catalysts
Article Mechanistic Insights into Visible Light-Induced Direct Hydroxylation of Benzene to Phenol with Air and Water over Pt-Modified WO3 Photocatalyst
Yuya Kurikawa 1, Masahiro Togo 1, Michihisa Murata 1 , Yasuaki Matsuda 1, Yoshihisa Sakata 2 , Hisayoshi Kobayashi 3 and Shinya Higashimoto 1,* 1 Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan; [email protected] (Y.K.); [email protected] (M.T.); [email protected] (M.M.); [email protected] (Y.M.) 2 Graduate School of Science and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan; [email protected] 3 Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-(0)6-6954-4283
Received: 2 May 2020; Accepted: 15 May 2020; Published: 18 May 2020
Abstract: Activation of C(sp2)-H in aromatic molecules such as benzene is one of the challenging reactions. The tungsten trioxide supported Pt nanoparticles (Pt-WO3) exhibited hydroxylation of benzene in the presence of air and H2O under visible-light (420 < λ < 540 nm) irradiation. The photocatalytic activities (yields and selectivity of phenol) were studied under several experimental conditions. Furthermore, investigations of mechanistic insight into hydroxylation of benzene have 18 been carried out by analyses with apparent quantum yields (AQY), an H2 O isotope-labeling experiment, kinetic isotope effects (KIE), electrochemical measurements and density functional theory (DFT) calculations. It was proposed that dissociation of the O–H bond in H2O is the rate-determining step. Furthermore, the substitution of the OH derived from H2O with H abstracted from benzene by photo-formed H2O2 indicated a mechanism involving a push-pull process for the hydroxylation of benzene into phenol.
Keywords: Pt-WO3 photocatalyst; visible light; hydroxylation of benzene; phenol formation; isotope 18 H2 O-labeling
1. Introduction Visible light-driven photocatalysts for environmental conservation, securing energy resources and selective organic synthesis have received considerable attention because they can utilize unlimited solar energy [1–7]. Tungsten trioxide (WO3) is one of the promising visible-light responsible photocatalyst having a direct band-gap excitation at ca. 2.7 eV. Recently, WO3 has been proven to be an effective strategy for improving the photocatalytic degradation of volatile organic compounds (VOCs) such as acetaldehyde [8–11], toluene [12], acid [13]; water splitting to form O2 in the presence of sacrificial agent [1]; as well as selective organic conversion such as oxidation of alcohol [14]. Phenols are important precursors for many chemicals and industrial products such as dyes and polymers, and they are currently produced from benzene by a three step cumene process. The cumene process for phenol formation exhibits low activity (~5% yield of phenol) and emission of large amounts of waste although the reactions require high temperature, high pressure and strong acidic conditions. Therefore, it is a great challenge to develop a one-step synthesis of phenol from benzene by using homogeneous and heterogeneous inorganic catalysts such as palladium membrane, titanium-containing
Catalysts 2020, 10, 557; doi:10.3390/catal10050557 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 557 2 of 12 mesoporous molecular sieves and vanadium-substituted phosphomolybdate by hydrogen peroxide (H2O2)[15–17]. Although H2O2 is often used as environmentally friendly oxidant only to produce H2O in catalytic hydroxylation of benzene, O2 is more ideal oxidant than H2O2 due to its abundance in nature. In a recent study, one-step direct hydroxylation of benzene toward phenol has been extensively II 2+ studied using homogeneous photocatalysts such as quinolinium ions and [Ru (Me2phen)3] ions, [18–21], and heterogeneous semiconductor photocatalyst such as TiO2 [22–27] and WO3 [28,29] have been employed for selective photocatalytic hydroxylation of benzene to phenol under photo-irradiation ultraviolet (UV) light and/or visible-light. Yoshida et al. previously reported that platinum (Pt)-loaded TiO2 (Pt-TiO2) photocatalyst exhibited direct hydroxylation of benzene to form phenol and H2 in the absence of O2 under UV-light irradiation [22]. Also, Tomita et al. reported that the Pt-deposited WO3 (Pt-WO3) photocatalyst exhibited selective hydroxylation of benzene to phenol in the presence of O2 and H2O under light irradiation of both UV light and visible light (300 < λ < 500 nm) [28,29]. It was also confirmed that OH derived from H2O is included in phenol by 18 employing an H2 O labeling experiment [29]. However, in a previous study, photocatalytic activities and reaction mechanisms for hydroxylation of benzene on the Pt-WO3 have not been demonstrated under only irradiation of visible-light (λ > 420 nm). In this study, we focus on understanding the photocatalytic activities for hydroxylation of benzene on the Pt-WO3 photocatalyst under irradiation of only visible light (420 < λ < 540 nm). Furthermore, investigation of mechanistic insight has been carried out by combination with apparent quantum 18 yields (AQY), H2 O isotope labeling experiment, kinetic isotope effects (C6D6,D2O), electrochemical measurements and density functional theory (DFT) calculations.
2. Results
2.1. Preparation of Pt-Deposited Tungsten Trioxide (WO3) and Its Characterization
The Pt-WO3 photocatalyst was characterized by X-ray diffraction (XRD), ultraviolet–visible (UV–Vis) spectroscopy, scanning transmittance electron microscope with energy-dispersed X-ray emission spectroscopy (STEM-EDS) and X-ray photoelectron spectroscopy (XPS) measurements. The XRD patterns of WO3 and Pt-WO3 were identified with a monoclinic structure in accordance with standard XRD profile of JCPDS cards No. 43-1035, and no other phase of Pt species was observed (see Figure1 [I]). The WO 3 exhibited absorption spectrum in the visible-light region. Bandgap of the WO was estimated to be ca. 2.6 eV from tauc plots υof (F(R) hυ)2 vs. hυ (See Figure S1). 3 × Furthermore, the Pt-WO3 with different amounts of Pt species exhibited optical absorbance above 450 nm, which is attributable to the scattering effects from the Pt particles [30] or surface resonance [31]. The optical absorbance significantly increased as an increase of the Pt species deposited on the WO3 (See Figure1 [II]). The scanning transmittance electron microscope with the STEM-EDS image of Pt(0.4)-WO3 confirmed that Pt nano-particles were dispersed on the WO3 surface (See Figure1 [III]). The XPS analysis indicated that the Pt(0.4)-WO3 photocatalyst was observed to possess two different 0 types of doublet peaks (4f5/2 and 4f7/2) at 74.8 and 71.6 eV for Pt as a major species, at 75.8 and 72.5 eV for Pt2+ as a minor species (See Figure1 [IV]). These results indicated that Pt nano-particle deposition on the WO3 was successfully performed by photo-electrochemical deposition methods. Catalysts 2020, 10, 557 3 of 12 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 12
Figure 1. X-ray diffraction (XRD) patterns [I] of tungsten trioxide (WO3) (a) and Pt(0.4)-WO3 Figure 1. X-ray diffraction (XRD) patterns [I] of tungsten trioxide (WO3) (a) and Pt(0.4)-WO3 (platinum-tungsten trioxide) (b); ultraviolet–visible (UV–Vis) absorption spectra [II] of WO3 (c), (platinum-tungsten trioxide) (b); ultraviolet–visible (UV–Vis) absorption spectra [II] of WO3 (c), Pt(0.1)-WO3 (d), Pt(0.2)-WO3 (e), Pt(0.4)-WO3 (f); scanning transmittance electron microscope (STEM) Pt(0.1)-WO3 (d), Pt(0.2)-WO3 (e), Pt(0.4)-WO3 (f); scanning transmittance electron microscope (STEM) image (g) and energy-dispersed X-ray emission spectroscopy (EDS) (h–j) [III] of the Pt(0.4)-WO3: O (h), image (g) and energy-dispersed X-ray emission spectroscopy (EDS) (h-j) [III] of the Pt(0.4)-WO3: O W (i) and Pt (j); and XPS [IV] of Pt (4f) of the Pt(0.4)-WO3. (h), W (i) and Pt (j); and XPS [IV] of Pt (4f) of the Pt(0.4)-WO3.
