Phosphate-Functionalized CeO2 Nanosheets for Efficient Catalytic

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Remediation and Control Technologies

Phosphate-Functionalized CeO2 Nanosheets for Efficient Catalytic Oxidation of Dichloromethane Qiguang Dai, Zhiyong Zhang, Jiaorong Yan, Jinyan Wu, Grayson Johnson, Wei Sun, Xingyi Wang, Sen Zhang, and Wangcheng Zhan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05002 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Environmental Science & Technology

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Phosphate-Functionalized CeO2 Nanosheets for

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Efficient Catalytic Oxidation of Dichloromethane

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Qiguang Dai,†, # Zhiyong Zhang, ‡, # Jiaorong Yan,† Jinyan Wu,† Grayson Johnson,‡ Wei Sun,†

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Xingyi Wang,† Sen Zhang,*, ‡ and Wangcheng Zhan*, †

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†

Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of

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Chemistry & Molecular Engineering, East China University of Science & Technology, Shanghai

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200237, PR China.

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‡

Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United

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States.

ABSTRACT

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Tuning acid and basic sites nature and profile in the surface of redox-active metal oxide

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nanostructures is a promising approach to constructing efficient catalyst for the oxidative

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removal of chlorinated volatile organic compounds (CVOCs). Herein, using the dichloromethane

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(DCM) oxidation as a model reaction, we report that phosphate (POx) Brønsted acid sites can be

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incorporated onto CeO2 nanosheets (NSs) surface via an organophosphate-mediated route, which

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can effectively enhance the CeO2’s catalytic performance by promoting the removal of chlorine

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poisoning species. Based on the systematic study of the correlation between POx composition,

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surface structure (acid and basic sites) and catalytic properties, we find that the incorporated

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Brønsted acid sites can also function to decrease the amount of medium-strong basic sites (O2-),

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reducing the formation of chlorinated organic by-product monochloromethane (MCM) and

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leading to the desirable product HCl. At the optimized P/Ce ratio condition (0.2), the POx-CeO2

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NSs can perform a stable DCM conversion of 65-70% for over 10 hours at 250°C, and over 95%

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conversion at 300°C, superior to both pristine and other phosphate-modified CeO2 NSs. Our

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work clearly identifies the critical role of acid and basic sites over functionalized CeO2 for

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efficient catalytic CVOCs oxidation, guiding the future advanced catalyst design for

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environmental remediation.

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 INTRODUCTION

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Most of chlorinated volatile organic compounds (CVOCs), such as dichloromethane (DCM),

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1, 2-dichloroethane (DCE), vinyl chloride (VC), trichloroethylene (TCE) and chlorobenzene

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(CB), are deemed as hazardous pollutants that should be stringently monitored and addressed to

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minimize their negative impacts on environment and public health.1-4 Catalytic oxidation (also

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called combustion) is one of the most promising technologies for the removal of CVOCs by

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converting CVOCs to desirable product of HCl, H2O and CO2. In search of an efficient catalyst

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for this oxidative conversion process, various catalytic materials have been studied, such as solid

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acids,5-7 supported noble metals,8-10 and transition metal oxides.11-16 CeO2 is a rare-earth metal

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oxide with an easy conversion between CeO2 and CeO2-x.

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property allows CeO2 to facilely activate O2 and serve as an active O-carrier for the catalytic

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oxidation of many molecules including CVOCs. Its high activity, plus high earth abundance and

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low cost, makes CeO2 an excellent catalyst choice for CVOCs oxidation in mild conditions.11

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However, a discouraging issue associate with CeO2-catalyzed CVOCs oxidation is that inorganic

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chlorine species (Cl-) tend to over-strongly adsorbed on the active sites, leading to a rapid decay

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of CeO2 catalytic performance. Doping CeO2 with first-row transition metal cations (e.g. Mn, Fe,

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Such a unique Ce3+/Ce4+ redox


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or Co)14-16 or loading of Ru24 can alleviate this poisoning issue, as these dopants can function to

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promote the Deacon reaction (oxidation of Cl- into Cl2). But disappointingly, the organic-

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chlorinated by-products will be generated in the presence of highly reactive Cl2, especially at

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higher temperatures. It is of particular importance to develop a new strategy to enhance CeO2

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based catalysts’ ability to remove the poisoning species while maintaining their high activity and

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selectivity for optimized CVOCs oxidation.

