Redox and Catalytic Properties of RhxCe1âxO2âÎ´ Solid Solution

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The Redox and Catalytic Properties of RhCe O Solid Solution Lidiya S. Kibis, Tatyana Yu. Kardash, Elizaveta A. Derevyannikova, Olga A. Stonkus, Elena M. Slavinskaya, Valery Anatolyevich Svetlichnyi, and Andrei I. Boronin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09983 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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The Journal of Physical Chemistry

The Redox and Catalytic Properties of RhxCe1-xO2-δ Solid Solution Lidiya S. Kibis,†,‡ Tatyana Yu. Kardash,†,‡ Elizaveta A. Derevyannikova,†,‡ Olga A. Stonkus,†,‡ Elena M. Slavinskaya,†,‡ Valery A. Svetlichnyi,§ Andrei I. Boronin*,†,‡ †

Boreskov Institute of Catalysis, Novosibirsk, 630090, Russia ‡

Novosibirsk State University, Novosibirsk, 630090, Russia

§

Tomsk State University, Prospect Lenina 36, 634050 Tomsk, Russia

ACS Paragon Plus Environment

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ABSTRACT

In this work, a detailed study of the redox properties of solid solution RhxCe1-xO2-δ in correlation with its catalytic activity in CO oxidation reaction was carried out. The ex situ XPS technique was applied to follow the charging states of the elements on the surface during the redox treatments at a temperature range of 25-450°C. The results were compared with the data of temperature-programmed reduction by CO (TPR-CO). The dissolution of rhodium in the ceria bulk considerably increased the mobility of CeO2 lattice oxygen, with redox transitions Ce4+↔Се3+ and Rh3+↔Rhnδ+ observed already at low temperatures (below 150°C). The reduced rhodium clusters (Rhnδ+) formed during the reduction treatment significantly improved the catalytic activity of the RhхCe1-хO2-δ solid solution. The small size of the rhodium clusters (Rhnδ+) and high defectiveness of the fluorite phase provided the reversibility of Rhnδ+/CeO2↔RhхCe1-хO2-δ transitions upon redox treatment, resulting in the high reproducibility of the CO conversion curves in the temperature-programmed reaction CO+O2 (TPR-CO+O2). The homogeneous solid solution was stable up to 800°С. Above this temperature, the CeO2 volume was depleted of Rh3+ ions because of their partial segregation into the surface and/or sub-surface layers with the formation of Rh2O3. For these inhomogeneous samples, the oxygen mobility was considerably lower, while the redox transitions, Се4+↔Се3+ and Rh3+↔Rhnδ+, required higher temperatures.


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INTRODUCTION The metal-support interaction is one of the key factors to be addressed during the investigation of the supported catalytic systems.1–10 Over the last decades, the influence of the support on the size of the active component and its charging states have been shown even for supports like Al2O3 and SiO2, which traditionally were considered relatively inert due to their high thermal stability and absence of reducible oxygen species.11–13 Additionally, for reducible oxide like ceria, the importance of the metal-support interaction is well recognized,14–23 although its exact nature is still under discussion. Numerous experimental and theoretical studies have been focused on systems based on noble metals supported on CeO2,18,24–32 as these catalysts are active in important reactions such as CO oxidation,33 preferential CO oxidation (PROX),34 ethanol steam reforming,35,36 conversion of auto exhausts,37,38 synthesis gas conversion,39 water gas shift reaction,40 etc. The study of the metal-support interaction is especially important in the case of Rh-based catalysts, as this metal is known for its tendency to dissolve into the support, causing catalyst deactivation and a loss of the high-priced active component. For a detailed analysis of the metal-support interaction, the systems based on a solid solution of ceria with metals (MxCe1-xO2-δ) might play a key role. Solid solutions provide maximum dispersion of the active component as ionic species over a reducible oxide support, giving the highest degree of metal-support interaction. A number of works have been focused on the analysis of solid solutions of Rh with ceria.14,41–45 The incorporation of rhodium into the subsurface ceria layers,41,46 as well as into the CeO2 bulk,43,47,48 has been shown. The ionic dispersion of rhodium in the CeO2 matrix enhances the oxygen storage capacity (OSC) of the system and hinders the sintering of CeO241,43,47 improving the catalytic properties of the system.41


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Based on the literature data, one can conclude that for the synthesis of the solid solution, the preparation method has to insure the maximum level of interaction of the components during the preparation procedure. The formation of a RhxCe1-xO2-δ solid solution has been shown for samples prepared by the water-in-oil microemulsion method,43,44 RF magnetron sputtering,48 and solution combustion.49 In contrast, conventional impregnation methods give metallic or oxidized rhodium nanoparticles on the ceria surface.33,36,50,51 Additionally, the size of the CeO2 particles seems to be of high importance. For ceria with low surface area, the dissolution of Rh into the support has been shown to be limited.14,41 A significant part of the work on the properties of Rh/CeOx systems has been carried out for samples with a rhodium loading of approximately 1-2 wt.% because of the high price of Rh. However, the study of the RhxCe1-xO2-δ solid solution in a wide range of Rh content has revealed that for x being in the range of 0.05-0.16, the system retained homogeneity, while the microstrains of the cerium oxide lattice and, consequently, the reducibility of the system were at maximum levels.43 These samples demonstrated reversible redox properties after the reductionoxidation treatments at 500°C.44 This might have a great influence on the catalytic characteristics of the systems. In this regard, it is of interest to study the catalytic properties of RhxCe1-xO2-δ solid solutions with high rhodium content corresponding to the maximum oxygen mobility. In the present work, the Rh/CeO2 catalysts were prepared by a coprecipitation method. The effectiveness of the coprecipitation method for the preparation of MxCe1-xO2-δ solid solutions has been shown in our previous works.52–54 We carried out a detailed study of Rh/CeO2 catalysts with a rhodium content of 5.5 wt.%. This system can be considered as a model catalytic system based on the RhxCe1-xO2-δ solid solutions with x~0.1 and can be utilized for the collecting of fundamental data on the catalytic characteristics of the system in correlation with its redox


