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Ru/CeO2 Catalyst with Optimized CeO2 Support Morphology and Surface Facet for Propane Combustion Zheng Wang, Zhenpeng Huang, John Brosnahan, Sen Zhang, Yanglong Guo, Yun Guo, Li Wang, Yunsong Wang, and Wangcheng Zhan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01929 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

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Environmental Science & Technology

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Ru/CeO2 Catalyst with Optimized CeO2 Support Morphology and

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Surface Facet for Propane Combustion

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Zheng Wang,† Zhenpeng Huang,† John T. Brosnahan,‡ Sen Zhang,‡ Yanglong Guo,† Yun Guo,† Li

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Wang,† Yunsong Wang,† Wangcheng Zhan*,†

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†

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Chemistry and Molecular Engineering, East China University of Science and Technology,

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Shanghai 200237, P. R. China

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‡

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* Corresponding Author: Fax: +86-21-64252923, E-mail: [email protected] (W.C. Zhan)

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

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

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Abstract: Tailoring the interfaces between active metal centers and supporting materials is an

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efficient strategy to obtain a superior catalyst for a certain reaction. Herein, an active interface

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between Ru and CeO2 was identified and constructed by adjusting the morphology of CeO2 support,

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such as rods (R), cubes (C) and octahedra (O), to optimize both the activity and stability of Ru/CeO2

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catalyst for propane combustion. We found that the morphology of CeO2 support does not

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significantly affect the chemical states of Ru species, but controls the interaction between the Ru

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and CeO2, leading to the tuning of oxygen vacancy in the CeO2 surface around the Ru-CeO2

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interface. The Ru/CeO2 catalyst possesses more oxygen vacancy when CeO2-R with predominantly

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exposed CeO2{110} surface facet is used, providing a higher ability to adsorb and activate oxygen

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and propane. As a result, the Ru/CeO2-R catalyst exhibits higher catalytic activity and stability for

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propane combustion compared to the Ru/CeO2-C and Ru/CeO2-O catalysts. This work highlights a

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new strategy for the design of efficient metal/CeO2 catalysts by engineering morphology and

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associated surface facet of CeO2 support for the elimination of light alkane pollutants and other

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volatile organic compounds.

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Keywords: Volatile organic compounds; CeO2 morphology; Surface facet, Oxygen vacancy;

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Propane oxidation

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1. Introduction

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Emissions of volatile organic compounds (VOCs) have caused serious harm to the ecological

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environment and human health in recent years, due to their toxicity and involvement in the formation

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of photochemical smog. The elimination of VOC pollutants from the atmosphere therefore becomes

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imperative to protect environmental sustainability and human health1-2. Among various treatment

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methods, catalytic oxidation is considered as a promising technology to control the emission of

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VOCs, because of its high adaptability to exhaust gas concentration, no secondary pollution and

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moderate operation temperature3-5. Various kinds of catalysts including noble metal-based

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materials6-15, nonnoble metal oxides16-22 and zeolites23-24 have been synthesized and applied for

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VOC removal, greatly boosting development of the catalytic oxidation technique. Currently, a

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challenging issue in this area is the design of a robust catalyst for the total oxidation of light alkanes

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(the largest fraction of hydrocarbons in automobile exhaust of gas-powered vehicles) at a low

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temperature, because short carbon chain molecules are thermodynamically and chemically stable,

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and subsequently are difficult to be converted1,2,5. For example, propane, a primary short-chain

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pollutant from liquefied petroleum gas, requires intermediate/high reaction temperatures to be

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activated. With the increasing demand for propane as a fuel in transport vehicles, a high-

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performance catalyst which can totally oxidize propane at moderate operation temperature is

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essential to controlling VOC emissions.

