Surface-Dependence of Defect Chemistry of Nanostructured Ceria

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Surface-Dependence of Defect Chemistry of Nano-Structured Ceria Shilpa Agarwal, Xiaochun Zhu, Emiel J. M. Hensen, Barbara Louise Mojet, and Leon Lefferts J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 May 2015 Downloaded from http://pubs.acs.org on May 8, 2015

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The Journal of Physical Chemistry

Surface-dependence of Defect Chemistry of NanoStructured Ceria S. Agarwal, [a] X. Zhu, [b] E. J. M. Hensen, [b] B. L. Mojet, [a] L. Lefferts [a,c]* [a] Catalytic Processes and Materials, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands [b] Schuit Institute of Catalysis, Laboratory of Inorganic Materials Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands [c] Aalto University, School of Chemical Technology, Department of Biotechnology and Chemical Technology, Research Group Industrial Chemistry, P.O. Box 16100, FI-00076 Aalto, Finland. Corresponding author: L. Lefferts, Tel: +31534892858, email: [email protected]

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Abstract The defect chemistry of reduced ceria nanoshapes was investigated using in situ Raman and FTIR spectroscopy. Octahedral- and rod-shaped particles behave similarly upon exposure to CO in terms of formation of anion Frenkel pair defects and oxygen

vacancies. This similarity is attributed to the preferential exposure of (111) surfaces in both type of particles. Cube-shaped particles terminated with (100)-oriented surfaces exhibit very different defect behavior in CO, revealing formation of oxygen vacancy defects at the expense of existing anion Frenkel pairs. Octahedra and rods, prereduced in H2, can be further reduced with CO. In contrast, pre-reduced cubes can reactively adsorb CO, forming surface-bicarbonate, only via converting Frenkel-pair defects to oxygen vacancies, without any further net reduction of the ceria. Keywords Raman spectroscopy, CeO2, octahedra, cubes, FTIR, catalysis, defects


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Introduction Ceria (CeO2) based materials are widely used for a range of applications such as, biomedicine,

1

solid oxide fuel cells,

2

solar fuel systems,

3

O2 sensors,

4-5

catalysis,

etc. In catalysis, CeO2 is used as a promoter in three-way catalysts (TWCs) for treatment of toxic exhaust gases, pollutants from waste water.

8

6

to remove soot from automotive engines

12-13

and

Another significant catalytic application includes low

temperature water gas shift reaction (LT-WGS), from bio-oil

7

9-11

steam reforming of oxygenates

and preferential oxidation of CO (PROX).

14-15

In all these

applications, highly mobile lattice oxygen is involved in the process that leads to facile creation of defects (e.g. oxygen vacancies) in the lattice, hence affecting the electronic and chemical properties of ceria.

16

Furthermore, defect sites in the lattice

act as active sites during the catalytic reaction. 16 Therefore, the control of the density and the nature of vacant sites could provide a means for tuning the reactivity of ceriabased catalysts. One of the widely accepted pathways to increase the activity and reducibility of ceria catalyst is through the development of robust synthesis methods to obtain ceria nanostructures with controlled crystallographic planes. 17 CeO2 nanoshapes have been successfully synthesized in various morphologies such as cubes, rods, wires, octahedra, spindle-like, etc, and examined for catalytic reactions, in which morphology-dependent performance has been reported. 18-23 Previously, the common consensus was that ceria rods had better catalytic reactivity than ceria octahedra and cubes, due to the presence of exposed (110) and (100) planes.

19, 21, 23-24

However, we recently demonstrated that both rod and octahedron

have (111) exposed planes; as compared to (100) for cubes. The exposed planes are



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consistent with observed WGS (water gas shift) reactivity trend: Rods ≈ octahedra < cube; the shapes are stable up to at least 350oC under reaction conditions

25

These

reports on ceria nanoshapes agree well with the experimental literature for ceria films. So far, several fundamental investigations related to the adsorption and dissociation of various probe molecules (CO,

26

methanol,

27

formic acid,

28-29

and water

30

) have

been extensively studied on epitaxially grown ceria (111) and (100) surfaces. (100) surfaces are found to be more active in adsorbing and reacting with probe molecules than (111).

27

Furthermore, in order to relocate surface charge and attain stability,

(100) planes undergoes surface reconstruction and forms defects. 31 Based on the earlier reports, pretreatment/processing conditions that enhance the formation of clusters of oxygen vacancies result in better reducibility of ceria.

32

Generally, ceria supported catalysts are hydrogen pre-treated to reduce the metal particles on them. 33 During the reduction, ceria supports are also greatly affected. For instance, H2 dissociation on the metal particle can result in spillover of [H] species to the ceria support, generating bridging –OH groups.