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properties. The high content of rhodium also facilitated the application of physicochemical methods, allowing to correlate charging states and structural/morphological changes with the catalytic properties of the solid solutions. EXPERIMENTAL SECTION Samples were prepared by coprecipitation method from rhodium and cerium nitrates (Rh(NO3)3, Ce(NO3)3). Solutions were mixed and precipitated with a 1N KOH aqueous solution followed by filtering and drying at 110ºC in air. The dried samples were calcined in flowing air at 450ºC for 4 h with further potassium removal by washing and subsequent calcination of the samples in air at 450ºС-1000ºС. The obtained samples are denoted in the text as Rh/CeO2-T, where T- is temperature of the calcination. A reference sample of pure СеО2 was prepared from Ce(NO3)3 following the same procedure with calcination in air at 450ºC. The Atomic absorption spectrometry (АAS) method was applied for the determination of the bulk chemical composition of the sample using a Perkin Elmer ISP OPTIMA 4300DV atomic absorption spectrophotometer. The exact Rh content measured by AAS was 5.5 wt.%. Speciﬁc surface area of the samples was determined by the BET method from the low-temperature nitrogen adsorption isotherms obtained with a Micromeritics ASAP-2400 analyzer. The X-ray diffraction (XRD) patterns were collected using a Bruker D8 diffractometer, CuKα radiation. Diffraction intensities were measured with the LynxEye position sensitive detector. XRD patterns were collected in 2Θ range 20-90°, with 0.05° step size and 3 s collection time. Phase analysis was performed using ICDD PDF-2 data base. Rietveld refinement was performed using TOPAS v4.2 software. The diffraction line profile was analyzed by the fundamental parameter approach. The lengths of coherent scattering domain (CSD) were calculated using


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LVol-IB values (i.e. volume weighted mean column lengths based on integral breadth). External standard material — well-crystallized Si powder — was used to describe the instrumental broadening. The Transmission electron microscopy data (TEM) were collected using an electron microscope JEM-2200FS (JEOL, Japan) at operation voltage 200 kV and lattice-fringe resolution 0.14 nm. STEM HAADF mode was employed together with energy dispersive X-ray spectroscopy (EDX). Samples were deposited on a perforated carbon film mounted on a copper grid. The Raman spectra were obtained using a dispersive spectrometer Renishaw InVia (UK) equipped with a Leica optical microscope with a 50× objective. The excitation wavelength was 532 nm from a laser with a power of ~100 mW. To prevent the samples heating, only 5% laser power was employed together with the defocusing mode. The Raman spectra were measured in the 100-3000 cm-1 range with a 1 cm-1 spectral resolution. The X-ray photoelectron spectra of the as-prepared (initial) samples and samples after reaction were collected on an X-ray photoelectron spectrometer KRATOS ES300 (UK). Samples were fixed on the sample holder with carbon scotch tape and were analyzed without preliminary thermal pretreatment. The collection of the spectra of the samples under redox treatment in ex situ mode were performed on an X-ray photoelectron spectrometer VG ESCALAB HP (UK). The powder samples were pressed into pellets ca. 1.5 cm×0.7 cm and 0.5 mm in thickness. The pellets were mounted on the sample holder with tantalum foil (Ø~50 µm). The pellets were heated via resistive heating of the tantalum foil. The Cr-Al thermocouple was used to control the sample temperature. The gas treatment of the samples was performed in a preparation chamber


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connected to an analyzer chamber through a gate valve. For each sample, the following redox cycle was performed: 1) a step-by-step heating in a CO atmosphere (р(СО)=2 mbar) at a temperature range of 25-450°С with a step of 50°C. The time of the gas treatment was 1 h for each temperature. After each gas treatment, the sample was evacuated up to 10-8 mbar and transferred to the analytic chamber for spectra collection. 2) a step-by-step heating in an O2 atmosphere (p(O2)=10 mbar) at a temperature range 25-450°С with a step of 50°С. The time of the gas treatment was 1 h for each temperature. After each gas treatment, the sample was evacuated up to 10-8 mbar and transferred to the analytic chamber for spectra collection. For the Rh/CeO2-450 sample, a second step-by-step reduction by CO was performed at a temperature range 25°C-300°С. Prior to the redox experiments, all samples were heated in an O2 atmosphere (p(O2)=10 mbar) for 2 h for removal of the residual carbon impurities. The XPS measurements were performed using a MgKα (hν=1253.6 eV) X-ray source operating at a 13 kV×6 mA power regime. The spectra were calibrated using the U’’’ component of the Ce3d spectra set at a binding energy (Eb) of 916.7 eV.48,52,55 The experimental curves were fitted with a combination of Gaussian and Lorentzian peaks after the Shirley background subtraction procedure. For fitting the asymmetric peak of metallic rhodium, the Doniach-Sunjic function was applied. The data were processed and analyzed using the XPS-Calc program.56–58 The catalytic properties of the samples were analyzed using an automated setup equipped with a flow reactor and mass-spectrometric analysis of the gas mixture. Samples were pressed into pellets and crushed to produce particles of 0.15 to 0.25 mm. Capacity of the reactor was 0.25 cm3. 0.4-0.5 g of the sample was used depending on the volume density of the material. For temperature-programmed reaction CO+O2 (TPR-CO+O2) experiments, the reaction mixture containing 0.2 vol.% СО, 1.0 vol.% О2, 0.5 vol.% Ne, and helium as the balance was introduced