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For propane combustion, supported Pt or Pd catalysts25-30 have always been the focus of research

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among various kinds of catalysts. Recent studies have demonstrated that Ru-based catalysts31-36 can

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also deliver appealing catalytic activities for the total oxidation of propane. As a cheaper noble metal,

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Ru is regarded as a promising alternative to Pt and Pd with potential to reduce cost in industrial

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applications1,2. For example, the Ru/γ-Al2O3 catalyst exhibited a high activity for total oxidation of

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propane, and TOF of 0.0035 s-1 can be achieved at 175 °C for the Cl-free Ru/γ-Al2O3 catalyst (4.6

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wt.% Ru)35. Meanwhile, it was reported that the most active sites in the propane oxidation reaction

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consist of a few layers of surface oxide on the large Ru particles and small RuxOy clusters34-35. ACS Paragon Plus Environment

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Although previous studies have investigated the preparation method, catalyst pretreatment, and the

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addition of another component into Ru in correlation to Ru-based catalyst performance36, it is still

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unknown how supporting materials can influence the activity of Ru-based catalysts for propane

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oxidation and how to optimize it.

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Essentially, the nature of the support is a critical factor to determine the catalytic activity of

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supported metal catalysts. The support can either modify or determine the electronic state of active

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metal through the interaction between active metal and the support37-38, or take part in the reaction

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to enable more favorable kinetic pathways39-41. On the other hand, with the rapid development of

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synthetic methodology for nanomaterials, various metal and metal oxide nanocrystals with well-

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defined morphologies and structures have been successfully synthesized, making it possible to

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precisely modulate the interface between metal and metal oxide support at the atom level.42-52 For

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example, Li et al.51 studied CO adsorption and oxidation on Au/TiO2 catalysts with different TiO2

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morphologies predominantly exposing {001}, {100}, or {101} facets, and found TiO2 morphology-

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dependent dual-perimeter-sites catalysis by Au/TiO2 catalysts in CO oxidation. Hu et al.52

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demonstrated that ceria with different morphologies can affect the structure and chemical states of

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Pd species. The ceria {111} facets on Pd/CeO2-O (octahedrons) favors the existence of PdO, which

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are beneficial for propane activation and the total oxidation of propane. We envision that the Ru-

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support interaction also plays an essential role in propane oxidation, and may be optimized by

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carefully engineering support morphology and surface facet.

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In the present work, we report a superior catalyst for propane oxidation utilizing CeO2 nanorods

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(CeO2-R) with CeO2 {110} surface facets as a support for composite Ru nanocrystals. In order to

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understand and systematically evaluate the CeO2 support effect, CeO2 with different morphology,

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such as CeO2 nanocubes (CeO2-C) and nano-octahedra (CeO2-O), as well as CeO2-R, were

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synthesized and used to support Ru in the same condition. Their structures and catalytic activities

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for propane combustion were studied in detail, and then the mechanism of influence of CeO2 support

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morphology on the catalytic performances of Ru/CeO2 catalysts for propane combustion was refined.



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The interaction between the Ru species and CeO2, dependent on CeO2 support morphology-

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associated surface facet, can optimize oxygen vacancy on the CeO2 surface around the Ru-CeO2

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interface, providing a higher ability to adsorb and activate oxygen and propane, which leads to

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excellent catalytic activity and stability for propane combustion. This work exemplifies a promising

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strategy for developing robust supported catalysts for the efficient removal of short-chain

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hydrocarbon VOCs.

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2. Materials and Methods

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2.1. Catalyst preparation

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CeO2-R, CeO2-C and CeO2-O were synthesized by hydrolysis of Ce(NO3)3 in an alkaline medium,

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followed by a hydrothermal treatment42,52. To obtain CeO2-R and CeO2-C, 4 mmol of

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Ce(NO3)3·6H2O and 480 mmol of NaOH were dissolved in 10 and 70 mL of deionized water

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seperately, and then these two solutions were slowly mixed and kept stirring for 30 min.

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Subsequently, the mixed solution was transferred into a Teflon-lined stainless steel autoclave and

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heated at 100 and 180 °C for 24 h to get CeO2-R and CeO2-C, respectively. After the hydrothermal

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treatment, the precipitates were washed with distilled water and ethanol several times until pH=7,

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then dried at 120 °C overnight and calcined in air at 400 °C for 4 h. For CeO2-O, 2 mmol of

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Ce(NO3)3·6H2O and 0.02 mmol of Na3PO4 12H2O were dissolved in 80 mL of distilled water. After

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being stirred at room temperature for 30 min, the solution was transferred into a Teflon-lined

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stainless steel autoclave and heated at 170 °C for 10 h. After being cooled to room temperature, the

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precipitates were washed with distilled water and ethanol, then dried at 120 °C overnight and

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calcined in air at 400 °C for 4 h.

