TiO2 Nanocomposite Formation Leads to Improvement in

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Co3O4/TiO2 nanocomposite formation leads to a tremendous improvement in UV-Vis-IR driven thermocatalytic activity due to novel photoactivation and photocatalysis-thermocatalysis synergetic effect Zhengkang Shi, Lan Lan, Yuanzhi Li, Yi Yang, Qian Zhang, Jichun Wu, Gequan Zhang, and Xiujian Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03602 • Publication Date (Web): 21 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Co3O4/TiO2 nanocomposite formation leads to a tremendous improvement in UV-Vis-IR driven thermocatalytic activity due to novel photoactivation and photocatalysis-thermocatalysis synergetic effect Zhengkang Shi, Lan Lan, Yuanzhi Li,* Yi Yang, Qian Zhang, Jichun Wu, Gequan Zhang, Xiujian Zhao State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China. Email: [email protected] Abstract: Co3O4/TiO2 nanocomposites with different Co/Ti molar ratios were prepared by hydrolysis of cobalt acetate with urea in the presence of TiO2, and then calcined at 260 oC. Compared to pure TiO2, the Co3O4/TiO2 nanocomposite with the optimum Co/Ti molar ratio of 0.30 demonstrates significantly enhanced catalytic activity as well as excellent catalytic durability for abatement of refractory poisonous benzene (a typical air pollutant) with UV-Vis-IR irradiation. It also exhibits effective catalytic activity for benzene abatement even with λ > 830 nm IR irradiation. Its very high catalytic activity derives from light-driven thermocatalytic benzene oxidation on nano Co3O4 in the Co3O4/TiO2 nanocomposite. A novel synergetic effect among light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite is discovered to remarkably promote catalytic activity and improve catalytic durability by inhibiting the formation of refractory carbonaceous intermediates on TiO2 by photocatalysis: energetic species produced by UV photocatalysis on TiO2 move from TiO2 to Co3O4 through the interface between nano Co3O4 and TiO2, thus accelerating the light-driven thermocatalytic benzene oxidation on nano Co3O4. A novel photoactivation, completely different from photocatalysis on TiO2, is discovered to further considerably accelerate light-driven thermocatalytic activity of Co3O4: Irradiation not only promotes the activity of lattice oxygen of nano Co3O4, but also accelerates the re-oxidation of the reduced cobalt oxide (Co3O4-x), resulting in a considerable enhancement in the light-driven thermocatalytic activity of Co3O4. The light-driven themocatalysis together with the novel photocatalysis-thermocatalysis synergetic effect and photoactivation in the Co3O4/TiO2 nanocomposite cause a tremendous enhancement of 489 times in benzene mineralization rate (initial production rate of CO2) as compared to the photocatalytic benzene abatement under UV-Vis-IR irradiation with the same light intensity at near room temperature. Keywords: Co3O4/TiO2 nanocomposite, photocatalytic, VOCs abatement, photothermocatalytic, photoactivation. 1

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Introduction TiO2 has been attracting tremendous interests for decades because of its unique properties for various applications such as photocatalytic abatement of diverse water and air pollutants, photocatalytic water reduction to produce hydrogen, photocatalytic CO2 reduction to produce chemicals and fuels, and so on.1-5 However, its wide practical photocatalytic applications are greatly blocked by several drawbacks: First, its quantum efficiency is low because of low separation efficiency of photogenerated holes and electrons. Second, it is easily deactivated for the photodegration of refractory organic pollutants (e.g. aromatic compounds) becuase of the deposition of refractroy carbonaceous intermediates on its surface. Third, it is only excited by UV photons becuase of its wide band gap. Enormous works have been doned to solve the problems. The reported approaches of improving photocatalytic efficiency include formation of anastase/rutile junction,6,

7

elevating specific surface area by synthesizing mesoporous TiO2,8,

9

surface

fluorination of TiO2,10, 11 decreasing bulk/surface defect ratio of TiO2 nanocrystals,12, 13 tuning the exposed facets of TiO2 nanocrystals (e.g.{001}),14-16 and so on. The reported approches of improving photocatalytic durability invovle decreasing the amount of refractory carbonaneous intermediates by depositing precious metal nanoparticles on TiO2,17 utilizing the synergetic effects between TiO2 and Ag–AgBr to improve photocatalyhtic activity and durability,18 decreasing O2 transfer limitation by forming ordered TiO2 nanotubes,19 inhibing the formation of carbonaneous intermediates by synergetic photothermocatalysis.20, 21 The documented approaches of extending photocatalytic response of TiO2 to visible and/or infrared range involve doping or co-doping of nonmetals and/or metals,1,

2, 22-24

Ti3+ self-doping,25-27 formation of

nanocomposites with narrow band gap semiconductors,28-31 interaction between organic pollutant molecules and TiO2,32 formation of upconversion photocatalysts,33, 34 and so on. Among the approaches reported, in view of

photocatalysis principle, the heterojunction formation of TiO2 and semicondictor with narrow

band gap is promising because it is not only benificial to improve separation efficiency of photogenerated holes (h) and elctrons (e) via the charge transfer at the interface, but also is benificial to effectively utilize solar energy for photocatalysis by using wide optical absorption of narrow band gap semiconductor up to visible or even to near infrared region. The feasibility of the approach depends on band gap match of two semiconductors and appropriate redox potentials of photogenerated holes and electrons of the two semiconductors for a known photocatalytic reaction. 2


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Co3O4 is an unique metal oxide as it is good thermocatalyst as well as a p-type semiconductor photocatalyst. As thermocatalysts, Co3O4 and its nanocomposites were reported to show good thermocatalytic activity for the abatement of air pollutants such as CO, methane, ethylene, volatile organic compounds (VOCs), and so on.35-42 As a photocatalyst, it has a narrow bandgap of 1.2-2.1 eV, which provides a pre-rquisite to effectively utilize solar energy from UV and visible to near infrared light. It is documented that Co3O4 show photocatalytic activity for organic pollutant photodegradation,43-58 water reduction and oxidation,59-67 and CO2 reduction 68, 69 with UV or visible irradiation. But, pure bulk Co3O4 is not good photocatalyst due to its low photocatalytic activity. To improve its photocatalytic activity, several approaches were developed. The approaches invovle: increasing surface area by decreasing Co3O4 nanoparticle size and preparing mesoporous Co3O4,46,

