Light-Enhanced Carbon Dioxide Activation and Conversion by

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Light-Enhanced Carbon Dioxide Activation and Conversion by Effective Plasmonic Coupling Effect of Pt and Au Nanoparticles Hui Song, Xianguang Meng, Thang Duy Dao, Wei Zhou, Huimin Liu, Li Shi, Huabin Zhang, Tadaaki Nagao, Tetsuya Kako, and Jinhua Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13043 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Light-Enhanced Carbon Dioxide Activation and Conversion by Effective Plasmonic Coupling Effect of Pt and Au Nanoparticles Hui Songab, Xianguang Mengc*, Thang Duy Daobg, Wei Zhoud, Huimin Liub, Li Shiab, Huabin Zhangb, Tadaaki Nagaobgh, Tetsuya Kakob, and Jinhua Yeabef* a

Graduate School of Chemical Science and Engineering, Hokkaido University, Sapporo

060-0814, Japan. b

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute

for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan c

Photo-functional Materials Research Platform, College of Materials Science and

Engineering , North China University of Science and Technology, Tangshan 063210, P. R. China d

Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials

Physics, Preparing Technology Faculty of Science, Tianjin University, Tianjin 300072, P. R. China e

TU-NIMS International Collaboration Laboratory, School of Material Science and

Engineering, Tianjin University, Tianjin 300072, P. R. China f

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

300072, P. R. China

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g

CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,

Saitama 332-0012, Japan h

Department of Condensed Matter Physics, Graduate School of Science, Hokkaido

University, Kita-10 Nishi-8 Kita-ku, Sapporo 060-0810, Japan Corresponding Authors: [email protected] [email protected]


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ABSTRACT Photocatalytic reduction of carbon dioxide (CO2) is attractive for the production of valuable fuels and mitigating the influence of greenhouse gas emission. However, the extreme inertness of CO2 and the sluggish kinetics of photo-excited charge carrires transfer process greatly limit the conversion efficiency of CO2 photoreduction. Herein, we report that the plasmonic coupling effect of Pt and Au nanoparticles (NPs) profoundly enhance the efficiency of CO2 reduction through dry reforming of methane reaction assisted by light illumination, reducing activation energies for CO2 reduction ~30% below thermal activation energies and achieving a reaction rate 2.4 times higher than that of the thermocatalytic reaction. UV-vis absorption spectra and wavelength-dependent performances show that not only UV but also visible light play important roles in promoting CO2 reduction due to effective local surface plasmon resonances (LSPR) coupling between Pt and Au NPs. Finite-difference time-domain (FDTD) simulations and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra results further reveal that effective coupling LSPR effect generates strong local electric fields and excite high concentration of hot electrons to activate the reactants and intermediate species, reduce the activation energies and accelerate the reaction rate. This work provides a new pathway towards the efficient plasmon-enhanced chemical reactions via reducing the activation energies by utilizing solar energy.

KEYWORDS: plasmonic coupling effect, platinum, gold, photocatalytic CO2 reduction, activation energy



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1. Introduction The increasing of atmospheric carbon dioxide (CO2) coming from excessive consumption of fossil fuels have generated much concerns about the consequent effects on the global climate change.1-3 Photocatalytic CO2 reduction on semiconductors, utilizing photoinduced electrons and holes to propel the simultaneous processes of CO2 photoreduction and H2O photooxidation, offer a promising and attractive route to take the advantage of abundant solar energy and recycle CO2 resource.4-8 However, the extreme inertness of CO2 (∆fG0= -396 kJ mol-1) and the sluggish kinetics of multiple e-/H+ transfer process result in low energy conversion efficiency and uncontrollable selectivity for CO2 photoreduction, indicating that it is still a great challenge to reduce CO2 only using semiconductors and solar energy.9-11 Currently, dry reforming of methane (DRM) reaction based on Group VIII metal (Pt, Rh, Ru, Pd, Ni, etc.) catalysts can reduce CO2 by utilizing CH4, one of the most abundant greenhouse gases, to produce industrially important syngas (H2 and CO).12-14 However, this reaction requires operating temperatures of 1100-1300 K to activate the reactants and achieve high equilibrium conversion of CO2 and CH4 to syngas, which waste a lot of heat energy and shorten catalyst lifetime via sintering deterioration and carbon deposition.15 Great efforts have been devoted to improve CO2 conversion efficiency in DRM at lower temperatures over past few years,13 but most of the reported ways were limited and few strategies were reported to directly reduce the activation energies of DRM reaction. Significant catalytic advances to reduce activation energies of CO2 for simultaneously lowering reaction temperatures and boosting CO2 conversion efficiency are highly desirable.


