Comprehensive Kinetic and Mechanistic Analysis of TiO2

Article pubs.acs.org/JPCC

Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct−Indirect Model: (II) Experimental Validation Juan F. Montoya,† Mohamed F. Atitar,‡ Detlef W. Bahnemann,‡ José Peral,† and Pedro Salvador*,† †

Departamento de Química, Universidad Autónoma de Barcelona, 08193 Cerdanyola del Vallés, Spain Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany

‡

S Supporting Information *

ABSTRACT: As a continuation of the Direct−Indirect (D-I) model theoretical approach presented in Part I of this publication, concerning the photocatalytic oxidation of organic molecules in contact with TiO2 dispersions, a comparative photooxidation kinetic analysis of three model organic molecules, benzene (BZ) dissolved in acetonitrile (ACN), phenol (PhOH) dissolved in either water or acetonitrile, and formic acid (FA) dissolved in water, is presented to test the applicability of the D-I model under both equilibrium and nonequilibrium adsorption−desorption conditions. A previous analysis involving diffuse reflectance ultraviolet−visible (DRUVS) and Fourier transform infrared (FTIR) spectroscopy, combined with adsorption isotherm plots, shows that BZ chemisorption on the TiO2 surface is not allowed, physisorption being in this case the only possible adsorption mode. In line with D-I model predictions, BZ photooxidation is observed to take place via an adiabatic indirect transfer (IT) mechanism, with the participation of photogenerated terminal −O•− s radicals as oxidizing agents. In contrast, because of their strong chemisorption, FA species dissolved in water are found to be mainly photooxidized via inelastic direct transfer (DT) trapping of photogenerated valenceband free holes (h+f ). Finally, when dissolved in water, PhOH chemisorption is not favored because of the strong electronic affinity of water molecules with the TiO2 surface, while chemisorption strength considerably increases when PhOH is dissolved in ACN, as far as the electronic interaction of solvent molecules with the TiO2 surface is negligible. Consequently, as predicted by the D-I model, PhOH dissolved in water is photooxidized via a combination of IT and DT mechanisms, the IT photooxidation DT rate (vIT ox ) being about 1 order of magnitude higher than DT photooxidation rate (vox ). In contrast, when ACN is used as solvent, IT DT vox remains practically unchanged, while vox increases by about 2 orders of magnitude. These photooxidation results sustain the central D-I model hypothesis that the degree of substrate species interaction with the TiO2 surface is a decisive factor determining the kinetics of photocatalytic reactions. The effect of adsorption−desorption equilibrium rupture on the photooxidation kinetics of dissolved substrate species, predicted by the D-I model, is analyzed for the first time from experimental kinetic data concerning the photooxidation of PhOH dissolved in water under high enough illumination intensity (ρ ≈ 1017 cm−2 s−1).

1. INTRODUCTION As pointed out in Part I of this publication,1 due to their role as promoters in oxidation−reduction reactions, the dynamic study of photogenerated charge carriers is of seminal importance when a sound and comprehensive understanding of TiO2 photocatalysis phenomena is a priority objective.1,2 Once electron−hole pairs are photogenerated: hv ≥ 3.2eV

TiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→

hf+

+

ef−

or by terminal oxygen/hydroxyl ions, depending on the electrolyte pH:

(1)

(2)

or be trapped by either TiO2 chemisorbed substrate species (RH2)ads: hf+

•

+ (RH 2)ads → RH + H

+

© 2014 American Chemical Society

(4)

hf+ + ( >OH−s ) → ( −OH•s ) pH < pH zpc

(5)

While reaction 3 concerns the photooxidation of chemisorbed organic molecules via an interfacial, inelastic direct • transfer (DT) mechanism,1,3−5 −O•− s /−OHs radicals photogenerated trough reactions 4 and 5 are able to further oxidize dissolved substrate species ([RH2]liq) weakly interacting with

valence band free holes can either recombine with conduction band free electrons (hf+ + ef− → heat)

hf+ + ( >O2s −) → ( −O•‐ s ) pH > pH zpc

Received: December 12, 2013 Revised: May 9, 2014 Published: June 3, 2014

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dx.doi.org/10.1021/jp4121657 | J. Phys. Chem. C 2014, 118, 14276−14290

The Journal of Physical Chemistry C

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BZ dissolved in ACN, FA dissolved in water, and PhOH dissolved in water and ACN, respectively. The information so obtained, in combination with adsorption isotherm results, will allow us to propose plausible adsorption modes of these organic molecules at the TiO2 surface, as a basic element for a further photooxidation kinetics study. A direct correlation between adsorption modes and the kinetic behavior predicted by the D-I model will be obtained on the basis of the fitting between experimental kinetic data and those predicted by D-I kinetic equations corresponding to the different combinations of interfacial charge transfer mechanisms discussed in Part I of this publication, under adsorption−desorption equilibrium and equilibrium rupture of substrate species.1 This constitutes the experimental validation of the D-I model as a powerful tool for obtaining information about interfacial charge transfer mechanisms involved in photocatalytic reactions, from the kinetic analysis of experimental photooxidation data.

the semiconductor surface via an interfacial, indirect transfer (IT) mechanism1,3,4,6−10 • + − 2− [RH 2]liq + (− OH•s /− O•− s ) → RH + H + ( > OH s / > Os )

(6)

According to the D-I kinetic model, DT and IT photooxidation mechanisms present important differences: (1) DT involves ultrafast, inelastic transfer of nonthermalized, free valence band photogenerated holes to semiconductor chemisorbed substrate species, while IT involves adiabatic transfer of thermalized holes across the TiO2−liquid interface;3,4 (2) chemisorption of substrate species is a prerequisite for the DT mechanism to take place, while physisorption is consubstantial to the IT mechanism;3,4,6,7 (3) both photooxidation mechanisms are carried out with the participation of different active sites: DT mechanism involves Ti4+ surface ions acting as TiO2 chemisorption sites of dissolved substrate species. In contrast, • IT mechanism involves −O•− s /−OHs radicals able to adiabatically accept electrons of physisorbed substrate species.3,4,6−10 Despite that DT and IT are well differentiated photooxidation mechanisms, they can coexist and even lead to a similar distribution of photooxidation products, hindering to be clearly differentiated.3,4 However, a clear differentiation of both mechanisms is possible on the basis that they involve well differentiated photooxidation kinetic processes.1,3,4,6,7 Furthermore, from a practical point of view, the discrimination of the photooxidation mechanisms involved in the D-I kinetic model is specially important when the design and scaling-up of the reactors involved in practical, photocatalytic applications is taken into account.11−17 Even if the kinetic analysis of photocatalytic reactions have largely relied on rate equations based on the Langmuir− Hinshewoold (L-H) model,1,5,18−22 its applicability has been widely questioned.4,23−32 Our objective here is to show that the Direct−Indirect (D-I) kinetic model, based on the degree of interaction of dissolved substrate species with the semiconductor surface,3,4 is a reliable alternative to the L-H model. During the past decade, there has been an extraordinary interest in the application of surface science techniques (SST) to the study of chemical reactions taking place at semiconductor surfaces, including TiO2. Especially relevant are those studies devoted to understand the existing relationship between surface properties and photochemical events triggered under TiO2 photoexcitation.33−35 In this sense, a deeper understanding of TiO2 surface chemistry processes involved in photocatalytic reactions is specially interesting, but always keeping in mind that SST experiments are performed under ultrahigh vacuum (UHV) conditions while photocatalytic experiments concern the semiconductor−liquid phase/gas phase junction. Moreover, the use of Fourier transform infrared spectroscopy (FTIR)36−45 and diffuse reflectance UV−vis spectroscopy (DRUVS) techniques have been extensively used in monitor the interaction, structure, and reactivity of diverse organic substrates with the TiO2 surface in the presence of a solvent,46 proving to be very useful when applied to the study of adsorption modes involved in the specific interaction of organic species with the TiO2 surface. Because of the crucial role of adsorption phenomena, their study can be considered as an excellent starting point for further kinetic studies of photocatalytic reactions.3,4 With this purpose in mind, FTIR and DRUVS techniques have been employed here to better understand the electronic interaction of the TiO2 surface with three model compounds,