47

doping Co3O4 with F or La,62,

63

forming

nanocomposites with semiductor photoactalysts such as Ag3PO4,43 BiOCl,49 CuO,53 BiVO4,54 BiPO4,55 Bi2WO6,57 C3N4,58,

61

TiO2,50,

64-67

and so on. Among the works, several groups tried to combine the

advantages of both TiO2 and Co3O4 to improve the photocatalytic activity.50, 64-67 For example, Bala et al reported that Co3O4/TiO2 heterojunction formation greatly improved the photocatalytic activity for water reduction to produce H2 under UV irradiation, which was attributed to the improvement of e-h separation efficiecny due to the transfer of holes from TiO2 to Co3O4 at the heterojunction.66 But, the Co3O4/TiO2 heterojunction had no photocatalytic activity under λ > 420 nm visible irradiation as Co3O4 only acts as hole capture.66,

67

This means that the wide optical absorption function of Co3O4 as norrow band gap

semiconductor is not fully realized for visible photocatalytic reduction of H2O in the the Co3O4/TiO2 heterojunction. On the other hand, in spite of its strong absorption up to infrrared region due to its narrow band gap, photocatalysis on Co3O4 under infrared irradiation has not been documented. This is mainly ascribed lower oxidation potential of photogenrated holes in Co3O4 (2.44 eV vs NHE),43 insufficient for

to

oxidation

of hydroxyl group (•OH, H+/H2O = 2.73 V, pH=7)70 and refractory organic pollutants with large oxidation potential. This is evidenced by the reported works: Although Co3O4 and its nanocomposites show photocatalytic activity for the photodegration of easily degradable polutants such as dyes, isopropanol,43-58 for the photodegradation of refactory poisonous benzene as one of typical air pollutants, it has no photocatalytic activity due to the large oxidation potential of benzene.35 Therefore, it is urgently needed and 3



great challenging to develope novel stratedgy to perfectly utilize its good thermocatalytic activity and its wide optical absorption as a narrow band gap photocatalyst for solving the problems. Herein, we perfectly combine the advantages of both TiO2 and Co3O4 by preparing Co3O4/TiO2 nanocomposites with different Co/Ti molar ratios. The optimum Co3O4/TiO2 nanocomposite with Co/Ti molar ratio of 0.30 demonstrates significantly enhanced catalytic activity as well as excellent catalytic durability for benzene abatement under UV-Vis-IR irradiation as compared to pure TiO2. Even under λ > 830 nm IR irradiation, the Co3O4/TiO2 nanocomposite still demonstrates effective catalytic activity. The very high catalytic activity of the Co3O4/TiO2 nanocomposite derives from light-driven thermocatalytic benzene oxidation. A synergetic effect among light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite is found for first time to greatly enhance the catalytic activity and improve catalytic durability by inhibiting refractory carbonaceous intermediate deposited on TiO2. A novel photoactivation, completely different from UV photocatalysis on TiO2, is discovered to further considerably improve the light-driven thermocatalytic activity of Co3O4. The effective light-driven themocatalysis together with the novel photocatalysis- thermocatalysis synergetic effect and photoactivation cause a tremendous enhancement of 489 times in benzene mineralization rate (initial production rate of CO2) as compared to that of photocatalytic benzene oxidation of the Co3O4/TiO2 nanocomposite under UV-Vis-IR irradiation with the same light intensity at near room temperature. Based on the experimental evidences, the origin of the novel photoactivation and synergetic effect is revealed. To the best of our knowledge, there are no literatures about the Co3O4/TiO2 nanocomposite with effective catalytic activity with IR irradiation and the photocatalysis-photothermocatalysis synergetic effect for gas-phase abatement of organic air pollutants.

Experimental section. Preparation. The procedure of preparing Co3O4/TiO2 nanocomposite is as follows. 1.0 g of TiO2 (P25, 80% anatase, 20% rutile) was put into distilled water (100 mL) in a beaker and well dispersed by 40 min ultrasonication. The obtained suspension was mixed with 0.3122g of cobalt acetate tetrahydrate (Co(Ac)2•4H2O) and 0.1881g of urea, and ultrasonicated for 5 min. In reactants, the molar ratios of urea/Co(Ac)2 and Co/Ti were 2.5 and 0.10, respectively. The beaker was heated at 90 oC for 48 h in an 4


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electrical oven. The formed precipitate was filtered, washed, and dried at 60 oC for 12 h. Finally, the dried sample was calcined at 260 oC for 2 h in a muffle furnace. The obtained sample is denoted as Co3O4/TiO2-A. The procedure of preparing the Co3O4/TiO2 nanocomposites with Co/Ti molar ratios of 0.20, 0.30, and 0.40 was the same as that of Co3O4/TiO2-A except for adding different amounts of urea and Co(AC)2·4H2O. The amount of Co(Ac)2·4H2O is 0.6239, 0.9363, and 1.2484 g, respectively. The amount of urea is 0.3765, 0.5644, and 0.7522g, respectively. For preparing all the nanocomposites, the urea/Co(Ac)2 molar ratio is 2.5. The obtained samples are denoted as Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D, respectively. The procedure of preparing pure Co3O4 was the same as that of Co3O4/TiO2-A except for no addition of TiO2. Characterization. A Rigaku D/max diffractometer with Cu Ka radiation was used to record the XRD patterns of the Co3O4/TiO2 samples. A JEM-100CX transmission electron microscope was used to observe morphology of the Co3O4/TiO2 samples. The specific surface area of the Co3O4/TiO2 samples was measured on an ASAP2020 physisorption instrument by N2 absorption at -196 oC. A VG Multilab 2000X-ray photoelectron spectrometer was used to analyze element valence states of the Co3O4/TiO2 samples. A Lambda 750 spectrophotometer was used to record diffusive reflectance UV–Vis-IR absorption spectra of the Co3O4/TiO2 samples. Thermogravimetric analysis was conducted on a STA449F3 thermal analyzer. FTIR spectra were recorded on a Nicolet-6700 infrared spectrometer. CO temperature programmed reduction (CO-TPR) and O2 temperature programmed oxidation (O2-TPO) of the Co3O4/TiO2 samples under irradiation and in the dark were conducted on a TP5080 multifunctional adsorption apparatus monitored by a thermal conduction detector. The light source used is a 500 W Xe system. The procedure was described in details in the previous publications.71, 72 Photothermocatalytic activity. A cylindrical reaction reactor with a quartz window was used to measure photothermocatalytic activity and photocatalytic activity at near room temperature of the samples for gas-phase benzene oxidation. A GC9560 gas chromatograph was used to analyze the reactant and products. The sample amount is 0.1000 g, and 2.0 uL of benzene was injected into the reactor in the catalytic measurements. The procedure and GC analysis were described in details in the previous 5



publications.73 The light irradiation intensities of UV-Vis-IR, λ > 420 and 560 nm Vis-IR, and > 830 nm IR light are 490.5, 436.1, 463.0, 346.8 mw cm-2, respectively. Thermocatalytic activity. A WFS-2015 gas-phase reaction apparatus was used to test thermocatalytic activities of the samples for benzene oxidation in the dark and under the irradiation at the different temperatures. The sample amount was 0.0500 g. A stream of 2.0 g m-3 benzene balanced by 20.8 vol% O2/N2 was fed at 40 mL min−1 flow rate in the reactor. The detailed procedure was described in reported publication.72