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The Ru/CeO2-R catalyst with the Ru loading of 2 wt% was prepared by the deposition-

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precipitation (DP) method. Firstly, 1.0 g of CeO2-R was suspended in 50 mL of water, followed by

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adding desired amount of RuCl3 solution. The suspension pH was adjusted to 8.0 with NaOH

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aqueous solution (0.01 M), and the resulting suspension was aged at 25 °C for 3 h with stirring. The

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precipitate obtained was separated by centrifugation, washed thoroughly with distilled water, and


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dried at 60 °C overnight, followed by calcination in 10% H2/Ar mixed gas at 400 °C for 90 min.

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The Ru/CeO2-C and Ru/CeO2-O catalysts are prepared using the same procedure.

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2.2. Catalytic activity testing

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The catalytic performance of the catalysts prepared for C3H8 combustion was tested in a fixed

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bed quartz reactor. 100 mg of catalyst was used in the test. The feed gas consisted of 0.2 vol.%

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C3H8, 2 vol.% O2 and Ar balance with a flow rate of 50 mL/min. The temperature of reaction bed

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was ramped up to 300 °C at a heating rate of 2°C/min. The conversion of C3H8 was measured after

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the catalytic reaction by an online gas chromatograph (GC-2060) that was equipped with an FID.

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The C3H8 conversion (XC3H8) was calculated by X

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[C3H8]in and [C3H8]out are the C3H8 concentration in the inlet and outlet gas, respectively.

C3H8

=

[C3H8]in - [ C3H8]out × 100 %, where [C3H8]in

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The stability of the Ru/CeO2 catalyst for propane combustion was conducted at 200 °C. 100 mg

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of the catalyst was used. The feed gas consisted of 0.2 vol.% C3H8, 2 vol.% O2, 3 vol.% H2O (when

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used) balanced by Ar with a flow rate of 50 mL/min. An online gas chromatograph (GC-2060) was

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applied to measure C3H8 concentration in the inlet and outlet gas.

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2.3. Reaction Kinetics Measurement

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The kinetic parameters for C3H8 oxidation were measured at 155 °C in a fixed-bed reactor with

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C3H8 conversion below 15%. The catalytic reaction data were obtained after 30 min of stable

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reaction. The effects of the internal/external diffusion has been eliminated by changing the catalyst

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particle size and feed gas velocity, and also be excluded by making a Weisz-Prater analysis for

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Internal Diffusion and a Mears analysis for External Diffusion52-53. Turnover frequency (TOF) (s−1)

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was calculated with TOF=XC8H8∙VC3H8∙NA/N, where XC3H8 is the conversion of C3H8, VC3H8 is the

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gas flow rate (mol/s) of C3H8, NA is the Avogadro constant and N is the number of catalytically

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active sites (N=Ntotal ∙ DRu). The Ru dispersion (DRu) was measured by CO chemical adsorption at

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25 °C (CO/Ru=1/1) on a micromeritics AutoChem II 2920 chemical adsorption instrument. Ntotal is

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total atom numbers of Ru, which can be calculated with Ntotal = (mRu/MRu) × NA, where mRu is the

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mass of Ru and MRu is relative atomic mass of Ru.



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2.4. Catalyst Characterization

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Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus diffractometer

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with Cu Kα radiation (λ=1.54056 Å) operated at 40 kV and 40 mA. The specific surface areas of

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the samples were measured on a Micromeritics ASAP 2400 instrument at 77K and calculated by the

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Brunauer-Emmett-Teller (BET) method. Transmission electron microscopy (TEM) images of

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samples were recorded on a JEOL Model 2100F electron microscope operated at 200 kV.

