Understanding the Strong Metal-Support Interaction (SMSI) Effect in

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Understanding the Strong Metal-Support Interaction (SMSI) Effect in CuNi /CeO (0 < x < 1) Nanoparticles for Enhanced Catalysis x

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Wallace Torres Figueiredo, Guilherme Basso Della Mea, Maximiliano Segala, Daniel L. Baptista, Carlos Escudero, Virginia Perez-Dieste, and Fabiano Bernardi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00569 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Nano Materials

Understanding the Strong Metal-Support Interaction (SMSI) Effect in CuxNi1-x/CeO2 (0 < x < 1) Nanoparticles for Enhanced Catalysis Wallace T. Figueiredo,† Guilherme B. Della Mea,† Maximiliano Segala,§ Daniel L. Baptista,† Carlos Escudero,‡ Virginia Pérez-Dieste,‡ and Fabiano Bernardi†,* † Programa

de Pós-Graduação em Física, Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre 91501970, Brazil.

§

Departamento de Físico-Química, Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre 91501970, Brazil. ‡ ALBA

Synchrotron Light Source, Barcelona, 08290, Spain.

KEYWORDS: Strong Metal-Support Interaction (SMSI) effect, atomic arrangement, cerium oxide, Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS), in situ X-Ray Absorption Near Edge Spectroscopy (in situ XANES)

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ABSTRACT: The Strong Metal-Support Interaction (SMSI) effect plays a central role in catalysis by decreasing the catalytic activity or even improving it in some specific cases. In spite of the intense research, a detailed description of the SMSI effect in CeO2-based catalysts is still missing. In this work, CuxNi1-x/CeO2 (0 < x < 1) nanoparticles were exposed to a reduction treatment in H2 atmosphere followed by an oxidation treatment in CO2 atmosphere, both at 500 °C, and studied using state-of-the-art techniques (in situ time-resolved X-Ray Absorption Near Edge Structure (XANES) and Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAPXPS)). It was observed the migration of Cu (Ni) atoms towards the surface of Cu-Ni bimetallic nanoparticles during reduction (oxidation) treatments. The core-shell-like structure is dependent on the Cu/Ni ratio. It is observed the existence of a capping layer from the support (CeO2-x) surrounding the metallic nanoparticles after reduction treatment (characteristic of the SMSI effect) in some specific cases, depending on the Cu/Ni ratio as well. The surface of the nanoparticles presenting the SMSI effect is recovered to the initial state after exposure to CO2 atmosphere. Moreover, the nature of the SMSI effect was elucidated. The capping layer interacts with the Cu and Ni atoms via Ce3d10O2p6Ce4f0 and Ce3d10O2p6Ce4f1 initial states, depending on the case studied. As a consequence of the SMSI effect, the Cu atoms of the nanoparticles reduce at lower temperature than similar nanoparticles that do not present the SMSI effect. Therefore, the decreasing in reduction temperature is directly related to the interaction between the CeO2-x capping layer and Cu and Ni atoms.


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1. Introduction

In industrial applications, metallic nanoparticles are usually combined with oxide supports in order to increase their thermal stability and maintain their dispersion on the active phase. Nevertheless, the interactions arising from these associations may influence some of the catalyst’s properties. From expanding the catalyst chemisorption ability to extinguishing its activity, different outcomes have been linked to metal-support interactions. Among these, an effect known as the Strong Metal-Support Interaction (SMSI) has amazed the catalysis community for the past 40 years. First reported by Tauster et al.1 on a study about the reduction process of group 8 noble metals supported on TiO2, the SMSI effect is characterized by a disturbance on the metal’s catalytic behavior leading to a drastic decrease on its H2 and CO chemisorption capacity after exposure to high temperature reduction treatments in H2 atmosphere. In fact, the effect occurs only for reducible supports. Further investigations showed that the catalyst’s initial state can be recovered after a high temperature oxidation treatment. The studies of Tauster et al. on TiO2-supported systems suggested that the SMSI effect was associated to the formation of chemical bonds on the metal-TiO2 interface.1 This idea was further supported by a theoretical study on the Pt/TiO2 orbitals presented by Horsley.2 The SCF-Xα calculations performed by Horsley indicate that the suppression of the H2 chemisorption ability of the supported metal is due to Pt-Ti+3 bonds. Further, Horsley study points out that the O vacancy formation on the support surface is paramount for the Pt-Ti bondings formation. According to the calculations, the overlapping between Ti 3d and Pt 5d orbitals lead to charge transferring from Ti+3 ions to the Pt atoms.


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The electrons transfer model for the SMSI effect suitably explains several documented outcomes, however, as particles grow and their properties become less atom-like, small variations on the catalyst’s electron number should not be fully responsible for those alterations on their catalytic state.3 Distinct attempts to elucidate the nature of the SMSI effect emerged considering the encapsulation of the metallic nanoparticles by the support (geometrical factor). One of these models was introduced by Resasco et al. after the results obtained from his studies on the H2 chemisorption capability and hydrogenolysis activity of Rh/TiO2 catalysts under low and high-temperature reduction reactions.4 The authors propose that the mechanism promoting the metal-support bonding has a strong geometrical component in which reduced titania (TiO2-x) diffuses onto the Rh particles surface, forming a TiO2-x capping layer at the Rh surface.4 This model can easily explain the reversibility of the SMSI state via simple oxidation and the effect occurrence in small or large particles. Recently, Fu et al. suggested that the formation of the capping layer on top of metal clusters is thermodynamically driven by a minimization of the system’s surface energy.5 It occurs if the supported metal phase has larger surface energy and work function, and lower Fermi energy, than the support. The authors investigated Pd clusters supported on n-doped and undoped TiO2 by ex situ X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS) measurements. The results allowed the authors to correlate the existence of the capping layer to the electronic structure of the support proposing a mechanism for the SMSI effect. The mechanism is based on the fact that the high temperature reduction reaction promotes the migration of Ti+n ions towards the TiO2 surface, then TiOx is formed and transported to the metallic clusters surface to lower its surface energy. Moreover, the comparison of the XPS measurements at the Ti 2p electronic level of TiO2 and Pd/TiO2 samples showed a negative


