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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

A Structural Insight into Strong Pt–CeO Interaction: From Single Pt Atoms to PtO Clusters x

Elizaveta A. Derevyannikova, Tatyana Yu. Kardash, Andrey I. Stadnichenko, Olga A. Stonkus, Elena M. Slavinskaya, Valery A. Svetlichnyi, and Andrei I. Boronin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11009 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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The Journal of Physical Chemistry

A Structural Insight into Strong Pt–CeO2 Interaction: from Single Pt Atoms to PtOx Clusters Elizaveta A. Derevyannikova 1, Tatyana Yu. Kardash

1,2*,

Andrey I. Stadnichenko

1,2,

Olga A.

Stonkus 1,2, Elena M. Slavinskaya 1,2, Valery A. Svetlichnyi 3, Andrei I. Boronin 1,2 * 1

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt

Akademika Lavrentieva 5, Novosibirsk 630090, Russia 2

Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia

3

Tomsk State University, Lenina Av. 36, Tomsk, 634050, Russia

Abstract

The Pt–CeO2 nanocomposites were obtained by coprecipitation, varying the Pt loading over a wide range of 1–30 wt %. The samples were calcined in air at 450–1000 °C. The Pt–CeO2 nanocomposites were investigated by a set of structural (XRD, EXAFS, PDF, TEM) and spectroscopic (XPS, Raman) methods. Over the whole range of Pt loading the main species were Pt2+ and Pt4+. They were localised either in a single-atom state or in the form of PtOx clusters on the ceria surface. The joint PDF and EXAFS modelling based on the combination of [Pt2+O4] single-atom and Pt3O4 structural fragments allowed us to propose the local structure of the PtOx clusters. The formation of such surface structures is associated with a distorted ceria surface on

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the Pt–CeO2 nanocomposites. We assume that the close arrangement of platinum ions in the PtOx clusters could be responsible for the effective redox properties of the samples.

Introduction Cerium dioxide is an irreplaceable support for a wide class of heterogeneous catalysts. Due to a high reduction efficiency of the Ce4+–Ce3+ redox couple, ceria can provide additional oxygen release without structure change. Modification of ceria1,2 by a platinum group metal increases the oxygen storage capacity (OSC) and oxygen mobility. It allows obtaining effective catalysts for use in low-temperature CO oxidation. The distinctive feature of ceria is to stabilise ionic states of Pt, Rh, Pd and Ru by a strong interaction.3–8 Such ionic states are actively explored for Pt/CeO2 catalysts – so-called single-atom catalysts.9–12 The Pt–CeO2 system is a commonly used catalyst for many reactions, such as PROX,13,14 WGS15,16 and CO oxidation.17–20 Single-atom platinum catalysts are in the focus of current research because this approach allows achieving the most effective use of expensive noble metals for catalysis. Furthermore, stabilization of ionic species on the ceria surface could increase the sintering resistance of the catalysts.7-8 In single-atom catalysts, platinum may be presented as Pt2+ or Pt4+ ionic species. Studies have shown21 that platinum is stabilised mainly in the +2 oxidation state on CeO2 in the form of the Ptdoped CeO2 phase. Ionic platinum forms are shown to contribute to an increase in catalytic activity.21,22 Nie L. et al23 showed that atomically dispersed Pt2+ on ceria could be activated via high-temperature steam treatment to achieve low-temperature CO oxidation activity. The improved low-temperature activity is attributed to the activation of surface lattice oxygen in the vicinity of atomically dispersed Pt. In another paper,24 the formation Pt4+ species on thin films of


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CeO2 was observed. However, Pt4+ is less stable then Pt2+. After wet hydrogen annealing, the Pt4+ species disappear, while Pt2+ species on the ceria surface exhibit high electrocatalytic activity. Structural features of the active species based on ionic forms and CeO2 were investigated in several works by a DFT method. The authors25–27 showed that Pt4+ could be introduced into the CeO2 lattice. After calcination, Pt4+ is released to the CeO2 surface and reduced to Pt2+, which is in a square planar coordination [PtO4] on the (100) facet of ceria. In other works9,28 it was confirmed that a stable form of Pt2+ is surrounded by four oxygen atoms in square planar coordination and localised on the ceria (100) plane. Pd2+ ionic species in square planar coordination could also be stabilised on the ceria surface. DFT and HR-TEM studies showed that a square planar coordinated fragment [PdO4] were observed on the steps of the (110) planes of ceria.29 Thus, structural stabilisation of Pt2+ on the (100) facet of ceria is well established. According to the literature,30 the (100) facet of fluorite CeO2 is known to be less stable than the (111) facet. However, according to Wang and Feng 31, formation of an unstable (100) facet depends on the particle size of ceria. When the ceria particle size is smaller than 10 nm, both stable (111) and unstable (100) facets are observed. The increase in ceria particle size leads to the disappearance of the (100) facets, and ceria particles are represented only by (111) facets. Thus, to form ionic Pt species on the surface of CeO2 it is necessary to use defective and dispersed ceria. In the present work, Pt–CeO2 nanocomposites with different Pt loadings were obtained by a coprecipitation method from nitrates. Our previous study on Pd/CeO2 systems32,33 has shown that this method of synthesis leads to the formation of dispersed ceria particles. Defective ceria particles were able to stabilise Pd2+ ions both in the volume of the ceria and in surface oxygen clusters.