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Our previous experimental and theoretical studies reveal that Brønsted acid sites can shuttle

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the protons to facilitate the formation of HCl which are subject to desorption to eliminate Cl-

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poisoning species.25-26 Such a stability enhancement induced by Brønsted acid surface

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functionalization have been observed in our previously reported vanadium oxide-modified CeO2

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catalyst during DCE oxidation.25 But vanadium oxide is highly toxic, prohibiting its practical

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application for environmental catalysis. As a more promising alternative, we envision that

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grafting non-toxic Brønsted acid phosphate species in the surface of CeO2 can also deliver the

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enhanced tolerance to chloride poisoning. Although phosphate-modification has been well-

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established in other oxides studies such as MCM-41 and SBA-15 by reaction with inorganic

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phosphoric acid (H3PO4),27-29 the direct functionalization of CeO2 with H3PO4 is prone to

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generating CePO4 phase in catalyst surface that inhibits the catalyst’s redox property.30-33

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Recently, Slowing et. al. reported that Brønsted acidic sites could be generated by

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immobilization of organophosphates on CeO2 without lowering redox activity.34 In the present

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work, we find that, using the reaction of CeO2 nanosheets (NSs) with trimethylphosphate (TMP)

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and a subsequent calcination, phosphate (POx)-modified CeO2 NSs can be successfully prepared

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with controllable surface POx content and without impurity phases. Since such surface Brønsted

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acid sites are constructed without sacrificing the redox property of CeO2, our POx-modified CeO2


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NSs exhibit high activity and durability for the catalytic oxidation of CVOCs, using DCM as a

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model pollutant. More important, the surface acidity/basicity and redox property of POx-modified

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CeO2 are systematically investigated by employing a suite of probe techniques, including NH3-

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/CO2-temperature programmed desorption (TPD), H2-temperature programmed reduction (TPR)

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and pyridine (Py)-mediated diffuse reflectance infrared Fourier transform spectroscopy

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(DRIFTs). By carefully studying POx content-dependant catalytic properties, we find POx-CeO2

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NSs with a P/Ce ratio of 0.2 is an excellent catalyst for DCM oxidation with balanced activity

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and durability, superior to the pristine CeO2, POx-CeO2 NSs with other compositions and CeO2

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modified with inorganic phosphates (phosphoric acid, ammonium phosphate and sodium

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phosphate). More importantly, we refine the essential roles of Brønsted acid sites (P-OH, in

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removing poisoning species) and basic site (surface lattice O2-, in tuning the formation of

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chlorinated organic by-product) for optimized DCM oxidation performance, which can be

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generalized to other CVOCs catalytic oxidation and beyond, providing a guidance to designing

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and engineering catalyst with functional surface sites for advanced environmental catalysis.

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 EXPERIMENTAL SECTION

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Chemicals and materials. Cerium (III) nitrate hexahydrate (Ce(NO3)3•6H2O), ammonium

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bicarbonate (NH4HCO3), TMP, and sodium phosphate (SP) were purchased from Aladdin

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Industrial Corporation, China. Phosphoric acid (PA) and ammonium phosphate (AP) were

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obtained from Sinopharm Chemical Reagent Co., Ltd, China. Silica (SiO2, amorphous fumed)

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was purchased from Alfa Aesar. All the chemicals were used as-received without any further

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purification.

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Synthesis of ceria nanosheets. In a typical synthesis,35 2.78 g of cerium (III) nitrate

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hexahydrate (Ce(NO3)3•6H2O) and 1.50 g of ammonium bicarbonate (NH4HCO3) were dissolved


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separately, each in 100 ml of deionized water, at 30°C under magnetic stirring. These two

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solutions were then mixed together rapidly, stirred for 0.5 h, and statically aged for 24 h at 30°C.