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binding energy shift of the Ti core levels after the Pd deposition, which was related to an upward bending of the system’s bands due to the relative Fermi energies positions (EF (oxide) > EF (metal)). Recently, Zhang et al investigated the SMSI effect in Pd/TiO2 nanoparticles by combining in situ TEM measurements and DFT calculations.6 The authors observed the formation of an amorphous or crystalline TiOx capping layer at the Pd surface, depending on the reduction temperature employed. For low (300 oC) and high (500 oC) temperature reduction treatments the TiOx layer on Pd surface is amorphous and crystalline, respectively. Moreover, the crystalline capping layer induces a shape change in the Pd nanoparticles that become faceted. The SMSI effect has been explored in several metal oxides systems such as V2O3,7 Nb2O5,8 Ta2O5,9 and CeO2,10-16 besides the TiO2 supported systems.1-5 Among those, cerium oxide (CeO2) is the front-runner due to its ability to readily change between Ce(IV) (CeO2) to Ce(III) (Ce2O3) oxidation states by exchanging O atoms with the atmosphere.17 Reduction treatments tend to form non-stoichiometric CeO2-x oxides (with 0 < x ≤ 0.5) creating O vacancies through the surface oxide and oxide crystal.18 Besides the described influence of the SMSI effect on the catalytic properties, the SMSI effect on CeO2-based systems also influence on the atomic arrangement in bimetallic nanoparticles.13,19 Despite the plentiful investigations on numerous metals, such as Rh,20 Pt,15,16 Pd,11 Ni,12 Ir,21 and Ru,22 the precise nature of the SMSI effect on CeO2-supported systems is still contradictory. Meriaudeau et al.23 compared the SMSI effect on Pt/TiO2 and Pt/CeO2 nanoparticles by probing the H2 chemisorption results and analyzing the products of hydrogenation reactions on carbon compounds by Gas Chromatography. The authors observed that the H2 and CO chemisorption capacity and hydrogenolysis selectivity of both systems was very different. Given that the reduction process has similar effects on TiO2 and CeO2, i. e., the formation of Ti+3 (Ce+3) ions and O vacancies, the authors suggested that these


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metal oxides present different mechanisms for the SMSI effect. Datye et al.24 compared the SMSI effect on Pt/TiO2 and Pt/CeO2 catalysts by using High-Resolution Transmission Electron Microscopy (HRTEM) and their observations support this latter hypothesis due to the detection of the capping layer only for the TiO2 supported system. On the other hand, new metal-support interactions in the Pt/CeO2 system were observed recently by Mao et al.15,16 The authors report on the great enhancement of the catalytic activity for benzene oxidation and CO2 reduction by methane (driven by solar light) when Pt nanoparticles are partially confined in mesoporous CeO2 instead of being traditionally supported on the surface of CeO2. In a previous work, Matte et al.12 detected both geometrical and electronic features of the SMSI effect on an investigation of the changes on the reduction behavior of Cu/CeO2 and Ni/CeO2 nanoparticles in comparison to the non-supported Cu and Ni nanoparticles. By means of in situ X-ray Absorption Spectroscopy (XAS), Transmission Electron Microscopy (TEM) and XPS measurements, the electronic and structural properties of the Cu and Ni nanoparticles were monitored during a reduction treatment in H2 atmosphere, from room temperature to 500 °C. No differences were observed between the reduction behavior of supported and non-supported Cu nanoparticles. At the same time, the Ni/CeO2 nanoparticles reduced earlier than the nonsupported Ni nanoparticles. Moreover, it was verified by XPS and HRTEM measurements that the Ni nanoparticles presented their surface covered by a capping layer of CeO2-x support (geometrical factor) after the reduction treatment. The decrease on the Ni nanoparticles reduction temperature was associated to a possible charge transfer from the support to the metallic nanoparticles (electronic factor). It is interesting to note that the charge transfer effect in CeO2based systems is dependent on the kind of metallic nanoparticle supported.11 Recently, in a previous work,11 the charge transfer effect was empirically measured for the first time. It was


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studied the behavior of Au-Pt/CeO2, Au/CeO2, Au-Pd/CeO2, and Pd/CeO2 nanoparticles during reduction treatment in H2 atmosphere at 500 °C. The charge transfer effect was quantified by means of in situ XANES measurements and the results pointed out to the occurrence of electron transfer from the metallic nanoparticle to the CeO2 support in these cases. The results are supported by theoretical calculations on these systems.25 Over the past several years, CeO2-based systems have been heavily explored in environmental catalytic applications, including the construction of three way catalysts and the development of photocatalytic reactors for the H2O and CO2 thermal splitting for fuel generation.17 Since the SMSI effect may decrease or improve the catalytic activity,17 shedding light on the nature of the SMSI effect in CeO2-based catalysts is essential for the projection of new smart catalysts. In this work, a detailed investigation of the SMSI effect on Cu-Ni/CeO2 nanoparticles is conducted, then contributing to elucidating the precise nature of the SMSI effect in these systems. This system was chosen since it has applications on the Reverse Water-Gas Shift (RWGS) and WaterGas Shift (WGS) reactions.