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To study the structural features of Pt species, different structural and spectral methods were used: XRD, XPS, HRTEM and Raman spectroscopy. The local structure was investigated by the Pair Distribution Function (PDF) method. PDF data show the distribution of interatomic distances in the material. It is a powerful tool to study disordered materials, nanocrystals34 and catalytic systems.35,36 For a more detailed analysis of the local platinum environment, EXAFS spectroscopy on Pt LIII-edge was used. According to data obtained, structural models of the combined phase based on Pt ions and CeO2, and of the local structure of PtOx clusters are suggested. Experimental section Sample synthesis Coprecipitation method was used to obtain the Pt–CeO2 nanocomposites with different Pt loading. First, Ce(NO3)3 and Pt(NO3)4 solutions were mixed; this step was followed by precipitation with a 1 M KOH aqueous solution at pH 8.8–9.0 and room temperature with subsequent aging under the same conditions for 2 h. The precipitates obtained by filtering were dried in air and then at 110 °C for 12–14 h. The dried samples were calcined in flowing air at 450 °C for 4 h; potassium was then removed by washing with subsequent calcination in air at 450, 600, 800 and 900 °C. Pristine CeO2 was prepared by precipitation from a cerium nitrate solution followed by drying and calcination in air at 600 °C for 4 h. These samples were used as reference materials. The bulk chemical composition of the calcined catalysts was determined using a Perkin Elmer ISP OPTIMA 4300DV atomic absorption spectrophotometer (AAS). The exact Pt content measured by AAS was equal to 1.1, 4.9, 8.3, 14.6, 20.3 and 29.8 wt %. The notations 1Pt–CeO2, 5Pt–CeO2, 8Pt–CeO2, 15Pt–CeO2, 20Pt–CeO2, 30Pt–CeO2 were used. The concentration of the remaining K in the samples was controlled by AAS, the results are shown in the Supporting Information, Table S1.


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The specific surface (SBET) of the samples was determined by the BET method, using argon thermal desorption with a Sorbtometr-M adsorption analyzer. X-ray diffraction The phase composition was determined using powder X‐ray diffraction (XRD) patterns which were collected on a Bruker D8 diffractometer, Cu‐Kα 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 the ICDD PDF‐2 database. Rietveld refinement was performed using TOPAS v4.2 software.37 The diffraction line profile was analyzed by the fundamental parameter approach. The mean crystallite sizes were calculated using 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. High resolution transmission microscopy Electron microscopy investigation was performed using JEM-2200FS (JEOL Ltd., Japan) electron microscope operated at 200 kV for obtaining HRTEM images. STEM HAADF mode was employed together with EDX spectroscopy. Lattice spatial resolution was 1 Å. JEOL EDX analyzer makes it possible to obtain the 1 nm spatial resolution of EDX mapping. The samples for the TEM study were dispersed in ethanol by ultrasound and supported on holey carbon film mounted on a copper grid. Pair Distribution Function method The diffraction data for PDF were obtained using synchrotron radiation at ID22 beamline at ESRF, Grenoble, France. X‐ray scattering data were obtained using a Perkin Elmer disordered silicon detector and a wavelength of λ = 0.177005 Å. The wavelength was refined against a standard NIST


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CeO2 sample. A powder sample was measured in polyimide capillaries (1.5 mm). A background contribution was measured using an empty capillary. Data reduction was done with pyFAI program.38 The normalized scattering curves S(Q) and PDF G(r) curves were calculated using the PDFgetX3 program (see equation 1).39

G (r ) 

2





0

Q[ S (Q )  1] sin(Qr )dQ  4 [  (r )   0 ]

(1)

where Q = 4πsinΘ/λ, ρ(r) is the paired atomic density (a probability of finding the i atom at a distance r from the j atom), and ρ0 is the average density. The upper limit of integration was Qmax = 25.7 Å–1. X-ray Absorption Spectroscopy XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) spectra were collected at the ELETTRA Synchrotron (Trieste, Italy) at XAFS beamline. The measurements were carried out at the Pt LIII-edge (11 563.7 eV), and the incident beam was monochromatized with a Si (111) double crystal. Pt LIII-edge absorption spectra were recorded at room temperature in transmission and fluorescence detection mode depending on Pt-loading in the samples. The energy was calibrated by measuring the absorption edge of a platinum metallic foil (fixed at 11563.7 eV). For transmission mode, standard ionization chamber was used for detection of intensity. For the samples with low Pt concentration (1-5 wt %), SDD fluorescence detector with beryllium window was used. In this study, the background removal and extraction of the EXAFS oscillatory part from the experimental spectra and EXAFS modelling were performed in Athena and Artemis software.40 The Fourier transformation of EXAFS oscillations was carried out in the k-range of 3.01–14 Å−1. The fitting was performed in the r-range of 1.1–3.75 Å with k2weight.


