Unraveling the Low-Temperature Redox Behavior of Ultrathin Ceria

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Unraveling the Low-Temperature Redox Behavior of Ultrathin Ceria Nanosheets with Exposed {110} Facets by In-Situ XAFS/DRIFTS Utilizing CO as Molecule Probe Long Zhang, Kui Ma, Cheng Shao, Zheng Tang, Qingpeng Cheng, Peng-Fei An, Sheng-Qi Chu, Lirong Zheng, Xingang Li, and Jing Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05980 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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The Journal of Physical Chemistry

Unraveling

the

Low-Temperature

Redox

Behavior of Ultrathin Ceria Nanosheets with Exposed {110} Facets by in-situ XAFS/DRIFTS Utilizing CO as Molecule Probe Long Zhanga,b, Kui Mac, Cheng Shaoa, Zheng Tangd, Qingpeng Chengc, Pengfei Ana, Shengqi Chua, Lirong Zhenga, Xingang Lic,*, Jing Zhang a,* aBeijing

Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100049, P. R. China bUniversity cSchool

of Chinese Academy of Sciences, Beijing 100049, P. R. China

of Chemical Engineering & Technology, Tianjin University; Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science & Engineering, Tianjin 300072, P. R. China dCollege

of Physics and Space Sciences, China West Normal University, Nanchong

637002, P. R. China Corresponding Authors [email protected] [email protected], phone: +86-88235980-3

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Ultrathin ceria nanosheets with exposed reactive {110} facets are synthesized to evaluate the influence of the unique nanostructure on the low-temperature redox behavior. Via a systematic comparison of the ceria nanosheets with nanorods and nanoparticles, we demonstrate that the nanosheets show the enhanced redox reaction activity and the reduced reduction temperature. Through the in-situ XAFS experiments, we disclose that the ultrathin nanosheets intrinsically decrease the coordination number of Ce atoms and increase the disorder of ceria face-centred cubic fluorite structure, ultimately enhance the oxygen reactivity as well as mobility. Remarkably, the nanosheets exhibit the temperature-independent reducibility between 150 °C and 250 °C. Through the in-situ DRIFTS experiments, we demonstrate that the unidentate carbonates are probably the most active surface carbonates species on the redox reaction. The bidentate carbonates are speculated to be the main carbonate species as CO molecules adsorb and diffuse on the nanosheets surface. The blue shift of the band of the unidentate carbonates species on the nanosheets is consistent with the enhanced oxygen reactivity. We combine the oxygen reactivity with the variation of carbonate species to unravel the integral low-temperature redox behavior of the ultrathin ceria nanosheets.


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1. Introduction Cerium dioxide (Ceria, CeO2) and cerium-based materials are widely investigated for a variety of applications, such as catalysis, O2 sensors and biomedicine.1-6 In catalysis, CeO2 is relevant as an unique component in a wide range of catalytic applications, such as three-way catalysts,7-8 water-gas shift reaction,9-10 fuel cells,11-12 reforming processes,13-15 photo catalysis,16-17 organic reactions,18-20 electrochemical reactions,21-22 and so on. These applications generally take advantage of its excellent redox ability, i.e., the ability to quickly switch its oxidation state between Ce4+ and Ce3+ in the fluorite structure.23-24 Therefore, the research of ceria redox mechanism holds great significance for the synthesis and the application of ceria-based materials. Currently, the interaction between the CO and ceria has emerged as one of the most important issues to investigate the ceria redox mechanism. It is generally believed that the reaction proceeds via Mars-van Krevelen mechanism, which involves the rapid adsorption of CO molecules, CO2 desorption with surface oxygen vacancies formation, and subsequently replenishment of the oxygen vacancies by the migration of lattice oxygen.8, 25-27 Further investigation suggests that the interaction of ceria with CO also involves the formation of carbonate-like complexes (e.g. unidentate, bidentate, and bridged carbonate species) .26 Extensive experimental and theoretical studies have been devoted to understanding factors of the ceria redox reaction.25-33 A general consensus has been reached that CO interaction with ceria is structure-dependent. The redox properties of ceria are associated with the size, exposed facets, and intrinsic oxygen vacancies. The progress of fine modulation over synthesis procedures of ceria



nanocrystals provide a chance to effectively investigate this structure-dependent ceria redox mechanism. For instance, Xu et al. demonstrated a size-dependent oxygen buffering capacity of ceria: at sizes below 5 nm the total amount of reducible oxygen dramatically increases, which undoubtedly contributed to improving the redox property of ceria.34 Nanoscale crystallites are beneficial to the redox reaction due to their downsizing effect and relatively more available active surface sites. Mai et al. prepared single-crystalline and uniform nanopolyhedra, nanorods, and nanocubes of ceria and researched their oxygen storage capacity (OSC).30 The OSC of the ceria is proved to be shape-dependent. The oxygen storage takes place both at the surface and in the bulk for the ceria nanorods and nanocubes but is restricted at the surfaces for the nanopolyhedra just like the bulk one. Wu et al. reported that ceria rods with exposed {110} and {100} facets showed obviously better redox reactivity than the ceria cubes with exposed {100} facets and octahedra with exposed {111} facets.25 The exposed ceria facets affect surface oxygen vacancy formation energy, density of oxygen vacancy, and the lattice oxygen mobility, ultimately influence the CO interaction with ceria. Also, the cubes exhibited superior CO oxidation properties than the truncated octahedral ceria.35 In addition, density functional theory calculation corrected by on-site Coulomb interaction have been carried out to track down the lattice oxygen reactivity of ceria (111) and (110) surfaces in direct oxidation of a single CO.36 The theoretical calculations proved that the lattice oxygen of the (110) surface is more reactive that that of the (111) surface, which confirms the shape-dependent property of ceria. The density and nature of oxygen vacancies intrinsically determine the reducibility of ceria. Liu et al. prepared