2. Experimental and theoretical description For the synthesis of the CuxNi1-x (0 < x < 1) nanoparticles, the ionic liquid BMI·BF4 was prepared according to the procedure described previously.26 MeOH, CH2Cl2, and acetone were purified by standard procedures.27 The CuCl2·2H2O, NiCl2·6H2O, NaBH4, and CeO2 were purchased from Sigma-Aldrich and used without further purification. A solution of copper chloride dehydrate (CuCl2·2H2O, 134 mg, 0.79 mmol) and a solution of nickel chloride hexahydrate (NiCl2·6H2O, 203 mg, 0.85 mmol) dissolved in ionic liquid BMI·BF4 (3 mL each)


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were maintained under stirring at room temperature during 15 min. After this period, both Cu and Ni solutions were mixed. Then a solution of sodium borohydrade (NaBH4, 617 mg, 16.4 mmol) dissolved in methanol (6 mL) was added to the reaction mixture. The reaction mixture turned black due to the formation of bimetallic CuxNi1-x (0 < x < 1) nanoparticles, which were washed with methanol (3 times, 10 mL) and dichloromethane (3 times, 10 mL). After this procedure, the samples were isolated by centrifugation (3500 rpm) and dried under reduced pressure. In order to obtain CuxNi1-x (0 < x < 1) nanoparticles supported in CeO2 nanoparticles, acetone (5 mL) and CeO2 nanoparticles (50 mg) were added to the CuxNi1-x (0 < x < 1) nanoparticles (12.5 mg) previously prepared as described above, and the mixture was sonicated during 15 min. The samples were isolated by centrifugation (3500 rpm) and dried under reduced pressure in order to obtain CuxNi1-x/CeO2 (0 < x < 1) nanoparticles with 20% w/w. Figure 1 shows a schematic representation of the synthesis of the nanoparticles. The samples were synthesized with four different Cu/Ni atomic concentrations, as determined by Energy Dispersive Spectroscopy (EDS) measurements (Figure S1 of the SI). The corresponding x values in CuxNi1-x are equal to 0.25, 0.35, 0.60, and 1.00. The nanoparticle fraction supported on CeO2 was also determined from EDS (Figure S1 of the SI). A sample of non-supported monometallic Cu nanoparticles was synthesized and used for comparison purposes.


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Figure 1. Schematic representation of the synthesis procedure of the CuxNi1-x/CeO2 nanoparticles.

2.1 Transmission Electron Microscopy (TEM) measurements The size distribution histogram of the CuxNi1-x/CeO2 (0 < x < 1) nanoparticles was obtained by analyzing the TEM images, acquired with a CCD camera on an electron microscope (JEOL JEM 1200 EXll) at CMM-UFRGS, operated at an accelerating voltage of 80 kV. For these measurements, the supported nanoparticles were dispersed in deionized water, and a drop of each solution was deposited on a carbon-coated copper grid and dried.

2.2 X-Ray Photoelectron Spectroscopy (XPS) measurements


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The chemical components and atomic arrangement of the as prepared non-supported CuxNi1-x (0 < x < 1) nanoparticles were also characterized by means of XPS measurements. The measurements were conducted at the LNNano (CNPEM) using the Thermo Scientific K-Alpha photoelectron spectrometer. An Al-Kα X-ray source (hν = 1486.7 eV) operating at 12 kV with 45° emission angle of the photoelectrons with respect to the incident X-rays was used. For these measurements, a thin layer of powder from each sample was dispersed on a carbon tape, attached to the sample holder, and introduced in the ultra-high vacuum chamber (2x10-9 mbar). The measurements were performed in the long scan, Cu 2p, Cu 3p, Ni 2p, Ni 3p, O 1s, and C 1s regions of the samples. The electron analyzer was set at a pass energy of 200 eV and 50 eV with an energy step of 1 eV and 0.1 eV, and dwell time of 0.1 s, for the long scan and high resolution spectra, respectively. The analyzer’s energy calibration was performed using a standard Au foil (Au 4f7/2 peak at 84.0 eV). The C 1s peak position of adventitious carbon at 285.0 eV was also checked, to verify possible charging effects.

2.3 Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS) measurements Intending to evaluate the structural and chemical changes induced by the gaseous treatments on the nanoparticles, NAP-XPS measurements were performed at the CIRCE (BL24) beamline of the ALBA Synchrotron Light Source.28 The CIRCE is an undulator beamline with energy range of (100 - 2000) eV. For the measurements, 10 mg of the CuxNi1-x/CeO2 (0 < x < 1) and nonsupported Cu nanoparticles powder was compacted in order to produce pellets of around 3 mm diameter. The samples were heated to 500 °C by means of an infrared laser under a 5.0 mbar H2 atmosphere. At 500 °C, the H2 pressure was decreased to 1.0 mbar and the samples were left


