Strong Photothermal Effect of Plasmonic Pt Nanoparticles for Efficient

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Strong Photothermal Effect of Plasmonic Pt Nanoparticles for Efficient Degradation of Volatile Organic Compounds under Solar Light Irradiation Song-Cai Cai, Juan-Juan Li, En-Qi Yu, Xi Chen, Jing Chen, and Hongpeng Jia ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01578 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Nano Materials

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Strong Photothermal Effect of Plasmonic Pt Nanoparticles for Efficient

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Degradation of Volatile Organic Compounds under Solar Light Irradiation

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Song-Cai Cai,†,‡,§ Juan-Juan Li,†,‡ En-Qi Yu,†,‡,§ Xi Chen,†,‡,§ Jing Chen,‖, ⊥ and Hong-Peng

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Jia*,†,‡,§

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†

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Environment, Chinese Academy of Sciences, Xiamen 361021, China

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‡

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Academy of Sciences, Xiamen 361021, China

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§ University

CAS Center for Excellence in Regional Atmospheric Environment, Institute of Urban

Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese

of Chinese Academy of Sciences, Beijing 100049, China

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‖

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Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of

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Matter, Chinese Academy of Sciences, Fuzhou 350002, China

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⊥

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Xiamen 361021, China

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* Corresponding author

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E-mail address: [email protected].

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Tel: 86-592-6190767; fax: 86-592-6190767

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ABSTRACT: Recently, the photothermal effect of plasmonic nanometals has been increasingly

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studied in the biomedical field, but few in the environmental remediation, e.g., the elimination of

CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian

Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences,

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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volatile organic compounds (VOCs). In this article, the Pt/-Al2O3 has been synthesized with the

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tunable optical properties for efficient toluene decomposition under the full solar spectrum

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irradiation because of the plasmonic photothermal effect of the Pt nanoparticles (NPs), which

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simultaneously serve as the light absorber and the catalytically active site. Transmission electron

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microscope (TEM) images show that Pt NPs with average diameter around 1 nm are well

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dispersed on the -Al2O3. UV-vis absorption spectra of the Pt/-Al2O3 exhibit the strong

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absorbance in the wavelength range of 200-2500 nm due to the surface plasmon resonance (SPR)

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absorption of Pt NPs. As a result, the 1.81 Pt/-Al2O3 shows a highly efficient catalytic activity

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with toluene conversion of 87% and CO2 yield of 84% under solar irradiation intensity of 320

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mW/cm2, as well as a decent stability upon a continuous running for 30 h. This work highlights

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that Pt/-Al2O3 plasmonic catalyst shows great promise for VOCs elimination through plasmonic

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photothermal effect.

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KEYWORDS: plasmonic nanometals; photothermal effect; Pt nanoparticles; full solar

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spectrum; VOCs

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1. INTRODUCTION

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Over the past few decades, since the energy and environmental issue has become an increasingly

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prominent social problem, the photocatalysis has intensively attracted research interest in

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environmental remediation. As the key factor for the photocatalysis, substantial semiconductor

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photocatalysts have been extensively developed in water treatment and air purification.1-6 TiO2

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has become the commercialized semiconductor photocatalyst because of its excellent properties,


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however, it only responds to ultraviolet (UV) light with the wide bandgap (3.2eV).7 A variety of

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strategies have been devoted to modifying the TiO2, including doping noble metal and/or

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nonnoble metal, adopting dye sensitization and coupling with secondary semiconductors, to

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extend the light-response to visible region and resist electron-hole recombination.8-12

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Unfortunately, absorption toward infrared (IR) light and low apparent quantum efficiency still

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are still “bottleneck” for these photocatalysts to meet the requirement of practical application.

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Unlike the nanometal used in semiconductor photocatalysis as the cocatalyst, the plasmonic

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structures of noble metals (mainly Ag and Au), as a new class of efficient photocatalytic

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materials, can generate either the hot electrons or the photothermal effect to drive redox reactions

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due to their strong localized surface plasmon resonance (LSPR) effect.13, 14 LSPR refers to the

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collective electron oscillations in noble metal formed when the photon frequency matches with

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the natural frequency of surface electrons. The excited oscillations of electrons will be released

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rapidly on a femtosecond timescale through electron-electron scattering, leading to a thermal

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Fermi-Dirac distribution. If the energetic or hot electrons are not involved in the charge-transfer

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process of the photocatalysis, they will cool down through electron-phonon interactions at a time

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scale of 100 ps - 10 ns, causing a heat generation in the lattice temperature of the metal