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two ceria nanorods with different types and distributions of oxygen vacancies.29 The results show direct evidence of promoting the reducibility and activity of ceria nanorods with a high concentration of larger size oxygen vacancy clusters. Despite the intensive studies of variant ceria nanocrystals with differently exposed crystal planes and oxygen vacancies, the knowledge obtained from the previous ceria nanocrystals, such as cubes and rods, is an ensemble average of all atoms. As the nanocrystals size decreases, the number of the edge and vertex atoms increases, which prevents us from fundamental understanding the precise influence of the uniform exposed facets on the redox behavior and further designing better ceria-based catalysts. A better understanding of the structure-dependent redox performance of nanoceria below 5 nm with defined morphology is still in urgent need. It is highly significant and challenging to investigate the redox activity of the ceria nanocrystals below 5 nm with exposed uniform reactive facets. Here, we synthesized the ultrathin ceria nanosheets with exposed reactive {110} facets. The nanorods and nanoparticles were also prepared for comparison. Combined in-situ XAFS/DRIFTS spectroscopic investigation were employed to provide an atom-level insight into the factors of the redox reaction activity of the ultrathin ceria nanosheets. Via a systemic comparison of ultrathin ceria nanosheets with nanorods and nanoparticles, we demonstrate that the nanosheets own the prominent redox activity attributed to the enhanced oxygen reactivity as well as mobility. The unidentate carbonates are probably the most active carbonates species on the redox reaction. The blue shift of the unidentate carbonates of the nanosheets is consistent with its enhanced oxygen reactivity. These



results bring the new insights into the low-temperature redox behavior of the ceria nanocrystals below 5 nm. 2. Experimental Methods 2.1. Material synthesis CeO2 nanosheets were prepared by a modified synthetic method reported by Xie et al.37 The ultrathin CeCO3OH nanosheets were synthesized as the precursor of the ultrathin CeO2 nanosheets. In a typical synthesis procedure, 0.456 g sodium oleate (98%) was dissolved in 20 mL deionized water. Then 0.373 g CeCl3·7H2O (99%) was dissolved in 10 mL deionized water and dropwise added to the above solution using a syringe pump at an injection rate of 0.222 mL/min followed by vigorous stirring for 45 min. After adding 10 mL NH3·H2O (25 wt.%) to the above solution at an injection rate of 1 mL/min and stirring for 10 min, the mixture was transferred into a 50 mL Teflon-lined stainlesssteel autoclave, sealed and heated at 180 °C for 48 h. The system was allowed to cool down to room temperature. The light gray products were collected via centrifugation and subsequently washed by cyclohexane for several times, followed by drying in vacuum overnight. The obtained CeCO3OH nanosheets were directly heated at 400 °C for 2 min in air and then cooled to room temperature. The brilliant yellow CeO2 nanosheets were collected for further characterization. For comparison, the CeO2 nanorods and nanoparticles were also prepared via the hydrothermal methods according to the previous reports.38-39 2.2. Material characterizations X-ray power diffraction (XRD) patterns of the ceria samples were operated on a Bruker


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D8 Advance diffractometer (40 kV, 40 mA), using Cu Kα radiation (λ= 0.15406 nm). The high-resolution transmission electron microscopy (HRTEM) characterization was obtained on a JEM-2100F system operating at 200 kV. Brunauer-Emmett-Teller (BET) surface area of the ceria samples was measured via N2 adsorption-desorption experiment at liquid N2 temperature (77 K) by performing on a Quantachrome QuadraSorb SI apparatus. The samples were degassed at 150 °C under vacuum for 12 h prior to testing. 2.3. Carbon monoxide temperature-programmed reduction (CO-TPR) Temperature-programmed reduction by carbon monoxide over ceria samples (30 mg) were conducted on the micro reactor system. Before the experiment, the ceria samples were pretreated in flowing 21% O2/N2 (30 mL/min) at RT for 10 min and then ramped (10 °C/min) up to 300 °C and held for 30 min. Then the system was cooled down to RT and then switched to N2 purging for 30 min. The pretreated samples were exposed to 5% CO/He (30 ml/min) at RT for 30 min and then ramped (10 °C/min) up to 900 °C. The outlet gas was analyzed by a TCD detector. 2.4. X-ray absorption fine structure spectroscopy (XAFS) Ce L3-edge (5723 eV) X-ray absorption fine structure spectroscopy (XAFS) measurements of the ceria samples at RT were performed at the beam line (1W1B) of Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS). The ex-situ X-ray absorption spectra for the samples and reference compounds were detected in the transmission mode at room temperature. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray



absorption fine structure spectroscopy (EXAFS) were used to research the coordination environments of the Ce atoms in the ceria samples. To understand the ceria configuration during the redox reaction, in-situ XAFS spectra of the ceria samples were performed. The samples were pretreated in flowing 8% O2/N2 (30 mL/min) at 300 °C for 30 min, cooled down to RT and switched to N2 purging for 30 min. Then the samples were heated to different temperatures in the flowing N2 (30 mL/min) and then exposed to the reaction flow (30 mL/min 8% O2/N2 or 30 mL/min 5% CO/N2). The O2/N2 and CO/N2 flow were switched off and on separately to monitor the variation of valence state and the local structure of Ce atoms during the ceria redox reaction. 2.5. In-situ DRIFTS The in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) spectra were collected using a Nicolet Nexus spectrometer equipped with a MCT detector. A Harrick DRIFTS cell with nominal cell volume of 6 cm3 was used. Before each of the following DRIFTS experiments, the ceria samples were pretreated in flowing 21% O2/N2 (30 mL/min) at 300 °C for 1 h and then cooled to RT before switching to N2. The gas-phase signal of CO is corrected by the signal of CO over KBr. In CO adsorption experiments, the pretreated samples were purged with 21% O2/N2 (30 mL/min) flow for 30 min at RT before switching to 5% CO/N2 (30 mL/min) flow. In CO desorption experiments, the pretreated ceria nanosheets were exposed to flowing 5% CO/N2 (30 mL/min) for 30 min at RT and then sequentially ramped up to different temperature in the N2 purging and hold at each temperature until the IR spectra


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did not change. In CO oxidation experiments, the pretreated ceria samples were purged in N2 (30 mL/min) at different temperature and then exposed to reaction mixture (30 mL/min 21% O2/N2 and 30 mL/min 5% CO/N2). The O2/N2 and CO/N2 flows were switched off and on separately to allow monitoring surface species change. The difference spectrum was obtained by the differentials between the spectrum under CO/N2 and that under O2/N2. The DRIFTS spectra were continuously recorded until they did not change during these processes. Prior to the CO adsorption and oxidation experiment, the background spectra were collected at each temperature in N2 and O2, respectively. The intensity of adsorbates is in the form of absorbance instead of Kubelka-Munk function for the case of poorly absorbing adsorbates to obtain DRIFTS data proportional to the adsorbate surface concentrations.40 3. Results 3.1. Synthesis and characterizations of ultrathin ceria nanosheets As depicted before, the successful synthesis of ultrathin CeO2 nanosheets took advantage of the precursor ultrathin CeCO3OH nanosheets. X-ray diffraction pattern (XRD) of the as-synthesized CeCO3OH nanosheets is shown in Figure 1. It matches well with that of the pure hexagonal CeCO3OH (JCPDS card No.52-0352). The distinctly enhanced intensity in (300), (330) and (600) diffraction peaks and weakened intensity in (002) and (004) diffraction peaks verify that the CeCO3OH nanosheets are synthesized with preferred orientation. They are bound by {030} facets, and ultrathin along the [001] direction.