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Pt foil as a reference compound was used also to establish the value of the reduction factor S20. For this purpose the correlation between 𝑆20 and Debye-Waller (DW) parameter σ2 was reduced using different dependencies {σ2(𝑆20)}𝑘𝑛, which were obtained from the experimental Pt LIII-edge EXAFS in foil for the first coordination sphere under differently weighted functions knχ(k) (n=0, 1, 2), see Supporting Information (Figure S1). The crossing region of these curves was centered to obtain the value. This value of 𝑆20 was used and fixed in the following refinement of Pt–CeO2 atomic structure. Crystal structures were built using the VESTA software.41 X-ray Photoelectron Spectroscopy X-ray photoelectron spectra (XPS) were recorded on a KRATOS ES-300 electron spectrometer using nonmonochromatic MgKα radiation (hν = 1253.6 eV). The photoelectron spectra were recorded using a low-power X-ray source (80 W) to prevent X-ray damage to ceria.42,43 The spectrometer was calibrated using the Au4f7/2 (84.0 eV) and Cu2p3/2 (932.7 eV) lines of pure gold and copper metallic surfaces. Spectral calibration for the samples was performed using Eb (C3d U’’’) = 916.7 eV.44 Chemical composition was calculated from XPS data taking into account atomic sensitivity factors.45 The original XPS-Calc software, which was successfully tested in our earlier works44,46–48, was employed for deconvolution of experimental spectra into individual components. Spectral background of the Pt4f line was subtracted by the Shirley method, an approximation of the doublets was made using Lorentzian and Gaussian functions. Before the curve fitting, all experimental spectra were smoothed using a Fourier filter. No considerable difference between the smoothed and experimental curves was observed; the mean-square deviation was less than 1%.


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Raman spectroscopy Raman spectra were measured using an InVia (Renishaw, UK) confocal Raman dispersion spectrometer with a Leica microscope. Excitation was performed with continuous lasers: a semiconductor laser with 785 nm and 100 mW; and a solid-state Nd: YAG laser, second harmonic, 532 nm, 100 mW. The diffraction grating with 1200 pieces mm-1 ensured a spectral resolution of 1 cm-1 upon excitation at 785 nm, and 2 cm-1 at 532 nm. The microscope provided the excitation locality up to 2 μm2 with a 50× objective lens. To prevent changes in the samples owing to heating under laser radiation, the measurements were made using 5% of the maximum laser intensity, an additional 100% defocusing of the laser beam, and signal accumulation time up to 100 s. The study was performed in a range of 100–3200 cm-1. Data are reported for the range of 100–800 cm-1, in which important changes were observed in the spectra owing to calcination of the samples. TPR-H2 method Investigation of the synthesized samples was performed in an automated setup with a ﬂow reactor and mass-spectrometric analysis of the gas mixture using the temperature-programmed regime. A sample with the particle size of 0.25–0.5 mm was mounted in a stainless steel reactor. The reaction mixture containing 10.0 vol.% H2 and helium the balance was introduced at a ﬂow rate of 100 cm3/min to the catalyst sample (0.3–0.1g) preliminary cooled in the reactor to −10 °C. As the steady-state concentrations of H2 were established, the sample was heated from −10 to 450 °C at 10 °C/min heating rate. The concentration of H2 was measured during the reduction. Before each TPR-H2 experiment the catalysts were pretreated by 20%O2/He gas mixture at 450 °C during 2 h with subsequent cooling in this mixture.


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Results XRD According to XRD data, all samples calcined at 450 and 600 °C present only ceria phase with a fluorite-type structure (ICDD PDF‐2 #00–043–1002). Figure 1a illustrates the XRD patterns for the Pt–CeO2 samples with different Pt loadings calcined at 600 °C. Additional platinum metal or platinum oxide phases are not observed. Platinum metal phase (ICDD PDF-2 #00–004–0802) appears in the samples calcined at 800 °C with Pt loading of 8–30 wt %. For the samples with a low Pt loading (1–5 wt %) only CeO2 phase is detected even after calcination at 900 °C. As an example, in Figure 1b, the XRD patterns for the 15Pt–CeO2 sample calcined at different temperatures are presented. The XRD data for all samples calcined at different temperatures are presented in Figure S2 (Supporting Information).

Figure 1. XRD data for the Pt–CeO2 samples. (a) XRD patterns for the Pt–CeO2 samples, calcined at 600 °C; (b) XRD data for the 15Pt–CeO2 sample, calcined at 450–900 °C.

For all obtained XRD patterns, a full profile Rietveld refinement was performed. It was noted that for the samples calcined below 800 °C with Pt loading of 20–30 wt %, calculated curves do not


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match the experimental curves, especially in the (200) peak area. To improve fitting, the additional peak at 2θ = 32.93 ° is introduced for the 20Pt–CeO2 and 30Pt–CeO2 samples. Figure 2 shows calculated, experimental and difference curves for the 30Pt–CeO2 sample calcined at 600 °C after refinement with the additional peak (b) and without it (a). One can observe a significant improvement in the fit when diffuse scattering (i.e. additional peak) is taken into account. This may be an indication of a presence of a disordered phase in samples with high Pt loading.

