TiO2 Photocatalytic

Additive Effect of Sacrificial Electron Donors on Ag/TiO2 Photocatalytic Reduction of Bis(2-dipyridyl)disulfide to 2-Mercaptopyridine in Aqueous Media...
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Langmuir 1999, 15, 7084-7087

Additive Effect of Sacrificial Electron Donors on Ag/TiO2 Photocatalytic Reduction of Bis(2-dipyridyl)disulfide to 2-Mercaptopyridine in Aqueous Media Hiroaki Tada,*,† Kazuaki Teranishi,‡ and Seishiro Ito†,‡ Environmental Science Research Institute and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka 577-8502, Japan Received December 18, 1998. In Final Form: May 17, 1999

I. Introduction From the viewpoint of environmentally “benign” or “green” chemistry, it is of great importance to develop new processes for synthesizing useful organic compounds utilizing solar energy. Heterogeneous photocatalysts represented by TiO2, which exerts strong oxidizing and moderate reducing power upon illumination, appear to be quite appropriate for the purpose. A great deal of investigations on the TiO2 photocatalytic oxidation of organics were carried out mainly with the intention of application to air and water purification.1 On the other hand, much less attention has been paid to the reductive photochemistry of TiO2, derived from excited conduction band electrons. Several organic compounds including methyl viologen,2 acetylene,3 halogenated alkanes,4 chloroethylenes,5 and nitro compounds6 have photocatalytically been reduced. We have recently reported that bis(2-dipyridyl)disulfide (RSSR) is selectively reduced to 2-mercaptopyridine (RSH) by H2O using TiO2 as a photocatalyst, and this highly endothermic reaction is enhanced upon incorporation of nanometer-sized Ag particles on TiO2 (eq 1).7 hν(λ > 300 nm)

RSSR + H2O 9 8 2RSH + 1/2O2, TiO or Ag/TiO 2

2

∆H ) +207.8 kJ mol-1 (1) This paper describes a remarkable accelerating effect with addition of sacrificial electron donors and discusses their action modes at the molecular level. II. Experimental Section Anatase TiO2 particles were supplied from Ishihara Techno Co. (A100) and used without further activation. The specific surface area was determined to be 8.1 m2 g-1 from N2 gas * To whom correspondence should be addressed. Telephone: +81-6-721-2332. Fax: +81-6-721-3384. E-mail: h-tada@ apsrv.apch.kindai.ac.jp. † Environmental Science Research Institute. ‡ Department of Applied Chemistry, Faculty of Science and Engineering. (1) In Photocatalytic Purification and Treatment of Water and Air; Allies, F. D., Al-Ekabi, H., Eds.; Elsevier Science: Amsterdam, 1993. (2) (a) Duonghong, D.; Ramsden, J.; Graetzel, M. J. Am. Chem. Soc. 1982, 104, 2977. (b) Muzyka, J.; Fox, M. A. J. Photochem. Photobiol., A.: Chem. 1991, 57, 27. (3) Lin, L.; Kuntz, R. R. Langmuir 1992, 8, 870. (4) (a) Bahneman, M. J.; Moenig, J.; Chapman, R. J. Phys. Chem. 1987, 91, 3782. (b) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1995, 27, 1646. (5) Glaze, W. H.; Kenneke, J. F.; Ferry, J. L. Environ. Sci. Technol. 1993, 27, 177. (6) (a) Mahdavi, F.; Bruton, T. C.; Li, Y. J. Org. Chem. 1993, 58, 744. (b) Ferry, J. L.; Glaze, W. H. Langmuir 1998, 14, 3551. (7) (a) Tada, H.; Teranishi, K.; Inubushi, Y.-i.; Ito, S. Chem. Commun. 1998, 2345. (b) Tada, H.; Teranishi, K.; Inubushi, Y.-i.; Ito, S. To be published.

