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Photoelectrochemical Oxidation of Aqueous Sulfite on Ni-TiO2 Composite Film Electrodes Norma R. de Tacconi,* Maja Mrkic, and Krishnan Rajeshwar* Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received April 17, 2000. In Final Form: July 25, 2000 The photoelectrochemical oxidation of aqueous sulfite on composite electrode films comprising titania particles as the dispersed component and nickel as the matrix (Ni-TiO2) is described. These composite films exhibit unusual photocurrent enhancement for sulfite oxidation relative to the corresponding neat TiO2 films containing no nickel. Both the morphology of the composite films and their surface chemical composition (Ni:Ti ratio) are shown to play a role in the photocurrent enhancement mechanism. Two potential domains for sulfite photooxidation separated by the onset of dark oxidation of sulfite by the nickel matrix itself are considered.
Introduction The use of semiconductor electrodes to drive the photooxidation or photoreduction of a given substrate is a well-established practice at present.1 In the early days of photoelectrochemistry, single-crystal semiconductors mostly were used as the electrode materials.2 More recently, polycrystalline semiconductor films1b,3 and their nanocrystalline counterparts4 have been deployed for this purpose. Composite films containing metal and semiconductor components are an emerging new family of photoelectrode materials. They have shown enhanced activity (relative to the corresponding “neat” semiconductor films containing no metal) for the targeted photooxidation of organic substrates such as formate,5 methanol,5d and acetaldehyde.6 These prior studies involved photoreactions both in an aqueous electrolyte5 and in the gas phase,6 underlining the generality of the rate-enhancement mechanism. In both sets of studies, composite films comprising a metal (either nickel5 or zinc6) and TiO2 (as the semiconductor component) were employed. The mechanistic aspects of the photocatalytic performance of these composite films have not yet been completely elucidated. However, the Ni sites are envisioned to bring about an increase of the local substrate concentration at the interface via specific adsorption while the adjacent TiO2 sites then serve to * To whom correspondence should be addressed. Telephone, 817272-5034/3810; fax, 817-272-3808; e-mail,
[email protected] or
[email protected]. (1) (a) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York and London, 1980. (b) Finklea, H. O., Ed. Semiconductor Electrodes; Elsevier: Amsterdam, 1988. (c) Rajeshwar, K.; Iban˜ez, J. G. Environmental Electrochemistry; Academic Press: San Diego, 1997. (2) Rajeshwar, K. J. Appl. Electrochem. 1985, 15, 1. (3) Hodes, G.; Fonash, S. J.; Heller, A.; Miller, B. Adv. Electrochem. Electrochem. Eng. 1984, 113. (4) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (b) Peter, L. M. In Comprehensive Chemical Kinetics; Compton, R. G., Hancock, G., Eds.; Elsevier: Amsterdam, 1999; pp 223-280. (b) Peter, L. M.; Vanmaekelbergh, D. Adv. Electrochem. Sci. Eng. 1999, 6, 777. (5) (a) Zhou, M.; Lin, W.-Y.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1996, 402, 221. (b) Zhou, M.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1997, 421, 111. (c) de Tacconi, N. R.; Wenren, H.; McChesney, D.; Rajeshwar, K. Langmuir 1998, 14, 2933. (d) de Tacconi, N. R.; Carmona, J.; Rajeshwar, K. To be published. (6) Ito, S.; Deguchi, T.; Imai, K.; Iwasaki, M.; Tada, H. Electrochem. Solid State Lett. 1999, 2, 440.
