TiO2 Hybrids

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Remediation and Control Technologies

Coupled Palladium#Tungsten Bimetallic Nanosheets/ TiO2 Hybrids with Enhanced Catalytic Activity and Stability for the Oxidative Removal of Benzene Yijing Liang, Yuxi Liu, Jiguang Deng, Kunfeng Zhang, Zhiquan Hou, Xingtian Zhao, Xing Zhang, Kaiyue Zhang, Rujian Wei, and Hongxing Dai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00370 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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Environmental Science & Technology

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Coupled Palladium‒Tungsten Bimetallic Nanosheets/TiO2 Hybrids

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with Enhanced Catalytic Activity and Stability for the Oxidative

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Removal of Benzene

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Yijing Liang, Yuxi Liu*, Jiguang Deng, Kunfeng Zhang, Zhiquan Hou, Xingtian

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Zhao, Xing Zhang, Kaiyue Zhang, Rujian Wei, Hongxing Dai

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Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of

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Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional

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Materials, Education Ministry of China, Laboratory of Catalysis Chemistry and

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Nanoscience, Department of Chemistry and Chemical Engineering, College of

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Environmental and Energy Engineering, Beijing University of Technology, Beijing

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100124, China.

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* To whom correspondence should be addressed:

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Dr. Yuxi Liu ([email protected])

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Tel. No.: +8610-6739-6118; Fax: +8610-6739-1983

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ACS Paragon Plus Environment


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ABSTRACT:

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Since the conventional Pd-based catalysts often suffer severe deactivation by water,

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development of a catalyst with good activity and moisture-resistance ability is of

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importance in effectively controlling emissions of volatile organic compounds

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(VOCs). Herein, we report the efficient synthesis of ultrathin palladium–tungsten

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bimetallic nanosheets with exceptionally high dispersion of tungsten species. The

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supported catalyst (TiO2/PdW) shows good performance for benzene oxidation, and

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90 % conversion is achieved at a temperature of 200 °C and a space velocity of 40

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000 mL g1 h1. The TiO2/PdW catalyst also exhibits better water-tolerant ability than

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the traditional Pd/TiO2 catalyst. The high catalytic efficiency can be explained by the

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facile redox cycle of the active Pd2+/Pd0 couple in the close-contact PdOx‒WOx‒TiO2

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arrangement. We propose that the reason for good tolerance to water is that lattice

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oxygen of the TiO2/PdW catalyst can effectively replenish the oxygen in active PdOx

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sites consumed by benzene oxidation. A four-step benzene transformation mechanism

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promoted by the catalyst is proposed. The present work provides a useful idea for

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rational design of efficient bimetallic catalysts for the removal of VOCs under the

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high humidity conditions.

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KEYWORDS: Two-dimensional material; Palladium-tungsten bimetallic nanosheet;

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Volatile organic compound; Benzene oxidation; Water-resistant ability.

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INTRODUCTION

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Volatile organic compounds (VOCs) are important precursors in formation of fine

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particles and ozone in the atmosphere. Furthermore, most of VOCs are harmful to

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human health.1,2 Thus, effective control of VOCs emissions is beneficial for the

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sustainable development of human society. Benzene emitted from industrial processes

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is a dangerous substance, which can lead to human carcinogens and contribute to

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formation of secondary organic aerosols.3 Catalytic oxidation is an effective pathway

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for VOCs elimination due to its low operating temperatures, high efficiency, and no

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secondary pollution.4,5 The key point of such a technology is the development of

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high-performance catalysts. Generally speaking, large specific surface areas and

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abundant active sites are essential for good performance of heterogeneous catalysts.68

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Ultrathin two-dimensional nanomaterials with high specific surface areas show unique

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physicochemical properties9,10 due to the electron effect in two dimensions,11 strong

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in-plane covalent bond,12 and atomic thickness.13

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In the past years, supported noble metals (especially Pd) have been used for the

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total oxidation of VOCs due to their excellent low-temperature catalytic performance.

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However, the high cost and H2O inhibition seriously limit wide applications of the

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Pd-based materials. One efficient way to reduce the amount of precious metals is

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modification of noble metals with transition metals.14 Except for the interaction

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between metal and support, the second metal can improve the electronic and

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geometric structures, generate bimetallic active sites, and further enhance activity and

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stability of a catalyst. Recently, some reports have shown that noble metal alloys

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exhibited better catalytic performance for VOCs and CO oxidation, as compared with

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the single noble metal catalysts.15-19 For example, Hosseini et al. reported that the

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PdAu/TiO2 catalyst showed a significantly enhanced catalytic activity for toluene and

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propene oxidation, as compared with the supported Pd and Au catalysts.15 Zhong’s

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group found that the Pd50Cu50 nanoalloy catalyst with randomly mixed Pd and Cu

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atoms exhibited a better catalytic activity for CO oxidation, indicating that the atomic

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structure of PdCu could strongly influence the catalytic performance.17 Persson et al.

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demonstrated that addition of Pt to Pd was beneficial for improving catalytic stability

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for methane combustion.18 After investigating the supported Pd-Co catalysts with

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different Co loadings for methane combustion, Stefanov et al. pointed out that the

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presence of cobalt was beneficial for stabilization of palladium and replenishment of

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oxygen.19 However, alloys composed of Pd and other transition metal(s) for VOCs

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oxidation have been rarely seen in the literature, possibly due to the challenge in

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synthesis of multi-metal nanocrystals (NCs) with controlled sizes and compositions.

