement for direct electrical contact with the sample. Instead, the sample is placed in a cell consisting of electrically blocking parallel plate electrodes, one of which is transparent. The measurement can be viewed as the time-resolved charging of a capacitor, initiated by inhomogeneous light absorption within the sample. The resulting concentration gradient of charge carriers produces a transient diffusion current when the transport velocities of the oppositely charged species are unequal. Under these conditions, the measurement may also be viewed as a time-resolved Dember effect,10 depending only on diffusion currents to produce the observed surface potentials. However, the magnitude and direction of the photoinduced diffusion currents can be influenced by the presence of depletion layers, inversion layers, surface contact potentials, and dielectric confinement effects.8,11 Under any of these circumstances, the measurements can no longer be considered a pure diffusion or Dember effect and the more general description as a timeresolved photocharge measurement (TRPC) is considered more appropriate. For isolated particles of sufficiently small diameter, band bending is not expected.12 However, if the small particles are aggregated, the effective particle size may be in a size regime where band bending can influence charge carrier transport.8c,13 The light source used in the present experiments is a pulsed nitrogen laser, Molectron UV24, with emission at 337 nm, having a 10 ns pulse width and pulse energy of 9 mJ/pulse. The nitrogen laser can also be used to pump a Molectron DL II dye laser to obtain light pulses in the UV, visible, and IR spectral regions, depending on selection of the laser dye. The transient digitizer used is a Tektronix DSA 602, with 1 GHz band width and 2 GHz single shot sampling rate. Record length can range from 512 to 32 768 points. Vertical resolution is 8 bits, with 1 mV/div to 10 V/div dynamic range, and can be increased to 14 bits with signal averaging. Sweep speeds can range from 50 ps/div to 100 s/div. When higher sensitivity is required and bandwidth is not a factor, a dc to 2 MHz Ortec Brookdeal 5006 amplifier with 1000× gain has been employed. The measurements were performed at ambient conditions, using pressed pellets prepared with 0.3 g of catalyst powder. Pellet dimensions were approximately 12.5 mm diameter and 3 mm thick. Singlepulse photoinduced TRPC signals, with the 1000× amplifier, and an equivalent sensitivity setting of 20 µV/div, provide signals measurable above noise level. The sample may be any photoconductor (single crystal, thin film, particulate solid, particulate or colloidal suspension, or liquid). When liquid photoconductors or dispersions are used a special sample cell is employed.14 Dimensions of solid samples are quite flexible, since the electrodes are spring loaded to accommodate thicknesses up to ∼5 mm. The present sample chamber accommodates samples e25 mm diameter. As mentioned above, normally, an external field is not applied. However, using an available multiple pulse delay timing device, the apparatus can be run in the time-of-flight mode, with or without blocking contacts, in which a square wave field can be applied across the sample cell electrodes, synchronized with the initiation of signal capture by the transient digitizer and the laser light pulse. With an early version of this apparatus,8a CCSD of ∼60 Å has been reported for silver halide photographic emulsion grains of ∼1 µm diameter. Detection limits for estimation of CCSD at the time of the measurements was ∼5 Å and is improved by a factor of ∼2 with the present apparatus. Results and Discussion Ti/Fe Mixed Oxide Photocatalysts. Preliminary results of the present investigation dealing with the oxidative degradation
J. Phys. Chem., Vol. 100, No. 50, 1996 19469
Figure 4. Photocatalyzed degradation of methanol showing relative decrease in partial pressure as a function of time using: (O) TiO2 (Degussa P-25); (4) cofumed Fe2O3-TiO2 with 2 wt % Fe; (0) cofumed Fe2O3-TiO2 with (6-8 wt % Fe).
of CH3OH vapor compare the photocatalytic efficiencies of cofumed samples of mixed TiO2-Fe2O3 oxides containing 2% and 6-8% Fe, with P-25 TiO2 as a control, all supplied by Degussa and limited in availability to these specific Fe contents. The rate of degradation with the 2% Fe mixed oxide sample was approximately 10 times lower than with the 0% TiO2, P-25, while the sample with 6-8% Fe provided a considerably lower methanol consumption rate, i.e. >50 times lower than the P-25, as seen in Figure 4. These cofumed samples were characterized using time-resolved photocharge (TRPC) techniques, where the amplitudes of the signals, |Vm|, have been shown to be proportional to mean charge carrier separation distances, as discussed above. Using this criterion, uncorrected for possible differences in 337 nm absorption, the relative charge carrier separation in the 0% (P-25), 2% and (6-8)% Fe samples is 140/5/1, as is shown in Figure 5. Notwithstanding obvious differences in photocatalyst preparation and composition of the substrate undergoing degradation, it is of interest to speculate regarding the applicability of the Russell-like mechanism of Schwitzgebel et al.3 describing solid/ liquid systems to the system investigated by Bahnemann,5 and also to the solid/vapor systems of the present investigation in which the Russell-like mechanism is modified to include the role of CCSD in governing the proximal formation of reaction intermediates on the photocatalyst surface, in competition with charge carrier recombination. Assuming for the moment that the decomposition of CH3OH with the fumed Degussa samples has the same type of dependency on % Fe as found by Bahnemann in photocatalyzed decomposition of DCA,5 these results imply that the 2% and 6-8% Fe cofumed samples may already be too heavily loaded with Fe; i.e., they are beyond the corresponding maxima observed by Bahnemann, possibly due to the effectiveness of Fe in decreasing the mean charge carrier separation distance. This suggests that levels of Fe