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exposed to this atmosphere during 50 min. After this period, and still at 1.0 mbar H2 at 500 °C, the NAP-XPS measurements were performed. Later, the H2 atmosphere was removed and 1.0 mbar CO2 atmosphere was introduced instead at the same temperature, 500 °C, and during the same period, 50 min. A new set of NAP-XPS measurements were performed under these conditions. All the NAP-XPS measurements were performed with 2 different incident photon energies of hν = 1250 eV and hν = 2000 eV and with a beamspot size of (100 x 300) μm2. After these measurements, the CO2 atmosphere was pumped out and the system was cooled down to the room temperature. A differentially pumped electron hemispherical analyzer (SPECS GmbH PHOIBOS 150 NAP) was used to probe the photoelectrons from the long scan, Cu 2p, Ni 2p, Ce 3d, O 1s and C 1s regions of the supported samples, collecting the photoelectrons ejected at an exit angle equal to 54.7° with respect to the incident X-rays. The beamline exit slit was 20 μm. The electron analyzer was set at a pass energy of 20 eV and 10 eV with an energy step of 1 eV and 0.1 eV, and dwell time of 0.1 s, for the long scan and high resolution measurements, respectively. In that conditions, the total beamline plus analyzer energy resolution was better than 0.3 eV. It was also considered the C 1s peak value of adventitious carbon at 285.0 eV and the Ce 3d5/2 peak of CeO2 at 882.4 eV as references to verify possible charging effects.

2.4 In situ Time-Resolved X-Ray Absorption Near Edge Structure (XANES) measurements In order to contemplate the intermediate steps through the reduction process, in situ timeresolved XANES measurements were performed at the XDS (W09A) and DXAS (D06A) beamlines at LNLS (Brazilian Synchrotron Light Laboratory).


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The measurements at the Cu K edge (8979 eV) were performed at the XDS beamline in the transmission mode.29 Pellets containing around 45 mg of CuxNi1-x/CeO2 (0 < x < 1) and nonsupported Cu nanoparticles powder mixed with 55 mg of boron nitride (BN) were used during the measurements. The pellets were inserted into a tubular furnace, then exposed to 100 mL/min of 5% H2 + 95% He atmosphere and heated to 500 °C at 10 °C/min. At 500 °C, the samples remained exposed to the reduction treatment during 2 h. The XANES spectra were collected every 1.5 min during the full reduction treatment (from room temperature to 500 °C). The XANES spectra of Cu, CuO, Cu2O, and CuCl2·2H2O compounds were measured to be used as standards for the data analysis. The spectra were collected using a Si(111) double crystal monochromator and two ionization chambers. The measurements at the Ce L3 edge (5723 eV) were performed at the DXAS beamline in the transmission mode.30 Pellets containing 7 mg of CuxNi1-x/CeO2 (0 < x < 1) nanoparticles powder mixed with 35 mg of BN were submitted to the same reduction treatment employed for the measurements at the Cu K edge described above. The XANES spectra were collected every 5 s during the full reduction treatment (from room temperature to 500 °C). The XANES spectra of commercial CeO2 and CeOHCO3 compounds at room temperature were measured to be used as Ce(IV) and Ce(III) standards, respectively. The measurements made use of a curved Si(111) crystal (dispersive polychromator) and a CCD detector. The time resolution of the measurements was around 100 ms. 2.5 High Resolution Transmission Electron Microscopy (HRTEM) measurements HRTEM analysis of the Cu/CeO2 and Cu0.25Ni0.75/CeO2 nanoparticles after the reduction treatment and Cu/CeO2 nanoparticles after the oxidation treatment were performed using an


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XFEG Cs-corrected FEI Titan 80/300 microscope at INMETRO operated at 300 kV. The supported nanoparticles were dispersed in deionized water, and a drop of each solution was deposited on a carbon-coated copper grid and dried. The typical lateral resolution was greater than 0.01 nm.

2.6 Data analysis The size distribution histogram, resulting from the analysis of the TEM images, was obtained using the ImageJ 1.46r software. For each sample, the size of at least 2500 nanoparticles was measured. The analysis of the XPS measurements was performed using the XPSPeak 4.1 software. All the analyzed regions were fitted using a Shirley-type background and asymmetric Gaussian-Lorentzian function (55% Lorentzian contribution), as determined from the analysis of an Au standard. For the NAP-XPS measurements, the same software and method was applied, however, with a smaller Lorentzian contribution of 46%. The binding energy and FWHM values of a given chemical component were constrained to the same value for all the samples for a given photon energy. The analysis of the complex Ce 3d region was performed using the procedure described by Schierbaum et al.31 The Ce 3d spectrum was deconvoluted in 10 Gaussian peaks. The FWHM and binding energy values of the peaks were constrained to be the same for all the samples. These peaks are labeled following the notation introduced by Burroughs et al.32 The peaks referring to Ce 3d5/2 are labeled as v0, v, v’, v”, and v”’, and those referring to Ce 3d3/2 are named u0, u, u’, u”, and u”’. The surface Ce(III) fraction, χsurf, was determined from this fitting procedure using Eq. (1)31


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χsurf =

v0 + u0 + v′ + u′

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(1)

∑v + u

The in situ time-resolved XANES measurements were analyzed using the standard procedure of data reduction using the IFEFFIT package.33,34 The XANES spectra at the Cu K edge were fitted using a linear combination of Cu standards. The sum of the coefficients of the components used was constrained to be equal to 1.0 with the individual coefficients always higher or equal to zero. The Ce(III) fraction χ was obtained by the analysis of the XANES spectra at the Ce L3 edge using the approach proposed by Takahashi et al.35 A linear combination of three Lorentzians and one arctangent functions were used to fit a 20 eV energy range region after the edge, containing the single(double)peak(peaks) of the Ce(III)(Ce(IV)) compounds, describing the 2p → 5d (Lorentzians) and 2p → continuum (arctangent) electronic transitions. The values determined for the energy positions and width of the Lorentzian peaks in the Ce(III) and Ce(IV) standards were fixed for all the samples. The only variable parameter was the height of the Lorentzian functions in the XANES spectra of the samples. The Ce(III) fraction χ was obtained by Eq. (2)35

χ =

A1 A1 + A2 + A3

(2)

where A1, A2, and A3 represent the Lorentzian areas associated to the Ce(III) (A1) and Ce(IV) (A2 and A3) components.

