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J . Phys. Chem. 1988, 92, 2076-2078
Moreover, Ulmann et al.’ observe that the voltammetric cathodic peak corresponding to the electroreduction of hydrogen peroxide, both dissolved and preadsorbed, shifts toward positive potentials by about 250 mV with respect to the peak observed during the electroreduction of photogenerated surface species. This observation is considered by the authors as evidence that chemisorbed, photogenerated hydrogen peroxide is not responsible for these species. However, these results are not conclusive. Rather, the peak attributed to the electroreduction of surface peroxo species by Ulmann et al. is due to the reduction of oxygen photogenerated inside the pores of an extremely rough TiO, surface. This oxygen trapped at surface cavities is hardly affected by electrolyte stirring’, and should diffuse very slowly toward the bulk of the electrolyte, as inferred from Figure 1 of ref 1. On the other hand, the cathodic shift of the peak of electroreduction of photogenerated oxygen with respect to the peak corresponding to dissolved 0, (see Figure 4 of ref 1) can be attributed to 0, supersaturation of the electrolyte inside the pores. Again, the peaks that Ulmann et al. show in their Figure 6 seem to be due to the electroreduction of oxygen photogenerated during electrode illumination at +O. 17 V (RHE), which is clearly a too anodic potential for complete electroreduction of photogenerated 0,. Experiments should be more carefully designed, and a TiO, single crystal with a very smooth surface should be employed before any conclusion can be reached from the type of experiments performed by Ulmann et al.’ Finally, we would like to comment on the results of Ulmann et al.’ about the increase of the anodic photocurrent by addition of H,O, to the basic electrolyte. We have recently studied the interaction of T i 0 2 with hydrogen peroxide, at both acidic and basic pH, by impedance and electrolyte electroreflectance techniquess We showed that, at pH 11.3, the distribution of potential at the TiO,-electrolyte interface was noticeably perturbed by addition of H z 0 2to the electrolyte ( V , is shifted negatively by about 200 mV), while it remained apparently unperturbed at pH 3. This was attributed to the fact that H02-, the stable form of hydrogen peroxide at basic pH, interacts more strongly with TiO, than H202,which is stable at acidic pH. The shift of Vb toward negative potentials at basic pH, and the simultaneous band-bending increase under constant polarization potential, account for the observation of Ulmann et al.’ that the addition of H 2 0 2to the NaOH solution causes a marked increase of anodic photocurrent in the initial part of their photocurrent-voltage curve (Figure 11). Their conclusion that the effectiveness of H202in increasing the magnitude of the photocurrent must be the result of a large value of the rate constant for charge transfer’ seems therefore incorrect. Acknowledgment. This work was partially financed by the USA-Spain Joint Committee for Scientific and Technological Cooperation under Contract CCA 83/038. Registry No. TiO,, 13463-67-7; H,O, 7732-1 8-5. (12) Salvador, P.; Decker, F. J . Phys. Chem. 1984, 88, 6116.
Instituto de C6talisis y Petroleoquimica (CSIC),Serrano 119, 28006 Madrid, Spain
P. Salvador* D. Tafalla
Received: February 17, 1987; I n Final Form: August 31, 1987
Reply to Comments on “Behavior of Surface Peroxo Species In the Photoreactions at 110,” Sir: The cathodically reducible surface species (surface states), photogenerated during anodic polarization of an n-type TiO, photoelectrode, have originally been considered by Wilson’ to be the intermediates of the photooxidation of water. Referring to (1) Wilson, R. H . J . Electrochem. SOC.1980, 127, 228.