34

H2 exposure further results in

the change of the oxidation state of the ceria from Ce4+ to Ce3+ and removal of surface oxygen atoms to generate oxygen vacancies. 16 We recently reported that WGS activity of ceria nanoshapes as well as the interaction with CO/H2O as studied with FTIR is dependent on the exposed planes.

25

We then

selected rods with their exposed (111) planes to highlight the complex role of defects during CO adsorption and reaction with H2O, as a function of temperature.

39

We

demonstrated that, depending on temperature, formation of certain formate and carbonate surface species involves lattice oxygen from CeO2 ,thus forming defects. In contrast, other species like poly-dentate carbonates are formed exclusively at 350°C without creating any vacancies and these species are stable in water vapor. Our 4 ACS Paragon Plus Environment

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motivation for the present work was thus to see whether this strong dependency on exposed planes is also true for the role of defects on different ceria nanoshapes. In the present study, we provide a detailed understanding of the roles of defects in reduced cubes (i.e., the {100} plane) as compared to octahedra and rods (with {111} plane), with focus on the local defect structure of different exposed planes of ceria and the effects on the reducibility of ceria nanoshapes. Moreover, we also investigate the effect of reductive treatment influencing the behavior of defects on these ceria nanoshapes. Our results show that the reduced octahedra and rods exhibit very similar results. Therefore, we will restrict the discussion to a comparison between results obtained with cubes versus octahedra only; the experimental results obtained with rods are presented in the Supporting Information (figure S5, S7-10). In the remainder of this paper we used the term ‘defect chemistry’ to refer to the dynamic behavior of defects on ceria nanoshapes as a function of gas type.

Experimental Sample preparation and characterization CeO2 cubes and rods were synthesized in our lab via the hydrothermal procedure reported by Mai et al. 19 For details related to the synthesis procedure, we refer to our earlier work.

25

CeO2 octahedra (99.9% pure) with typical particle sizes smaller than

25 nm were obtained from Sigma-Aldrich. Both samples were calcined at 500oC (heating rate 5oC/min) for 4 hr in synthetic air (flow rate 50 ml/min). The BET surface areas were determined using a Micromeritics Tristar instrument. Prior to N2 physisorption, the samples were out-gassed in vacuum at 300 oC for 24 h.



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The BET surface areas of cubes, octahedra and rods were 10, 58 and 80 m2/g, respectively. For XRD patterns and HRTEM images of ceria cubes, rods and octahedra, the reader is referred to our earlier work. 25

In-situ Raman Spectroscopy A Jobin-Yvon T64000 triple-stage spectrograph equipped with CCD detector was used for the Raman measurements. The Raman spectra with resolution of 2 cm-1 were obtained using a Kinmon He-Cd 325 nm (UV) laser. The laser spot size on the sample was less than 0.5 mm. The output power of the laser was 30 mW. The power on the sample was maintained at 6 mW. It was carefully checked that at this laser power the sample was not damaged. Powdered samples (approximately 10 mg) were loaded in the homemade stainless-steel environmental cell and then slightly compressed to obtain a dense bed (for discussion related to the sample pressing techniques in the current study see supplementary information). The in-situ Raman cell was covered with a quartz assembly and fitted with a thermocouple placed close to the sample. For calibration of the Raman shift, Teflon was used. Raman spectra were recorded with two co-additions of 10 minutes laser exposure, hence recording each spectrum took 20 min. Preheated gases were led through the catalyst bed. All samples were pretreated in H2 (Linde 5.0) with flow rate of 20 ml/min at 450oC for 60 min. For CO reactivity studies, 33 vol% CO (Supplier: Linde 4.7) in He (Linde 5.0) was used (total flow: 20 ml/min). For H2O reactivity experiments, He (20 ml/min) was flowed through the saturator filled with H2O. To obtain the H2O vapor pressure of 9 mbar, the saturator


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temperature was kept at 5oC. The total flow of gases used during the Raman measurements was 20 ml/min.

In-situ FTIR Spectroscopy A Bruker Vector 22 with an MCT detector cooled at liquid nitrogen temperature was used for the transmission FTIR (Fourier transform infra-red) measurements. Typically, a spectrum was recorded by averaging 128 scans with a spectral resolution and time interval of 4 cm-1 and 120 s, respectively. The empty cell was used to collect the background spectrum. Self-supporting pellets of 13 to 15 mg catalyst were pressed and put into a purpose-built stainless-steel cell. Varian Chromopack CP17971 Gas Clean Moisture Filter was used to trap any moisture from the He (Linde 5.0) gas. Preheated gases were also flowed through the pressed sample pellets. Prior to CO/H2O studies, pressed ceria samples were pretreated in H2 (Linde 5.0) for 60 min at 450oC. For CO adsorption experiments, 33 vol% CO (Linde 4.7) in He (total flow 20 ml/min) was used. For obtaining the H2O flow with vapor pressure 9 mbar, He (20 ml/min) was bubbled through the saturator (temperature maintained at 5oC) filled with H2O.