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Herein, we report the design and fabrication of ultrathin bimetallic nanosheets, and

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find that the supported bimetallic catalyst exhibits a good catalytic activity and

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excellent water-resistant ability in total oxidation of benzene, which are much better

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than the supported Pd counterpart. To date, there have been no reports regarding the

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catalytic removal of VOCs using palladium-tungsten bimetallic catalysts in a high

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humidity atmosphere. The excellent catalytic performance is attributed to the

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atom-dispersed WOx islands embedded in the PdOx‒Pd nanosheets and the modified

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electronic structure of the noble metal.

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EXPERIMENTAL SECTION

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Catalyst Preparation. Synthesis of PdW-S1 nanosheets (NSs). 0.02 mmol of

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Palladium (II) 2,4-pentanedionate (Pd(acac)2) was dissolved in toluene (5 mL) and

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oleylamine (3.5 mL), the mixture is stirred at room temperature (RT) for 20 min. Then,

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0.14 mmol of W(CO)6 is added to the above mixture and stirred for 5 min. The

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resulting homogeneous solution is transferred into a 12-mL Teflon-lined autoclave

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and heated at 160 °C for 4 h. After being cooled to RT, the product is isolated by

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centrifugation, and washed with a ethanolcyclohexane mixture several times. The

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as-obtained NSs are re-dispersed in a nonpolar organic solvent (hexane or

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cyclohexane), thus obtaining the PdW-S1 NSs.

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Synthesis of PdW-S2 NSs. 0.02 mmol of Pd(acac)2 is dissolved in 1-octadecene (5

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mL) and oleylamine (3.5 mL), the mixture is stirred at RT for 20 min. Then, 0.17

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mmol of L-ascorbic acid and 0.14 mmol of W(CO)6 are added to the above mixture

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and stirred for 5 min. The resulting homogeneous solution is transferred into a 12-mL

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Teflon-lined autoclave and heated at 160 °C for 4 h, thus obtaining the PdW-S2 NSs.

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For comparison purposes, Pd nanoparticles (NPs) with a size of approximately 5.9

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nm are fabricated according to the procedures described in the literature.20 The

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as-synthesized PdW and Pd nanocrystals (NCs) were loaded on the nanosized

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commercial TiO2. The supported catalysts are prepared according to the following

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procedures: An appropriate amount of Pd NPs (theoretical palladium loading is 0.8

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wt%) is loaded on TiO2 in 20 mL of cyclohexane under stirring at RT for 12 h. After

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centrifugation and washing twice with ethanol, the obtained wet powders are dried in

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a vacuum drying chamber. Finally, the dried powders are transferred into a muffle

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furnace for calcination at 350 oC for 2 h. The as-prepared supported catalysts are

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denoted as TiO2/PdW-S1, TiO2/PdW-S2, and Pd/TiO2, respectively.

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Catalyst Characterization. All of the catalysts are characterized by means of

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techniques, such as X-ray diffraction (XRD), high angle annular dark field

scanning

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transmission electron microscopy (HAADF-STEM), and in situ diffuse reflectance

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Fourier transform infrared spectroscopy (in situ DRIFT). The detailed characterization

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procedures are described in the Supporting Information.

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Catalytic Evaluation. Activities of the catalysts for benzene oxidation are

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evaluated in a continuous flow fixed-bed quartz microreactor. The reactant feed is

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1000 ppm benzene + 20 vol% O2 + N2 (balance), and the space velocity (SV) is ca. 40

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000 mL g1 h1. The detailed evaluation procedures are stated in the Supporting

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Information.

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RESULTS

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Morphology, Crystal Structure, and Surface Area. The structures of the

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catalysts are determined by the TEM and HAADFSTEM technique, and the results

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are presented in Figures S1‒S3. Apparently, both PdW-S1 and PdW-S2 NSs display

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the high two-dimensional (2D) anisotropy with a lateral dimension in the

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sub-micrometer range. Aberration-corrected HAADFSTEM and corresponding

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elemental mappings of PdW-S1 are shown in Figure 1. Figure 1A and B shows the

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crumpled structure of NSs with a typical curly and thicken edges, and thickness of the

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edges is measured to be 1.24 and 2.57 nm (Figure 1C), respectively. Therefore, it can

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be deduced that thickness of the NSs is about 1 nm or even less than 1 nm. As shown

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in Figure 1D and E, several dispersed bright dots corresponding to W atoms can be

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clearly distinguished from the palladium substrate because of a different Z-contrast.

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The elemental mappings of the PdW-S1 NSs (Figure 1F) also reveal that W atoms

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(tungsten single atoms and/or WOx clusters) are well dispersed on the surface of Pd

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NSs. Indeed, the high-resolution STEM images clearly show existence of the isolated

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tungsten species with a high dispersion on the surface of NSs, as can be seen from the

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additional atomic STEM images within different regions of PdW-S1 NSs (Figure S4).

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In our present work, nanocrystal synthesis is carried out in the absence of inert gas

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protection, and isolation of nanocrystals are further undertaken in air, and no

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pretreatments are made prior to the TEM and XRD measurements. According to the

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literature,21 oxidation of surface W atoms is inevitable when the nanocrystals are

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exposed to the ambient atmosphere. Therefore, the PdW bimetallic nanosheet may be

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composed of a large amount of amorphous WOx clusters attached on palladium

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surface, and only a small amount of metallic tungsten atoms embedded in palladium

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substrate. As shown in Figure S5, when PdW-S1 is loaded on TiO2 and after

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calcination, the TiO2/PdW-S1 nanocomposite still possesses uniform distributions of

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Pd, W, and Ti elements (Figure S5(B‒E)). Most of the original Pd sheets seem to roll

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up (instead of to tile) to closely contact with TiO2. We also investigated the influence

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of different tungsten contents on the morphology of the as-obtained samples. As

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shown in Figure S6, the dispersity of the PdW-S3 (with a lower tungsten content)

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derived from the same method for PdW-S1 synthesis is much less than that of the

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PdW-S1 sample. This may be due to the fact that the CO released from W(CO)6 acts

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as a protective agent to be adsorbed on the palladium surface to protect the ultrathin

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nanosheet from aggregation. If the amount of hexacarbonyltungsten is too low, the

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highly dispersed ultrathin PdW nanosheets can not be formed under the same reaction

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conditions.