2. EXPERIMENTAL METHODS 2.1. Materials. Aldrich TiO2, 100% anatase, with an average particle size of 25 nm and a BET area of 55 m2/g was used in the kinetic study and spectroscopic measurements. Other analytical grade reagents were used as received without further purification. Aqueous solutions were prepared with deionized water obtained from a SARTORIUS ARIUM 611 apparatus (conductance = 18 MΩ cm−1) 2.2. High Performance Liquid Chromatographic (HPLC) Analysis. In kinetic and adsorption experiments, the concentration time evolution of organic substrates was monitored by HPLC. System details, chromatographic methods, and sample treatments are presented in Appendix A of the Supporting Information. 2.3. Photooxidation Kinetics. TiO2 powders were dispersed in 100 mL of the used solvent, either water or acetonitrile (Ccat = 0.5 g/L), by sonication and shaking in an ultrasonic cleaning bath for 15 min. In the case of aqueous suspensions a pH = 3 was adjusted by addition of a HClO4 standard solution. An aliquot of a stock solution (5000 ppm) of the organic substrate was further added to reach the desired initial concentration. The suspensions were covered with foil and left under stirring at 300 rpm overnight to reach adsorption equilibrium. Irradiation experiments were carried out under top illumination of a borosilicate glass cylindrical reactor (I.D. = 6 cm, h = 7 cm) with an assembly of six UV-A lamps (Philips Cleo 15 W; emission, 300 < λ < 400 nm; λmax = 365 nm). The photon flux arriving to the photoreactor was controlled by regulating its distance to the light source and then measured via an standard potassium ferrioxalate actinometry procedure.47 To guarantee the stability of the photon flow, the light source intensity was monitored with a UV-A radiometer. A photon flux range 1.5 × 10−6 < ρ < 8 × 10−5 einsteins min−1 was used. Before illumination, 1 mL of the previously equilibrated suspensions was analyzed, being considered as the initial equilibrium concentrations reported in Figures 3−6. The photoreactor was filled with 100 mL of the equilibrated suspension and magnetically stirred at 300 rpm while being bubbled with air. Afterward, the dark screen was removed and the reactor illuminated under different time intervals depending on the experimental conditions (solvent and organic substrate used). Photooxidation rate versus photon flux experiments were performed in triplicate, allowing initial reaction rates to be obtained with a mean experimental error of about ±12% (for details, see Appendix B of the Supporting Information). 14277



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Control experiments showed no substrate concentration decrease due to air stripping or photolysis. The temperature of the liquid phase, periodically monitored during the experiment, was 25 ± 2 °C. 2.4. Attenuated Transmission Reflectance Fourier Transform IR Spectroscopy (ATR-FTIR). The interaction of BZ, FA, and PhOH substrates with the TiO2 surface was monitored by in situ ATR-FTIR spectroscopy.36−45 The spectra were recorded with a BOMEM MB122 instrument equipped with a liquid N2-cooled MTC-A (mercury− tellurium−cadmium) detector and a Pike Technologies horizontal ATR unit equipped with a trapezoidal ZnSe-ATR crystal of 6.8 × 72 mm2 on the horizontal probe face, 4 mm thick, and 45° incidence angle, allowing nine upper face reflections with a spectral resolution of 4 cm−1. The interferometer and the infrared light path were constantly purged with argon. Final spectra represented the average of 80 scans. To eliminate minor fluctuations due to instrumental instability, baseline corrections were performed. Neither spectra smoothing nor ATR corrections were fulfilled. Details about the used solution compartment can be found elsewhere.41 To avoid spectral interferences due to materials degradation, POM (polyoxymethylene) and borosilicate glass, both chemically resistant to the employed solutions, were employed to construct the solution compartment and reactor windows, respectively. A volume of 10 mL of solvent, either water (pH 3, 10 mM KNO3) or ACN, was introduced into the solution compartment; after 2 min, a background spectrum was collected; the solvent was then substituted by 10 mL of the test solution, with sequential spectra being then collected during 60 min, at a rate of 60 s per spectrum. Spectra (a), corresponding to the pure solutions and bare ZnSe crystal, are shown in Figure 2. The ZnSe crystal was then covered with a TiO 2 thin layer according to a procedure previously described.41 The same procedure was used in TiO2-substrate adsorption studies, when acetonitrile instead of water was used as solvent. One hour was considered enough time for adsorption equilibrium to be reached, as no band intensity changes could be observed for longer times. 2.5. Diffuse Reflectance UV−Vis Spectroscopy (DRUVS). TiO2 samples for DRUVS analysis were prepared as follows: 2 g of TiO2 powder was dispersed in 100 mL of the solvent (either water or ACN) by simultaneous shaking and sonication for 15 min in an ultrasonic cleaning bath. An aliquot of 5 mL of a stock solution (10 000 ppm) of the organic substrate was subsequently added to the suspension, leading to a final substrate concentration of 476 ppm. With aqueous suspensions, pH = 3 was adjusted by adding a HClO4 standard solution. The suspensions were stirred in the dark at 300 rpm for 24 h; then the TiO2 powder was recovered by filtering the suspension with Nylon filter membranes (0.45 μm pore size) and further drying at 60 °C for 1 h. The UV−vis spectra of the resulting TiO2 powders were measured in a Varian Cary 100 spectrophotometer equipped with a diffuse reflectance Labsphere attachment. TiO2 spectra without adsorbed organic substrates were obtained in a similar way, without addition of the organic substrates to the TiO2 suspensions.

Figure 1. DRUVS spectra of anatase TiO2 powders previously equilibrated with pure solvents, either water or ACN (a), or in contact with different organic substrates: BZ dissolved in ACN (b), PhOH dissolved in water (c), and PhOH dissolved in ACN(d).

contact with solvents not containing dissolved organic substrates, are practically identical. Pure anatase absorption takes place in the λ < 390 nm region, with their spectra remaining unchanged when the TiO2‑anatase sample is in contact with the pure solvent (either water or ACN). When TiO2-anatase enters in contact with ACN dissolved BZ, the corresponding spectrum (b) shows identical absorbance in the λ < 390 nm region, indicating either no surface complex formation or absence of absorption in the visible region of generated surface complexes. In contrast, when TiO2-anatase is in contact with PhOH dissolved either in water or ACN, the corresponding (c) and (d) spectra show an onset light adsorption shift toward higher wavelengths. Remarkably, the magnitude of such a red shift depends on the solvent: 20 nm when water is used as solvent (spectrum c) and 90 nm when dissolved in ACN. The absorption in the visible region of TiO2/PhOH samples evidences the formation of a surface complex between PhOH species and the semiconductor surface.46 According to the literature, a covalent bonding between the oxygen atom of PhOH molecules and a terminal, Ti(IV) ions (phenolate link) is probably generated,43,48 the resulting surface complex being probably able to absorb in the visible region.49,50 Considering that the surface density of the visible light absorbing surface complex depends on the amount of chemisorbed PhOH, regardless of the solvent used, the observed red shift onset dependence on the used solvent (20 and 90 nm for water and ACN, respectively) may be attributed to the solvent influence on the generated TiO2/PhOH complex. While in the TiO2/ H2O/PhOH system PhOH molecules are less efficiently chemisorbed onto the TiO2 surface, due to the higher competition of water molecules for occupying Ti(IV) sites, when water is substituted for ACN (TiO2/ACN/PhOH sample), PhOH molecules are expected to be more strongly chemisorbed because of the low affinity of ACN molecules with Ti(IV) sites.51 Consequently, a higher surface density of PhOH species, leading to a higher red shift in the absorption spectra, is to be expected for TiO2 exposed to ACN dissolved PhOH, than in contact with PhOH dissolved in water. It is worth noting that used TiO2/substrate samples were obtained by filtration of the TiO2 powder under equilibrium with a substrate solution, in