Figure 2. Experimental and calculated XRD patterns for the 30Pt–CeO2 sample calcined at 600 °C refined by (a) only CeO2 fluorite phase and (b) with additional diffuse scattering (green curve). The calculated pattern obtained by the Rietveld structure refinement for the fluorite structure. The differential curve is shown at the bottom.

Lattice parameter and coherent scattering region (CSR) for the CeO2 phase were calculated from the Rietveld refinement results. Obtained data for all samples are shown in Table 1, which also shows the specific surface area of the samples. The increase in Pt content from 15 to 30 in the samples calcined at 450 and 600°C results in the decrease in specific surface area of the samples.


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However, the CSR size also decreases: the increase of Pt loading from 1 to 30 wt % results in a decrease in ceria particle size from 9 to 4 nm. This indicates and supports the XRD Rietveld modeling data on the formation of defective phase in the samples with high Pt loading. The appearance of this phase results in the formation of the particle agglomerates, which results in the decrease in the surface area of the samples, measured by the BET method. Figure 3a represents the dependence of fluorite lattice parameter and CSR on Pt loading in the samples calcined at 600 °C. Decrease in CSR size in the samples with high Pt loading is accompanied by the increase in the ceria lattice parameter. The increased ceria lattice parameter compared with the standard value of 5.411 Å (ICDD PDF‐2 #00‐043–1002) is often observed for defective ceria nanoparticles with sizes less than 5 nm.49–51 Figure 3b shows the dependence of ceria lattice parameter and CSR for the sample with 20 wt % Pt loading on calcination temperature. As was mentioned above, a metallic platinum phase with ~67 nm particle size appears after sample calcination at 800 °C. However, the quantitative analysis shows that the amount of metallic Pt is less than Pt content in the sample after calcination at 800 °C, see Figure S3 (Supplementary Information). It indicates that part of Pt remains in a highly dispersed state, which preserves CeO2 nanoparticles growth at this temperature. The lattice parameter of CeO2 after calcination at 800°C is not changed. However, further calcination at 900 °C results in the transition of all Pt into a metallic state. As a result, after calcination at 900 °C ceria sinters up to particle size >88 nm and ceria lattice parameter decreases to the value of pristine ceria (5.411 Å). It should be noted that calcination at 450–800 °C of pristine ceria (dashed line) leads to the generation of larger ceria particles52 compared to the platinum–ceria samples (Figure 3b). The amount of platinum metallic phase in the samples calcined at 800 °C is less than platinum analytical value (Figure S3, Supporting Information).


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Thus, XRD data show that coprecipitation of Pt with ceria results in the formation of a Pt–CeO2 nanocomposite. Therein, ceria nanoparticles are stabilised and their sintering up to 800 °C is restrained.

Figure 3. Lattice parameters (a, Å), coherent scattering region (CSR, nm) of CeO2 for (a) the Pt– CeO2 samples with different Pt-loading, calcined at 600 °C, and for (b) the 20Pt–CeO2 sample calcined at different temperatures. The dashed line corresponds to parameters of pristine CeO2 nanoparticles.


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Table 1. Structural characteristics of ceria in the Pt–CeO2 samples. Pt loading

1%

5%

8%

T, °C

a, Å

CSR, nm

SBET, m2/g

a, Å

CSR, nm

SBET, m2/g

a, Å

CSR, nm

SBET, m2/g

450

5.417 (1)

7.4 (3)

135

5.416 (1)

6.2 (3)

156

5.417 (2)

4.4 (1)

135

600

5.414 (1)

8.6 (3)

93

5.416 (1)

6.5 (3)

132

5.422 (3)

5.9 (6)

134

800

5.413 (1)

20 (1)

26

5.417 (1)

11.0 (6)

49

5.425 (2)

11 (1)

42

900

5.414 (1)

40 (2)

1.3

5.413 (1)

42 (1)

6.4

5.414 (1)

46 (2)

6.3

Pt loading

15%

20%

30%

T, °C

a, Å

CSR, nm

SBET, m2/g

a, Å

CSR, nm

SBET, m2/g

a, Å

CSR, nm

SBET, m2/g

450

5.419 (2)

4.9 (1)

148

5.419 (3)

4.4 (3)

102

5.419 (3)

3.7 (3)

81

600

5.422 (2)

5.2 (3)

131

5.423 (3)

4.7 (3)

98

5.429 (3)

3.9 (3)

77

800

5.421 (1)

14.1 (9)

25

5.421 (1)

16 (1)

18

5.413 (1)

62 (2)

6.6

900

5.413 (1)

58 (1)

5.4

5.413 (1)

88 (3)

4.3

5.412 (1)

112 (3)

