The Key Role Played by Metallic Nanoparticles on the Ceria Reduction

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The Key Role Played by Metallic Nanoparticles on the Ceria Reduction Alisson Steffli Thill, Alex Sandre Kilian, and Fabiano Bernardi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09013 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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The Journal of Physical Chemistry

The Key Role Played by Metallic Nanoparticles on the Ceria Reduction Alisson S. Thill,† Alex S. Kilian, ‡ Fabiano Bernardi*,† †

Programa de Pós-Graduação em Física, Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS, Brazil. ‡

Departamento de Tecnologia, Universidade Estadual de Maringá (UEM), Umuarama, PR, Brazil.

KEYWORDS: Charge transfer, ceria, in situ XAS, support reduction, catalyst

ABSTRACT: In this work, the reduction properties of ceria (CeO2) used as support of metallic nanoparticles (Au, Pd, Au0.9Pd0.1 and Au0.8Pt0.2) were elucidated. The catalysts were exposed to a reduction treatment in H2 atmosphere at 500 °C. In situ X-Ray Absorption Spectroscopy (in situ XAS) and in situ time-resolved XAS measurements at the Ce L3 edge were used to probe the local atomic order around Ce atoms and the Ce oxidation state. In this way, it was observed that the supported metallic nanoparticles improve the reduction process of the support. Moreover, the reduction of ceria is dependent on the composition of the metallic nanoparticle supported. By means of in situ XAS measurements at the Au and Pt L2,3 edges, it was possible to obtain information concerning the fractional change in the number of 5d-band electron holes relative to

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a reference material for the Au and Pt containing nanoparticles. In this way, it was observed an evidence for the charge transfer effect from the nanoparticles to the support, which is responsible for the improved reduction of the CeO2 support in the presence of nanoparticles. This result is corroborated by the observation of energy shifts on the Au 4f, Pd 3d and Pt 4f binding energy values of the measured X-Ray Photoelectron Spectroscopy (XPS) spectra. In this way, this work contributes on elucidating the physical mechanisms responsible for the enhanced support reduction effect existing in modern ceria based catalyst.


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INTRODUCTION The cerium oxide (CeO2-x, 0 < x < 0.5) or ceria (CeO2) is widely used currently in several fields of knowledge such as catalysis, fuel cells and optics.1-4 The main characteristic that makes it of great interest is the facility on creating and eliminating oxygen vacancies at the surface in a reversible way. This property is responsible for the high capacity of oxygen storage of ceria. Moreover, this property is important for catalytic applications because defects such as oxygen vacancies are one of the most reactive sites existing in metal oxides.4,5 It makes catalysts containing ceria much more effective, for example, for the control of pollutant gas emission in the automotive industry when compared to catalysts without ceria.5,6 In fact, there are several examples in the literature demonstrating the important role played by the ceria support on the catalytic reaction mechanism, usually due to the oxygen vacancies existing at the surface of ceria.7,8 At the same time, there is still a lack of knowledge on several catalytic events occurring in ceria based catalysts. It occurs because a catalytic reaction is a sequence of several complex steps starting with the molecule adsorption at a specific site at the surface of the catalyst and ending up with the formation of the final product. The detailed understanding of the catalytic events at the atomic level occurring during a catalytic reaction plays a major role on the optimization and generation of new catalysts to be applied in the industry. An important effect that influences on the catalytic activity is the Strong Metal-Support Interaction (SMSI) effect.9 The effect was first described almost 40 years ago by Tauster et al.9 but it is still subject of intense research nowadays.10,11 The effect is characterized by an almost complete inhibition of the capacity of CO and H2 adsorption by the metallic nanoparticles. It occurs only for reducible supports, like ceria, during H2 reduction process which is a primordial stage typically used in the chemical industry in order to bring the catalyst to the active form,


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prior to the catalytic reaction.12 The SMSI effect may present a geometric or electronic factor.13 The geometric effect occurs with the migration of functional groups coming from the support to the surface of the supported metallic nanoparticles making a capping layer around the nanoparticle. In this way, it can block the catalytic active sites of the reaction, thus decreasing the catalytic activity.14 On the other hand, in some cases the capping layer creates new catalytic active sites thus improving the catalytic activity of the reaction.14 The electronic effect occurs with the modification of the electronic properties of the catalyst without the migration of functional groups from the support.15,16 In this case, it is proposed a charge transfer between nanoparticle and support. Recently, it was studied the surface atomic population of Rh0.5Pd0.5/CeO2 bimetallic nanoparticles subjected to reducing (H2) and oxidizing (O2) atmospheres at 300 °C and 480 °C by using the Ambient Pressure X-Ray Photoelectron Spectroscopy (AP-XPS) technique.10 It was observed the formation of the cerium oxide capping layer surrounding the nanoparticles (geometric factor of the SMSI) when performing thermal treatment at 480 °C under H2 atmosphere. It is also demonstrated that in this case the gas molecules can penetrate trough the cerium oxide capping layer and react in the catalytic active sites of the catalyst.10 Moreover, it was verified that at 300 °C occurs a switching between a Rh (Pd)-rich skin layer for oxidizing (reducing) atmospheres. However, if the thermal treatment at 300 °C in these atmospheres is performed after a high temperature pre-treatment (480 °C) in H2 atmosphere, the switching between Pd and Rh atoms does not occur anymore. In this case, Pd atoms are frozen at the surface of the nanoparticles, independent of the reducing or oxidizing atmosphere employed during the thermal treatment. The explanation for this behavior was associated to the existence of the geometric factor of the SMSI effect when exposing the nanoparticles to the H2 atmosphere at