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nanocrystal. The thermal energy will be dissipated to the surrounding medium, resulting in

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plasmonic photothermal conversion.15-19 The plasmonic nanometals exhibit the high absorption

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coefficients in the UV-Vis spectral region and the excellent catalytic activities.20-22 However, the

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hot-electron effect is influenced by the species, structures, and morphologies of the nanometals



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and induced by the limited excitation wavelengths.23 Therefore, if the hot electrons are not

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involved in the charge-transfer process, the energy of the LSPR will yield an increase in the

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temperature of the metal NPs.16 It was found that the catalytic activity induced by the

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photothermal effect in noble metal NPs had no difference with the process driven by the

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conventional heating except the manner of energy source, moreover, the photothermal effect

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provides an immediate, energy-efficient and targeted heating method to the active site where a

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catalytic reaction exactly occurs.15,

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particular shape are totally black and absorb the whole solar spectral region, which is the

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prerequisite for highly effective solar-to-thermal conversion.25 The photothermal effect has been

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designed for the selective catalysis, the biomedical applications and solar fuel production.26-28 Pt

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NPs have been found excellent photothermal conversion but showed the poor performance in the

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photothermal conversion of CO2 into CH4 due to the poisoning effect of CO.25 But there are very

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few reports about the plasmonic photothermal effect of Pt NPs for the removal of VOCs.

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It was also demonstrated that the nanometals without a

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Herein, the x Pt/-Al2O3 catalysts (x = 0.45, 0.92, 1.81 and 2.81 wt%), synthesized by a facile

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impregnation, calcination and hydrogen reduction method (Scheme 1), exhibit excellent

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photo-thermocatalytic activity for the removal of gaseous toluene under the full solar spectrum

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irradiation as compared with pure -Al2O3. The high-efficiency catalytic activity is actually

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similar to that of the conventional heating process, which is attributed to the fact that the Pt NPs

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not only produce the plasmonic photothermal effect under the solar irradiation but also serve as

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active sites to the catalytic reaction (Scheme 2). This work highlights that Pt/-Al2O3 catalyst


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shows great promise for VOCs decomposition through the plasmonic photothermal effect and

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thermocatalytic activity of Pt NPs induced by solar energy.

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Scheme 1. Synthetic process of Pt/-Al2O3.

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Scheme 2. Light-driven plasmonic photo-thermocatalytic decomposition of gaseous toluene over the Pt/-Al2O3.

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2. EXPERIMENTAL SECTION

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2.1. Catalyst Preparation. A modified method was utilized to synthesize the catalyst.29 0.5 g

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of the -Al2O3 purchased from the Beijing JAH TECH. Co. were immersed in 20 mL deionized

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water containing stoichiometric amounts of an aqueous solution of H2PtCl6 (3.8 g/L) (theoretical 5 ACS Paragon Plus Environment


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weight ratio of Pt/-Al2O3 = 0, 0.5%, 1.0%, 2.0% and 3.0%). -Al2O3 was dispersed

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homogeneously in the solution by ultrasonic treatment and stirred at 80 °C until the water was

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vaporized. The Pt precursors were loaded onto the surfaces of -Al2O3. The mixture was dried in

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an oven at 120 °C for 3 h and calcinated at 500 °C for 4 h. Afterward, the dried solid was

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reduced in H2 flow at 300 °C for 4 h.

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2.2. Catalyst Characterizations. Powder X-ray diffraction (XRD) was detected on the X’Pert

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Pro instrument using Cu Kα radiation. The specific surface area and pore size distribution were

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tested on the Quantachrome Autosorb IQ instrument. X-ray photoelectron spectroscopy (XPS)

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analyses were performed on a PHI Quantum 2000 Scanning ESCA Microprobe. TEM images of

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the samples were given in JEM-2100F transmission electron microscopy. The high-resolution

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transmission electronic microscopy (HRTEM) and high-angle annular dark-field scanning

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transmission electronic microscopy (HAADF-STEM) images were taken by FEI Tecnai F20

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transmission electronic microscopy. The element distribution of the sample was performed on

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energy dispersive spectroscopy (EDS). Diffuse reflectance spectra (DRS) were carried on a

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Shimadzu

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spectroscopy (ICP-OES, PerkinElmer Optima 7000DV) was used to test the chemical

17

composition.

UV-2550

spectrophotometer.

