Light-induced oxidative transformation of diphenylamine on ZrO2. Synergism by ZnO and ZnS J. Serb. Chem. Soc. 80 (11) 1411–1421 (2015) UDC 547.551.2+544.526.5:546.831–31: JSCS–4807 546.47–31+546.47’22 Original scientific paper 1411 Light-induced oxidative transformation of diphenylamine on ZrO2. Synergism by ZnO and ZnS CHOCKALINGAM KARUNAKARAN*, SWAMINATHAN KARUTHAPANDIAN** and PAZHAMALAI VINAYAGAMOORTHY Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India (Received 24 August, revised 13 December, accepted 15 December 2014) Abstract: Diphenylamine (DPA) in ethanol on the surface of ZrO2 undergoes photoinduced oxidative transformation yielding N-phenyl-p-benzoquinonimine (PBQI). The light-induced transformation on ZrO2 enhanced with DPA con- centration, ZrO2-loading, airflow rate and photon flux. The formation of PBQI on ZrO2 is larger on illumination at 254 nm than at 365 nm. The ZrO2 is reusable without any treatment. The mechanism of light-induced oxidative transformation of DPA on ZrO2 is discussed with an appropriate kinetic law. ZnO and ZnS enhance the UV light-induced transformation of DPA on ZrO2, indicating synergism. Keywords: photooxidation; wide band gap; sub-band gap illumination; photo- catalysis. INTRODUCTION Semiconductor-photocatalyzed selective organic oxidative transformations have gained interest because of their environmental benign nature and the reviews by Lang et al.,1 Palmisano et al.2 and Shiraishi and Hirai3 present the different kinds of organic transformations performed photocatalytically. TiO2- -based materials are widely used as photocatalysts for organic synthesis.1–3 Like TiO2, ZrO2 is a nontoxic and photostable IV–VI semiconductor but with a larger band gap energy (≈5 eV). However, this large band gap enables the photocatal- ytic splitting of water4 and reduction of CO2.5,6 The band gap of combustion synthesized ZrO2 is not large (3.5 eV) and Madras and coworkers7 reported degradation of dyes with illumination at 365 nm; defects in the crystal lattice are the likely reason for the decreased band gap. Here, for the first time, the photo- induced oxidative transformation of diphenylamine (DPA) on untreated ZrO2, * Corresponding author. E-mail: karunakaranc@rediffmail.com ** Present address: Department of Chemistry, VHNSN College, Virudhunagar 626001, Tam- ilnadu, India. doi: 10.2298/JSC140824122K 1412 KARUNAKARAN, KARUTHAPANDIAN and VINAYAGAMOORTHY which is reusable, is reported. DPA is used in post-harvest treatment of apple and pear8 and the photosensitized oxidation of DPA is known; cyanoanthracenes9 and benzophenone10 are some of the photosensitizers used. The unsensitized photo-oxidation of DPA to N-phenyl-p-benzoquinonimine (PBQI) is slow.11 The chemical transformation of DPA was studied in the absence and presence of ZrO2 and the difference in the rates provides the rate of PBQI formation on ZrO2. The oxidative transformation on ZrO2 surface was investigated under UV light and under natural sunlight at different experimental conditions to obtain the kin- etic law and to elucidate the reaction mechanism. ZnO is a promising II–VI semi- conductor photocatalyst with a band gap of 3.2 eV and large excitonic binding energy (60 meV) at room temperature. Although the band gap energy of ZnO is not different from that of TiO2 and the conduction band (CB) energy levels of the two semiconductors do not differ significantly and so are the valence band (VB) edges, there are reports that ZnO is a better photocatalyst than TiO2.12 ZnS is also a II–VI semiconductor but with a wide band gap (≈3.6 eV). Semiconductor mixtures are reported to enhance photocatalytic mineralization of organic mole- cules13,14 and here the photoconversion of DPA to PBQI on ZrO2 was enhanced on mixing ZnO or ZnS with ZrO2. EXPERIMENTAL Materials and measurements ZrO2, ZnS and ZnO (Merck) were used as received and their specific surface areas, obtained by BET method, were 15.1, 7.7 and 12.2 m2 g-1, respectively.15 The mean particle sizes (t) of ZrO2, ZnS and ZnO, obtained using the formula