3.5

HRTEM According to TEM data, only ceria nanoparticles with a polyhedral shape are detected in the samples calcined at 450 and 600 °C. Crystallites of Pt-containing phases (platinum metal or oxides) are not observed. It is in a good agreement with XRD data. Figure 4 illustrates TEM data (a) and EDX mapping (b) for the 20Pt–CeO2 sample calcined at 600 °C. EDX mapping shows a uniform distribution of Pt and Ce elements over the entire volume of the samples. The size of ceria crystallites decreases with the increase in Pt loading. For instance, the sample with 5 wt % Pt loading contains agglomerates of ceria with sizes 5–10 nm. There are also larger nanoparticles with sizes up to 20 nm. For the sample with 15 wt %. Pt loading, ceria


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nanoparticle size is 4–10 nm, with 30 wt % Pt it is 3–7 nm. In 5Pt–CeO2, ceria is porous, while in the samples with larger Pt loading, micropores are not observed. (Figure S4, Supporting Information).

Figure 4. (a) TEM and (b) HAADF-STEM images for the 20Pt–CeO2 sample calcined at 600 °C with (c-d) corresponding EDX mapping patterns of PtM (red) and CeL (green).

In all samples we were able to detect metallic platinum clusters, which were mainly localised on CeO2 defects and/or epitaxially bonded to the ceria surface. The metallic clusters were formed under the action of the electron beam. Rh-doped CeO2 nanoparticles demonstrated similar behaviour, as reported in our previous work.53 The increase in Pt loading results in the formation of a higher number of platinum metallic clusters generated by the electron beam. Figure 5 displays TEM data for 15Pt–CeO2 (a) and 30Pt–CeO2 (b) samples calcined at 450 °C. The red arrows point to Pt metallic clusters. Thus, TEM analysis shows that a mixed phase containing both Pt and CeO2 is formed in Pt–CeO2 nanocomposites. This phase is located on the surface and in defects of the CeO2 nanoparticles. In the samples with a high Pt loading, a rapid reduction of the mixed phase to metal under the electron


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beam is observed. This may be an indication of the presence of Pt-containing clusters in these samples.

Figure 5. HRTEM images for the (a) 15Pt–CeO2 and (b) 30Pt–CeO2 samples calcined at 450 °C. Small Pt° clusters can be seen on the defects of ceria. Indicated spacing (2.3 Å) correspond well to d111 Pt° (2.265 Å for bulk platinum).

XPS In an earlier work,48 it was shown that the samples synthesised by a coprecipitation method contain only ionic forms of platinum. The XPS data obtained in the present work support these findings. Figure 6 displays Pt4f spectra for Pt–CeO2 samples calcined at 600 °C with 1, 15 and 30 wt % Pt loading (curves 1–3). Figure S5 (Supporting Information) displays Ce3d spectra for Pt–CeO2 samples calcined at 600 and 800 °C with different Pt loading. According to XPS data, in all obtained samples (1–30 wt % Pt) calcined at 600 °C, platinum is only in ionic states. Table 2 represents data on the quantitative composition of the samples obtained by XPS. The main platinum state is characterised by a binding energy Eb(Pt4f7/2) = 72.8–72.5 eV. According to the


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literature, this binding energy can be attributed to Pt2+ ions bonded directly to the ceria surface in a square planar structure form [PtO4].54

Figure 6. Fitted Pt4f XPS spectra for the 1Pt–CeO2 (1, multiplied 3 times), 15Pt–CeO2 (2) and 30Pt–CeO2 (3) samples calcined at 600°C.

Additionally, for all samples, a platinum state with binding energy Eb(Pt4f7/2) = 74.5–74.1 eV was observed. Such binding energy corresponds to the highly oxidised form Pt4+. Nevertheless, based on previous experiments dedicated to XPS investigation of PtO2 nanoparticles,46 our Pt4+ state cannot be attributed to nanoparticles of platinum dioxide. Furthermore, XRD and TEM data do not show the presence of crystallised forms of platinum oxide. There are no reliable literature data on the interpretation of this platinum state. In one paper,55 the formation of a PtxCe1-xO2-δ solid solution possessing the similar platinum species as in the samples was discussed. One can assume that Pt4+ enters the fluorite lattice in a subsurface layer. It was previously shown that the ionic forms of other platinum group metals, such as Pd2+7,8,32,56 and Rh3+53 ions, can incorporate into the CeO2 lattice.


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Table 2 indicates that the Pt/Ce atomic ratios obtained by XPS are underestimated with respect to the analytical platinum concentration in the samples with high Pt loading. Considering the fact that the separate Pt-containing oxides are not detected in the samples, we can assume that Pt can be located in the ceria volume and/or subsurface. In this case, the XPS signal of Pt could be decreased due to shielding by ceria. Table 2. Quantitative composition of the samples obtained by XPS, atomic %. Sample