Inductively

coupled

plasma/optical

emission

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CO chemisorption was carried out on a ChemStar chemisorption analyzer to analyze the Pt

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dispersion of the catalyst. 0.2 g of the catalyst without hydrogen treatment was heated at 300 °C

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for 4 h with a heating rate of 5 °C/min in H2 flow. Then the catalyst was purged by pure He at


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200 °C for 1 h. After cooling to 35 °C, CO pluses were introduced to the catalyst until surface

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saturation and chemisorbed CO was quantitatively determined by a TCD placed at the exit of the

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reactor.

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Toluene temperature programmed desorption (toluene-TPD) was performed on a ChemStar

5

chemisorption analyzer equipped with a thermal conductivity detector. The gaseous toluene was

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generated by heating the vapor generator at 50 oC, and the He flow carried the gaseous toluene

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into the tube for toluene on catalysts. 0.2 g of the catalyst without hydrogen treatment was placed

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in a quartz tube and heated at 300 °C for 4 h with a heating rate of 5 °C/min in H2 flow. Then the

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catalyst was purged by pure He at 200 °C for 1h and cooled down to 35 °C prior to the

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adsorption of toluene for 1 h. After saturation with toluene, the sample was again flushed with

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pure He for 1 h at 35 °C. Then toluene-TPD were recorded online from 0 to 300 oC at a rate of

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10 °C/min.

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In situ DRIFTS was performed on a FTIR spectrometer (Nicolet Nexus 670) equipped with a

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smart collector and a MCT/A detector. The background spectrum was recorded in flowing pure

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N2 and automatically deducted from the sample spectrum. Thereafter, 1000 ppm toluene/N2

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flowed into the cell at a rate of 50 mL/min at room temperature. After the absorption saturation

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of toluene, the pure air was introduced to blow off the redundant toluene. Then the DRIFTS

18

spectra were recorded while the sample was irradiated by the simulated sunlight with 340

19

mW/cm2 intensity.



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The temperature-programmed desorption (TPD) of the spent catalyst was taken on a

2

Quantachrome chemisorption instrument. The detailed procedure was described in our previous

3

work.30

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2.3. Photo-thermocatalytic Activity. The photo-thermocatalytic activities for toluene

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oxidation upon the catalysts were examined in a continuous flow reactor under the irradiation of

6

the simulated sunlight (CHF-XM500, Beijing Changtuo Co.). 0.1 g of the catalyst coated on the

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fiberglass membrane with a diameter of 50 mm was placed on the bottom of the reactor. A

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thermocouple was placed upon the catalyst to monitor the temperature of the catalyst layer. To

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measure the catalytic activity driven by the visible-infrared light, a cutoff filter of 400 nm was

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placed between the lamp and the the reactor to filter out the wavelengths below 400 nm. The

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reactants and products were analyzed by a GC9160 gas chromatograph (GC) equipped with an

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FID and a TCD. Prior to irradiation, 200 ppm toluene/air firstly flowed into the reactor with a

13

rate of 50 mL/min in order to reach the adsorption-desorption equilibrium of the catalyst in the

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dark condition. The toluene conversion and CO2 yield were calculated by the following Eqs.

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Toluene conversion (%) = 100 × ([Toluene]in-[Toluene]out)/[Toluene]in

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CO2 yield (%) = 100 × [CO2]produced/[CO2]theoretical

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The incident light intensity was measured by an optical power meter (CEL-NP2000-2). The

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light intensity of the full solar spectrum on the catalysts was 320 mW/cm2. And another intensity

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of vis-infrared light was 390 mW/cm2.


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1 2


The stability test was the same as above except the irradiation time, which comes up to 30 hours.

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2.4. Photocatalytic Activity. The experiment of measuring the photocatalytic activity of the

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catalyst for toluene oxidation at room temperature was conducted as follows: 0.1 g of the catalyst

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was uniformly coated on the bottom of the reactor and then dried at 40 °C. A thermocouple was

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placed upon the catalyst to monitor the temperature of the catalyst. The reactor was put in an

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ice-water bath to maintain the reaction temperature at room temperature under the irradiation of

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the 320 mW/cm2 simulated sunlight.

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2.5. Thermocatalytic Activity. The procedures for measuring the thermocatalytic activity of

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the catalysts for toluene oxidation in the dark condition with various temperatures were the same

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as that of the photo-thermocatalytic activity described above. And the reaction temperatures were

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controlled by wrapping the reactor with heating tape and thermocouple.