3. RESULTS AND DISCUSSION 3.1. Spectroscopic Study of TiO2 Interaction of BZ Dissolved in ACN, FA Dissolved in Water, and PhOH Dissolved in Water and ACN. As shown in Figure 1, DRUVS spectra (a) corresponding to TiO2‑anatase, either clean or in 14278



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such a way that the expected density of the generated surface complex should correspond to adsorption equilibrium conditions. However, our DRUVS measurement procedure differs from that used by Kim and Choi,46 who used TiO2 samples obtained by solvent evaporation, forcing the adsorption of large amounts of PhOH molecules. Therefore, a higher surface complex density than that obtained under adsorption equilibrium conditions is to be expected. This may explain the higher red shift observed by Kim and Choi for the absorption spectrum of the TiO2/H2O/PhOH sample (λ < 530 nm), with respect to the red shift observed in Figure 1c. In summary, a higher absorption spectrum red shift is observed for the TiO2/ACN/PhOH sample by comparison to the TiO2/ H2O/PhOH one. A similar conclusion is obtained from the ATR-FTIR TiO2− substrate interaction study. As shown in spectrum (a) of Figure 2A, seven bands located at 1153, 1169, 1223, 1261, 1366, 1474, and 1501 cm−1, respectively assigned to δ(CH), δ(OH), δ(OH) + δ(CH), ν(CO), ν(CC), ν(CC) + ν(CO), and δ(CH) + ν(CC) vibration modes, identify ACN dissolved PhOH species.43,52−55 In contrast, the main bands characterizing an acidic, aqueous PhOH dissolution (Figure 2B, spectrum a) are located at 1242 cm−1 (ν(CO)), 1377 cm−1 (ν(CC) + ν(CO)), 1477 cm−1 (ν(CC)), and 1501 cm−1 (δ(CH) + ν(CC)), together with two shoulders at 1192 cm−1 (δ(OH)) and 1269 cm−1 (ν(CO)). The band at 1223 cm−1 in spectrum (a) of Figure 2A, attributed to δ(OH) mode PhOH vibrations,57 evidences the presence of protonated species, while the band at 1261 cm−1 corresponds to the ν(CO) mode of PhOH deprotonated species. Thus, the spectra show that, in ACN dissolved PhOH, an equilibrated mixture of protonated and deprotonated PhOH species must exist. On the other hand, only a small shoulder at 1269 cm−1, attributed to the scarce presence of PhOH deprotonated species, can be observed in spectrum (a) of Figure 2B, while a broad, intense band corresponding to the ν(CO) mode of protonated species can be observed at 1242 cm−1. This is in agreement with the wellknown fact that PhOH dissolved in water presents a pKa = 9.98,56 indicating that protonated PhOH species must predominate at pH 3. Under contact of PhOH solutions with the TiO2 surface (spectra b−d of Figure 2A and B), some spectral changes can be observed, together with an intensity increase of ATR-FTIR bands, directly related to the PhOH coverage degree of the TiO2 surface.57 A comparison of these spectra with those corresponding to pure solutions help us to envisage the structure and TiO2 covering degree by PhOH species. The spectral differences between Figure 2A and B, in the region between 1100 and 1300 cm−1, can be reasonably attributed to the different behavior of dissolved PhOH molecules in water and in ACN, while the nonexistence of appreciable changes between FTIR absorption bands in Figure 2B and C indicates that both spectra correspond to solution species. Indeed, when a molecule is not absorbed (benzene) or is weakly adsorbed (phenol in water), there are very little changes in the FTIR spectra (Figure 2B), or no changes at all (Figure 2C) if those are obtained for the same solution in the presence of a TiO2 layer on the surface of the ATR detector. This means that FTIR spectra of the solution are virtually identical with or without the TiO2 layer, indicating the nonexistence of an important amount of chemisorbed adsorbate molecules in the presence of the catalyst. However, when the molecule is strongly adsorbed (phenol in ACN, Figure 2A), the presence of the TiO2 layer

Figure 2. ATR-FTIR spectra of (A) a 100 mM solution of ACN dissolved PhOH in the absence (a) and in the presence of an anatase film under different TiO2−PhOH contact times (b−d). (B) A 100 mM solution of water dissolved PhOH (pH 3) in the absence (a) and in the presence of a TiO2 anatase film, under different TiO2−PhOH contact times (b−d). (C) A 3365 mM solution of ACN dissolved BZ in the absence (a) and in the presence of a TiO2 anatase film under different TiO2−BZ contact times (b, c).

completely changes the spectra of the solution alone. The noticeable differences are due to the large amount of chemisorbed molecules that involve band absorption increase and band frequency displacement. Two main adsorption modes, molecular and dissociative, are inferred. While under molecular adsorption PhOH species weakly interact with the TiO2 surface (physisorption) and their structure remains unchanged, under dissociative adsorption PhOH molecules 14279



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Table 1. TiO2 Langmuir Adsorption Isotherm Constants of FA Dissolved in Water and PhOH Dissolved in Water and ACN isotherm phenol in water (TiO2/PhOH/H2O) formic Acid in water (TiO2/FA/H2O) phenol in acetonitrile (TiO2/PhOH/ACN)

Kads (cm3/molecule) −17

1.328 × 10 8.915 × 10−18 3.961 × 10−18

b (molecule/nm2)

Kadsb (cm)

0.9390 13.91 9.234

0.00125 0.0124 0.003 66

atoms of FA molecules and terminal Ti(IV) ions.34 The Ti−O bond length of FA chemisorbed on TiO2, obtained via the scanned energy mode of photoelectron diffraction (PHD), is known to be 2.08 Å, while the bond length of Ti−O for chemisorbed water is of 2.21 Å,34 indicating a weaker covalent interaction. It explains why FA dissolved in water is able to be chemisorbed at the TiO2 surface, in competition with an amount of water molecules orders of magnitude greater (more details can be found in the Supporting Information, Appendix A). 3.2. Adsorption Isotherms of FA Dissolved in Water and PhOH Dissolved in Water and ACN. The information concerning TiO2 structure and coverage degree, obtained by means of spectroscopic techniques, must be complemented with that obtained from adsorption isotherms. Experimental details, figures, and calculations dealing with adsorption isotherms are presented in the Supporting Information (Appendix A). Constant values of Langmuir type isotherms obtained from the TiO2 adsorption study of FA and PhOH dissolved in water, as well as PhOH dissolved in acetonitrile, are shown in Table 1. Adsorption isotherms data about BZ dissolved in ACN are not included in Table 1 as adsorbed BZ concentrations were found to be far bellow the sensitivity of the used experimental procedure, as BZ electronic interaction with the TiO2 surface responds to a physisorption mechanism. It is clear from experimental data shown in Table 1 that the “b” parameter, associated with the surface density of adsorbed substrate species forming a monolayer, follows the trend: (TiO2/FA/H2O) > (TiO2/PhOH/ACN) > (TiO2/PhOH/ H2O). Different trends are apparently observed for the Kads Langmuir adsorption constant. However, the “Kadsb” parameter follows the same tendency as the “b” parameter, with “Kadsb” being a parameter that indicates the tendency of the substrate to form a monolayer under adsorption equilibrium conditions. Congruently with the spectroscopy results shown in Figures 1 and 2, adsorption isotherm data indicate that FA adsorption on TiO2 is highly favored with respect to BZ and PhO adsorption, as it reaches a maximum covering degree of about 70% of TiO2 adsorption sites (remember that T = 20 sites/nm2 is the maximum number of available surface adsorption sites for anatase nanoparticles dispersed in water).59 With respect to PhOH adsorption constants, they are found to be highly dependent on the used solvent: while chemisorption of PhOH dissolved in ACN reaches a maximum covering degree of about 46% of the total number of available Ti(IV) adsorption sites, only about 4.7% is obtained for PhOH dissolved in water. As expected, a good agreement is found between adsorption isotherms results and spectroscopy data. In summary, physisorption is the only possible interaction mechanism between BZ molecules and the TiO2 surface, while chemisorption is found to be the prevalent adsorption mode for FA species dissolved in water. With respect to PhOH, physisorption and chemisorption coexist, with phenol chemisorption degree strongly depending on the used solvent: chemisorption is hindered to a great extent in the presence of water, because of the strong competition of PhOH and H2O for

