Nanorods Evaluated by in Situ Raman Spectroscopy

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Reversible Oxygen Vacancies Generation on Pure CeO Nanorods Evaluated by in Situ Raman Spectroscopy Isaias de Castro Silva, Fernando Aparecido Sigoli, and Italo Odone Mazali J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 28 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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The Journal of Physical Chemistry

Reversible Oxygen Vacancies Generation on Pure CeO2 Nanorods Evaluated by in situ Raman Spectroscopy

Isaías de Castro Silva, Fernando Aparecido Sigoli and Italo Odone Mazali*

Laboratory of Functional Materials – Institute of Chemistry, University of Campinas - UNICAMP, P.O. Box 6154, 13083-970, Campinas - SP, Brazil

*[email protected]

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Abstract In order to observe oxygen vacancies influence over pure CeO2 Raman spectrum profile, nanorods were synthesized and submitted to an in situ strategy to generate or suppress reversibly the oxygen vacancies. No changes were observed in CeO2 nanorods Raman spectra. However, after the comminution of these nanorods, a reversible frequency downshift in CeO2 T2g Raman band was observed. The catalytic activities of CO oxidation were compared for both pristine and comminuted samples. The results indicate that the comminuted sample show higher catalytic activity than the pristine one. This enhancement on catalytic activity is attributed to the exposure of the CeO2 {110} crystals facets after the comminution process.


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Introduction Cerium oxide (CeO2) is a compound that arouses interest in areas such as heterogeneous catalysis1, where it can be the catalyst itself2 or the support3,4, solid oxide fuel cells5 and oxygen sensors.6 For these applications, distinct features of CeO2 are its nonstoichiometry, expressed by the presence of CeIV ions reduced to CeIII ions and oxygen vacancies, and its capacity to transport these oxygen vacancies.1 The CeIV/CeIII pair can be coupled with another redox pair in order to achieve a better catalytic performance.7,8,9 Oxygen vacancies induce changes in CeO2 crystal lattice, especially its expansion due to the higher ionic radius of CeIII in comparison to CeIV.10 Due to changes in the crystal lattice, Raman spectroscopy may be employed to characterize the oxygen vacancies. Some properties of CeO2 crystals, besides oxygen vacancies, affect their Raman spectrum profile, like its size11 and crystallinity12. The Raman band near to 460 cm-1, attributed to a T2g mode, is the only first-order Raman active mode of CeO2,13 that can be viewed as the breathing mode of the eight oxygen anions around each CeIV ion in the fluorite structure.14,15 An attempt to correlate oxygen vacancies and changes in CeO2 Raman spectrum was made by McBride et al.15 They observed a broadening and higher asymmetry of the T2g band as well as the downshift of its frequency after doping CeO2-bulk with trivalent lanthanides. The authors attributed the band frequency downshift to both lattice distortion, due to lanthanide(III) ions replacing CeIV ions, and the presence of oxygen vacancies, proposing a model to distinguish the contribution of these two effects. Based on this model, oxygen vacancies by themselves would induce an upshift of the T2g band frequency. This model was also applied in the interpretation of Raman spectra of pure CeO2 nanoparticles that exhibited the same feature changes in comparison to the lanthanide doping case as the particle size decreases.11 Lee et al.16 proposed that oxygen vacancies would induce a downshift of the T2g band maximum frequency, allowing the determination of the oxygen deficit with the T2g frequency downshift. Computational methods show that the energy related to oxygen vacancy formation varies according to the exposed facets.17,18 This may be the main reason for the different values of frequency downshift 3 ACS Paragon Plus Environment