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into the reactor at a rate of 1000 cm3/min. The space velocity was 240 000 h-1. The catalysts were heated in the reaction mixture from –10°C to 450°C at a rate of 10°C/min, followed by cooling and a second heating in the reaction mixture. The temperature-programmed reduction by CO (TPR-CO) was performed in the reaction mixture containing 1 vol.% CO and 0.5 vol.% Ne balanced in He. The volume rate of the flow was 100 cm3/min. The samples (0.3 g) were heated at a rate of 10°C/min. Before TPR-CO experiments, the samples were heated in 20 vol. %O2 in He at 450°C for 2 h to remove surface admixtures. RESULTS The synthesized Rh/CeO2 samples calcined at different temperatures were analyzed by a number of structural and spectroscopy methods of characterization. XRD data. Figure 1а shows the X-ray diffraction patterns of the Rh/CeO2 samples, calcined at 450-1000оС. Only the peaks corresponding to the ceria phase with the fluorite structure (ICDD PDF-2 #00-029-1484) can be detected in the diffraction patterns of the samples calcined up to 900°С (Figure 1а, curves (1)-(4)). The calculated parameters of the samples (lattice parameter (a), CSD, and microstrains (e0)) are presented in Table 1. The CSD for the samples does not increase substantially upon sample calcination up to 900°C and is in the range of 9-14 nm. It is known that the decrease of the particle size of CeO2 crystallites results in the increase of the lattice parameter.43,59,60 However, in the case of the of Rh/CeO2 samples, a decrease of the lattice parameter in comparison with pure nanosized CeO2 is observed. The decrease of the lattice parameter of the fluorite phase upon Rh introduction can be related to the substitution of cerium Ce4+ ions with Rh3+ ions with a smaller ionic radius (97 pm versus 66.5 pm, respectively).41,43 Additionally, the microstrain parameter in these samples is high, pointing to the distortion of the


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fluorite structure of ceria upon Rh incorporation into the lattice. Calcination of the sample at 1000°C results in the appearance of new peaks in the diffraction pattern corresponding to the phases of α-Rh2O3 (ICDD PDF-2 #00-041-0541) and β-Rh2O3 (ICDD PDF-2 #00-043-0009) (Figure 1a, curve (5)). The appearance of the reflections of rhodium oxides is accompanied by the decrease of the microstrains and the increase of the lattice parameter up to the value typical for pure CeO2. The obtained results imply the temperature-induced segregation of the rhodium ions out of the ceria lattice with the formation of the rhodium oxide phases on the surface or within subsurface layers, in good agreement with literature data.41,43,47 The presence of the rhodium in the ceria lattice is known to inhibit the sintering of the CeO2 crystallites.43,47 Thus, the substantial increase of the mean crystalline size is observed for a fluorite phase depleted of Rh (Rh/CeO2-1000 sample). Table 1. Structural and microstructural characteristics, TPR-CO data for Rh/CeO2 samples. XRD data T, °С

Rh/CeO2

CeO2

SBET, m2/g

CeO2, wt.%

βRh2O3 wt.%

αRh2O3 wt.%

а, Å

CSD, nm

e0

СО2/Rh, TPR-CO

450

138

100

0

0

5.406(1)

8.7

0.28

4.7

600

114

100

0

0

5.405(1)

8.3

0.25

3.8

800

61

100

0

0

5.407(1)

11.1

0.23

3.4

900

37

100

0

0

5.411(1)

13.9

0.162

3.1

1000

9

90,6

5,9

3,5

5.414(1)

45.5

0.007

2.7

450

112

100

0

0

5.414(1)

11.5

0.087

-


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Figure 1. (a) XRD data for Rh/CeO2 samples calcined at (1) 450°C, (2) 600°C, (3) 800°C, (4) 900°C, and (5) 1000°C. (b) TEM data for Rh/CeO2-450: (1) HRTEM and (2) HAADF-STEM; EDX mapping of the area (2) obtained from (3)- Rh L and (4)- Ce L emission lines. TEM data. TEM analysis of Rh/CeO2-450 sample reveals only CeO2 nanoparticles with size of approximately 4-8 nm (Figure 1b insertion (1)). Particles have a spherical shape. No phases of rhodium oxides can be detected. However, the EDX-mapping shows the homogeneous distribution of rhodium over the entire area (Figure 1b insertions (2)-(4)). The rhodium content is estimated to be approximately 10 at.%.