2.2. Photocatalytic Hydroxylation of Benzene on the Pt-WO3 under Visible-Light Irradiation 2.2. Photocatalytic Hydroxylation of Benzene on the Pt-WO3 under Visible-Light Irradiation The photocatalytic hydroxylation of benzene in the presence of air and H2O was performed on 2 the WOThe3 photocatalyst photocatalytic under hydroxylation visible-light of irradiation. benzene in It the was presence confirmed of air that and the H hydroxylationO was performed reaction on doesthe WO not take3 photocatalyst place under under photo-irradiation visible-light without irradiation. a photocatalyst It was con norfirmed with athat photocatalyst the hydroxylation without irradiation,reaction does i.e., not both take photocatalyst place under photo-irradiation and irradiation arewithout required a photocatalyst in combination nor with for hydroxylationa photocatalyst without irradiation, i.e., both photocatalyst and irradiation are required in combination for reaction to occur. The WO3 exhibited very low activity for phenol formation under visible-light irradiationhydroxylation (See reaction Table S1). to occur. After theThe photocatalyticWO3 exhibited reaction, very low the activity color offorsuspension phenol formation was slightly under visible-light irradiation (See Table S1). After the photocatalytic reaction, the color of suspension was bluish, suggesting that the WO3 was partially reduced and the ability for oxygen reductive reaction slightly bluish, suggesting that the WO3 was partially reduced and the ability for oxygen reductive (ORR) was retarded. On the other hand, Pt-WO3 exhibited significant improvement of photocatalytic activitiesreaction for(ORR) the formation was retarded. of hydroxylated On the other products hand, such Pt-WO as phenol3 exhibited and catechol. significant The yieldsimprovement of phenols of significantlyphotocatalytic improved activities as for an increasethe formation of loading of hydrox amountsylated of Ptproducts species upsuch to as 0.2 phenol atom%, and and catechol. the activity The wasyields saturated of phenols at 0.4 significantly atom%. The improved selectivity as of an phenol increase was of optimized loading amounts at 0.1 atom% of Ptand species then up slightly to 0.2 decreasedatom%, and as anthe increase activity ofwas Pt-deposition saturated at (see 0.4 Figureatom%.2 [I],The Table selectivity S1). Reaction of phenol time was profile optimized for phenol at 0.1 atom% and then slightly decreased as an increase of Pt-deposition (see Figure 2 [I], Table S1). Reaction evolution on the Pt(0.2)-WO3 photocatalyst was shown in Figure2 [II]. The photocatalytic activity increasedtime profile as anfor increasephenol ev ofolution reaction on time: the Pt(0.2)-WO producing3 29photocatalystµmol of phenol was withshown 47 in % Figure selectivity 2 [II]. after The μ photocatalyticphotocatalytic reaction activity for increased 20 h, and as 40 anµ increasemol phenol of reaction with 64% time: selectivity producing for 70 29 h. (Seemol distributionof phenol with of 47 % selectivity after photocatalytic reaction for 20 h, and 40 μmol phenol with 64% selectivity for 70 side products shown in Table S2). After the reaction for 70 h, the amount of photo-formed H2O2 was h. (See distribution of side products shown in Table S2). After the reaction for 70 h, the amount of only 0.15 µmol, which are much less than hydroxylated products assuming two electron reduction of O2 photo-formed H2O2 was only 0.15 μmol, which are much less than hydroxylated products assuming to form H2O2. It can be considered that the H2O2 may be self-decomposed on the photocatalyst and/or two electron reduction of O2 to form H2O2. It can be considered that the H2O2 may be self-decomposed participate in the reaction for the hydroxylation of benzene. The role of H2O2 will be discussed later. Moreover,on the photocatalyst when the volume and/or ofparticipate solvent in in the the reaction reaction became for the small,hydroxylation the yields of of benzene. phenol decreased,The role of whileH2O2 thewill selectivitybe discussed of later. phenol Moreover, improved when over the 70% volu (Seeme Tableof solvent S3). in Therefore, the reaction a trade-obecameff small,relation the betweenyields of the phenol yields decreased, and selectivity while of phenolthe selectivit was observed.y of phenol The surfaceimproved coverage over 70% of the (See photocatalyst Table S3). byTherefore, condensation a trade-off of the relation adsorbent between could the be consideredyields and selectivity to prevent of over phenol oxidation was observed. of benzene The leading surface coverage of the photocatalyst by condensation of the adsorbent could be considered to prevent over
Catalysts 2020, 10, x 557 FOR PEER REVIEW 4 4of of 12 12 Catalysts 2020, 10, x FOR PEER REVIEW 4 of 12 oxidation of benzene leading to high selectivity of phenol, of which the similar phenomena was oxidationto high selectivity of benzene of phenol,leading of to which high selectivit the similary of phenomena phenol, of was which previously the similar reported phenomena on the TiOwas2 previously reported on the TiO2 photocatalyst [25]. previouslyphotocatalyst reported [25]. on the TiO2 photocatalyst [25].
[I] 40 100 [II] 50 100 [I] 40 100 [II] 50 100
(a) 80 40 80 30 (a) 80 40 (c) 80 30 (c) 60 30 60 60 30 60 20 20 40 20 (d) 40 (b) 40 20 40 (b) (d) 10 10 20 10 20 20 10 20 Selectivity of phenol / % Selectivity of phenol of Selectivity / % Amounts of phenol / µmol Selectivity of phenol / % Selectivity of phenol of Selectivity / % Amounts of phenolof Amounts / µmol
0 0 Amounts of phenol / µmol 0 0 Amounts of phenolof Amounts / µmol 0 0 0 0 00.10.20.30.4 020406080 00.10.20.30.4 020406080 Amounts of Pt added to WO / atomic% Time / h Amounts of Pt added to WO3 / atomic% Time / h 3 Figure 2. Dependence of loading amounts of Pt deposited on the WO3 [I]; and reaction time profile FigureFigure 2. Dependence of loadingloading amountsamounts ofof Pt Pt deposited deposited on on the the WO WO3 [3I ];[I]; and and reaction reaction time time profile profile on on the Pt(0.2)-WO3 [II] for hydroxylation of benzene to form phenol (yields: a, c; and selectivity: b and onthe the Pt(0.2)-WO Pt(0.2)-WO3 [II3 ][II for] for hydroxylation hydroxylation of benzeneof benzene to formto form phenol phenol (yields: (yields: a, c;a, andc; and selectivity: selectivity: b andb and d) d) under visible-light irradiation. d)under under visible-light visible-light irradiation. irradiation.