t = 6/ρS, where ρ is the material density and S is the specific surface area, are 68, 190 and 87 nm, respectively. The UV–visible diffuse reflectance spectra (DRS) of the semiconductors were obtained using a Shimadzu UV-2600 spectrophotometer with an ISR-2600 integrating sphere attachment. The Kubelka– –Munk (KM) plots provide the band gaps of ZnS and ZnO as 3.57 and 3.15 eV, respectively. Potassium tris(oxalato)ferrate(III), K3[Fe(C2O4)3]·3H2O, was prepared using a standard pro- cedure.16 DPA, AR (Merck) was used as received. The infrared spectra were recorded on a Nicolet iS5 FT-IR spectrometer. Commercial ethanol was purified by distillation with calcium oxide. UV light-driven transformation The UV light-driven transformation on ZrO2 was performed in a multilamp photoreactor equipped with eight 8 W mercury UV lamps (Sankyo Denki, Japan) emitting at 365 nm. The lamps were shielded by a highly polished anodized aluminum reflector. Four cooling fans mounted at the bottom of the reactor dissipated the generated heat. A borosilicate glass tube of 15-mm inner diameter was used as the reaction vessel and was placed at the center of the photoreactor. The UV light-induced reaction was also studied with a micro-photoreactor fixed with a 6 W 254 nm low-pressure mercury lamp and a 6 W 365 nm mercury lamp. Quartz and borosilicate glass tubes were employed as reaction vessels for 254 and 365 nm lamps, res- pectively. The light intensity (I) was measured by ferrioxalate actinometry. The volume of the reaction solution was always maintained as 25 mL in the multilamp photoreactor and 10 mL in the micro-photoreactor. Air was bubbled through the solution at a flow rate measured by PHOTOOXIDATION OF DIPHENYLAMINE ON ZrO2 1413 the soap bubble method. The UV–Visible spectra were obtained with a Hitachi U-2001 UV– –Vis spectrophotometer. The solution was diluted 5-times to lower the absorbance to the Beer–Lambert law limit. The PBQI formed was estimated from its absorbance at 450 nm. Sunlight-driven transformation The sunlight-induced transformation on ZrO2 was performed under clear sky in summer (March–July) at 11.30 am–12.30 pm. The solar irradiance (440 W m-2) was measured using a Global pyranometer, supplied by Industrial Meters, Bombay, India. Ethanolic solutions of DPA of the required concentration were prepared afresh and taken in wide cylindrical glass vessels of uniform diameter. The entire bottom of the vessel was covered with ZrO2 powder. Air was bubbled using a micro-pump without disturbing the ZrO2 bed. The volume of DPA solution was 25 mL and the loss of solvent because of evaporation was compensated periodically. The formed PBQI was estimated spectrophotometrically. RESULTS AND DISCUSSION UV light-induced oxidative transformation on ZrO2 The UV light-promoted oxidative transformation of DPA in ethanol on ZrO2 surface was realized by bubbling air in a multilamp photoreactor fixed with UV lamps emitting at 365 nm. The UV-visible spectra of the DPA solution recorded at different illumination times show the formation of PBQI (λmax = 450 nm). The time spectra are displayed in Fig. 1. The illuminated solution is EPR silent showing the absence of the formation of diphenylnitroxide. In addition, a thin layer chromatographic experiment revealed the formation of a single product. The illuminated DPA solution was evaporated to dryness after recovery of the particulate ZrO2 and the solid was dissolved in chloroform to develop the chro- matogram on a silica gel G-coated plate using benzene as eluent. The PBQI formed was estimated from its absorbance at 450 nm using the reported molar absorptivity.17,18 The linear increase of the concentration of PBQI with illumin- ation time, as seen in the inset to Fig. 1, provides the PBQI formation rate and the Fig. 1. UV light-induced PBQI formation with ZrO2 in ethanol: the UV-visible spectra of reac- tion solution (5-times diluted) at 0, 30, 60, 90 and 120 min (increasing A); [DPA] = = 20 mM, ZrO2- loading = 1.0 g, airflow rate = 7.8 mL s-1, I = 13.7×10-24 J L-1 s-1, volume