0022-3654/88/2092-2076$01.50/0
Wilson’s paper, Salvador, has proposed the surface hydroxyl groups a t TiOz to be the surface states involved in both photoassisted (anodic) and dark (cathodic) electrode reactions. Subsequently, Salvador and GutiErrez’ have investigated by cyclic voltammetry the cathodic reduction of the species photogenerated at polycrystalline Ti02 electrodes (obtained by sintering a rutile powder), both in alkaline (1 M KOH) and in acidic (1 M Na2S04acidified to pH 3) solutions. For all measurements the solutions were saturated with oxygen, which rendered the resolution of possible multiple signals rather difficult. The increase of the cathodic voltammetric peak, following a few-seconds exposure of the TiO, electrode to UV irradiation under anodic bias, has been attributed to a local supersaturation of the solution by photogenerated molecular oxygen.’ At that time, the authors have underlined that the electrochemical reduction of the photogenerated 0, in alkaline as well as in acidic media takes place at the both being mediated by same potential as that of “dissolved 02”, basic OH groups at the TiO, ~ u r f a c e . ~ In a later paper,4 Salvador and Gutiirrez report again on analogous voltammetric measurements, carried out with sintered Ti02 electrodes in air-saturated 1 M aqueous N a 2 S 0 4at pH 3 (the solution used already in previous experiments). Due presumably to different experimental conditions (prepolarization of the illuminated T i 0 2 electrode at a more positive potential, different potential sweep range, higher sweep rate), the authors4 discover a new voltammetric peak, shifted ca. 0.2 V to more positive potentials with respect to the peak assigned previously’ to the reduction of photogenerated oxygen species. This new peak is explained in terms of the reduction of H202molecules chemisorbed at the TiO, surface, such an assignment being based on M H20, the fact that the TiO, electrode, first immersed in a solution and then dried in air, exhibited a similar reduction peak.4 The authors have suggested that hydrogen peroxide, photogenerated in an intermediate stage of the photooxidation of water, goes, in part, into the solution and, in part, becomes readsorbed (chemisorbed) at the TiO, surface, forming stable surface states! In order to check the latter hypothesis, Salvador and Decker5 have performed a series of experiments with a rotating Pt ringilluminated TiO, disk electrode (RRDE). In spite of the high sensitivity of the RRDE technique6 for the detection of even unstable reaction intermediates, the formation of H 2 0 2 at the illuminated TiO, disk photoanode could not be confirmed. Rather surprisingly, the authorsS conclude that hydrogen peroxide is actually photogenerated as an intermediate product of photoelectrolysis of water at n-type Ti02, but most of it undergoes further photooxidation before diffusing away from the electrode. Despite the clearly negative results of the above RRDE study, the validity of the previously proposed model, involving the photogeneration and the subsequent chemisorption of H202at titanium d i ~ x i d e , ~ have not been reconsidered. We were prompted to reexamine the electrochemical behavior of the surface species, formed in the course of photooxidation of water at Ti02, by the reports dealing with the photogeneration of surface-bonded peroxo complexes during photocleavage of water in aqueous suspensions of TiO, Under the latter conditions, the surface-bound peroxides are, in fact, the main final product of the photooxidation of water. This situation is clearly in contrast with the case of the anodically biased T i 0 2 photoelectrode at which molecular oxygen is photogenerated with high faradaic efficiency. Our recent cyclic voltammetric measurements,1° to which the (2) Salvador, P. J . Electrochem. SOC.1980, 127, 2650. (3) Salvador, P.; Gutitrrez, C. Chem. Phys. Left. 1982, 86, 131. (4) Salvador, P.; Gutitrrez, C. J . Phys. Chem. 1984, 88, 3696. (5) Salvador, P.; Decker, F. J . Phys. Chem. 1984, 88, 61 16. (6) Albery, W. J.; Hitchman, M. L. Ring-disc Electrodes; Clarendon: Oxford, 197 1 . (7) Yesodharan, E.; Gratzel, M. Helu. Chim. Acta 1983, 66, 2145. (8)Yesodharan, E.; Yesodharan, S.; Griitzel, M. Sol. Energy Mafer. 1984, 10, 287. (9) Gu, B.; Kiwi, J.; Gratzel, M. N o w . J . Chim. 1985, 9, 539. (10) Ulmann, M.; de Tacconi, N. R.; Augustynski, J. J . Phys. Chem. 1986, 90, 6523.