2.7 Density Functional Theory (DFT) calculations

Density Functional Theory (DFT) calculations were performed within the Quantum Espresso 5.436 suite of codes, through the plane waves self-consistency field (PWSCF) method. These


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calculations were carried out in the generalized gradient approximation (GGA), using PerdewBurke-Ernzerhof (PBE) exchange-correlation functionals.37 Projector-Augmented Waves (PAW) pseudopotentials from the Standard Solid-State Pseudopotentials (SSSP)38 library were used to describe the Ce (4f, 5s, 5p, 5d, 6s) and O (2s, 2p) electronic states. The Cu (3d, 4s, 4p) and Ni (3d, 4s, 4p) electronic states were described by PAW pseudopotentials available on the pslibrary (version 0.3.1).39 A sequence of energy minimization and geometry optimization calculations were performed, relaxing the system to a force per atom of 10-5 eV/Å. The kinetic energy cutoff for the wave functions was set to 40 Ry, and the charge density cutoff to 440 Ry. The Brillouin zone was integrated using an 8x8x8 k-points mesh for the materials bulk representation, yielding the representation of the electronic band gap. Gaussian smearing, with σ value equal to 0.03, was enabled to improve the convergence of the system. The electronic ground states were determined after an electronic energy convergence of 10-6 eV. This procedure reproduced the Fm3m structure of CeO2, with a lattice parameter equal to 5.412 Å, which agrees to the experimental value of 5.411 Å.40 For the interaction between nanoparticles and support, a tetrahedral cluster (Cu2Ni2) was inserted on an oxygen terminated CeO2 (100) surface, constructed as a 2x2 supercell of (5.5059 x 5.5059 x 28.1755) Å3. A vacuum slab of 20 Å was considered in order to separate the neighboring slabs (perpendicularly to the CeO2 surface) and to avoid charge effects. The integration on the Brillouin zone of the cluster/slab model proceeded with a 4x4x1 k-points mesh. 3. Results Figure 2 shows a typical TEM image, in this case obtained for the Cu0.60Ni0.40/CeO2 bimetallic nanoparticles, where an almost circular projection in 2D is observed, probably due to the almost


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spherical shape of the nanoparticles. The size distribution histograms show a typical mean diameter of (4 ± 1) nm for all the samples analyzed.

Figure 2. Typical TEM image for the bimetallic nanoparticles synthesized and the corresponding size distribution histogram (inset). The image was obtained from the Cu0.60Ni0.40/CeO2 nanoparticles.

The analysis of the XPS measurements of the as prepared samples provided information about the chemical components at the surface region of the nanoparticles. The long scan measurements (not shown here) demonstrated the presence of Cu, Ni, Ce, O, C, and Cl atoms. The Cl atoms come from the synthesis procedure, as observed in the previous work.12 Figure 3 shows a comparison of the (i) Cu 2p3/2 and (ii) Ni 2p3/2 XPS region between the as prepared CuxNi1-x (0 < x < 1) nanoparticles. The Cu 2p3/2 region shows the presence of three


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distinct components, which are associated to Cu(0) at 932.6 eV,41 and Cu(II) components. The Cu(II) components are probably associated to a Cu(II)-O bonding at 933.4 eV and a Cu(II)-Cl bonding at 934.9 eV.42,43 The Ni 2p3/2 XPS region presents two main components associated to Ni(II) oxidation states.44 These Ni(II) components are probably associated to Ni(II)-O bondings at 855.1 eV and Ni(II)-Cl bondings at 856.7 eV.44,45 The intensity at the Cu 2p3/2 and Ni 2p3/2 regions is directly related to the x value obtained from the EDS measurements. It is observed that the nanoparticles surface is mainly composed of Cu(II) and Ni(II) components for all the samples. The Cu(0) component has an important contribution at the Cu 2p3/2 region but the same trend is not observed for the Ni(0) component at the Ni 2p3/2 region, where no significant contribution appears for the samples analyzed. Table S1 of the Supporting Information (SI) gives the fraction of each chemical component.

Figure 3. XPS spectra at the (a) Cu 2p3/2 and (b) Ni 2p3/2 regions of the non-supported CuxNi1-x nanoparticles for (i) x = 1.00, (ii) x = 0.60, (iii) x = 0.35, and (iv) x = 0.25. The black points represent the measurement, the solid black line the Shirley background, the solid red, blue, and


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green lines the peaks associated to the metal, oxide and chlorine components, the solid purple lines the satellite peaks, and the grey line the best fitting obtained.