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The XRD patterns of the as-obtained PdW and Pd NCs are shown in Figure S1(E).

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The Pd NPs, PdW-S1, and the PdW-S2 NSs display three diffraction peaks at 2θ =

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40.1o, 46.7o, and 68.1o, corresponding to the (111), (200), and (220) planes of the Pd

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phase (JCPDS PDF# 46-1043). Figure S1(F) shows the XRD patterns of the

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supported Pd-based catalysts. All of the catalysts exhibit several diffraction peaks

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characteristic of the anatase TiO2 phase. Furthermore, there are no diffraction peaks

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corresponding to the palladium or tungsten-containing crystal phases owing to the

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lower Pd and W loadings and high TiO2 dispersion on the bimetallic NSs. Figure S7

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shows the Raman spectra of Pd/TiO2, TiO2/PdW-S1, and TiO2/PdW-S2. The Raman

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bands at 142, 392, 513, and 636 cm1 are assigned to the anatase TiO2 phase, but other

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species are not detected.22 The nitrogen adsorptiondesorption isotherms and

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pore-size distributions of the catalysts are shown in Figure S8. The BET surface areas

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of the supported catalysts are approximately 86 m2/g and their pore sizes are around

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10 nm.

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Catalytic Performance. Benzene, widely used as an industrial solvent, is one of

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typical VOCs. We evaluate activities of the catalysts for benzene oxidation. The

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TiO2/PdW-S catalysts exhibit better activity than the Pd/TiO2 catalyst, among which

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TiO2/PdW-S1 shows the best catalytic activity (T50% = 178 oC and T90% = 200 oC).

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Activity of the TiO2/PdW-S catalysts increases with a rise in tungsten doping, and is

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higher than those reported previously.23,24 To estimate the inherent performance of the

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catalysts, apparent activation energies (Ea) and turnover frequencies (TOFs) are

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calculated. The reaction with a lower Ea can take place more readily over the catalyst.

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Hence, the Ea is calculated to compare the low-temperature activity of the as-obtained

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catalysts. The Arrhenius equation was k = Aexp(Ea/RT), where k, A, and Ea represent

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the rate constant (s1), pre-exponential factor, and apparent activation energy (kJ

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mol1), respectively. Figure 2B shows the Arrhenius plots as a function of reaction

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temperature over the catalysts at SV = 40 000 mL g‒1 h‒1, and the calculated Ea data

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are provided in Table 2. The Ea increases in the sequence of TiO2/PdW-S1 ˂

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TiO2/PdW-S2 ˂ Pd/WOx/TiO2 ˂ PdW/TiO2 < Pd/TiO2. The increasing trend in Ea is

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consistent with the improvement in low-temperature activity of the catalysts. The

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TOF can be calculated according to TOF = xC0/nPdDPd, where x is benzene conversion,

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C0 (mol/(g s)) is initial benzene concentration, n (mol/g) is Pd molar amount, and DPd

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is palladium dispersion determined by CO chemisorption. Benzene oxidation rates are

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calculated according to the activity data and Pd amounts in the catalysts, and the

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results are summarized in Table 2. Among all of the catalysts, the TiO2/PdW-S1

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catalyst exhibits the highest TOFPd of 22.2 × 10‒3 s‒1 and the highest benzene

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oxidation rate of 13.8 μmol gPd1 s1 at 170 oC, which are much higher than those over

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TiO2/PdW-S2 (7.5 × 10‒3 s‒1 and 5.8 μmol gPd1 s1), Pd/WOx/TiO2 (2.0 × 10‒3 s‒1 and

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3.4 μmol gPd1 s1), PdW/TiO2 (3.0 μmol gPd1 s1), and Pd/TiO2 (1.0 × 10‒3 s‒1 and

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1.8 μmol gPd1 s1), respectively.

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To probe the influence of moisture on activity during the catalytic reaction process,

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we conduct 40-h on-stream benzene oxidation in the presence of 1.0 or 5.0 vol%

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water vapor over the TiO2/PdW-S1, TiO2/PdW-S2, and Pd/TiO2 catalysts, and the

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results are shown in Figure 3. The TiO2/PdW-S catalysts show much better

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water-resistant ability than the Pd/TiO2 catalyst. Introduce of 1.0 vol% H2O almost

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does not affect the catalytic activity of TiO2/PdW-S1, only a slight decrease by ca.

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5 % in benzene conversion is detected over the TiO2/PdW-S2 catalyst; however,

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benzene conversion decreases by ca. 20 % over the Pd/TiO2 catalyst after a certain

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reaction time. After water vapor was cut off, benzene conversion almost returned to

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the initial value over all of the catalysts. When 5.0 vol% H2O is introduced, benzene

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conversion over Pd/TiO2 decreases by ca. 30 %, which is more than that over

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TiO2/PdW-S2 (ca. 10 %) and TiO2/PdW-S1 (ca. 2 %). When water vapor was cut off,

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the catalytic activity is almost restored, proving that the drop in activity caused by

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water vapor is reversible.