become deprotonated, with oxygen atoms of the alcohol moiety becoming covalently bonded to Ti(IV) sites (chemisorption). The pronounced intensity of the band at 1261 cm−1 in the presence of the TiO2 film (spectra b−d of Figure 2A) evidences a density increase of deprotonated, adsorbed PhOH species, as corresponds to dissociative adsorption. In contrast, the intensity increase of the band at 1223 cm−1 can be attributed to the modification that the chemical interaction with TiO2 surface produces on protonated PhOH species (inner sphere complexation). This interpretation of FTIR spectra is in agreement with spectral DRUVS features of Figure 1. On the other hand, the changes observed in bands at 1474 and 1501 cm−1 (spectrum a, Figure 2A), associated with the aromatic ring, after PhOH molecules enter in contact with the anatase film (spectra b−d, Figure 2A), constitute a further indication of surface complexes generation between the TiO2 surface an PhOH species. Typical bands of pure PhOH are replaced by a broader and more intense band centered at 1485 cm−1 attributed to the aromatic ring vibrations. With respect to the scarce influence of the contact time of dissolved PhOH species with the semiconductor surface on FTIR spectra (Figure 2A, spectra b−d), it constitutes proof that adsorption equilibrium is very rapidly reached, as ACN molecules do not compete with PhOH species for adsorption at Ti(IV) sites.51 In contrast, when an aqueous PhOH solution enter in contact with the TiO2 layer, an important competition between PhOH and water molecules for the Ti(IV) sites is produced. Such a competitive adsorption explains the higher time needed for adsorption equilibrium to be reached (spectra b−d, Figure 2B). Finally, it is relevant the general intensity increase of FTIR bands observed in Figure 2B, maintaining unchanged their relative intensities, as an indication that the adsorption mode of PhOH dissolved in water mainly corresponds to physisorption. The lack of phenol chemisorption on the TiO2 surface is in agreement with the conclusion inferred from DRUVS spectra of Figure 1. With respect to BZ interaction with the TiO2 surface, the ATR-FTIR spectra of Figure 2C present bands at 1377 cm−1 (ν(CC)), 1396 cm−1 (ν(CC)), and 1481 cm−1 (ν(CC)), which remain unchanged when ACN dissolved BZ (Figure 2C, spectrum a) is exposed to the TiO2 layer (Figure 2C, spectra b and c), indicating absence of chemical interaction of BZ species with the TiO2 surface (note that the spectral baseline appears deliberately shifted in order to avoid spectra superposition). In agreement with our thesis that physisorption is the only possible BZ adsorption mode on TiO2, are the results of a recent study of BZ adsorption on TiO2 via a combination of scanning tunneling microscopy (STM) and temperatureprogrammed desorption (TPD) techniques by Baddorf and co-workers.58 ATR-FTIR spectra of water dissolved FA were previously reported.6,7 There, it was shown that when dissolved in water, FA strongly interacts with the TiO2 surface, with chemisorption being the main adsorption mode, even being able to displace chemisorbed water molecules, giving rise to TiO2-formiate complexes via generation of covalent bonds between oxygen 14280



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chemically interacting with Ti(IV) sites, while it is favored when ACN is used as solvent, because of its weak electronic interaction with the TiO2 surface. Scheme 1 illustrates TiO2 surface interactions modes of the studied model compounds. Scheme 1. Interaction Modes of BZ, FA, and PhOH, Model Organic Compounds with the TiO2 Surfacea

a

(a) FA chemisorption; (b) BZ physisorption; (c) PhOH chemisorption; and (d) PhOH physisorption. Solid lines represent covalent bonding, while dashed lines represent weak interactions, such as van der Waals forces.

3.3. Photooxidation Kinetic Study of BZ Dissolved in ACN, FA Dissolved in Water, and PhOH Dissolved in Water and ACN. On the basis of the theoretical kinetic− mechanistic approach concerning the photocatalytic photooxidation of dissolved organic substrates presented in Part I of this publication,1 and taking into account the just described implications of the aforementioned TiO2−substrate interaction, a photooxidation kinetic study of those model compounds previously considered (BZ dissolved in ACN, FA dissolved in water, and PhOH dissolved in water and ACN) is presented, specially emphasizing on the analysis of the reaction rate dependence on the photon flow and substrate concentration. From now on, equation numbers followed by the notation (-I) refer to mathematical expressions of Part I of this publication.1 I. BZ Dissolved in ACN. Taking into account that physisorption is the only possible interaction mode of BZ molecules with the TiO2 surface, the photooxidation of ACN dissolved BZ should take place via a pure IT mechanism, the initial photooxidation rate under adsorption−desorption equilibrium conditions being therefore expressed by eq 1-I IT 2 IT (vIT ox = (d[(RH2)liq)]/dt)(t→0) = [(a [(RH2)liq]) ] + 2a k0ρ1/2 IT [(RH2)liq]] − a [(RH2)liq]), under low enough illumination IT 1/2 1/2 flux, and by eq 12-I (vIT ox ≈ (2a k0[(RH2)liq]) ρ ) under high enough illumination flux.1 Experimental plots of vIT ox versus ρ 1/2 and vIT versus ρ and their respective fitting to eqs 1-I and 12-I ox are shown in Figure 3A and B, respectively. According to eq 1a-I, it can be written that aap ≅ a IT =

IT 2 (vox ) 2k 0ρ[(RH 2)liq ] IT

Figure 3. Initial experimental photooxidation rate (vox) of BZ dissolved in ACN, as a function of ρ (A) and ρ1/2(B). The representation of the experimental parameter aap versus ρ is shown in (C). Solid lines represent the best fitting to the D-I model eq 1-I in (A), eq 12-I in (B), and eq 11-I in (C). Fitting parameters are k0 = 0.1; Acat = 1 × 104 cm2, and aIT = 5 × 10−9 cm/s. The [BZ]ACN values are 1.4 × 1017 (1) and 3.8 × 1016 cm−3 (2). Calculation details can be found in Appendix B of the Supporting Information. Correlation coefficients can be found in Appendix C of the Supporting Information.

It has to be pointed out that a large number of data set adjustments were carried out through Figures 3A−6A. The ones shown in the figures were the best adjustments after numerous trials using the different models (I, D, or I + D equations). Due to the large number of adjustable parameters, the possible combinations were virtually infinite. Nevertheless, the choice of starting parameters was always based on values that had physical sense, thus constraining those values to the range of orders of magnitude reported in each figure caption. After doing that, it was found that only the combinations shown in Figures 3A−6A provided a reasonable adjustment of all the parameters.

