J. Chem. Soc., Faraday Trans. I , 1989, 85(6), 1279-1290 Spectroscopic Characterisation and Photochemical Behaviour of a Titanium Hydroxyperoxo Compound G. Munuera, A. R. Gonzalez-Elipe, A. Fernandez, P. Malet and J. P. Espin6s Instituto de Ciencias de Materiales (Centro Mixto CSIC- Univ. Sevilla) and Dpto. de Quimica Inorganica, P.O. Box 1115. 41071-Sevilla, Spain In order to obtain more information about the generation of peroxide species during water photo-cleavage using M/TiO, (M = noble metal) systems in which an 0, evolution is normally not observed, a titanium hydroxyperoxo compound has been prepared by the reaction of TiCl, with H,O,. The structure and the thermal behaviour of this compound have been examined using TG-MS, XRD and TPD techniques. XPS, i.r., e.s.r. and u.v.-visible reflectance spectroscopy have been used to characterise the types of peroxo species present in this sample.The photochemical decomposition of this compound, which leads to 0, evolution, has been studied compared to a TiO, (Degussa P25) sample. A model is proposed to explain the lack of 0, evolution during water photo-cleavage on M/TiO, systems where peroxo species, similar to those observed in the titanium hydroxyperoxo compound, become stabilised against photodecomposition. Introduction During irradiation of aqueous suspensions of metal-loaded TiO, powders or colloids (M/TiO,, M = Pt, Rh, Ni, etc.), evolution of H, is always observed due to water cleavage though 0, could not be detected even after long periods of irradiation (60 h). To explain this behaviour, we have proposedl that peroxo-species are formed according to the stoichiometric reaction 2h' + 20H; + 20Hi,, + H202,ds where OH; refers to basic surface OH groups on TiO,., The resulting peroxide is retained by the TiO, support due to reactions such as / o A-0, Ti=O+H,O, + Ti, I +H,O (2) and 2Ti-OH + H,O, + Ti Ti + 2H,O.(3) 0 Detection of such peroxo species in these systems has been carried out by Kiwi and Gratze13 by the reaction with o-toluidine, a compound which reacts specifically with such species. However, though the amount of peroxo species was quantitatively equivalent to the evolved H,, even after such long periods of irradiation (90 h), most of the peroxo species are expected to be incorporated into the bulk of the TiO, colloidal particles according to reaction (2), as we have recently sh0wn.l Though Kiwi and Morrison4 and Augustynski et aL5 have suggested some models for the actual structure of these peroxo species, such models are considered only as tentative since they are not supported by spectroscopic characterisation.We have previously shown, using XPS,' that Rh/TiO, photocatalysts, used for water photocleavage, show the growth of a new O( 1s) peak at 532 f 0.1 eV which was ascribed 12791280 Photochemical Behaviour of a Titanium Hydroxyperoxo Compound to peroxo- species by comparison with the XPS spectra recorded for a titanium hydroxyperoxo compound, prepared by the reaction of TiCl, with H,O, as reported in the literature.' The aim of this work is to examine in more detail the nature of such peroxo species in this reference compound as well as their photochemical behaviour, in order to explain the lack of oxygen photogeneration during water photo-cleavage when using M/TiO, systems.Experimental A titanium hydroxyperoxide sample [hereafter referred to as TiO(O,)] was prepared as described by Schwarzenbach et al.' by the reaction of a H,O, acidified solution with TiCl,. From TG-MS analysis, the formula of the resulting yellow solid was established as Ti,O,~,(OH)o~,~xH,O, thus corresponding to ca. 1 f0.2 peroxo species per Ti ion, while the B.E.T. surface area, after evacuation at 473 K during 1 h, was 83 m2 g-l, decreasing to 64 m2 g-l after decomposition at 723 K during 4 h. A TiO, sample (anatase P25 from Degussa, SBET = 50 m2 g-') was used as reference in this work.Colloidal rhodium particles, used in some experiments, were prepared by flowing H, at room temperature through a 0.1 mol dm-, RhCl, aqueous solution in 1 mol dm-, NaOH. Portions of the resulting Rho suspension were immediately injected