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To further examine poisoning effect of the catalyst, we also perform benzene

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oxidation in the presence of 5 % CO2 and SV = 40 000 mL g‒1 h‒1 over TiO2/PdW-S1

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and Pd/TiO2, and the results are shown in Figure S9. When 5 vol% CO2 is added,

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benzene conversion does not change obviously over TiO2/PdW-S1, but decreases by

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ca. 11 % over Pd/TiO2 after 40 h of on-stream reaction. After the two catalysts are

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treated in oxygen at 300 oC for 1 h, their activities are recovered to the original values

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in the absence of CO2. When CO2 is introduced again, the activity of the two catalysts

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show the similar change trend, as compare with the fresh counterparts. The results

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indicate that the TiO2/PdW-S1 catalyst exhibit better carbon dioxide-resistant ability

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than the Pd/TiO2 catalyst.

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Surface Composition, Metal Chemical State, and Oxygen Species. XPS is used

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to gain surface properties of the catalysts. Figure 4 shows the Pd 3d5/2, W 4f, and O 1s

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XPS spectra of the catalysts. As shown in Figure 4A, the asymmetrical signal of Pd

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3d5/2 is decomposed into two components at binding energy (BE) = 334.8 and 336.5

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eV, ascribable to the surface Pd0 and Pd2+ species,25,26 respectively. Table 1 lists the

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surface Pd0/Pd2+ molar ratios of the catalysts. The Pd0/Pd2+ molar ratios (0.16‒0.21)

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of the TiO2/PdW-S catalysts are much higher than that (0.05) of the Pd/TiO2 catalyst,

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demonstrating that the Pd in TiO2/PdW-S is present in a mixed oxidation state,

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whereas Pd2+ is the dominant species in the Pd/TiO2 catalyst. As presented in Figure

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4B, the asymmetrical W 4f spectra of the TiO2/PdW-S catalysts are deconvoluted into

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five components at BE = 34.2, 35.7, 36.3, 37.0 and 38.1 eV, in which the two

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components at BE = 35.7 and 38.1 eV are due to the 4f7/2 and 4f5/2 final states of the

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surface W6+ species,27,28 while the two components at BE = 34.2 and 36.3 correspond

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to the 4f7/2 and 4f5/2 final states of the surface W5+ species,27,28 and the peak at BE =

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37.0 eV could be ascribed to the final state of the Ti 3p signal. Figure 4C illustrates

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the O 1s spectra of the catalysts. The oxygen species at BE = 530.0, 531.5, and 533.2

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eV are attributed to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads, e.g. O2,

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O22, O or OH), and adsorbed water species,29,30 respectively. The results of XPS

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characterization demonstrate that Pd/TiO2 and TiO2/PdW-S possess a similar amount

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of the surface adsorbed oxygen species.

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Low-Temperature Reducibility. The H2-TPR experiment was conducted to study

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reducibility of the supported Pd-based catalysts, and their H2-TPR profiles and H2

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consumption are shown in Figure 5A and Table 1, respectively. There is only one

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broad reduction peak at 278 oC of the Pd/TiO2 catalyst, which can be assigned to the

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reduction of Pd2+ to Pd0.31 For the supported bi-metallic catalysts, the first peak of the

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TiO2/PdW-S catalysts is divided into three peaks: the one at 229 oC is attributable to

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the reduction of Pd2+ to Pd0, the one at 260 oC is due to the reduction of a small

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amount of PdOx located at the interface between the active component and support

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(which would have a strong interaction with WOx and TiO2), and the ones centered at

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352 and 339 oC are ascribable to the reduction of WOx homogeneously embedded in

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the palladium substrate.32 As the tungsten loading increases, the reduction peak of

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WOx slightly shifts to a lower temperature. The reduction peak centered at 497 oC can

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be assigned to reduction of the surface Ti4+ to Ti3+ via the spillover of H atoms

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chemically adsorbed on the reduced Pd. A more amount of hydrogen spillover

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represents a stronger metalsupport interaction.33 Therefore, there is a stronger

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interaction between the PdW NSs and TiO2 support. As for the Pd/WOx/TiO2 catalyst,

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there is only one broad reduction peak centered at 294 oC, ascribable to the reduction

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of Pd2+ to Pd0 and the reduction of a small amount of WOx, in which the reduction

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temperature is slightly higher than that of the Pd/TiO2 catalyst. As shown in Table 1,

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the calculated H2 consumption of TiO2/PdW-S is much higher than that of Pd/TiO2,

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suggesting that part of surface WOx and/or Ti4+ species are reduced. Obviously, the

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reduction is promoted by presence of the unsaturated WOx clusters and close-contact

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PdOx‒WOx‒TiO2 via a hydrogen spillover process.

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Oxygen Mobility. The O2-TPD experiment is also conducted to estimate the

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amount of oxygen species and the mobility of lattice oxygen, and the results are

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shown in Figure 5B. The peak at 80 oC can be attributed to desorption of the

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physically absorbed oxygen species,34 the one at 200 oC is due to desorption of the

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surface chemisorbed oxygen species,35 and the ones at 350‒550 oC are mainly

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ascribed to desorption of the active lattice oxygen from PdOx and WOx clusters.36

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Addition of tungsten oxide species can increase the oxygen (especially the active

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lattice oxygen) desorption amount. It should be mentioned that the peak attributable to

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desorption of the lattice oxygen species from PdOx in the TiO2/PdW-S1 sample shifts

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to a lower temperature, indicating the enhancement of oxygen mobility.