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Recently, it has been reported that plasmonic metallic nanostructures can efficiently drive chemical reactions with photo-excited charge carriers to overcome activation barriers, exhibiting a linear or super-linear dependence of reaction rates on incident light intensity.16-20 Plasmonic nanostructures of metals (such as Ag, Au, Cu, Pd, Pt and Rh) exhibit strong light absorption through an excitation of localized surface plasmon resonances (LSPR), which can be spectrally tuned throughout the UV or visible light by modulating the shape and size of metallic nanostructures.21,

22

Interestingly, LSPR

excitation produces a large number of energetic electrons (hot electrons) at the nanostructured surfaces that can transfer to adsorbates and promote reaction rates.23-25 Recent studies have shown that Pt nanoparticles as photocatalysts can drive catalysis and enhanced catalytic performances under light irradiation.26, 27 We recently also found that plasmonic Au NPs could effectively enhance the catalytic activity of Rh/SBA-15 in DRM reaction under visible light irradiation because Au could act as a plasmonic promoter to facilitate DRM.28 Meanwhile, the reaction rate enhancement is closely related to the energy and distribution of hot electrons, which rely on the energy of the incident photon and the LSPR of metallic nanostructures as well as the local density-of-states in the nanostructures and the associated band structure. Therefore, it is expected that surface plasmonic coupling of two metal NPs with different excited LSPR peaks may be effectively utilize solar energy and induce more energetic hot electrons to inject into the adsorbates and/or specific reaction intermediates, thereby reducing activation energies and promoting reaction rates.29-31 Herein, we report that the plasmonic coupling effect of Pt and Au NPs can significantly reduce activation energies and enhance reaction rates of CO2 reduction in DRM reaction.



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Under low-intensity light (300~800 nm, ~0.6 W cm-2) irradiation, the activation energies for CO2 reduction are reduced ~30% below thermal activation energies and the reaction rate is 2.4 times higher than that of the thermal-catalytic reaction rate at 673 K. The wavelength-dependent performances show that not only UV but also visible light play important roles in facilitating CO2 conversion rate. Finite-difference time-domain (FDTD) simulation indicate that effective coupling LSPR effect of Pt and Au NPs generates intense electric fields and hot electrons by absorbing UV and visible light. The transfer of hot electrons generated on the photoactivated Pt and Au surfaces to the adsorbed reactants and intermediate species can weaken the chemical bonds and reduce surface coverage of adsorbed species, reducing the activation energies of CO2 and facilitating the reaction rates. The lower species coverage was detected by in situ DRIFTS analysis. Our research that effective coupling LSPR effect of Pt and Au NPs promote the DRM reaction via reducing the activation energies opens a new avenue for photo-mediated reactions on plasmonic metal NPs by utilizing solar energy.