Result and discussion Characterization. The Co3O4/TiO2 nanocomposites were synthesized by hydrolysis of cobalt acetate with urea in the existence of TiO2(P25), and then calcined at 260 oC. The Co/Ti molar ratios in the obtained nanocomposites are 0.10, 0.20, 0.30, and 0.40, respectively. They are labeled by Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D, respectively.

XRD result shows that TiO2 in

Co3O4/TiO2 nanocomposites is a mixture of anatase and rutile (Figure 1). Cobalt oxide in all the Co3O4/TiO2 nanocomposites exists in the form of Co3O4 with a spinel structure (JCPDS 09-0418). With elevating the Co/Ti molar ratios, the XRD peaks of Co3O4 in the Co3O4/TiO2 nanocomposites are intensified. The average crystal size of Co3O4 in Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D, calculated in accordance to the Scherrer formula at 31.25o (corresponding to {220} facets), are 13.9, 13.9, 14.1, 14.3 nm, respectively. The average crystal sizes of anatase and rutile in the Co3O4/TiO2 nanocomposites, calculated in accordance to the Scherrer formula at 28.28o (corresponding to {101} facets of anatase) and 27.45o (corresponding to {110} facets of rutile), are 22.1 and 38.3 nm, respectively. The morphology of the Co3O4/TiO2 nanocomposites is observed by TEM (Figure 2). For Co3O4/TiO2-A with the lowest Co/Ti molar ratio (0.10), larger TiO2 nanoparticles (23.5~72.5 nm) with clear surface are mainly observed (Figure 2). There are only a small amount of smaller Co3O4 nanoparticles (pointed by white circles) around TiO2 particles observed. A larger amount of smaller Co3O4 nanoparticles around TiO2 nanoparticles are observed for Co3O4/TiO2-B and Co3O4/TiO2-C with higher Co/Ti molar ratio (0.20 and 0.30, respectively). For Co3O4/TiO2-D with the largest Co/Ti molar ratio (0.40), there are a lot of smaller Co3O4 nanoparticles around TiO2 particles observed. The interface area around Co3O4 and TiO2 6


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nanoparticles for Co3O4/TiO2-C are observed by HRTEM.

Co3O4 nanoparticle with 0.286 nm lattice

spacing of {220} facets closely contacts to anatase nanoparticle with 0.352 nm lattice spacing of {101} facets. N2 adsorption was used to measure the surface area of the Co3O4/TiO2 nanocomposites. The BET surface area of Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D is 78.9, 104.6, 124.1, 118.0 m2 g-1, respectively.

Figure 1. XRD patterns of the Co3O4/TiO2 nanocomposites. *, ■, and●, represent XRD peaks of Co3O4, rutile, and anatase, respectively.

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Figure 2. TEM images of the Co3O4/TiO2 nanocomposites and HRTEM image of Co3O4/TiO2-C around the interface area of nano Co3O4 and TiO2. 8


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The Ti and Co valence states of the Co3O4/TiO2 nanocomposites were determined with XPS. Co in all the nanocomposites of Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D shows typical XPS spectra of Co3O4 (Figure 3A). A strong peak around 779.2 eV is attributed to Co2p3/2 of cobalt ions. The weak broad peak (800~807 eV) is attributed to the satellite peaks of Co2p3/2.35,74 With increasing the Co/Ti molar ratio, the peaks are gradually intensified. Interestingly, the formation of the Co3O4/TiO2 nanocomposites causes a decrease in the binding energy of Co2p3/2 to lower value (779.2 eV) as compared to that of Co2p3/2 in pure Co3O4 (779.6 eV). As shown in Figure 3B, Ti in Co3O4/TiO2-A (Co/Ti ratio, 0.10) shows a strong peak at 458.4 eV, which is the same as the value of Ti2p3/2 of Ti4+ in pure TiO2 (P25).75 With elevating the Co/Ti ratio to 0.20, 0.30, and 0.40, the binding energy of Ti2p3/2 in Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D shifts to higher value of 458.5, 458.6, and 458.7 eV, respectively. The decrease in the binding energy of Co2p3/2 and increase in the binding energy of Ti2p3/2 induced by the Co3O4/TiO2 nanocomposite formation indicate the presence of a strong interaction among Co3O4 and TiO2 nanoparticles. As shown by HRTEM (Figure 2), Co3O4 nanoparticle closely contacts to anatase nanoparticle in the Co3O4/TiO2 nanocomposites. The work function of Co3O4 (5.7 eV)76 higher than rutile (4.8 eV) and anatase (5.1 eV)77 causes a decrease in the binding energy of Co2p3/2 and an increase in the binding energy of Ti2p3/2.

9



Figure 3. XPS spectra of the Co3O4/TiO2 nanocomposites and Co3O4.