of reaction solution = 25 mL; inset: Linear increase of formed PBQI with illumination time. 1414 KARUNAKARAN, KARUTHAPANDIAN and VINAYAGAMOORTHY rates were reproducible to ±6 %. The photoformation of PBQI by direct photo- oxidation of DAP in the absence of ZrO2 was slow11 and the rate of PBQI formation on ZrO2 was obtained by measuring the rates of PBQI formation in the presence and absence of ZrO2. The enhancement of PBQI formation on ZrO2 with concentration of DPA is displayed in Fig. 2. The observed enhancement conforms to Langmuir–Hinshelwood (LH) kinetics with respect to the DPA con- centration. The rate of surface reaction increased with ZrO2 loading in the DPA solution and the rate reached a limit at a high ZrO2-loading, as could be seen in Fig. 3. A study of PBQI formation on ZrO2 as a function of the airflow rate showed enhancement of the surface reaction by oxygen and the rate dependence on the airflow rate conformed to the LH kinetic law, as could be seen in Fig. 4. Fig. 2. Light-induced PBQI formation on ZrO2 as a function of DPA concentration; ZrO2-loading = 1.0 g, volume of reaction solution = 25 mL; UV: λ = 365 nm, I = = 13.7×10-24 J L-1 s-1, airflow rate = 7.8 mL s-1; Solar: bed area = 11.36 cm2, airflow rate = 4.6 mL s-1. Fig. 3. Photoinduced PBQI formation on ZrO2 at different ZrO2-loading; [DPA] = 5.0 mM, airflow rate = 7.8 mL s-1, λ = 365 nm, I = 13.7×10-24 J L-1 s-1, volume of reaction solution = 25 mL. Fig. 4. Photoformation of PBQI on ZrO2 as a function of airflow rate; [DPA] = 5.0 mM, ZrO2-loading = 1.0 g, volume of reaction sol- ution = 25 mL; UV: λ = 365 nm, I = 13.7× ×10-24 J L-1 s-1; solar: bed area = 11.36 cm2. PHOTOOXIDATION OF DIPHENYLAMINE ON ZrO2 1415 The formation of PBQI on ZrO2 was also determined without bubbling air but the solution was not deoxygenated. The dissolved oxygen itself enabled the light-induced surface reaction, but the reaction was slow. PBQI formation on ZrO2 was studied at different intensities of illumination. The chemical transform- ation was studied with two, four and eight lamps and the angles sustained by adjacent lamps were 180, 90 and 45°, respectively. The dependence of the sur- face reaction rate on photon flux is displayed in Fig. 5. PBQI was not formed in the absence of illumination. A study of PBQI formation on ZrO2 under UV-A and UV-C light, using a 6 W 365 nm mercury lamp (I = 10.0×10-24 J L–1 s–1) and a 6 W 254 nm low-pressure mercury lamp (I = 4.09×10-24 J L–1 s–1), separ- ately in a micro-photoreactor under identical conditions showed that UV-C light was more efficient than UV-A light in inducing the organic transformation on ZrO2. The rate of PBQI formation with UV-A and UV-C light were 9.2 and 20.6 nM s–1, respectively ([DPA] = 5.0 mM, ZrO2 suspended: 1.0 g, airflow rate = = 7.8 mL s–1). The ZrO2 retained its activity on usage. Reuse of ZrO2 showed sustainable light-induced PBQI formation. The azide ion (5 mM), a singlet oxy- gen quencher, failed to suppress PBQI formation, showing the absence of an involvement of singlet oxygen in the light-induced organic transformation on ZrO2. This finds literature support; Fox and Chen19 ruled out the possibility of singlet oxygen in the TiO2-photocatalyzed olefin-to-carbonyl oxidative cleavage. Fig. 5. Influence of photon flux on ZrO2-promoted PBQI formation; [DPA] = 5.0 mM, ZrO2-loading = 1.0 g, airflow rate = 7.8 mL s-1, λ = 365 nm, volume of reaction solution = 25 mL. Sunlight-induced oxidative transformation on ZrO2 The ZrO2-mediated oxidative transformation of DPA into PBQI also occurs under natural sunlight. The UV–visible spectrum of sun-shined DPA solution in ethanol in the presence of ZrO2 and air was similar to that with UV light (λmax = = 450 nm). Furthermore, the sun-shined solution was EPR silent revealing the absence of diphenylnitroxide. In addition, TLC analysis shows the formation of a single product. Determination of the solar irradiance (W m–2) showed fluctuation of the sunlight intensity during the experiment, even under a clear sky. Hence, the solar experiments