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480 oC. The capping layer allows monitoring the surface properties of the nanoparticles and it enables the formation of a Pd rich-shell even in the presence of an oxidizing atmosphere. Alayoglu et al.17 exposed the Pt/CeO2 system to a thermal treatment in H2 or O2 atmosphere and studied the catalyst by means of AP-XPS and in situ X-ray Absorption Spectroscopy (XAS) techniques. The authors showed that the CeO2 support has a significant reduction when it is heated under a H2 atmosphere and it oxidizes completely when it is heated under an O2 atmosphere. Then, it is showed that the support reduction is reversible. Moreover, the authors observed a contraction and expansion of the CeO2 unit cell when the system was exposed to an O2 and H2 atmosphere, respectively. This effect was attributed to the elimination and creation of oxygen vacancies in the crystalline structure of CeO2 due to the oxidation and reduction processes. The authors concluded that there is an important metal-support interaction effect which can be associated to changes in the catalytic activity in hydrogenative hydrocarbon transformations where the Pt/CeO2 system is used. Matte et al.11 studied the effect of CeO2 support on the reduction properties of Cu and Ni nanoparticles when exposed to a reduction treatment at 500 °C in a H2 atmosphere by in situ XAS, ex situ Transmission Electron Microscopy (TEM) and ex situ XPS techniques. It was observed that the Ni nanoparticles were covered at the surface by functional groups coming from the support after the thermal treatment in H2 atmosphere, which is associated to the geometric factor of the SMSI effect in the Ni/CeO2 system. The same phenomenon is not observed in the Cu/CeO2 system. Moreover, it was showed that the SMSI effect has great influence on the reduction properties of the Ni nanoparticles supported on CeO2, decreasing the reduction temperature of the Ni nanoparticles in comparison to the non-supported Ni nanoparticles case. It was observed no difference in the reduction temperature for the Cu/CeO2 system in comparison


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to the non-supported Cu nanoparticles case. A possible charge transfer effect from the CeO2 support to the Ni nanoparticles was suggested to explain the change on the reduction properties of the system. On the other hand, there are only few works in the literature addressing the influence of the metallic nanoparticles on the reduction of ceria support. Piotrowski et al.18 and Tereshchuk et al.19 studied the effect of the supported nanoparticles on the reduction properties of the CeO2 support using Density Functional Theory (DFT) calculations. Piotrowski et al.18 considered clusters of 4 atoms formed with a unique atom type of Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au supported on CeO2. Tereshchuk et al.19 considered clusters of 13 atoms formed with a unique atom type of Au, Pt, Pd and Ag. Thereby, the authors found that these metal clusters supported on CeO2 change the oxidation state of the support surface. It was explained that, due to the electronegativity difference between the nanoparticle atoms and the oxygen present in the CeO2 support, it occurs a charge transfer from the metallic cluster to the support. Because of this, some Ce atoms of the support change their oxidation state from Ce(IV) to Ce(III). As the transition metal atoms have a lower electronegativity than oxygen atoms, the authors proposed that this explanation can be considered general and valid for any transition metal supported on CeO2. From the experimental point-of-view, Rodriguez et al.20 observed an improved reduction of ceria support in Cu/CeO2(111) and Au/CeO2(111) systems exposed to a CO reducing atmosphere at 625 K if compared to the single CeO2(111) system exposed to the same atmosphere by means of XPS measurements. However, no detailed investigation about this feature was performed in this work. Xu et al.21 studied the reduction of the CeO2 support by exposure to H2 gas at 330 °C in Rh nanoparticles supported on CeO2 films by using ex situ XPS measurements. The authors


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observed that H2 gas is ineffective on reducing the CeO2 or Rh/CeO2 system. However, it was observed that the exposure to O2 at room temperature before the exposure to H2 at 130 °C improves the reduction of the CeO2 support. Furthermore, the amount of reduction resulting from the O2–H2 exposure cycle increases as the quantity of Rh on the CeO2 support increases. The authors interpreted that these results are caused by the formation of an interfacial Rh–OH species, which transfers hydrogen to support and it removes oxygen from the CeO2, thus improving the support reduction. The majority of works in the literature address the ceria effect on the metallic nanoparticles properties during the activation treatment in H2 atmosphere. However, for a detailed and complete elucidation of the catalytic events during a given reaction, it is crucial to study the influence of the metallic nanoparticles on the ceria support since it plays a central role in several catalytic reactions7,8 but there is still a lack of knowledge on this point. In this work it was performed a detailed experimental investigation on the influence of transition metal nanoparticles supported on CeO2 in the support reduction properties.