Apparent quantum yields (AQY) on thethe Pt(0.2)-WOPt(0.2)-WO3 were measured.measured. Assuming one photon Apparent quantum yields (AQY) on the Pt(0.2)-WO3 were measured. Assuming one photon producing one phenol, the AQY for phenol formation reached over 2% by excitation at 400 nm as producing one phenol, the the AQY AQY for for phenol formation formation reached reached over over 2% 2% by by excitation excitation at at 400 400 nm nm as shown in Figure 3. The AQY profile was found to be very similar with the absorption spectrum of shownshown in FigureFigure3 .3. The The AQY AQY profile profile was was found found to beto verybe very similar similar with with the absorption the absorption spectrum spectrum of WO of3, WOsuggesting3, suggesting that phenol that phenol formation formation is strongly is strongly correlated correlated with the with light the absorbance light absorbance of WO ,of not WO with3, not Pt WO3, suggesting that phenol formation is strongly correlated with the light absorbance3 of WO3, not with Pt scattering [30] and/or resonance absorbance [31]. withscattering Pt scattering [30] and [30]/or resonanceand/or resonance absorbance absorbance [31]. [31].
3 0.8 3 0.8
0.6 2 0.6 2 (c) 0.4 (c) 0.4
AQY / % 1 Absorbance AQY / % 1 (a) (b) 0.2 Absorbance (a) (b) 0.2
0 0.0 0 0.0 400 450 500 550 600 400 450 500 550 600 Wavelength / nm Wavelength / nm Figure 3. DependenceDependence of of wavelength wavelength photo-irradiated photo-irradiated on on apparent apparent quantum quantum yields, yields, AQY AQY (a) (a) for Figurehydroxylation 3. Dependence of benzene of towavelength phenol, and photo-irradiated UV–Vis absorbance on apparent of WO (b) quantum and Pt(0.2)-WO yields, AQY(c). (a) for hydroxylation of benzene to phenol, and UV–Vis absorbance of WO33 (b) and Pt(0.2)-WO33 (c). hydroxylation of benzene to phenol, and UV–Vis absorbance of WO3 (b) and Pt(0.2)-WO3 (c). 2.3. Role of H2O on the Photocatalytic Reactions 2.3. Role of H2O on the Photocatalytic Reactions 2.3. Role of H2O on the Photocatalytic Reactions 18 In order to understand role of H2O, the H2 O-labeling experiments for photocatalytic In order to understand role of H2O, the H218O-labeling experiments for photocatalytic hydroxylationIn order ofto benzeneunderstand were role performed of H2O, by liquidthe H chromatography–mass218O-labeling experiment spectrometrys for photocatalytic (LC-MS), hydroxylation of benzene were performed by liquid chromatography–mass spectrometry (LC-MS), hydroxylationand results are of shown benzene in Tablewere 1performed and Figure by S2. liquid It is chromatography–mass noted that the atomic exchangesspectrometry between (LC-MS), O 2 and results are shown in Table 1 and Figure S2. It is noted that the atomic exchanges between O2 and andand results H2O, and are betweenshown in O Table2 and 1 phenol and Figure are very S2. It slow is noted even that under the photo-irradiation atomic exchanges [23 between,32]. Therefore, O2 and 2 2 218 H O,18 and between O and phenol are very slow even under photo-irradiation [23,32]. Therefore, H 18O HH22O, Oand was between used forO2 and tracing phenol O species are very incorporated slow even under in phenol. photo-irradiation When the photocatalytic[23,32]. Therefore, reactions H2 O was used for tracing O species incorporated16 in phenol. When the photocatalytic reactions were waswere used carried for outtracing in the O presencespecies incorporated of H2 O (100%), in phenol. the photo-formed When the photocatalytic phenol was only reactions observed were at 216 carried out in the presence of H 16O (100%), the photo-formed phenol was only observed16 at the mass carriedthe mass out number in the presence of 93 (m /ofz), H which2 O (100%), is attributed the photo-formed to the phenolic phenol anion was (Conly6H 5observedO−). Furthermore, at the mass 6 516 - number of 93 (m/z), which is attributed to the phenolic16 anion18 (C H 16O-). Furthermore, when the numberwhen the of photocatalytic 93 (m/z), which reactions is attributed by employing to the Hphenolic2 O (90%) anion/H2 (CO6 (10%)H5 O were). Furthermore, carried out, when the peaks the 216 218 photocatalytic reactions by16 employing H 16O (90%)/H 18O (10%)18 were carried out, the peaks at 93 (m/z) photocatalyticat 93 (m/z) for reactions the C6H5 byO employing− as well as H2 95O (m (90%)/H/z) for C2 6OH 5(10%)O− werewere carried observed. out, Thethe peaks peak intensitiesat 93 (m/z) 16 - 18 - for the C6H5 O as well as18 95 (m/16z) for C6H5 O were observed. The peak intensities indicated that the forindicated the C6H that516O the- as ratio well ofas 95O ( tom/z)O for involved C6H518O in- were the photo-formed observed. The phenol peak intensities was 9.9% underindicated visible-light that the ratio of 18O to 16O involved in the photo-formed phenol was 9.9% under visible-light (420 < λ < 540 ratio of 18O to 16O involved in the photo-formed phenol was 9.9% under visible-light (420 < λ < 540 nm), while 9.1% under UV-light irradiation (300 < λ < 400 nm). Assuming that H216O and H218O nm), while 9.1% under UV-light irradiation (300 < λ < 400 nm). Assuming that H216O and H218O
Catalysts 2020, 10, 557 5 of 12 Catalysts 2020, 10, x FOR PEER REVIEW 5 of 12 exhibited same activity for hydroxylation of benzene, it was concluded that hydroxyl groups 16in (420 < λ < 540 nm), while 9.1% under UV-light irradiation (300 < λ < 400 nm). Assuming that H2 O phenol is18 almost derived from H2O under both irradiation of visible-light and UV-light. and H2 O exhibited same activity for hydroxylation of benzene, it was concluded that hydroxyl groups in phenol is almost derived from H2O under both irradiation of visible-light and UV-light. Table 1. H218O isotope labeling test for photocatalytic hydroxylation of benzene on the Pt(0.2)-WO3. 