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Acidity. Acidic properties of the catalysts are investigated using the NH3-TPD

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technique, and their profiles are shown in Figure S10. The peaks below 200 oC are

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related to the weak acid sites, the ones in the range of 270–400 oC are ascribed to the

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medium acid sites, and the ones in the range of 420–600 oC belong to the strong acid

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sites,37,38 respectively. The Pd/TiO2 catalyst presents three peaks at 277, 434, and 477

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oC,

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noticing that the TiO2/PdW-S catalysts display a weak peak below 200 oC, which can

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refer to the weak acid sites. In addition, desorption peaks of the WOx-containing

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catalysts belong to the strong acid sites, and the peak position shifts to higher

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temperatures (491 and 518 oC, respectively), indicating that the acidic strength is

corresponding to the medium and strong acid sites, respectively. It is worth

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enhanced after the doping of tungsten species. Introduction of WOx leads to a

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decrease in the amount of acid sites because of the exchanging of acidic protons

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(Brønsted acid sites) with ionic tungsten species entering into crystal TiO2 support.

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Since intensity of the NH3-TPD peaks can directly reflect the amount of surface

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acidity, all of the peaks in the TPD profiles are quantitatively analyzed. If the area of

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total NH3 desorption peak of Pd/TiO2 is assumed to be 1.00, the amount of NH3

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desorption (0.660.72) of the TiO2/PdW-S catalysts decreases. The data reported here

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indicate that benzene oxidation activity is affected by acidic property of the catalyst.

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Reaction Pathway. The in situ DRIFTS experiments are conducted to determine

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the intermediate species during benzene oxidation process, and the spectra are shown

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in Figure 5C, D, and S11. TiO2/PdW-S1 and Pd/TiO2 are taken as examples to obtain

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more evidence of benzene oxidation steps. As shown in Figure 5C and D, the bands at

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672 and 848 cm1 are due to the C–H deformation vibrations, and the bands at 3036

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and 3093 cm1 are due to the C–H stretching vibrations in the benzene rings.39 With a

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rise in reaction temperature, some new IR bands appear. The similar bands at 1189

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and 1200 cm1 are attributed to the C–O stretching vibrations. The bands at 1432 cm1

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is ascribed to the maleate species,40 and the one at 1342 cm1 is assigned to the CH3

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stretching vibration of the acetate species.41 The band at 1506 cm1 is due to the

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skeleton stretching vibrations of the aromatic ring,42 the one at 1584 cm1 is

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attributable to the C=C stretching vibrations of the phenolate species,43 and the one at

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1714 cm1 is assignable to the C=O stretching vibrations of the quinone or other

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ketone species.40 The above results reveal that the adsorbed phenol, benzoquinone,

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and maleate (acetate) species are formed during benzene oxidation process. For the

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TiO2/PdW-S1 catalyst, a relatively weak band corresponding to the phenol species is

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detected, and the typical bands assignable to the benzoquinone species are clearly

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observed, indicating that the phenolate species rapidly transform into the quinone

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species. For the Pd/TiO2 catalyst, however, the characteristic bands of the phenolate

315

and quinone species are clearly observed, suggesting that the two main intermediate

316

species are strongly adsorbed on the surface of the supported Pd catalyst. Figure

317

S11(A) and (B) shows desorption behaviours of benzene at RT over the Pd/TiO2 and

318

TiO2/PdW-S1 catalysts. The bands at 1479 and 1599 cm1 are assigned to the skeleton

319

stretching vibrations of the benzene ring, 42 whereas the ones at 1812 and 1963 cm1

320

are attributed to the C–H out-of-plane bending.39 With the extension of desorption

321

time, the bands referring to the benzene features disappear.

322

An additional in-situ DRITF study is also performed over the as-obtained samples

323

at 200 oC in the presence or absence of water. As can be seen from Figure 6, when

324

water is introduced, a broad peak appears at 3200‒3800 cm‒1, which is attributable to

325

the hydroxyl species adsorbed on the sample surface. The peak at 3566 cm‒1 is

326

assigned to the hydroxyl species,18 and the one at 3717 cm‒1 is attributed to the

327

bridging OH species.44,45 The Pd/TiO2 sample shows the OH species with higher

328

intensity than the TiO2/PdW-S1 sample, and after water is cut off, the peaks

329

assignable to the OH species almost disappear on the TiO2/PdW-S1 sample, whereas

330

the OH desorption proceeds more slowly from the Pd/TiO2 sample. Therefore, it is

331

possible that accumulation of the surface hydroxyls inhibits benzene conversion over

15



332

the Pd/TiO2 sample, and the TiO2/PdW-S1 sample exhibits better water tolerance

333

ability, as compared with the Pd/TiO2 sample.

334

DISCUSSION

335

Generally speaking, total oxidation of VOCs over the transition-metal oxide

336

catalysts follows a Mars–van Krevelen (MVK) mechanism, in which lattice oxygen is

337

involved in the combustion process and the reduced transition-metal oxide is

338

re-oxidized by gas-phase O2 molecules. For instance, catalytic performance of Co3O4

339

was superior to that of the other transition-metal oxides for CO oxidation, a result due

340

to the lowest metal–oxygen bond energy of Co3O4.46 Tang et al. observed existence of

341

a synergistic action between manganese and cerium oxides in the MnOx–CeO2

342

composite catalyst for formaldehyde oxidation.47 As shown in the H2-TPR and

343

O2-TPD results of the present work, the downshift in reduction temperature and lattice

344

oxygen desorption after doping of tungsten oxide clusters can be ascribed to the facile

345

redox cycle and enhanced lattice oxygen mobility. Therefore, participation of lattice

346

oxygen in benzene oxidation is supported by the H2-TPR and O2-TPD results. In

347

addition, the higher H2 consumption of TiO2/PdW-S indicates that more amounts of

348

lattice oxygen in WOx and TiO2 can be extracted at lower temperatures.