EXPERIMENTAL PROCEDURE The Pd, Au, Au0.9Pd0.1 and Au0.8Pt0.2 metallic nanoparticles were synthesized using the ionic liquid BMI.PF6 as reducing agent following procedure described previously.22 After synthesis, the metallic nanoparticles were supported with 50% wt on commercial CeO2 standard. For supporting the metallic nanoparticles on cerium oxide, 50 mg of the metallic nanoparticles in the powder form were mixed with 100 mg of commercial cerium oxide and 10 mL of acetone. The resulting dispersion was maintained under ultrasonic bath for 10 min, and centrifuged at 3500 rpm during 3 min. The supernatant solution was removed and the residual solid was dried under


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reduced pressure. The samples were characterized with TEM, in situ XAS, in situ time-resolved XAS and XPS techniques. The TEM analyses of the metallic nanoparticles were carried out on an electron microscope at CMM-UFRGS operating at an accelerating voltage of 80 kV (JEOL JEM-1200 EXII). For the measurements, the synthesized nanoparticles were dispersed in deionized water, and a drop of the solution was spotted on a carbon-coated Cu grid. The in situ XAS measurements were performed at the D08B-XAFS2 beamline of LNLS (Brazilian Synchrotron Light Laboratory)23 in transmission mode at the Ce L3 edge (5723 eV), Au L2 (13734 eV) and L3 (11919 eV) edges and Pt L2 (13273 eV) and L3 (11564 eV) edges. All the samples, including the CeO2 commercial standard for comparison purposes, were analyzed. For the Ce L3 edge, in situ X-Ray Absorption Near Edge Spectroscopy (XANES) and in situ Extended X-Ray Absorption Fine Structure (EXAFS) measurements were acquired while at the Pt and Au L2,3 edges only in situ XANES measurements were recorded. For the measurements, about 15 mg of nanoparticles in the powder form were mixed with 40 mg of boron nitride (BN) and this mixture was compacted to produce homogeneous pellets with 5 mm diameter. The measurements were performed with the samples at room temperature and during a thermal treatment in H2 reducing atmosphere. The thermal treatment consisted of heating the samples using a tubular furnace from room temperature (RT) to 500 °C (10 °C/min heating rate) with the samples exposed to 290 ml/min of 5% H2 + 95% He reducing atmosphere. At 500 °C, the sample temperature was maintained under H2 atmosphere for 50 minutes before starting the measurements. A Cr and Au foil were used as references for the measurements at Ce L3 edge, Au and Pt L2,3 edges, respectively, to calibrate the energy of the beamline. The XANES spectra of commercial CeO2 and CeOHCO3 compounds at room temperature were used as Ce(IV) and


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Ce(III) standards, respectively. The spectra were collected using a Si (111) double crystal monochromator and three ionization chambers filled with Ar gas. The average interval between spectra was around 40 min. Two to four spectra were averaged for each case. In situ time-resolved XAS measurements were performed at the D06A-DXAS beamline of LNLS (Brazilian Synchrotron Light Laboratory)24 in transmission mode at the Ce L3 edge (5723 eV) in the XANES region. All the samples were measured, including the CeO2 commercial standard. For the measurements, around 15 mg of nanoparticles in the powder form were mixed with 40 mg of boron nitride (BN) and this mixture was compacted to produce homogeneous pellets with 5 mm diameter. The measurements were performed during a thermal treatment in H2 atmosphere. During the thermal treatment, the samples were exposed to 290 ml/min of 5% H2 + 95% He reducing atmosphere. The thermal treatment consisted of heating the samples using a tubular furnace from room temperature (RT) to 500 °C (10 °C/min heating rate). In this case, however, the thermal treatment consisted of isothermal steps during 15 min at 100 °C, 200 °C, 300 °C, 400 °C or 500 °C. Moreover, the XANES spectra of commercial CeO2 and CeOHCO3 compounds at room temperature were considered as Ce(IV) and Ce(III) standards, respectively. The monochromator consists of a curved Si (111) crystal (dispersive polychromator). The average interval between spectra was around 5 s and the time resolution of the measurements was around 100 ms. For the XPS measurements, the reduction treatment was accomplished using the same parameters (gas composition, flux, reduction time, temperature, and heating rate) and the same reactor used during the in situ XAS measurements. After the reduction treatment, the samples were exposed to the atmosphere and introduced into the analysis chamber at the D04A-SXS beamline endstation25 at LNLS. The samples were investigated using the long scan, Au 4f, Pd


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3d, Pt 4f, Ce 3d, and C 1s scan regions. The spectra were collected using an InSb (111) double crystal monochromator at fixed photon energy of 1840 eV. The hemispherical electron analyzer (PHOIBOS HSA500 150 R6) was set at a pass energy of 30 eV, and the energy step was 0.1 eV, with an acquisition time of 100 ms/point. The base pressure used inside the chamber was around 5.0 x 10-9 mbar. The monochromator photon energy calibration was done at the Si K edge (1839 eV). An additional calibration of the analyzer’s energy was performed using a standard Au foil (Au 4f7/2 peak at 83.8 eV). We also considered the C 1s peak value of 284.6 eV and the lower binding energy of the Ce 3d5/2 peak of CeO2 at 882.4 eV as references to verify possible charging effects. The XPS measurements were obtained at a 45o takeoff angle at room temperature.

DATA ANALYSIS The TEM images were analyzed using the ImageJ 1.46r software from which the size distribution histogram was obtained. The Ce(III) fraction present in each case, before and during the thermal treatment, was obtained by analyzing the in situ XANES spectra at Ce L3 edge using a method proposed in the literature.26 The method consists on adjusting the in situ XANES spectra by using two arctangents and three Lorentzian functions. The energy range used in the fitting procedure includes the single (double) peak characteristic of the Ce(III) (Ce(IV)) compounds. The two Lorentzian functions correspond to the 2p3/2 → (4fƺ)5d (around 5730 eV) and 2p3/2 → (4f0)5d (around 5738 eV) transitions and are associated to the Ce(IV) component while the other Lorentzian function corresponds to the 2p3/2 → (4f1)5d transition (around 5726 eV) and can be associated to the Ce(III) component. The symbol ƺ denotes that an electron in the 2p orbital of oxygen was transferred to 4f orbital of Ce. The arctangent functions are associated to the


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transitions from the Ce 2p3/2 electronic level to the continuum for each component. The parameters of the arctangent and Lorentzian functions (energy position, width and height) were first determined by fitting the XANES spectra of the Ce(IV) and Ce(III) standards at RT. In the adjustment of the in situ XANES spectra of the nanoparticles the only parameter variable was the height of each Lorentzian function. The other parameters are constrained to the values determined by fitting the XANES spectra of the Ce(IV) and Ce(III) standards at RT. At the end, the fitting was refined allowing that the width and the energy positions of the Lorentzian functions varied 0.5 eV around their previous fixed positions. The Ce(III) fraction (Ξ) present in the ceria support is determined by the Eq. (1).