adsorption at -196 °C on the basis of the BET equation. The other materials were used as received. Water was used after being passed through ion-exchange resin. A KI-I2 aqueous solution was added dropwise to a solution of 2-mercaptopyridine (40.4 g, 360 mmol, >95%, Tokyo Kasei) in a 2 M NaOH aqueous solution (500 mL) until the reaction mixture turned brown. To the above solution, a Na2S2O3 aqueous solution was added and then extracted into benzene (200 mL) three times. The benzene layer was washed with water and dried over MgSO4. After the solvent was evaporated, the residue was purified by recrystallization from a mixed solvent of n-hexane and benzene (3:2 v/v) to yield bis(2-dipyridyl)disulfide: yield, 72%; mp, 57-58 °C, 1H NMR (60 MHz, CDCl3, δ ppm), 7.03-7.50 (m, 3Py, 3′Py, 5Py, 5′Py, 4H), 8.60 (d, 4Py, 4′Py, 2H), 8.57 (d, 6Py, 6′Py, 2H), 8.60 (d, 4Py, 4′Py, 2H).8 The TiO2 particles (20.1 g) were suspended in a 5.05 × 10-2 M AgNO3 (>99.8%, Mitsuwa Chem.) aqueous solution (200 mL) in a double-jacket type reaction cell (31 mm in diameter and 175 mm in length, transparent for the light of λ > 300 nm). After the suspension had been purged with Ar for 15 min, irradiation was carried out with a 400-W high-pressure mercury arc (H-400P, Toshiba);9 the light intensity integrated from 320 to 400 nm (I320-400) was measured to be 4.39 mW cm-2 using a digital radiometer (DRC-100X, Spectroline). The TiO2 suspension was magnetically stirred during the irradiation. The resultant Agloaded TiO2 particles (Ag/TiO2) were washed by repeating the washing-centrifugation (1.3 × 104 rpm, 2 min) cycle twice and dried in a vacuum oven at room temperature. The amount of Ag deposited on TiO2 was determined to be 0.24 wt % by induced coupled plasma spectroscopy (ICPS-1000, Shimadzu). The photoreaction solution of RSSR (5.41 × 10-5 M) was prepared by diluting a CH3CN solution (5.41 × 10-4 M) with H2O. The concentration of CH3CN in this standard solution was 0.19 M. To check the additive effect of organic compounds, their concentrations were varied in the range below 5 M with maintaining the concentrations of RSSR and CH3CN. For the use of CH3CN as an additive, the concentration was expressed by the increment from that in the standard solution (0.19 M). After the suspension of Ag/TiO2 (50 mg/50 mL) had been purged with N2 for 15 min, irradiation (λ > 300 nm, I320-400 ) 0.69 mW cm-2) was started in the same reaction cell as used in the photodeposition of Ag. N2 gas bubbling (6.1 mL min-1) and magnetic stirring of the suspension were continued throughout the irradiation. The cell was kept at 30 °C by circulating thermostated water through an outer jacket around the cell. The pH of the suspension was adjusted by adding 0.1 M NaOH or HCl. Electronic absorption spectra of the supernatants, obtained by centrifugation of the suspensions after the irradiation, was performed in the 200-500 nm range on an ultraviolet-visible spectrophotometer (U-4000, Hitachi). The concentration of RSH generated was determined from the absorbance at 342 nm (max ) 7.18 × 103 M-1 cm-1).7b The average rate of the reaction v was calculated from the concentration of RSH generated after 20 min of irradiation. The amount of CO2 evolved in a closed reaction system was determined by gas chromatography using a Shimadzu GC-8APT equipped with a TCD column SHINCARBON T (6 m × 2 mm). The carrier gas was Ar (450 kPa) at an injection temperature of 200 °C and a column temperature of 150 °C.