photooxidize these sequestered species.5c Indeed, the morphology of these composite films, and specifically cocluster formation of interspersed Ni and TiO2 sites on their surface, were shown to facilitate this rate-enhancement mechanism.7 In this paper, we further elaborate on these ideas using aqueous sulfur dioxide (sulfite) as the targeted substrate. We show that the nickel and TiO2 sites in our composite Ni-TiO2 films play a concerted and complementary role in promoting the initial adsorption and subsequent photooxidation of sulfite. Photocurrent enhancements up to 3-fold the corresponding level typical of “neat” TiO2 films (containing no Ni sites) were observed under certain conditions for our composite Ni-TiO2 films. Sulfite-loaded aqueous media have been employed previously as facile hole scavengers in photoelectrochemical experiments.8 The photocatalytic oxidation of these species in UV-irradiated aqueous dispersions containing a variety of semiconductors (e.g., TiO2, ZnO, CdS, Fe2O3, or WO3) has also been reported.9 The mechanistic aspects of the photocatalytic oxidation of aqueous sulfur dioxide have been examined for phthalocyanine-modified TiO2 particle sufaces.10 Finally, we note the rather extensive literature that exists on the anodic oxidation11 and catalytic oxidation12 of dissolved sulfur dioxide in aqueous media. Pertinent aspects of this earlier body of work are noted in a subsequent section below. (7) de Tacconi, N. R.; Boyles, C. A.; Rajeshwar, K. Langmuir 2000, 16, 5665. (8) (a) Tenne, R.; Hodes, G. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 74. (b) de Tacconi, N. R.; Wenren, H.; Rajeshwar, K. J. Electrochem. Soc. 1997, 144, 3159. (9) Frank, S. N.; Bard, A. J. J. Phys. Chem. 1977, 81, 1484. (10) Hong, A. P.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 6245. (11) (a) Seo, E. T.; Sawyer, D. T. J. Electroanal. Chem. 1964, 7, 184. (b) Lu, P. W. T.; Ammon, R. L. J. Electrochem. Soc. 1980, 127, 2610. (c) Lyke, S. E.; Langer, S. H. J. Electrochem. Soc. 1991, 138, 1682. (d) Brevett, C. A. S.; Johnson, D. C. J. Electrochem. Soc. 1992, 139, 1314. (e) Quijada, C.; Rodes, A.; Vazquez, J. L.; Perez, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 394, 217. (f) Quijada, C.; Rodes, A.; Vazquez, J. L.; Perez, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 398, 105. (g) Quijada, C.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1996, 414, 229. (12) (a) Calvert, J. G., Ed. SO2, NO and NO2 Oxidation Mechanisms: Atmospheric Considerations; Butterworth: Boston, 1984. (b) Kotronarou, A.; Sigg, L. Environ. Sci. Technol. 1993, 27, 2725. (c) Sedlak, D. L.; Hoigne, J. Environ. Sci. Technol. 1994, 28, 1898.
10.1021/la000573p CCC: $19.00 © 2000 American Chemical Society Published on Web 09/21/2000
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Experimental Section Polycrystalline gold disk electrodes (Bioanalytical Systems, West Lafayette, IN; geometric area 0.018 cm2) were used to support the electrosynthesized Ni-TiO2 films. A single-compartment cell was utilized for both electrosynthesis and voltammetry. A platinum wire was used as counter electrode, and Ag/AgCl/3M KCl was used as the reference electrode. All potentials are quoted with respect to this reference unless specified otherwise. Photoelectrochemical experiments used a similar cell fitted with a quartz window. A 75-W xenon arc lamp was used as the light source, and an infrared water filter was interposed between the light source and the cell window. The incident light flux was measured using an Oriel radiant power meter (model 70260) and was nominally ∼2.5 mW/cm2. All chemicals were from commercial sources and were of the highest purity available; they were used as received. All solutions were made with Corning Megapure deionized water. Freshly prepared sulfite solutions were always used, and the supporting electrolyte solutions (0.1 M NaNO3) were thoroughly purged with N2 before sodium sulfite was added. The titania (P-25) was donated by Degussa; the particles were mainly anatase, with a mean size in the range 50-150 nm. The Ni-TiO2 composite films were prepared by the use of an occlusion electrosynthesis procedure discussed elsewhere.5a,b A nickel Watts bath was used for this purpose and was dosed with the requisite amount of TiO2. All films in this study were grown from baths