Ξ=

Aଵ Aଵ + Aଶ + Aଷ

(1)

where A1 is the area under the Lorentzian function associated to the Ce(III) component and A2 and A3 are the areas under each Lorentzian function associated to the Ce(IV) component. In order to analyze the EXAFS region of the absorption spectra at Ce L3 edge, firstly it was performed the multielectron photoexcitations (MPE) correction27 by using the Ce(III) fraction values obtained from the in situ XANES measurements. The MPE effect in Ce atoms corresponds to the generation of an atomic signal in the absorption spectrum, at around 5850 eV, together with the structural signal, due to coexcitations of valence and subvalence electrons of the atoms. For the Ce atom, this atomic signal is strong and should be removed. After this, it was performed the standard procedure of data reduction28 using the IFEFFIT package.29 FEFF8 was used to obtain the phase shift and amplitudes30 considering a cluster of CeO2 with 10 Å radius. The Fourier Transform of the EXAFS oscillations was performed using a Kaiser-Bessel window


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with ∆k = 5.2 Å and all the data were k2 weighted. The Fourier Transform of the k2 weighted EXAFS oscillations was adjusted using the model presented previously.17 Only the coordination shell around the Ce absorbing atom was adjusted in this procedure. This model considers that the cerium oxide can be modulated as a linear combination of two regions. One region corresponds to the crystalline structure of the CeO2 (fluorite) with eight oxygen atoms and zero oxygen vacancies in the coordination shell of the Ce atoms. The second region corresponds to the crystalline structure of the CeO2 (fluorite) with four oxygen atoms and four oxygen vacancies in the coordination shell of the Ce atoms. The adjustment was performed for the data corresponding to each sample at RT and at 500 °C simultaneously and considering constrains between the samples. For all the cases, the amplitude reduction parameter (S02) was fixed at 0.63. This value was obtained from the previous adjustment of the FT of the CeO2 standard. For the same region, the parameters correspondent to the distance of the absorbing Ce atom to the scattering oxygen atom (RCe-O) and the Debye-Waller factor (σ2) are considered the same for all the samples at a given temperature. For the same region, the Ce-O distance (RCe-O) for the heated sample is constrained to the value obtained by the thermal expansion effect from the RCe-O value at RT. The NCe-O coordination number of the regions with zero and four oxygen vacancies for the ceria is given by, respectively, 8(1-xi) e 4xi, where xi is given by the Eq. (2)

‫ݔ‬௜ = Ξ௜ ܽ + ܾ

(2)

where Ξi is the Ce(III) fraction of the sample i, obtained from the XANES analysis (Table 1), and a and b are adjustable parameters constrained to be the same for all the cases. Besides the parameters presented with the model, two other parameters related to the cumulant expansion (c3 and c4) were used31 for the measurements performed at 500 °C.


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XPSPeak version 4.1 was used to fit the XPS results. All peaks were adjusted using a Shirleytype background and an asymmetric Gaussian-Lorentzian sum function (33% Lorentzian contribution).

RESULTS AND DISCUSSION Figure 1 shows typical TEM images obtained for each sample synthesized and the respective size distribution histogram (inset). It is possible to observe a monomodal size distribution for all the nanoparticles with exception of the Au0.8Pt0.2/CeO2 nanoparticles that presents a bimodal distribution. The images show a similar mean diameter around 4.0 nm for all the different nanoparticles synthesized.


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Figure 1. Typical Transmission Electron Microscopy (TEM) images of (a) Au0.8Pt0.2/CeO2, (b) Pd/CeO2, (c) Au/CeO2 and (d) Au0.9Pd0.1/CeO2 nanoparticles. The inset represents the size distribution histogram of the nanoparticles.

Figure 2(a) shows the in situ XANES spectra at Ce L3 edge of the samples measured (i) at room temperature (RT) and (ii) at 500 °C in H2 atmosphere. It is possible to observe the Ce(III) and Ce(IV) characteristic fingerprints present in the XANES measurements of the standard compounds. The Ce(III) component has an important contribution at 5725.9 eV, while the Ce(IV) component shows a doublet in this region. It can be concluded that in all cases the ceria support have initially approximately the same oxidation state of the Ce(IV) standard sample (CeO2). However, by observing the region indicated by the dashed line in the in situ XANES spectra of the samples at 500 °C in H2 atmosphere (Figure 2(a)), it is possible to observe that each case presents a different Ce(III) fraction. Aiming to determine the Ce(III) fraction in the ceria support quantitatively, the method presented in the Data Analysis section was used to adjust the in situ XANES spectra at the Ce L3 edge of the samples. The adjustment performed in the in situ XANES spectra of the samples measured at room temperature (RT) and at 500 °C in H2 atmosphere is showed in the Figure 2(a). The results obtained for the Ce(III) fraction by fitting the in situ XANES spectra are showed in Table 1. The R² factor obtained in the adjustment was always higher than 0.998. One can observe in Table 1 that the ceria presents essentially the same Ce(III) fraction in each case at room temperature (RT), before the thermal treatment in H2 reducing atmosphere. At 500 °C, during the thermal treatment in H2 reducing atmosphere, the ceria presents a different Ce(III) fraction, where the CeO2 standard presents always smaller Ce(III) fractions than the case of metallic nanoparticles