18 Table 1. H2 O isotope labeling test for photocatalytic hydroxylation of benzene on the Pt(0.2)-WO3. Products/µmol 18O/16O 1 Ratios Entry Products/µmol 18 16 1 EntryPH RE BQ CA HQ PL PG O/ O inRatios Phenol/% PH RE BQ CA HQ PL PG in Phenol/% 1 22.0 1.4 0.01 4.3 0.1 3.4 1.9 9.9 1 22.0 1.4 0.01 4.3 0.1 3.4 1.9 9.9 2 218.0 18.00.4 0.40.03 0.035.9 5.91.3 1.35.5 5.5 1.7 1.7 9.1 9.1 1 16 18 μ 1H2 16O (90%)/H218O (10%) (5 mL), benzene: 300 mol, visible-light irradiation (entry 1) and UV-light H2 O (90%)/H2 O (10%) (5 mL), benzene: 300 µmol, visible-light irradiation (entry 1) and UV-light irradiation irradiation(entry 2) for (entry 20 h. Abbreviation: 2) for 20 h. phenol Abbrev (PH),iation: resolcinol phenol (RE), (PH),p-benzoquinone resolcinol (BQ), (RE), catechol p-benzoquinone (CA), hydroquinone (BQ), catechol(HQ), phloroglucinol (CA), hydroquinone (PL), pyrogallol (HQ), (PG). phloroglucinol (PL), pyrogallol (PG). The rate determining step for hydroxylation of benzene was investigated by the kinetic isotope The rate determining step for hydroxylation of benzene was investigated by the kinetic isotope effect (KIE) using D2O and C6D6 (See Figure 4). The reaction rate constants for formation of phenol effect (KIE) using D2O and C6D6 (See Figure4). The reaction rate constants for formation of phenol−6 from normal benzene (C6H6) in the presence of H2O and D2O were roughly estimated to be 3.75 × 10 6 from normal benzene (C6H6) in the presence of H2O and D2O were roughly estimated to be 3.75 10− and 1.42 × 10−6 s−1 for (a) and (b), respectively. The kinetic isotope effect (kH2O/kD2O) was estimated× to and 1.42 10 6 s 1 for (a) and (b), respectively. The kinetic isotope effect (k /k ) was estimated to be ca. 2.7 ×(see− Figure− 4 [I]). On the other hand, the reaction rate constantsH2O for D2Oformation of phenol be ca. 2.7 (see Figure4 [I]). On the other hand, the reaction rate constants for formation−6 of phenol from-6 −1 from C6H6 and C6D6 in the presence of H2O were roughly estimated to be 3.75 × 106 and 3.77 × 106 s 1 C6H6 and C6D6 in the presence of H2O were roughly estimated to be 3.75 10− and 3.77 10− s− for (c) and (d), respectively. That is, the KIE of kC6H6/kC6D6 was estimated to× be 1.0 (see Figure× 4 [II]). for (c) and (d), respectively. That is, the KIE of kC6H6/kC6D6 was estimated to be 1.0 (see Figure4 [II]). These results suggest that the dissociation of O–H bond in the H2O plays an important role in the rate These results suggest that the dissociation of O–H bond in the H O plays an important role in the rate determining step for the hydroxylation of benzene. 2 determining step for the hydroxylation of benzene.
Figure 4. Kinetic isotope effects (KIE) on photocatalytic hydroxylation of benzene in the presence of
air and H2O on Pt(0.2)-WO3 (20 mg) under visible light irradiation. Hydroxylation of C6H6 in the Figure 4. Kinetic isotope effects (KIE) on photocatalytic hydroxylation of benzene in the presence of presence of H2O (a) and D2O (b) [I]; and that of C6H6 (c) and C6D6 (d) in the presence of H2O[II]. air and H2O on Pt(0.2)-WO3 (20 mg) under visible light irradiation. Hydroxylation of C6H6 in the 2.4.presence Role of O of2 on H2 theO (a) Photocatalytic and D2O (b) [ ReactionsI]; and that of C6H6 (c) and C6D6 (d) in the presence of H2O [II]. When the half reaction for the photocatalytic hydroxylation of benzene was performed on 2.4. Role of O2 on the Photocatalytic Reactions + the Pt-WO3 in the presence of Ag ions as electron scavenger instead of O2 under visible-light irradiation,When the no half products reaction could for the be photocatalytic detected under hydroxylation visible-light of irradiation benzene was (Table performed2). This on result the 3 + 2 Pt-WOindicated in thatthe presence both H2O of and Ag O ions2 (air) as inelectron combination scavenger are instead essential of for O hydroxylation under visible-light of benzene irradiation, under novisible-light products could irradiation. be detected under visible-light irradiation (Table 2). This result indicated that both H2O and O2 (air) in combination are essential for hydroxylation of benzene under visible-light irradiation.
Catalysts 2020, 10, 557 6 of 12
+ Table 2. Effects of Ag ions added on the photocatalytic hydroxylation of benzene on the Pt(0.2)-WO3 photocatalyst under photo-irradiation.
Reaction Products/µmol Selectivity of Condition PH RE p-BQ CA HQ PL PG PH/% Visible light/air 29.0 1.2 0.2 18.3 0.3 19.0 2.4 41.0 (1) Visible light/N2 0 0 0 0 0 0 0 - (1) UV light/N2 1.1 0.5 0.5 0.6 0.2 6.8 0.06 11.7 (1) + presence of Ag (50 µmol) under N2 purge.
The flat band potential of WO3 was obtained from capacitance versus voltage measurements. The WO3 powder was deposited on fluorine-doped tin oxide (FTO) coated glass. Mott–Schottky plots indicated that the flatband potential ( conduction band) of the WO /FTO was estimated to be +0.40 V ≈ 3 vs. reversible hydrogen electrode (RHE) (Figure S3). LSV measurements of WO3 and Pt-WO3 were carried out in O2-saturated 0.1 M Na2SO4 aq. The cathodic current was obtained at the potential more cathodic than the conduction band (+0.40 V vs. RHE) of WO3 (See Figure S4). This cathodic current would be attributed to the reduction of WO3. Furthermore, the cathodic current significantly increased by deposition of Pt on WO3. A deposition of Pt species as a co-catalyst caused an improvement of the ORR. It is noted that multi-electron reduction of O2 to H2O2, thermodynamically occurred at 0 E (O2/H2O2) = +0.68 V [33]. An increase of cathodic current is attributed to the effective electron transfer to O2 via Pt species to form H2O2 [34]. It was confirmed that the hydroxylation of benzene in the presence of H2O2 did not take place under photoirradiation without a photocatalyst nor with photocatalyst without irradiation. However, both photocatalyst and visible-light irradiation are required for the hydroxylation of benzene (See Table S4). In order to understand the role of H2O2, the hydroxylation of benzene (300 µmol) in 16 18 the presence of H2O2 (150 µmol) and H2 O (90%)/H2 O (10%) was conducted on the Pt-WO3 under visible-light irradiation. As a result, the ratio of 18O to 16O was determined to be 8.3%, suggesting that major contribution of OH groups in phenol came from H2O even in the presence of H2O2.