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observed for various morphologies explicating why some morphologies of CeO2 nanocrystals are more suitable to work as catalysts for reduction or oxidation reactions.19 However, Lee et al.16 do not performed any in situ experiment to evaluate the reversibility of oxygen vacancies generation and suppression, and thereafter they build their argument just based on the comparison of Raman spectra acquired in different temperatures of CeO2 samples exposed solely to carbon monoxide (CO) atmosphere. In situ experiments were performed by Agarwal et al.,20 employing resonant Raman spectroscopy for different CeO2 nanocrystals morphologies. UV Raman radiation source provides information from the surface constituents rather than bulk ones.21,22,23 A frequency upshift of T2g Raman band was observed after exposure of CO-reduced-CeO2 nanorods to H2O at 350 °C, but no further discussion was presented. Since no studies about reversible generation/suppression of oxygen vacancies in CeO2 are found in the literature, this work reports on the evaluation of CeO2 nonstoichiometry by Raman spectroscopy in undoped CeO2 nanorods using an in situ reversible strategy, based on gas-solid reaction, that allows oxygen vacancies generation and suppression. Raman results are supported by X-ray powder diffraction and catalytic measurements.

Experimental section Synthesis of CeO2 nanorods The synthesis procedure performed was the same as reported by Mai et al..24 Cerium nitrate hexahydrate was used as CeIII source and 2 mmol (0.8684 g) of it was dissolved in 5 mL of deionized water. This solution was slowly added to another one consisted of 0.24 mol (9.6 g) of NaOH in 35 mL of deionized water. The final suspension was stirred for 30 min open to air before being transferred to a poly(tetrafluoroethene)-lined stainless steel autoclave, reaching 80 % of its volumetric capacity. The autoclave was held at 100 °C for 24 h. After cooling, the obtained solid was washed several times with ethanol and deionized until neutralization of the media. Once washed, the solid was dried at 60 °C for 12 h. Finally, the dried solid was calcined in a tubular furnace 4 ACS Paragon Plus Environment

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at 600 °C for 2 h, with heating rate of 5 °C min-1 and synthetic air flow (150 mL min-1). This sample was denoted as CeO2-NR. A little amount of CeO2-NR grains was effectively comminuted until the obtention of a very fine powder. This sample was denoted as CeO2-cNR.

Characterization X-ray powder diffraction (XRD) The samples were analyzed in a Shimadzu XRD-7000 with Cu Kα (1.5418 Å) as incident radiation generated by an X-ray tube operating at 40 kV and 30 mA. The scan speed was 2 ° (2θ) min-1. The analyses requiring in situ control of temperature and atmosphere were performed at the D10B-XRD line of the Brazilian Synchrotron Light Laboratory-LNLS, using the Arara furnace and a Mythen linear detector [resolution of 0.05 ° (2θ)]. The samples were slightly pressed on stainless steel sample holders, and the incident radiation wavelength used was 1.5498 Å. All diffractograms were collected with the same number of incident photons per detector position. The analyses were conducted at 19°C and 400 °C, with heating rate of 20 °C min-1. The gases used for atmosphere control were He, 5 % O2/He or 5 % H2/He. All of them were provided by White Martins, and the flow rate was adjusted to 100 mL min-1 with a mass flow controller (Brooks Instruments 0254). The samples were alternately exposed for 30 min to the 5% O2- or 5% H2-containing He atmospheres, with a purge of 100% He between atmosphere changes, as shown in Figure 1. The diffraction data were acquired just before each He purge steps. A total number of three cycles of atmosphere change was carried out at 400 °C. A Pfeiffer Vacuum QMA 200 M mass spectrometer was coupled to furnace outlet.



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Figure 1. Experimental scheme adopted for in situ characterization.

Transmission electron microscopy(TEM) The low resolution TEM images used for particle counting were obtained in a Zeiss Libra 120 with LaB6 electron source operating at 120 kV. The high resolution TEM images were obtained in a JEOL JEM-2100 with LaB6 electron source operating at 200 kV.

Surface area measurements A Micromeritics ASAP 2020 was used in the determination of surface areas by BET method. Samples were outgassed at 30 °C until reaching a pressure of 1 µmHg before nitrogen physisorption.