into the irradiation flask. XPS spectra were recorded with a Leybold-Heraeus LHS- 10 spectrometer working in the AE = cte. mode with a pass energy of 50 eV using Mg Ka radiation as an excitation source. The samples, in the form of pellets, were mounted on a tantalum holder which can be heated resistively while temperature was measured by a thermocouple placed at its rear. Gases evolved were monitored with a 4-200 mass quadrupole. The binding energy reference level was taken to be the O(1s) line of the oxide species in TiO, at 530.0 eV, which gives a value for C( 1s) of 284.6 & 0.1 eV due to carbon impurities which are always present in the samples.An HP 1000-E computer on line to the spectrometer was used for data handling (background subtraction, area calculation and fitting). Atomic percentages were estimated using sensitivity factors supplied with the spectrometer, which agree well with those reported in the literature.8 1.r. spectra were carried out on a self-supported disc of the sample using a Perkin- Elmer 684 spectrometer with a data-station on line which allows subtraction of the spectra in the absorbance mode. E.p.r. spectra were recorded, under previously reported condition^,^ with a JEOL JES-3P-X spectrometer working in the X-band (9.6 GHz).XRD diffractograms and u.v.-visible reflectance spectra were recorded with a Philips PW 1060 XR diffractometer and a Perkin-Elmer 554 UV-V spectrometer (equipped with an integration sphere and using BaSO, as reference), respectively. The irradiation experiments were carried out at ca. 313 K in a pyrex flask, described elsewhere,lO using 25 cm3 of a 1 mol dm-3 NaOH solution. Aqueous suspensions (ca. 50 mg of the sample) were irradiated with a 200 W Osram HBO bulb, with a total energy output at the flat window of the flask of 470 mW ern-,. Analyses of H, and 0, evolved into the flask free volume (ca. 17.6 cm3) were made by GC.1° Prior to each irradiation period, the suspension was deaerated with argon until N, and/or 0, were not detected. In some of the experiments, measured doses of 0, were injected in the cell before irradiation.Results Sample Characterisation Fig. 1 shows TPD results recorded for two portions of the sample. For the first one, the evolved gases were passed through a trap at 77 K to remove water (and any other condensable gases), while for the second portion the trap was removed; thus all theG. Munuera et al. 1281 40 - I 7 30 E 2 8 .a" 1 b) ; 20 Y * 10 350 450 550 650 T/K Fig. 1. TPD profile of the TiO(0,) sample. (-) Total signal; (....> H,O; (----I 0,. desorbed products were measured. MS-analysis showed that H,O and 0, were the products desorbed during TPD with only traces of condensable impurities (mainly hydrocarbons and CO,). The TPD profile of water evolution (obtained by subtraction of the two TPD spectra) showed that most of the molecular water was evolved at T < 473 K while the second peak at ca.550 K was probably due to the condensation of OH groups. Meanwhile, oxygen evolution gave two peaks, one broad at 540 K and a sharp one at ca. 600 K, decomposition being completed at 673 K. XRD spectra depicted in fig. 2 for the sample heated in flowing N, at different temperatures, show that the original material was amorphous but a crystalline phase was generated when it was heated at T > 573 K (coinciding with the sharp peak of loss of 0, in fig. 1). This crystalline phase corresponds to the anatase form of TiO, which remains stable without any vestige of rutile, even after heating at 623 K for 4 h. In order to characterise the peroxo species present in the original TiO(0,) sample, XPS spectra were recorded. The original compound was quite stable at 300 K in the pretreatment chamber of the XPS spectrometer (under P < lo-' Torr?)), except for a small loss of H,O and 0,, but heating at T > 473 K readily produced large amounts of H,O and 0,, as expected from the TPD results and the changes in the photoelectron spectrum shown in fig.3 for the O( 1 s) level. After heating at 673 K, the O( 1 s) spectrum, in the figure, coincided with that of the TiO, P25 sample used as reference, which agrees