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The final product was collected by filtration, washed with deionized water to remove any

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possible ionic remnants, dried at 80°C for overnight, and calcined at 500°C for 4 h in flowing air

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to obtain the belt-like CeO2 nanosheets.

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Synthesis of phosphate-modified ceria. Phosphate modified CeO2 nanosheets was

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synthesized through an incipient wetness impregnation route. Typically, a desired amount (the

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molar ratio of P/Ce was controlled to be 0.05, 0.1, 0.2 or 0.3) of TMP, PA, AP or SP was

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dissolved in 1.1 ml of deionized water. 0.86 g of the CeO2 nanosheets was then added to the

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phosphoric solution, statically aged for 24 h at room temperature, dried at 80°C, and calcinated at

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450°C for 4 h with a 5°C/min ramp rate in flowing air. Catalysts prepared with TMP, PA, AP,

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SP were named as POx-CeO2-n, PA-CeO2-n, AP-CeO2-n, and SP-CeO2-n respectively, where n

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is the molar ratio of P/Ce. The same approach was applied to prepare POx-functionalized SiO2.

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Catalyst characterization. Scanning electron microscopy (SEM) images were collected on a

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Hitachi S-3400N at 8.0 kV. Transmission electron microscopy (TEM) images and mapping

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analyses were acquired from a JEM-2100 operated at 200 kV. X-ray powder diffraction (XRD)

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patterns were recorded on a Rigaku D/MAX 2550 VB/PC X-ray diffractometer with Cu Kα

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radiation (λ= 0.154056 nm). The surface areas of catalysts were measured on a Micrometrics

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ASAP 2460 adsorption apparatus at -196°C and calculated by the Brunauer-Emmett-Teller

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(BET) method. FT-IR spectra were obtained in transmission mode on a Nicolet 6700

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spectrometer with pressed KBr pellets.

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CO2-, and NH3-TPD were performed in a quartz tube reactor system equipped with a

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quadrupole mass spectrometer (MS, Hiden, HPR20, for CO2-TPD) or a thermal conductivity


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detector (TCD, for NH3-TPD). Prior to the TPD analysis, 100 mg of the sample was first pre-

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heated at 400°C in 20 vol. % O2/Ar flow (30 ml min-1) for 1 h, cooled to 100°C, and swept by Ar

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for another 1 h. Pure CO2 or NH3 was introduced into the catalyst bed at 10 ml min-1 for 30 min.

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An Ar flow (30 ml min-1) was then purged through the catalysts for 1 h to remove physiosorbed

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CO2 or NH3. After that, the sample was heated from 100 to 600°C, at a ramp of 10°C min-1. For

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CO2 detection, mass signals at m/z = 44 was recorded. H2-TPR was carried out using the same

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apparatus. The sample (100 mg) was pre-treated following a similar procedure as the TPD

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experiments, and was heated from 50 to 750°C at 10°C min-1 in a 5 vol.% H2/Ar flow (30

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ml/min). The consumption of H2 was determined by a TCD.

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FT-IR spectrum of the pyridine adsorption was measured on a Nicolet 6700 spectrometer

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equipped with an MCT detector in diffuse reflection mode (DRIFTS). The sample was first

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purged with O2/Ar (20 vol. %) at 400 ºC for 1 h, and background measurements were collected at

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300, 200, and 100°C, respectively. The pyridine vapor was then admitted to the IR cell for 15

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min at 100°C. After sweeping with Ar for 30 min, the spectra of adsorbed pyridine were

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collected at the desired desorption temperature (100, 200, or 300°C).