Figure 2. TEM data Rh/CeO2-1000: (1) HRTEM; (2),(3) HAADF-STEM; EDX mapping of the areas (2) and (3) obtained from (4), (5)- Rh L; (6), (7)- Ce L emission lines.


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The homogeneous distribution of Rh is observed in the samples calcined up to 800°C. For the Rh/CeO2-1000 sample, the TEM data shows the formation of CeO2 particles approximately 3050 nm in size with well-crystallized grains (Figure 2 insertion (1)). The inhomogeneous distribution of Rh and Ce is observed. There are areas where big Rh2O3 crystallites (~1 µm) can be detected (insertions (2), (4) and (6) in Figure 2). Additionally, there are areas with a homogeneous distribution of Rh (insertions (3), (5), and (7) in Figure 2), but the Rh content in these areas is lower (~1 at.% Rh). Thus, the TEM data are in good agreement with the XRD results confirming the formation of a RhxCe1-xO2-δ solid solution with homogeneous distribution of rhodium in the CeO2 lattice for the sample calcined at 450°C. The high temperature heating of the samples results in the partial segregation of Rh with the formation of Rh2O3 particles. However, a small fraction of dispersed rhodium is still present even after calcination at 1000°C. Raman Spectroscopy data. Figure 3 shows the Raman spectra of Rh/CeO2 samples depending on the calcination temperature. The most intensive peak at ~460 cm-1 observed in the spectra of all samples can be ascribed to the triply degenerate F2g mode of the СеО2 lattice, a symmetric breathing mode of the oxygen atoms around cerium ions.61 This mode is sensitive to the distortion of the oxygen sublattice caused by changes in the CeO2 microstructure. The shift of the F2g mode to lower energies and broadening of the line along with an increase of its asymmetry point to the decrease of the particle size.62,63 For the sample calcined at 450°C, the half-width of the F2g mode is ~35 cm-1 that, according to the literature data,62 corresponds to СеО2 crystallites of approximately 5-6 nm in size. Calcination of the sample up to 1000°C results in the decrease of the half-width of the Raman line, implying the increase of particle size. Along with the F2g mode there is a band in the Raman spectra at 500-700 cm-1 with a maximum


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at ~585 cm-1, which can be associated with the localized vibrations (D mode) induced by the presence of oxygen vacancies within the ceria lattice.42,63 The enhanced defect-induced mode is usually observed for ceria-doped systems64–66 and might be an indication of the incorporation of Rh3+ species into the CeO2 lattice causing its distortion. After calcination at 1000°C, the D mode is depleted, pointing to a decrease of the number of defects in the fluorite lattice. The narrowing of the F2g line and depletion of the D mode intensity at 1000°C might point to a segregation of rhodium out of the ceria lattice accompanied by the crystallization of the latter. At 800°C, a small shoulder appears at ~650 cm-1, which can be related to the A1g mode of rhodium oxide αRh2O3.67 Therefore, the formation of rhodium oxide can be detected by Raman at temperatures higher than 800°C. It should be noted that reflections of Rh2O3 oxide phases are not observed in XRD patterns up to 1000°C, therefore, Rh2O3 oxide particles detected by Raman should be of a very small size or have an amorphous structure. Furthermore, there is a peak in the lowfrequency region at ~235 cm-1. The precise nature of this peak seems to be unclear at the moment. However, we can tentatively assign it to the 2TA mode of ceria in the presence of dopants.68,69


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Figure 3. Raman data for Rh/CeO2 samples calcined at (1) 450°C, (2) 600°C, (3) 800°C, and (4) 1000°C. Thus, XRD, TEM and Raman spectroscopy data show that homogeneous a RhxCe1-xO2-δ solid solution can be prepared by the coprecipitation method. Rhodium is incorporated into the CeO2 lattice causing its distortion. Calcination of the samples at a temperature of 800°C and higher results in the diffusion of rhodium ions out of the ceria lattice with the formation of Rh2O3 particles accompanied by the crystallization of CeO2 structure. One can expect the change of the redox and catalytic properties of the system with a decrease of solid solution homogeneity. TPR-CO. The mobility of the oxygen species in the Rh/CeO2 samples was analyzed by temperature-programmed reduction by CO (TPR-CO). Figure 4 presents the temperature dependence of СО2 evolution in TPR-CO experiments for Rh/CeO2 samples calcined at different temperatures and pure СеО2, given for comparison (The data on СО consumption in TPR-CO experiments are given in Supporting Information (SI), Figure S1). For the catalyst calcined at 450°С, the main peaks of CO2 evolution are observed at the low temperature region (below 250°С). An increase of the calcination temperature results in the shift of the CO2 evolution curves towards higher temperatures. The TPR-CO profile of the Rh/CeO2-1000 sample shows a strong two-peak CO2 evolution at ~300°C. The peaks below 250°C have low intensity. It should be noted that pure CeO2 demonstrates a minor CO2 evolution (CO uptake) only at temperatures higher than 250°C (See curve (1) in Figure 4). The quantified amount of evolved CO2 molecules per Rh atoms (CO2/Rh) are given in Table 1. In the case of the reduction of Rh2O3, 3 CO2 molecules would be expected per two Rh3+ ions. The estimated CO2/Rh ratio for Rh/CeO2 samples greatly exceed 1.5 pointing to the participation of the CeO2 lattice oxygen in the reduction process. The obtained results are in good agreement