(7)

(kIT ox k′red[(O2)liq])/(4k′r,1) 1

Moreover, we know that a = is a parameter with real physical meaning. Figure 3C shows the photon flux dependence of the aap parameter experimentally obtained from eq 7. 14281



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A linear dependence of BZ photooxidation rate, vox, on ρ1/2, for ρ > 1013 cm−2 s−1, and a photon flux independent value of parameter aap are evident in Figure 3. In contrast, for ρ < 0.5 × 1013 cm−2s−1, when the condition ρ ≫ (aIT[(RH2)aq])/(2k0) is not fulfilled and aIT is found to decrease (aIT → 0) as ρ → 0, clearly indicates that BZ photooxidation kinetics is compatible with the D-I model when chemisorption is not allowed and the IT mechanism governs the interfacial transfer of photogenerated holes to physisorbed substrate species. As expected, these results are in agreement with those obtained from spectroscopy techniques (Figures 1 and 2) and adsorption isotherm measurements (for further details, the reader is directed to Appendix A of the Supporting Information). Apparently, a similar behavior can be inferred from literature photooxidation kinetics data concerning chloroform60 and methylviologen,61 two compounds unable to be chemisorbed at the TiO2 surface. We would like to stress that kinetic data of Figure 3 are not compatible with the analytical expression associated with the LH model, where the photon flux dependence of the photooxidation rate is not explicit, contradicting the generally accepted dogma that kinetic data of photocatalytic reactions generally fit L-H type kinetic equations.26 It may be surprising that the experimental error involved in parameter aap of Figure 3C is higher than that observed in Figure 3A and B for vox. This apparent contradiction can be explained by taking into account that according to eq 7 a ∝ (vox)2, in such a way that the experimental error associated with aap duplicates the error associated with vox (see Appendix B in the Supporting Information for further details). II. FA Dissolved in Water. FA is able to strongly interact with the TiO2 surface.62 This is just the behavior observed in Table 1, where a monolayer of chemisorbed FA, with a recovering degree of about 70%, is generated. Therefore, a DT photooxidation mechanism is expected to prevail, and consequently, a linear dependence of the initial photooxidation rate on the photon flux intensity should be expected according to eqs 2-I and 2a-I,1 as in fact observed in Figure 4A. IT 1/2 1/2 Since according to eq 1b-I vIT ox ≈ (2a k0 [(RH2)liq]) ρ , DT DT 2− while according to eq 2a-I vox ≈ (k0kox [RH2]adsρ/k′1[Os ], the IT prevalence of vDT ox on vox for FA dissolved in water should be attributed, at least partially, to a relatively low kIT ox versus a high IT kDT ox rate constant. In fact, we know that kox = zσ exp −[(Ered − E(h+s )2)/(4λkT)];4 that is, kIT ox depends exponentially on the square of the activation energy term Δ = (Ered − E(h+s )), which depends on the degree of overlapping between the energy level associated with h+s surface trapped holes, just located above the valence band top level (Ev), and energy levels associated with the reduced substrate species in the liquid phase (see Figure 1-i of Part I of this manuscript1). In contrast, an inelastic electron transfer rate constant, kDT ox , governs the DT mechanism. As schematically shown in Table 2, for FA dissolved in water it is −21 (Ered − Es) ≈ 1.1 eV and kIT cm3 s−1, while kDT ox ≈ 1.6 × 10 ox ≈ −9 3 −1 10 cm s is several orders of magnitude higher. Charge carrier dynamics studies of TiO2−adsorbate systems have demonstrated that DT interfacial hole transfer is an ultrafast process occurring in femtoseconds, a time scale similar to that of interfacial electron−hole recombination,2 so that in IT most cases kDT ox ≫ kox . In addition, if the TiO2 surface becomes saturated with chemisorbed substrate molecules, as it is the case for FA dissolved in water, it can be assumed that kDT ox Cads ≫ DT IT kIT ox Caq, which necessarily implies that vox ≫ vox according to eqs 1a-I and 2-I. This thesis is confirmed from the analysis of

Figure 4. (A) Experimental photon flux (ρ) dependence of the initial photooxidation rate (vox) of FA dissolved in water. Solid lines IT correspond to the theoretical representation of vDT ox + vox calculated from eq 4-I.1 (B) Experimental photon flux dependence of the + IT ” defined by eq 23-I.1 Solid lines correspond to the parameter “aDT ap + IT vsρ calculated from eqs 2-I and theoretical representation of aDT ap 2 2 −18 = 1; A 22-I, respectively for k0 cat = 6 × 10 cm ; Kads = 8.91 × 10 −8 cm3; b = 1.39 × 1015 cm−; kDT cm3 s−1; aIT = 10−14 cm ox = 5.85 × 10 8 −1 s−1; and k1′[O2− s ] = 2.36 × 10 cm s . The [FA]aq values are 6.14 × 1017 (1), 3.01 × 1017 (2), and 1.20 × 1017 cm−3 (3). Correlation coefficients can be found in Appendix C of the Supporting Information.

the aap versus ρ plot represented in Figure 4B, obtained from experimental data of Figure 4A. As was shown in Part I of this manuscript, when DT and IT + DT mechanisms coexist, the following apparent aIT parameter ap 1 can be defined: IT + DT = aap

=

IT DT 2 + vox (vox ) 2k 0ρ[(RH 2)liq ] IT 2 DT 2 IT DT (vox ) + (vox ) + 2vox vox 2k 0ρ[(RH 2)liq ]

DT ≡ a IT + aap +

vDT ox

IT DT vox vox k 0ρ[(RH 2)liq ]

(22-I)

≫ vIT ox for FA dissolved be aIT+DT ≈ aDT ap ap . Further,

But considering that in water, according to eq 22-I, it must we know from eq 20-I that when adsorption−desorption equilibrium conditions are maintained under illumination, aDT ap depends 1 DT linearly on ρ, daDT ap /dρ > 0 and aap = 0 for ρ = 0. Indeed, this is the behavior observed in Figure 4B, indicating that, in fact, FA dissolved in water is preferentially photooxidized via a DT mechanism. 14282


If the fitting parameters of Figure 4A and B are introduced IT DT into eqs 11-I, 12-I, 21-I, and 12-I, it is found that aDT ap /aap ≈ vox / 9 IT DT IT vox ≈ 10 , confirming that vox ≫ vox . In conclusion, FA can be considered a paradigmatic case where DT is the prevailing photooxidation mechanism, even if DT and IT mechanisms coexist. Let us now apply the definition of photonic efficiency of a photooxidation process:68 v ξ(%) = ox × 100 ρ (8)

2 −12 vox values correspond to ρ = 1014 cm−2 s−1; [(RH2)liq] = 1017 cm−3’ and kIT cm3 s−1; λ ≈ 0.6 eV}. bReference 63. cReference 64. dReference 65. eReference 66. ox = zσ exp −(Δ /(4λkT)) {zσ ≈ 10 Reference 67.

0.11 20 0.13 1.2 2 × 1013 1.3 × 1010 1.2 × 1012 5.8 × 10−8 1.4 × 10−8 1.5 × 10−5 2.44 2.75b 2.75b 2.44b 2.64 2.95b 2.95b 2.64b