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Ruthenium content was determined by X-ray fluorescence (XRF) using a Shimadzu (XRF-1800)

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wavelength dispersive X-ray fluorescence spectrometer. The Raman spectra were obtained on a

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Renishaw in Viat + Reflex spectrometer equipped with a CCD detector at ambient temperature and

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moisture-free conditions. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics) was

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focused, analyzing spot about 1 mm, on the sample under the microscope. The power of the incident

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beam on the sample was 3 mW. Time of acquisition was varied according to the intensity of the

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Raman scattering. The wave numbers obtained from spectra were accurate to within 2 cm−1. X-ray

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photoelectron spectroscopy (XPS) spectra were obtained at 25 °C on a PHI-Quantera spectrometer

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under ambient conditions, and the 514 nm line of a Spectra Physics Ar+ laser was used for excitat

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SXM spectrometer with Al Kα (1486.6 eV) radiation as the excitation source under ultra-high

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vacuum (6.7×10−8 Pa). All binding energies (BE) were determined with respect to the C1s line

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(284.8 eV) originating from adventitious carbon.

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The temperature-programmed desorption of O2 (O2-TPD) was conducted using a Micromeritics

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AutoChem II 2920 equipment associated with a computer-interfaced quadruple mass spectrometer

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(Hiden HPR 20). 50 mg of catalyst was pre-treated with 3 vol.% O2/He (40 mL/min) at 400 °C for

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30 min, then cooled to room temperature and being purged with He for 1 h, the sample was heated

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from 30 to 700 °C at a rate of 10 °C/min in a flow of He (40 mL/min). The signal of O2 desorbed

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from the samples was measured by the quadruple mass spectrometer.

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For measurement of the temperature programmed surface reaction with C3H8 (C3H8-TPSR), 50

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mg of the sample was pretreated with 3% vol. O2/He (40 mL/min) at 400 °C for 30 min, and then


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cooled to 30 °C. After being swept with 5% vol. C3H8/Ar for 1h, the sample was heated from 30 to

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600 °C at a rate of 10 °C/min in 5 vol.% C3H8/Ar flow (40 mL/min). A mass spectrometer (HPR20

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QIC) was used to monitor the effluent gas and the MS signals of both C3H8 (m/z = 43) and CO2

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(m/z =44) were recorded.

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Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data were recorded on a

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Nicolet Nexus 670 FTIR spectrometer with 64 scans at an effective resolution of 4 cm−1. Prior to

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the measurements, the sample was heated at 250 °C in Ar for 1 h and then cooled to 200 °C. After

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the background spectrum was recorded at 200 °C, Ar was replaced by the mixed gas (30 mL/min)

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of 0.5 vol.% C3H8 - 99.5 vol.% Ar and in situ DRIFTS spectra of the samples were taken at a certain

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time. At the C3H8 + O2 reaction stage, the gas of 0.5 vol.% C3H8/Ar was changed to 0.5 vol.% C3H8

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- 5 vol.% O2 - 94.5 vol.% Ar (30 mL/min) and the DRIFTS spectra of the samples were collected at

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a specific time.

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3. Results and Discussion

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3.1. Catalytic activity of the catalysts

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The catalytic activities of CeO2 and Ru/CeO2 samples were evaluated for C3H8 combustion, and

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are shown in Figure 1A. All pure CeO2 supports show a negligible catalytic activity for C3H8

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combustion under the experimental conditions. After Ru loading on the CeO2 supports, C3H8

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conversion improved drastically. The following oxidation performance, in terms of total-conversion

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temperature, was observed: Ru/CeO2-R (198 °C) > Ru/CeO2-C (226 °C) > Ru/CeO2-O (255 °C). In

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order to further compare the catalytic activity of three catalysts, C3H8 conversion rates were

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calculated by normalizing over Ru weight (Figure 1B and Table S1). At a reaction temperature of

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155 °C, Ru/CeO2-R displays a rate of 21.26106 molecules C3H8 gRu−1 s−1, while Ru/CeO2-C and

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Ru/CeO2-O present the lower values of 7.30106 and 1.25106 molecules C3H8 gRu−1 s−1,

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respectively. The apparent activation energy (Ea) for Ru/CeO2-R, Ru/CeO2-C, and Ru/CeO2-O is

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28.6, 58.5, and 74.9 kJ/mol, respectively.