Figure 1. XRD pattern of as-synthesized ultrathin CeCO3OH nanosheets. XRD pattern of the calcined ceria nanosheets is shown in Figure 2. The diffraction peaks can be indexed to (111), (200), (220), (311), (222) and (400) crystal faces, corresponding to a face-centred cubic fluorite structure of CeO2 (JCPDS card No.655923). The sheets and particles exhibit broadened XRD peaks compared with the rods, especially the (220) peak, which indicates that they exhibit relatively lower crystallinity. The average crystal parameters of the samples are summarized in Table 1. The mean particle sizes, determined from the Scherrer equation, are 11.0, 6.1, and 5.1 nm for rods, sheets, and particles, respectively. The sequence of crystallinity is particles< sheets < rods. The BET areas of the ceria sheets, rods, and particles are 73.8, 98.2, and 130.2 m2g-1, respectively.


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Figure 2. XRD patterns of as-synthesized ceria nanosheets, nanorods, and nanoparticles. Table 1. Average Crystal Parameters of Different Ceria Samples. Sample

Crystallite size (nm)

Lattice constant (nm)

Surface area (m2g-1)

sheets

6.1

0.543

74

rods

11.0

0.543

98

particles

5.1

0.546

130

bulk

--

0.541

7

Figure 3 shows the TEM and HRTEM images of the ultrathin CeCO3OH nanosheets and three CeO2 samples. The CeCO3OH nanosheets are about 50-150 nm across (see Figure 3a1) and 2-3 nm thick which can be inferred from Figure S1. The interplanar spacing of 0.193 nm corresponding to the d spacing of (330) facets and the corresponding dihedral angle of 60° is fairly consistent with the calculated angle between different (330) facets. The results indicate that the CeCO3OH sheets are undoubtedly bound by {330} facets, or {300} facets, which has been verified by their enhanced intensity in (330) and (300) diffraction peaks in Figure 1. The yellow circles (Figure 3a2) show that the sheets are made up of many small CeCO3OH particles with a mean particle diameter about 5 nm by statistical analysis, and there are lots of surface concentrated unsaturated sites between the small particles which are marked by the red circles (Figure 3a3). The structure of the calcined ceria sheets can be clearly observed in the TEM images in Figure 3b1-3. The mean diameter size of ultrathin ceria sheets is 100 nm and they are overlapped with several layers due to the calcination. The HRTEM images reveal the spacing of the lattice fringe is 0.316 nm, which proves the interplanar of (111) facets. The angle of the lattice fringe is 70°. The orientation of the nanosheets



should be orthogonal to the lattice fringe, which means it should be [110] orientation. The red circles indicate that there are lots of surface pits with a mean diameter of 1 nm on the CeO2 nanosheets.37

Figure 3. TEM and HRTEM images of as-synthesized CeCO3OH nanosheets (a1-3), ceria nanosheets (b1-3), nanorods (c1-3) and nanoparticles (d1-3). The TEM images (see Figure 3c1-3) shows the nanorods are about 10 nm across and 50-250 nm long. It is widely accepted that the synthesized nanorods grow along the [110] direction and prefer to expose {110} and {100} surfaces. The nanoparticles (Figure 3d1-3) show less uniform shape and size distribution, exposing {100}, {110} and {111} facets, so they are composed of loose packed particles which are dominated by complex surface termination. The particle size of the ceria nanoparticles is in the range of 3-10 nm. The interplanar spacing of 0.280 nm, 0.195 nm and 0.325 nm respectively corresponding to the d spacing of (100), (110) and (111) facet are wider than the relevant spacing of the lattice fringe on nanosheets and nanorods. The TEM observations are in the good accord with previous studies of ceria


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nanomaterials, indicating successful synthesis of the ceria nanocrystals with desired morphology. To keep the morphology and surface structure unchanged, measurements on the three ceria samples were carried out at temperatures lower than 400 °C, except in some temperature-programmed experiments (such as CO-TPR) where sacrificed by heating to higher temperatures.25 3.2. CO-TPR The reducibility of the synthesized ceria nanocrystals was characterized by CO-TPR measurement. The TCD signals during CO-TPR over the samples are shown in Figure 4.

Figure 4. CO consumption during CO-TPR experiment over ceria nanosheets, nanorods, and nanoparticles. With the temperature elevating, various types of reactive oxygen have been identified on ceria, e.g. surface lattice oxygen, surface hydroxyl oxygen and bulk lattice oxygen.25 The ceria samples exhibit three main reduction peaks Os1, Os2 and Ob, respectively. The peak Os1 below 330 °C belong to CO reaction with reactive surface lattice oxygen of ceria. CO + OL→CO2