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3. RESULTS AND DISCUSSION

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Figure 1. X-ray diffraction patterns of pure -Al2O3 and x Pt/-Al2O3. 9 ACS Paragon Plus Environment


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3.1. Structure and Properties Characterization of x Pt/-Al2O3. For realizing the optimal

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loading amount of Pt NPs on -Al2O3 for excellent catalytic activity,31 samples with various

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weight ratios of Pt/Al2O3 were synthesized. The ICP-OES was first conducted to determine the

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actual chemical composition of the x Pt/-Al2O3. As shown in Table 1, the Pt/Al2O3 weight ratio

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of x Pt/-Al2O3 are 0.45, 0.92, 1.81 and 2.81%, respectively, thus, the as-obtained catalysts were

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named as 0.45 Pt/-Al2O3, 0.92 Pt/-Al2O3, 1.81 Pt/-Al2O3, and 2.81 Pt/-Al2O3. Then, XRD

7

was employed to investigate the crystal phase structure of the samples. As Figure 1 shown, it

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exhibits the typical peaks of aluminum oxide, where the peaks at 2 = 19.5o, 31.9o, 37.6o, 39.5o,

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45.9o, 60.9o, 67.0o, and 85.0o are assigned to (111), (220), (311), (222), (400), (511), (440) and

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(444) crystal planes of Al2O3 (JCPDS card no. 00-010-0425), and the structure of the -Al2O3

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support hasn't been altered by the deposition of Pt NPs. Moreover, as shown in Figure S1, the

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more evidence given by the images of HRTEM further confirm the morphology of Al2O3, which

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is corresponding to the specific lattice fringes. Notably, no typical diffraction peaks

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corresponding to the oxide or metal states of platinum are observed in 0.45 Pt/-Al2O3, 0.92

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Pt/-Al2O3 and 1.81 Pt/-Al2O3. However, when the Pt amount is increased to 2.81%, three

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distinct diffraction peaks at 39.8o, 46.2o and 81.2o corresponding to Pt species are clearly

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observed. Combining the under-mentioned average particle size and dispersion of Pt NPs, no

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detected diffraction peaks of Pt may be ascribed to its relatively low loading content and/or high

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dispersity.32, 33

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Table 1. Some properties and characteristics of x Pt/-Al2O3.


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Entry

-Al2O3

0.45

0.92

1.81

2.81

Pt/-Al2O3

Pt/-Al2O3

Pt/-Al2O3

Pt/-Al2O3

BET(m2/g)

130

126

135

128

126

Total Pore Volume (cm3/g)

0.876

0.757

0.754

0.710

0.766

Pt loading (wt%)a

/

0.45

0.92

1.81

2.81

Pt dispersion (%)b

/

88.61

73.13

60.87

40.07

Average Pt Size (nm)c

/

1.0

1.2

1.4

1.5

Active Pt sites (wt%)d

/

0.399

0.673

1.102

1.126

1

a

The Pt amount was determined from ICP-OES.

2

b

The Pt dispersion was calculated by CO chemisorption.

3

c

The average Pt size was determined by TEM.

4

d

The product of Pt loading and Pt dispersion.

5

To better confirm the distribution of Pt NPs over -Al2O3, CO chemisorption was performed

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and the result is shown in Table 1. The Pt dispersion is 88.61, 76.13 and 60.87% on 0.45

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Pt/-Al2O3, 0.92 Pt/-Al2O3 and 1.81 Pt/-Al2O3, respectively, indicating that the Pt NPs are

8

highly dispersed on the support surface. However, the Pt dispersion is decreased to 40.07% on

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2.81 Pt/-Al2O3, which should be attributed to the agglomeration of Pt NPs.34 In other words, the

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Pt dispersion gradually decreases with the increasing loading amount.



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Figure 2. (A-D) TEM images of x Pt/-Al2O3 (x = 0.45, 0.92, 1.81, 2.81) and (E-H) corresponding Pt particle size

3

distributions. (I) HAADF-STEM image and (J-L) corresponding element mapping images of 1.81 Pt/-Al2O3.

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TEM measurement was used to confirm the particle size and dispersion of Pt NPs. As depicted

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in Figure 2A-H, it can be clearly observed that small and homogeneous Pt NPs have uniformly

6

loaded on -Al2O3 and the density of Pt NPs over 2.81 Pt/-Al2O3 is larger than other samples.