The NAP-XPS measurements allowed the identification of the chemical components of the nanoparticles in situ, that is, during the 1 mbar H2 and 1 mbar CO2 treatment of the samples at 500 °C. Figure 4 shows the NAP-XPS spectra at the Cu 2p3/2 and Ni 2p3/2 regions during H2 treatment at 500 °C for both incident photon energies of 1250 eV and 2000 eV. It is possible to observe clearly a significant change on the Cu 2p3/2 and Ni 2p3/2 NAP-XPS spectra if compared to those shown in Figure 3. In this case, the major component, for both Cu 2p3/2 and Ni 2p3/2 regions, is associated to the metallic state (Cu(0), at 932.6 eV, and Ni(0), at 852.8 eV), although small contributions from oxide components (Cu(II)-O at 933.4 eV, and Ni(II)-O, at 855.1 eV) still remained in the nanoparticle surface. Table S2 of the SI gives the fraction of the chemical components found for both incident photon energies. The Ni(II)-O/Ni(0) intensity ratio is higher than the Cu(II)-O/Cu(0) intensity ratio for all the bimetallic nanoparticles analyzed. No signals associated to chlorine bonding were detected at the Cu 2p3/2 or Ni 2p3/2 regions. The change on the incident photon energy allows the study of the dependence of the metallic and oxide components with the probed depth. Interestingly, when increasing the photon energy, probing deeper depths, there is an enhancement of the Cu(II)-O component fraction but essentially the same Ni(II)-O component fraction. It is also clear that, in the bimetallic cases, the Cu(II)-O fraction at the nanoparticles is directly related to the amount of Cu on the samples. This is probably associated to a synergistic effect due to the presence of Ni atoms, since no Cu oxidation is observed for supported, nor non-supported monometallic nanoparticles. Moreover, the Ni(0) component fraction is proportional to the Ni amount in the bimetallic nanoparticles. In other


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words, a lower Ni concentration of the nanoparticle gives a stronger Ni(II)-O component. The same trend is observed for Cu, as the nanoparticles with a lower amount of Ni (higher amount of Cu atoms) remains more oxidized during the reduction process. This effect is directly related to the Cu and Ni atomic population at the surface of the nanoparticles as well as to the atomic structure of the nanoparticles, as shown later in this work. The removal of the H2 atmosphere and insertion of the CO2 atmosphere does not produce a strong change on the chemical components present at the Cu 2p3/2 and Ni 2p3/2 NAP-XPS spectra (Figure S2 of the SI). The Cu and Ni atoms are mainly in the metallic state for both incident photon energies. Table S3 of the SI gives the fraction of the chemical components for each case. There it is possible to find a significant change only for the Ni atoms, which are more oxidized when exposed to the CO2 atmosphere than in the H2 atmosphere.


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Figure 4. NAP-XPS spectra at the (a) and (b) Cu 2p3/2 and (c) and (d) Ni 2p3/2 regions measured with h𝜈 = 1250 eV ((a) and (c)) and h𝜈 = 2000 eV ((b) and (d)) during 1 mbar H2 treatment at 500 °C for CuxNi1-x/CeO2 nanoparticles for (i) x = 1.00, (ii) x = 0.60, (iii) x = 0.35, and (iv) x = 0.25. The black points represent the measured data, the solid black line the Shirley background,


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the solid red and blue lines the peaks associated with metal and oxide components, the solid purple line the satellite peak, and the grey line the best fitting obtained.

Furthermore, the NAP-XPS measurements also probed the Ce 3d region, as shown in Figure 5. In spite of the complex Ce 3d XPS spectra, the peak located at around 917 eV, labeled u’’’, gives a direct indicative of the Ce(III) fraction existing in the sample. The higher the height of this peak, the smaller the Ce(III) fraction in the sample. It is possible to observe a change on the degree of support reduction depending on the nature of the metallic nanoparticle supported, as expected from a previous study that investigated the dependence of the CeO2 reduction with the kind of metallic nanoparticles supported on CeO2.11 Moreover, the increase on the depth probed by comparing the NAP-XPS measurements at hν = 1250 eV and hν = 2000 eV during H2 reduction and CO2 oxidation (Figure 5 and Figure S3 of the SI) gives an increase on the relative height of the u’’’ peak, associated to a decrease on the Ce(III) fraction. However, the intensity of the u’’’ peak alone does not allows to quantify the Ce(III) fraction present in each sample. Aiming to quantify it, the method described in Section 2.6 was applied. Table S4 of the SI gives the surface Ce(III) fraction χsurf as a function of the sample and incident photon energy used. A common trend is observed in Table S4 concerning the Ce(III) fraction values, in which the samples surface is more reduced than the inner regions. This is expected since the reduction of the cerium oxide starts at the surface region and then it propagates to the inner region of the sample.18 The only exception is the Cu/CeO2 sample that presents an almost constant Ce(III) fraction value for both incident photon energies. Moreover, the measurements under CO2 atmosphere give a significant decrease on the Ce(III) fraction only for the Cu/CeO2 sample.