Photothermocatalytic activity. Benzene, utilized in large quantity as an organic solvent and feedstock in diverse fine chemical synthesis, is a typical refractory poisonous air pollutant. The photothermocatalytic activities of the pure TiO2(P25) and Co3O4/TiO2 samples for gas-phase benzene abatement with UV-Vis-IR irradiation were evaluated. Among the samples, the pure TiO2(P25) sample demonstrates the lowest catalytic activity. After 40 min UV-Vis-IR irradiation, only 38.6% of benzene is oxidized (Figure 4A) and its CO2 concentration is only 1761.1 mg m-3 (Figure 4B). The Co3O4/TiO2 nanocomposite formation greatly improves the photothermocatalytic activity. For Co3O4/TiO2-A (Co/Ti ratio, 0.10), after 40 min UV-Vis-IR irradiation, 49.3% of benzene is oxidized and its CO2 concentration rises to 2546.8 mg m-3. With elevating the Co/Ti molar ratio in the Co3O4/TiO2 nanocomposites, the photothermocatalytic activity considerably enhances. For Co3O4/TiO2-B (Co/Ti ratio, 0.20), after 40 min 10


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UV-Vis-IR irradiation, benzene conversion increases to 70.1% and its CO2 concentration rises to 6318.0 mg m-3. Co3O4/TiO2-C (Co/Ti ratio, 0.30) demonstrates the optimum photothermocatalytic activity. After 40 min UV-Vis-IR irradiation, its benzene conversion increases to 95.0% and CO2 concentration is 9778.7 mg m-3. A further increase of the Co/Ti molar ratio to 0.40 leads to a minor reduction in the catalytic activity. We calculated the carbon mass balance of benzene oxidation on the Co3O4/TiO2-C sample with the optimum catalytic activity according to the formula: Cbalance=MCO2/(6×MC6H6) Where MCO2 and MC6H6 are the mole numbers of the produced CO2 and the reacted benzene. Cbalance is equal to 1.0, indicating that no intermediates of carbon species strongly adsorbed on the used Co3O4/TiO2-C sample after reacted for 40 min are detected. To confirm the conclusion, the used Co3O4/TiO2-C sample was characterized by FTIR (Figure S1, Supporting information) and TG-MS (Figure S2). FTIR shows that the used Co3O4/TiO2-C sample has spectra similar to that of the fresh Co3O4/TiO2-C sample besides the attenuation of the broad peak of O-H stretching of water or hydroxyl groups adsorbed on the sample around 3380 cm-1, which is attributed to the desorption of water and/or decomposition of hydroxyl groups under the UV-Vis-IR irradiation. No FTIR peaks of C-O stretching in the region of 1000~1300 cm-1 and the peaks of C-H stretching in the region of 2800~3100 cm-1 are observed. The result further confirms that no intermediates of carbon species are strongly adsorbed on the used Co3O4/TiO2-C sample. TG profile of the used Co3O4/TiO2-C sample shows that there is a weight increase, indicating no combustion of the strongly adsorbed intermediates of carbon species that would cause a weight loss. The weight increase of the used Co3O4/TiO2-C sample is attributed to the oxidation of the reduced Co3O4-x due to the participation of lattice oxygen of Co3O4 in the benzene oxidation (see Scheme 1, discussed later) and/or the decomposition of hydroxyl groups (Figure S1). We further compared the initial production rate of CO2 (rCO2, in initial 5 min) of the pure TiO2(P25) and Co3O4/TiO2 samples. As shown in Figure 4C, the pure TiO2(P25) sample has a very low rCO2 of 4.2 μmol g-1 min-1). The Co3O4/TiO2 nanocomposite formation greatly promotes the rCO2 values. The rCO2 values of Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D increase to 8.3, 23.7, 51.3, and 45.8 μmol g-1 min-1, respectively. Compared to that of TiO2 (P25), the rCO2 values of Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D are enhanced by 2.0, 5.6, 12.1, 10.8 times, respectively. 11



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Catalytic stability is very important to the practical application of a catalyst. It is reported that TiO2 and it nanocomposites usually experience a quick deactivation for the photocatalytic oxidation of aromatic compounds (e.g. benzene) because of refractory carbonaceous intermediates deposited on the photocatalyst surface.17-21 The quick deactivation greatly hampers wide practical application of TiO2 and it nanocomposites in VOCs purification. Therefore, we evaluated the photothermocatalytic stability of the optimum Co3O4/TiO2-C sample for benzene oxidation with UV-Vis-IR irradiation. In this case, a stream of about 810 mg m-3 benzene balanced by 20 vol% O2/N2 was constantly fed in the reactor at

20 mL min-1

flow rate. After 33 h UV-Vis-IR irradiation, the photothermocatalytic activity of the Co3O4/TiO2-C sample remains unchanged (Figure 4D). The result indicates that the Co3O4/TiO2-C sample demonstrates excellent photothermocatalytic durability.

Figure 4. The time course of benzene (A) and CO2 (B) concentration, initial production rate of CO2 (C): (a) TiO2(P25), (b) Co3O4/TiO2-A, (c) Co3O4/TiO2-B, (d)Co3O4/TiO2-C, and (e) Co3O4/TiO2-D with UV-Vis-IR irradiation. The time course of benzene conversion for Co3O4/TiO2-C with UV-Vis-IR irradiation (D). 12


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The photothermocatalytic activities of the optimum Co3O4/TiO2-C sample for benzene oxidation with under irradiation of Vis-IR and IR light were evaluated by using corresponding long wave pass filters. With λ > 420 nm Vis-IR irradiation, Co3O4/TiO2-C demonstrates efficient photothermocatalytic activity. After 40 min irradiation, its CO2 concentration is 6195.9 mg m-3 (Figure 5A). Its rCO2 value is 27.4 μmol g-1 min-1 (Figure 5B). With λ > 560 nm Vis-IR irradiation, the CO2 concentration is 3226.1 mg m-3 after 40 min irradiation. Its rCO2 value is 15.2 μmol g-1 min-1. Co3O4/TiO2-C still demonstrates photothermocatalytic activity even with λ > 830 nm IR irradiation. In this case, its CO2 value is 4.4 μmol g-1 min-1.

Figure 5. The time course of CO2 concentration (A) and initial production rate of CO2 (B) of Co3O4/TiO2-C for benzene abatement with Vis-IR and IR irradiation.