Experimental sequence Raman and FTIR spectra were recorded after flowing gases in the sequence as shown in the experimental scheme in figure 1. The samples were heated in H2 flow (20 ml/min) to 450oC (heating rate 5oC/min) followed by an isothermal period of 60 min. After pretreatment, ceria shapes were cooled to 150oC in He flow and then heated to the reaction temperature (350oC) at the rate of 5oC/min in He flow (20 ml/min). The samples were exposed to He, CO and H2O for 30 min in the order as presented in figure 1. It was ensured that exposure during 30 minutes was sufficient to achieve



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equilibration, i.e. both Raman and IR spectra did not change anymore with time. For simplicity, all the gas flows are labeled with a sequential number that will be used as a reference for each gas in result and discussion section.

Figure 1. Experimental scheme for Raman and FTIR experiments at 350oC Data analysis Raman and FTIR spectra were analyzed using Bruker OPUS (version 7.0) spectroscopy software. All the Raman spectra presented in this work were baseline corrected (concave rubberband correction, 1 iteration) and smoothened (using Savitzky-Golay algorithm, 25 points). In a similar way, FTIR spectra (baseline correction-2 points, smoothened- 21 points) were processed. To verify that changes observed after smoothing are real, figures S1 (supplementary information) compare the raw and smoothened spectra obtained on exposure of He and CO for octahderon and cube samples. Furthermore, Raman spectra in figure 2 were normalized with respect to the F2G band at 455 cm-1. This is in accordance with common practice reported in literature to investigate the relative defect concentration (oxygen vacancy and Frenkel-type anion defects) in the ceria nanoshapes.38, 41, 58-60

Results and Discussion


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In-situ Raman and FTIR spectra of H2-reduced ceria octahedra and cubes samples in He flow at 350oC are shown in figure 2. The combination of these spectroscopic techniques allows investigation of lattice defects and adsorbates on ceria nanoshapes, assuming the total defect concentration is semi-quantitatively associated with the relative Raman intensities. The Raman spectra in figure 2a were normalized to the 455 cm-1 (IF2G) peak. The Raman spectra of both nanoshapes consisted of two broad peaks centered at 455 and 594 (ID) cm-1. Based on the literature and our earlier work, the peaks at 455 and 594 cm-1 can be assigned to the symmetric stretch mode of CeO8 in the fluorite crystal

35-36

and Frenkel-type anion defects in the ceria lattice,

37-38

respectively. For further details related to the assignments, we refer the reader to our previous work. 39 As suggested in our earlier work for unreduced ceria nanorods, the shoulder bands at 404 and 487 cm-1 on both sides of the fluorite peak are due to distortions in the fluorite structure.

39

These distortions are the result of an imperfect fluorite lattice

with Ce-Ox crystal units with x = 5-7. The shoulder peaks at 540-560 cm-1 region have previously been assigned to the presence of oxygen vacancy defects.

38, 40-41

IF2g

peak for octahedra is less broad than the corresponding peak for the cubes, as can be seen in figure 2a. The broadening of this peak at 350oC for ceria cubes is due to the increase in intensity of the side features at 404 and 487 cm-1. Similar features were previously reported for the unreduced ceria nanoshapes.39 This interpretation is further supported by the XRD and Raman data, where cubes were found to have more strain and distortion in their lattice than octahedra. 25, 42 The relative intensities of ID to IF2G (ID/IF2G) for cubes is higher than that of the octahedra, which is consistent with the previous studies indicating that cubes have a higher fraction of defects.

38, 42

In addition, the ID/IF2G ratio for the reduced ceria 9

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samples (figure 2a) was higher in comparison to the unreduced nanoshapes (Figure S2a). This observation was more evident in case of cubes, pointing out that cubes are more easily reduced. Furthermore, the shoulder bands at 404 and 487cm-1 next to fluorite peak (455cm-1) are more prominent for the unreduced ceria nanoshapes (figure S2a). In-situ transmission FTIR spectra of the reduced ceria octahedra (black) and cubes (dash-dotted blue spectrum) are shown in figure 2b. It must be noted that the peak intensity in FTIR cannot be directly correlated with the amount of sample as the light scattering properties are not only affected by the amount of sample, but also by the shape and size of particles. 43 Three distinct sets of peaks can be observed: OH stretching vibrations between 3800 and 3000 cm-1,

44-45

(2200-2000 cm-1)

45

stretching linear vibration of CO adsorbed on Ce4+ and Ce3+ and COx- (x=2,3) stretching and deformation between 1700 and

800 cm-1. 46-49 The peaks in the OH and COx- regions for reduced ceria nanoshapes have been extensively discussed in our previous work.