under different reaction conditions were performed in a set 1416 KARUNAKARAN, KARUTHAPANDIAN and VINAYAGAMOORTHY to maintain the quantum of sunlight incident on a unit area the same. This enabled comparison of the solar results. A pair of solar experiments performed simultaneously under identical reaction conditions yielded results within ±6 %, which was also the case on different days. The effect of the operational para- meters on the solar-promoted oxidative transformation was studied by perform- ing the given set of experiments simultaneously and the results displayed in each figure represent identical solar irradiance. The rate of PBQI formation was obtained by shining the DPA solution on ZrO2 bed for 60 min. The dependence of PBQI formation rate on the concentration of DPA is shown in Fig. 2. The observed increase of PBQI formation with DPA concentration is characteristic of the LH kinetic law. The double reciprocal plot of the PBQI formation rate versus DPA concentration was a straight line with a positive y-intercept (figure not shown), which confirmed the LH kinetic model. The rates of PBQI formation on ZrO2 at different airflow rates are shown in Fig. 4. The observed enhancement of the PBQI formation by oxygen revealed that the surface reaction also conformed to LH kinetics with respect to oxygen. The double reciprocal plot of reaction rate versus airflow rate was linear with a finite y-intercept (figure not presented). The PBQI formation on ZrO2 was measured without bubbling air but the solution was not deoxygenated. The dissolved oxygen was sufficient to effect the chemical transformation on ZrO2 during the experimental period. However, the transform- ation was slow. The PBQI formation on ZrO2 increased linearly with the apparent area of the ZrO2-bed, as could be seen in Fig. 6. The oxidative trans- formation did not occur in the absence of sunlight. ZrO2 does not lose its activity on usage. Reuse of ZrO2 showed sustainable activity. Fig. 6. Dependence of sunlight-driven PBQI formation rate on ZrO2-bed area; [DPA] = 5.0 mM, ZrO2-loading = 1.0 g, airflow rate = 4.6 mL s-1, volume of reaction solution = 25 mL. Mechanism The DRS of ZrO2 is presented in Fig. 7, from which it could be observed that the absorption edge of the employed pristine ZrO2 was 320 nm. Illumination of ZrO2 with light of wavelength 365 nm is energetically unviable to bring about PHOTOOXIDATION OF DIPHENYLAMINE ON ZrO2 1417 band gap excitation and hence the operation of the usual semiconductor-photo- catalysis mechanism is ruled out. DPA is likely to be adsorbed on the surface of ZrO2. The FT-IR spectra of DPA and DPA adsorbed on ZrO2 are displayed in Fig. 8. The shift of the >N–H stretching vibrational frequency from 3408 to 3404 cm–1 and bending vibrational frequency from 1595 to 1591 cm–1 indicate bind- ing of DPA with ZrO2 through the amine hydrogen. The DRS of DPA-adsorbed ZrO2 shows a shift of the absorption edge to the visible region (413 nm; Fig. 7). This absorption is likely due to electronic excitation of the adsorbed DPA. The excited electron may move to Zr4+ resulting in the formation of the radical cation Ph2NH •+. The reduced form of Zr4+ (i.e., Zr3+) may lose an electron to the adsorbed molecular oxygen yielding superoxide radical ion (O2 •–). The reaction of the formed radical cation with the superoxide radical ion may yield the product PBQI. Fig. 7. DRS of bare ZrO2 and DPA-adsorbed ZrO2. Fig. 8. FT-IR spectra of DPA and DPA-adsorbed ZrO2. Ali et al.20 studied the photodegradation of methylene blue on ZnO films, deposited on glass slides by either the hydrothermal method or the magnetron sputtering technique, with UV-C light and the results conformed to the LH kinetic law. Leaching of zinc due to photocatalysis was reported under oxygen limited conditions but the ZnO films were observed to be more stable under oxygen-rich conditions. It was suggested that the oxygen from the ZnO lattice 1418 KARUNAKARAN, KARUTHAPANDIAN and VINAYAGAMOORTHY was removed and used in the radical initiation and