III. Results and Discussion Figure 1 shows the additive effect of some organic compounds on the rate of the reduction of RSSR to RSH (v); C0 denotes the initial concentration of the additive. Positive effects are observed for the addition of CH3OH (b), C2H5OH (c), and HCOOH (d), whereas the reduction is completely inhibited by adding CH3CN at C0 > 0.29 M (a).10 With increasing C0, the value of v increases in systems (8) Inubushi, Y.-i. Ph.D. Thesis, Kinki University, 1995. (9) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. (b) Reiche, H.; Dunn, W. W.; Bard, A. J. J. Phys. Chem. 1979, 83, 2248.

10.1021/la981728k CCC: $15.00 © 1999 American Chemical Society Published on Web 07/23/1999

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Figure 1. Additive effect of some organic compounds on the rate of the reduction of RSSR to RSH (v) at 30 °C: (a) CH3CN; (b) CH3OH; (c) C2H5OH; (d) HCOOH. The rate of v was calculated from the concentration of RSH formed after 20 min of illumination. C0 denotes the initial concentration of additives; however, in the case of CH3CN, it was denoted as the inclement from the original value (0.19 M). In every reaction system, the pH of the reaction solution was approximately 5.8.

Figure 2. Time courses of CO2 evolution with illumination at 30 °C. In these experiments, [HCOOH]0 was fixed at 0.26 M, and [RSSR]0 and CH3CN ([AN]0) were varied ((a) [RSSR]0 ) 5.4 × 10-5 M, [AN]0 ) 0.19 M; (b) [RSSR]0 ) 1.6 × 10-4 M, [AN]0 ) 0.58 M; (c) [RSSR]0 ) 2.7 × 10-4 M, [AN]0 ) 0.96 M; (d) [RSSR]0 ) 5.4 × 10-4 M, [AN]0 ) 1.9 M; (e) [RSSR]0 ) [AN]0 ) 0; [AN]0 denotes the inclement from the original CH3CN concentration of 0.19 M).

Scheme 1. Proposed Reaction Mechanism in the Absence of Sacrificial Electron Donors

recombination to increase the lifetime of the cb electron (∼seconds order) because of effective hole scavenging.13 Consequently, the sacrificial electron donors (SEDs) are thought to increase the efficiencies of steps 5 and 6 (effect I). It is well-known that HCOOH as well as CH3OH and C2H5OH undergoes the vb hole oxidation followed by another prompt electron injection into the cb of TiO2.14 In the presence of the SED, steps 5′-10′ can be substituted for steps 5 and 6, as shown in Scheme 2 (SED ) HCOOH). Figure 2 shows time courses of CO2 evolution with illumination at 30 °C and the initial concentration of HCOOH ([HCOOH]0) ) 0.26 M; this reaction was carried out in a closed system. In the absence of RSSR, no CO2 was evolved (e). In the other cases (a-d), the amount of CO2 increases monotonically with illumination time. Also, the rate rises as the initial concentration of RSSR increases. The molar ratio of CO2/RSH for system a was calculated to be approximately 0.52 from the amount of CO2 evolved in the gas phase at t ) 70 min. The value is in good agreement with that expected from eq 2 (0.5). These facts indicate that HCOOH acts as a two-electron donor.