containing a constant TiO2 dose of 0.64 g/10 mL, and were nominally 500 nm thick (as assessed coulometrically from the charge consumed during the potentiostatic deposition). The rate of nickel deposition (and thence the composite film morphology; see below) was tuned by sequential dilution of the initial nickel-ion content of the Watts bath. Thus three different nickel levels (0.1 M, 0.01 M, and 0.005 M) were used, and the resultant films are designated as Ni(0.1)-TiO2, Ni(0.01)-TiO2, etc. in what follows. All other electrosynthesis conditions used optimized parameters as established in the previous studies from this laboratory.5b For comparison purposes, TiO2 films containing no nickel were also prepared. The pyrolysis of aqueous TiCl4 at 550 °C was utilized for this purpose,4a and the films were built up to ∼0.86.0 µm thickness by application of multiple coats on a Ti disk support. The instrumentation and relevant details for scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) have been described elsewhere.7
Results Photovoltammetry, i.e., voltammetry performed with simultaneous interrupted irradiation of the working electrode, is a useful tactic for examining the dark current-potential profile and the photoelectrochemical counterpart in the same experiment.13 Figure 1 shows photovoltammograms for a Ni(0.01)-TiO2 film in an indifferent electrolyte, 0.1 M NaNO3 (Figure 1a), and in the same medium containing, additionally, 0.1 M Na2SO3 (Figure 1b). The photoanodic current flow is significantly enhanced when sulfite is present in the electrolyte. The scan in Figure 1b is shown only for an abbreviated potential window (relative to its counterpart in Figure 1a) because the subsequent (photo)oxidation of sulfite occurs on both TiO2 and nickel sites, as is described later. Data corresponding to Figure 1 for a neat TiO2 film are shown in Figure 2. Notably little rate enhancement in sulfite photooxidation (relative to the baseline electrolyte level) is seen in this case when the metal sites are omitted from the film. Although the data in Figure 2 are shown for a 0.8-µm-thick TiO2 film, little variation in the photocurrent magnitude was seen for films up to ∼5 µm (13) (a) Da Silva Pereira, M. I.; Peter, L. M. J. Electroanal. Chem. 1982, 131, 167. (b) Mishra, K. K.; Rajeshwar, K. J. Electroanal. Chem. 1989, 273, 169. (c) Son, Y.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1993, 345, 135.
Figure 1. Photocurrent-potential profiles under chopped irradiation for Ni(0.01)-TiO2 composite film in 0.1 M NaNO3 (a) and in 0.1 M Na2SO3 + 0.1 M NaNO3 (b). Geometric electrode area, 0.018 cm2. Potential scan rate, 2 mV/s. Chopper frequency ) 0.1 Hz.
thick, signaling that electron-hole recombination was not a significant factor in the observed lack of photocurrent enhancement when sulfite was present. On the other hand, the trends in Figure 2 are those expected for a well-behaved semiconductor/electrolyte junction. Thus, note that the sigmoid shape of the photocurrent-potential profile near the onset potential regime (-0.4-0.0 V) is less pronounced when sulfite ions are present in the electrolyte. The photocurrent onset also shifts to more negative potentials in the latter case. On the other hand, the photocurrent magnitude in the “plateau” potential regime (0.2-0.8 V) is not influenced by the presence of sulfite, because the current flow in this regime is controlled only by the photon flux and not by the interfacial charge-transfer kinetics.1 Thus the photocurrent enhancement seen in Figure 1 is rather unusual in that it cannot be reconciled within the usual frame of photoelectrochemistry concepts (see below). The behavior seen here for sulfite mimics what we observed earlier for formate.5 In the absence of sulfite, the photogenerated holes presumably undergo reaction pathways involving surface hydroxyl groups or adsorbed water.4 The remarkable influence of nickel on the photoelectrochemical behavior of the composite Ni-TiO2 films is further underscored by the data in Figure 3 and Table 1.
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Figure 2. Photocurrent-potential profiles under chopped irradiation for a neat TiO2 thin film in 0.1 M NaNO3 (a) and in 0.1 M Na2SO3 + 0.1 M NaNO3 (b). Geometric electrode area, 0.29 cm2. Chopper frequency ) 0.2 Hz. All other conditions as in Figure 1.