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supported on CeO2. The Au0.8Pt0.2/CeO2 nanoparticle presents the higher Ce(III) fraction between all the nanoparticles analyzed. These results reveal that the metallic nanoparticles supported on CeO2 improves the support reduction and the value of the Ce(III) fraction is dependent on the metallic composition of the nanoparticle. Aiming to investigate the local atomic order around Ce atoms for each case, the in situ EXAFS oscillations at Ce L3 edge were analyzed. The EXAFS oscillations χ(k) were extracted from the absorption spectra at Ce L3 edge of the as-prepared and reduced samples and are showed in Figure 2(b). It can be observed that the k2 weighted EXAFS oscillations for each case at 500 °C are damped if compared to the oscillations of the same case at room temperature (RT). This is caused by the higher atomic disorder existing due to the increase of the sample temperature. This phenomenon is reflected in the width and intensity of the peaks in the Fourier Transform of the k2 weighted EXAFS oscillations (Figure 2(c)). The intensity of the peaks in the Fourier Transform of the k2 weighted EXAFS oscillations also depends on the Ce-O coordination number (NCe-O). In order to observe a possible relation of the Ce(III) fraction with the NCe-O coordination number, it is possible to adjust the Fourier Transform of the k2 weighted EXAFS oscillations using the model discussed in Data Analysis section. The R factor obtained in the adjustment was lower than 0.006, which demonstrates the excellent quality of the fit. Figure 2(b) and 2(c) shows the fitting result of the k2 weighted EXAFS oscillations and the respective Fourier Transforms. The parameters obtained from the adjustment of the Fourier Transform of the k2 weighted EXAFS oscillations have values of a = (0.9 ± 0.2), b = (-0.03 ± 0.01), c3 = (4 ± 1) x 10-4 and c4 = (3 ± 4) x 10-5. The values of the Ce-O distance (R) and Debye-Waller factor (σ2) obtained from the adjustment are separated in two regions. For the region with zero oxygen vacancies in the coordination shell, the values for these parameters are R = (2.307 ± 0.002) Å, σ2 = (2.1 ± 0.3) x


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10-3 Å-1 at room temperature and R = (2.308 ± 0.005) Å, σ2 = (8.8 ± 0.9) x 10-3 Å-1 at 500 °C. For the region with four oxygen vacancies in the coordination shell the values for these parameters are R = (2.22 ± 0.04) Å, σ2 = (6 ± 6) x 10-3 Å-1 at room temperature and R = (2.23 ± 0.05) Å, σ2 = (16 ± 14) x 10-3 Å-1 at 500 °C.


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Figure 2. (a) Normalized absorption spectra of the nanoparticles and the Ce(III) and Ce(IV) standards measured at the Ce L3 edge, (b) k2 weighted EXAFS oscillations and (c) the corresponding Fourier Transform for the measurements at (i) room temperature (RT) and at (ii) 500 °C in H2 atmosphere. The experimental points and the best adjustment result are showed in black point and gray lines, respectively.

Table 1. Ce(III) fraction at room temperature (RT) and at 500 °C (during reduction treatment) as a function of the sample. The results were obtained with the adjustment of the in situ XANES spectra at the Ce L3 edge.

Sample

Au0.8Pt0.2/CeO2

Pd/CeO2

Au/CeO2

Au0.9Pd0.1/CeO2

CeO2 standard

Ξ at RT

0.05 + 0.02

0.05 + 0.03

0.05 + 0.02

0.05 + 0.04

0.05 + 0.04

Ξ at 500 °C

0.29 + 0.01

0.24 + 0.01

0.21 + 0.01

0.18 + 0.03

0.14 + 0.02

It can be observed that the R and σ2 values for a specific region increases with the temperature, as expected. Moreover, by comparing the two regions at the same temperature it can be observed that the R value is smaller and the value of σ2 is higher for the region with four oxygen vacancies. It occurs because when increasing the number of oxygen vacancies the Ce-O distance decreases and it causes an increase on the structural disorder of the system. Figure 3 shows a comparison between NCe-O, obtained from the EXAFS analysis, and Ce(IV) fraction (1 - Ξ), obtained from the Ce(III) fraction values from in situ XANES analysis, as a function of the sample. Despite uncertainties, it is observed a relation between the Ce(IV)


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fraction values and the coordination numbers of the coordination shell around the Ce absorbing atom (NCe-O). As the value of the Ce(IV) fraction decreases the NCe-O value decreases as well.

Figure 3. Average number of oxygen atoms in the coordination shell around the Ce atoms (black points) compared with the results of Ce(IV) fraction (1 – Ξ) obtained by in situ XANES analysis (blue points) for the samples at 500 °C during thermal treatment in H2 atmosphere.