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The particle distribution and average diameter are also calculated by the images. As a result, the

8

average size of Pt NPs is 1.0, 1.2 and 1.4 nm on 0.45 Pt/-Al2O3, 0.92 Pt/-Al2O3 and 1.81

9

Pt/-Al2O3, respectively, while that on 2.81 Pt/-Al2O3 slightly increases to 1.5 nm. This

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indicates that the particle size of Pt NPs gradually increases as the Pt loading content increases,

11

but the enhancement is smaller when the loading content is high. This is due to the high Pt 12 ACS Paragon Plus Environment

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content, resulted in the aggregation and growth of Pt clusters.35 Additionally, the high-angle

2

annular dark-field scanning transmission electron microscopy (HAADF-STEM) and element

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mapping have also been applied to further confirm the element distribution of Pt/-Al2O3. As

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shown in Figure 2I-L, the overlapping element mapping obviously manifests that Pt NPs are

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evenly dispersed on the -Al2O3. Considering the dispersion and particle size of Pt NPs, it is

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found that the composite samples also have relatively high dispersity and small particle size even

7

though the loading content of Pt is 2.81%. This may be the result of well impregnation,

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calcination and hydrogen reduction condition.36

9



1

Figure 3. (A) XPS Pt 4d5/2 spectra of the x Pt/-Al2O3 samples. (B) N2 adsorption-desorption isotherm and (C) pore

2

distribution of pure -Al2O3 and x Pt/-Al2O3. (D) MS signal of toluene-TPD of pure -Al2O3 and x Pt/-Al2O3.

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XPS analysis was carried out to concretely confirm the chemical state of Pt NPs on the

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catalyst surface. Considering that the Pt 4f is overlapped with the Al 2p, the Pt 4d is selected to

5

analyze the chemical state of Pt. As Figure 3A shown, the spectra of Pt 4d on x Pt/-Al2O3

6

present only one peak at 314.3 eV, which corresponds to the metallic Pt.37 Wang et al. had

7

previously studied that the hydrogen treatment at 200 oC for 2 h could lead to complete reduction

8

of the oxidized platinum to metallic platinum.29

9

The N2 adsorption-desorption isotherms of all samples show the typical type III curve that is

10

characteristic of the mesoporous structure, which is in accordance with the corresponding

11

pore-size distribution (Figure 3B and 3C). Meanwhile, as shown in Table 1, the BET surface

12

areas of all samples are about 130 m2/g, and the total pore volume decreases slightly. Obviously,

13

the loading of Pt NPs has no significant effect on the surface area and pore volume of -Al2O3. It

14

is known that adsorption is one of the important steps for a catalytic reaction and the high surface

15

area provides more adsorption sites, which are in favor of VOCs catalytic degradation.36 To

16

better understand the influence of Pt NPs, the toluene-TPD over the samples has been performed

17

and the results show that the introduction of Pt significantly improves the adsorption capacity of

18

gaseous toluene by catalyst, especially for the 2.81 Pt/-Al2O3 (Figure 3D).


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1 2

Figure 4. (A) Toluene conversion and (B) CO2 yield over pure -Al2O3 and x Pt/-Al2O3 under the full solar

3

spectrum irradiation with 320 mW/cm2 light intensity. (C-D) The durability of 1.81 Pt/-Al2O3 under the full solar

4

spectrum irradiation with 320 mW/cm2 light intensity and in the dark condition with similar temperature for 30 h

5

(Toluene concentration = 200 ppm, GHSV = 30480 mL/(g·h)).

6

3.2. Evaluation of Catalytic Activity. The catalytic activities of x Pt/-Al2O3 for degradation

7

of gaseous toluene were evaluated under the simulated sunlight with 320 mW/cm2 of light

8

intensity. For comparison, the controlled trial was also carried out over the pure -Al2O3 under

9

the identical reaction condition. As seen in Figure 4A and 4B, the toluene conversion and CO2

10

yield of 0.92 Pt/-Al2O3 are 31% and 19% in 90 min, respectively, whereas no toluene evolution



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1

is obviously noticed over pure -Al2O3 and x Pt/-Al2O3 with low Pt loading amount (0.45

2

Pt/-Al2O3). It intuitively reveals that the -Al2O3 as the support has no contributions for the

3

degradation of gaseous toluene. Clearly, -Al2O3 can’t be stimulated by any wavelength of light

4

to generate neither charge carriers nor photothermal effect to induce the catalytic reaction.25, 38

5

Additionally, the catalytic activity of the 1.81 Pt/-Al2O3 has a considerable enhancement, where

6

toluene conversion and CO2 yield can dramatically increase to 87% and 84%, respectively. And

7

the 2.81 Pt/-Al2O3 shows the best catalytic activity of the catalysts, in which both toluene

8

conversion and CO2 yield achieve 94%. Generally, the amount of Pt active sites on catalyst plays

9

the crucial role in a thermal-driven catalytic reactions,39,

40

which can be calculated by the

10

dispersion and loading amount of Pt. As shown in Table 1, the catalyst with more Pt loading

11

amount possesses the higher amount of Pt active sites, and 2.81 Pt/-Al2O3 shows the highest

12

value for Pt active sites. Moreover, the turnover frequency (TOF) per unit amount of Pt NPs is

13

used to evaluate the catalyst, which is calculated by the CO2 yield below 20% and the actual

14

loading amount of Pt NPs. Clearly, as shown in Figure S2, the TOFPt value of 2.81 Pt/-Al2O3 is

15

larger than that of 1.81 Pt/-Al2O3, confirming the Pt content plays a main role in the catalyst.