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Catalytic oxidation of dichloromethane. Catalytic oxidation of DCM was carried out in a

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temperature-controlled flow reactor (U-shaped quartz tube with an inner diameter of 3 mm)

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containing 200 mg of the as-prepared catalyst. The flow rate of feeding gas (air containing 500

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ppm of DCM) was controlled by mass flow controllers (Alicat), and the space velocity was

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always fixed to 15,000 ml/g•h. Here, air (dried by silica gel and 5A zeolite) and dichloromethane

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(Macklin, 99.9%, with molecular sieves) were used to avoid the presence of water in the feeding

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gas. The effluent gases were analysed on-line using a GC equipped with an FID detector, and the

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conversion of DCM was calculated based on the peak area after running for 15 min at the setting


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temperature. The amount of MCM by-product was calculated from the DCM peak using a

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relative correction factor of 0.92.

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 RESULTS AND DISCUSSION

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CeO2 NSs were synthesized through an aqueous phase precipitation reaction of Ce(NO3)3 and

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NH4HCO3 at room temperature followed by a calcination in air at 500°C, according to our

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previously reported method.35 Subsequent incipient wetness impregnation of TMP allows the

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generation of POx-functionalized CeO2 nanosheets, as modified from Slowing’s route

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(Experimental Section).34 SEM and TEM images in Figure 1a & S1b indicate that as-

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synthesized CeO2 samples have a belt-like nanosheet morphology with a thickness of 20-50 nm,

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a length of 5-10 µm and a width of 0.5-1.2 µm. The NS structure is well retained after the

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incorporation of functional POx species in POx-CeO2-0.05 (Figure S1a and S1c) and POx-CeO2-

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0.2 (Figure 1b and 1c) samples. Meanwhile, these pristine and POx-modified CeO2 NSs are

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polycrystalline, which are confirmed by the concentric circles of the selected area electron

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diffraction (SAED) patterns (insets of Figure 1c, S1b and S1c).36. Such morphological and

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crystallographic similarities between pristine and POx-modified CeO2 NSs allow us to

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unambiguously refine POx species effect on sample’s catalytic properties.

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SEM-EDS elemental mappings of POx-CeO2-0.05 and POx-CeO2-0.2 NSs demonstrate that P

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is uniformly distributed over our samples regardless of the P concentration (Figure 1d and S1d).

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Based on the SEM-EDS analysis, the element atomic ratios of P/Ce on POx-CeO2-0.05, POx-

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CeO2-0.1, POx-CeO2-0.2 and POx-CeO2-0.3 are quantified to be 0.04, 0.09, 0.15 and 0.28,

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respectively, only slightly lower than the precursor values. N2-physisorption measurement

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indicates that the POx-modification leads to the decline of the specific surface areas. The BET

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specific surface areas (SBET) are 75, 71, 65 and 63 m2/g for POx-CeO2-0.05, -0.1, -0.2 and -0.3,


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respectively, lower than that of pristine CeO2 (85 cm2/g). The decrease is related with the

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formation of phosphate34, which is bonded on the surface of CeO2, resulting in the monotonously

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decrease in specific surface area with the P contents.

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Figure 1. (a) SEM images of CeO2 NSs; (b-d) SEM (b) and TEM images with the

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corresponding SAED pattern (c), and SEM-EDS elemental mapping (d) of POx-CeO2-0.2 NSs.

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All POx-CeO2 NSs with different POx contents show the typical cubic fluorite structure

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identical to CeO2 (JCPDS no. 34-0394),37 with no impurity diffraction peaks observed in their

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XRD patterns (Figure 2a). This indicates that our TMP modification process does not lead to the

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formation of crystalline phosphate impurity (such as CePO4 or CeP2O7) which are usually

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observed in the traditional approach via inorganic phosphoric acid treatment.30 FT-IR was


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conducted to confirm the successful POx-grafting on CeO2 surface. As seen in Figure 2b, all

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POx-modified CeO2 present three new IR bands at 1158 cm-1, 1000 cm-1 and 950 cm-1 compared

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to the pristine CeO2, and the intensities of these peaks increase with POx content. These peaks

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are attributed to the symmetric stretching of non-bridging (O-P-O)sym oxygen atoms in Q2

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phosphate units, the stretching vibrations of P-O- in Q0 species and the antisymmetric stretching

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mode of non-bridging (P-O-P)antisym oxygen in isolated orthophosphate groups, respectively.38

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The band at 1625 cm-1 is assigned to the bending motions of physiosorbed water molecules

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and/or (P-OH) groups,38 while the latter contribution is more dominant as the peak is absent on

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the pristine CeO2. More importantly, we find that the band at 1324 cm-1, which can be assigned

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to carbonate species adsorbed on CeO2 basic sites,39 diminishes with increasing POx content and

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eventually disappears on the POx-CeO2-0.2 sample, suggesting that the basic surface sites are

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fully consumed by POx-modification in high POx content samples.