b, c, and d and decreases in system a as compared to that of the original system (v0 ) 4.2 × 10-7 M min-1), reaching an approximate plateau (vs) at C0 > 1 M. The ratio of vs/v0 was ∼0 (CH3CN), 5.0 ( 0.5 (CH3OH), 4.3 ( 0.5 (C2H5OH), and 15.2 ( 0.5 (HCOOH). In all the reaction systems, photoillumination of Ag/TiO2 (or TiO2) was required to reduce RSSR. Also, previously some control experiments precluded the possibility of Ag-photoinduced reduction of RSSR.7a A reaction mechanism in the absence of organic additives was previously proposed (Scheme 1).7a In the initial stage of the reaction, selective RSSR adsorption on the surface of Ag accompanied by S-S bond cleavage takes place (S1).11 Electron-hole pairs are generated by the band gap excitation of TiO2 (S2). Most of the pairs are lost by recombination (S3), but a portion of the electrons excited to the conduction band (cb) flow into Ag (S4), while the holes are left in the valence band (vb) of TiO2. Disdier et al. showed the migration of the excited electrons to the metal particles by photoconductance measurements for Pt-deposited TiO2.12 The Schottky barrier at the Ag/TiO2 interface would assist the charge separation. The hole has enough potential to oxidize H2O to yield H+ and O2 (S5). The coupling of H+ and RS- produced by reduction of RS adsorbed (RSad) forms RSH (S6). The rate-determining step (rds) of the reaction was revealed to be the oxidation of H2O by the hole.7b Warman et al. confirmed by pulse-radiolysis time-resolved microwave conductivity experiments that the addition of i-C3H7OH retards the (10) It should be noted that the original system contains 0.19 M CH3CN. (11) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (12) Disdier, J.; Herrmann, J.-M.; Pichat, P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 651.

2RSSR + HCOOH f 2RSH + CO2

(2)

There is also an alternative possibility that the radical generated by the one-electron oxidation directly reduces RSSR in the bulk solution (RSSRsol). Cr2+ ions reduce disulfides to the corresponding thiols (the standard electrode potential, E°(Cr3+/Cr2+) ) -0.665 V versus SCE).15 Since the oxidation potentials of R-hydroxyalkyl radicals are more negative than -1.0 V versus SCE,16 disulfides can thermodynamically be reduced by the intermediate radicals. In this case, step 8′ is further replaced by step 8′′ (Scheme 2). At the initial concentration of RSSR ([RSSR]0) < 2 × 10-5 M, almost all the molecules adsorb on Ag/TiO2 (0.05 g) due to the strong specific bonding of Ag-S.7a Figure 3 shows the change in the electronic absorption spectra before and after 10 min of (13) Warman, J. M.; de Haars, M. P.; Pichat, P.; Serpone, N. J. Phys. Chem. 1991, 95, 8858. (14) (a) Morrison, S. R.; Freund, T. Electrochim. Acta 1968, 13, 1968. (b) Schwitzgebel, J.; Ekerdt, J. G.; Sunada, F.; Lindquist, S.-E.; Heller, A. J. Phys. Chem. B 1997, 101, 2621. (15) Overman, L. E.; Petty, S. T. J. Org. Chem. 1975, 40, 2779. (16) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data 1996, 25, 709.

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Notes

Scheme 2. Proposed Reaction Mechanism in the Presence of Sacrificial Electron Donors

Figure 4. pH dependence of v at 30 °C: (a) [HCOOH]0 ) 0.026 M; (b) [CH3OH]0 ) 0.026 M; (c) without additive. The pH’s of the solutions were adjusted by adding NaOH(aq) or HNO3(aq). Figure 3. Change in the electronic absorption spectra before and after 10 min of illumination for the HCOOH nonadded system (a) and added system (b, [HCOOH]0 ) 0.026 M). Photoirradiation (λ > 300 nm) on a suspension consisting of Ag/TiO2 (50 mg) and a 5.41 × 10-6 M RSSR solution (50 mL) was carried out after complete adsorption of RSSR at 30 ( 1 °C and pH ) 4. The spectra of the RSSR solutions before (c, broken line) and after (d) adsorption are also shown.

illumination for the HCOOH nonadded system (a) and added system (b); [RSSR]0 ) 5.41 × 10-6 M. In each system, the absorption band of free RSSR molecules, which is observed at 281 nm before adsorption (spectrum c, broken line), disappears after adsorption (spectrum d). As indicated by spectrum b, illumination causes two absorption bands due to RSH at 272 and 342 nm. Comparison of spectra a and b indicates HCOOH addition remarkably accelerates the rate of reduction also under these conditions. This finding is strong evidence for the surface reaction mechanism (S8′-S10′). Ferry and Glaze have recently drawn the same conclusion in the photocatalytic reduction of nitrobenzene to aniline on the basis of the results of the competitive kinetic measurements.6b The comparable values of vs/v0 in the CH3OH- and C2H5OH-added systems probably result from effect I.