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To highlight the superior catalytic performance of the Ru/CeO2 catalysts in propane combustion,

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Table S2 compares catalytic activity of the Ru/CeO2 catalysts with different morphology to that of ACS Paragon Plus Environment


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catalysts reported in references. Excitingly, the reaction rate and TOF of the Ru/CeO2-R are much

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higher than those of other supported Ru catalysts and even the well-known noble metal-based

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catalysts (e.g., Ru/CeO232, Ru/TiO233, Pt/ZSM-527 and Pd/Al2O328). Meanwhile, to evaluate the

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durability of Ru/CeO2 for propane combustion, C3H8 conversion over three catalysts are detected at

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200 °C within 60 h. The results show that the C3H8 conversion over the Ru/CeO2-C catalyst

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dramatically decreases with the extension of the reaction time, while C3H8 conversion over other

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two catalysts is retained well, indicating that the Ru/CeO2-R and Ru/CeO2-O catalysts have high

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stability for propane combustion.

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Figure 1. Catalytic activity (A), ln r as a function of 1/T (B) and the stability at 200 °C (C) of the

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Ru/CeO2 catalysts for propane combustion. (The feed gas is consisted of 0.2 vol.% C3H8 + 2

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vol.% O2 + 3 vol.% H2O (when used) balanced by Ar, and GHSV was 30000 mL·h-1·gcat-1 for A

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and C, and 120000 mL·h-1·gcat-1 for B).

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3.2. Catalyst structures and chemical states of Ru species

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Figures 2A, D and H show the typical transmission electron microscopy (TEM) images of three

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CeO2 supports above mentioned, confirming the formation of CeO2 nanorods, nanocubes and nano-

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octahedra, respectively. High-resolution TEM (HRTEM) analysis indicates that CeO2-R

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predominantly exposes well-defined {110} facets, while CeO2-C and CeO2-O have {100} and {111}

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surface facets, respectively54-55 (Figure S1B, D and F). TEM images in Figures 2B, E and I show

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the typical morphologies of Ru/CeO2 samples and corresponding Ru particle size distributions. The

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morphology of the CeO2 supports can be well preserved after Ru loading. Ru nanocrystals are

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uniformly distributed on the CeO2 support with an average size of 2.8, 1.2 and 1.7 nm for the

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Ru/CeO2-R, Ru/CeO2-C and Ru/CeO2-O catalysts, respectively (Table S3). On the other hand, as is

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well known, thin RuOx layers can be easily formed when Ru metal is exposed to air at room ACS Paragon Plus Environment

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temperature, leading to the formation of Ru@RuOx species56-57, which would be confirmed by the

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following X-ray photoelectron spectroscopy (XPS) results. However, only the interplanar spacing

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of 0.21 nm assigned to the Ru {101} plane is observed in the FFT patterns, and the thin oxide layer

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was absent in the HRTEM images of the Ru nanocrystals over Ru/CeO2 catalysts (Figures 2C, F

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and J). This may be attributed to the amorphous phase of RuOx58-60.

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Figure 2. TEM images of CeO2 (A, D and H) and Ru/CeO2 (B, E and I) and high-resolution TEM

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images of Ru/CeO2 (C, F and J): rod (A, B and C); cube (D, E and F); octahedra (H, I and J).

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The XRD patterns of the CeO2 and Ru/CeO2 catalysts are shown in Figure 3A. All CeO2 supports

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display a cubic fluorite-type CeO2 phase (JCPDS 34-0394). Their grain sizes as calculated by the

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Scherrer equation were 9.1, 30.2 and 39.8 nm for CeO2-R, CeO2-C and CeO2-O, respectively (Table

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S3). After Ru loading, XRD patterns of the Ru/CeO2 catalysts are almost the same as those of the

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corresponding CeO2 supports. There is no diffraction peak of Ru species (metallic Ru at 44º, RuOx

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at 35º and 54º) for the Ru/CeO2 catalysts, due to the small particle size of Ru species as shown in

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TEM results58-60.



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The chemical states of Ru and their concentrations on the surface of the catalysts were detected

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by X-ray photoelectron spectroscopy (XPS). Figure 3B shows the Ru 3p XPS spectra of the

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Ru/CeO2 catalysts. The peaks at about 461.5 and 463.0 eV could be assigned to Ru0 and Run+ (0
Ru/CeO2-C (0.13) > Ru/CeO2-O (0.07). The

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relative concentration of oxygen vacancy was further confirmed by calculating with the nature of

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the major peak at 459 cm-1 based on the spatial correlation model53,73. The relative values are

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4.721021 cm−3 (Ru/CeO2-R), 4.081021 cm−3 (Ru/CeO2-C) and 3.771021 cm−3 (Ru/CeO2-O)

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respectively, which is consistent with the order of the ID/IF2g ratio. These results reveal that different

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CeO2 surface facets can affect the interaction between the Ru species and CeO2, leading to different

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amounts of oxygen vacancy on the CeO2 surface around the Ru-CeO2 interface.