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with literature data33,41,70 discussing the reduction of Ce4+ species together with Rh3+ ions at the low temperature region in TPR-Н2 and TPR-CO experiments. The CO2/Rh ratio quantified for the main CO uptake of the Rh/CeO2-1000 sample (peak at T~300°C) is 1.3, implying the reduction of structures close to Rh2O3 at this temperature region. The obtained TPR-CO results reveal the enhanced oxygen mobility at low temperatures for the CeO2 lattice modified with Rh ions. The calcination of the RhxCe1-xO2-δ solid solution with the formation of Rh2O3 oxide species leads to the substantial decrease of oxygen mobility at T250°C. TPR-CO+O2. The catalytic activity of the samples was tested using the temperatureprogrammed reaction of CO oxidation. The temperature dependence of CO conversion during


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TPR-CO+O2 experiments is given in Figure 5. The sample calcined at 450°C demonstrates high low-temperature catalytic activity with the temperature of a 50% conversion of CO (T50) being ~50°C and 100% conversion of CO below 100°C (Figure 5a curve (1)). The Rh/CeO2-1000 sample oxidizes CO at higher temperatures with T50~180°C and 100% conversion achieved at 235°C. For pure CeO2 the catalytic activity is observed only at T>200°C with T10~250°C and Т50~360°C (Figure 5a, curve (3)). Note that for the Rh/CeO2 samples, the CO conversion curves are smooth and do not have any shoulders and pronounced local peaks. Such behavior indicates the uniformity of the active centers in the operating temperature region. The Rh/CeO2-450 and Rh/CeO2-600 samples are also characterized by the high reproducibility of the catalytic characteristics. Figure 5b demonstrates the behavior of CO conversion curves for the Rh/CeO2450 sample during its heating in the reaction mixture, followed by cooling and second heating. The complete coincidence of the curves can be seen. (The TPR-CO+O2 data for heating1/cooling/heating-2 cycle for the Rh/CeO2-1000 sample are given in SI, Figure S2).

Figure 5. The temperature-programmed CO+O2 data for (a) Rh/CeO2 catalysts calcined at (1) 450°C and (2) 1000°C; (3) CeO2 calcined at 450°C (second heating in CO+O2 mixture), (b) Rh/CeO2-450 sample: heating-1/cooling/heating-2 cycle.


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XPS. The Rh3d spectra for the Rh/CeO2-450 and Rh/CeO2-1000 samples are presented in Figure 6 (curves (1)). The Rh3d spectra show only one main doublet with Eb(Rh3d5/2) near 309 eV.

Figure 6. The Rh3d spectra of Rh/CeO2 samples calcined at (а) 450°С and (b) 1000°C. (1) Initial samples. (2) Samples after TPR-CO. Such an Eb value is higher than those known for the bulk rhodium oxide, Rh2O3 – 308.0-308.3 eV58,71,72 and Rh4+ species- 308.4 eV.58,73 The oxidized rhodium nanoparticles deposited on the relatively inert supports (SiO2, TaOx) are also characterized by a similar Eb value- 308.3 eV.58,74 According to the literature data, the observed value Eb(Rh3d5/2)~309.0 eV can be assigned to the Rh3+ species located in the ceria lattice.41,44,48 Rh3+ species can be stabilized in a form of single ions within the ceria lattice or in –O-Rh-ORh-O- clusters surrounded by cerium ions. As TEM, XRD and Raman data do not reveal any Rh2O3 oxide structures in Rh/CeO2 samples calcined at T800°C stimulates the diffusion of Rh3+ ions with the formation of –O-Rh-O-Rh-O- clusters and nanoparticles, which can be detected by structural methods. The


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temperature-induced increase of the number of -Rh-O-Rh- bonds over -Rh-O-Ce- fragments should result in the shift of Rh3d spectrum towards Eb(Rh3d5/2) typical for rhodium oxide (308.3 eV). Following this assumption, the slight decrease in the Eb(Rh3d5/2) of the Rh/CeO2-1000 sample with respect to the Rh/CeO2-450 sample might be related to the increase of the number of -Rh-O-Rh- bonds. The large Rh2O3 crystallites detected in the Rh/CeO2-1000 sample by XRD and TEM should have negligible impact to the Rh3d spectrum. The estimation of the rhodium content based on XPS data was performed and compared with AAS results. XPS gives information about the surface/subsurface content, whereas AAS provides the total volume concentration. For the Rh/CeO2-450 sample, the XPS and AAS data are rather close: 4 wt.% and 5.5 wt.%, respectively, pointing to a quite uniform distribution of Rh3+ ions within the CeO2 lattice. Calcination of the sample at 1000°C results in an increase of the rhodium concentration on the surface, the estimation based on XPS data gives 8.6 wt.% for the Rh/CeO2-1000 sample. The substantial enrichment of the surface with rhodium implies the segregation of rhodium ions out of the ceria lattice with their localization in surface/subsurface layers. Similar results were reported for Ce1-xFexO2-x solid solution.75 Authors have shown that for sample calcinated at 750°C iron is enriched at the surface of the ceria nanoparticles, as well as in the voids. No changes are observed in the Rh3d spectra of the samples after TPR-CO+O2 experiments (see SI, Figure S3). The Rh3d spectra of the samples after TPR-CO experiments are given in Figure 6 (curves (2)). As a substantial amount of CO is consumed in TPR-CO experiments, one would expect the complete reduction of rhodium species. However, in the Rh3d spectrum of the Rh/CeO2-450 sample, the component corresponding to the oxidized rhodium species remains dominant. For the Rh/CeO2-1000 sample, the reduction is more noticeable, and the main Rh3d5/2