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Figure 2. (a) XRD patterns and (b) FT-IR spectra of the pristine CeO2 and POx-CeO2 NSs with

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different POx contents.

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The nature of surface O and P over POx-CeO2 NSs was probed via XPS, as shown in Figure 3.

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The O1s XPS spectra for both POx-CeO2-0.05 and POx-CeO2-0.2 can be deconvoluted into three


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peaks: O2- (529 eV, lattice oxygen species, denoted as OⅠ), O22-/O- (530.8 eV, surface active

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oxygen species, denoted as OⅡ) and -OH (532.5 eV, hydroxyl species and adsorbed water,

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denoted as OⅢ).

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functionalized CeO2 NSs can retain their redox properties. Moreover, the relative content of

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oxygen species (OⅠ, OⅡ, OⅢ/Ototal) calculated from XPS measurement is 67.2, 22.1 and 10.8%,

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61.8, 21.0 and 17.2% for POx-CeO2-0.05 and POx-CeO2-0.2, respectively, which indicates that

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O22-/O- species decreases (redox sites) with the increasing of POx content while -OH species

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increases (Brønsted acid sites). The P2p XPS spectrum for POx-CeO2-0.2 is deconvoluted into

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two peaks. While the one at 132.5 eV is assigned to the monodentate surface complex, the one at

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a higher binding energy (134.2 eV) is assigned to the bidentate surface complex, in which POx is

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bound to two surface hydroxyl groups. Such a co-existence of surface mono- and bidentate POx

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has also been reported in other POx-modified metal oxide.41 Surprisingly, however, POx-CeO2-

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0.05 only shows a single peak of monodentate phosphate complex, indicating the bidentate is not

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favourable under low concentration surface modification.

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The presence of surface O22-/O- on the two samples suggests that our POx-

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Figure 3. O1s and P2p XPS spectra of the (a) POx-CeO2-0.05 and (b) POx-CeO2-0.2 NSs.


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The redox properties of POx-CeO2 NSs were further evaluated by H2-TPR. As shown in

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Figure 4a, the pristine CeO2 and POx-CeO2-0.05 present a broad peak between 325 and 600°C in

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their H2-TPR profiles, which are attributed to the reduction of surface oxygen species (adsorbed

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oxygen species over O-vacancy and surface lattice oxygen species of CeO2).42 This peak

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becomes more symmetric when incorporating POx, along with the disappearance of adsorbed

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oxygen species due to POx coverage on CeO2 surface. Meanwhile, a small reduction peak at

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lower temperatures (200 to 300°C) is detected on POx-CeO2-0.05 and POx-CeO2-0.1, which can

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be ascribed to oxygen species adsorbed on the interface of POx and CeO2, as illustrated in

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Scheme S1. Interestingly, we note this low-temperature peak becomes negligible on the POx-

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CeO2-0.2 and POx-CeO2-0.3, since the high coverage of POx over CeO2 surface can effectively

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mitigate the interface-mediated oxygen adsorption. Such high surface coverage of POx when

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P/Ce>0.2 is consistent with aforementioned FT-IR analysis wherein surface carbonate species

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absorption is also prohibited.


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Figure 4. (a) H2-TPR, (b) NH3-TPD, and (c) CO2-TPD profiles of POx-CeO2 NSs with different

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POx contents; (d) Schematic illustration of surface acidic and basic sites on POx-CeO2 NSs.