Mechanism: photocatalysis and light-driven thermocatalysis. Why do the Co3O4/TiO2 nanocomposites demonstrate effective catalytic activity with UV-Vis-IR irradiation? To answer the question, the optical absorption spectra of TiO2(P25) and Co3O4/TiO2 nanocomposites were recorded. 13



Figure 6A shows the optical absorption spectra of the samples, of which the corresponding Kubelka-Munk functions are shown in Figure S3. As shown in Figure 6A and Figure S3, the pure TiO2(P25) sample demonstrates strong absorption below 410 nm as it is composed of rutile (band gap 3.0 eV, 411 nm) and anatase (band gap 3.2 eV, 386 nm). The Co3O4/TiO2 nanocomposite formation causes the strong absorption in the Vis-IR region above 410 nm. Due to the presence of nano Co3O4 with spinel structure in the Co3O4/TiO2 nanocomposites, the 420~820 nm band is ascribed to the absorption of low-spin Co3+ in octahedral sites and the metal-metal charge transition of Co3+-Co2+ (MMCT). The 1040~1780 nm band is ascribed to the absorption of Co2+ in tetrahedral sites and MMCT.35, 78 With increasing the amount of Co3O4 in the Co3O4/TiO2 nanocomposites, the absorption of Co3O4/TiO2-B in the Vis-IR region above 420 nm increases as compared to that of Co3O4/TiO2-A. Further increasing the amount of Co3O4 in the Co3O4/TiO2 nanocomposites does not cause an obvious increase in Vis-IR absorption of Co3O4/TiO2-C and Co3O4/TiO2-D. As TiO2 has photocatalytic activity for organic compound photodegradation with UV irradiation at room temperature, to reveal whether the wide optical absorption of the Co3O4/TiO2 nanocomposites causes photocatalytic activity, we tested photocatalytic benzene abatement on the optimum Co3O4/TiO2-C sample at near room temperature with UV-Vis-IR irradiation, of which light intensity is the same as that for measuring photothermocatalytic activity (Figure 4A). As shown in Figure 6B, Co3O4/TiO2-C demonstrates photocatalytic activity for benzene oxidation. After 40 min UV-Vis-IR irradiation, although its benzene conversion is about 16.0%, the CO2 concentration is only 38.9 mg m-3. The photocatalytic activity of Co3O4/TiO2-C is much lower than its photothermocatalytic activity (Figure 4). This result suggests that the very high photothermocatalytic activity of Co3O4/TiO2 mainly derives from light-driven thermocatalytic benzene oxidation on Co3O4 (Scheme 1): The strong optical absorption of Co3O4/TiO2 causes surface temperature increase because of photothermal conversion. Once the temperature is elevated above the light-off temperature (Tlight-off) for thermocatalytic benzene oxidation on Co3O4/TiO2, the thermocatalytic benzene oxidation occurs. To corroborate the mechanism, the surface temperatures of the pure TiO2(P25) and Co3O4/TiO2 samples under the irradiation and reaction conditions identical to those in the photothermocatalytic activity measurement (Figure 4 and 5) were measured. Upon the irradiation, the surface temperatures of the pure 14


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TiO2(P25) and Co3O4/TiO2 samples are rapidly elevated to equilibrium temperatures (Teq) within several minutes. With the UV-Vis-IR irradiation, the Teq value of TiO2(P25) is 150 oC due to its strong UV absorption (Figure 6A) and the IR heating effect of the Xe lamp. The Co3O4/TiO2 nanocomposite formation considerably increases their Teq values owing to its strong UV-Vis-IR absorption. With the UV-Vis-IR irradiation, the Teq values of Co3O4/TiO2-A, Co3O4/TiO2-B, Co3O4/TiO2-C, and Co3O4/TiO2-D are 227, 223, 229, and 227 oC, respectively (Figure 6C). We also measured the Teq values of the optimum Co3O4/TiO2-C sample with Vis-IR and IR irradiation. With λ > 420 and 560 nm Vis-IR irradiation, the Teq values of Co3O4/TiO2-C are 227 and 218, respectively. With λ > 830 nm IR irradiation, the Teq value of Co3O4/TiO2-C is 184 oC. To corroborate if the Teq values of the Co3O4/TiO2 samples under the irradiation exceed their Tlight-off values, the activity of TiO2(P25) and the Co3O4/TiO2 samples for thermocatalytic benzene abatement at different temperatures in the dark was measured. TiO2(P25) has very low thermocatalytic activity (Figure 6D). Only with the elevation of temperature to 340 oC does the thermocatalytic benzene oxidation proceed. The Co3O4/TiO2 nanocomposite formation significantly accelerates the thermocatalytic activity as nano Co3O4 has good thermocatalytic activity. For Co3O4/TiO2-A (Co/Ti ratio, 0.10), when the temperature is elevated to 180 oC, benzene oxidation occurs (Tlight-off, 180 oC). When the temperature is elevated to 252 oC, its benzene conversion is 50% (T50 , 252 oC). When the temperature is elevated to 280 oC, its benzene conversion is 88.8% (T90 > 280 oC).

Compared to Co3O4/TiO2-A, the thermocatalytic activity of

Co3O4/TiO2-B (Co/Ti ratio, 0.20) is considerably enhanced. Its Tlight-off, T50, and T90 values are reduced to 160, 232, and 255 oC, respectively. A further increase in the Co/Ti molar ratio to 0.30 and 0.40 only causes a minor enhancement in the activity. The Tlight-off, T50, and T90 values of Co3O4/TiO2-C are 160, 228, and 255 oC, respectively. The Tlight-off, T50, and T90 values of Co3O4/TiO2-D are 160, 224, and 258 oC, respectively. The catalytic activity of a commercial supported precious metal catalyst (0.5% Pt/Al2O3) for the thermocatalytic abatement of benzene was also evaluated under the same reaction conditions because supported precious metal catalysts are believed to be the most efficient catalysts for thermocatalytic VOCs abatement. The Tlight-off, T50, and T90 values of 0.5% Pt/Al2O3 are 140, 204 °C, and 260 °C, respectively. This shows that the Co3O4/TiO2-C and Co3O4/TiO2-D nanocomposites have very good thermocatalytic 15



activity, comparable to the high-priced 0.5% Pt/Al2O3 catalyst. With the UV-Vis-IR irradiation, all the Teq values of the Co3O4/TiO2 nanocomposites exceed their corresponding Tlight-off values, thus, light-driven thermocatalytic benzene oxidation proceeds. With the Vis-IR and IR irradiation, the Teq values of Co3O4/TiO2-C exceed its Tlight-off value, thus, the Vis-IR and IR light-driven thermocatalytic benzene oxidation proceeds. The higher photothermocatalytic activity of Co3O4/TiO2-C than Co3O4/TiO2-A and Co3O4/TiO2-B (Figure 4) is ascribed to the fact that Co3O4/TiO2-C has higher thermocatalytic activity than Co3O4/TiO2-A and Co3O4/TiO2-B (Figure 6D) according to the light-driven thermocatalytic mechanism.