349

The electronic state of palladium in a supported noble metal catalyst plays a key

350

role in influencing its catalytic performance for the combustion of organics. For

351

instance, Bell group found that catalytic activity of PdO for CH4 combustion could be

352

enhanced by producing a small amount of metallic Pd0 on the surface of PdO.48

353

Bounechada et al. pointed out that the presence of a Pd2+/Pd0 couple (rather than a

16


Page 16 of 40

Page 17 of 40


354

fully oxidized Pd2+) was responsible for the enhancement in CH4 conversion.49 The

355

intrinsic stability of Pd0 was the reason for high activity of Pd/CoO for the oxidative

356

removal of o-xylene.50 In the present work, Pd2+ is the dominant palladium species in

357

the Pd/TiO2 catalyst but both metallic Pd0 and oxidized Pd2+ species co-exist in the

358

TiO2/PdW-S catalysts. In other words, Pd0 species is stabilized by the close-contact

359

WOx islands on the NSs in an oxidative atmosphere. Thus, the reason for the high

360

catalytic activity can be ascribed to existence of the Pd2+/Pd0 couple in the

361

TiO2/PdW-S catalysts.

362

The distributions of acidity in the catalysts are measured using the NH3-TPD

363

technique, as shown in Figure S10. The Pd/TiO2 and TiO2/PdW-S catalysts display a

364

strong desorption peak ( 400 oC), which can be ascribed to strong Brønsted acid sites

365

of the protons (H+) in catalysts.51 As described in the previous work, PdOx is regarded

366

as a basic oxide, which can be mounted on the Brønsted acid sites by the acid−base

367

driving force. For example, Lou et al. found that the acid−base interaction between

368

PdO and acid sites of the zeolite promoted the fixation of highly dispersed PdO.52 On

369

the basis of this finding, the oxygen from the PdOx is strongly anchored at Brønsted

370

acid sites of the TiO2 support and cationic palladium maintains in a high valence state.

371

The mixture of metallic Pd0 and oxidized Pd2+ (PdO) in the TiO2/PdW-S catalysts

372

with moderate surface acid amounts is more active than the pure oxidized Pd2+ (PdO)

373

(Table 1). The experimental results prove that the decrease in amount of Brønsted

374

sites leads to an increase in benzene conversion, since surface acidic sites can

375

facilitate formation of the active sites and further modify the electronic property of

17



376

noble metal.

377

According to the above results, a synergistic action between Pd(O)‒WOx‒TiO2 is

378

present. Figure 7 shows the benzene oxidation mechanism. On the catalyst surface,

379

benzene first reacts with the surface active lattice oxygen species to produce the

380

phenolate species. Then, the phenolate species are easily transformed into the

381

benzoquinone (e.g., o-benzoquinone and p-benzoquinone) species. The carboxylate

382

(e.g., acetate and maleate) species are generated via the breaking of benzene rings

383

which are attacked by the active oxygen species. Finally, the carboxylate species are

384

oxidized into CO2 and H2O, and the catalyst is recovered.

385

Although the supported palladium catalysts show excellent catalytic activity for

386

VOCs oxidation, the reversible inhibition by H2O adsorption causes a significant drop

387

in catalytic activity, especially at low temperatures. The deactivation was thought to

388

be associated with the existence of PdO (s) + H2O (g)  Pd(OH)2 (s), where the PdO

389

represents an active phase, whereas the Pd(OH)2 is an inactive phase.53 In the present

390

work, however, deactivation by Pd(OH)2 formation is unlikely in the lower reaction

391

temperatures, which is in agreement with the observation that Pd(OH)2 in Pd(OH)2/C

392

could decompose in N2 at 250 oC.54 Using the isotopic labeling technique, Schwartz

393

and Pfefferle55 pointed out that Pd/PdO was an active site for molecular oxygen to

394

dissociate, migrate to the support, and exchange with oxygen from the support. The

395

migration and exchange process allowed oxygen originally located at the support to

396

participate in the catalytic oxidation process, and adsorbed water molecules inhibited

397

the exchange of oxygen between the support and PdO. Ciuparu et al.56 reported that

18


Page 18 of 40

Page 19 of 40


398

the inhibition effect of H2O was dependent upon the oxygen mobility of the support.

399

Comparing PdO supported on metal oxides (surface oxygen mobility increased in the

400

order of Al2O3 < ZrO2 < Ce0.1Zr0.9O2), the H2O-resistant ability increased in the same

401

trend in methane combustion. As evidenced from the FTIR results, the H2O inhibition

402

mechanism is likely via the hydroxyls blocking of surface re-oxidation, and bulk

403

oxygen and oxygen from the support could be used for re-oxidation of the PdO

404

surface after reduction by organic molecules. Based on the results reported in the

405

literature and in the present work, we propose that the support with a higher oxygen

406

mobility can decrease the inhibitory effect of H2O on the Pd-based catalysts for

407

benzene oxidation. If the support possesses a lower oxygen mobility, the PdO NCs

408

rapidly become oxygen-deficient, and re-oxidation of the reduced catalyst becomes

409

difficult, since the surface vacancies exposed to the reaction environment are easily

410

blocked by the adsorbed hydroxyls. On the other hand, the support with a higher

411

oxygen mobility can exchange oxygen with the PdOx NCs to reach the oxygen

412

balance of the PdO phase. For the supported bimetallic catalyst, existence of PdO/Pd

413

at the interface between WOx and TiO2 favors the enhancement in oxygen mobility.

414

On the contrary, PdO on TiO2 possesses lower oxygen mobility, leading to

415

suppression of catalytic activity because of accumulation of hydroxyls on the catalyst

416

surface. The results of the in-situ DRIFT characterization provide the evidence of less

417

extensive surface hydroxyl formation and faster water desorption on the PdW

418

bimetallic catalysts. Hence, formation of the PdOx‒WOx‒TiO2 nanocomposite in

419

TiO2/PdW-S1 influences the water deactivation and regeneration.