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Surface acidity and basicity of POx-CeO2 NSs were systematically investigated by using NH3-

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TPD and CO2-TPD. The NH3-TPD profile (Figure 4b) on pristine CeO2 NSs displays a peak in

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the range of 125 to 250°C, which is resulted from the desorption of NH3 adsorbed on Ce4+/3+

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(dominant) and surface acidic hydroxyl group (bridged OHad, as illustrated in Figure 4d), in

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agreement with the mild acidic nature of CeO2.34 The POx modification significantly enhances

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the acidic strength of CeO2 NSs, and correspondingly, NH3 desorption peak is broadened to

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450°C, confirming the generation of the new medium-strong acid sites (P-OH).34 However, the

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overall number of surface acid sites on POx-CeO2 NSs is reduced with the increase of POx

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content. Based on integrated peak area, the total acid sites decrease drastically from 7.5x for


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POx-CeO2-0.05 (relative to pristine CeO2) to 6.0x for POx-CeO2-0.1, 5.7x for POx-CeO2-0.2, and

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4.7x for POx-CeO2-0.3. This is probably due to the transformation from monodentate to bidentate

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POx when more species are grafted, as implied by XPS and IR results.

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The CO2-TPD results in Figure 4c clearly exhibit two CO2 desorption peaks on pristine CeO2.

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The one at 175°C is correlated to surface basic hydroxyl group (weak basic sites, as illustrated in

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Figure 4d), while another one around 375°C reflects basic oxygen ion (O2-, medium-strong basic

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site).

234

strong basic sites on pristine CeO2. Once POx is grafted, the density of weak basic sites is

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substantially reduced and becomes even negligible on POx-CeO2-0.2 and POx-CeO2-0.3 NSs.

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This can also be explained by the aforementioned mono-to-bidentate POx transition when

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increasing POx content.34,

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converted to bidentate -HPO4 through the dehydration reaction between acidic P-OH and the

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adjacent surface basic hydroxyl groups over CeO2,27, 41 consuming these weak basic sites. More

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interestingly, the CO2 desorption peak corresponding to the medium-strong basic sites O2- is

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heightened and broadened after POx modification, especially for the POx-CeO2-0.05 NSs, which

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is an indication that more of the medium-strong basic sites are formed. The lower

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electronegativity of P (2.19) than O (3.44) and their interfacial effect are probably responsible for

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the basicity change over POx-CeO2-0.05 NSs.

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Moreover, the total number of weak basic sites is clearly much higher than medium-

41, 44-45

As illustrated in Scheme S2, monodentate -H2PO4 can be


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Figure 5. DRIFT spectra of pyridine adsorption on (a) Al2O3, pristine CeO2, and POx-CeO2 NSs

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with different POx contents at 200°C and (b) POx-CeO2-0.2 NSs at different temperatures.

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Furthermore, the nature of acid sites (Brønsted or Lewis acid sites) and their corresponding

249

strengths were revealed in detail via pyridine (Py) adsorption coupled with IR spectroscopy.

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Figure 5a shows the DRIFT spectra for Py adsorption on POx-CeO2 NSs with different POx

251

contents at 200°C, with Al2O3 as a reference. For pristine CeO2 and POx-CeO2, the presence of

252

Lewis acid sites is proven by two bands at approximately 1595 cm-1 (ν8a vibration mode of

253

adsorbed Py) and 1440 cm-1 (ν19b mode), both shifting to lower frequencies compared with

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Al2O3 standard (1620 cm-1 and 1450 cm-1), suggesting the relatively weak strength of Lewis acid

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sites on both pristine and POx-modified CeO2.46 The Brønsted acid site is undetectable on

256

pristine CeO2, but becomes evident on POx-CeO2 NSs due to the existence of P-OH as

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demonstrated by the peak at 1525 cm-1.34 By comparing the peak area, we notice that the number

258

of Lewis acid site is significantly increased after the POx-grafting, with POx-CeO2-0.05 being the

259

highest one. Meanwhile, POx-CeO2-0.05 and POx-CeO2-0.3 NSs present fewer Brønsted acid

260

sites than the POx-CeO2-0.1 and POx-CeO2-0.2, probably due to the low POx content for POx-

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CeO2-0.05 and the formation of bidentate POx (less P-OH) for POx-CeO2-0.3. In addition, these


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acid sites are thermally stable. Temperature-dependant DRIFT spectra of POx-CeO2 (Figure 5b

263

and S2) show that the strong Py adsorption on the acid sites can still be observed even at 300°C,

264

which is consistent with our NH3-TPD results (the higher and broader NH3 desorption

265

temperature on POx-CeO2).