However, the as much as 15-fold increase in v with HCOOH addition cannot be explained only in terms of that. When HCOOH was added at C0 > 0.26 M, the pH of the reaction solution decreased below 2.1. Figure 4 shows the dependence of v on the pH of the solution: (a), [HCOOH]0 ) 0.026 M; (b) [CH3OH]0 ) 0.026 M; (c) without additive. At 4.5 < pH < 6.2, the value of v is almost constant in systems a and b, and the former value is approximately 2 times as large as the latter value. This is attributable to the difference in the half wave oxidation potential (E1/2ox) of CH3OH (E1/2ox(CH2O/•CH2OH) ) -0.98 V versus SCE) and HCOOH (E1/2ox(HCOO•/CO2) ) -1.2 V versus SCE).17 The stronger reducing power of the latter will increase the rate of electron transfer between the radical and the surface, again increasing the steady-state population of cb electrons. At 3 < pH < 4.5, the v value of system a steeply increases with decreasing pH, while that of system b is still unchanged. The ζ-potential of Ag/TiO2 in the solvent (H2O/CH3CN ) 99:1 v/v) was determined to be 4.2. Accordingly, at pH < 4.2, the electrostatic attraction between the surface (Tis-OH2+) and HCOO(17) Lilie, V. J.; Beck, G.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1971, 75, 458.

Notes

is thought to increase the rate of its adsorption or the adsorption equilibrium constant (effect II).18 The strong adsorption of HCOO- may also aid the successive twoelectron transfer to illuminated TiO2.19 On the other hand, since CH3OH is electrically neutral, the rate should be hardly affected by the surface charge of TiO2 in system b. In both systems, the increases in v are seen at pH < 3. A reversible change in the electronic absorption spectrum of RSSR was confirmed above and below pH ) 3, indicating that the protonation on the N atom of RSSR occurs at pH < 3. The protonation on the N atom of RSad is also expected in this pH region, which would increase its electron affinity to make the reduction of RSad easier (effect III). IV. Conclusions The rate of Ag/TiO2 photocatalytic reduction of bis(2dipyridyl)disulfide (RSSR) to 2-mercaptopyridine (RSH) (18) (a) Tada, H.; Kubo, Y.; Akazwa, M.; Ito, S. Langmuir 1998, 14, 2936. (b) Tada, H.; Akazawa, M.; Kubo, Y.; Ito, S. J. Phys. Chem. B 1998, 102, 6360. (19) Gerfin, T.; Graetzel, M. In Molecular Level Artificial Photosynthetic Materials; Meyer, G. J., Ed.; John Wiley & Sons, Inc.: New York, 1997.

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in a mixed solvent (H2O/CH3CN ) 99:1 v/v) has been found to increase upon addition of sacrificial electron donors including CH3OH, C2H5OH, and HCOOH. Particularly, a small addition of HCOOH (C > 0.1 M) increased the rate by a factor of approximately 15. This striking promotion of the reaction could be attributed to both the increase in the adsorption amount or the adsorption rate of HCOO(effect II) and the protonation of the N atom of RSad (effect III), caused by the decrease in pH, in addition to its strong reducing power (effect I). Acknowledgment. The authors express sincere gratitude to Dr. M. Iwasaki of Kinki University for valuable comments and Ishihara Techno Co. for the gift of the TiO2 particles (A-100). Also, support of this work by the ESRI (Kinki University) under the artificial photosynthesis program is gratefully acknowledged. Supporting Information Available: Plot of absorbance versus wavelength. This material is available free of charge via the Internet at http://pub.acs.org. LA981728K