25

Briefly, the following observations

were made: The main hydroxyl (OH) peak for the cubes and octahedra was observed at 3638 cm-1, assigned to bridging hydroxyl species.

44-45

Broad bands (at approx.

3450 and 3173 cm-1) related to multi-bonded and hydrogen bonded hydroxyl species were observed at 3500-3000 cm-1.

45

Both of the ceria nanoshapes exhibit these

distinct OH peaks, however with different relative intensities. For instance, the ratio of the peak maximum I3638/I3450 was lower for cubes in comparison to octahedra. Furthermore, on comparing the reduced (figure 2b) and unreduced ceria cubes (figure S2b in supplementary information), it is clear that during hydrogen pretreatment the


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isolated hydroxyl species at 3710 cm-1 completely disappear along with major decrease in OH3638 species, again indicating cubes are highly reducible. 44-45

Figure 2. In-situ normalized Raman (a) and FTIR (b) spectra of H2 reduced ceria octahedra (black spectrum) and cubes (dash-dotted blue spectrum) in He flow at 350oC.

The carbonate peaks in the COx- region for the cube and octahedron are at different position, and the peaks were sharper for the cubes. These differences can be attributed to the different exposed planes on cubes ({100}) versus octahedra ({111}). Furthermore, the relative ratio of carbonates (1700-1200 cm-1) with respect to hydroxyl (3800-3000 cm-1) region for cubes was higher in comparison to octahedra consistent with the reported preferential formation of carbonates on the (100) plane in comparison to the (111) plane. 42, 50 CO on Ce3+ and Ce4+ – For the reduced samples, a small peak at 2112 and a shoulder band at 2150 cm-1 were observed (figure 2b inset). These features have been attributed to CO adsorption on the Ce3+ and Ce4+ sites.

45, 51

We believe that the CO

adsorbed on the cerium cations is formed during the H2 pretreatment, where



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carbonates (formed on exposure to atmospheric CO2) species partially decompose into CO and CO2 giving rise to some adsorbed surface species on the samples.

45, 52

The CO adsorbed peak is not observed when ceria samples are O2 pretreated that can be attributed to complete oxidation of decomposed species from carbonates into CO2 (figure S12 in supplementary information). To summarize, Raman and FTIR data reveal that ceria cubes contain more defects, as well as higher amount of surface carbonates with respect to hydroxyl species as compared to octahedra, after reduction at 350oC.

In-situ Raman: CO adsorption and reactivity with water To investigate the defect chemistry of the ceria nanoshapes at 350oC, the H2-reduced sample was exposed to the series of gas flows as outlined in figure 1, resulting in the spectra shown in figure 3. The left and right hand side plots are related to the octahedron and cube samples respectively, with each plot containing spectra from ‘before’ and ‘after’ each stage of the gas treatments.


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Figure 3. In-situ Raman spectra at 350oC of reduced ceria octahedra (left plots) and cubes (right plots) in; (a) & (b) He (1, solid black), followed by CO (2, red dashdotted); (c) & (d) CO (2) followed by He (3, solid green); (e) & (f) He (3) followed by H2O/He (4, dash-dotted orange); (g) & (h) H2O/He followed by He (5, solid blue) flow.



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Raman spectra of the reduced octahedra and cubes obtained in He (1) and CO (2) flow are shown together in figure 3a and b. After 30 min in He (1) flow, CO was adsorbed on the sample (red dash-dotted spectrum). In CO, the fingerprint fluorite peak at 455 cm-1 and the side peaks at 404 and 487 cm-1 decreased in intensity for both nanoshapes. In the case of octahedra, the intensities of the defect bands, including the peaks in region 540-560 cm-1 (oxygen vacancy, Ovac) (Frenkel-anion pair, ID)

37-38

38, 40-41

and 594

cm-1 increased indicating the formation of new defects.

For cubes, new CO-induced defect bands were observed in 540-560 cm-1. Unlike for octahedra, in cubes the Frenkel-anion defect peak at 594 cm-1 decreased in intensity on introduction of CO. This observation for the cubes hints towards the formation of oxygen vacancy at the expense of Frenkel-anion pair defects in CO. Comparing the CO-induced defects for reduced (figure 3) and unreduced (figure S3 in supplementary information) nanoshapes, it is immediately apparent that the reduced samples form relatively higher amount of defects, especially for cubes. We believe that the present work is the first experimental evidence for clearly showing that the CO induced defects are formed differently on ceria shapes with different exposed planes. In the past few years, Wu et al. have extensively studied similar ceria nanoshapes for the reactivity towards CO, methanol, and ethanol.