2.5. Reaction Mechanisms for Photocatalytic Hydroxylation of Benzene to Phenol In order to evaluate the oxidation power of photo-induced holes, photocatalytic half reactions on + the Pt(0.2)-WO3 photocatalyst were carried out. It was observed that addition of Ag ions instead of O2 showed no products under visible-light irradiation, while the phenolic compounds were detected under UV-light irradiation (See Table2). These results suggest that water oxidation by UV-light irradiation induced strong oxidation power possibly to form free OH radicals, which would attack benzene directly [29], but by visible-light irradiation would not form OH radicals. In fact, we employed the tertiary butyl alcohol (TBA) as scavengers of OH radicals. As a result, the TBA added in the photocatalytic system on the Pt-WO3 under visible-light irradiation did not influence to the yields of phenol (Table3). However, the amounts of catechol (CA) and phloroglucinol (PL) decreased by the addition of TBA (See Table3). The di- and tri- OH groups involved in phenolic compounds may be 18 partially derived from O2. Research is under way using an O2 labeling test.
Table 3. Effects of tertiary butyl alcohol (TBA) added on the photocatalytic hydroxylation of benzene
on the Pt(0.2)-WO3 photocatalyst under visible light irradiation for 20 h under air.
Products/µmol Selectivity of TBA Added/µmol PH RE BQ CA HQ PL PG PH/% 0 29.0 1.2 0.19 18.3 0.34 19 2.4 41.0 50 29.2 1.39 0.25 13.3 0.27 7.9 5.0 51.0 Catalysts 2020, 10, 557 7 of 12
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 12 As mentioned above, H2O2 was confirmed to form as intermediate species by multi-electron reductionAs mentioned of O2 during above, photocatalytic H2O2 was confirmed hydroxylation to form of as benzene. intermediate The reactionspecies by mechanisms multi-electron were investigatedreduction of byO2 DFTduring calculations. photocatalytic Figure hydroxylation5 [Ia] showed of benzene. the optimized The reaction structures mechanisms of the reactant, were transitionalinvestigated state by DFT (TS) calculations. and product Figure by the interactions5 [Ia] showed of the benzene, optimized H2O structures with H2O 2of. Thethe O–Hreactant, bond lengthtransitional in H2 O,state O–O (TS) in Hand2O product2 and C–H by inthe benzene interactions were observedof benzene, to increaseH2O with through H2O2. The the reactionO–H bond path. Itlength was found in H2O, that O–O H 2inO H2 could2O2 and assist C–HC-H in benzene bond dissociation were observed from to increase benzene. through Subsequently, the reaction OH path. species 2 2 derivedIt was found from that H2O H wasO incorporatedcould assist C-H within bond phenol dissociation in the finalfrom product.benzene. Moreover,Subsequently, the OH reaction species path derived from H2O was incorporated within phenol in the final product. Moreover, the reaction path in the presence of the Pt co-catalyst was investigated (See Figure5 [Ib]). Strong interaction of H 2O2 in the presence of the Pt co-catalyst was investigated (See Figure 5 [Ib]). Strong interaction of H2O2 with Pt3 was observed to dissociate the O–O bond to form O species, and subsequently, the C–H bond with Pt3 was observed to dissociate the O–O bond to form O species, and subsequently, the C–H bond in benzene was dissociated by an assist of the O species and/or Pt3. The energy changes for overall reactionsin benzene were was indicated dissociated in by Figure an assist5 [II]. of The the hydroxylationO species and/or of benzenePt3. The energy proceeded changes on muchfor overall lower reactions were indicated in Figure 5 [II]. The hydroxylation of benzene proceeded on much lower potential energy surface with the Pt3 co-catalyst than that without Pt3 co-catalyst. Thus, we have demonstratedpotential energy the surface possible with reaction the Pt path3 co-catalyst for hydroxylation than that ofwithout benzene Pt3 toco-catalyst. form phenol Thus, including we have OH demonstrated the possible reaction path for hydroxylation of benzene to form phenol including OH groups derived from H2O. groups derived from H2O.
400
200 -1
TS 0 (a)
-200 Reactant Energy kJ/ mol Energy -400 (b) Products -600 01234 Reaction coordinate [II]
Figure 5. Optimized structures at the reactant, TS and product between benzene, H2O and H2O2 Figure 5. Optimized structures at the reactant, TS and product between benzene, H2O and H2O2 system in the absence [Ia] and presence [Ib] of Pt3 as co-catalyst for the hydroxylation of benzene to system in the absence [Ia] and presence [Ib] of Pt3 as co-catalyst for the hydroxylation of benzene to formform phenol; phenol; and and energy energy potentials potentials for for intrinsic intrinsic reaction reaction coordinates coordinates (IRC) (IRC) [II]. [ TheII]. stabilizationThe stabilization energy atenergy each stateat each was state calculated was calculated by the di ffbyerence the differe fromnce the energyfrom the of energy isolated of system. isolated The system. bond The distance bond (Å) distance (Å) of O–H in H2O (blue), O–O in H2O2 (green) and C–H in benzene (red) were indicated. of O–H in H2O (blue), O–O in H2O2 (green) and C–H in benzene (red) were indicated.
Catalysts 2020, 10, 557 8 of 12
CatalystsThe 2020 reaction, 10, x FOR mechanism PEER REVIEW for the direct hydroxylation of benzene to phenol on the Pt-WO8 of 123 was proposed (See Figure6). This reaction is initiated by visible-light irradiation of the WO 3. Theinduced photo-induced holes and electrons holes and are electrons generated are in generatedthe VB and in CB the of VBthe WO and3, CB respectively. of the WO It3, was respectively. assumed Itthat was the assumed photo-induced that the holes photo-induced can oxidize holesH2O to can form oxidize O2 [1], H as2O well to form as activated O2 [1], as(H well2O)* intermediate as activated (Hspecies2O)* intermediateof which the species O–H ofbond which length the O–Hwould bond be lengthincreased. would On be the increased. other hand, On the the other electrons hand, theeffectively electrons transfer effectively from transfer CB to O from2 via Pt CB species to O2 via to form Pt species peroxide to form species peroxide such as species H2O2, such which as would H2O2, whichbe further would activated be further on the activated catalyst on surface. the catalyst The activated surface. (H The2O)* activated species (Hwould2O)* speciesattack the would π electrons attack theof theπ electrons aromatic of benzene the aromatic ring, benzeneassisting ring, the activated assisting the(H2 activatedO2)* to dissociate (H2O2)* tothe dissociate C–H bond the in C–H benzene. bond inIt benzene.was, thus, It proposed was, thus, that proposed the hydroxylation that the hydroxylation of benzene of occurs benzene by occurs a mechanism by a mechanism involving involving a push- apull push-pull process, process, which promotes which promotes the formation the formation of phenol. of phenol.