19



Page 20 of 40

420

In conclusion, the supported palladium–tungsten bimetallic catalysts with a high

421

catalytic activity and a good water-resistant ability are successfully developed for the

422

oxidative removal of benzene. The TiO2/PdW-S1 catalyst performing the best for

423

benzene oxidation: the T90% was 200 oC (SV = 40 000 mL g1 h1), and the TOFPd was

424

22 times higher than that over the Pd/TiO2 sample. Such a high activity of this catalyst

425

is due to presence of a facile redox cycle of the active mixed Pd2+/Pd0 couple, which

426

is responsible for the enhancement in benzene conversion. Deactivation of the

427

supported Pd catalysts by water vapor (benzene conversion over Pd/TiO2 decreases by

428

ca. 30 % with 5.0 vol% H2O) is due to the fact that the adsorbed hydroxyls suppress

429

the oxygen exchange between the support and palladium active sites. The ternary

430

mixed oxide with excellent oxygen mobility can decrease the inhibitory effect of H2O

431

for benzene oxidation (benzene conversion over TiO2/PdW-S1 decreases by ca. 2 %

432

with 5.0 vol% H2O). Based on the results, a four-step reaction‒transformation

433

mechanism promoted by the bimetallic catalysts is proposed. The novel bimetallic

434

NCs with a 2D structure are considered to be a potential candidate for catalytic

435

oxidation applications.

436

ASSOCIATED CONTENT

437

The Supporting Information is available free of charge on the ACS Publications

438

website at DOI: 10.1021/acs.est.XXXXXXX.

439

Catalyst preparation; catalyst characterization procedures; catalytic evaluation

440

procedures; TEM images of PdW-S1 NSs and PdW-S2 NSs; XRD patterns of Pd NCs,

441

PdW-S2

NSs,

PdW-S1

NSs,

Pd/TiO2,

TiO2/PdW-S2,

20


TiO2/PdW-S1,

and

Page 21 of 40


442

Pd/WOx/TiO2; TEM images and corresponding EDS elemental mapping images of

443

PdW-S1 NSs and PdW-S2 NSs; TEM images of the as-obtained Pd NPs;

444

Aberration-corrected HAADFSTEM images and corresponding EDS elemental

445

mappings

446

aberration-corrected HAADFSTEM images of PdW-S1 NSs; NH3-TPD profiles of

447

the as-obtained catalysts; Raman spectra of Pd/TiO2, TiO2/PdW-S2, and

448

TiO2/PdW-S1; nitrogen adsorptiondesorption isotherms and pore-size distributions

449

of Pd/TiO2, TiO2/PdW-S2, TiO2/PdW-S1, and Pd/WOx/TiO2; effect of 5 vol% CO2 on

450

catalytic activity of TiO2/PdW-S1 and Pd/TiO2 and SV = 40 000 mL g1 h1; In-situ

451

DRIFT spectra of benzene adsorption at 30 oC in a N2 flow of 30 mL min1 on

452

Pd/TiO2 and TiO2/PdW-S1, which are pretreated in a gas mixture (1000 ppm benzene

453

+ 20 vol% O2 + N2 (balance)) flow of 16.7 mL min1 at 30 oC (PDF).

454

AUTHOR INFORMATION

455

Corresponding Author

456

*Telephone: +86-10-6739-6118. Fax: +86-10-6739-1983. E-mail: [email protected].

457

ORCID

458

Hongxing Dai: 0000-0003-1738-0348

459

Notes

460

The authors declare no competing financial interest.

461

ACKNOWLEDGMENTS

462

This work was supported by the National Natural Science Foundation of China

463

(21876006, 21677004, 21607005, and 21622701), National Key R&D Program of

of

TiO2/PdW-S1;

TEM

images

of

21


PdW-S3

NSs;

additional


464

China (2016YFC0204800), Natural Science Foundation of Beijing Municipal

465

Commission of Education (KM201710005004).

466

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(47) Tang, X. F.; Chen, J. L.; Huang, X. M.; Xu, Y. D.; Shen, W. J. Pt/MnOx–CeO2

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catalysts for the complete oxidation of formaldehyde at ambient temperature. Appl.

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(48) Carstens, J. N.; Su, S. C.; Bell, A. T. Factors affecting the catalytic activity of Pd/ZrO2 for the combustion of methane. J. Catal. 1998, 176, 136–142.

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(49) Bounechada, D.; Groppi, G.; Forzatti, P.; Kallinen, K.; Kinnunen, T. Effect of

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periodic lean/rich switch on methane conversion over a Ce-Zr promoted Pd-Rh/Al2O3

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catalyst in the exhausts of natural gas vehicles. Appl. Catal., B 2012, 119–120, 91–99.

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nanocatalysts with high performance for o-xylene combustion. Catal. Sci. Technol.

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catalytic combustion of methane over PdO/ZSM-5: Dominant or negligible? J. Catal.

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2018, 357, 29–40.

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644

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645

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30


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Table 1. BET Surface Areas, Actual Metal Loadings, Surface Element Compositions, and Reducibility of the As-Obtained Catalysts. BET surface area (m2/g)

Actual Pd loadinga (wt%)

Actual W loadinga (wt%)

TiO2/PdW-S1

86.0

0.65

TiO2/PdW-S2

86.7

Pd/TiO2 Pd/WOx/TiO2

Catalyst

Surface element compositionb (mol/mol)

H2 consumptionc (μmol/g)

Pd0/Pd2+

Oads/Olatt

W6+/W5+

0.93

0.21

0.35

1.70

39.1

0.67

0.13

0.16

0.36

1.37

30.6

86.9

0.67

–d

0.05

0.34

–d

9.89

86.6

0.63

0.98

–d

–d

–d

13.4

a

Determined by the ICPAES technique; Determined by the XPS technique; c Calculated by quantitatively analyzing the reduction peaks of the H -TPR profiles of the catalysts; 2 d Not determined. b

31



Page 32 of 40

Table 2. Catalytic Activities, Specific Reaction Rates at 170 oC, and Apparent Activation Energies (Ea) of the As-Obtained Catalysts for Benzene Conversion at SV = 40 000 mL g1 h1.