−0.56 −0.25b −0.25b −0.56b [BZ]ACN [FA]aq [PhOH]aq [PhOH]ACN

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to the experimental data of Figures 3A and 4A. We found that while for BZ 0.063 < ξ(%) < 0.35, for FA it is 12.9 < ξ(%) < 22.7. The great difference between BZ and FA photonic efficiency further illustrates of the differences existing between DT and IT photooxidation mechanisms. FA is directly oxidized via single, inelastic, hole trapping DT mechanism (CO2H2 + h+f → CO2H + H+), while BZ photooxidation involves a more complex two step process: a first step where TiO2 terminal oxygen ions are inelastically oxidized with valence band free + •− holes (O2− s + hf → Os ), followed by a second step where adiabatic transfer of charge between oxygen radicals and physisorbed BZ species takes place (O•− + C6H6 → O2− s s + C6H5 + H+).10 This is the main reason why FA photooxidation is efficiently enough to compete with electron−hole recombination (e−f + h+f → heat), yielding a high ξ value. Finally, let us compare the experimental results shown in Figures 3 and 4, respectively, concerning BZ and FA photooxidation within a similar illumination intensity range. We see that vox ∝ ρ1/2 for BZ, while vox ∝ ρ for FA. This behavior contradicts the common assertion stating that, in general, vox ∝ ρ under low enough ρ values, while vox ∝ ρ1/2 for high enough ρ values. In contrast, our kinetic analysis demonstrate that the dependence of vox on ρ is determined by the type of interfacial electron transfer mechanism, either DT or IT, involved in the photooxidation of dissolved substrate species, which in turn depends on the degree of electronic interaction of substrate species with the semiconductor surface. Summing up, FA, strongly chemisorbed on the TiO2 surface, is oxidized via a DT mechanism with a high photonic efficiency and a linear dependence of the initial photooxidation rate on photon flow (vox ∝ ρ). In contrast, BZ, physisorbed via van der Waals forces, becomes photooxidized via an IT mechanism, showing a linear dependence of the initial photooxidation rate on the photon flux square root III. PhOH Dissolved in Water. The photocatatalytic degradation of PhOH dissolved in water has been an important attention center for different researchers.6,25,69−74 However, to our knowledge, the only detailed kinetic study of phenol photooxidation on TiO2 is due to Emeline et al.25 Recently, some of us analyzed the experimental data reported by these authors,6 arriving to the conclusion that even if the specific interaction of PhOH molecules with the TiO2 surface in the presence of water is very weak compared with that of FA, PhOH photooxidation kinetics cannot be explained with the only participation of an IT mechanism, as it is the case for BZ dissolved in ACN, but as a mixture of both DT and IT mechanisms. To verify this hypothesis, we will analyze here our own experimental results, shown in Figure 5, from a kinetic point of view. On the one hand, Figure 5A shows a nonlinear dependence of vox on ρ, indicating that DT is not the prevalent mechanism, as it was the case for FA in Figure 4A. On the other hand, it can

f

a

ζ (%)

× × × × 2.54 1.07d 1.28e 0.46f

0.5 1.08 0.87 1.38

10−14 1.6 × 10−21 2 × 10−18 1.4 × 10−27

1.1 2 1.3 1.2

× × × ×

1011 1013 1011 1012

1.1 1.9 1.3 3.6

1011 108 1011 1011

−2 −1 vIT s ) ox (cm −2 −1 vDT s ) ox (cm

vox (cm−2s−1)a −3 −1 kDT s ) ox (cm −2 −1 kIT s ) ox (cm

Δ (V) Eredox (V vs NHE)

c b

E(O•− s ) (V vs NHE) Ev (V vs NHE)

b b

Ec (V vs NHE) entry

Table 2. Estimated Thermodynamic and Kinetic Parameter Values Involved in the Photooxidation of Benzene (BZ) Dissolved in Acetonitrile (ACN), Formic Acid Dissolved in Water (aq), and Phenol (PhO) Dissolved in ACN and Water


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water, where DT prevailed on IT. Note that the aap behavior shown in Figure 5C presents some similarities with Figures 3C and 4B, indicating that the photooxidation of PhOH dissolved in water takes place via a complex IT+DT mechanism, with vDT ox ≈ vIT ox . In fact, according to eqs 1-I and 2-I, it can be written: DT vox IT vox

=

DT kox [(RH 2)ads ] IT (kox [(RH 2)aq ])1/2

k 01/2 ⎛ k k1′[O2s −]⎜ [Ored] ⎝

⎞1/2 ⎟ 2 aq ⎠

ρ1/2 (9)

and introducing into eq 9 the fitting parameters of Figure 5, we IT DT IT obtain that 0.3 < vDT ox /vox < 1, or in other words that vox ≈ vox in the whole experimental range of the studied photon flow (1.5 × 1014 ≤ ρ ≤ 6.7 × 1014 photons cm−2 s−1). Since in IT DT general it is kDT ox ≫ kox , the apparent incongruence that vox ≈ DT IT vIT for PhOH, while v ≈ v for FA, disappears if we consider ox ox ox that under an analogous concentration rang of water dissolved substrate species it is [PhOH]ads ≪ [FA]ads, in agreement with experimental adsorption data presented in Appendix A IT (Supporting Information). In summary, even if kDT ox ≫ kox , IT and v can be obtained if the difference similar values of vDT ox ox DT between kIT ox and kox kinetic constants is compensated with a lower coverage of the TiO2 surface with substrate species, or, in other words, by a lower [(RH2)ads]/[(RH2)liq] ratio. As can be seen in Table 2 for phenol dissolved in water, a low TiO2 IT DT IT surface coverage leads to vDT ox < vox , despite that kox ≫ kox . IV. PhOH Dissolved in ACN. If DT mechanism is partially hindered as a consequence of the low TiO2 surface coverage by PhOH molecules in the presence of water, the substitution of water by acetonitrile as solvent should produce a promotion of DT mechanism, as the competition of acetonitrile molecules with PhOH substrate species for TiO2 chemisorption drastically diminishes (compare the drastic increase of PhOH, “b” recovering parameter in Table 1 when water is substituted by acetonitrile as solvent) . With this idea in mind, the kinetics study shown in Figure 5, using acetonitrile instead of water as solvent, was performed. The corresponding experimental kinetic data are reported in Figure 6. As it was the case for PhOH dissolved in water, a nonlinear dependence of vox on ρ and vox on ρ1/2 is inferred from Figure 6A and B, respectively, indicating that vox ∝ ρn with 0.5 < n < 1, as just expected when photooxidation takes place via a mixture of both DT and IT mechanism. However, the slopes dvox/dρ and dvox/dρ1/2 are higher than those obtained for the photooxidation of PhOH dissolved in water shown in Figure 5A and B, indicating a higher contribution of the DT mechanism for photooxidation of PhOH when dissolved in acetonitrile than when dissolved in water. The aap dependence on the photon flow and PhOH concentration is shown in Figure 6C. It can be observed that daap/dρ > 0 for any experimental photon flux value, as corresponds to a pure DT photooxidation mechanism under adsorption−desorption equilibrium conditions.1 Although this behavior is similar to that observed in Figure 5C for the photooxidation of PhOH dissolved in water, two important differences appear in Figure 6C: (1) the daap/dρ slope is clearly higher in Figure 6C than in Figure 5C; (2) a clear tendency of aap → 0 for ρ → 0 takes place for PhO dissolved in ACN (Figure 6C), but not for PhOH dissolved in water (Figure 5C), indicating that in the last case DT does not prevail on IT, as it was the case in Figure 4B for FA photooxidation.

Figure 5. Initial photooxidation rate (vox) of PhOH dissolved in water as a function of the photon flow ρ (A) and ρ1/2 (B). The solid lines IT correspond to the theoretical representation of vDT ox + vox calculated 1 from eq 4-I. The photon flux dependence of the experimental parameter aap = (vox)2/(2k0[(RH2)liq]ρ) is shown in (C). Solid lines + IT versus ρ correspond to the theoretical representation of aDT ap calculated from eq 22-I,1 with the following fitting parameters: k0 = 0.1; Acat = 1.2 × 103 cm2; Kads = 1.33 × 10−17 cm3, b = 9.39 × 1013 8 −1 DT −8 cm−2, k′1[O2− cm3 s−1; and aIT s ] = 2.36 × 10 cm s ; kox = 1.42 × 10 −9 −1 = 3.3× 10 cm s . The [PhOH]aq values are 2.77 × 1017 (1), 1.08 × 1017 (2), 4.22 × 1016 (3), and 1.02 × 1016 cm−3 (4). Correlation coefficients can be found in Appendix C of the Supporting Information.

be seen in Figure 5B that vox does not depend linearly on ρ1/2, as it was the case for the photooxidation of BZ dissolved in ACN shown in Figure 3B. Therefore, it must be vox ∝ ρn with 0.5 < n < 1, as corresponds to a complex photooxidation case where DT and IT are competing mechanisms (see eq 4-I).1 To confirm this hypothesis, the experimental kinetic data shown in Figure 5A were used to determine the photon flow dependence of the aap parameter shown in Figure 5C. As can be seen, aap does not linearly depend on ρ, as it was for FA dissolved in 14284