Pt

Pt0

Pt2+

Pt4+

1Pt–CeO2 600°C

0.56

-

0.49 0.07 17.8 48.3 33.3

0.12

0.01

0.01

15Pt–CeO2 600°C

2.07

-

1.45 0.62 12.8 49.9 35.1

0.12

0.06

0.16

30Pt–CeO2 600°C

3.58

-

2.51 1.07 13.6 52.6 30.0

0.19

0.12

0.38

1Pt–CeO2 800°C

0.56

-

0.56

16.7 51.0 31.7

0.20

0.01

0.01

15Pt–CeO2 800°C

1.49 0.30 1.04 0.15 10.9 50.4 35.9

0.17

0.04

0.16

30Pt–CeO2 800°C

1.55 0.19 1.24 0.12 13.1 55.5 28.9

0.21

0.05

0.38

-

C

O

Ce

Ce3+/Cetot Pt/Ce

Pt/Cean

According to the XRD data (Figure 1b), calcination of the samples with Pt loading of >5 wt % at 800 °C results in the formation of platinum metal with particle size >60 nm. However, XPS data show that in addition to the formation of Pt0 state in these samples the major platinum states remain ionic forms, mainly Pt2+. After calcination at 800 °C, the amount of Pt4+ decreases significantly compared to Pt2+, which can indicate the substantially higher stability of the Pt2+ state. The value of the Pt/Ce atomic ratio in the high-percentage samples decreases, which can be explained by the formation of large platinum metallic particles. The values of Pt/Ce atomic ratio obtained by XPS for 1Pt–CeO2 coincide with the analytical. This implies the uniform distribution of platinum ions on the ceria surface. At a calcination temperature


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of 800 °C, platinum metal is not observed in 1Pt–CeO2, and all platinum is in the Pt2+ state, which is an indication of the high thermal stability of such Pt species in Pt–CeO2 composites.

Raman spectroscopy The Raman spectra for the Pt–CeO2 samples calcined at 450–1000 °C were recorded in the range 100–1000 cm–1 with laser excitation wavelengths of 532 nm (Figure 7a) and 785 nm (Figure 7b). The peak at 463 cm–1 corresponds to the F2g vibrational mode of oxygen ions in a cubic [CeO8] subcell of the CeO2 fluorite structure.57 Additional peaks at 153, 308, 550 and 680 cm–1 for 532 nm excitation (Figure 7a) are observed. In the case of 785 nm excitation, the last two peaks appear at 540 and 670 cm-1 (Figure 7b). Some authors have that wide peaks at 550 and 680 cm–1 are associated with asymmetric and symmetric vibration of Pt–O–Ce in surface mixed-oxide species, respectively.58 The presence of these peaks can be attributed to the existence of the strong interaction between Pt and CeO2. With the increase of calcination temperature to 800 °C, the intensities of the peaks at 550 and 680 cm–1 decrease and completely disappear at 1000 °C, due to the decrease of active surface platinum forms, which reduce to the metal. However, CeO2 defect modes characterised by a broad peak in the region of 550–600 cm–1 could also contribute to the spectra. These defect modes were observed for CeO2 doped by different cations59,60 substituting for Ce4+. It should be noted that the spectra contain peaks below 350 cm–1. The high intensity resonance peak at 153 cm–1 is observed especially with laser excitation wavelength of 785 nm. This resonance is especially pronounced in the samples with high Pt loading. For the 1Pt–CeO2 sample, this peak in the Raman spectra is also observed. However, its intensity is much lower (see Figure


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S6, Supporting Information). The additional peak at 308 cm–1 (Figure 7b) correlates in intensity with the peak at 153 cm-1 and is its overtone. The nature of this peak is not clear and is barely discussed in the literature. The similar set of peaks at 188 and 698 cm–1 is observed in Raman spectra of Pt3O4 and platinum bronze MxPt3O4 phases, which have similar structures.61 In these phases, square planar coordinated platinum constructs a three-dimensional structure. We have previously shown32 that the Pd–CeO2 sample, which has the Pd2+ square planar environment on the CeO2 facet, contains the similar Raman vibration at 187 cm– 1.

Therefore, the observed peak at 153 cm–1 could be attributed to the vibration of Pt2+ ion square-

planar-coordinated on the CeO2 facet. On the other hand, with an increase in calcination temperature the intensity of the peak at 153 cm–1 also increases. According to XRD and TEM data, an increase in calcination temperature should be accompanied by a sintering of Pt species. Moreover, in another study44 the peak at 153 cm–1 in Pt/CeO2 catalysts is interpreted as vibration in PtOx clusters formed on CeO2. Thus, Raman data show that PtOx clusters with Pt in a square planar coordination are formed in the Pt–CeO2 nanocomposites. Calcination of the nanocomposites at 800 °C results in the aggregation of single-site Pt ions on CeO2 and an increase in a concentration of PtOx clusters on the ceria surface.


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Figure 7. Raman spectra for the 20Pt–CeO2 samples calcined at 450–1000 °C temperatures with a laser excitation wavelength of 532 nm (a) and 785 nm (b).

