Localized Surface Plasmon Resonance Assisted Photothermal

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Localized Surface Plasmon Resonance Assisted PhotoThermal Catalysis of CO and Toluene Oxidation over Pd-CeO2 Catalyst Under Visible Light Irradiation Jinshuo Zou, Zhichun Si, Yidan Cao, Rui Ran, Xiaodong Wu, and Duan Weng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08630 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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The Journal of Physical Chemistry

Localized Surface Plasmon Resonance Assisted Photo-Thermal Catalysis of CO and Toluene Oxidation over Pd-CeO2 Catalyst under Visible Light Irradiation Jinshuo Zou,†,‡ Zhichun Si,∗,† Yidan Cao,‡ Rui Ran,‡ Xiaodong Wu,†,‡ Duan Weng†,‡ †

Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

‡

State Key Laboratory of New Ceramics & Fine Process, School of Materials Science and

Engineering, Tsinghua University, Beijing 100084, China

ABSTRACT:

The extinction peak of Pd particles generally locates at the ultraviolet

light region, suggesting that only 4% of solar light can be absorbed. Furthermore, the efficiency of LSPR hot electrons converted to chemical energy of reaction is very low due to the fast relaxation of carriers. It is extremely valuable to design Pd based catalysts which have strong response to the visible light irradiation and high efficiency in photon to chemical energy. The Pd-CeO2 catalyst was synthesized via hexadecyl trimethyl ammonium bromide (CTAB) assisted liquid-phase reduction method to generate more active interfaces. The significant extinction of Pd-CeO2 in visible to near infrared region indicates the strong electron interaction between Pd and CeO2. LSPR hot electrons, transferring from the Pd metal particles to the conduction band of ceria, promote the dissociation of adsorbed oxygen. Therefore, the reaction temperature of CO and toluene oxidation can be significantly lowered by visible light irradiation. The maximum light efficiencies of Pd-CeO2 catalyst for toluene oxidation and CO oxidation are obtained as 0.42% and 1%, which benefit from the effective

∗ Corresponding author, Tel: +86 755 26036861; Fax: +86 755 26036417 E-mail address: [email protected] (Z. Si)

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Pd-O-Ce interfaces. 1. INTRODUCTION When metal particles such as Au, Ag, Cu, and Pd are struck by light, collective oscillations of the conduction electrons called surface plasmon can be excited from the surface of the metal.1-4 The “hot ” electrons generated by collective oscillation have far more energy than they would have in standard thermal equilibrium condition.1 The ability to concentrate such large amounts of energy in tight spaces has led to the emergence of initial applications in photo-catalysis.5, 6 However, because of the competition between the electron-molecule reaction process and the fast carrier relaxation process, typically only a small fraction of photo-excited hot electrons can be used in practice. For efficient extraction of hot carriers from a metal, total energy is not the only condition. Once created, the hot carriers must move perpendicularly towards the interface.2 This problem can be solved by the introduction of metal-semiconductor system, since the process of electrons transferring from metal surface into a semiconductor can extend the lifetime of hot electrons.2, 3 To a certain extent, the existence of a potential energy barrier known as the Schottky barrier can inhibit the motion of a charge carrier through a metal-semiconductor interface. However, the key benefit of many Schottky barrier-based devices is that the height of the barrier is smaller than the bandgap of the semiconductor. In such cases, to be extracted, an excited electron does not necessarily need to possess a certain amount of energy greater than the semiconductor’s bandgap. It would also be expected that a good electric contact, 2