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Although it is apparently surprising the great influence of the solvent, either water or ACN, in PhOH photooxidation kinetics, this behavior can be easily rationalized in terms of the solvent influence of on the PhOH−TiO2 chemical interaction. As previously discussed, a higher TiO2 surface coverage by PhOH species is expected when PhOH is dissolved in ACN than when dissolved in water, which indicates a higher [RH2]ads]/[RH2]liq ratio when ACN is substituted for water, as IT well as a simultaneous increase of vDT ox /vox , because of the DT DT important vox increase when [RH2]ads raises.75−77 The vox growth should lead to a simultaneous ξ growth, as is in fact observed. The claim that water can be directly photooxidized with TiO2 valence band free holes is a mater of controversy.32,78−80 In this context, experimental evidence has been recently presented, indicating that the participation of OH• radicals originated from direct photooxidation of water species with free photogenerated holes is not operative in the photooxidation of BZ dissolved in ACN.10 However, it is generally accepted that dissolved molecular oxygen brings a way for free OH• radical generation via a multistep O2 photoreduction reaction: O2 + 2H+ + 3ef− → OH− + OH•

(10)

But if protons participation is required in eq 10, it seems reasonable to assume that this reaction should be favored when water at acid pH is used as solvent, and hindered when water is substituted by an organic solvent like ACN. Contradicting this expectation, we have shown that the initial PhOH photooxidation rate is about 1 order of magnitude higher when dissolved in ACN than when dissolved in water, indicating that, at least at initial steps of the photocatalytic process (for t → 0), the intervention of free OH• radicals generated via reaction 10 in PhOH photooxidation is not significant. Moreover, if OH• radicals were generated via direct photooxidation of water species with valence band free holes, as commonly accepted,5 PhOH photooxidation should be favored when water instead of ACN is used as solvent, contradicting the experimental evidence shown here. Therefore, it seems reasonable to conclude that free OH• radicals generated from either oxidation of water species with valence band free holes, h+f , or via electroreduction of dissolved O2 molecules with free conduction band electrons, e−f , can be considered as a plausible mechanism for PhOH photooxidation. However, the higher PhOH photooxidation efficiency observed when ACN instead of water is used as solvent should not be exclusively analyzed in terms of PhO−TiO2 surface interaction. It must be also taken into account the simultaneous participation of two competitive photooxidation mechanisms as primary reaction steps: a DT mechanism with participation of photogenerated free holes (C6H5OH + h+f → C6H5O• + H+) and an IT mechanism with intervention of surface trapped holes (C6H5OH+ Os•− → C6H5O• + Os2− + H+). In contraposition, the commonly invoked photooxidation mechanism with intervention of OH• radicals photogenerated from water molecules would produce a behavior just opposite to that observed here experimentally. Summing up, the photooxidation kinetic results presented here provide additional evidence against the crucial role generally attributed to OH• radicals directly photogenerated from water species as main photooxidizing species.5,80 The main physical parameters involved in the photooxidation of the different substrate species studied here, together with the

Figure 6. Initial, experimental photooxidation rate (vox) of PhOH dissolved in ACN as a function of the photon flow ρ (A) and ρ1/2 (B). IT Solid lines correspond to the theoretical representation of vDT ox + vox 1 calculated from eq 4−1. The experimental photon flux dependence of the parameter aap = (vox)2/(2k0[(RH2)liq]ρ) is shown in (C). Solid versus ρ lines correspond to the theoretical representation of aDT+IT ap calculated from eq 22-I,1 with the following fitting parameters: k0 = 0.012; Acat = 1.0 × 104 cm2; Kads = 3.96 × 10−18 cm3, b = 9.23 × 1014 8 −1 DT −5 cm−2, k′1[O2− cm3 s−1; and aIT s ] = 2.36 × 10 cm s ; kox = 1.5 × 10 −7 −1 = 6 × 10 cm s . The [PhOH]ACN values are 2.77 × 1017 (1), 1.08 × 1017 (2), 4.22 × 1016 (3), and 1.02 × 1016 cm−3 (4). Correlation coefficients can be found in Appendix C of the Supporting Information.

On the one hand, when the fitting parameters used in Figure 6 are introduced into eqs 1b-I and 2-I, we found that 1.5 < vDT ox / 13 vIT ox < 6.5 within the experimental photon flux range (1.7 × 10 ≤ ρ ≤ 7.8 × 1013 photons cm−2 s−1). On the other hand, the photooxidation photonic efficiency (ξ) obtained from the kinetic data of Figure 6A is found to be in the range 1.1 < ζ (%) IT < 2.2. Also remarkable is the fact that both vDT ox /vox and ξ are about 1 order of magnitude higher than the respective values for PhOH dissolved in water of Figure 5. All these features IT indicate that vDT ox > vox for PhOH dissolved in acetonitrile (see specific values in Table 2). 14285



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corresponding experimental photooxidation rates, are summarized in Table 2 and rationalized in the energy diagram of Figure 7.

Figure 7. Schematic representation of valence band (VB) and conduction band (CB) energy levels of anatase TiO2 in contact with water at pH 3 (dashed lines) or with acetonitrile (solid lines). The solid lines at the liquid phase represent the redox potential (Eredox) of the model compounds dissolved in water (aq) at pH 3 or in acetonitrile (ACN).

V. Photooxidation of Water Dissolved PhOH under Adsorption−Desorption Equilibrium Rupture. As is shown in Figure 8 at Part I of this publication,1 under high enough photon flow, adsorption−desorption equilibrium rupture of dissolved substrate species takes place, the commonly observed linear dependence of aDT ap on ρ is lost, and a maximum value of aDT ap parameter is observed at a photon flow ρ = ρmax. In addition, under low enough photon flux, aDT ap is found to decrease when [(RH2)liq] increases, while ρmax is observed to shift toward higher photon flux values. However, this tendency can be inverted under high enough photon flux, in such a way that aDT ap increases as [(RH2)liq] rises (such an anomalous behavior was illustrated in Figure 8-I).1 Since no maxima appear in aap versus ρ plots of Figures 3C, 4B, 5C, and 6C, it can be concluded that the corresponding experimental kinetic data were obtained with low enough photon flow values (ρ ≤ 1015 cm−2 s−1), under adsorption−desorption equilibrium conditions. However, these results contrast with water dissolved PhOH photooxidation kinetic data reported by Emeline et al.,25 where much higher photon flux values were used (ρ ≤ 1017 cm−2 s−1). Figure 8 illustrates the aap photon flow dependence, previously obtained by some of us,6 from PhOH experimental photooxidation rate dependence data reported by Emeline et al.; the great similarity of the experimental aap versus ρ plot of Figure 8 with the theoretical dependence predicted in Figure 8 in Part I of this publication1 allows us to state that PhOH adsorption−desorption equilibrium rupture takes place under the high photon flux values used by Emeline et al. We also proposed that PhOH dissolved in water is photooxidized via a pure IT mechanism.6 However, a contradiction appeared, as the experimental a parameter was not found to be photon flux independent, as expected for a pure IT mechanism. Instead, we found that parameter a depends on both the photon flow and the PhOH concentration. To explain such an apparent contradiction, it was suggested that, despite the weak interaction of aqueous phenol with the TiO2 surface, the contribution of the DT mechanism to the initial reaction rate is not negligible, as

Figure 8. (A) Initial photooxidation rate (vox) dependence on the photon flux of PhOH dissolved in water, for ρ0 = (1.1 ± 0.3) × 1017 photons cm−2 s−1, reported by Emeline et al.25 (Reproduced from ref 25 with permission. Copyright 2000 Elsevier.) (B) aap versus ρ representation obtained from experimental data shown in (A). Initial concentrations reported by Emeline et al. were C(1) = 0.027 mM, C(2) = 0.053 mM, C(3) = 0.106 mM, C(4) = 0.213 mM, C(5) = 0.427 mM, C(6) = 0.638 mM, and C(7) = 0.851 mM.25 aap parameters of panel (B) were obtained with the help of eq 22-I, for k0 = 0.1, Acat = 1 × 103 cm2, and Airr = 100 cm2 (see Apendix B in the Supporting Information for photooxidation rate units conversion).