Benzene conversion at 170 oC

Benzene conversion Catalyst

a Not

Specific reaction rate

Metal

TOFPd

dispersion

Ea (kJ/mol)

T50% (oC)

T90% (oC)

TiO2/PdW-S1

178

200

13.8

22.2

6.6

56

TiO2/PdW-S2

203

215

5.8

7.5

8.2

62

Pd/TiO2

224

232

1.8

1.0

18.9

69

Pd/WOx/TiO2

212

228

3.4

2.0

18.1

65

PdW/TiO2

215

235

3.0

‒a

‒a

67

(×

106

mol gPd

1 s1)

determined.

32


(×

103 s1)

(%)

Page 33 of 40


(A)

100 nm (E)

(F)

(F-1)

Pd

(F-2)

W

10 nm

Figure 1. Aberration-corrected HAADF‒STEM images (A‒E) and the corresponding EDS elemental mappings (F) of the PdW-S1 NSs sample. W single atoms (white circles) are seen to be uniformly dispersed on the Pd sheet.

33



Figure 2. (A) Benzene conversion and (B) Arrhenius plots as a function of reaction temperature over the catalysts at SV = 40 000 mL g1 h1.

34


Page 34 of 40

Page 35 of 40


Figure 3. Effect of water vapor with different concentrations on the catalytic activity of (A) TiO2/PdW-S1 and TiO2/PdW-S2, and (B) Pd/TiO2 and SV =40 000 mL g1 h1.

35



(A)

(B)

Pd 3d5/2

336.5

35.7

Page 36 of 40

(C)

W 4f

36.3

530.0

O 1s

37.0 34.2

531.5

38.1

533.2

TiO2/PdW-S2

Pd/TiO2

Intensity (a.u.)

Pd/TiO2

Intensity (a.u.)

Intensity (a.u.)

334.8

TiO2/PdW-S2

TiO2/PdW-S2

TiO2/PdW-S1

TiO2/PdW-S1

332

334

336

338

Binding energy (eV)

340

30

32

34

36

38

40

Binding energy (eV)

42

TiO2/PdW-S1

44

528

530

532

Binding energy (eV)

Figure 4. (A) Pd 3d5/2, (B) W 4f, and (C) O 1s XPS spectra of the as-obtained catalysts.

36


534

536

Page 37 of 40


(B)

(A)

294oC

483 oC o

80 C

Pd/WOx/TiO2

Pd/WOx/TiO2

o

483 C o 267oC 352 C 229oC

TiO2/PdW-S1 497oC

425 oC

Intensity (a.u.)

Intensity (a.u.)

260oC 339oC

TiO2/PdW-S1 o

435 C

511 oC TiO2/PdW-S2

TiO2/PdW-S2

278oC

Pd/TiO2

411 oC Pd/TiO2

100

200

300

400

500

600

700

800

50

900

150

250

1503 1412 1585 1342 1720 1199

650

750

672 1495 1432

1589 1724

852

3036 2871

o

240 C 220oC 200oC 180oC 160oC 130oC 100oC

3500

550

(D)

Absorbance (a.u.)

Absorbance (a.u.)

672

2965 3088

450

Temperature ( C)

Temperature ( C) (C)

350

o

o

852

1342 1194

3036 3088 2965 2871 240oC 220oC 200oC 180oC 160oC 100oC

3000

2500

2000

1500

1000

3500

500

3000

2500

2000

1500

1000

-1

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 5. (A) H2-TPR profiles and (B) O2-TPD profiles of the as-obtained catalysts, in situ DRIFT spectra of (C) Pd/TiO2 and (D) TiO2/PdW-S1 during benzene oxidation process at different temperatures (reaction conditions: 1000 ppm benzene + 20 vol% O2

+

N2

(balance);

SV

=

40

37


000

mL

g1

h1).

500


(A)

(B) 3566

(c) (b)

3566

Absorbance (a.u.)

3717

(d)

3717

(d) Absorbance (a.u.)

Page 38 of 40

(c)

(b)

(a)

(a)

4000

3800

3600

3400

3200

3000

2800

4000

3800

-1

3600

3400

3200

3000

2800

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 6. In-situ DRIFT spectra of benzene oxidation in a 1000 ppm benzene + 20 vol % O2 + N2 (balance) flow of 16.7 mL/min at 200 oC over (A) Pd/TiO2 and (B) TiO2/PdW-S1: (a) baseline, (b) in the absence of water, (c) in the presence of 5 vol% water, and (d) after water is cut off for 1 h.

38


Page 39 of 40


Figure 7. Proposed mechanism of benzene oxidation over the TiO2/PdW-S catalyst.

39



Page 40 of 40

Benzene conversion (%)

TOC: 110 100 90 80 70 60 50 40 30 20 10 0

Pd(O)-WOx-TiO2

T = 200 oC 1.0 vol% H2O off 1.0 vol% H2O on

5.0 vol% H2O off T = 220 oC

Pd(O)-TiO2

5.0 vol% H2O on

+

+

+ 0

5

10

15

20

25

30

On-stream reaction time (h)


35

40

TiO2 Hybrids

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