2. Experimental section 2.1. Materials Chloroplatinic acid hexahydrate (H2PtCl6 • 6H2O), gold chloride trihydrate (HAuCl4 • 3H2O), sodium carbonate (Na2CO3) and urea (CN2OH4) were purchased from Wako Co. Silicon dioxide, (SiO2, nanopowder 5-15 nm (BET), 99.5% metals basis) was purchased from Sigma-Aldrich. 2.2. Preparation of the catalysts Preparation of Au/SiO2: Au/SiO2 catalysts with different loadings were prepared with urea-precipitation method. In general, 1.0 g SiO2 was added into 100.0 mL of an aqueous


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solution of HAuCl4 (0.51 mM) and urea (11.4 g). The suspension was maintained at 80 ℃ with vigorous stir for 4 h, then centrifuged, washed, dried and calcined at 400 ℃ for 4 h. The theoretical loadings of Au was 1.0 wt. %. Preparation of Pt-Au/SiO2: Pt was loaded on Au/SiO2 via the impregnation method. In general, 1.0 g Au/SiO2 was added into 10.0 mL of an aqueous solution of H2PtCl6 (2.56, 5.13 and 7.69 mM)) and Na2CO3 (the weight ratio of Na and Pt is 2).

After

impregnation, excess water was removed at 50 ℃ until dryness. Then samples were calcined at 400 ℃ for 4 h. The theoretical loadings of Pt were from 0.5 to 1.5 wt. %. Preparation of Pt/SiO2 and Pt (No Na)/SiO2: For Pt/SiO2, 1.0 g SiO2 was added into10.0 mL of an aqueous solution of H2PtCl6 (2.56, 5.13 and 7.69 mM) and Na2CO3 (the weight ratio of Na and Pt is 2). Then, the sample was dried and calcined at 400℃. After that, Pt/SiO2 with loadings of Pt from 0.5 to 1.5 wt. % were obtained. The preparation of Pt (No Na)/SiO2 was similar to the above method in addition to not adding Na2CO3. In this paper, all the catalysts were in-situ reduced for one hour during DRM reaction. Unless otherwise stated, Pt-Au/SiO2, Pt/SiO2 and Au/SiO2 were referred to the catalysts with Na modification and the theoretical Pt and Au loading of 0.5 and 1.0 wt.%. 2.3 Characterization The crystallographic phases of as-prepared samples were analyzed by X-ray diffraction (XRD) method on an X’Pert PRO diffractometer with Cu Kα radiation. TEM and HRTEM were employed on a FEI Tecnai G 2 F20 transmission electron microscope operated at 300 kV. HAADF-STEM images were taken on a JEOL 2100F microscope. The XPS spectra were analyzed on a Thermo ESCALAB-250 spectrometer using a



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monochromatic Al Kα radiation source (1486.6 eV) and the binding energies determined by XPS were corrected by reference to the adventitious carbon peak (284.7 eV) for every sample. The diffuse reflection spectra of the catalysts were investigated by ultraviolet– visible spectrophotometer (UV-2600, SHIMADZU Co., Japan). N2 adsorption-desorption experiments were performed at 77 K to examine the Brunauer-Emmett-Teller surface area. 2.4 Catalyst evaluation The photocatalytic reaction was carried out at atmospheric pressure in a fixed-bed reactor system (the reactor system is home-made by ourselves, and see details in Figure S1). Typically, 20 mg photocatalyst was used for each test and was placed uniformly in the alumina cell (diameter 8.5 mm, height 3 mm). In the experiments, the Pt and Au NPs and the SiO2 were in well thermal equilibrium with each other due to their good thermal contact. The temperature was precisely measured and controlled by a thermocouple and TC-1000 temperature controller (JASCO), conducting the resistive heater to mitigate heating induced by light irradiation and keep the catalyst at the desired temperature (Figure S1 c and d). LA-251 Xe lamp equipped with HA30 filter (remove the IR light) was used to provide the light energy input. CO2 and CH4 (the molar ratio of 1:1) flowed into the reactor at a total flowrate of 20.0 ml/min. Firstly, the catalysts were in-situ reduced in DRM reaction for one hour. After the catalytic reactions were carried out for one hour, the effluent gas streams were analyzed using gas chromatograph (GC) equipped with a thermal conductivity (TCD) and a flame ionization detector (FID) to gain the relative amounts of CO2, CH4, CO and H2. 2.5 In situ DRIFTS analysis