V vs. RHE
WO3 O2 2H+ 3e- +0.4 Pt * * H (H2O2) O H H O OH Visible-light pull H (λ > 420 nm) OH H H push
+ 2H2O + 2H H H 1/2 O2 * (H2O) + 2H+ 2h+ h+ H +3.0 H O H2O 2
FigureFigure 6.6. ReactionReaction mechanismsmechanisms for for the the hydroxylation hydroxylation of benzeneof benzene into into phenol phenol in the in presencethe presence of O 2ofand O2 Hand2O H on2O the on Pt-WO the Pt-WO3 under3 under visible visible light irradiation.light irradiation. (*) indicates (*) indicates formation formation of activated of activated H2O and H2O H 2andO2 intermediatesH2O2 intermediates by photo-excitation. by photo-excitation.
3.3. Materials and Methods 3.1. Materials 3.1. Materials Tungsten trioxide (WO3, 99.9%) from Kojundo Chemical Laboratory (Osaka, Japan), H2PtCl6 6H2O Tungsten trioxide (WO3, 99.9%) from Kojundo Chemical Laboratory (Osaka, Japan),· (99.9%) from Wako Pure Chemical Industries (Osaka, Japan); and water-18O (97 atom% 18O) and H2PtCl6∙6H2O (99.9%) from Wako Pure Chemical Industries (Osaka, Japan); and water-18O (97 atom% titanium (IV) oxysulfate solution (99.99%) from Sigma-Aldrich (St. Louis, MO, USA) were purchased. 18O) and titanium (IV) oxysulfate solution (99.99%) from Sigma-Aldrich (St. Louis, MO, USA) were All chemicals were used without further purification. purchased. All chemicals were used without further purification.
3.2. Photoelectrochemical Deposition of Pt Species as Co-Catalyst on the WO3 3.2. Photoelectrochemical Deposition of Pt Species as Co-Catalyst on the WO3 Photoelectrochemical deposition of Pt species was conducted on the WO3 surface. WO3 powder Photoelectrochemical deposition of Pt species was conducted on the WO3 surface. WO3 powder (1.0 g) was suspended in distilled water (25 mL) involving desired amounts of H2PtCl6 (1.0 g) was6 suspended in5 distilled water (25 mL) involving desired amounts of H2PtCl6 (4.32 × 10−6 ~ (4.32 10− ~ 2.16 10− mol), and the suspension was photo-irradiated by a light-emitting diode 2.16 ×× 10−5 mol), and× the suspension was photo-irradiated by a light-emitting diode (LED) lamp (420 (LED) lamp (420 < λ < 540 nm) for 1 h under stirring in order to disperse photo-adsorbed Pt species. < λ < 540 nm) for 1 h under stirring in order to disperse photo-adsorbed Pt species. Subsequently, Subsequently, methanol (5 mL) as reductant was added into the suspension, and then it was continuously methanol (5 mL) as reductant was added into the suspension, and then it was continuously photo- photo-irradiated for 4 h. The solid products were separated by centrifuge (LC-200, TOMY, 4500 rpm), irradiated for 4 h. The solid products were separated by centrifuge (LC-200, TOMY, 4500 rpm), followed by washing with distilled water and acetonitrile, and drying under vacuum condition followed by washing with distilled water and acetonitrile, and drying under vacuum condition at at ambient temperature over night. The photocatalyst was referred to be as Pt(x)-WO3(x: atom%). ambient temperature over night. The photocatalyst was referred to be as Pt(x)-WO3(x: atom%). The The photocatalyst was kept in the ambient temperature, and the photocatalytic reactions were carried photocatalyst was kept in the ambient temperature, and the photocatalytic reactions were carried out out without further treatment of photocatalyst. without further treatment of photocatalyst.
3.3. Characterizations
The X-ray diffraction (XRD) patterns were obtained with a RIGAKU RINT2000 using Cu Kα radiation (λ = 1.5417 Å) (RIGAKU, Tokyo, Japan). The oxidative states of Pt species were analyzed by an X-ray photoelectron spectroscope (XPS), KRATOS, AXIS Ultra, using Al Kα radiation (E = 1486.8 eV) (Shimadzu, Kyoto, Japan). The UV–Vis spectroscopic measurements were carried out on diffuse
Catalysts 2020, 10, 557 9 of 12
3.3. Characterizations The X-ray diffraction (XRD) patterns were obtained with a RIGAKU RINT2000 using Cu Kα radiation (λ = 1.5417 Å) (RIGAKU, Tokyo, Japan). The oxidative states of Pt species were analyzed by an X-ray photoelectron spectroscope (XPS), KRATOS, AXIS Ultra, using Al Kα radiation (E = 1486.8 eV) (Shimadzu, Kyoto, Japan). The UV–Vis spectroscopic measurements were carried out on diffuse reflectance with a UV–Vis scanning spectrophotometer (UV-3100PC, Shimadzu, Kyoto, Japan). The elemental distribution images were taken with scanning transmittance electron microscope with energy-dispersed X-ray emission spectroscopy (STEM-EDS; JEOL JSM-2100, Tokyo, Japan).
3.4. Photocatalytic Reactions Photocatalyst (20 mg) was suspended in distilled water (2, 5 or 10 mL) in a pyrex reaction tube (volume: 20 mL) under air capped with precision seal septum. If specific description was not given, the distilled water (10 mL) was introduced. And subsequently, 26.8 µL of benzene (300 µmol) was dropped into the suspension by a syringe under vigorous stirring. The reaction cell was photo-irradiated by a blue LED (visible light: 420 < λ < 540 nm) or black light (UV light: 300 < λ < 400 nm) at 298 K. The light energy intensities were shown in Figure S5. After the reaction, the catalysts were immediately separated from the suspension by filtration through a 0.20 µm membrane filter (Dismic-25JP, Advantec, Tokyo, Japan). The solution was, then, analyzed by High Performance Liquid Chromatography (HPLC, Shimadzu LC10ATVP, Kyoto, Japan, UV–Vis detector, column: Chemcopak, mobile phase: a mixture of acetonitrile and 1.0% formic acid aqueous solution), and several products were identified (see Figure S6). The gas phase (CO2) was analyzed by Gas Chromatography–thermal conductivity detector (GC-TCD, Shimadzu GC-8A, Kyoto, Japan; column: porapak Q). Titanium sulfate (100 µL) was added to 1 mL of the reaction solution, and amounts of H2O2 formed in solution were analyzed by colorimetry from the value of absorbance at 410 nm in the UV–Vis absorption spectrum. 18 For the tracer experiment, H2 O was used as the oxygen isotope source. The photocatalytic hydroxylation of benzene or phenol was carried out over Pt-WO3 photocatalyst in the presence of 18 16 H2 O (10%)/H2 O (90%) as solvent and air under visible-light irradiation for 20 h. The photocatalytic 16 18 16 hydroxylation of benzene was performed in the presence of H2 O2 (100 µmol) and H2 O (10%)/H2 O (90%) under visible-light irradiation for 2 h. Phenol in aqueous solutions were extracted by toluene, and the molar ratios of Ph-18OH and Ph-16OH were identified by LC-MS (Shimadzu LCMS-2020 spectrometer, Kyoto, Japan).