24, 38, 53-54

Unfortunately, direct

comparison of their results with our data cannot be made as the experimental conditions are very different. After CO exposure, the sample was exposed to He (3) to remove weakly adsorbed CO. Subtle changes were observed for both nanoshapes. Firstly, the peak at 455 cm-1 increased in intensity, whilst peaks at 404 and 487 cm-1 showed almost no change. Secondly, the overall defect band showed subtle increase in intensity for the octahedra (at the level of noise), whereas for cubes the Ovac (540-560 cm-1) decreased with an 14 ACS Paragon Plus Environment

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increase in intensity of ID (594 cm-1). It must be noted that the changes observed in the defect region (530-650 cm-1) for the cubes are more significant in comparison to octahedra. In the presence of H2O/He (4), the fluorite peaks at 404, 455 and 487 cm-1 showed increased intensities accompanied by partial disappearance of peaks assigned to defects that were induced by CO treatment, for both ceria octahedra and cubes (dashdotted orange spectra in figures 3e and 3f). Specifically, the ID peak in cubes further increased in intensity accompanied by a decreasing Ovac peak, whereas for octahedra both peaks intensities were significantly reduced in H2O/He flow (4). Subsequently, He (5) was flowed after H2O/He (4) to remove physisorbed water from the sample. The spectra obtained for ceria octahedra and cubes in He (5, solid blue spectrum) and H2O/He (4) are shown in figure 3g and 3h respectively. For octahedra, the 455 cm-1 peak showed a subtle increase in intensity with the further disappearance of CO-induced defects, whereas for cubes a very minor decrease in intensity of the fluorite and defect peaks was observed. Note that for octahedra, the major disappearance of defects occurred in two stages: First in H2O/He (4) and the remaining in He (5) after H2O flow, whereas in cubes, the removal of defects initiates in He (3) flow and continues in subsequent stages.

In-situ FTIR: CO and H2O adsorption To investigate the types of species involved/formed during defect creation on reduced ceria nanoshapes, in-situ FTIR spectroscopy was performed under identical conditions as the in-situ Raman experiments. The plots resulting from subtracting the spectra obtained after two consecutive gas treatments are shown in figure 4. Positive peaks in the difference spectra correspond to the species formed, whilst negative



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peaks indicate that species have disappeared upon introduction of CO. (The original un-subtracted spectra are shown in figure S4-6 in the Supporting Information). In CO, the peaks corresponding to formates and carbonates species increased significantly in intensity (solid red spectrum in figure 4a and 4b respectively) along with decreasing intensity of the OH peaks (bridging, multi and hydrogen bonded) for both octahedra and cubes. On comparing the CO – He FTIR spectrum of cubes with octahedra, the CO vibration of formed formate species was observed at the same peak position (1580 cm-1), whereas the CH vibration was located slightly higher at 2843 cm-1 (2838 cm-1 for octahedra). Unlike octahedra, carbonate-related positive peaks were observed for ceria cubes at 1491 and 870 cm-1 (1494 and 854 cm-1 for octahedra) and a positive OH peak related to bi-carbonates species (CO2 (OH)-) was observed at 3592 cm-1 (for octahedra at 3618 cm-1).

25, 39

In addition, the pronounced negative

OH3173 peak was also observed for cubes. As can be seen in figure 4c and 4d, on subsequent treatment with He (3, solid green spectrum), the peaks related to formate species (2945 and 1580 cm-1) disappeared with a shift and partial re-appearance of the OH3674 peak (OH3670 for cubes). The disappearance of formate peaks is clearly evident in figures S4b and S6b (supplementary information). This observation indicates a backward reaction of formates to CO. Furthermore, partial decomposition of bi-carbonate species, evident from negative peaks at 1402 cm-1 and 3592 cm-1 for cubes (3618 cm-1 for octahedra) was observed for the reduced ceria nanoshapes. Similar to our previous findings for ceria rods, on exposing octahedra to H2O vapor (solid orange spectrum, figure 4e), mono and bi-dentate carbonate species decomposed (evident from negative peaks at 1394, 854 cm-1) leaving behind stable poly-dentate carbonates (1388 cm-1 - figure S4c).

39

In addition, an increase in 16

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intensity of OH peaks was observed, pointing towards hydroxyl group regeneration. On subsequent exposure of the cubes to H2O vapor (figure 4f), decomposition of carbonates (evident from 1387, 870 cm-1) was observed together with regeneration of hydroxyl species (3670, 3629 and 3450 cm-1).