Thus, it is clear that the nature of the metallic nanoparticle influences not only on the oxidation state of Ce but also on the local atomic order around the Ce atoms. In order to understand the influence of the metallic nanoparticle on the ceria reduction properties, it is important to get information about the activation energy related to the reduction process for each case. The in situ time-resolved XANES measurements were analyzed in order to obtain this information. Figure 4 presents a typical time evolution of the normalized absorption spectra at Ce L3 edge during reduction treatment (Au0.8Pt0.2/CeO2 nanoparticle). Again, it is possible to see the increase of the Ce(III) component at around 5725.9 eV with the increase of the temperature. Bulfin et al.32 proposed a model that describes the rate of the oxygen vacancy concentration change as a


18

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function of the time as the difference between the O atoms leaving rate (reduction) and O atoms arriving rate (oxidation) at cerium oxide. The authors obtained the Eq. (3)

ln ቀ

Δ‫ܧ‬ ‫ݔ‬ ‫ܣ‬௥௘ௗ ቁ= + ݈݊ ൬ܲைି௡ ൰ మ ܴܶ ‫ܣ‬௢௫ 0.35 − ‫ݔ‬

(3)

where ∆E is the difference between the activation energy of reduction process and the activation energy of oxidation process, R is the ideal gas constant, T is the temperature, and Ared and Aox are constants. This proposed model was verified experimentally in the literature for high temperatures (T > 500 °C).32 The value of x is obtained from the oxidation state of the support (CeO2-x), which is obtained from the Ξ value given by the analysis of the in situ time resolved XANES spectra at Ce L3 edge. Therefore, the Ce(III) fraction values as a function of the inverse of the temperature allowed to obtain the activation energy value for each case. It was considered the Ce(III) fraction values at the end of each isothermal step (see Experimental Procedure section) in order to ensure the samples reaching a stationary state. The R2 factor obtained in the in situ XANES spectra adjustment was always higher than 0.984.


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Figure 4. Normalized in situ time-resolved XANES spectra at Ce L3 edge for different temperatures of the Au0.8Pt0.2/CeO2 nanoparticle during reduction treatment.

Figure 5 shows the graph relating the value of ln(x/(0.35 - x)) as a function of the inverse of the temperature, as proposed in the model.32 In each graph a straight line is expected, and the slope of this straight line is proportional to the activation energy of the reduction process. However, the graphs in this work clearly do not present only one straight line but two distinct regions with linear behaviors (Figure 5). The temperature range of each linear region was chosen between all possibilities for 2 straight lines (since containing three data points at least) to give a minimum in the residual of the fit. Figure 5 shows the linear adjust of the two regions. The first straight line was adjusted in the temperature range from room temperature to 200 °C and the second straight line was adjusted in the temperature range between 300 °C and 500 °C. According to the results presented in Figure 5, the first region (RT – 200 °C) can be considered as corresponding to the desorption process of molecules at the surface and then the slope value of the straight line can be interpreted as the desorption energy (ED). The region of higher temperatures corresponds to the reduction process itself and the slope value of the straight line is


20

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directly related to the activation energy (∆E). Moreover, it can be observed that the values of the activation energy change significantly between the samples, but there is an apparent inconsistence with the values observed in Table 1. The samples that present a higher Ce(III) value at the end of the reduction process would present a smaller value of activation energy, which is not observed. It is possible to observe that the case of ceria with supported metallic nanoparticles present a higher ∆E value than the CeO2 standard without metallic nanoparticles. Furthermore, by comparing only the cases with supported metallic nanoparticles, it is possible to observe no significant change on the ∆E values between the samples if considering the uncertainty associated to the measurement. It indicates that the metallic nanoparticles are responsible for the increasing in the activation energy value. This increase can be explained observing that the metallic nanoparticles are covering a given area of the support. It causes the existence of surface regions at support of hard access for the H2 molecules to react with the CeO2 support atoms. Thereby, there is an increase on ∆E value since the oxygen vacancies and the oxygen atoms need to diffuse through CeO2 to these hidden regions in order to occur the support reduction process. It results in an activation energy value higher than that of the CeO2 standard sample. However, this analysis cannot explain the different Ce(III) fraction values obtained for the ceria with different supported metallic nanoparticles and the reason for the improved reduction of ceria support when using metallic nanoparticles. In order to investigate the possibility of changes in the electronic properties of the metallic nanoparticles, the in situ XANES measurements at Au and Pt L2,3 edges were analyzed.


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Figure 5. Linear fit (red solid and dashed lines) performed in the graphs relating the Ce(III) fraction to the temperature, which was obtained by analysis of the in situ time-resolved XANES spectra at Ce L3 edge.


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The intensity of the peak at the edge region of the XANES spectrum depends on the density of empty states at a given electronic level of the absorbing atom (depending on the electronic transition studied). Based on this, Mansour et al.33 demonstrated the possibility of calculating the fractional change in the number of 5d-band electron holes relative to a reference material (fd) for Pt using the XANES measurement at Pt L2,3 edges. After this work, the same procedure was applied for XANES measurements at the Au L2,3 edges.34-36 Aiming to investigate the reason for the dependence of the oxidation state of the CeO2 support with the type of supported nanoparticle, the fractional change in the number of 5d-band electron holes relative to a reference material (fd) was calculated for Au and Pt atoms of the nanoparticles during reduction treatment at 500 °C. The reference material was chosen as the standard metallic sample, Pt or Au, at room temperature. The method described by the authors33 consists on calculating the area under the Pt or Au XANES spectrum at the L2,3 edges from -10 eV to +13 eV relative to the respective absorption edge energy. Figure 6 shows a comparison between the Au or Pt L2 and Au or Pt L3 absorption spectra referent to the standard and nanoparticles under H2 atmosphere at 500 °C. The fd value obtained is included as well. The area under the Pt or Au absorption spectrum at the L2,3 edges for all the samples used in the calculation is highlighted. The fd value is obtained by the Eq. (4) derived in the literature33

݂ௗ =

ሺ‫ܣ‬ଷ௦ − ‫ܣ‬ଷ௥ ሻ + 1.11ሺ‫ܣ‬ଶ௦ − ‫ܣ‬ଶ௥ ሻ ‫ܣ‬ଷ௥ + 1.11‫ܣ‬ଶ௥

(4)

where A2s and A3s are the areas under the absorption spectrum at the Pt or Au L2,3 edges of the sample. In the same way, A2r and A3r are the areas under the absorption spectrum at the L2,3 edges of the reference. Figure 6 shows the fd results found for each case.