16

From the above-mentioned particle size distribution, the samples possess the similar size (1.0 -

17

1.5 nm), indicating that the particle size has no obvious effect on the catalytic activity.

18

Combining the catalytic performance, it can be included that Pt NPs are the primary photoactive

19

ingredient while the inert -Al2O3 acts as support for the high dispersion of Pt NPs.


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1

Moreover, prolonged light irradiation was tested to measure the stability of the catalyst, which

2

is as important as the activity for the practical application of VOCs abatement. As shown in

3

Figure 4C and 4D, the continuous experiment of 1.81 Pt/-Al2O3 is conducted under the

4

irradiation of the simulated sunlight. Apparently, it still maintains the average toluene conversion

5

of ~ 87% and CO2 yield of ~ 84%, respectively, after the consecutive irradiation for 30 h,

6

suggesting that the catalyst has a remarkable catalytic stability.

7

3.3. Catalytic Mechanism. In order to make clear the reason why the catalyst with enhanced

8

loading amount of Pt exhibited efficient catalytic activity under the same light intensity, the DRS

9

was first tested to evaluate the optical properties of the catalysts. As depicted in Figure 5A, the

10

pure -Al2O3 shows no optical absorption over the whole wavelength range (200-2500 nm). The

11

curves of x Pt/-Al2O3 show a remarkable enhancement of light absorption in the whole range,

12

unlike Ag and Au nanostructures with strong surface plasmon bands in the UV-Vis region.41-43

13

The light absorption increases for composites with high Pt content obeying the following order:

14

2.81 Pt/-Al2O3 > 1.81 Pt/-Al2O3 > 0.92 Pt/-Al2O3 > 0.45 Pt/-Al2O3. It is obvious that the Pt

15

content has the decisive effect on the light absorption characteristics of the composite samples.

16

Accordingly, from the inset photograph as shown in Figure 5A, we can see that the colors of the

17

samples are different. The pure -Al2O3 is white, but gradually darkens as the Pt NPs amount

18

increases. Thus, one can easily maximize the optical absorption of the Pt/-Al2O3 in the whole

19

UV-Vis-NIR region for efficient utilization of solar energy. This extraordinary optical absorption



1

further substantiates the formation of metallic Pt on the -Al2O3 and perhaps results in either the

2

hot-electron effect or the photothermal effect to drive the catalytic reaction.

3 4

Figure 5. (A) The UV/Vis/NIR diffuse reflectance absorption spectra, (B) the temperature curves of pure -Al2O3

5

and x Pt/-Al2O3 under the light on/off.

6

On the other hand, the temperature variation of the catalyst layer is also monitored during the

7

process. Under the irradiation, the temperature rapidly increases to a plateau temperature in a

8

short time without extra heat supply (Figure 5B), at which the thermal equilibrium between the

9

solar energy absorption and the heat dissipation from the nanocomposites to the surrounding has

10

been formed, indicating that the Pt NPs can effectively transfer the solar energy to thermal

11

energy. As shown in Figure 5B, the plateau temperatures of 0.45 Pt/-Al2O3, 0.92 Pt/-Al2O3,

12

1.81 Pt/-Al2O3 and 2.81 Pt/-Al2O3 are 119, 132, 165 and 169 oC, respectively, whereas the

13

temperature of pure -Al2O3 support is measured to be only 78 oC. This temperature gradient is

14

consistent with the trend of toluene evolution, both of which closely related to the loading

15

amount of Pt. The strong photothermal property along with the excellent catalytic ability of Pt


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1

NPs enable the high-efficiency photo-induced conversion of toluene. Obviously, with increasing

2

of Pt loading amount, more light absorbers and catalytic active sites can be possessed to transfer

3

solar energy into thermal energy and further remarkably enhance the catalytic activity. This

4

result can confirm that the plasmonic photothermal effect of Pt NPs plays a crucial role in the

5

catalytic reaction.

6 7

Figure 6. (A) Toluene conversion and (B) CO2 yield of 1.81 Pt/-Al2O3 under the full solar spectrum irradiation

8

with different light intensities (red plots) and at different heating temperatures (black plots). (C) Catalytic activity

9

under the full solar spectrum (320 mW/cm2) and visible-infrared (390 mW/cm2) irradiation of simulated sunlight.