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peak with Eb(Rh3d5/2)=307.3 eV is typical for bulk metal rhodium.71 However, approximately 30% of rhodium remains in an oxidized state. It should be noted that analysis of the Ce3d spectra of the samples does not reveal any substantial changes after TPR-CO or TPR-CO+O2 experiments as well (see SI, Figure S4). The reasonable explanation of such spectral behavior is the effect of the samples contact with air upon sample transfer from the catalytic reactor to the XPS chamber. Therefore, for the correct interpretation of the charging states of the surface under redox treatment we applied the XPS technique in an ex situ regime. The reduction (CO) and oxidation (O2) treatments of the samples were performed directly in the preparation chamber of the photoelectron spectrometer with a subsequent registration of XPS spectra. Ex situ XPS. Redox properties of the Rh/CeO2-450 sample. Figure 7 shows a set of Rh3d and Ce3d spectra of the Rh/CeO2-450 sample after its sequential reduction by CO at temperatures from 25°C to 450°C.

Figure 7. (a) Rh3d and (b) Ce3d spectra of the Rh/CeO2-450 sample: (1) initial sample, and after reduction by CO at (2) 50°C (3) 100°C, (4) 150°C (5) 200°C, (6) 250°C, (7) 300°C, (8) 350°C, and (9) 450°C.


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The changes in the Rh3d spectra are insignificant upon interaction with CO at temperatures below 150°C (Figure 7a). One main doublet is observed with a maximum at Eb(Rh3d5/2)=309.2 eV related to the Rh3+ state in the CeO2 lattice. At temperatures above 150°C, an additional component appears in the Rh3d spectra with a binding energy 307.6-307.8 eV. Figure 8a presents the fitted Rh3d spectra for the initial sample (curves (1)) and the sample after reduction by CO at 200°C and 4500C (curves (2) and (3), respectively). After interaction with CO at T>150°C, the Rh3d spectra clearly reveal the reduced form of rhodium with Eb(Rh3d5/2)=307.8 eV, which can be related to the small metal/partially oxidized clusters Rhnδ+.48 With further temperature increase, the peak of the reduced species grows in intensity and slightly shifts to lower binding energies, which may indicate the increase of the size of the reduced cluster size. The quantitative data on the impact of the Rhnδ+ component to the overall Rh3d spectrum as a function of the temperature of interaction with CO are given in Figure 9a. After interaction with CO at 300°C and above, the reduced form of rhodium becomes dominant in the Rh3d spectrum (Figure 8a curves (3)).


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Figure 8. (a) Rh3d and (b) Ce3d spectra of Rh/CeO2-450: (1) initial sample, and after reduction by CO at (2) 200°C and (3) 450°C. In contrast to rhodium, the noticeable changes in the shape of the Ce3d lines are observed already at 100°C (Figure 7b). Figure 8b presents the deconvolution of the Ce3d spectra into individual components of Се4+ species (V, V’’,V’’’ and U, U’’, U’’’) and Се3+ species (V0, V’ and U0, U’), in accordance with the literature data.55,76,77 Using this curve fitting procedure, the quantity of Ce3+ species was estimated for each step of the reduction. Data in Figure 9a show the rapid increase of Ce3+ concentration at the temperature region 50°C-250°C, implying the reduction of cerium at a significantly lower temperature, in contrast to rhodium. Above 250°C, the Ce3+ concentration changes insignificantly. The sample reduced at 450°C contains approximately 35% of Ce3+ indicating a high concentration of oxygen vacancies in the fluorite lattice.

Figure 9. (a) TPR-CO curve and XPS data on Rhnδ+ and Ce3+ impacts depending on the temperature of the reaction with CO for Rh/CeO2-450 sample. (b) Rh3d spectra of Rh/CeO2-450 sample after reoxidation in O2 at (1) RT and (2) 450°C.