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Figure 6. In situ XANES spectra at the Au and Pt L2,3 edges of the Au and Pt standards (at RT) and of the samples containing Au and Pt during H2 reduction treatment at 500 oC. The area under the absorption spectra used in the fd calculation is highlighted. The results obtained for the fd value are showed too.

A negative value for fd indicates the occurrence of a decreasing in the number of 5d-band electron holes in the Au or Pt atoms relative to the Au or Pt standard, respectively. In other words, it is consistent with the existence of a charge transfer from the support to the metallic nanoparticle. A positive value for fd indicates that occurs an increasing in the number of 5d-band electron holes in these atoms, i.e., a charge transfer occurs from the metallic nanoparticles to the support. It can be observed that all the nanoparticles presented a positive value for fd, indicating that there is an electron transfer from the nanoparticles to the CeO2 support. As the ceria with


24

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supported nanoparticles present a higher Ce(III) fraction during thermal treatment in H2 atmosphere than the CeO2 standard sample (Table 1), one can conclude that the charge transfer improves the reduction of the CeO2 support. This charge transfer was predicted theoretically by Piotrowski et al.18 and Tereshchuk et al.19 and it is being showed experimentally for the first time in the present work. Moreover, the fd values calculated shows that there is a direct relation of the amount of charge transfer with the Ce(III) fraction during thermal treatment in H2 atmosphere for each case. The Au/CeO2 nanoparticle at 500 oC in H2 atmosphere presents an intermediate value for fd and Ce(III) fraction. The case where Pd is added to the Au nanoparticle (Au0.9Pd0.1/CeO2 nanoparticle) presents the smallest value for fd and Ce(III) fraction. In other words, the system with monometallic Au nanoparticles presents a higher value for fd and Ce(III) fraction than the system where Pd is added to the monometallic Au, indicating an anti-synergistic effect for the system Au0.9Pd0.1. On the other hand, the opposite trend is found when adding Pt to the Au nanoparticle (Au0.8Pt0.2/CeO2 nanoparticle) where there is an increase of the combined fd value (Au + Pt) in relation to the Au monometallic case and, consequently, the Au0.8Pt0.2/CeO2 nanoparticles present a higher Ce(III) fraction than the Au/CeO2 nanoparticles. In fact, it shows a charge transfer from both Au and Pt atoms to the CeO2 support. The interpretation for the fd values found is valid if neglecting the charge transfer from the Pd atoms. Then, the indicative of charge transfer obtained from the in situ XANES measurements should be consistent with an energy shift on the Au 4f, Pt 4f and Pd 3d binding energies obtained by XPS measurements. Figure 7 shows the (a) Pt 4f, (b) Au 4f and (c) Pd 3d XPS spectra for all the samples after the reduction treatment at 500 °C in H2 atmosphere. For the Au 4f region, it is observed only one chemical component associated to Au0 for all the cases. The comparison between the Au 4f7/2 binding energy associated to Au0 chemical component for the different


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cases shows an energy shift, indicated by the dashed lines. Considering the Au0.9Pd0.1/CeO2 nanoparticles as reference, it is possible to state that the Au0 component presents an energy shift of +0.2 eV and +0.3 eV for the Au/CeO2 and Au0.8Pt0.2/CeO2 nanoparticles, respectively. Then, as smaller is the Ce(III) fraction of the ceria support as smaller is the Au0 binding energy value. It is a further evidence of the charge transfer effect, previously observed by means of in situ XANES measurements at Au L2,3 edges. The Pd 3d region shows the presence of Pd0 and Pd+2 components for the Pd/CeO2 nanoparticles. On the other hand, only the Pd0 component is found in the Au0.9Pd0.1/CeO2 nanoparticles. The Pt 4f region presents two distinct chemical components, associated to Pt0 and Pt+2. The same tendency is observed when analyzing the Pd0 and Pt0 binding energy values. The Pt0 component shows an energy shift of +0.3 eV in comparison to the expected value for this component.37 This energy shift, associated to the energy shift observed for the Au0 component, confirms the strong effect of charge transfer from the Au0.8Pt0.2 nanoparticles to the support. Also, the Pd0 component from Pd/CeO2 nanoparticles shows a binding energy shift of +0.3 eV in comparison to the Au0.9Pd0.1/CeO2 nanoparticles. The energy shifts presented in the Pd 3d and Pt 4f regions is one more time consistent with the charge transfer from the nanoparticles to the ceria support. It demonstrates clearly the role played by metallic nanoparticles on improving the ceria reduction.


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Figure 7. XPS data at the (a) Pt 4f, (b) Au 4f and (c) Pd 3d electronic levels of the samples after exposition to the reduction treatment.