with the total decomposition to form TiO, suffered by this compound after this thermal treatment . The best-fitting O(1s) spectra, shown in fig. 3, were obtained for two bands at 530.0 eV (01) and 532.1 eV (011).The small shoulder at higher binding energy in the spectrum of the sample heated at 673 K, which also appeared in TiO, P25, can be attributed6 to OH groups and/or oxide ions on the surface of the sample. From the area of the peaks, the atomic percentages and O/Ti ratios (given in table 1) were obtained. Besides the changes induced in the O(1s) spectrum by the thermal treatment, a shift to higher binding energies (ca. 0.3 eV) of the Ti(2p) level was recorded, suggesting a higher density of electrons on the titanium ions in the original TiO(0,). The final value t 1 Torr = 101 325/760 Pa.1282 Photochemical Behaviour of a Titanium Hydroxyperoxo Compound I I I I I 1 70 60 50 40 30 20 28 Fig. 2. X-Ray diffractograms of the TiO(0,) sample heated in a N, flow. (a) Original; (b) 523 K, 8 h; (c) 573 K, 4 h; ( d ) 623 K, 4 h.at 458.5 eV (after decomposition at 673 K) corresponds to that of the Ti(2p) level for TiO, P25. Further information about the nature of the 011 peak at 532.1 eV was obtained by assessment of the O(2s) and O(2p) valence photoelectron regions. Fig. 4 shows the spectra in this region for the hydroxyperoxide sample and P25 together with their difference spectrum, after complete subtraction of the 01 peaks at 530.0eV. The resulting O(2s) and O(2p) features in the difference spectrum are ascribed to species 011. In addition to the XPS data, i.r. spectra in the range 110MOO cm-l showed some new vibration modes in the TiO(0,) sample when compared with the corresponding spectrum of P25. The difference spectra in fig.5 showed that heating up to 623 K produced loss of bands at 1040,890,770 and 710 cm-l which may be ascribed to the 011 species in the original sample and growth of a broad band centred at ca. 950 cm-'. E.p.r. spectra showed a very intense signal due to 0; species bonded to Ti4+ ions (gl = 2.026, g , = 2.009 and g , = 2.002)11 which was not affected by a high oxygen pressure (P > 10 Torr) though it vanished after heating at ca. 473 K. Fig. 6 shows that the main difference in the u.v.-visible reflectance spectra of TiO, and TiO(0,) was the presence of a weak band at ca. 420 nm (see inset in the figure) which completely disappeared when the sample was heated at 673 K, thus indicating that this band was characteristic of the 011 species detected by XPS.On the other hand, both samples showed a similar absorption pattern at R < 400 nm. Accordingly, a comparative study of their photochemical behaviour under band-gap irradiation is feasible.G. Munuera et al. 1283 0 536.0 532.0 528 .O binding energy/eV Fig. 3. The O(1s) XP spectra of the TiO(0,) sample outgassed (P = Torr) at: (a) 300, (b) 473 and (c) 673 K. (....) Experimental curve; (----) fitted spectrum. Photochemical Behaviour Suspension of 50 mg of the TiO(0,) sample in 25 cm3 of 1 mol dm-3 NaOH did not give any oxygen evolution in the dark. However, band gap irradiation in the U.V. (1 < 360 nm) led to a fast evolution of oxygen which immediately stopped when the lamp was switched off, thus indicating that the TiO(0,) compound was photochemically1284 Photochemical Behaviour of a Titanium Hydroxyperoxo Compound Table 1.Atomic percentages and O/Ti ratios obtained from XPS analysis sample 0 Ti O/Ti OJTi O,,/Ti ~~ ~ TiO,(P25) 67.2 32.8 2.05 1.98" - Ti0(0,)/300 K 73.4 26.5 2.80 1.85 0.95 Ti0(0,)/473 K 69.8 30.2 2.31 1.94 0.37 Ti0(0,)/673 K 67.2 32.8 2.05 1.95 - a Corrected for OH/H,O at the surface. (61 ( C ) 1 : : : : : : : I 40 32 24 16 8 0 binding energy/eV Fig. 4. The O(2s) and O(2p) XP spectra of (a) TiO, and (b) TiO(0,). (c) Difference spectrum b -0.66~ [a factor of 0.66 was used to obtain complete subtraction of the 01 peaks of both samples in the O(1s) region]. decomposed leading to 0, evolution under illumination, while it remained stable in the dark (or under vacuum), though the initial rate of 0, evolution (ca.38pmol h-l) slowly decayed with time. In order to show whether oxygen