Figure 3. Plot of photocurrent ratio as a function of sulfite concentration in the 0.1-M NaNO3 electrolyte for Ni(0.1)-TiO2, Ni(0.01)-TiO2, and Ni(0.005)-TiO2 films. The photocurrent was measured at 0.15 V. Table 1. Atomic Concentrations of Ni and Ti in Ni-TiO2 Composite Films as Determined by XPS Analysis sample
Ti (%)
Ni (%)
Ni(0.1)-TiO2 Ni(0.01)-TiO2 Ni(0.005)-TiO2
10.5 23.4 37.8
89.5 76.6 62.2
Figure 3 maps the photocurrent enhancement as a function of sulfite concentration in the NaNO3 electrolyte and at a fixed potential of 0.15 V for three Ni-TiO2 composite films, as well as for a neat TiO2 film containing no Ni. The surface-metal compositions of the three composite films (as probed by XPS) are listed in Table 1. Several points are worthy of note in Figure 3. First, the photocurrent ratio, i.e., (Iph(sulfite) + Iph(water))/Iph(water), can attain
values as high as ∼3 under certain conditions. Second, this ratio quickly rises above 1 even for very low sulfite levels in the supporting electrolyte and for all of the three composite films considered in Figure 3. Thereafter, the ratio tends toward saturation at higher sulfite levels. The extent of photocurrent enhancement for sulfite oxidation is ordered thus: Ni(0.1)-TiO2 < Ni(0.01)-TiO2 < Ni(0.005)-TiO2. The neat TiO2 film shows less than 20% enhancement (relative to the baseline electrolyte level) when sulfite is present. Control experiments with “pure” Ni electrodes (i.e., containing no TiO2) also show no photocurrents in the presence of sulfite. The observed rate enhancement for the composite films is rooted in factors related to both surface composition (Table 1) and morphology (Figure 4). As Table 1 shows, a lower initial content of the nickel ions in the original bath translates to a concomitant decrease in the surface Ni concentration in the resultant film. Importantly, the Ti:Ni ratio is better “balanced” in the Ni(0.005)-TiO2 relative to its Ni(0.1)-TiO2 counterpart. That is, an optimal balance must be struck between the number of photooxidation sites (TiO2) and the main adsorption sites (Ni) on the film surface. Equally important, as our companion study also demonstrates,7 is how these sites are juxtaposed relative to one another on the composite film surface. The SEM pictures in Figure 4 show that “mixing” of the TiO2 and Ni sites becomes progressively more intimate in the series Ni(0.1)-TiO2 < Ni(0.01)TiO2 < Ni(0.005)-TiO2. What happens in the potential regime where the sulfite species can also undergo oxidation on the Ni surface in the dark? Cyclic voltammetry data (not shown) indicate that this happens at potentials greater than ∼0.3 V. No current flow is also seen in the potential range from -0.4 to 0.9 V when sulfite is omitted from the NaNO3 electrolyte. Voltammograms for Ni(0.005)-TiO2 are shown in Figure 5 under periodic illumination (Figure 5a) and in the dark (Figure 5b). Unlike in the previous cases, a rather low concentration (0.001 M) of sulfite was employed here and in the potential-step experiments (see below) so that the intermediate steps in the oxidation of sulfite would be better differentiated. Several points about the data in Figure 5 are worthy of note. First, the photoanodic current flow is shifted to potentials negative of that observed on the Ni sites in the dark (cf. Figure 5a,b). Such behavior is typical of a situation in which the light energy provides the (additional) driving force for a reaction on a semiconductor surface relative to a metal.1 Second, the photocurrent-potential profile in Figure 5a appears superimposed on a high dark background at potentials positive of ∼0.3 V. The latter background, of course, is associated with the nickel matrix component in the composite. Third, the photocurrent profile is considerably more spiked at potentials greater than ∼0.3 V. In fact, cathodic current transients are observed when the irradiation is interrupted, i.e., in the dark periods. This observation is further illustrated in the transient current data in Figure 6a at 0.42 V. Four components may be distinguished in the overall current flow as identified in the figure: two steady-state components consisting of the photocurrent, Iph and the dark current, INid (associated with the TiO2 and Ni sites, respectively), and two non-steady-state terms, Itron and Itroff, differing in polarity. Figure 6b contains a “reconstructed” voltammogram obtained from the latter two components by repeating the potential-step experiment at different potentials in the range from -0.6 to 0.9 V. This reconstructed voltammogram features an initial, irreversible
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Figure 4. Scanning electron micrographs of (a) electrodeposited nickel, (b) Ni(0.1)-TiO2, (c) Ni(0.01)-TiO2, and (d) Ni(0.005)TiO2. Films were grown at -0.8 V to a total thickness of 500 nm in each case.