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The fitting of the Ce 3d region was performed by including constraints on the energy positions and FWHM values of the 10 Gaussian peaks used. However, by using a single set of parameters for the Gaussian peaks is not possible to reach a reasonable good fitting for all the samples. In order to achieve a good fitting for all the samples at both incident photon energies, a relaxation process of the binding energy values was performed, resulting in a different set of parameters for the samples. Tables S5 - S8 of the SI gives the energy position used for all the Gaussian peaks of all the cases studied. Some peaks needed energy shifts to fit the Ce 3d NAP-XPS region during H2 reduction treatment in comparison to the CO2 atmosphere case. It occurs only for the Cu/CeO2 (v’ and u’ peaks) and Cu0.60Ni0.40/CeO2 nanoparticles (v’’ and u’’ peaks). The energy shifts correspond to the case where a lower χ2 value was found in the fitting procedure between several possible peak parameters used. The energy shift values in the v′, u′, v′′, and u′′ peaks correspond to, respectively, 0.9 eV, 1.9 eV, 0.3 eV, and 0.4 eV. It is known that the Ce 4f orbitals may overlap to the O 2p ones.31,46 The electronic states associated to the v′, u′ and v′′, u′′ peaks are Ce3d10O2p6Ce4f1 and Ce3d10O2p6Ce4f0, which correspond to Ce(III) and Ce(IV) oxidation states, respectively. The interactions between Ce 4f and O 2p electronic levels are probably associated to changes in the Ce atoms neighborhood, shifting these peaks position to higher binding energies in H2 atmosphere in comparison to the CO2 atmosphere.


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Figure 5. NAP-XPS spectra at the Ce 3d region measured with h𝜈 = 1250 eV during (i) 1 mbar H2, and (ii) 1 mbar CO2 treatment at 500 °C for CuxNi1-x/CeO2 nanoparticles for (a) x = 1.00, (b) x = 0.60, (c) x = 0.35, and (d) x = 0.25. The black points represent the measured data, the solid


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black line the Shirley background, the solid red and blue lines the peaks associated with the Ce(III) and Ce(IV) components, and the grey line the best fitting obtained.

The in situ time-resolved XANES measurements provided an overview of the fraction of the chemical components during the reduction treatment. A typical fitting in the Cu K edge XANES spectra is shown in Figure S4 of the SI. Firstly, the room temperature XANES spectra at the Cu K edge were analyzed in order to compare the fractions of the different chemical components with those found in the XPS measurements. Table S9 of the SI shows the results where the as prepared samples are composed essentially by Cu(0), CuO and CuCl2·2H2O compounds. This result is consistent with the results found by XPS for the chemical components composition at the surface region (see Table S1 of the SI). During H2 and CO2 treatment all the samples exhibit 100% of Cu(0) chemical component, which is again consistent with the NAP-XPS results, more surface-sensitive, since the XANES technique probes the bulk region of the sample. Figure 6(a) shows a typical time-resolved XANES measurement at the Cu K edge during the reduction treatment, in this case for the Cu0.60Ni0.40/CeO2 nanoparticles. The XANES spectrum at the Cu K edge is associated to an allowed electronic transition from the 1s to 3p electronic state. Figure 6(a) shows clearly the decrease on the height of the first peak after the absorption edge meaning the decrease on the number of empty states at the Cu 3p electronic level, in accordance with the reduction of the Cu atoms. Figure 6(b) shows the corresponding time evolution of the fraction of each chemical component as obtained from the fitting of the XANES spectra individually during the reduction treatment. The sample reduces from CuO to Cu2O and then to Cu(0) metallic state. It is a general behavior and was observed for the other samples as well (not


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shown here). Figures 6(c) and (d) show a comparison between the samples for the Cu(0) and CuO fraction time evolution. It is possible to observe a dependence of the inflection point with the sample composition. Then, the data show the influence of the sample composition and support on the Cu reduction temperature. The Cu(0) fraction reached its maxima at different temperatures, depending on the sample, where the in situ XANES spectra presented a typical metallic fingerprint. The changes on the reduction properties of the nanoparticles may be directly related to the redox activity of the cerium oxide support.


25


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Figure 6. (a) In situ time-resolved XANES spectra at the Cu K edge for the Cu0.60Ni0.40/CeO2 sample during the reduction treatment in H2 atmosphere and the correspondent (b) time evolution of the compounds fraction determined by linear combination of XANES spectra (part (a)). Time evolution of the (c) Cu(0) and (d) CuO compound fractions for the CuxNi1-x/CeO2 (0 < x < 1) and non-supported Cu nanoparticles during reduction treatment in H2 atmosphere. The red and black XANES spectra from (i) correspond to CuO and Cu(0) standard samples for comparison purposes.


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In order to investigate the reduction process of the support, XANES spectra at the Ce L3 edge were measured and analyzed during the reduction treatment in H2 atmosphere (not shown here). Figure 7 presents the time evolution of the Ce(III) fraction (χ) during reduction treatment obtained by the XANES spectra analysis, described in Section 2.6, of the CuxNi1-x/CeO2 (0 < x < 1) nanoparticles. The reduction temperature of the support depends on the composition of the bimetallic nanoparticle supported on the cerium oxide. It can be seen also that all the samples presented a two step reduction process of the support. The first stage is marked by a step-like increase of the Ce(III) fraction around 300 °C, and the last one by a second step-like increment around 420 °C. An unexpected behavior of the cerium oxide reduction process is observed for the Cu/CeO2 and Cu0.60Ni0.40/CeO2 nanoparticles around 200 °C, where a decrease of the Ce(III) fraction is detected, even with the sample exposed to a H2 atmosphere.

Figure 7. Time evolution of the Ce(III) fraction, χ, obtained from the analysis of the XANES spectra at the Ce L3 edge for CuxNi1-x/CeO2 (0 < x < 1) nanoparticles during reduction treatment in H2 atmosphere.