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facilitated by a relatively large contact between the metal and the semiconductor, should favor the electron transference. And consequently, a dependence of the photocatalytic activity on the area of the metal–oxide interface can be foreseen.7 Noble metal nanoparticles are promising in harvesting photon energy for chemical reactions due to their extraordinary and tailorable LSPR properties.8-10 However, compared with Au and Ag, the plasmonic properties of Pd nanoparticles have been much less explored due to the fact that the extinction peak of Pd particle generally located at the ultraviolet light region. It’s a great challenge to develop a new strategy to properly tune the LSPR of Pd nanoparticles in the visible region. Pd based catalysts have drawn much attention due to the excellent activity for the low temperature oxidation of CO and hydrocarbons. Therefore, it’s meaningful to combine the LSPR of the Pd particles with their excellent catalytic performance. Ceria has been widely studied as catalysts because of its excellent oxygen storage capacity. When Pd is incorporated with ceria, the dispersion of supported metal Pd will be significantly improved and the active Pd-CeO2 interface will form extensively due to the interaction of Pd and ceria particles. Thus the photocatalytic activity of Pd for oxidation reactions will be obviously enhanced because of the improved oxygen storage and release characters of CeO2.11, 12 In the present work, PdO and CeO2 were homogeneously mixed by CTAB-assisted liquid-phase reduction method. Pd-CeO2 interface was obtained after in-situ H2 reduction of PdO-CeO2. Hot electrons excited by irradiation visible-near infrared light irradiation could transfer from Pd to the conduction band of ceria and promote the 3



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activation of oxygen on Pd-CeO2 interface. The catalyst showed significantly enhanced photo-thermal catalytic activity for CO and toluene oxidation, compared with thermal catalysis in dark.

2. RESULTS AND DISCUSSION 2.1 Synthesis and characterization The Pd-CeO2 nanoparticles were synthesized via CTAB-assisted liquid-phase reduction method. Thereafter, the formed particles were supported on the inactive α-Al2O3 and thermally treated to produce a homogeneously distributed Pd-CeO2 catalyst. The obtained yellowish PdO-CeO2/Al2O3 catalyst powder (Pd: 1 wt %, CeO2: 3 wt %) and the gray Pd-CeO2/Al2O3 powder were denoted as PCA-R and PCA. The CeO2/Al2O3 catalysts were obtained by adding the support into the CeO2 colloidal solution directly. Similarly, the CeO2/Al2O3 (CeO2: 3 wt %) and H2-reduced CeO2/Al2O3 catalysts were denoted as CA and CA-R. Synthesis details are given in the Supporting Information. The physical properties of the obtained catalysts were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), scanning electronic microscope (SEM) and the energy dispersive spectrum analysis (EDS), Raman spectroscopy, UV-visible diffuse reflectance spectra (DRS), and diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS). Details on the instrumental part are given in the Supporting Information. The experimental apparatus for the catalytic performance tests in Scheme S1 was used to measure the activities of 4


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different catalysts for CO and toluene oxidation. The concentrations of CO and CO2 were analyzed by a Fourier transform infrared spectroscopy. The concentrations of toluene were analyzed by a GC-7920 gas chromatograph (CEAULIGHT, Beijing) equipped with a flame ionization detector. Related details are given in the Supporting Information.

2.2 Structural properties The XRD patterns of the catalysts are shown in Figure 1. Both fluorite structure ceria (JCPDS 34-0394) and the α-Al2O3 structure (JCPDS 10-0173) can be recognized from all catalysts. In the enlarged view of XRD patterns in Figure 1c, the characteristic peak of crystalline state PdO at 2θ = 32-35o can only be observed clearly from the PCA catalyst. It can be inferred that, after the thermal treatment at 773 K, palladium exists in the crystalline PdO form. However, in Figure 1d, the characteristic diffraction peak of crystalline state metallic Pd at 2θ = 39-41o can be observed only from PCA-R catalyst, suggesting that the crystalline PdO is reduced to crystalline Pd completely by hydrogen at 423 K.

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Figure 1. XRD patterns of the catalysts: full patterns (a) and regional patterns of the characteristic diffraction peaks of metallic CeO2 (b), PdO (c), Pd (d).

The TEM images in Figure 2 shows the morphology of Pd and CeO2 nanoparticles supported on inactive α-Al2O3. The size of the Pd-CeO2 nanoparticles is homogeneously below 8 nm, suggesting the excellent dispersion of the catalyst on α-Al2O3 support (200 nm, shown in Figure S1). By measuring the crystalline interplanar spacing of non-overlapping particles at the regions of interest in Figure 2, CeO2 (111) and Pd (110) phases can be found in the PCA-R catalysts. The strongly disturbed crystalline interplanar spacing on the interfaces shown in Figure 2 indicates that Pd nanoparticles are closely contacted with ceria (

Localized Surface Plasmon Resonance Assisted Photothermal

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