Considering a metal/oxide interface, the charge transfer occurs from the metallic nanoparticle to the O atoms of ceria. It occurs because the metal atoms have lower electronegativity than the O atoms since the electronegativity values are 3.44 (O), 2.54 (Au), 2.28 (Pt), and 2.20 (Pd). Tereshchuk et al19 studied metallic clusters composed by 13 atoms of Pt, Au, Pd or Ag supported on CeO2(111) by DFT calculations. The authors proposed that the charge transfer from the metallic nanoparticle to the ceria support should occur via the topmost O layer of the support. It was observed that the charge transferred is not localized at the O atom that interacts directly with the metal atom of the nanoparticles but it spreads out to the neighborhood due to the pressure exerted by the metallic nanoparticle on the support. Due to the charge transfer from the metallic nanoparticles to the O atoms, the Ce charge changes as well with Ce(IV) turning into Ce(III) oxidation state. In the present work, the H2 molecules interact with these O atoms at the topmost


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layer forming H2O molecules (as observed by mass spectrometry measurements, not shown here) and leaving O vacancies at the surface. The trend observed in the electronegativity values explains the charge transfer observed for Au/CeO2 and Pd/CeO2 samples in this work. It is confirmed when looking for the Bader charge studied theoretically, as demonstrated in the literature.19 In the study, the authors calculated the effective charge transferred from the Pd and Au clusters to the CeO2(111) surface as 1.04e and 0.79e, respectively. It follows the trend observed in this work. However, the case of the bimetallic nanoparticles used in this work is still not analyzed theoretically for comparison purposes. On the other hand, by observing the electronegativity values it is expected the Au0.9Pd0.1/CeO2 and Au0.8Pt0.2/CeO2 nanoparticles presenting an intermediate charge transfer effect but, in fact, it corresponds respectively to the lowest and highest charge transferred observed in this work. It can evidence an anti-synergic (Au0.9Pd0.1/CeO2) and synergic (Au0.8Pt0.2/CeO2) effect occurring for these systems, which should be subject of a further work.

CONCLUSIONS In this work, it was possible to study the reduction properties of ceria used as support of metallic nanoparticles. In conclusion, the switch from Ce(IV) to Ce(III) is enhanced in the ceria support in the presence of a supported metallic nanoparticle in comparison to the case where no metallic nanoparticle is supported. It shows that the nanoparticle has a key role in reducing the CeO2 support. It was shown that the reduction of ceria is dependent on the composition of the supported metallic nanoparticle. Moreover, it was demonstrated that the fundamental role played by metallic nanoparticles is due to charge transference from the nanoparticles to the support. It should be taken into account for elucidating catalytic events in ceria based systems since the


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oxygen vacancies are one of the most active defects of metal oxides for several catalytic reactions.

AUTHOR INFORMATION Corresponding Author *Address correspondence to Fabiano Bernardi, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources CNPq – Project number 487071/2013-1

ACKNOWLEDGMENTS This work was supported by CNPq (project number 487071/2013-1). F. B. and A. S. T. thank CNPq for the research grant. The authors thank the LNLS and CMM staff for their assistance, Lívia P. Matte, Raquel W. Cunha and Denise R. B. Kobelinsky for the technical assistance, and Prof. Jairton Dupont (IQ-UFRGS) for providing the samples used in this work.

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(2)

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TABLE OF CONTENTS


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Figure 1. Typical Transmission Electron Microscopy (TEM) images of (a) Au0.8Pt0.2/CeO2, (b) Pd/CeO2, (c) Au/CeO2 and (d) Au0.9Pd0.1/CeO2 nanoparticles. The inset represents the size distribution histogram of the nanoparticles. 94x103mm (300 x 300 DPI)


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Figure 2. (a) Normalized absorption spectra of the nanoparticles and the Ce(III) and Ce(IV) standards measured at the Ce L2 edge, (b) k2 weighted EXAFS oscillations and (c) the corresponding Fourier Transform for the measurements at (i) room temperature (RT) and at (ii) 500 °C in H2 atmosphere. The experimental points and the best adjustment result are showed in black point and gray lines, respectively. 163x323mm (600 x 600 DPI)



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Figure 3. Average number of oxygen atoms in the coordination shell around the Ce atoms (black points) compared with the results of Ce(IV) fraction (1 – Ξ) obtained by in situ XANES analysis (blue points) for the samples at 500 oC during thermal treatment in H2 atmosphere. 58x41mm (300 x 300 DPI)


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Figure 4. Normalized in situ time-resolved XANES spectra at Ce L3 edge for different temperatures of the Au0.8Pt0.2/CeO2 nanoparticle during reduction treatment. 66x52mm (600 x 600 DPI)



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Figure 5. Linear fit (red solid and dashed lines) performed in the graphs relating the Ce(III) fraction to the temperature, which was obtained by analysis of the in situ time-resolved XANES spectra at Ce L3 edge. 194x245mm (600 x 600 DPI)


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Figure 6. In situ XANES spectra at the Au and Pt L2,3 edges of the Au and Pt standards (at RT) and of the samples containing Au and Pt during H2 reduction treatment at 500 oC. The area under the absorption spectra used in the fd calculation is highlighted. The results obtained for the fd value are showed too. 100x121mm (600 x 600 DPI)



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Figure 7. XPS data at the (a) Pt 4f, (b) Au 4f and (c) Pd 3d electronic levels of the samples after exposition to the reduction treatment. 90x47mm (600 x 600 DPI)


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TOC graphic 47x26mm (300 x 300 DPI)


The Key Role Played by Metallic Nanoparticles on the Ceria Reduction

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