photo-uptake occurred simultaneously or not, experiments were carried out after introducing 1 cm3 of 0, in the cell (giving 40pmol in the gas phase) before irradiation. The initial rate of 0, photogeneration became 28 pmol h-l, indicating that two simultaneous processes (photoadsorption and photodesorption) are occurring in this sample. This was confirmed by the results, depicted in fig. 7, using portions of the sample which were partially decomposed in N, at different temperatures and/or times. A change, from a net oxygen photodesorption to photoadsorption, was observed when the TiO(0,) samples were irradiated in presence of 40 pmol of 0,. It is worth noting that 0, photo-uptake observed for the TiO(0,) sample which had been decomposed at 623 K was of the same order as that observed for the P25 reference," which agreed with theG.Munuera et al. 1285 1 I 1 1000 800 600 Wcm- ' Fig. 5. (A) 1.r. spectra, in transmittance mode, of the TiO(0,) sample outgassed (P < 10-5 Torr) at: (a) 298, (b) 423 and (c) 623 K. The dashed lines indicate base lines in each case. (B) Difference spectra in absorbance mode. \\ J n 30 0 400 500 Alnm Fig. 6. Diffuse reflectance spectra of (a) TiO, (P-25), (b) TiO(O,), (c) Difference spectrum b-a. decomposition of the TiO(0,) sample to TiO, (anatase). Moreover, irradiation of the latter sample in argon did not give any 0, evolution as previously observed for the P25. Since the presence of a metal (Pt, Rh, etc.) is required to generate H, under u.v.- irradiation of TiO,, experiments were carried out with two TiO(0,) samples. The original and the sample decomposed at 623 K were irradiated in the presence of colloidal metallic rhodium.As shown in fig. 8, a slow increasing evolution of H, was observed for the sample decomposed at 623 K. The presence of Rh neither modified the 0, evolution in the original sample nor did it generate H,.1286 Photochemical Behaviour of a Titanium Hydroxyperoxo Compound 100 - 80 - - - 8 60- =t 2 - LO- - 20- I I 1 L ;i ;1 10 tirnelh Fig. 7. Gas-phase 0, evolution during the irradiation in 1 mol dm-3 NaOH of the TiO(0,) sample heated at the following temperatures and times in a N, flow: (a) original; (b) 523 K, 2 h; (c) 523 K, 4 h; ( d ) 573 K, 4 h; (e) 623 K, 4 h.0 2 4 6 8 timelh Fig. 8. Gas-phase evolution during the irradiation in 1 mol dm-3 NaOH of (a) the TiO(0,) original sample, (b) the TiO(0,) sample in the presence of colloidal Rh, and (c) the TiO(0,) sample decomposed at 623 K in the presence of colloidal Rh.G. Munuera et al. 1287 Discussion Sample Characterisation To our knowledge, a detailed spectroscopic study of titanium hydroxyperoxide compounds of the type used here has not been previously reported. The two XPS bands at 530.5 (01) and 532.1 eV (011) may be ascribed to 0x0 (02-) and peroxo (0;-) species in agreement with the binding energies reported for similar species in other systems.12 Nevertheless, some contribution of H,O/OH- species (present in our compound) to the 011 peak cannot be ruled out.6 As shown in table 1, the excess of oxygen (O/Ti ca.2.80 compared to 2.05 for TiO,) must be associated with such 011 species (OII/Ti ca. 0.9), corresponding to ca. 0.45 peroxo species per Ti ion. The difference from the 1 0.2 value obtained from the TG-MS analysis may be ascribed to partial decomposition at 298 K of the peroxo species present at the surface of the TiO(0,) sample under the ultra-high vacuum conditions required for XPS recording. The assignment in the valence band region of the molecular orbital levels of the 011 species to a peroxide species (&, a::, oip, nip, n2*,") agrees with the UPS spectra of diatomic oxygen adsorbed on different metals reported by Kamath and Rao.13 The pattern of these M.O. levels is similar to that ascribed to the Si- species by Endo et a1.l4 in the XPS spectra of TiS, and TiS,.Several i.r. bands in the range 1040-890 cm-l, which disappeared upon heating, may be ascribed to different types of peroxo species. l5 Thus, q2-peroxo (T-shaped) complexes show vibrations ascribed to the uo-o mode in the range 932-800 cm-l l6 while solid-state peroxides give bands in the region 