0 1988 American Chemical Society
Comments comments of Salvador and Tafalla are addressed," performed with polycrystalline anatase Ti02 film electrodes in alkaline (0.1 M and 1 M NaOH) solutions, have shown for the first time that the surface species having closely similar electrochemical characteristics were formed at a Ti02 electrode illuminated with near-UV light under the following conditions: (a) under anodic bias (including very large bias) in a continuously deaerated solution; (b and c) in the absence of any external polarization in a deaerated (b) and in an 02-saturated (c) solution. All the above species exhibited not only practically identical reduction potentials but also very similar decay rates. These observations corroborated the hypothesis according to which the surface-bound peroxo species, photogenerated in aqueous Ti0, suspensions, are the direct product of the photooxidation of water by the positive holes and not necessarily the result of reduction of the photogenerated oxygen by the conduction band electrons. The above-mentioned experiments, performed both in thoroughly deaerated and in 02-saturated solutions, have permitted us to separate clearly the voltammetric peaks associated with the reduction of surface-bonded peroxo species, that of dissolved molecular oxygen and that of hydrogen peroxide, either present in the solution or preadsorbed at the electrode surface. The latter reaction has been shown to require the least cathodic potential, the two other peak potentials being 0.15 V (for the 0, reduction) and 0.30 V (for the reduction of the surface-bound peroxo species) more negative.I0 Such a large difference between the reduction potentials of H 2 0 2and of the surface-bonded peroxides photogenerated at the TiOz electrode, together with the observation that hydrogen peroxide undergoes rapid, preferential (versus water and/or OH- ions) photooxidation at a Ti0, photoanode (cf. Figure 11 from ref lo), brought us to conclude that H202is a very unlikely intermediate of the photooxidation of water. This conclusion is entirely consistent with a large series of experimental data published in the literature since 1953. Instability of H 2 0 2 in the Presence of T i 0 2 Irradiated with N e a r - W Light. Early experiments performed with 02-saturated aqueous suspensions of ZnO and TiO,, illuminated with UV light, have demonstrated the formation of easily detectable amounts of hydrogen peroxide at the former but not at the latter semiconductor.I2 Those observations have been confirmed by a recent work of Harbour et al.I3 involving the use of an in situ method to monitor the UV-irradiated, 02-containing, aqueous dispersion of titanium dioxide for the eventual presence of hydrogen peroxide. The method consisted of measuring, by means of a Clark electrode, the changes in oxygen concentration consecutive to the injection of catalase to the system. The only case where the authors have obtained evidence for the transitory generation of H202(in the form of a slight increase of the oxygen concentration) was in the presence, but not in the absence, of sodium formate added to the irradiated TiO2 dispersion. Importantly, no H20, was detected in the final reaction mixture, after the light had been switched off. This work shows that, unlike water and/or OH- ions, the formate ions are able to compete efficiently for the positive holes at the TiO, surface with the H202molecules formed through the reduction of dissolved 0, by the conduction band electrons. In this connection, it is particlarly important to recall the measurements, effected by Brown and Darwent,I4 regarding competitive photooxidation of Methyl Orange and hydrogen peroxide in colloidal T i 0 2 suspensions. In the absence of H,02 from the solution, the photoreactions occurring at irradiated (250 < X < 380 nm) Ti02consisted of the oxidation of Methyl Orange and the simultaneous reduction of dissolved oxygen. Addition of H202to the reaction mixture resulted in a pronounced inhibition of the Methyl Orange photooxidation, replaced by that of the hydrogen peroxide. The kinetic analysis of the reaction demonstrated that H 2 0 2reacts directly with a precursor to the species responsible for the dye oxidation, Le., with the photogenerated (1 1) Salvador, P.; Tafalla, D. J. Phys. Chem., preceding paper in this issue. (12) Markham, M. C.; Laidler, K. J. J . Phys. Chem. 1953, 57, 363. (13) Harbour, J. R.; Tromp, J.; Hair, M. L. Can.J . Chem. 1985,63, 204. (14) Brown, G. T.; Darwent, J. R. J . Phys. Chem. 1984, 88, 4955.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 2077 positive holes, h'.