The peak Os1 of sheets at 220 °C for releasing weakly bound surface lattice oxygen shifts to lower temperature by approximately 20 °C compared with the rods and 100 °C compared with the particles, which prove the presence of most reactive oxygen on the surface of sheets. As the previous report,31 the bound oxygen on the ceria (110) facets own the highest activity. The nanosheets and unsaturated Ce sites on the (110) plane could enable the weakly bound oxygen to be more easily released. The particles exhibit the lowest reduction peak Os1, which could be attributed to their complex exposed facets including less activity {111} facets. Another reduction peak Os2 at 400 °C could be attributed to the reaction between CO and adsorbed surface OH groups via water-gas shifted type reactions:41-42 CO + OH→0.5H2 + CO2 CO + 2OH→H2 + CO2 + OL The Os2 peaks of sheets and rods present at 400 °C, which suggests that the surface OH groups on the sheets and rods own the similar activity. Further analyzing the TPR curves, as shown in Figure S2 and table S1, the consumption of CO per gram at low temperature is 1.34, 1.66, and 1.81 mmol for particles, rods and sheets, respectively.43 Accordingly, the low-temperature reducibility of the surface oxygen species of ceria was enhanced in the sheets. The bulk lattice oxygen species of ceria, Ob, has different reducibility and follows the order: sheets > particles > rods. The Figure 3 has exhibited that the surfaces of sheets present large amount of the pits. We speculate that the ultrathin nanostructure and surface pits together lead to the highest low-temperature oxygen reactivity of ultrathin ceria nanosheets.29


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3.3. XAFS spectroscopy To monitor the valence state and the local structure of Ce ions, XAFS measurements at the Ce L3-edge of the ceria samples were carried out in transmission mode at the X-ray absorption station (beam line 1W1B) of Beijing Synchrotron Radiation Facility (BSRF) using a double crystal Si (111) monochromator. The extend X-ray absorption fine structure (EXAFS) functions were Fourier transformed to R space with k2-weight in the range 2.5-9.6 Å-1. The X-ray absorption near-edge structure (XANES) spectra are shown in Figure 5a. The peaks A (5738.5 eV), B (5731.7 eV) and C (5721.2 eV) are present, which suggests that all samples are face-centred cubic fluorite structure. The virgin samples give typical doublet white lines characteristic of Ce4+ species in CeO2. The peaks A and B of the sheets decrease remarkably compared with those of the rods and particles which could be attributed to the structure distortion of the sheets. As shown in Figure 5b, the Ce L3-edge k2χ(k) oscillation curve for the CeO2 sheets displays obvious differences compared with the rods, particles and bulk. It is further verified by their corresponding Fourier transformed FT (k2χ(k)) functions in Figure 5c. The intensities of all peaks of sheets decrease remarkably and the peak at 3.6 Å shifts slightly to the low R direction compared with that of rods and particles, which qualitatively reveals their distinct local atomic arrangement. The FT (k2χ(k)) curve of bulk CeO2 exhibits three main peaks at 1.95, 3.65, and 4.20 Å, corresponding to the nearest path Ce-O, next nearest Ce-Ce, next to next nearest Ce-O1 and multiple scattering path Ce-O-Ce-O. The peak at 4.2 Å is attributed to the superposition of path Ce-O1 and Ce-O-Ce-O. Moreover, to achieve quantitative structural parameters around



Ce atoms, a least-squares curve fitting for the Ce-O, Ce-Ce, Ce-O1 and Ce-O-Ce-O paths was conducted.

Figure 5. Ce L3-edge (a) XANES spectra, (b) extended XAFS oscillation function k2(k), and (c) the corresponding Fourier transformed FT (k2(k)) of ceria nanosheets, nanorods, and nanoparticles collected at RT. The IFEEFIT package was employed to analyze EXAFS data using a theoretical model generated by the FEFF 8.4 code.44 The corresponding fitting results of the Fourier transformed FT (k2(k)) are displayed in Figure S3. The best fits for the EXAFS


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data of samples are shown in Table 2. Table 2. EXAFS Curve Fitting Results of Different Ceria Samples. Sample

Sheets

Rods

Particles

Bulk

Path

R (Å)

N

σ2 (10-3 Å2)

ΔE0 (eV)

Ce-O

2.323

5.9

6.7

8.2

Ce-Ce

3.829

9.1

6.7

5.0

Ce-O1

4.440

15.7

11.6

6.3

Ce-O-Ce-O

4.637

5.9

13.4

8.2

Ce-O

2.323

7.8

7.8

7.8

Ce-Ce

3.829

10.3

6.2

4.1

Ce-O1

4.443

16.5

8.1

5.2

Ce-O-Ce-O

4.643

7.8

15.6

7.8

Ce-O

2.322

7.1

7.7

7.7

Ce-Ce

3.833

9.6

6.3

4.2

Ce-O1

4.447

16.1

7.7

5.2

Ce-O-Ce-O

4.645

7.1

15.4

7.7

Ce-O

2.336

8.0

4.0

8.4

Ce-Ce

3.836

12.0

3.7

4.7

Ce-O1

4.444

24.0

7.0

5.2

Ce-O-Ce-O

4.667

8.0

8.0

8.4

R-factor

0.0167

0.0100

0.0078

0.0029

Structural parameters around Ce atoms extracted from EXAFS curve-fitting for ceria nanosheets, nanorods, nanoparticles and bulk, respectively.

Rietveld analysis of CeO2 nanosheets reveals that the bond length and coordination number of the Ce-O path are 2.323 Å and 5.9, respectively, while rods and particles have Ce-O path coordination numbers of 7.8 and 7.1. Besides, the coordination number of other paths like Ce-Ce, Ce-O1 and Ce-O-Ce-O in sheets also decrease remarkably compared with those in rods and particles. The sheets exhibit the highest oxygen vacancy concentration. Meanwhile the fits show that Ce-O1 distances of sheets are less



than those of rods and particles, which has been verified by the Figure 5c. The disorder degrees of all paths of sheets increase significantly, suggesting a noticeable distortion of coordination environment of Ce ions, which is consistent with the above result. 3.4. In-situ XAFS spectroscopy To elucidate specific information about the valence state and coordination environment change of Ce ions of the ultrathin CeO2 nanosheets on the redox reaction, the in-situ Ce L3-edge XAFS measurements were also performed at different temperatures and atmosphere. The Ce L3-edge XANES spectra and the corresponding Fourier transforms FT (k2χ(k)) spectra of the sheets and rods are displayed in Figure 6.