770-700 cm-l l 7 normally ascribed to peroxo species bonded to several metal cations (p-peroxo compounds). Our data in fig. 5, clearly suggest the existence of q2-peroxo and p-peroxo complexes in the TiO(0,) sample as proposed by Gratzel et al.3 Heterogeneity in the 0-0 bond order is also suggested by the strong band at 1040cm-' which does not correspond exactly to reported wavenumbers of titanium peroxo-complexes.15 On the other hand, the band at 950 cm-l, which appeared during the thermal decomposition of the TiO(0,) sample, may be associated with vTip0 modes of 0x0 Ti=O groups at the TiO, surface, in agreement with the values reported for mononuclear titanium-oxo complexes.'' A similar set of bands in the region 1180-900 cm-l and 900-730 cm-' have been found by Davidov et all9 upon adsorption of oxygen on reduced TiO, samples. We have reported the growth of the same set of i.r.bands, which were ascribed to peroxo species, during 0, photoadsorption on a TiO, P25 sample,,' which agree with the results of Augustynski et aL5 who showed the formation, during 0, photoreduction or H,O photo-oxidation, of the same set of peroxo species bonded to T14+ ions in TiO, photoelectrodes.The shift to lower binding energies observed for the Ti(2p) level in our sample, compared to TiO,, may be related to a degree of donation to the titanium ions from the 71 and n* molecular orbitals of the peroxide, as has been observed in the case of peroxo-complexes in solution. 21 The study of the stability of the Ti(p-0,) and Ti(q2-0,) species in our TiO(0,) compound, examined by TPD and XRD, indicates that they start to decompose only at ca. 423 K. It is worth noting that under all conditions used both types of peroxo compounds were identified, indicating a similar stability. Photochemical Behaviour The identical optical absorption at A < 400 nm of TiO(0,) and P25, allows a comparative photochemical study of both samples. The results shown in fig.7 clearly indicate that the net observed 0, evolution (i.e. 0, photodesorption or photoadsorption), during irradiation of partially decomposed TiO(0,) samples, corresponds to differences1288 Photochemical Behauiour of a Titanium Hydroxyperoxo Compound Fig. 9. Diagram of the modulated potential barriers expected for an M/TiO, particle irradiated under alkaline conditions. between two simultaneous opposite processes, i.e. photodecomposition of peroxo species and 0, photoadsorption. These two processes probably occur during water photo-cleavage on M/TiO, (M = Pt, Rh, etc.) where only a net 0, photo-uptake is normally observed10*22 as in pure TiO,. This indicates the incorporation of peroxo species into the bulk of the TiO, support in these samples, as previously suggested,' to account for the lack of 0, evolution during water photo-cleavage.It is important to understand why photodecomposition of the peroxide does not occur in such M/TiO, systems. Using TiO, with preadsorbed H,O,, we have a Fenton-like reaction for the decomposition of the peroxide upon irradiation which involves photoelectrons according to (4) 3H,O, + 2e;i,2 + 2H,O + 0, + 20H-. The presence of metallic particles of Rh or Pt on the TiO, seems to prevent this reaction, probably due to trapping the photoelectrons by the metallic particles, which then act as efficient electron scavengers." Nakato and TsubomuraZ4 have concluded that M/TiO, photoelectrodes containing small metallic particles ( < 5 nm) widely dispersed on TiO,? show a modulation of the semiconductor bands.Meanwhile, Aspnes and Heller25 have found that Rh particles on TiO, develop ohmic contacts with the support in a H, atmosphere. From these two observations, a Schottky barrier might exist at the liquid/TiO, interface under our alkaline conditions. An ohmic contact could be set up between the metallic particles and TiO, in the presence of gaseous H,, as shown in the scheme in fig. 9. This latter situation has been recently confirmed26 using e.s.r. which t A situation fulfilled by the Rh/TiO, sample previously used by us in water photocleavage experiments.'OG. Munuera et al. 1289 detected a non-activated and reversible electron transfer (i.e. Ti3+ generation) from weakly adsorbed H, to the TiO, indicating the ohmic nature of the Rh-TiO, contact.According to this model, during irradiation photoelectrons should be trapped at the potential well generated by the metallic particles (where the H+ reduction takes place), while peroxide is expected to be formed [according to eqn (l)] at the free TiO, surface, where photoholes are driven by the Schottky barrier at the liquid/TiO, interface. This would imply the oxidation of the basic OH; groups covering the free TiO, surface under these alkaline condition^.