Figure 6. Absorption spectra of TiO2 (P25) and the Co3O4/TiO2 nanocomposites (A). Time course of benzene and CO2 concentration for photocatalytic benzene oxidation on Co3O4/TiO2-C with UV-Vis-IR irradiation at near room temperature (B). Teq values of TiO2(P25) and the Co3O4/TiO2 nanocomposites with UV-Vis-IR irradiation (C). Temperature dependent benzene conversion of TiO2(P25) and Co3O4/TiO2 nanocomposites for thermocatalytic benzene oxidation in the dark (D): (a) TiO2(P25), (b) Co3O4/TiO2-A, 16


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(c) Co3O4/TiO2-B, (d) Co3O4/TiO2-C, and (e) Co3O4/TiO2-D.

Scheme 1. Schematically illustrated photocatalysis on TiO2, light-driven thermocatalysis and photoactivation on Co3O4, and synergetic effect among light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2.

Synergetic effect. The Co3O4/TiO2 nanocomposites are composed of nanoTiO2 and nano Co3O4 (Figure 1 and 2).

Photothermocatalytic result shows that nano Co3O4 in the Co3O4/TiO2-C nanocomposite

has efficient light-driven thermocatalytic activity (Figure 4), while photocatalytic result shows that nano TiO2 in the Co3O4/TiO2-C nanocomposite has photocatalytic activity (Figure 6B). Do the light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in Co3O4/TiO2 nanocomposites proceed independently or is there a synergetic effect among them?

As can be seen from Figure 7A and 7B, in the

case of photocatalytic benzene oxidation on Co3O4/TiO2-C at near room temperature with UV-Vis-IR irradiation (of which light intensity is the same as that in the photothermocatalytic measurement), there is only photocatalysis on nano TiO2 in Co3O4/TiO2-C as nano Co3O4 doesn’t show photocatalytic activity for the abatement of refractory benzene.35 The values of rCO2 and initial benzene reaction rate (rC6H6, in the initial 10 min) of Co3O4/TiO2-C are very low (only 0.105 and 1.36 μmol g-1 min-1). The rCO2/rC6H6 ratio (0.08) of Co3O4/TiO2-C much less than the stoichiometric ratio of rCO2/rC6H6=6 according to the complete 17



oxidation of benzene (C6H6+7.5O2=6CO2+3H2O) indicates that most of the reacted benzene is converted to refractory carbonaceous intermediates. This observation is in agreement to the reported observation of refractory carbonaceous intermediates easily deposited on TiO2 during the photodegradation of aromatic compounds.17-21 This is confirmed by FTIR. After 50 min UV-Vis-IR irradiation at near room temperature, the used Co3O4/TiO2-C sample demonstrates strong absorption peaks in the region of 1000~1150 cm-1 and strong peaks in the region of 2850~3050 cm-1 (Figure 7C). They are attributed to C-O stretching and C-H stretching of deposited carbonaceous intermediates, respectively.79 In the case of photothermocatalytic benzene oxidation under the λ > 420 nm Vis-IR irradiation by using a λ > 420 nm long wave pass filter, only light-driven thermocatalytic benzene oxidation on nano Co3O4 in Co3O4/TiO2-C proceeds as the photocatalytic oxidation on nano TiO2 in Co3O4/TiO2-C cannot proceed due to the large band gap of TiO2(P25) (3.2 eV for anatase, 3.0 eV for rutile). The values of rCO2 and rC6H6 of Co3O4/TiO2-C are 27.4 and 4.57 μmol g-1 min-1, respectively. In the case of photothermocatalytic benzene oxidation under the UV-Vis-IR irradiation, both light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite simultaneously proceed. Interestingly, in this case, Co3O4/TiO2-C demonstrates much higher values of rCO2 and rC6H6 (51.3 and 8.56 μmol g-1 min-1, respectively). As discussed above, the light-driven thermocatalytic activity of Co3O4/TiO2 depends on the Teq value under the irradiation. The Teq value of Co3O4/TiO2-C under UV-Vis-IR irradiation (229 oC) is slightly higher than that under the λ > 420 nm Vis-IR irradiation (227 oC).

The slight higher Teq value under the UV-Vis-IR irradiation should only cause a slight increase of

~10% in the light-driven thermocatalytic activity as shown in Figure 6D. However, as compared to the photocatalytic rate at near room temperature with the UV-Vis-IR irradiation and the photothermocatalytic rate with the λ > 420 nm Vis-IR irradiation, the photothermocatalytic rCO2 value of Co3O4/TiO2-C with the UV-Vis-IR irradiation are significantly increased by 489, and 1.87 and times, respectively, while the photothermocatalytic rC6H6 value with the UV-Vis-IR irradiation significantly increase by 6.3 and 1.87 times, respectively. As compared to the corresponding summations of the photocatalytic rate with the UV-Vis-IR irradiation and the photothermocatalytic rate with the λ > 420 nm Vis-IR irradiation (rCO2, photocatalysis

+rCO2, photothermocatalysis and rC6H6,photocatalysis +rC6H6, photothermocatalysis), the values of photothermocatalytic

rCO2 and rC6H6 with the UV-Vis-IR irradiation considerably increase by 1.86 and 1.44 times, respectively. 18


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The result clearly indicates the existence of a synergetic effect among light-driven thermocatalysis on nano Co3O4 and UV photocatalysis on nano TiO2 that considerably promotes the light-driven thermocatalytic activity of Co3O4/TiO2-C. On the other hand, in the case of photothermocatalysis with UV-Vis-IR irradiation, the stoichiometric ratio of rCO2/rC6H6=6 of Co3O4/TiO2-C indicates that benzene is completely mineralized to CO2. This is confirmed by FTIR. After 50 min UV-Vis-IR irradiation, the used Co3O4/TiO2-C sample only demonstrates strong broad peak around 3350 cm-1 that is attributed to O-H stretching of adsorbed H2O produced by benzene oxidation. No strong C-O stretching peak in the region of 1000~1150 cm-1 and strong C-H stretching peak in the region of 2800~3050 cm-1 79 are observed (Figure 7C). This suggests almost no carbonaceous intermediate deposited on the used Co3O4/TiO2-C in the process of the UV-Vis-IR photothermocatalytic oxidation. This is in striking contrast to the severe deposition of carbonaceous intermediates on the used Co3O4/TiO2-C during the photocatalytic oxidation as discussed above. The result indicates that the synergetic effect not only considerably increases light-driven thermocatalytic activity, but also inhibits carbonaceous intermediate deposition on TiO2. In principle, the synergetic effect between light-driven thermocatalyis on Co3O4 and UV photocatalysis on TiO2 in Co3O4/TiO2-C should be realized by the effective migration of active species through the interface between nano TiO2 and Co3O4. To firmly support this, we measured the photothermocatalytic activity of a mechanic mixture of nano Co3O4 and TiO2(P25) (Co/Ti molar ratio, 0.30, the same as that of Co3O4/TiO2-C) for benzene abatement with UV-Vis-IR irradiation, of which the light intensity is the same as that in the photothermocatalytic activity measurement of Co3O4/TiO2-C. In this case, light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2(P25) in the mechanic mixture proceed independently due to the absence of closely contacted Co3O4/TiO2 interface for the effective interface migration of active species produced by UV photocatalysis on TiO2. The values of rCO2 and rC6H6 of the mechanic mixture are 7.95 and 2.55 μmol g-1 min-1, respectively (Figure 7A and 7B). Compared to those of Co3O4/TiO2-C, the rCO2 and rC6H6 values of the mechanic mixture are reduced by 6.5 and 4.4 times, respectively. This result indicates that the effective migration of photogenerated active species via closely contacted Co3O4/TiO2 interface in the Co3O4/TiO2-C nanocomposite plays an important role in the photocatalysis-thermocatalysis synergetic effect. 19