oxidation wave peaking at ∼0.3 V, followed by another reversible wave centered at ∼0.5 V. The latter has a peak separation (∆Ep) close to 0 mV, diagnostic of surfaceconfined redox processes.14 It must be emphasized that these spiked current profiles are unique to Ni-containing composite TiO2 films (Figures 5a, 6a); these are conspicuous by their absence in the corresponding TiO2 films containing no Ni (see, for example, Figure 2b).
In eq 1, n is the electron stoichiometry, F is the Faraday constant, k is the heterogeneous (bimolecular) rate constant for hole transfer across the semiconductorelectrolyte interface, and the last two concentration terms correspond to the substrate (sulfite in the present case) and holes, respectively. The photon flux dependence of
the photocurrent is embodied in the hole concentration term of eq 1. The photocurrent enhancement seen in Figures 1 and 3 for the composite Ni-TiO2 films (relative to their neat TiO2 film counterparts) can reside with any one of the last three terms (or combinations thereof) in eq 1. Of these, the rate constant can be ruled out as a causal factor because the enhancement is largely seen in a potential regime where the interfacial charge-transfer kinetics are not expected to play a dominant role. This then leaves the two concentration terms in eq 1. The trends seen in Figure 3 coupled with the crucial role that surface composition and film morphology appear to play in the photocurrent enhancement (see above) suggest that an increase in the local concentration of sulfite at the film-electrolyte interface (via adsorption largely on the nickel sites) can explain the observed behavior. There is literature precedence for adsorption of sulfite on the nickel surface.16 Sulfite groups were postulated to adsorb mainly through oxygen coordination on the nickel surface.16b Our previous surface analysis of the composite Ni-TiO2 films used in this work was consistent with a surface oxy-hydroxy (passivating) layer on the Ni surface.17 Such a hydroxylated metal surface would facilitate
(14) For example: Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1983; Vol. 13, p 191. (15) Rajeshwar, K. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, in press.
(16) (a) Persson, D.; Leggraf, C. J. Electrochem. Soc. 1993, 140, 1256. (b) Persson, D.; Leggraf, C. J. Electrochem. Soc. 1995, 142, 1459. (17) de Tacconi, N. R.; Wenren, H.; Rajeshwar, K. J. Electrochem. Soc. 1997, 144, 3159.
Discussion In general, the photoanodic current density, ja, on an n-type semiconductor electrode under illumination is given by the expression15
ja ) nFkcredch
(1)
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Figure 5. Photovoltammogram under periodic illumination (a), and dark voltammogram (b) for a Ni(0.005)-TiO2 film in 0.001-M Na2SO3 + 0.1-M NaNO3 solution. Conditions as in Figure 1.