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Figure 4B shows the Ce 3d XPS spectra. It consists of two series of spin-orbit lines u and v. Ce

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3d3/2 spin-obit components which correspond to u lines include three characteristic peaks labeled as

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u (901.1 eV), u" (907.1 eV) and u‴ (917.1 eV). Ce 3d5/2 spin-orbit components which correspond to

276

v lines include three peaks labeled as v (882.4 eV), v" (888.7 eV) and v‴ (898.5 eV). These three

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pairs of peaks are attributed to the characteristic peaks of Ce4+. Moreover, the residual four spectral

278

lines labeled as u0 (899.2 eV), v0 (880.5 eV), u' (903.7 eV) and v' (885.3 eV) belong to the Ce3+

279

species. The Ce3+ concentration that correlates with the amount of oxygen vacancy on CeO2 surfaces,

280

can be estimated by calculating the ratio of the area of Ce3+ peak to the area of all peaks54, 58-59. As

281

show in Table S4, the Ru/CeO2-R catalyst has the highest atomic ratio of Ce3+/Ce (30.9%), followed

282

by the Ru/CeO2-C (27.9%) and Ru/CeO2-O (26.3%) catalysts. Since the Ce3+ concentration on CeO2

283

surface correlates with the amount of oxygen vacancies on CeO2 surfaces54, 58-59, it can be concluded

284

that the Ru/CeO2-R catalyst has more oxygen vacancies on the catalyst surface consistent with the

285

Raman results.

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Figure 4. Raman spectra (A), Ce 3d (B) and O1s (C) XPS of the Ru/CeO2 catalysts

288 289

The O 1s XPS spectra of the Ru/CeO2 catalysts are shown in Figure 4C. All the catalysts display

290

one strong peak (OL) at 529.5 eV and a wide shoulder peak (OS) at 531.5 eV, which can be mainly

291

attributed to the lattice oxygen and surface-adsorbed oxygen belonging to carbonate (CO32−),

292

hydroxyl (OH-) species and/or the contributions from O of RuOx, respectively74-80. Since the content

293

of Ru in the Ru/CeO2 catalysts was very low, the ratio between the area of OS and OL (OS/OL) for

294

the Ru/CeO2 catalysts is analogous to the amount ratio between surface-adsorbed oxygen belonging

295

to defect oxides and the lattice oxygen. As shown in Table S4, the OS/OL ratio decreases in the order


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of Ru/CeO2-R (43.2%) > Ru/CeO2-C (33.8%) > Ru/CeO2-O (31.9%), indicating that the Ru/CeO2-

297

R catalyst possesses the highest amount of surface-adsorbed oxygen due to more oxygen vacancy

298

on the surface of the Ru/CeO2-R catalyst. The Ru/CeO2-C catalyst comes second, and the Ru/CeO2-

299

O catalyst is the least. These results is consistent with those of Raman and the Ce 3d XPS spectra.

300

3.4. Amount of active oxygen

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Since an oxygen vacancy is a good site for oxygen adsorption and activation during oxidation

302

reactions54,60,76, the presence of oxygen vacancies on the surfaces of the Ru/CeO2 catalysts is

303

beneficial for improving their catalytic activity in the total oxidation of propane. To further

304

determine the amount of active oxygen species on the surface of the Ru/CeO2 catalysts, the

305

temperature-programmed desorption of O2 (O2-TPD) and the temperature programmed surface

306

reaction with C3H8 (C3H8-TPSR) were conducted, and the results are summarized in Figure 5. In

307

the O2-TPD spectra of the Ru/CeO2 catalysts (Figure 5A), there is a broad overlapping desorption

308

peak for all catalysts, assigned to the chemisorbed oxygen and lattice oxygen of CeO270, 81. The

309

Ru/CeO2-R catalyst exhibits a desorption peak in the temperature range of 100-500oC, while the