^' The H,O, generated in this way would then react with the TiO, surface according to eqn (2) and (3). This process seems to progress far into the bulk thus leading to corrosion of the TiO,. Moreover, the unchanged 0, evolution and the lack of H, evolution during irradiation of the original TiO(0,) sample in the presence of Rh (fig.8) seems to suggest that such peroxo species prevent a good electric contact between the TiO(0,) surface and the metallic particles, contrary to the situation for the sample which had been decomposed into TiO,, where H, slowly evolved. These results clearly suggest that the photostationary situation normally reached for H, evolution after long irradiations during water photocleavage experiments using M/TiO, systems,lo* 28 might be due to a progressive loss of ohmic contact between the metal particles and the TiO,, even in the presence of hydrogen. This effect occurs when the peroxide is incorporated into the TiO, close to the metallic particles and explains that thermal decomposition of such species restores H, evolution to its original Conclusions The XRD and TPD study of a TiO(0,) sample prepared from TiC1, and H,O, shows that the resulting amorphous material is rather stable at 298 K even under ultra-high vacuum, while it' decomposes by heating at T > 423 K to give 0,, H,O and TiO, (anatase).The characterisation of the peroxo species in this compound, carried out by the analysis of the XPS [O(ls), O(2s) and O(2p) levels], e.p.r. and i.r. spectra, confirm the existence of 0;- (and 0,) species bonded to Ti4' ions as ~ ~ - 0 , and p - 0 , ligands which are readily decomposed evolving 0, under band gap (A < 400 nm) irradiation. Experiments with added colloidal Rh particles to the TiO(0,) sample, either in its original state or after decomposition to TiO,, indicate that such peroxo compounds prevent a good electric contact between the metal and the photosupport.According to these results a mechanism is proposed for water photocleavage on M/TiO, which involves modulation of the semiconductor bands by the metal, as proposed by Nakato and T s ~ b o m u r a , ~ ~ leading to the formation of potential wells at the metallic particles which act as electron traps. This fact prevents the photodecomposition into 0, of the generated peroxide, through a Fenton-like process, and allows the incorporation of peroxide into the TiO, destroying the electrical contact at the metal/TiO, interface and stopping H, evolution. Authors thank Dr A. Navio for the preparation of the TiO(0,) sample, the CAICYT, the 'Junta de Andalucia ' and the ' Fundacion Ramon Areces ' for financial support and the referees for their suggestion to improve the manuscript.References 1 A. Fernandez, A. R. Gonzalez-Elipe, J. P. Espinos and G. Munuera, 6th Int. Conf. on Photochemical 2 G. Munuera and F. S. Stone, Faraday Discuss. Chem. Soc., 1971, 52, 205. 3 J. Kiwi and M. Gratzel, J . Mol. Catal., 1987, 39, 63. 4 J. Kiwi and C. Morrison, J . Phys. Chem., 1984, 88, 6146. 5 M. Ulmann, N. R. de Tacconi and J. Augustynski, J . Phys. Chem., 1986, 90, 6523. Conversion and Storage of Solar Energy, Paris (1986), Book of abstracts D-133.1290 Photochemical Behaviour of a Titanium Hydroxyperoxo Compound 6 G. Munuera, A. R. Gonzalez-Elipe, J. P. Espinos and A. Navio, J. Mol. Struct., 1986, 143, 227. 7 J. Miihlebach, K. Muller and G.Schwarzenbach, Inorg. Chem., 1970, 9, 2381. 8 C. D. Wagner, L. E. Davies, M. V. Zeller, J. A. Taylor, R. M. Raymond and L. H. 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