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The DRIFTS analysis were measured at 673 K by a FT/IR-6300 system (JASCO Corp.), equipped with an insitu DR cell and a liguid nitrogen cooled mercury-cadmiumtelluride detector (MCT) (see details in Figure S2). The wavenumber resolution is 4 cm-1. The catalyst (20 mg) was placed uniformly in the DR cell, then Ar (20 mL min-1) was introduced into the cell at 673 K for 30 min to acquired the background spectrum of the sample. Then CO2+CH4+Ar (1:1:2) mixture (10.0 mL min-1) was introduced into the cell at 673 K for 60 min. After that, the spectra were recorded at 673 K under CO2+CH4+Ar (1:1:2) mixture (10.0 mL min-1) in the dark and with the irradiation of UV-visible light (300~800 nm, ~0.6 W cm-1). 2.6 Electromagnetic field simulation The electromagnetic fields distributed around plasmonic Pt and Au nanoparticles were conducted

using

finite-difference

time-domain

(FDTD)

methods

(FullWAVE,

Synopsys’RSoft). The dielectric functions of Rh, Au and SiO2 were taken from the literature.32 2.7 DFT theoretical calculations: All calculations were carried out with VASP code based on the density-functional theory (DFT).33 The generalized gradient approximation (GGA) was used for the exchange-correlation energy. A plane-wave expansion for the basis set with a cutoff energy of 450 eV was employed. To the CO2/Pt and CH4/Pt interface models, we constructed a slab model containing a CO2 or CH4 molecule on a (111) Pt surface with five atomic layers, and a vacuum region of 20 Å was included. 5×5×1 Monk-horst kpoint meshes was used for the Brillouin-zone integrations of the slab model, respectively. All atoms, excluding the last two atomic layer of Pt, were relaxed until the residual force



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was less than 0.01 eV/Å. In addition, the DFT-D2 method proposed by Grimme was adopted to describe the van der Waals (vdW) interactions.34

3. Results and discussions Pt and Au with different loadings were loaded on SiO2 (BET surface area, 436 m2 g-1, see Figure S3) by impregnation and precipitation method and the actual Pt and Au loadings characterized by ICP-OES method were similar to the theoretical values (Table S1). Note that sodium ions (Na+) were added in the process of loading Pt, since the addition of sodium ions could stabilize the dispersed Pt NPs and improve the catalytic activity for DRM in thermal process (see Figure S4 and 5).35,

36

The representative high-angle

annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pt/SiO2 are presented in Figure 1a and S4, which clearly show that Pt NPs with an average diameter of about 2.0 nm dispersed on SiO2. Figure S6 present the representative HAADF-STEM and TEM images of Au/SiO2. The size of Au NPs was in the range of 10-30 nm. Figure 1b shows the HAADF-STEM image of Pt-Au/SiO2. The size of Pt NPs almost remains unchanged. From the HAADF-STEM images (Figure 1b and S7), Pt NPs were indeed deposited on Au NPs except those directly deposited on SiO2 support, while not all the Au NPs were loaded with Pt NPs (Figure S8a). EDX mapping spectra (Figure S8) and HRTEM image (Figure S9) of Pt-Au/SiO2 can distinguish Pt and Au NPs. It is noted that this Pt-Au contact aggregate structures are thermodynamically stable and AuPt surface alloy is difficult to form.37, 38 The crystalline structures of the catalysts were determined by XRD analysis (Figure 1c and S10). The broad peak centered at 20-30o was ascribed to the amorphous SiO2. The peaks located at 38.2o, 44.3o, 64.6o and 77.5o were assigned to the Au NPs. No peaks of Pt can be observed because of its low amount and 10 ACS Paragon Plus Environment

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small size. But XPS (Figure 1d) and ICP-OES analysis (Table S1) of the catalysts can detect the presence of Pt. XPS data also show that Pt and Au existed in a metallic state when loaded on the SiO2 support.39, 40 The catalytic activities of the catalysts in DRM reaction with light irradiation (300 nm

Light-Enhanced Carbon Dioxide Activation and Conversion by

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