Raman spectroscopy Raman spectra were obtained in a Horiba Jobin Yvon T64000 coupled with Olympus BX41 microscope, charge-coupled device detector and incident radiation wavelength of 514 nm provided by an argon-ion laser (Melles Griot). Laser output was adjusted to 5 mW at the sample compartment, and the spectral resolution was 0.62 cm-1. Spectra were obtained with 4 accumulations of 1 min each. For CeO2-cNR the laser output was varied from 0.7 mW to 21 mW to investigate any possible effect of laser heating on the sample. A Linkam TS1500 stage coupled with Linkam T95-HT controller was used for in situ temperature and atmosphere control (resolution of 1 °C). In a similar manner to XRD (Figure 1), the atmosphere was alternated between 2 % H2/Ar and O2 at 400 °C, always with a N2 purge between atmosphere changes.


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The flow rates were adjusted to 150 mL min-1 with flow meters and the heating rate was 20 °C min-1. Three cycles of atmosphere changes were carried out.

Oxygen storage capacity measurements (OSC) The samples were analyzed in a TA Instruments SDT Q600. The experimental procedure was similar to that of Figure 1, employing different temperatures to perform the three cycles of atmosphere changes. The gases mixtures used were 2.5 % H2/N2 and 2.5 % O2/N2 with no purge of inert gas between the atmosphere changes. Approximately 12 mg of sample were used, the flow rate was adjusted to 100 mL min-1 with a mass flow controller (MKS 247), and the heating rate was 10 °C min-1. The OSC value was calculated with the final value of the mass before each change from H2- to O2-containing atmosphere.

Catalytic activity The catalytic activities of the two samples were compared according to carbon monoxide (CO) oxidation to carbon dioxide (CO2). The experimental setup was composed by a MKS 247 mass flow controller, a fixed bed reactor inside a furnace and an Agilent 7890A gas chromatograph. The feed flow was composed of 1% CO/4% O2/16% N2/79% He, and the flow rate was 100 mL min-1. CO2 production was analyzed in the temperature range from 30 °C to 400 °C, with heating rate of 1 °C min-1. An amount of approximately 50 mg of sample was mixed with 150 mg of grounded quartz and placed inside reactor with quartz wool. The chromatograph was equipped with Agilent J&W HP-Plot Q and Agilent J&W HP-Plot MoleSieve capillary columns and thermal conductive detector. The operating variables were set as: He as carrier gas, dilution at injection valve of 10:1, columns pressure of 17 psi and temperature of the chromatograph furnace of 60 °C. Under these conditions chromatography run time was 7.5 min. Quantitative determination was obtained by integration of peak areas and compared to injections of known gas quantities.

Results & Discussion



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The XRD diffractograms of both CeO2-NR and CeO2-cNR indicate the formation of CeO2 cubic phase (Figure 2). According to it, the comminution process does not substantially affect the crystallinity of CeO2-cNR sample. The crystallite sizes were estimated using the Scherrer equation,25 considering (111), (200) and (220) peaks. In both samples the estimated size is about 10 nm.

Figure 2. XRD patterns for: a) CeO2-NR and b) CeO2-cNR. The CeO2 file JCPDS 34-0394 is shown in the bottom of the figure as vertical lines.

One can see in TEM images (Figures 3 and 4) the CeO2 nanorods, and that the comminution process affects nanorods length, specially its size distribution.


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Figure 3. TEM images for CeO2-NR (a,b) and its particle size distribution (c).



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Figure 4. TEM images for CeO2-cNR (a,b) and its particle size distribution (c).

As discussed by Agarwal et al.,26 the stains observed in CeO2-NR TEM images correspond to voids in crystals. Particle size counting shows that CeO2-


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NR has a size distribution of 22-891 nm in length and 5-82 nm in width. The mode size range (c. 20 %) is

Nanorods Evaluated by in Situ Raman Spectroscopy

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