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Figure 4. In-situ FTIR subtracted spectrum at 350oC of reduced ceria octahedron (left plots) and cube (right plots) obtained by subtracting (a) and (b) He (1, solid black) from CO (2, solid red); (c) and (d) CO (2) from He (3, solid green); (e) and (f) He (3) from H2O (4, solid orange); (g) and (h) H2O (4) from He (5, solid blue). Insets in figure 4g and 4h are expanded spectra of carbonate regions obtained in H2O (dashdotted orange) and He (5, solid blue) flow.


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The FTIR difference spectra He (5)-H2O (4) for octahedra and cubes are shown in figure 4g and 4h respectively. The spectra (carbonate region 1700-1200 cm-1) obtained in H2O (4, dash-dotted orange spectrum) and He (5, solid blue spectrum) are plotted in the insets of figure 4g and 4h. Negative OH peaks were observed for octahedra and cubes due to the removal of weakly adsorbed H-bonded water (figure 4g and 4h). Interestingly, for octahedra, the band in the 1700-1200 cm-1 region became narrower along with an increase in the peak maxima (figure 4g inset). These induced changes are better reflected in the difference spectrum (figure 4g) where negative peaks at 1553 and 1530 cm-1 and positive peaks at 1440, 1388, 1318 cm-1 were observed. These peaks are clearly associated with carbonates and not with formate species, since no C-H vibrational bands were found in the 3000-2800 cm-1 region. Based on the peak positions, we assign negative and positive peaks to bidentate and poly-dentate carbonate species respectively.

55

From this observation, it

seems that rearrangements of carbonate species on ceria octahedra occurred, which resulted in the formation of stable poly-carbonates at the expense of bi-dentate carbonates. This agrees well with the theoretical prediction of Vayssilov et al., who have reported the transformation of bi-dentate carbonates in the vicinity of oxygen vacancies to stable poly-carbonates due to the enhanced structural flexibility next to the oxygen vacancy defects.

56

In addition, a change in the dipole moment of the

carbonate C-O bond upon desorption of water may also affect the peak position and intensity of carbonates. For cubes, an increase in intensity of poly-dentate carbonate peaks (1471, 1388 cm-1) was observed with a subtle decrease in bi-dentate species (1553 and 1530 cm-1, figure 4h). Notably, the overall carbonate band in the region (1700-1200 cm-1) did not become narrower on introduction of He (5), see figure 4h inset.



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In-situ FTIR: CO adsorption on Ce4+ and Ce3+ To investigate the reducibility of ceria nanoshapes (pretreated in H2) after exposure to CO and H2O, we plotted the FTIR spectra in the range 2250-2000 cm-1 (figure 5). As indicated earlier, the observed peaks at 2150 and 2112 cm-1 for both ceria nanoshapes in He (1, solid black line) flow are due to CO adsorbed on accessible Ce4+ and Ce3+ respectively. See figure 1 for the gas flow labels. In the presence of gaseous CO, these peaks are usually hidden within the very intense vibrational peak of CO(g). On flowing He after CO (3, dashed green line in figure 5), peaks related to CO(g) disappeared and peaks due to CO adsorbed on cerium ions can now be observed. In the case of octahedra, increases in intensities of the 2112 and 2150 cm-1 peaks were observed. The increase in intensity of these peaks is due to the interaction of CO with the accessible cerium ions formed either during H2 pretreatment or on exposure to CO (2). For cubes, a subtle increase in intensity of 2150 cm-1 with a slight decrease in intensity of 2112 cm-1 peak was observed, indicating the absence of accessible cerium ions for interaction with CO. On comparing the results of both nanoshapes and keeping in mind the possibility that CO exposure can further form more bare cerium ions, we suggest that in the presence of CO octahedra further reduce, whereas cubes are not further reduced in CO. Finally, decreasing intensities at 2112 and 2150 cm-1 were observed as a result of exposure to H2O, followed by flushing with He (5, dash-dotted blue line in figure 5). The intensity of the Ce3+ and Ce4+ peaks were lower after H2O exposure (blue spectra) than the intensities of the respective peaks obtained for the freshly reduced ceria samples (black spectra). In addition, the decrease was much more pronounced for the cube sample, indicating towards their higher extent of re-oxidation in H2O in comparison to other ceria nanoshapes, which agrees well with our previous work. 42 20 ACS Paragon Plus Environment

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Figure 5. In-situ FTIR of reduced ceria (a) octahedron and (b) cube at 350oC obtained in He (1), He (3) and He (5) flow. Key: He (1) black solid line is obtained for the freshly reduced samples in He flow, He (3) dashed green line is obtained in He flowed after exposure to CO gas and He (5) dash-dotted blue line is recorded in He flow after exposure to H2O vapor.