initially assumed. The experimental data reported in Table 2 for water dissolved PhOH photooxidation corroborate our IT hypothesis, since vDT ox /vox ≈ 0.1. It must be noted that photooxidation rates appearing in Figure 8A are expressed in mol min−1 and not in cm−2 s−1 as required by the D-I model. Moreover, the aap versus ρ plot shown in Figure 8B is only estimated, as the experimental parameters corresponding to the irradiated area and reactor volume were not reported by Emeline et al.25 (calculation details are given in the Supporting Information, Appendix B). In spite of this, and beyond quantitative aap values, it must be realized that the aap on ρ dependence shown in Figure 7B accurately reproduces the behavior predicted by the D-I model (see Figure 8-I and 9-I).1 Further, it must be taken into account that independently of the values assigned to k0, Acat, and Airr parameters, the qualitative behavior observed in Figure 8B remains unchanged. This reinforces the utility of using aap versus ρ plots from the experimental kinetic data, regardless of physical units used, as an interesting tool to elucidate the nature of the interfacial charge transfer mechanism involved in photocatalytic oxidation reactions. 14286



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4. CONCLUSIONS The interaction of the TiO2 surface with three model compounds, formic acid (FA), benzene (BZ), and phenol (PhOH), dissolved in water and/or acetonitrile, have been analyzed by means of DRVS and ATR-FTIR spectroscopy and adsorption isotherm techniques. It has been found that FA strongly interacts with the TiO2 surface, even when dissolved in water, with chemisorption being observed to be the main adsorption mode, which, according to the D-I photooxidation model, implicates that FA dissolved in water is preferentially photooxidized via a DT mechanism. In contrast, BZ dissolved in ACN is exclusively physisorbed the TiO2 surface, and consequently photooxidized via a pure IT mechanism. Finally, electronic interaction of PhO with the TiO2 surface was found to strongly depend on the solvent used, either water or ACN, because of the different competition between phenol and solvent molecules for interacting with TiO2 adsorption sites (terminal Ti(IV) ions). In aqueous dissolution, PhOH chemisorption is disfavored because of the strong electronic affinity of water molecules with the TiO2 surface. Contrarily, when dissolved in ACN, PhOH chemisorption strength considerably increases, as far as the electronic interaction of ACN molecules with the TiO2 surface is negligible. In terms of the D-I model, PhOH dissolved in water is photooxidized via a combination of both mechanisms IT and DT, although vDT ox is about 1 order of magnitude greater than vDT ox . In contrast, when ACN is used as solvent, vDT ox remains practically unchanged, while vDT ox increases by about 2 orders of magnitude. The kinetic analyses based on experimental vox versus ρ, vox versus ρ1/2, and aap versus ρ plots ratify these conclusions. The effect of adsorption−desorption equilibrium rupture of substrate species on photooxidation kinetics was analyzed for the first time from experimental kinetic data concerning the photooxidation of water dissolved PhOH under high enough illumination intensity (ρ ≈ 1017 cm−2 s−1). The photon flux dependence predicted by the D-I model for the parameter defined as aap = (vox)2/(2k0[(RH2)liq]ρ), according to which aap versus ρ plots show an atypical maximum value when a critical ρmax photon flux value is reached, is experimentally verified, confirming that substrate adsorption−desorption equilibrium rupture takes place for ρ > ρmax. The experimental evidence collected here gives support to the central D-I model hypothesis that the degree of organic substrate−TiO2 surface interaction plays a crucial role in determining photocatalytic reaction kinetics. Depending on the interaction type and interaction degree, organic substrate species will be photooxidized either via of IT, DT, or a combination of both mechanisms. The experimental procedure and methodological analysis presented here can be applied to the kinetic study of any photocatalytic reaction, allowing one to elucidate the type of involved photooxidation mechanism and participation extent, both crucial data to design the strategy leading to improved photocatalytic efficiency. To this respect, it must be taken into account that charge transfer mechanism efficiencies can be easily improved via TiO2 surface modifications.7

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be found in Appendix A. Additional details about numerical procedures employed in initial reaction rate and a parameter calculations, as well as unit conversion of intervening physical parameters, can be found in Appendix B. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 34 93 581 2772. Fax: 34 93 581 2920. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

Financial support from the Spanish “Ministerio de Ciencia e Innovación”, through both project CTQ 2008-00178 and a research fellowship granted to J.F.M.

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ASSOCIATED CONTENT

S Supporting Information *

Details of HPLC analytical methods, adsorption isotherms, adsorption constants calculations and additional discussions about adsorption of model molecules on the TiO2 surface can 14287

LIST OF ACRONYMS a : indirect transfer parameter defined by eq 1a-I1 aDT ap : apparent, direct charge transfer parameter defined by eq 12-I1 IT+DT aap : apparent, (direct + indirect) charge transfer parameter defined by eq 22-I1 Acat: total area of liquid phase dispersed TiO2 catalyst nanoparticles b: TiO2 catalyst surface density of sites available for chemisorption of dissolved substrate species Cads: TiO2 catalyst surface concentration of adsorbed substrate species present in the liquid phase Caq: liquid phase concentration of dissolved substrate species DT: electric charge, interfacial direct transfer mechanism e−f : conduction band free electrons Ered: electrochemical reduction potential of liquid phase dissolved substrate species E(h+s ): energy levels associated with h+s surface trapped holes h+f : valence band free holes IT: electric charge, interfacial indirect transfer mechanism kads: substrate species adsorption constant k1: diffusion constant of dissolved substrate species from the electrolyte toward the TiO2 surface k−1: diffusion constant of dissolved substrate species from the sc surface toward the electrolyte kIT ox : indirect transfer photooxidation rate constant kDT ox : direct transfer photooxidation rate constant k′red: recombination rate constant of conduction band free electrons with electrolyte dissolved O2 molecules k1′ : diffusion constant of dissolved substrate species from the electrolyte toward the semiconductor surface k0: empirical photon absorption constant kr′1: interfacial recombination rate constant of conduction band free electrons with O·− s radicals λ: reorganization energy of substrate redox couple [(O2)liq]: concentration of dissolved O2 molecules in the liquid phase (water or any other solvent) >O2− br : 2-fold coordinated terminal bridging oxygen ions at the TiO2 surface −O2− br : 1-fold coordinated terminal bridging oxygen radicals at the TiO2 surface, generated via photooxidation of >O2− br ions with valence band free holes IT



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>O2− s : in general, low coordinated terminal oxygen ions present at kinks and steps at the surface of suspended TiO2 nanoparticles 2− [O2− s ]: TiO2 surface concentration of Os terminal oxygen ions O•− s : terminal oxygen radicals at the TiO2 surface, generated via photooxidation of >O2− s ions with valence band free holes [(RH2)liq]: concentration of dissolved substrate species in the liquid phase (water or any other solvent) [(RH2)ads]: semiconductor surface concentration of adsorbed substrate species RH: radical generated from the photooxidation of RH2substrate species ρ: photon flux intensity Sg: TiO2 photocatalyst specific surface area σ: hole capture cross section vDT ox : initial, direct transfer photooxidation rate of semiconductor chemisorbed substrate species, defined by eq 2 vIT ox : initial, indirect transfer photooxidation rate of semiconductor physisorbed substratespecies, defined by eq 1-I vox: global, initial photooxidation rate of dissolved substrate DT species (vox = vIT ox + vox ) Θa: equilibrium concentration of TiO2 surface sites covered with chemisorbed substrate species ξ: photooxidation photonic efficiency Xcat: TiO2 photocatalyst mass z: electrons thermal rate

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