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A comparison of the TPR-CO and ex situ XPS experimental results allows distinguishing three temperature regions of interaction with CO: 1) The first one at 25-100°С is characterized by a low-temperature peak of CO2 evolution and reduction of Ce4+ species without noticeable changes in the rhodium charge state; 2) at 100-250°С, the most intensive CO uptake occurs, and XPS spectra show the reduction of both Ce4+ and Rh3+ species; and 3) at T>250°C, the evolution of CO2 is mainly related to the reduction of Rh3+. After reduction up to 450°C, the sample was exposed to oxygen. The nearly complete reoxidation of cerium to Ce4+ (see SI, Figure S5) is observed already at room temperature. The reoxidized sample has approximately 10% of Ce3+ close to the Ce3+ concentration in the initial sample. The reoxidation of the reduced Rh forms is also observed. At room temperature, approximately 60% of the rhodium is in the Rh3+ state (Figure 9b, curve (1)). As the temperature of interaction with O2 increases, oxidation proceeds further with rhodium being completely reoxidized at T>300°C (Figure 9b, curve (2)). It should be mentioned that the reactivity of the oxygen of the sample reoxidized at 450°C nearly completely coincides with the reactivity of the initial sample (see SI, Figure S6), thus, indicating the effective reversibility of the system during redox cycles. The obtained results are in good agreement with the TPR-CO+O2 data on the reproducibility of the catalytic characteristics of the sample (Figure 5b). Ex situ XPS. Redox properties of the Rh/CeO2-1000 sample. Figure 10 shows the fitted Rh3d and Ce3d spectra for the initial Rh/CeO2-1000 sample and the sample after ex situ reduction by CO at 200°C and 450°C (The complete set of Rh3d and Ce3d spectra for this sample depending on the temperature of interaction with CO is given in SI, Figure S7).


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Similar to the Rh/CeO2-450 sample, the reduction of Rh3+ ions occur at T~200°C. However, for the Rh/CeO2-1000 sample, the amount of the reduced rhodium species (Eb(Rh3d5/2)=307.7 eV) at this temperature is smaller (Figure 10a, curves (2)). Additionally, the rhodium reduction for the Rh/CeO2-1000 sample occurs in narrower temperature range (200°C-350°С) in comparison with the Rh/CeO2-450 sample (temperature range: 150°-450°С) (see Figure 9a and Figure 11a). The sharper reduction profile might indicate the reduction of more crystallized phases. The reduced rhodium species of the Rh/CeO2-1000 sample has Eb(Rh3d5/2)=307.4 eV (Figure 10a, curves (3)), which is close to the Eb value typical for Rh bulk metal. The temperature profiles of the Ce4+ reduction for Rh/CeO2-450 and Rh/CeO2-1000 samples differ significantly. For the Rh/CeO2-1000 sample, the reduction of cerium is insignificant at T 200°C, with a gradual reduction of cerium over the entire temperature range and duplication of the reduction profile of rhodium (Figure 11a).

Figure 10. Rh3d and Ce3d spectra of Rh/CeO2-1000 sample: (1) initial sample, and after reduction by CO at (2) 200°C, and (3) 450°C.


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The treatment of the reduced Rh/CeO2-1000 sample with O2 causes its reoxidation. The Ce3+ species practically completely reoxidizes at room temperature, similar to the reoxidation of the Rh/CeO2-450 sample, the amount of Ce3+250°C in the temperature region of the reduction of


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both Ce4+ and Rh3+ species. The weak peaks of CO2 evolution at 120°C and 210°C most likely originate from the reduction of weekly bonded oxygen species in the surface/subsurface layers of ceria. The influence of reduction pretreatments on the catalytic properties of samples. As reduction of the samples leads to the formation of Rhnδ+ species that might serve as active centers for CO adsorption, it was of interest to investigate the effect of the reduction pretreatment on the catalytic properties of the samples. Figure 12 shows the TPR-CO+O2 curves for the initial Rh/CeO2-450 (Figure 12a) and Rh/CeO2-1000 (Figure 12b) samples and samples after pretreatment with CO at 300°C.

Figure 12. TPR-CO+O2 for (а) Rh/CeO2-450 and (b) Rh/CeO2-1000 catalysts: (1) initial samples; (2) samples after pretreatment with CO at 300°C, first run; and (3) samples after pretreatment with CO at 300°C, second run. The reduction pretreatment significantly improves the catalytic properties of the samples. For the Rh/CeO2-450 sample, a 30% conversion of CO already at T=-100С is observed. To our knowledge, such high activity at low temperatures for Rh/CeO2 systems has not been previously reported in the literature. However, the 100% conversion of CO is achieved only at 80°C – the


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same temperature as for the initial sample. Moreover, the CO conversion curves of the prereduced sample heated in the reaction mixture for the second time (Figure 12a, curve (2)) and the initial sample (Figure 12a, curve (3)) nearly completely coincide. This result means that active sites formed upon reduction are reoxidized completely during heating in the reaction mixture up to 450°C, thus demonstrating the complete reversibility of the system upon redox treatment. For the Rh/CeO2-1000 sample, the reduction pretreatment also influences the catalytic characteristics of the sample. A significant decrease of T50 is observed (Figure 12b). However, the activity at low-temperature is not high. The subsequent second heating in the reaction mixture up to 450°C leads to a deterioration of the catalytic activity, although it is still better than the activity of the initial sample. Thus, in the case of the Rh/CeO2-1000 sample, complete reversibility of the catalytic properties after the preliminary reduction is not observed. It should be noted that the reduction pretreatment of the samples in hydrogen (used instead of CO) has nearly identical influence on the catalytic properties of the systems (see SI, figure S9). For the Rh/CeO2-450 sample, the complete reversibility of the system is observed, while for the Rh/CeO2-1000 sample, the reduction pretreatment induces irreversible changes in catalyst activity. DISCUSSION Based on the obtained results, we can conclude that the coprecipitation method can be applied for the synthesis of homogeneous solid solution RhxCe1-xO2-δ. Coprecipitation of the rhodium and cerium precursors provides the interaction of the components of the system at the atomic level during the preparation procedure, thus, facilitating the formation of a homogeneous solid solution. These results are in good agreement with the literature data on the formation of solid