. Figure 7. The initial CO2 production rate (A) and benzene reaction rate (B) in the different cases: photothermocatalytic oxidation benzene on Co3O4/TiO2-C with UV-Vis-IR (a) and λ > 420 nm Vis-IR (b) irradiation. Photocatalytic oxidation benzene on Co3O4/TiO2-C with UV-Vis-IR irradiation at near room temperature (c). Photothermocatalytic oxidation benzene on a mixture of nano Co3O4 and TiO2(P25) (Co/Ti molar ratio, 0.30) with UV-Vis-IR irradiation (d). FTIR spectra (C) of fresh Co3O4/TiO2-C (Curve 1), the 20


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used Co3O4/TiO2-C sample after the photocatalytic test at near room temperature (Curve 2), and the used Co3O4/TiO2-C sample after the photothermocatalytic test (Curve 3). Based on the above evidences, a mechanism of the synergetic effect among light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite is proposed (Scheme 1). Upon UV absorption, electrons are promoted from the valence band of TiO2 to its conduction band. The adsorbed O2 molecules on TiO2 are reduced by the photogenerated electrons (e) to form active O2- species because of the oxidation potential of e (-0.18 V vs NHE, pH = 1) lower than that of O2 (O2/O2-,−0.16 V vs NHE).80, 81 Hydroxyl groups and benzene adsorbed on TiO2 are oxidized by photogenerated holes (h) to form OH• radicals and C6H6+ active species because of the lower oxidation potentials of hydroxyl groups (•OH, H+/H2O=2.73 V, pH=7)70 and benzene (C6H6+/C6H6, 2.995 V vs NHE) than that of h (3.02 eV vs NHE).73 The photogenerated active species migrates through the Co3O4/TiO2 interface to nano Co3O4. On the other hand, as discussed in Section 3.4, light-driven thermocatalytic benzene oxidation on nano Co3O4 in Co3O4/TiO2 nanocomposite proceeds upon UV-Vis-IR irradiation. Thermocatalytic oxidation on Co3O4 complies with Mars-van Krevelen redox mechanism:35. 39 C6H6 is oxidized with the surface lattice oxygens of Co3O4, and O2 subsequently re-oxidizes the reduced cobalt oxide (Co3O4-x). As the photogenerated C6H6+ species are more active than C6H6 molecules in accordance to the molecule orbit theory,82 the reduction of Co3O4 by benzene is accelerated. As O2- species are more active than O2 molecules in accordance to the molecule orbit theory82 and OH• radicals are strong oxidant due to its high oxidation potential, the Co3O4-x re-oxidation is accelerated. Thus, the light-driven thermocatalytic benzene oxidation is significantly accelerated.

Photoactivation. Does the very high photothermocatalytic activity of Co3O4/TiO2-C with UV-Vis-IR irradiation (Figure 4) just arise from light-driven thermocatalysis on nano Co3O4 and its synergetic effect with UV photocatalysis on TiO2? As shown in Figure 6D, the catalytic activity of Co3O4/TiO2 nanocomposite for light-driven thermocatalytic benzene abatement depends on the reaction temperature. To address the question, we investigated benzene abatement on Co3O4/TiO2-C with the irradiation and in the dark at the same temperature and otherwise identical conditions. To eliminate the synergetic effect by UV photocatalysis on TiO2, we investigated benzene abatement on Co3O4/TiO2-C with λ > 420 nm Vis-IR 21



irradiation by using a λ > 420 nm long wave pass filter because TiO2 cannot be photoactivated with the Vis-IR irradiation. As can be seen from Figure 8A, compared to the catalytic activity in the dark, the λ > 420 nm Vis-IR irradiation causes a significant increase in the catalytic activity of Co3O4/TiO2-C at the same temperature higher than 160 oC. Its rCO2 values at 180, 200, 220, and 240 oC with λ > 420 nm Vis-IR irradiation are 18.2, 48.3, 93.1, and 195.7 μmol g-1 min-1, respectively. They are 4.3, 3.9, 2.3, and 2.3 times higher than the corresponding rCO2 values in the dark at 180, 200, 220, and 240 oC, respectively. Due to the absence of UV photocatalysis on TiO2 in this case, the great catalytic activity increase induced by >420 nm Vis-IR irradiation affirms the existence of a photoactivation on nano Co3O4 in Co3O4/TiO2-C. We further investigated benzene abatement on Co3O4/TiO2-C at the same temperature with UV-Vis-IR irradiation and the conditions identical to those with λ > 420 nm Vis-IR irradiation. In this case, photocatalysis on TiO2 with UV irradiation and thermocatalysis on Co3O4 in Co3O4/TiO2-C proceed simultaneously. With UV-Vis-IR irradiation, the rCO2 values of Co3O4/TiO2-C at 180, 200, 220, and 240 oC are 29.1, 72.0, 126.7, and 226.9 μmol g-1 min-1, respectively (Figure 8A). They are 6.9, 5.8, 3.2, and 2.7 times higher than the corresponding values of Co3O4/TiO2-C in the dark. Compared to the rCO2 values with λ > 420 nm Vis-IR irradiation at the same temperature above 160 oC, UV-Vis-IR irradiation results in a considerable enhancement in the rCO2 values. This result further affirms the presence of the synergetic effect among light-driven thermocatalysis on nano Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite because the photocatalytic rCO2 value of Co3O4/TiO2-C with UV-Vis-IR irradiation is very low (0.105 μmol g-1 min-1) (Figure 7A). As discussed above, thermocatalytic oxidation on Co3O4 complies with Mars-van Krevelen redox mechanism. The catalytic activity of Co3O4 for the thermocatalytic oxidation is mainly decided by the reducibility or lattice oxygen activity of Co3O4.35 To put an insight into the photoactivation on Co3O4 in Co3O4/TiO2, we used CO-TPR to investigate whether the reducibility of Co3O4 in Co3O4/TiO2-C is affected by λ > 420 nm Vis-IR irradiation (Figure 8B). In the dark, Co3O4/TiO2-C shows two weak CO consumption peaks around 103 and 239 oC, and two strong CO consumption peaks at 388 and 460 oC. The weak peak around 103 oC is ascribed to CO oxidation by oxygen species adsorbed on Co3O4. The weak peak around 239 oC is ascribed to CO oxidation by active surface lattice oxygen of Co3O4. The strong peak at 388 oC is ascribed to Co3O4 reduction to CoO, while the strong shoulder peak at 460 oC is ascribed to CoO reduction 22