General Discussion Raman and FTIR spectroscopy results (figure 2) confirm that reduced ceria cubes contain more intrinsic defects and have a higher ratio of surface carbonates to hydroxyl species than octahedra. This is in agreement with the literature and is attributed to the difference in surface termination on these ceria nanoshapes.

25, 38, 50

Recently, Lin et al. reported that the octahedra with (111) planes are O terminated, whereas (100) facets, comprising the surface of cubes, are terminated by both Ce and O. 57 Due to the difference in geometry and coordination number (see figure S11 in supplementary information), the interaction of different exposed planes with gases varies. These differences in the interaction of the surface of these ceria nanoshapes with CO and H2O are illustrated in figure 6 and 7, using a simplified representation of the surface for clarity reasons.



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Octahedra – Reduced ceria octahedra have inherent defects (ID and Ovac) as can be seen in figure 2a. When the surface is exposed to CO, more oxygen vacancy (Ovac) and anion Frenkel pair (ID) defects are created (dash-dotted red spectrum in figure 3a) along with a significant increase in amounts of formates, carbonates {mono (I) and bi (II)-dentate} and bi-carbonates (solid red spectrum in figure 4a). This is illustrated in figure 6; oxygen moves from the lattice position to interact with CO to form carbonates (I and II) while creating vacant site ( V ) at its original lattice position (Ovac defect). At the same time, another oxygen atom leaves its lattice position (forming vacant site D ), moving to an interstitial site, this increasing the number of ID defects. In addition to defect-related carbonates, bi-dentate (II) carbonates (bonded to two oxygen of the ceria lattice), bi-carbonates and formates are also formed in CO (not shown in Figure 6). Interestingly, on flushing with He (3) after CO adsorption, in Raman data no change within the experimental error in the ID defect band is observed (solid green spectrum in figure 3c). At the same time in the FTIR data, the disappearance of formate species with partial re-appearance of the hydroxyl peak is observed (figure 4c). In addition, the partial decomposition of bi-carbonate species is also observed. As no change in the defect band is observed with the disappearance (partial) of formate (bi-carbonate) species, it is clear that the formation of formates and bicarbonates via the interaction of OH species with CO does not involve creation of defects in the octahedra lattice. 39 When exposing to H2O/He (4), the ID and Ovac defects partly disappear (figure 3e) with concurrent decomposition of mono (I) and bi-dentate (II) carbonates (figure 4e), accompanied by the regeneration of hydroxyl species. As illustrated in figure 6, the carbonate species related to the defects disappear and the vacant sites ( D and V )


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associated with ID and Ovac defects respectively, are refilled with oxygen accompanied by the regeneration of OH species on exposure to H2O/He. Finally, further disappearance of CO-induced defects is observed in He after exposure to H2O (5 in figure 3g) along with the formation of stable poly-carbonates at the expense of bi-dentate carbonates (figure 4g). Thus, we propose that bi-dentate (II) carbonate species remaining after H2O treatment further interact with the neighboring vacant ( V ) sites in the lattice to form stable poly-dentate (III) carbonates, resulting in the filling of the vacant oxygen site. This experimental finding agrees well with the reported theoretical calculations. 56

Figure 6: Schematic representation of the interaction at the surface of ceria octahedra with CO and H2O at 350oC. OD represents oxygen that moves from the lattice position to an interstitial site, forming an oxygen vacancy ( D ), which together form a Frenkel-pair anion defect. OV denotes oxygen in ceria lattice, interacting with CO, forming a carbonate species while creating a vacant site at its original lattice position (symbolized by V ).

Cubes – Similar to octahedra, formates, carbonates and bi-carbonates form on cubes on exposure to CO (figure 4b). However, unlike octahedra, new CO-induced defects (Ovac) are created at the expense of existing ID defects (figure 3b). Figure 7 provides a



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schematic representation. Note that formates cannot be associated with defects as they are formed by the interaction of CO with only one oxygen of the ceria lattice. Combining the Raman and FTIR findings, it is apparent that the formation of mono/bi-dentate carbonates and bi-carbonates induces the formation of oxygen vacancy site ( V by interacting with interstitial oxygen (OD) associated with an existing anion Frenkel pair defects. As a result, the Frenkel pair defect (ID) disappears while forming and oxygen vacancy V .