3.5. Electrochemical Measurements Flatband potentials of the photoelectrodes were measured by a Potentio/Galvanostat (PGSTAT204, Autolab). The electrolysis cell was constructed with three electrodes, the WO3 photoelectrode as working electrode, the platinum wire as auxiliary, and Ag/AgCl as the reference electrode. The WO3 photoelectrodes were prepared as follows: the WO3 (20 mg) was suspended in ethanol (0.2 mL) by a super-sonification to form a paste. The paste was then spread onto the FTO (active space 1.0 cm2, Aldrich) by spin coating (2200 rpm, 30 s) for 3 times, followed by heat-treatment at 773 K for 1 h in air. Prior to measurements, 0.10 M Na2SO4 aqueous solution was vigorously bubbled by N2 gas for 20 min in order to remove O2. 1 Linear sweep voltammetry (LSV) was conducted at the rate of 10 mV s− . The photocatalyst (20 mg) was added into ethanol (0.2 mL), followed by a super-sonification for 5 min to disperse. The paste was casted by spin-coating (2200 rpm, 30 s) on the conductive transparent glass (FTO, 10 Ω/, Sigma Aldrich, St. Louis, MO, USA) 3 times, followed by heat-treatment at 373 K overnight. Prior to LSV measurements, 0.10 M Na2SO4 aq. was vigorously bubbled by O2 for 20 min. Catalysts 2020, 10, 557 10 of 12
3.6. Density Functional Theory (DFT) Calculation Gaussian 09 program [35] and the hybrid B3LYP functional [36] were used. For Pt atoms, the Los Alamos effective core potentials [37] were employed along with the corresponding valence double basis sets [38]. For other atoms, a 6-311G(d,p) basis set was used. After the transition state (TS) was characterized, the intrinsic reaction coordinate (IRC) analysis [39] was carried out for both directions, reactant and product sides. The IRC analyses were followed by normal optimization runs, and the reactant and product were optimized as local minima.
4. Conclusions In this study, we provided an understanding of photocatalytic activities for the hydroxylation of benzene under irradiation of only visible-light (420 < λ < 540 nm). It was demonstrated that hydroxylation of benzene on the Pt-WO3 photocatalyst in the presence of air and H2O produced several products such as phenol, catechol and phloroglucinol etc., and the selectivity of phenol improved over 70%. One of novelties in our findings was to confirm the reaction mechanisms for photocatalytic hydroxylation of benzene into phenol by experimental and theoretical studies. An investigation of the mechanistic insights has been carried out by the combination with apparent quantum yields 18 (AQY), an H2 O isotope-labeling experiment, kinetic isotope effects (C6D6,D2O), electrochemical measurements, and DFT calculations. It was proposed that the substitution of the OH derived from H2O with H abstracted from benzene by photo-formed H2O2 indicated a mechanism involving a push-pull process for the hydroxylation of benzene into phenol. Our results showed new perspectives for the enhancement of yields and selectivity of phenol, as well as deeper understanding of the reaction mechanisms for the hydroxylation of benzene into phenol on the Pt-WO3 photocatalyst.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/5/557/s1: 18 Figure S1: Tauc plots of WO3, Figure S2: H2 O isotope labeling experiments, Figure S3: Mott–Schottky plots of WO3, Figure S4: LSV measurements of WO3 and Pt-WO3, Figure S5: Photo-intensities emitted from light-emitting diode (LED) and UV lamp, Figure S6: Molecular structures of products, Table S1: Photocatalytic activities depending on the amounts of Pt-deposition, Table S2: Time profiles in the photocatalytic reaction, Table S3: Effects of volumes in the reaction on the photocatalytic activity, Table S4: Role of H2O2 for the photocatalytic reactions. Author Contributions: Conceptualization, S.H.; investigation, Y.K., M.T.; writing—original draft preparation, Y.K.; writing—review and editing, S.H., Y.S., H.K., M.M., Y.M.; supervision, S.H.; project administration, S.H. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
References
1. Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D.J.; Higashi, M.; Kong, D.R.; Abe, R.; Tang, J. Mimicking
natural photosynthesis: Solar to renewable H2 fuel synthesis by Z-scheme water splitting systems. Chem. Rev. 2018, 118, 5201–5241. [CrossRef][PubMed] 2. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D.W. Understanding
TiO2 photocatalysis: Mechanisms and materials. Chem. Rev. 2014, 114, 9919–9986. [CrossRef][PubMed] 3. Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem. Rev. 2014, 114, 9987–10043. [CrossRef][PubMed] 4. Kou, J.; Lu, C.; Wang, J.; Chen, Y.; Xu, Z.; Varma, R.S. Selectivity enhancement in heterogeneous photocatalytic transformations. Chem. Rev. 2017, 117, 1445–1514. [CrossRef][PubMed] 5. Yamashita, H.; Mori, K.; Kuwahara, Y.; Kamegawa, T.; Wen, M.; Verma, P.; Che, M. Single-site and nano-confined photocatalysts designed in porous materials for environmental uses and solar fuels. Chem. Soc. Rev. 2018, 47, 8072–8096. [CrossRef] 6. Higashimoto, S. Titanium dioxide-based visible-light sensitive photocatalysis: Mechanistic insight and applications. Catalysts 2019, 9, 201. [CrossRef] Catalysts 2020, 10, 557 11 of 12
7. Asahi, R.; Morikawa, T.; Irie, H.; Ohwaki, T. Nitrogen-doped titanium dioxide as visible light-sensitive photocatalyst: Designs, developments, and prospects. Chem. Rev. 2014, 114, 9824–9852. [CrossRef] 8. Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. Pristine simple oxides as visible light driven photocatalysts: Highly efficient decomposition of organic compounds over platinum-loaded tungsten oxide. J. Am. Chem. Soc. 2008, 130, 7780–7781. [CrossRef] 9. Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Reaction mechanism and activity
of WO3-catalyzed photodegradation of organic substances promoted by a CuO cocatalyst. J. Phys. Chem. C 2009, 113, 6602–6609. [CrossRef] 10. Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Complete oxidation of acetaldehyde