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Figure 6. Ce L3-edge (a) XANES spectra and (b) the corresponding Fourier transformed FT (k2(k)) of ceria nanosheets collected at different temperature in CO/N2 or O2/N2 atmosphere. Ce L3-edge (c) XANES spectra and (d) the corresponding Fourier transformed FT (k2(k)) of ceria nanorods collected at different temperature in CO/N2 or O2/N2 atmosphere. As shown in the Figure 6a and b, the XANES spectra of ceria sheets at different temperatures (150-250 °C) show quite similar features when exposed to the O2 flow. The reduction degree doesn’t change much exposed to CO flow until the temperature



rise up to 150 °C. Above 150 °C (142 °C, precisely), the sheets could be reduced quickly which is derived from the white line left shift by 3.2 eV and the appearance of the peak D at 5728.3 eV in the Figure 6a. This peak is commonly accepted as a symbol of Ce3+ in the ceria. The reduction of ceria sheet at 150 °C is consistent with the peak Os1 of sheets at the CO-TPR. The corresponding Fourier transforms FT (k2χ(k)) spectra show that all the peaks decrease remarkably due to the synergistic effect of oxygen vacancies formation and the structure disorder degree increase. Importantly, with the temperature rising, reduction degree of nanosheets at 150-250 °C temperature range did not increase. Hence, the reactivity and mobility of oxygen in the ultrathin nanosheets remain less temperature relevant at this range temperature, which is due to the ultrathin sheet structure made up by only several atomic layers. Due to the quantum size effect, the oxygen atoms on the sheets surface are activated easily above 150 °C and migrate to the adjacent oxygen vacancies sites. Therefore, the oxygen mobility of ceria nanosheets is temperature-independent at the low-temperature range. When the temperature arises above 250 °C, the reducibility of the ceria nanosheets would get enhanced continuously. Compared with the ultrathin nanosheets, the rods don’t get reduced until the temperature rise up to 250 °C, which is remarkably higher than the initial reduction temperature of the ceria sheets (see Figure 6c). With the temperature rising continuously at the range between 250 °C and 350 °C, the reduction degree of the rods got gradually deepened. This is verified by their corresponding Fourier transformed FT (k2χ(k)) functions in Figure 6d. The peaks at 300 °C exposed to the CO/N2 decreased


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much more than that at 250 °C. This implied an obvious viewpoint that with the temperature rising, more oxygens of the rods could be activated, migrate to the oxygen vacancies, and then react with the CO adsorbed on the surface. It differs markedly from the temperature-independent redox reaction of sheets at low-temperature range shown in Figure 6b. The XAFS analysis prove the unique local structure of the sheets compared with the rods which is the main reason for higher surface oxygen reactivity and better redox property. 3.5. In-situ DRIFTS of CO adsorption In-situ DRIFTS technology was performed to investigate the surface species variation on the ceria nanosheets using CO as molecule probe. Figure 7a shows the time-resolved spectra of CO adsorption on the ceria nanosheets at room temperature. The spectra of CO adsorption on the ceria nanorods and nanoparticles are shown in Figure 7b and Figure 7c for comparison, respectively. Notably, bands in the region of 2600-1800 cm-1 are associated with the linearly or bridgingly adsorbed CO on the surface Ce sites.25 The intensity of the two sharp bands at 2172 and 2118 cm-1, which can be assigned to the overlap of the gas phase CO and the adsorbed CO on the surface unsaturated Ce sites, gradually strengthens with the CO continuously introducing. More importantly, from the variation tendency of the bands in the range of 1800-800 cm-1 and 3800-2600 cm-1, we imply that CO interacts strongly with the surface oxygen of ceria, thereby forming a variety of carbonaceous species.26, 45 The corresponding bands assignments are summarized in Table 3. Over the ceria nanosheets, the bicarbonate species (3710, 3665, 3343, 1617, 1289, 1217, 1025 and 860 cm-1), formates species (2937, 2843, 2725,



1572, 1550, 1373 and 1358 cm-1), bidentate carbonate species (1598, 1289, 1025, and 860 cm-1), and unidentate carbonate species (1434 cm-1) are observed.25, 27, 33,45-48 The band 1122 cm-1 could be attributed to the bridged carbonate species and the corresponding higher wave number band around 1700 cm-1 probably is covered by the wide band at 1617 cm-1. Initially, the CO adsorbed on the ceria surface as the bidentate carbonate species, and then reacted with the adjacent hydroxyl to form bicarbonate.27 Then the bands due to the bridged carbonate species and unidentate carbonate species appeared. The positive bands of formates formed at first and the corresponding bands turned negative with the CO continuously introducing. The unstable formates probably reacted with CO molecule and transform into the bicarbonate on the nanosheets at room temperature.48 Moreover, the two negative sharp bands at 3710 and 3665 cm-1, can be assigned to the type Ⅰ and Ⅱ hydroxyl groups.25


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Figure 7. In-situ DRIFTS spectra of CO adsorbed on ceria samples at RT as a function of time after start of CO adsorption (a) nanosheets (b) nanorods (c) nanoparticles. (d) Comparison of CO adsorbed on the three ceria nanocrystals at RT after 30 min adsorption. Spectra are referenced to pretreated samples in N2 prior to introducing the CO flow. Table 3. Assignment of IR Bands Observed upon Room Temperature CO Adsorption on Ceria Nanocrystals.37,45-48 Sample

Species

(CO3)

(CO3)

δ(OH)

(OH)

Sheet

Bicarbonate

1617,1289,1025

860

1217

3710,3665,3343

Bidentate carbonate

1598,1289,1025

860

Unidentate carbonate

1434

Formate

1572,1550,1372,1358

Bicarbonate

1614,1297,1034

865

1217

3712,3674,3258

Bidentate carbonate

1585,1297,1034

865

Rod



Particle


1419

Formate

1570,1552,1372,1361

Bicarbonate

1607,1293,1045

854

Bidentate carbonate

1588,1294,1044

854


1393

Formate

1572,1552,1372,1358

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1216

3712,3669,3268

The original DRIFTS contrast of the pure CO gas signal and three ceria samples at 2300-2000 cm-1 is shown in Figure S4. There is little difference of the bands at 2171 and 2118 cm-1 among the three samples and only a little decrease of band at 2171 cm-1 on nanosheets. The band 2045 cm-1 on the nanoparticles could be part of the wave number at 2118 cm-1 shifted to the lower wave number,45 which can be attributed to the enhanced Ce-CO bond, indicating a stronger interaction between CeO2 nanoparticles and CO. Figure 7d shows the DRIFTS spectra of steady-state CO adsorption at RT over the ceria sheets, rods and particles. The CO gas signal has been subtracted here.49 Obviously, similar locations of carbonate species are observed on the surfaces of all three nanocrystals, but the corresponding bands, especially that at 1289 and 1025 cm-1 over the sheets are much sharper. It should be attributed to that the sheets with exposed (110) facets possess a uniform local coordination environment, providing the similar CO adsorption sites. Moreover, the vibrational band at 1434 cm-1 attributed to unidentate carbonates on the sheets exhibits a blue shift as comparison with that on the rods at 1419 cm-1 and particles at 1393 cm-1, implying that the bond between the adsorbed CO and the surface of sheets is different from those of rods and particles, which is probably related to the surface oxygen reactivity. 3.6. Temperature stabilization of the carbonates on ceria nanosheets