Figure 7: Schematic representation of the interaction of the surface of ceria cubes with CO and H2O at 350oC. OD represents oxygen at an interstitial site forming a Frenkelpair anion defect together with oxygen vacancy ( D ). D and V symbolize vacant lattice sites created due to the formation of Frenkel-pair anion and oxygen vacancy defects respectively. The key difference in the interaction of ceria nanoshapes with CO is that the (111)terminated octahedra form both anion Frenkel pair and oxygen vacancy defects, whereas the (100)-terminated cubes form oxygen vacancy defects at the expense of anion Frenkel pairs. In other words, octahedra can be further reduced in CO, whereas cubes cannot be reduced further and, instead, undergo vacancy reorganization. This observation is further supported by FTIR data on CO chemisorbed on accessible 24 ACS Paragon Plus Environment

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cerium ions (figure 5). Even after pretreatment with H2, octahedra can be further reduced, forming unshielded cerium ions as can be seen in figure 5a (dashed green spectra), while forming more defects (Ovac and ID). In contrast, cubes cannot be further reduced when exposed to CO (dash green spectrum in figure 5b) and hence undergo lattice rearrangement to interact with CO. In He after adsorption of CO (figure 3d), the Ovac concentration decreases whereas the amount of ID increases on cubes. Similar to octahedra, the disappearance of formate species with partial re-appearance of the OH groups and partial decomposition of bicarbonate species are observed (figure 4d). Based on the above findings, it is clear that with the disappearance of bi-carbonate species, Ovac defects also partly disappear while restoring part of ID defects in the cube lattice. Comparing the results with octahedra, it is evident that bi-carbonate species do not create defects in the octahedra lattice. On subsequent exposure to H2O vapor (4), the ID band further increases in intensity with a concurrent decrease in the Ovac peak (figure 3f), followed by decomposition of carbonates (figure 4f) and regeneration of hydroxyl species. This observation indicates that carbonates related to defects decompose and disappear in the presence of H2O while refilling the vacant site ( V ) with oxygen (or OH group) and subsequently regenerate the anion-Frenkel pair defects in the lattice of cube-shaped particles (illustrated in figure 7), as discussed above. On the other hand, in case of octahedra (figure 7), regeneration of both defects (Ovac and ID) is observed in H2O (4) vapor. The re-oxidation of ceria nanoshapes after exposure to H2O vapor is further supported by the observation that the intensity of peaks associated with CO adsorbed on bare cerium ions decreases (dash-dotted blue spectrum in figure 5).



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Finally, in He after H2O (5) a minor decrease in Ovac peak is noted (figure 3h) along with the rearrangements of bi-dentate to stable poly-dentate carbonate species (figure 4h). Similar to octahedra, we suggest that the oxygen vacancies are further filled during the interaction of bi-dentate (II) carbonates with the neighboring vacant ( V ) sites to form poly-carbonates (III) in the cube lattice. Results for octahedra and rods are very similar (figure 3 left plots and figure S7), which reinforces the recent assertions that they share the same exposed (111) planes, 25

despite the fact that rods exhibit the presence of dark pits ("intrinsic" defects)

demonstrated in HRTEM images of rods but not in octahedra. 32 The present work contributes to understand the defect chemistry of ceria nanoshapes. It is well known that reducibility of ceria, and thus the defect chemistry, influences the performance of ceria as a catalyst as well as a catalyst support in e.g. water-gasshift, CO oxidation and steam reforming. This work shows that the formation of defects depends on the exposed plane. On the one hand, cubes are more reducible than octahedra, explaining why cubes are more active. On the other hand, the fact that cubes form one defect type (oxygen vacancies) at the expense of another type (Frenkel pair defects) indicates that correlations between reducibility and catalytic activity can be rather complex.

Conclusion A detailed description is presented of the defect chemistry of ceria nanoshapes during interaction with CO and H2O for cube-shaped particles (exposing {100} planes) as compared to octahedral- and rod-shaped particles (with mainly {111} planes exposed). CO-induced defects on rods and octahedra were found to be very similar, in 26 ACS Paragon Plus Environment

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contrast to formation of defects formation on cubes. Oxygen vacancy (Ovac) defects were formed in CO at the expense of existing anion-Frenkel pair (ID) defects on cubes, whereas both defect types (Ovac and ID) form on octahedral and rods. FTIR findings confirm that H2-reduced ceria rods and octahedra can be further reduced in CO, simultaneously creating bare cerium ions and new defects (Ovac and ID). In contrast, CO cannot further reduce cubes subjected to the same pretreatment and oxygen is made available to form adsorbates by converting Frenkel-pair defects (ID) to oxygen vacancies (Ovac). Defect chemistry on ceria nanoshapes depends directly on the surface terminations.

Acknowledgments The authors gratefully acknowledge ADEM Innovation Lab (Project Number 30961318) for financial support. The work was performed under auspices of NIOK. We thank Karin Altena-Schildkamp for BET measurements. Special thanks to Ruben Lubkemann and Bert Geerdink for technical assistance.

Supporting Information Available: Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.



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Surface-Dependence of Defect Chemistry of Nanostructured Ceria

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