310

Ru/CeO2-C and Ru/CeO2-O in 250-500 oC. The integrated areas of desorption peaks are 1.41×10-7,

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8.25×10-8, 5.56×10-8 for the Ru/CeO2-R, Ru/CeO2-C and Ru/CeO2-O, respectively. On the basis of

312

the temperature and area of desorption peaks, it can be concluded that the Ru/CeO2-R has the highest

313

ability to adsorb oxygen among all catalysts, while the Ru/CeO2-C catalyst is middle and the

314

Ru/CeO2-O catalyst is least. As indicated by Raman and XPS, the morphology of CeO2 support can

315

affect the amount of oxygen vacancy on the CeO2 surface around the Ru-CeO2 interface, leading to

316

a different ability to adsorb oxygen for the Ru/CeO2 catalyst.

317

Figure S3 and 5B shows the C3H8-TPSR results over CeO2 support and the Ru/CeO2 catalysts.

318

The mass spectrometry (MS) signals of both C3H8 (m/z = 43) and CO2 (m/z =44) were recorded.

319

For CeO2 supports, almost no reaction product can be obtained in the temperature range 100-400 °C

320

(Figure S3), indicating that all CeO2 supports are poorly active for propane combustion at < 400 °C.

321

After Ru was introduced on CeO2, a strong peak (m/z = 44) and a negative peak (m/z = 43) appeared

322

at about 200 °C, indicating that total oxidation of propane occurred (Figure 5B). Although the curve ACS Paragon Plus Environment


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323

is similar for the different Ru/CeO2 catalysts, the area of CO2 desorption peak (m/z = 44) is 4.54,

324

2.18 and 1.70 for the Ru/CeO2-R, Ru/CeO2-C and Ru/CeO2-O catalyst, respectively. This result

325

further confirms that the amount of active oxygen on the catalysts’ surfaces is in the order of

326

Ru/CeO2-R > Ru/CeO2-C > Ru/CeO2-O, in agreement with the results of O 1s XPS spectra.

327 328

Figure 5. O2-TPD (A) and C3H8-TPSR (B) of the Ru/CeO2 catalysts

329 330

3.5. C3H8 adsorption and activation

331

Figure 6 shows the DRIFT spectra of C3H8 adsorption and reaction on CeO2 and Ru/CeO2

332

catalysts at 200 °C. When the chamber was purged with feed gas consisting of 0.5 vol % C3H8-99.5

333

vol % Ar at 200 °C for 30 min, absorption bands are detected at 2800-3000 cm−1 for the CeO2-R

334

support, assigned to the absorption of gaseous propane (the details are listed in Table S5). In addition,

335

several absorption bands assigned to the carboxylate and carbonate species are present at 1200-1700

336

cm-1 (listed in Table S5), indicating that propane reacted with the oxygen of the CeO2-R support to

337

a certain degree. When the feed gas was replaced with 0.5 vol % C3H8-5 vol % O2-94.5 vol % Ar,

338

the intensity of the corresponding absorption bands at 1200-1700 cm-1 on the CeO2-R support

339

increases after exposure for 30 min at 200 °C, due to the enhancement of oxidation reaction.

340

However, the DRIFT spectra of C3H8 adsorption and reaction on CeO2-C and CeO2-O supports are

341

much different from that on CeO2-R support. The intensities of all absorption bands on CeO2-C and

342

CeO2-O supports are very weak, and the absorption bands at 1200-1700 cm-1 almost even

343

disappeared on CeO2-O support, after they were exposed to feed gas with and without O2 at 200 °C

344

for 30 min, indicating the low ability of CeO2-C and CeO2-O supports to adsorb and activate C3H8.


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345

After Ru was introduced on CeO2, the ability to absorb and activate C3H8 is significantly

346

improved for CeO2-R and CeO2-C supports (a different scale of values on the vertical axis), and

347

intermediate species have changed to some extent. However, the relative order of the ability to

348

absorb and activate C3H8 has not changed, i.e. Ru/CeO2-R >> Ru/CeO2-C > Ru/CeO2-O. An

349

absorption band at 1850 cm-1 appears on the Ru/CeO2-C catalyst, and the Ru/CeO2-O catalyst only

350

has trace amounts of carboxylate species in its DRIFTS spectra. In contrast, the strong absorption

351

bands at 1200-1900 cm-1 (the details are listed in Table S5) are present on the Ru/CeO2-R catalyst,

352

and O2 in the feed gas can also affect the intermediate species. Compared to the feed gas without

353

O2, the absorption bands at 1430 and 1544 cm-1 related to the carboxylate species became more

354

intense on the Ru/CeO2-R catalyst after exposed to feed gas containing O2 due to the enhancement

355

of oxidation degree.