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As discussed in Figure 7, the adsorbed CO on the surface Ce4+ sites and a variety of surface carbonaceous species (e.g. bicarbonates, formates, bidentate and unidentate carbonates) are observed at RT. To investigate the stability and reactivity of the carbonate species on the ceria nanosheets, in-situ DRIFTS was employed to monitor the evolution of the surface species on the ceria samples. Figure 8 shows the temperature-dependent in-situ DRIFTS spectra of the ceria nanosheets.

Figure 8. In-situ DRIFTS spectra of CO adsorbed on ceria nanosheets at different temperatures in N2 after CO adsorption at RT. Spectra are referenced to the background spectra collected at the indicated temperature during cooling down from pretreatment process of the ceria nanosheets. In the region of 1800-800 cm-1, the intensity of bands tremendously changes as a function of temperature. With the temperature elevating, the characteristic band assigned to the unidentate carbonate species at 1433 cm-1 continuously weakens and finally disappears above 200 °C, which is the consistent with the initial reduction temperature of the nanosheets. Simultaneously, the intensity of the bands at 1598 and 1289 cm-1 assigned to the bidentate carbonates also decreases rapidly, but they could still be observed at 250 °C. Notably, the negative bands at 1551, 1370 and 1358 cm-1



attributed to the formates disappear and transform into positive bands, which suggests that the proportion of formate species in the surface carbonaceous species on the sheets increases with the elevated temperatures at this range. The wide band in 3346 cm-1 and 1126 cm-1 respectively ascribed to the bicarbonates and the bridged carbonates disappears above 200 °C. The presence of the band at 2136 cm-1 above 200 °C might be attributed to an electronic transition band caused by the presence of Ce3+ in the reduced ceria. In summary, the stability of the carbonaceous species could be concluded from the observed variation of the corresponding bands over the ultrathin ceria nanosheets, following the order of formates > bidentate carbonates > unidentate carbonates > bicarbonates ≈ bridged carbonates. 3.7. In-situ DRIFTS study of carbonates in ceria redox reactions In-situ DRIFTS study of CO adsorption and subsequent oxidation on the ceria nanosheets was performed to identify the reactive surface species in the redox reaction of ceria. The ceria nanosheets experienced a redox cycle, i.e., the initial CO reduction and subsequent reoxidation by O2 over the ceria. The corresponding in-situ DRIFTS spectra collected at the different temperatures are shown in Figure 9a-c. In order to give a clear interpretation on the oxidation process of carbonates over the sheets, we also make the corresponding difference spectrum at each temperature.


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Figure 9. In-situ DRIFTS spectra collected during gas switching on ceria nanosheets at different temperatures. (a) 50 °C; (b) 100 °C; (c) 150 °C. The difference spectra were obtained by subtracting the spectrum in O2 from the spectrum in CO. As discussed above, CO adsorbs on the ceria in the form of the carbonaceous species, which was observed to maintain relatively stable under N2 or O2 purging at 50 °C due to the slight changes of the corresponding bands as comparison with those under CO condition. After switching from CO to O2 at 50 °C, the bands at 1553-1613, 1289, 1022, and 858 cm-1 ascribed to the bidentate carbonate species decreased slightly shown in



Figure 9a, which is the result of the partial weak absorption of the bidentate. Other bands attributed to unidentate carbonates and formates exhibited little change. The redox reaction cannot occur at 50 °C. After O2 purging at 100 °C for 5 min, it is obviously observed that there is a remarkable decrease for all of the carbonate species, implying that the oxidation process induced by O2 probably occurs on the surface at 100 °C. Specifically, upon O2 purging at 100 °C, the unidentate carbonates (1429 cm-1) are almost absent and the bidentate carbonates (1581-1622, 1288, 1026 and 859 cm-1) become dominant. Additionally, the formate (1541 and 1369 cm-1) and the bridged carbonate (1108 cm-1) are stable and inert to the redox reaction, implying that the unidentate and bidentate carbonates participate in the redox reaction while the formate and bridged carbonate species not. The corresponding differential spectra at 100 °C are also presented in Figure 9b. It is almost attributed to the bands of the unidentate carbonates and bidentate carbonates. Upon the switch between CO and O2 at 150 °C, the carbonaceous species undergo significant changes as shown in Figure 9c. With the introduction of O2, the observed unidentate carbonates bands decreased rapidly, while the bands assigned to the bidentate carbonates decreased slowly. After purging O2 for 10 min, the bands of the bidentate (1614, 1288, 1005-1043 and 853 cm-1) and unidentate (1414-1441 cm-1) carbonate species remarkably decrease, while the bands at 1572, 1547, 1371 and 1359 cm-1 ascribed to the formate remain unchanged, which confirms that the formates are considered as the spectator in the redox reaction of the ceria nanosheets. The spectra after O2 purging in Figure 9c also verify that the bidentate carbonates would decrease


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slowly after most of the unidentate carbonates had vanished. All the carbonaceous species except for the formates and some bidentate carbonates would finally disappear under the oxidizing condition at 150 °C. For further evaluation, the bands at 1200-1800 cm-1 are fitted with Gaussian peaks. The absorbance at 1589, 1540 and 1423 cm-1 are measured to present the population of bidentate carbonates, formates and unidentate carbonates, respectively. The fitting results are shown in Figure S5. Figure S5f provides the quantitative variation of three adsorbed carbonates with the O2 gas purging, which further verify that the higher reactivity of the unidentate carbonates compared with the bidentate carbonates at 150 °C. The bands due to the bidentate carbonate species continuously decrease might be attributed to their conversion into unidentate carbonate species at high temperature.33 To clarify the intrinsic changes of the carbonate species during the redox reaction of ceria nanocrystals, the CO adsorption and oxidation on the rods and particles at 150 °C were also performed for comparison as shown in Figure 10.