266 267

Figure 6. Catalytic oxidation of DCM on different catalysts: (a) effect of POx content, (b)

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stability of catalysts, (c) effect of phosphate precursors, and (d) MCM selectivity on pristine

269

CeO2 and POx-CeO2 NSs with different POx contents.

270

With acid and basic sites over CeO2 being fine-tuned through POx-incorporation, we have

271

systematically studied its effect on catalytic oxidation of DCM. As shown in the light-off curves

272

in Figure 6a, the pristine CeO2 NSs exhibit an irregular increase of DCM conversion with two


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plateaus, which is caused by the deactivation/poisoning of catalyst during the reaction.

The

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complete oxidation of DCM is not achieved even at 400°C. The POx-modification can effectively

275

eliminate these plateaus and clearly improve the conversion of DCM when temperature is above

276

175°C. Among catalysts with different POx content, POx-CeO2-0.05 shows the maximum

277

increase rate in DCM conversion, while POx-CeO2-0.2 and POx-CeO2-0.3 present almost

278

overlapping curves. We believe that such a performance enhancement is attributed to the

279

inhibition of catalyst deactivation, given the facts that the pristine CeO2 shows a better initial

280

activity than POx-CeO2 while POx itself is not catalytically active towards DCM oxidation (only

281

20% DCM conversion was obtained over POx-SiO2-0.2 catalyst even at 400 °C, Figure S3).28-29

282

Such Brønsted acid site-induced anti-poisoning effect can be further demonstrated in the

283

stability testing (Figure 6b). At 250°C, a rapid deactivation of the pristine CeO2 NSs is observed

284

with DCM conversion decreased from 87% to 20% within 4 hours. For POx-CeO2 NSs with low

285

POx content (POx-CeO2-0.05 and POx-CeO2-0.1), although the deactivation is still recorded at

286

the initial stage (the conversion of DCM drops from 85-90% to 65-70% within 90 min), the

287

DCM conversion can be eventually stabilized at a much higher level compared with the pristine

288

CeO2 (approximately 65% vs. 20%). Very encouragingly, the decay of initial activity is

289

negligible on POx-CeO2-0.2 and POx-CeO2-0.3 NSs, and a stable DCM conversion of 65-70% is

290

delivered. The initial activity loss on POx-CeO2-0.05 and POx-CeO2-0.1 is correlated to their

291

relatively low coverage of POx species. With more POx attached on CeO2, the adsorbed Cl-

292

poisoning species can be rapidly removed in the form of HCl under the assistance of Brønsted

293

acid sites. Meanwhile, the re-dissociation of formed HCl to Cl- can be inhibited by Brønsted acid

294

site, which further strengthens the anti-poisoning property of the catalyst.25 It is noteworthy that

295

the higher reaction temperature is favourable to the subsequent removal of HCl as a final


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Page 17 of 26


296

product. Slightly increasing the stability testing temperature from 250 to 300°C leads to a

297

significantly enhanced DCM conversion stabilized at approximately 95%.

298

Our POx-CeO2 NSs obtained via a TMP process show significantly higher activity than CeO2

299

treated with inorganic phosphates, such as PA, AP and SP, as summarized in Figure 6c. The SP-

300

CeO2 catalyst presents the lowest activity (the conversion of DCM is only 40% even at 400°C),

301

which is probably due to the presence of Na+ and unsuccessful grafting of Brønsted acid sites.