356 357 358 359 360 361

Figure 6. DRIFT spectra of C3H8 adsorption (0.5 vol % C3H8-99.5 vol % Ar, black line) and C3H8 + O2 reaction (0.5 vol % C3H8-5 vol % O2-94.5 vol % Ar, red line) on the CeO2 and Ru/CeO2 catalysts at 200 °C. The feed gas was 30 mL/min. All spectra were obtained after the catalysts were exposed in the feed gas for 30 min.

362

To conclude, CeO2 supports with different morphologies (nanorods, nanotubes, and nano-

363

octahedra, exposing {110}, {100} and {111} surface facets, respectively) were prepared, and the

364

effect of CeO2 morphology on the catalytic performances of Ru/CeO2 catalysts for propane

365

oxidation were investigated. It is revealed that the morphology of CeO2 support mainly influences ACS Paragon Plus Environment


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366

the amount of oxygen vacancy in the CeO2 surface around the Ru-CeO2 interface due to the

367

difference in the interaction between the Ru species and CeO2 with different surfaces. The Ru/CeO2-

368

R catalyst possesses more oxygen vacancies in the CeO2 surface, leading to a high ability to adsorb

369

and activate oxygen and propane. As a result, the Ru/CeO2-R catalyst exhibits the higher catalytic

370

activity and reaction stability for propane combustion compared to the Ru/CeO2-C and Ru/CeO2-O

371

catalysts. In contrary, among three Ru/CeO2 catalysts, the Ru/CeO2-O catalyst possesses the least

372

oxygen vacancies in the CeO2 surface, leading to a lowest ability to adsorb and activate oxygen and

373

propane, as well as the lowest catalytic activity for propane combustion. In order to confirm

374

influence mechanism of surface oxygen vacancies, the difference in vacancy concentration, the

375

amount of surface-adsorbed oxygen, and the TOF on three catalysts has been evaluated. As shown

376

in Figure 7, the TOF at 155 ºC is correlated well with vacancy concentration (ID/IF2g), the amount

377

of surface-adsorbed oxygen (peak area in O2-TPD and Os/OL). This work can open up a facile and

378

reliable strategy for the design and construction of efficient metal/CeO2 catalysts by engineering the

379

surface facets of CeO2 support for the elimination of VOC pollutants. 45

0.3

15

12

0.2

15

9

40

36

OS/OL XPS

30

O2-TPD area

ID/IF2g Raman

TOF S-1 102 at 155 C

44

0.1 6

32

0

380 381

Ru/CeO2-R

Ru/CeO2-C

Ru/CeO2-O

Figure 7. Correlation between TOF (at 155 ºC) and oxygen vacancy concentration of catalysts.

382 383 384

ASSOCIATED CONTENT

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Supporting information ACS Paragon Plus Environment

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The reaction kinetics measurements are provided, together with catalytic activity, lnA as a function

387

of Ea and TEM images. The supporting information is available free of charge on the ACS

388

Publications website.

389 390

AUTHOR INFORMATION

391

Corresponding authors

392

*Email: Wangcheng Zhan ([email protected])

393

ORCID

394

Wangcheng Zhan: 0000-0002-0712-4917

395

Sen Zhang: 0000-0002-1716-3741

396

Yun Guo: 0000-0003-4778-6007

397

Yanglong Guo: 0000-0003-0021-9128

398

Notes

399

The authors declare no competing financial interest.

400 401

ACKNOWLEDGEMENTS

402

Z.W. and Z.H. appreciate the financial support from the National Key Research and Development

403

Program of China (2016YFC0204300), the National Natural Science Foundation of China

404

(21577034) and the Fundamental Research Funds for the Central Universities (222201717003).

405

Y.G. thanks the National Natural Science Foundation of China (21571061).

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References



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