propagation phases of the photocatalysis under oxygen-limited conditions. That is, the observed deform- ation of the ZnO lattice over time was ascribed to the use of lattice oxygen in the photocatalytic process. Oxygen-rich conditions either minimize the release of or replace the last ZnO lattice oxygen and the authors proposed the operation of a Mars Van Krevelen (MVK) type mechanism in the photocatalytic degradation of methylene blue under oxygen-limited conditions. In a similar study with UV-A and UV-C light, the same authors suggested the predominance of the MVK mechanism under UV-C illumination.21 However, Delmon22 stated that the re- oxidation step in the MVK mechanism is often too slow and consequently the catalysts become reduced in the corresponding reduction–oxidation cycle. That is, the photocatalytic activity is decreased remarkably on reuse. Furthermore, irrespective of the operative mechanism, leaching of zinc or a small decrease in the photocatalytic activity of ZnO on repeated reuse is well known.23,24 How- ever, ZrO2 is chemically unreactive and hence operation of a MVK type mech- anism in the present study is unlikely. In addition, the fact that ZrO2 shows sus- tainable photocatalytic activity (PBQI formation rate was not lowered on reuse of ZrO2) does not support the operation of a MVK type mechanism in the title reaction. Serpone and co-workers25–28 stated that both the LH and Eley–Rideal (ER) models are applicable for semiconductor photocatalytic reactions. The LH model presents the adsorption of both the reactant molecules on the surface29 while the ER model represents adsorption of one of the reactants on the surface; the other reactant molecule interacts with adsorbed reactant molecule to form the product. The LH model requires saturation kinetics with respect to both the reac- tants whereas the ER model demands saturation kinetics with respect to one of the reactants and first order dependence of the reaction rate with respect to the other reactant. The studied photocatalyzed reaction on ZrO2 surface shows satur- ation kinetics with respect to DPA as well as oxygen (Figs. 2 and 4) and hence operation of the ER reaction mechanism is ruled out. Although Bansal and Sidhu30 stated that singlet oxygen is the oxidant in dye-sensitized photooxidation of DPA, this has been ruled out in the present photocatalytic transformation as azide ion does not suppress the formation of PBQI (vide supra); de Lasa et al.31 also proposed the formation of reactive species superoxide radical anion in semiconductor photocatalyzed reactions. Kinetic law The heterogeneous photoinduced reaction occurring in a continuously stirred tank reactor (CSTR) conforms to the kinetic law:32 Rate of PBQI formation on ZrO2 = [ ] [ ]( ) ( ) 1 2 1 2 DPA 1 DPA 1 kK K SIC K K γ γ+ ⋅ + (1) PHOTOOXIDATION OF DIPHENYLAMINE ON ZrO2 1419 where K1 and K2 are the adsorption coefficients of DPA and O2 on the illu- minated ZrO2 surface, k is the specific rate of oxidation of DPA on the ZrO2 sur- face, γ is the airflow rate, S is the specific surface area of ZrO2, C is the amount of ZrO2 suspended per liter and I is the light intensity. The data-fit to the LH kinetic curve, drawn using a computer program,32 confirmed the kinetic law (Figs. 2 and 4). The linear double reciprocal plots of surface reaction rate versus the DPA concentration and the airflow rate supports the LH kinetic law. The data-fit provides the adsorption coefficients K1 and K2 as 76 L mol–1 and 0.039 mL–1 s, respectively, and the specific reaction rate k as 2.75×109 mol L m–2 J–1. However, the rate of PBQI formation on ZrO2 surface fails to increase linearly with ZrO2-loading. This is because of the high ZrO2 loading. At high ZrO2 load- ings, the surface area of the ZrO2 exposed to illumination does not correspond to the weight of ZrO2. The quantity of ZrO2 used is beyond the critical amount cor- responding to the volume of the reaction solution and reaction vessel; the whole quantity of ZrO2 is not exposed to light. The photoinduced transformation lacks