10

(Toluene concentration = 200 ppm, GHSV = 30480 mL/(g·h)).

11

The question remains whether the favorable catalytic activity of the catalyst under irradiation

12

is solely dominated by the plasmonic photothermal effect. If so, the photo-thermocatalytic

13

activity should be consistent with its thermocatalytic activity in dark condition with the similar

14

temperature. To verify this assumption, the photo-thermocatalytic experiments in several

15

irradiation intensities (220, 250, 270, 320 and 350 mW/cm2) and the thermocatalytic experiments

16

at several temperatures (120, 130, 142, 153, 164 and 175 oC) on the 1.81 Pt/-Al2O3 were

17

conducted, and the plateau temperatures were also recorded at the same time. As seen in Figure 19 ACS Paragon Plus Environment


1

6A and 6B, the red plots of the toluene conversion and CO2 yield show that the increase of light

2

intensity leads to the enhancement of the catalytic activity. Meanwhile, the black plots show that

3

the toluene conversion and CO2 yield increase as a function of temperatures under the thermal

4

condition. Distinctly, the fitting curve of the thermocatalytic activities is nearly consistent with

5

that of the photo-thermocatalytic activities. In addition, to further investigate whether the

6

photocatalytic process has any effect on the toluene evolution, the photocatalytic activity of 1.81

7

Pt/-Al2O3 was also conducted at room temperature under the full solar spectrum irradiation (320

8

mW/cm2). With this method, the photothermal effect is effectively restrained. As depicted in

9

Figure S3, it is found that no toluene conversion or CO2 yield is observed, and the temperature

10

remains at 26.5 oC. This result reveals the fact that the photocatalysis has no contribution to the

11

photo-induced toluene degradation. All of above results suggest that the photothermal effect of

12

LSPR should solely dominate the process without the hot-electron transfer effect. And as

13

depicted in Figure 4C and 4D, the 1.81 Pt/-Al2O3 has also shown the excellent stability of

14

toluene removal under the light irradiation or in the dark condition with the similar temperature.

15

Moreover, the photothermal conversion efficiency () is defined as the ratio of the thermal

16

energy, which is converted by the absorption of the irradiation light, to the energy of the incident

17

photons, where the efficiency reaches 10.57% for the 1.81 Pt/-Al2O3 under the full solar

18

spectrum irradiation (320 mW/cm2), which is depicted in Figure S4.

19

Furthermore, to better examine the contribution of spectral region to the photothermal effect,

20

1.81 Pt/-Al2O3 is irradiated by the segmental simulated sunlight with 390 mW/cm2 light


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1

intensity, cut off the wavelength below 400 nm (Figure 6C). Notably, in absence of ultraviolet

2

light, the toluene evolution can still proceed. Meanwhile, the visible-infrared irradiation has

3

almost the same catalytic activity with the approximate plateau temperature induced by different

4

light intensities. Therefore, the plasmonic Pt NPs have the advantages over the conventional

5

photocatalysts because of the efficient utilization of solar energy, especially the low-energy

6

photons of visible/infrared light. It proves that the catalytic activity is driven by the plasmonic

7

photothermal effect of the Pt NPs.

8 9

Figure 7. In situ DRIFTS spectra on 1.81 Pt/-Al2O3 for the toluene oxidation under the light irradiation.

10

To further disclose the potential reaction pathway of gaseous toluene oxidation on 1.81

11

Pt/-Al2O3, in situ DRIFTS spectra of toluene degradation under the light irradiation are shown

12

in Figure 7. Obviously, at room temperature, the toluene is easily adsorbed on the surface of

13

catalyst in dark. The band at 3028 cm–1 is assigned to C-H stretching vibrations of the aromatic

14

ring,44 and the C-H stretching from the toluene methyl group are detected at 2926 and 2878 cm–

15

1.45

The bands at 1600 and 1495 cm–1 are attributed to the C=C stretching in the toluene aromatic 21 ACS Paragon Plus Environment


1

ring.45 The new bands at 1583, 1456 and 1427 cm–1 assigned to benzaldehyde46 or benzoic

2

acid,47 and methylene group (–CH2) of benzyl species.48 The benzaldehyde, benzoic acid, and

3

maleic acid are also detected in the outlet gas as shown in Figure S5, and no CO signal is

4

detected by TCD. From the TPD-MS result (Figure 8A), the spent catalyst (1.81 Pt/-Al2O3