Figure 10. In-situ DRIFTS spectra collected during gas switching on ceria samples at 150 °C. (a) nanorods (b) nanoparticles The bands of the bidentate (1588, 1293, 1045 and 864 cm-1) and unidentate (14001411 cm-1) carbonate species remarkably decrease with the O2 gas introducing. The corresponding fitting results of spectra on nanorods are shown in Figure S6. The fitting result of the particles are not shown because of the negative band at 1550 and 1357 cm-1, which may result in the unsatisfactory fitting results. The Figure S6f shows that the unidentate carbonate species decrease more quickly than the bidentate on the rods. The previous report proposed that the population among bicarbonates, unidentate carbonates, bidentate carbonates, and formates might be vital to the redox of ceria.27 From the fitting result of DRIFTS spectra of the ceria oxidation process at 150 °C in Figure S5 and Figure S6, the ratio of unidentate carbonates (1405-1441 cm-1) to


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bidentate carbonates (1611-1619, 1289-1308 cm-1) for the sheets at 150 °C is 0.317, which much higher than that for the rods (0.103) and particles (0.189). The relatively more unidentate carbonates is related with the ultrathin sheet structure and the concentrated oxygen vacancies on the surface and would probably contribute to the higher reducibility of nanosheets at low temperature. Besides, the variation rate of the unidentate carbonate species of sheets is higher than that of rods, which is coincident with the more CO2 band area of DRIFTS spectra on sheets shown in Figure S7. Therefore, the unidentate carbonates species are probably the most active among all carbonaceous species at low-temperature oxidation of the ceria nanosheets. It is noteworthy that the amount of unidentate carbonates might not be the single critical factor in the redox reactivity. The unidentate carbonates (1414-1441 cm-1) on the sheets are found to shift blue toward that of rods (1400 cm-1) and particles (1411 cm-1), which is also consistent with the lowest surface lattice oxygen reducible temperature of the sheets. As the vibrational band of unidentate carbonates on the sheets at 1423 cm-1 shifts blue, the oxygen reactivity of the sheets shows enhanced. The bond property between the adsorbed CO molecules and the oxygens on the ceria also has a significant impact on the redox properties of the ceria nanocrystals. 3.8. Low-temperature activity and mobility of oxygen in ultrathin ceria nanosheets With the increasing demand of the low-temperature ceria-based catalysis in the energy and environment field, the low-temperature activity of oxygen in ceria should be investigated. Oxygen mobility plays an important role in the redox reaction of ceria nanocrystals, because the high oxygen mobility can enhance the surface oxygen species



reactivity and oxygen storage capability.25 It is widely accepted that the migration of oxygen in ceria and ceria-based materials takes place via a vacancy hopping mechanism.29 Wu et al. found that the interaction of CO with ceria (adsorption and reduction) and the reactivity and mobility of ceria surface and bulk oxygen are closely related to the exposed surface structure of ceria nanocrystals.25 Here, the reduction temperature from both CO-TPR, in-XAFS and in-DRIFTS demonstrates structuredependent oxygen reactivity with the sequence of sheets > rods > particles below 600 °C and sheets > particles > rods at higher temperatures. The result shows that the ultrathin ceria nanosheets own the highest oxygen reactivity among the three samples. Esch et al. proved that clusters of more than two vacancies exclusively expose the reduced cerium ions, primarily by including subsurface vacancies, which therefore play a crucial role in the process of vacancy cluster formation.23 The presence of oxygen vacancies on the sheets below 5 nm provides relative freedom for the movement of lattice oxygen and thus increases the mobility of oxygen in ceria. Hence, the underlying reason for the high mobility of lattice oxygen on sheets is that the mount of oxygen vacancies in the ultrathin nanosheets is higher than those of other two samples verified by the lowest coordination number of Ce atoms on nanosheets. The reducibility of the ultrathin ceria nanosheets at 150, 200 and 250 °C keep unchanged, which proves that the oxygen mobility on the sheets is temperature-independent at the low-temperature range. On the contrary, the oxygen mobility of the ceria rods increases continuously with the temperature elevating, which imply that the oxygens in both ceria samples are different. The oxygens in the sheets are highly activated and uniform, while the oxygens


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in the rods are various and attributed to the complex exposed surface facets. The prominent activity and mobility of the oxygens in the ultrathin ceria nanosheets gives a new sight to understand the oxygen migration mechanism and provide potential implications for the design of new ceria-based catalysts. 3.9. Low-temperature redox mechanism of ultrathin ceria nanosheets It has been well accepted that the redox reaction proceeds on ceria with CO molecule via the Mars-van Krevelen mechanism. Here the synthesized ultrathin ceria nanosheets and the following in-situ spectroscopic investigation shed light on the atomic-level the redox mechanism of the ultrathin ceria nanocrystals. Specifically, we found that the chemical bonds of CO reaction with ceria are closely related to the exposed surface structure of ceria nanocrystals. The corresponding carbonates by CO reaction with surface oxygens are crucial to the reducibility of the ceria nanocrystals. The unidentate proportion might be a crucial factor in determining the redox reactivity. The bicarbonates are unstable with the temperature changing and the bands didn’t obviously change on the sheets during the oxidation test, thus bicarbonates might be the inactive species during the redox process. The formates decrease with CO adsorption at RT and increase at higher temperature, which is the result of the mutual conversion with bicarbonate. During the oxidation reaction, the intensity of formates on sheets remain unchanged, which proves that the formates are stable and inert to the redox reaction and they are believed to be the spectators upon CO oxidation. The bidentate carbonates prove to be the initial species for CO adsorption and more stable than the unidentate carbonates, which is consistent with previous experimental and theoretical studies.27