PDF study Figure 8 displays the PDF curves for the samples with different Pt loadings calcined at 600 °C. The PDF curve for pristine CeO2 with 5 nm particle size is presented for comparison. For the samples with low Pt loading (1–5 wt %) additional peaks compared to pristine CeO2 are not observed. The additional peaks at 2.01 and 3.18 Å appear on the PDF curve for the 15Pt–CeO2 sample. Based on the structural models available in the literature, these distances could be associated with the distances formed by Pt2+ in the square planar environment on the (100) CeO2 facet (Figure 9a). Accordingly, the 2.01 Å peak can be attributed to Pt–O distance, while the peak at 3.18 Å corresponds to the Pt–Ce distance. In the literature, it is being actively discussed that such a Pt2+ position on the CeO2 surface is stable according to DFT calculations.25,54 In the samples


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with low Pt loading, these Pt–O and Pt–Ce peaks are probably not observed due to their low intensity. According to XPS data, there is the Pt4+ state. If Pt4+ occupies the Ce4+ position in the fluorite structure, it would not generate new peaks in the PDF curves. It is also worth considering that the Pt4+ concentration is less than that of Pt2+, and the contribution from Pt4+ distances may not be observed in PDF curves.

Figure 8. (a) X-ray PDF data for the 1-5-15Pt–CeO2 samples, calcined at 600 °C; (b) X-ray PDF data for the 15-20-30Pt–CeO2 samples, calcined at 600 °C. X-ray PDF data for the nanocrystalline CeO2 are presented for comparison (dot curve).

The increase in Pt loading to 20–30 wt % in the samples leads to an increase in the intensity of additional peaks (Figure 8b). The intensity of the 2.01 Å peak increases due to a higher number of surface Pt2+ species. The Pt–Ce distance changes from 3.18 to 3.10 Å, which could be associated either with the bond shortening or with the formation of a new type of bond. Based on the Raman spectra above, the formation of Pt3O4 structural fragments, as shown in Figure 9b, is proposed. Two different Pt–Pt distances, 2.81 and 3.44 Å, are represented in the Pt3O4


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structure. The short 2.81 Å distance is not observed in PDF curves of the samples, which could be explained by a localisation of Pt2+ on the CeO2 surface. On the other hand, a Pt–Pt distance of 3.44 Å in the Pt3O4 structure could correspond to the 3.47 Å distance observed in the PDF curves. Thus, this comparison allows us to make a reasonable assumption about the formation of oxide clusters with Pt3O4 structure in the CeO2 surface layer. We assume that Pt3O4 structures are ‘embedded’ in a ceria matrix. Therefore, due to boundary interactions, the stoichiometry in Pt3O4 is most likely not constant and depends on Pt2+/Pt4+/Pt0 ratios in the sample. Therefore, we will use the notation PtOx for these clusters, where x varies depending on the oxidation/reduction state of the surface. XRD and TEM data show that 20–30 wt % Pt loading in the samples leads to the formation of an additional, distorted phase on ceria defects. We suggest that PtOx clusters are embedded within this distorted phase. This phase could present either a disordered structure based on fluorite or a phase with a different local structure. On one hand, the structure based on α-PtO2 might be suggested. It has been shown62 that during ageing of platinum nitrate solution, the cluster [Pt4(μ3-OH)2(μ2-OH)4(NO3)10] (Pt4) is formed. The structure of this cluster is a monolayer of αPtO2 (Figure 9c). The basic distances in this cluster are close to those observed in PDF curves: Pt-O 2.0 Å, Pt–Pt 3.09 Å and Pt–O 3.52 Å. Platinum within this cluster is in the 4+ state and coordinated in an octahedron. The formation of such clusters on the ceria surface has been discussed.63 One can assume that during the synthesis in nitrate solution with an excess of platinum, such Pt4 clusters are formed. After rapid precipitation, the clusters retain their local structure, forming an additional phase on ceria defects. On the other hand, our XPS data show that the main platinum state is Pt2+, not Pt4+ as in the model presented in Figure 9c. Although Pt4+ was detected in the sample, its amount is too small compared


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to Pt2+ to give such a significant contribution to the intensity of the peaks in PDF. Moreover, according to Raman spectroscopy, no additional peaks characterising this type of platinum oxide structural fragments are observed. Hence, the model-based α-PtO2 monolayer was not used to construct the structural model. Instead, a model based on a disordered fluorite structure and platinum ions stabilising on the (100) CeO2 facet was used.

Figure 9. Prospective structures: (a) single atom Pt2+ on (100) CeO2 facet, (b) Pt3O4, (c) Pt4 cluster.62

XAS study For detailed analysis of the local environment around the platinum atoms and construction of the most suitable model of PtOx clusters, X-ray absorption spectroscopy was applied. At the Pt LIII-edge, the absorption intensity of the white line reflects the vacancy in the 5d orbital of Pt atoms. Therefore, it is possible to estimate the average oxidation state of the Pt atoms in each sample. As standard samples of Pt0 and Pt4+ states, we took Pt foil and H2[Pt(OH)6]. Regarding the experimental XANES spectra for standards and the literature data for Pt(AcAc) standard12 containing Pt2+ state, it could be concluded that all samples calcined at 600 °C contain Pt2+ and Pt4+ (Figure 10a), which is in good agreement with XPS data (Table S1, Supporting Information).