-
The rate constant for this reaction H202 2h+ O2 + 2H+(aq)
+
determined at pH 11.2 appears to be 1000 times larger than that for the formation of Ti,-0' radicals Ti,-0- + h+ Ti,-0' (2) considered by the above authors as intermediates in the oxidation of Methyl Orange. Now, the absence of oxygen evolution over irradiated Ti02 suspensions indicates that the rate of photooxidation of water is even lower than that of Methyl Orange. Very similar observations, consistent with a high rate of photocatalytic destruction of H,02 on the T i 0 2 surfaces, have very recently been reported by Hong et al.Is All the above results, including also those of Salvador and D e ~ k e rconduce ,~ to exclude the hydrogen peroxide as a possible intermediate of the photooxidation of water at TiO2-responsible, according to Salvador and Tafalla," for the formation of stable, chemisorbed surface species. It is, in fact, not conceivable that a reactign intermediate, characterized by the rate of (further) oxidation a t least 1000 times higher than the rate at which it is being formed,I4 could accumulate at the electrode surface. Now, in contrast to the behavior of hydrogen peroxide, the photooxidation of the surface-bonded peroxo species is actually a slow process requiring significant anodic bias.I6 Coverage of Ti02 by the Photogenerated Surface Peroxo Species versus Adsorption Isotherm of H 2 0 2 . Recent spectrophotometric analyses, using o-tolidine as a redox indicator, have shown1' that during photolysis of water over a TiO, photocatalyst the hydrogen evolution was accompanied by the formation of a stoichiometric amount (in a 1:I molar ratio) of surface-bonded peroxo species. The latter experiments were performed with Pt-loaded anatase P-25 particles (having a surface area of ca. 55 m2/g) dispersed in a solution having an initial pH of 10. Under these conditions, the total amount of photogenerated peroxide was observed to be associated with the TiO, photocatalyst. The saturation coverage, determined after the dispersion had been illuminated for 48 h with a solar simulator, amounted to ca. 4.6 peroxo groups per nm2. In the case of a TiOz photoelectrode the surface coverage of the peroxo species, photogenerated under anodic bias, may be estimated from the corresponding cathodic voltammograms, like those shown in Figure 2 of ref 10. Thus, the determined amount of charge, which is still less than a saturation coverage, exceeds 10'' electrons per cm2 of geometrical surface area of the electrode. Dividing the latter value by the approximate surface roughness factor, 200, and by the number of electrons involved in the reduction of one peroxo group, 2, one obtains the coverage equal to 2.5 peroxo groups per real nm2. On the other hand, hydrogen peroxide is known to undergo chemisorption at Ti02 leading to a significant modification of its reflection spectrum.18 In the case of a large-surface-area anatase P-25 sample, being frequently used in water photosplitting experiments," the saturation coverage has been shown'* to not exceed ca. 1.8 molecules of H202per real nm2. (It was reached in equilibrium with 0.08 M H202solution.) In M aqueous H202 the coverage was already of 0.7 molecule/nm2 and decreased rapidly in less concentrated solutions." These results show clearly that the coverage of T i 0 2 with adsorbed hydrogen peroxide becomes significant only in relatively concentrated solutions and that it is unfounded to consider Ti02as a kind of sink for H202. The most important point is that for the same anatase P-25 photocatalyst the maximum coverage of hydrogen peroxide (including
-
(1.5) Hong, A. P.; Bahnemann, D. W.; Hoffmann, M. R. J . Phys. Chem. 1987, 91, 2109.
(16) It is useful to recall, in this connection, that the surface peroxo species are the final product of photodissociation of water at Ti02 particles as well as the major initial product of the photooxidation of water at anodically biased TiO, electrodes (cf. Table I in ref 10). ( 1 7 ) Kiwi. J.; Gratzel, M. J . Mol. Card. 1987, 39, 63. (18) Boonstra, A. H.; Mutsaers, C. A. H. A. J . Phys. Chem. 1975, 79, 1940.