5

treated in a continuous toluene flow (1000 ppm) at 120 oC for 19 h) releases a series of species,

6

including CO2, H2O, CO, toluene, maleic acid, benzoic acid, and benzaldehyde. Vividly, the

7

gaseous toluene can be effectively absorbed by the catalyst, which is consistent with the result of

8

toluene-TPD. And the absorbed toluene can be partially oxidized to the intermediates, CO, CO2

9

and H2O at low temperature. As shown in TPO-MS (Figure 8B), the toluene and intermediates

10

are further oxidized in 20% O2/He stream at relatively high temperature and finally decomposed

11

into CO2 and H2O. Combining the above results, a proposed reaction pathway is depicted in

12

Scheme 3. The chemisorbed toluene molecules react with the activated O atoms which

13

dissociated on the Pt NPs supported -Al2O3 by the pre-adsorbed O2 molecules, to form

14

benzaldehyde and benzoic acid, then further convert into maleic acid and CO species, and finally

15

oxidize into CO2. It can be concluded that, under the simulated sunlight irradiation, the highly

16

dispersed Pt NPs over -Al2O3 could strongly absorb the full solar spectrum and efficiently

17

transfer into heat through the plasmonic photothermal effect of Pt NPs, providing the energy for

18

toluene catalytic oxidation.


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1 2

Figure 8. (A) TPD-MS and (B) TPO-MS results of the spent 1.81 Pt/-Al2O3 treated at 120 oC in a continuous

3

toluene flow (1000 ppm) for 19 h, while the released species including CO2 (m/z = 44), CO (m/z = 28), H2O (m/z =

4

18), toluene (m/z = 91), benzoic acid or benzaldehyde (m/z = 50), and maleic acid (m/z = 45).



1

Scheme 3. Illustration of reaction pathway.

2 3

4. CONCLUSION

4

In summary, the highly-dispersed Pt NPs supported on Al2O3 display excellent

5

photo-thermocatalytic activity and stability towards the toluene degradation reaction under the

6

full solar spectrum irradiation. Even with visible-infrared irradiation, the Pt/-Al2O3 still shows

7

efficient catalytic activity. The mechanism of the highly efficient catalytic activity is primarily

8

ascribed to the strong plasmonic photothermal effect of plasmonic Pt NPs, which has remarkable

9

light absorption over the UV-Vis-NIR region and efficient photothermal conversion. Meanwhile,

10

the photo-thermocatalytic activity is identical to the traditional thermocatalytic activity without

11

an additional photoactivation mechanism based on hot-electron transfer and injection. This

12

finding could open a novel orientation in VOCs elimination by using the inexhaustible solar

13

energy. 24 ACS Paragon Plus Environment

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1



2

Supporting Information Available: HRTEM images of the 1.81 Pt/-Al2O3; TOF values of Pt

3

NPs in the 1.81 Pt/-Al2O3 and 2.81 Pt/-Al2O3; toluene oxidation of the 1.81 Pt/-Al2O3 at room

4

temperature under the light irradiation; the light on/off temperature curves and TG-DSC plots of

5

the 1.81 Pt/-Al2O3, along with the calculation of photo-thermal conversion efficiency; MS

6

spectrum of the organic components on the spent 1.81 Pt/-Al2O3.

7



8

This work was supported by Nature Science Foundation of Fujian Province of China [No.

9

2016J01079, No. 2016J05049], “One Hundred Talent Project” and “Key Program for Frontier

10

Sciences-Youth Scientist” from Chinese Academy of Sciences [No. QYZDB-SSW-DQC022],

11

“Xiamen High-level Overseas Innovation Talent”, and the National Natural Science Foundation

12

of China [21501175, 21703233].

13



14

(1) Li, J. J.; Weng, B.; Cai, S. C.; Chen, J.; Jia, H. P.; Xu, Y. J., Efficient Promotion of Charge

15

Transfer and Separation in Hydrogenated TiO2/WO3 with Rich Surface-Oxygen-Vacancies for

16

Photodecomposition of Gaseous Toluene. J. Hazard. Mater. 2018, 342, 661-669.

17

(2) Xue, X. Y.; Zang, W. L.; Deng, P.; Wang, Q.; Xing, L. L.; Zhang, Y.; Wang, Z. L.,

18

Piezo-Potential Enhanced Photocatalytic Degradation of Organic Dye Using ZnO Nanowires.

19

Nano Energy 2015, 13, 414-422.

ASSOCIATED CONTENT

ACKNOWLEDGMENT

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Strong Photothermal Effect of Plasmonic Pt Nanoparticles for Efficient

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