302

The limited performance of PA-CeO2 and AP-CeO2 catalysts might be related to the formation of

303

cerium phosphate over their surfaces, which is detrimental to the surface redox property for

304

catalysis.

305

The surface basic sites on CeO2 and POx-CeO2 NSs are found to be important for controlling

306

the production of monochloromethane (MCM) by-product during DCM oxidation reaction

307

(Figure 6d). MCM is barely generated over the pristine CeO2 NSs with less than 0.5%

308

selectivity at all temperatures. The incorporation of surface POx drastically increases the

309

production of MCM at relatively low temperatures, reaching a maximum of 18% selectivity at

310

200°C for POx-CeO2-0.05 NSs. The transition from DCM to MCM normally involves two steps

311

in the absence of gaseous hydrogen: the dissociation of C-Cl bond and hydride transfer. The

312

latter one is rate-determining under oxidative condition and is widely observed on the medium-

313

strong basic sites in previous studies.47-48 Our TPD and Py-IR results have indicated that among

314

all POx-CeO2 NSs samples, POx-CeO2-0.05 presents the largest amount of medium-strong

315

surface basic site O2-, which well explains its highest selectivity for MCM. The higher POx

316

content can prohibit MCM generation, which is decreased to 5% at 200°C and 2% at 250°C on

317

POx-CeO2-0.2. Coupled with the excellent stability as aforementioned, it makes POx-CeO2-0.2

318

the most promising catalyst for DCM oxidation in our study. In addition, MCM by-product


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Page 18 of 26

319

disappears above 300°C on all the POx-modified CeO2 catalysts. All these point to a principle

320

that, when working at low temperatures, an ideal transition metal oxide catalyst for CVOCs

321

oxidation should minimize the medium-strong surface basic sites to efficiently prevent the

322

production of chlorinated organic by-products.

323

Based on the above discussion, mechanism of the enhanced oxidative DCM removal over the

324

as-prepared POx-CeO2 NSs catalysts is clearly derived. The employment of TMP ensures a

325

successful surface POx grafting without any change in surface CeO2 phase or sacrifice of surface

326

redox site (Ce4+/3+), leading to a balanced Lewis – Brønsted acid sites combination, which is the

327

key to achieve the highly efficient oxidative DCM removal on the CeO2-based catalysts: While

328

the Lewis acid sites (Ce4+/3+, also as the active redox sites) facilitate the C-Cl bond dissociation

329

and catalyze DCM oxidation, the presence of POx Brønsted acid sites effectively lower the

330

medium-strong basic sites (O2-) intensity, reducing the hydrogen transfer rate as well as the

331

MCM by-product formation. In addition, the Brønsted acid sites can accelerate the removal of

332

adsorbed chlorine species on active redox sites, substantially enhancing the catalyst stability. Our

333

study highlights that the rational surface decoration should be an effective way to improve the

334

catalysts’ performance in DCM removal, and also underlies a promising approach for other air

335

pollution treatment.

336

ASSOCIATED CONTENT

337

Supporting Information. Addition Schemes, SEM and TEM images, SEM-EDS elemental

338

mapping, and in situ DRIFTS spectra are provided. The supporting information is available free

339

of charge on the ACS Publications website.

340

AUTHOR INFORMATION

341

Corresponding Author


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Page 19 of 26


342

*E-mail: Sen Zhang ([email protected]), Wangcheng Zhan ([email protected])

343

Author Contributions

344

The manuscript was written through contributions of all authors. All authors have given approval

345

to the final version of the manuscript. #These authors contributed equally.

346

Notes

347

The authors declare no competing financial interest.

348

ACKNOWLEDGMENT

349

This work was supported by the National Key Research and Development Program of China

350

(No. 2016YFC0204300) and the National Natural Science Foundation of China (No. 21777043)

351

and Jeffress Trust Awards Program in Interdisciplinary Research from Thomas F. and Kate

352

Miller Jeffress Memorial Trust.

353

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Phosphate-Functionalized CeO2 Nanosheets for Efficient Catalytic

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