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the totality of chemisorbed and physically adsorbed H20218)is 2.5 times lower than that of the photogenerated peroxo species. In addition, the chemisorption of any significant amount of H202 in the course of a photoreaction at Ti0, would imply necessarily the buildup of a substantial local concentration of H202,easily detectable by means of the rotating ring-disk electrode technique. Finally, we wish to make the following comments with regard to the specific points raised by Salvador and Tafalla: (i) The latter authors expound a series of arguments against the postulatedI0 formation of the surface-bound peroxo species through a purely surface process. Unfortunately, for this purpose, Salvador and Tafalla use, at the beginning of their demonstration, a distorted version (eq 4, 5, and 6 of ref 11) of the reaction sequence given in our original paper (cf. eq 4,5, and 6 in ref IO). There are other erroneous citations concerning, in particular, the radicals at the surface of TiO,. This makes the further remarks of Salvador and Tafalla pointless as regards the discussion of our paper. In order to avoid any ambiguity, we will again point out that about 90% of all the surface hydroxyls present at TiO, (i.e., all the acidic and 80%of so-called basic OH groups) are subjected to neutralization adsorption of alkaline hydroxide^.^^*'^ In other terms, in alkaline solutions (like those used in our experiments) there exists at the T i 0 2 surface a sufficiently large amount of ionized Ti,-O- groups originating from singly coordinated hydroxyls to account for the proposed sequence of reactions 5 and 6.1° In this connection, it seems useful to specify that the simplified formulas, like Ti,-OH and Ti,-O-, largely employed in the lite r a t ~ r e , ' ~ .are ' ~ ,obviously ~~ not intended to reflect the exact nature of the bonding between the surface titanium ions and hydroxyl groups. The latter is represented properly by using fractional charges.20,21 (ii) Salvador and Tafalla claim hydrogen peroxide to be the main product of the oxygen reduction at TiO, in alkaline solutions. This assertion is manifestly in contrast with the results of a RRDE study, performed by Parkinson et a1.,2, showing that the formation of H202accounts for less than 5% of current efficiency for the O2reduction at the T i 0 2 disk. The conclusion of the latter work was that hydrogen peroxide is probably produced through the reaction sequence parallel to the main four-electron pathway. (iii) Without presenting any evidence to support their view, Salvador and Tafalla argue that the cathodic voltammetric peak, investigated in detail in our work and assigned to the peroxo species bound to the TiO, surface,1° would be due to the electroreduction of oxygen supersaturating the electrolyte present inside the pores P.Discuss. Faraday SOC.1971, 52, 264. (20) Waldsax, J C R.; Jaycock, M J Discuss. Faraday SOC 1971, 52,
(19) Boehm, H.
23 1 (21) Jaycock, M. J.; Waldsax, J C R J Chem SOC.,Faraday Trans. I 1974, 70, 1501. (22) Parkinson, B.: Decker, F.: JuliIo. J. F.: Abramovich. M.; Chagas, H . C . Elecrrochim. Acta 1980, 25, 521.
Comments in the electrode surface. It is to be recalled that the above peak was ca. 0.15 V more negative than the peak due to the reduction of O2dissolved in the solution and still ca. 0.3 V more negative than the peak associated with the reduction of H202.'0 Now, the local supersaturation of the solution with oxygen, if any, would obviously lead not to a negative but to a positive shift of the cathodic voltammetric peak-toward that corresponding to the H202 reduction. This could be caused not only by the local increase of the partial oxygen pressure, pol, but very likely also by the OH- concentration gradient created during anodic 0, evolution (3) The remark of Salvador and Tafalla regarding the use of porous Ti02electrodes in our experiments is something which is difficult to understand. Salvador et al.'s own m e a s ~ r e m e n t s ,serving ~ . ~ as a basis for the present discussion, have been performed with sintered TiO, electrodes of probably very large surface area. (No indication regarding the electrode morphology is given in the above-mentioned papers.) Our choice of the TiO, film electrodes was dictated by the purpose to simulate the behavior of powdered photocatalysts.10 (iv) Our interpretation of Figure 11 in ref 10 is based not only on the shift of the apparent onset potential of the anodic photocurrent but, principally, on the steep initial increase of the photocurrent consecutive to the H202addition to the solution (cf. curve d in Figure 11). It is entirely consistent with the just-mentioned independent experimental evidence.13-15 This kind of behavior may, in fact, be paralleled by the negative shift of the semiconductor flat-band potential caused by the specific adsorption of
anion^.^^^^^ In conclusion, we wish to point out that the involvement of hydrogen peroxide as an intermediate of the photooxidation of water at titanium dioxide, suggested by Salvador et a1.3" remains an unfounded hypothesis, inconsistent with (a) the fact that H202 is a highly unstable species at irradiated Ti02,'3-15(b) the amount of the observed coverage of TiOz with the photogenerated peroxo s p e c i e ~ , ' ~ Jand ' , ~ ~(c) the failure to detect H202,expected to form at illuminated Ti02 disk, in the RRDE experiments5 Registry No. TiOz, 13463-67-7; HzO, 7732-18-5. (23) Augustynski, J. Strucr. Bonding, in press. (24) Gerischer, H. J . Electroanal. Chem. 1983, 150, 5 5 3 .
DZpartement de Chimie MinPrale, Analytique et AppliquZe UniversitP de GenPve I21 1 GenPve 4, Switzerland Received: January 12, I988
J. Augustynski* M. Ulmann