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Figure 10b shows Fourier-transformed EXAFS spectra for 1Pt–CeO2, 15Pt–CeO2, 20Pt–CeO2 and 30Pt–CeO2 samples calcined at 600 °C. It can be seen that the EXAFS spectra, regardless of Pt loading, look similar, which indicates the formation of similar local platinum structures. The first intense peak corresponds to the first platinum coordination sphere. This short distance of 1.6 Å on EXAFS curves corresponds to 2.0 Å Pt–O distance and is typical for platinum oxides.64 The similar distance was observed on the PDFs. The remaining peaks corresponding to the second and subsequent coordination spheres have a rather low intensity, which indicates a decrease in coordination number of these coordination spheres. These peaks refer to the distances between platinum and metal in the second and subsequent coordination spheres. As detected by PDF data, the distances can be identified as 3.10 and 3.48 Å. The low coordination numbers in these spheres indicate two facts. Firstly, platinum is localised mainly on the ceria surface. Secondly, platinum can be included in the composition of the distorted structure, which is characterised by a broad distance distribution. As the distribution around average values broadens, the intensity of peaks falls off.


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Figure 10. Pt LIII-edge XANES spectra (a, c) and EXAFS spectra (b, d) of 1-15-30Pt–CeO2 and reference samples, calcined at 600 °C (a, b) and 800 °C (c, d).

XANES and EXAFS spectra for standard samples and Pt–CeO2 nanocomposites calcined at 800 °C are shown in Figure 10c,d. When samples are calcined at 800 °C, Pt0 species are observed. The higher the Pt loading in the sample, the more metal phase is formed (Table S2, Supporting Information), which points to different strengths of Pt–O bonds in the samples with different Pt loadings. As a result of the formation of a strong and thermally stable [PtO4] single-atom form on ceria surface in 1Pt–CeO2, no sintering to the metal occurs. In contrast, for 30Pt–CeO2 sample there are only 2% of Pt ionic state. The increase of Pt loading leads to the formation of Pt–O–Pt


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interactions in PtOx clusters. In these clusters there is a general weakening of the Pt–O bonds, resulting in reductions in the thermal stability of the Pt ionic forms.

Structure modelling The 1Pt–CeO2, 15Pt–CeO2 and 30Pt–CeO2 samples calcined at 600 °C were selected for further modelling by EXAFS spectra. Based on the experimental and literature data, the models presented in Figure 11 were considered. In the case of 1Pt–CeO2, the model considers Pt2+ ions locating on the (100) ceria facet and Pt4+ ions substituting at the Ce position in the subsurface layer. This model does not generate Pt–Pt interactions in the local environment. These Pt centres can be considered as single-atom sites. To model EXAFS spectra of 1Pt–CeO2, single scattering Pt–O with 2.0 Å distance and Pt–Ce with 3.16 Å distance were chosen. The amplitude-reduction factor 𝑆20 was established for Pt foil reference (Figure S1, Supporting Information) and was fixed for EXAFS modelling. Double scatterings at 3.42 and 4.02 Å distances were also added to the modelling (Figure S7, Supporting Information), which are related to the first coordination sphere. The distance corresponding to the second coordination sphere Pt4+–Ce at 3.8 Å was not added, since the concentration of Pt4+ states is very small compared to the Pt2+ states, and its scattering amplitude can be neglected. As a result, the selected scattering paths were sufficient for a satisfactory description of the EXAFS spectrum.


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Figure 11. Structures of the platinum local atomic order in the studied samples (a) 1Pt–CeO2, (b) 15Pt–CeO2 and (c) 30Pt–CeO2.

With the increase in Pt loading in the samples, the probability of platinum being located nearby other platinum atoms increases. To model the high-Pt-loading samples, Pt–Pt distances were added to the set of scattering paths of 1Pt–CeO2. Single scattering Pt–Pt at 3.45 Å represents a distance between two Pt2+ ions located on equivalent (100) and (010) ceria facets. A similar distance was observed in the PDF curves. This structural fragment is also observed in the structure of Pt3O4. For better modelling of 30Pt–CeO2, the formation of Pt2+–Pt4+ interaction at 3.16 Å was assumed, where Pt4+ replaces the Ce4+ position in the fluorite structure. The EXAFS modelling results are displayed in Table 3. Comparisons of the experimental and model curves in r- and k-space are shown in Figure 12. It should be noted that all refined distances


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were observed in the PDF curves. The Table 3 shows that the coordination number (CN) of the Pt–O sphere is between 4 and 6. A CN value of 4 is typical for Pt2+, while Pt4+ is characterised by octahedral65 or cubic66 environments. In our case, Pt4+ in the first coordination sphere has a short Pt–O distance as well as Pt2+, but with large CN. Thus, according to our model, Pt2+ ions are in square planar coordination. Meanwhile, Pt4+ substitutes for Ce4+ in the fluorite structure but with a shorter Pt–O distance than in fluorite. The resulting EXAFS spectrum is a superposition of these two states. The CNs for the second and subsequent coordination spheres are

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