linear dependence on the illumination intensity; a lower than first power depend- ence of a surface-photoreaction rate on the light intensity at high photon flux is well known.33 Synergism by ZnO and ZnS Vectorial transfer of electrons and holes from one semiconductor to another is possible in semiconductor mixtures under band gap-illumination. This charge separation enhances the photocatalytic activity.13,14 However, what was obs- erved in this study was enhanced phototransformation due to the presence of the semiconductor ZnO or ZnS nanoparticles with ZrO2 nanoparticles; the wave- length of illumination could effect the band gap excitation of ZnO and ZnS but not ZrO2. The enhanced formation of PBQI with ZrO2 mixed with ZnO or ZnS is displayed in Fig. 9; the two nanoparticles were kept under suspension and under continuous motion by bubbling air through the illuminated solution. Aggregation of nanoparticles under suspension is known.34 The particle size distribution of ZrO2, ZnO and ZnS under suspension, determined by the light scattering method, are presented in Fig. 10. Examination of Fig. 10 along with the size of the particles obtained from the XRD and BET methods revealed aggregation of the nanoparticles. As observed in the individual ZrO2, ZnO and ZnS suspensions, aggregation in the ZrO2–ZnO and ZrO2–ZnS mixtures under suspension is likely, and both ZrO2 and ZnO or ZnS nanoparticles are likely to be present in the aggregate. This may lead to transfer of the generated hole from the illuminated ZnO or ZnS to the DPA molecule adsorbed on ZrO2 surface, resulting in enhanced photooxidation. The densities and particle sizes of ZrO2, ZnO and ZnS are differ- ent and this may be a reason for not observing maximum enhanced photo-oxi- dation at 50 % composition. 1420 KARUNAKARAN, KARUTHAPANDIAN and VINAYAGAMOORTHY Fig. 9. Enhanced PBQI formation on mixing ZrO2 with ZnO or ZnS; [DPA] = 5.0 mM, nanoparticles-loading = 1.0 g, airflow rate = 7.8 mL s-1, λ = 365 nm, I = 13.7×10-24 J L-1 s-1, illumination time = 30 min, volume of reac- tion solution = 25 mL. Fig. 10. Particles aggregation. CONCLUSIONS ZrO2 mediates photoinduced oxidative transformation of DPA to PBQI. The formation of PBQI on ZrO2 enhances with DPA concentration, airflow rate and photon flux and conforms to the Langmuir–Hinshelwood kinetic law. The PBQI formation on ZrO2 is greater with UV-C light than with UV-A light. ZrO2 mixed with ZnO or ZnS affords more PBQI than the individual nanoparticles due to synergism. Acknowledgements. Prof. C. Karunakaran is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi for the Emeritus Scientist Scheme (21(0887)/ /12/EMR-II). И З В О Д ФОТО-ИНДУКОВАНА ОКСИДАТИВНА ТРАНСФОРМАЦИЈА ДИФЕНИЛАМИНА НА ZrO2. СИНЕРГИЗАМ ZnO И ZnS C. KARUNAKARAN, S. KARUTHAPANDIAN и P. VINAYAGAMOORTHY Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India Дифениламин (DPA) у етанолу на површини ZrO2 подлеже фото-индукованој окси- дативној трансформацији дајући N-фенил-p-бензохинонимин (PBQI). Фото-индукована трансформација на ZrO2 се повећава са порастом [DPA], количине ZrO2, брзине протока ваздуха и флукса фотона. Стварање PBQI на ZrO2 је веће при озрачивању на 254 nm него на 365 nm. ZrO2 се може поново користити без икаквог третмана. Мехнизам фото- индуковане оксидативне трансформације DPA на ZrO2 је разматран применом одгова- PHOTOOXIDATION OF DIPHENYLAMINE ON ZrO2 1421 рајућег кинетичког закона. ZnO и ZnS повећавају UV фото-индуковану трансформацију DPA на ZrO2 указујући на синергизам. (Примљено 24. августа, ревидирано 13. децембра, прихваћено 15. децембра 2014) REFERENCES 1. X. Lang, X. Chen, J. Zhao, Chem. Soc. Rev. 43 (2014) 473 2. G. Palmisano, E. Garcia-Lopez, G. Marci, V. Loddo, S. Yurdakal, V. Augugliaro, L. Palmisano, Chem. Commun. 46 (2010) 7074 3. Y. Shiraishi, T. Hirai, J. Photochem. Photobiol., C 9 (2008) 157 4. M. J. Poston, A. B. Aleksandrov, D. E. Sabo, Z. J. Zhang, T. M. Orlando, J. Phys. Chem., C 118 (2014) 12789 5. S. Yoshida, Y. Kohno, Catal. Surv. 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