and toluene over a Pd/WO3 photocatalyst under fluorescent- or visible-light irradiation. Chem. Commun. 2008, 43, 5565–5567. [CrossRef] 11. Tomita, O.; Sugimoto, T.; Sako, K.; Hayakawa, S.; Katagiri, K.; Inumaru, K. Enhanced photocatalytic
activity of Pt/WO3 photocatalyst combined with TiO2 nanoparticles by polyelectrolyte-mediated electrostatic adsorption. Catal. Sci. Technol. 2015, 5, 1163–1168. 12. Higashimoto, S.; Katsuura, K.; Yamamoto, M.; Tahakashi, M. Photocatalytic activity for decomposition of
volatile organic compound on Pt-WO3 enhanced by simple physical mixing with TiO2. Catal. Commun. 2020, 133, 105831. [CrossRef]
13. Higashimoto, S.; Ushiroda, Y.; Azuma, M. Electrochemically assisted photocatalysis of hybrid WO3/TiO2 films: Effect of the WO3 structures on charge separation behavior. Top. Catal. 2008, 47, 148–154. [CrossRef] 14. Tomita, O.; Otsubo, T.; Higashi, M.; Ohtani, B.; Abe, R. Partial oxidation of alcohols on visible-light-responsive WO3 photocatalysts loaded with palladium oxide cocatalyst. ACS Catal. 2016, 6, 1134–1144. [CrossRef] 15. Niwa, S.; Eswaramoorthy, M.; Nair, J.; Raj, A.; Ito, N.; Shoji, H.; Nanba, T.; Mizukami, F. A one-step conversion of benzene to phenol with a palladium membrane. Science 2002, 295, 105–107. [CrossRef] 16. Tanev, P.T.; Chibwe, M.; Pinnavaia, T.J. Titanium-containing mesoporous molecular sieves for catalytic oxidation of aromatic compounds. Nature 1994, 368, 321–323. [CrossRef] 17. Chen, J.; Gao, S.; Xu, J. Direct hydroxylation of benzene to phenol over a new vanadium-substituted phosphomolybdate as a solid catalyst. Catal. Commun. 2008, 9, 728–733. [CrossRef] 18. Ohkubo, K.; Kobayashi, T.; Fukuzumi, S. Direct oxygenation of benzene to phenol using quinolinium ions as homogeneous photocatalysts. Angew. Chem. Int. Ed. 2011, 50, 8652–8655. [CrossRef] 19. Fukuzumi, S.; Ohkubo, K. One step selective hydroxylation of benzene to phenol. Asian J. Org. Chem. 2015, 4, 836–845. [CrossRef] 20. Han, J.W.; Jung, J.; Lee, Y.M.; Nam, W.; Fukuzumi, S. Photocatalytic oxidation of benzene to phenol using dioxygen as an oxygen source and water as an electron source in the presence of a cobalt catalyst. Chem. Sci. 2017, 8, 7119–7125. [CrossRef] 21. Fukuzumi, S.; Lee, Y.M.; Nam, W. Photocatalytic oxygenation reactions using water and dioxygen. ChemSusChem 2019, 12, 3931–3940. [CrossRef][PubMed] 22. Yoshida, H.; Yuzawa, H.; Aoki, M.; Otake, K.; Itoh, H.; Hattori, T. Photocatalytic hydroxylation of aromatic ring by using water as an oxidant. Chem. Commun. 2008, 38, 4634–4636. [CrossRef][PubMed] 23. Bui, T.D.; Kimura, A.; Ikeda, S.; Matsumura, M. Determination of oxygen sources for oxidation of benzene
on TiO2 photocatalysts in aqueous solutions containing molecular oxygen. J. Am. Chem. Soc. 2010, 132, 8453–8458. [CrossRef][PubMed] 24. Ide, Y.; Matsuoka, M.; Ogawa, M. Efficient visible-light-induced photocatalytic activity on gold-nanoparticle-supported layered titanate. J. Am. Chem. Soc. 2010, 47, 16762–16764. [CrossRef]
25. Ide, Y.; Torii, M.; Sano, T. Layered silicate as an excellent partner of a TiO2 photocatalyst for efficient and selective green fine-chemical synthesis. J. Am. Chem. Soc. 2013, 135, 11784–11786. [CrossRef] 26. Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M.-H. Facile in situ synthesis of visible-light
plasmonic photocatalysts M@TiO2 (M = Au, Pt, Ag) and evaluation of their photocatalytic oxidation of benzene to phenol. J. Mater. Chem. 2011, 21, 9079–9087. [CrossRef]
27. Goto, T.; Ogawa, M. Efficient photocatalytic oxidation of benzene to phenol by metal complex-clay/TiO2 hybrid photocatalyst. RSC Adv. 2016, 6, 23794–23797. [CrossRef] 28. Tomita, O.; Abe, R.; Ohtani, B. Direct synthesis of phenol from benzene over platinum-loaded tungsten(VI) oxide photocatalysts with water and molecular oxygen. Chem. Lett. 2011, 40, 1405–1407. [CrossRef] Catalysts 2020, 10, 557 12 of 12
29. Tomita, O.; Ohtani, B.; Abe, R. Highly selective phenol production from benzene on a platinum-loaded tungsten oxide photocatalyst with water and molecular oxygen: Selective oxidation of water by holes for generating hydroxyl radical as the predominant source of the hydroxyl group. Catal. Sci. Technol. 2014, 4, 3850–3860. [CrossRef] 30. Chen, C.W.; Tano, D.; Akashi, M. Synthesis of platinum colloids sterically stabilized by poly(N-vinylformamide) or poly(N-vinylalkylamide) and their stability towards salt. Colloid Polym. Sci. 1999, 277, 488–493. [CrossRef]
31. Arabatzis, I.M.; Stergiopoulos, T.; Andreeva, D. Characterization and photocatalytic activity of Au/TiO2 thin films for azo-dye degradation. J. Catal. 2003, 220, 127–135. [CrossRef] 32. Buchachenko, A.L.; Dubinina, E.O. Photo-oxidation of water by molecular oxygen: Isotope exchange and isotope effects. J. Phys. Chem. A 2011, 115, 3196–3200. [CrossRef][PubMed] 33. Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press Ltd.: London, UK, 1966. 34. Panchenko, A.; Koper, M.T.M.; Shunbina, T.E.; Mitchell, S.J.; Roduner, E. Ab Initio calculation of intermediates of oxygen reduction on low-index platinum surfaces. J. Electrochem. Soc. 2004, 151, A2016–A2027. [CrossRef] 35. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09; Revision D.01; Gaussian, Inc.: Wallingford, CT, USA, 2013. 36. Becke, A.D. A new mixing of Hartree–Fock and local density-functional theories. J. Chem. Phys. 1993, 98, 5648–5652. [CrossRef] 37. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283. [CrossRef] 38. Dunning, T.H.; Hay, P.J.; Schaefer, H.F. Modern Theoretical Chemistry; Schaefer, H.F., III, Ed.; Plenum Press: New York, NY, USA, 1976; Volume 3, pp. 1–28. 39. Fukui, K.; Kato, S.; Fujimoto, H. Constituent analysis of the potential gradient along a reaction coordinate. Method and an application to methane + tritium reaction. J. Am. Chem. Soc. 1975, 97, 1–7. [CrossRef]
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