the sulfite adsorption process.16b Adsorption of the sulfite species on the TiO2 surface also undoubtedly occurs,10 and probably accounts for the small but noticeable enhancement (∼20%) of the photoanodic currents even for the neat TiO2 films as a function of sulfite concentration in the electrolyte (Figure 3). Photocurrent enhancement arising from an increase in the surface-hole population can be envisioned for the NiTiO2 films via a mechanism involving rapid injection of the photogenerated electrons in TiO2 into the adjacent Ni phase. Simply put, e--h+ pair separation in Ni-TiO2 films is considered to be more efficient than in their TiO2 counterparts.5b,7 The present data do not enable us to differentiate which of these two mechanisms (i.e., enhanced sulfite adsorption on nickel or enhanced e--h+ separation in Ni-TiO2 vis-a`-vis TiO2) plays a dominant role. The situation appears to be even more complex in the potential regime positive of 0.2 V. In this regime, the dark voltammetric data clearly show that sulfite oxidation can occur on the nickel surface as well as on the irradiated TiO2 sites. The oxidation of aqueous sulfite is known to proceed via either one-electron free radical or two-electron nonradical pathways.10,12 The unmistakable presence of spiked current transients in the potential regime positive of 0.2 V (Figures 5a and 6a) for the composite Ni-TiO2 films, but not in their neat TiO2 film counterparts (Figure 2b) leads to the interesting scenario that oxidation reaction intermediates are stabilized to a greater degree in the composite film matrix than in neat TiO2. Once again, although the present data do not shed light on the chemical makeup of these intermediates, it is reasonable to envision a reaction scheme involving free radicals. Thus the main anodic wave in Figure 5b is assigned to solution-confined sulfite species. However, this peak does
Figure 6. Photocurrrent transients at 0.42 V for Ni(0.005)TiO2 in 0.001-M Na2SO3 + 0.1-M NaNO3 solution (a). Reconstructed voltammogram from potential-step experiment data as in Figure 6a (b). Thus the nonstationary current transients Itron (under light) and Itroff (under dark) were measured with respect to the corresponding steady-state photocurrent at each potential.
not scale in amplitude in a linear manner with sulfite concentrations higher than ∼0.01 M. We attribute this to passivation of the electrode surface by strongly adsorbed sulfite species. The latter are oxidized at more positive potentials in a scheme modeled by two consecutive electron-transfer steps:
(SO32-)ads f SO3•- + e-
(2)
SO3•- + H2O f SO42- + 2H+ + e-
(3)
The sulfite radical intermediate can back-inject a hole into the Ni matrix, giving rise to the cathodic current transient spikes seen in Figures 5a and 6a. Such a notion is also supported by the pair of reversible “waves” that appear at 0.5 V in the reconstructed “voltammogram” in Figure 6b. Finally, the anodic current transients in Figures 5a and 6a can be explained by the stabilizing environment provided by the Ni component to the adsorbed sulfite species and reaction intermediates in the composite film matrix. A portion of the accumulated holes then simply recombine with the photogenerated electrons in the TiO2 component instead of being utilized for sulfite oxidation. Conclusions Careful consideration of the results presented and discussed in the preceding sections leads to the inevitable conclusion that the intermixing of nickel and TiO2 sites within the composite film matrix is a prerequisite to
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securing rate enhancement for substrates such as sulfite. The present data along with the companion results presented elsewhere5,7 illustrate the generality of the tactic for building complementary functions within a composite film. Simply put, a photoelectrochemical matrix can be built by dispersing and mixing separate but complementary sites for substrate adsorption and subsequent photooxidation within it. The library of existing knowledge on the surface chemistry of metal surfaces can be deployed to select a metal as the matrix component in the film matrix. Examples include the nickel-sulfite combination considered here and the platinum-methanol pair in our companion study.5d In closing, it is worth pointing out that our composite M-TiO2 films (M ) metal) differ in important respects from the metal-modified TiO2 particles that have been considered by the photocatalysis community.1c,18 Not only are the morphologies vastly different in the two cases, but the metal candidate in the latter instance is chosen for its
catalytic property rather than its adsorption affinity toward a targeted substrate. We also note studies of strong metal support interactions (SMSIs) involving welldispersed metals in a TiO2 matrix.19 Although the composite film configuration is inverted in these cases from our films (where TiO2 is the dispersed component), it is nonetheless interesting that drastic changes occur for the sorption properties of a metal when a support such as TiO2 is used instead of (the more conventional) SiO2 and Al2O3. We observe comparably drastic photoelectrochemical behavior for TiO2 when a metal is present as the support. Whether the modified behavior has similar mechanistic origins as SMSI remains to be established.
(18) Wang, C.-M.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114, 5230.
(19) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170.
Acknowledgment. This research was funded in part by a grant from the U.S. Department of Energy, Office of Basic Energy Sciences. LA000573P