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solutions in the Rh/CeOx systems.43,47,48 In these works, the techniques that ensured maximum interaction of the components during the preparation procedure were applied. The introduction of rhodium into the ceria lattice leads to a considerable distortion of the CeO2 lattice because of the substitution of Ce4+ species by Rh3+ ions with a smaller ionic radius. The resulting homogeneous solid solution is stable up to 800°С. With an increase of the calcination temperature above 800°С, the decomposition of the solid solution occurs in good agreement with data on the thermal stability of the RhxCe1-xO2-δ solid solution studied by Kurnatowska et al.43 and Hosokawa et al.47 XRD, TEM and Raman results reveal the formation of large particles of rhodium oxide (Rh2O3) on the surface of CeO2. However, the XPS data points to the presence of Rh3+ ions dispersed in the ceria lattice or RhOx clusters in contact with CeO2. We can propose that calcination of the solid solution at T>800°C results in the diffusion of rhodium ions out of the ceria lattice and "nucleation" of small rhodium oxide clusters, which upon further heating form large particles of Rh2O3 on the surface of CeO2. A part of the Rh3+ ions remains in the surface/subsurface layers either in the form of individual Rh3+ ions in the ceria lattice or in a form of sub-nanosized rhodium oxide (RhOx) surrounded by ceria. These species have a major impact in the overall Rh3d spectra of the sample. It should be noted that large Rh2O3 crystallites should have a negligible impact in Rh3d spectra. Kurnatowska et al.43 have also shown that even after calcination at 1000°C, a part of Rh remained in the ceria lattice or was dispersed as an amorphous oxide phase on the ceria surface. The diffusion of the rhodium ions out of the CeO2 lattice at elevated temperatures results in a crystallization of the ceria structure. The size of CeO2 particles increase, and the number of defects decreases, leading to a decrease of oxygen mobility. The TPR-CO data show that the homogeneous solid solution RhxCe1-xO2-δ has high oxygen mobility at low temperatures T≤250°C. The comparison of the TPR-CO data and ex situ XPS


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results allows us to conclude that reduction of the solid solution by CO includes three different redox routes in accordance with the involved redox transitions. The first one at low temperature (T250°C) related to the reduction of Rh3+, while the reduction of Ce4+ to Ce3+ slows down. The Rh deficient ceria lattice has less distortions and defects, and accordingly, the oxygen mobility decreases. We propose that at this temperature region, the reduction of Rh-O-Rh fragments takes place. These species might be present in the initial sample originally in a small amount or could be formed during the restructuring of the system upon heating during TPR experiments. Reoxidation of the reduced homogeneous solid solution by molecular oxygen is very effective. The Се3+→Се4+ transition is nearly instantaneous upon O2 treatment at low temperatures. Reoxidation of rhodium species is also noticeable already at room temperature, and after oxygen


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treatment at 450°С, the system returns to the initial oxidized state. Reoxidation of the reduced metal particles with their redispersion within the CeO2 surface was demonstrated by a number of authors with both the experimental22,33,50,78 and theoretical79 approaches. Kurnatowska et al.43,44 have shown the dissolution of reduced rhodium nanoparticles (~1 nm) in CeO2 upon heating in O2 at 500°C. Miyazawa et al46 have shown that the oxidation of rhodium metal particles on the surface of ceria resulted in the formation of Rh-O-Ce bonds at a lower temperature than the RhO-Rh bonds, which can be a reason for dispersing of the clusters with their dissolution in the support. The re-dispersion of the metal particles in an oxidizing atmosphere has been shown for other metals as well. Gänzler et al.22 have recently analyzed in detail the dynamic structural behavior of Pt nanoparticles in the CeO2 surface under redox treatment. Authors have shown that for a strongly interacting support the noble metal dispersion can be adjusted by tuning the reduction conditions followed by oxidation treatment. Tanaka et al. have observed the reversible transition of Pd into and out of the perovskite lattice.80,81 Authors have shown that because of the structural peculiarities of palladium-containing perovskite samples, the small metallic particles (1-3 nm) observed in the aged catalyst move back inside the perovskite crystal as Pd cations upon oxidation. Such a self-regenerative function greatly increased the samples durability that allowed the authors to introduce the term “intelligent catalyst.”80 The XPS data shows the complete reversibility of charging states of rhodium after the redox cycle. Based on the literature data, we can propose that the interaction of reduced Rhnδ+ clusters with oxygen results in their oxidation followed by redispersion in the sub-surface layers of cerium oxide with further increase of the temperature. Following the works of Kurnatowska et al.,43,44 we can reliably conclude that the reversible RhxCe1-xO2-δ↔Rhnδ+/CeOx transition takes place at the surface and near-surface region of ceria. It is reasonable to assume that for such a


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reversible transition, the reduced rhodium particles should be of very small size (

Redox and Catalytic Properties of RhxCe1âxO2âÎ´ Solid Solution

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Redox and Catalytic Properties of RhxCe1âxO2âÎ´ Solid Solution