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to metallic Co.35, 40 Compared to the corresponding peaks of Co3O4/TiO2-C in the dark, there are obvious shifts to lower temperatures for the peaks of CO oxidation by active surface lattice oxygen of Co3O4 and Co3O4 reduction to CoO upon λ > 420 nm Vis-IR irradiation. This indicates that the λ > 420 nm Vis-IR irradiation promotes the reducibility of Co3O4 in Co3O4/TiO2. The result affirm the existence of a photoactivation that promotes the lattice oxygen activity of Co3O4 in Co3O4/TiO2-C upon λ > 420 nm Vis-IR irradiation, thus accelerating the catalytic activity as shown in Figure 8A. We also recorded the CO-TPR profile of Co3O4/TiO2 under UV-Vis-IR irradiation. As can be seen from Figure 8B, compared to the profile under λ > 420 nm Vis-IR irradiation, there are obvious shifts to lower temperatures for the peaks of CO oxidation by active surface lattice oxygen of Co3O4 and Co3O4 reduction to CoO upon λ > 420 nm Vis-IR irradiation. This indicates that UV irradiation promotes the reduction of Co3O4 in Co3O4/TiO2. The reason is as follows: Upon UV irradiation, CO molecules adsorbed on TiO2 are oxidized by the photogenerated holes (h) in TiO2 to produce CO+ species because of the oxidation potentials of CO (0.64 V vs RHE) lower than that of h (3.02 eV vs NHE).73 The CO+ species are more energetic than CO molecules in accordance to the molecular theory.82 They migrate through the Co3O4/TiO2 interface to nano Co3O4, thus promoting the reduction of Co3O4. We also used O2-TPO to investigate whether the oxidation of the Co3O/TiO2-C sample pre-reduced (Co3O4-x/TiO2) is affected by λ > 420 nm Vis-IR irradiation. For the Co3O4-x/TiO2-C sample pre-reduced at 270 oC for 30 min by 5%CO/He in the dark, there is a strong O2 consumption peak at 136 oC that is attributed to the Co3O4-x re-oxidation (Figure 8C). The negative peak below 100 oC is attributed to the desorption of O2 adsorbed on Co3O4-x/TiO2-C.

Upon λ > 420 nm Vis-IR irradiation, the strong O2

consumption peak slightly shifts to a lower temperature, indicating that λ > 420 nm Vis-IR irradiation also accelerates the Co3O4-x re-oxidation. We also recorded the O2-TPO profile of the pre-reduced Co3O4-x/TiO2-C sample with UV-Vis-IR irradiation. As can be seen from Figure 8C, upon UV-Vis-IR irradiation, the strong O2 consumption peak further slightly shift to a lower temperature as compared to that with λ > 420 nm Vis-IR irradiation, indicating that the UV irradiation further promotes the Co3O4-x re-oxidation. This is ascribed to the fact that upon UV irradiation, the photogenerated electrons in TiO2 transfer to the adsorbed O2 molecules on TiO2 to produce O2- active species. The O2- active species migrate through the Co3O4-x/TiO2 interface to Co3O4-x and 23



promote the re-oxidation of Co3O4-x. Therefore, the photoactivation derives from the promotion of the lattice oxygen activity of Co3O4 in Co3O4/TiO2-C as well as the acceleration of the Co3O4-x re-oxidation upon the UV-Vis-IR irradiation.

24


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Figure 8. The catalytic activity of Co3O4/TiO2-C for benzene abatement (A), CO-TPR profiles of Co3O4/TiO2-C, and O2-TPO profiles of the pre-reduced Co3O4-x/TiO2-C sample (C) in the dark and with the irradiation of λ > 420 nm Vis-IR and UV-Vis-IR light .

Conclusion In summary, we synthesized the Co3O4/TiO2 nanocomposite with the Co/Ti molar ratios by a facile approach. With UV-Vis-IR irradiation, the optimum Co3O4/TiO2 nanocomposite (Co/Ti molar ratio, 0.3) demonstrates very high catalytic activity and excellent durability for benzene abatement.

It also

demonstrates effectively catalytic activity even with λ > 830 nm IR irradiation. The very high catalytic activity the optimum Co3O4/TiO2 nanocomposite arises from light-driven thermocatalytic benzene oxidation on nano Co3O4. A novel synergetic effect among light-driven thermocatalysis on Co3O4 and UV photocatalysis on TiO2 in the Co3O4/TiO2 nanocomposite is found to tremendously promote the catalytic activity and improve durability by inhibiting the deposition of refractory carbonaceous intermediates on TiO2 by photocatalysis. A novel photoactivation is discovered to further considerably accelerate light-driven thermocatalytic activity of Co3O4. The strategy is applicable to rationally design nanocomposites of semiconductor photocatalyst and thermocatalytically active metal oxides for environmental abatement with excellent catalytic performance by fully using renewable solar light.

Acknowledgements We acknowledged National Natural Science Foundation of China (21473127, 21673168) for financial support. Z. K. Shi and L. Lan have contributed equally to the paper.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website: FTIR, TG-DSC, and Kubelka-Munk functions.

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Co3O4/TiO2 nanocomposite exhibits high catalytic activity for benzene purification under UV-Vis-IR irradiation due to novel photoactivation and photocatalysis-thermocatalysis synergetic effect.

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TiO2 Nanocomposite Formation Leads to Improvement in

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