After purging O2 feed, the unidentate species decreased quickly, while the bidentate species decreased slowly in comparison. Finally, the bidentate species on ultrathin ceria nanosheets would almost disappear while that of the rods and particles remain their intensities. The unidentate carbonates formed on the ceria surface might be the most active species for the ceria nanocrystals and the bidentate carbonates migrate to defect sites and transfer to the unidentate by the CO molecules migration after overcoming a diffusion barrier. The ultrathin sheet structure of the ceria nanosheets and the highly coordination unsaturated Ce sites ensure the oxygen activity and mobility, simultaneously enhance the CO fast diffusion as the bidentate along the surface and facilitate the CO oxidation and the ceria reduction. At the same time, the blue shift of the unidentate carbonates on the ultrathin nanosheets compared with the rods and particles, indicating the stronger C=O bonds and the weaker C-O interface bonds between CO and ceria surface, which could induce the CO2 molecules easier to escape from the ceria surface. Therefore, the variety and strength of the interaction between ceria surface and the CO would have an impact on in the redox property of the ultrathin ceria nanosheets. 4. Conclusions Ultrathin ceria nanosheets exposed reactive {110} facets were synthesized and further investigated in detail by in-situ spectroscopy experiments including XAFS and DRIFTS coupled with CO-TPR to evaluate the influence of the unique nanostructure on the lowtemperature redox reaction using CO as a probe molecule. The ceria nanorods and particles were also synthesized as the references. We demonstrate that the synthesized


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ultrathin ceria nanosheets own the prominent redox property due to the defective surface exposed the reactive {110} facets with excellent oxygen activity and mobility. Remarkably, the nanosheets exhibit the temperature-independent reducibility at continuous elevated low-temperatures range, which is the result of the ultrathin sheet structure. The unidentate carbonates species reduced quickly under the oxidation circumstance among the carbonaceous species. We demonstrate that the unidentate carbonates might be the most active surface carbonates on the ceria nanosheets redox reaction. The bidentate carbonates are speculated to be the main carbonate species as CO molecules adsorb and diffuse along the ceria nanosheets surface and transform into the unidentate carbonate. Besides, the blue shift of the unidentate carbonates bands in the nanosheets is consistent with the enhanced oxygen reactivity and mobility, which will ultimately enhance the ultrathin ceria nanosheets redox activity.

Supporting Information Supporting information available: (TEM images of as-synthesized CeCO3OH nanosheets. The curve fitting results of the TPR experiment of the ceria samples. The corresponding CO consumption and active oxygen of different ceria samples from the TPR experiment. The corresponding fitting results of the Fourier transformed FT (k2(k)) of ceria samples. The original DRIFTS spectra contrast of the pure CO gas signal and three ceria samples at 2300-2000 cm-1. The fitting result of the DRIFTS spectra of the adsorbed CO oxidation process on the nanosheets and nanorods at 150 °C. The variation of the bands due to CO2 gas in the DRIFTS spectra over the CO oxidation



process on the nanosheets and nanorods.) This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments We appreciate the financial support from the National Key Research and Development Program of China (No. 2017YFA0403403) and the National Natural Science Foundation of China (No. 21476159). References (1) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. The Utilization of Ceria in Industrial Catalysis. Catal. Today 1999, 50, 353-367. (2) Kašpar, J.; Fornasiero, P.; Graziani, M. Use of CeO2-Based Oxides in the ThreeWay Catalysis. Catal. Today 1999, 50, 285-298. (3) González-Castaño, M.; Saché, E. L.; Ivanova, S.; Romero-Sarria, F.; Centeno, M. A.; Odriozola, J. A. Tailoring Structured WGS Catalysts: Impact of Multilayered Concept on the Water Surface Interactions. Appl. Catal. B 2018, 222, 124-132. (4) Jasinski, P.; Suzuki, T.; Anderson, H. U., Nanocrystalline Undoped Ceria Oxygen Sensor. Sens. Actuators, B 2003, 95, 73-77. (5) Liao, L.; Mai, H. X.; Yuan, Q.; Lu, H. B.; Li, J. C.; Liu, C.; Yan, C. H.; Shen, Z. X.; Yu, T. Single CeO2 Nanowire Gas Sensor Supported with Pt Nanocrystals: Gas Sensitivity, Surface Bond States, and Chemical Mechanism. J. Phys. Chem. C 2008, 112, 9061-9065. (6) Patil, S.; Sandberg, A.; Heckert, E.; Self, W.; Seal, S. Protein Adsorption and Cellular Uptake of Cerium Oxide Nanoparticles as a Function of Zeta Potential.


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Artefacts and Correction of Gas-Phase Species Signal during Diffuse Reflectance FTIR Analyses of Catalysts at Variable Temperatures. Appl. Catal. A- Gen. 2015, 495, 1722



Table of Contents Image


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Figure 1. XRD pattern of as-synthesized ultrathin CeCO3OH nanosheets. 55x38mm (600 x 600 DPI)



Figure 2. XRD patterns of as-synthesized ceria nanosheets, nanorods, and nanoparticles. 55x38mm (600 x 600 DPI)


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Figure 3. TEM and HRTEM images of as-synthesized CeCO3OH nanosheets (a1-3), ceria nanosheets (b1-3), nanorods (c1-3) and nanoparticles (d1-3). 78x77mm (600 x 600 DPI)



Figure 4. CO consumption during CO-TPR experiment over ceria nanosheets, nanorods, and nanoparticles. 79x55mm (600 x 600 DPI)


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Figure 5. Ce L3-edge (a) XANES spectra, (b) extended XAFS oscillation function k2χ(k), and (c) the

corresponding Fourier transformed FT (k2χ(k)) of ceria nanosheets, nanorods, and nanoparticles collected at RT. 150x284mm (600 x 600 DPI)



149x161mm (300 x 300 DPI)


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149x109mm (300 x 300 DPI)



Figure 8. In-situ DRIFTS spectra of CO adsorbed on ceria nanosheets at different temperatures in N2 after CO adsorption at RT. Spectra are referenced to the background spectra collected at the indicated temperature during cooling down from pretreatment process of the ceria nanosheets. 55x38mm (600 x 600 DPI)


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Figure 9. In-situ DRIFTS spectra collected during gas switching on ceria nanosheets at different temperatures. (a) 50 °C; (b) 100 °C; (c) 150 °C. The difference spectra were obtained by subtracting the spectrum in O2 from the spectrum in CO. 153x293mm (600 x 600 DPI)



Figure 10. In-situ DRIFTS spectra collected during gas switching on ceria samples at 150 °C. (a) nanorods (b) nanoparticles 79x106mm (600 x 600 DPI)


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Unraveling the Low-Temperature Redox Behavior of Ultrathin Ceria

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