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Figure S5 of the SI shows the DOS calculation for the bulk CeO2. It is worth noting that the valence band is predominantly associated to the O 2p states while the conduction band is primarily given by Ce 5d states. Nevertheless, significant apparent orbital hybridization is observed between the O 2p and Ce 5d orbitals on both valence and conduction bands. The ~5.4 eV band gap between the valence and conduction bands accommodates the narrow and empty Ce 4f band, located ~2 eV upper than the last occupied O 2p state of the valence band. These calculated band gaps are smaller than the experimental values47,48 but such underestimation is expected for GGA calculations.49 Figure 8(a) shows the DOS calculations for the O terminated CeO2 (100) surface. It is clear the break of crystalline symmetry in comparison to the bulk calculation which is manifested as the emergence of an O 2p state between the valence band and the Fermi level. It approximates the valence band to the conduction band, narrowing the band gap between the valence and conduction bands to ~4.8 eV. The insertion of the Cu2Ni2 cluster on the CeO2 (100) surface induces changes in the DOS, as shown in Figure 8(b). The presence of Cu 4p and Ni 4p states at the Ce 4f state region spreads the Ce 4f band in a 0.5 eV wider band. The metallic cluster contributed significantly to the increase on the density of occupied states at the valence band, mainly by the expressive apparent orbital hybridization of O 2p with Cu 3d and Ni 3d states. Due to the presence of the Cu2Ni2 cluster, the surface O 2p localized state detaches from the valence band and connects to the Ce 4f band. Considering also the spreading of valence band and Ce 4f band, the band gap between the O 2p and Ce 4f states is essentially closed, while that between the Ce 4f and Ce 5d states decreases to ~0.7 eV (2.3 eV on the clean surface).


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Figure 8. Total (black line) and partial (colored lines) density of states (DOS) of the (a) CeO2 and (b) Cu2Ni2/CeO2 surface. Valence band (VB) and conduction band (CB) are indicated in the graph. Both DOS have the same y scale for comparison purposes.

4. Discussion The results obtained by XPS and NAP-XPS measurements (Figure 2) allowed probing the atomic arrangement of the bimetallic nanoparticles. It is obtained the intensity ratio of the Cu/Ni signal when probing different depths in the nanoparticles. For the as prepared nanoparticles, the different depths were probed by keeping a fixed photon energy (hν = 1486.7 eV) and analyzing the Cu 2p3/2, Ni 2p3/2, Cu 3p (not shown here), and Ni 3p (not shown here) electronic regions, then changing the kinetic photoelectron energy. On the other hand, the NAP-XPS measurements were performed analyzing only the Cu 2p3/2 and Ni 2p3/2 electronic regions but changing the incident photon energy. Again, the ejected photoelectrons have different kinetic energies, then allowing probing different depths. The photon energies were chosen aiming to have approximately the same inelastic mean free path (obtained using the TPP2M formula)50 for the XPS and NAP-XPS measurements, that is, 11 Å (surface region, XPS measurements), 8 Å


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(surface region, NAP-XPS measurements), 20 Å (inner region, XPS measurements), 17 Å (inner region, NAP-XPS measurements). Since the XPS intensity is dependent of the photoionization differential cross section, the data were normalized by this quantity in order to make possible to compare the Cu/Ni ratios. Table 1 shows the Cu/Ni normalized ratio for the different regions probed and for each sample and condition studied. In the case of as prepared samples, the Cu/Ni normalized ratio increases when changing the region probed from the surface to the inner region of the nanoparticles. It means there is a higher Cu concentration in the inner region than in the surface region of the nanoparticles. Since the Cu/Ni normalized ratio is higher than 1.0 (lower or equal to 1.0) in the inner (surface) region of the nanoparticle, this result is consistent with the existence of bimetallic nanoparticles with a Ni-rich surface and a Cu-rich core. Moreover, the Ni concentration at the surface region follows the same trend expected from the amount of Ni obtained by EDS measurements, i.e., the Ni-richer nanoparticles also present the Ni-richer surfaces.

Table 1. Cu/Ni ratio normalized by the photoionization differential cross section obtained from the XPS analysis at Cu 2p3/2, Ni 2p3/2, Cu 3p, and Ni 3p regions (as prepared samples) for the CuxNi1-x (0 < x < 1) nanoparticles and from the NAP-XPS analysis at Cu 2p3/2 and Ni 2p3/2 (in 1 mbar H2 treatment and 1 mbar CO2 treatment at 500 °C) regions for the CuxNi1-x/CeO2 (0 < x < 1) nanoparticles. Cu/Ni normalized ratio Sample

As prepared 𝜆IMFP ≈ 11 Å

𝜆IMFP ≈ 20 Å

H2 atmosphere

CO2 atmosphere

𝜆IMFP ≈ 8 Å

𝜆IMFP ≈ 8 Å

𝜆IMFP ≈ 17 Å

𝜆IMFP ≈ 17 Å


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Cu0.60Ni0.40 nanoparticles

1.0

2.7

1.3

1.6

1.3

1.8


0.5

1.1

1.9

1.5

1.3

1.5


0.4

1.3

2.0

2.1

0.9

1.2

Ruban et al. performed a theoretical calculation aiming to determine the surface segregation energies of several different host-solute metal combinations.51 The authors showed that the CuNi system presents moderate segregation of Cu atoms to the surface. This result is in contradiction to those found in Table 1. The same tendency is observed theoretically elsewhere.52 However, it is important to stress out that the theoretical calculations were performed for surface clean systems that do not correspond to the system used in the present work. It is well known from the literature that nanoparticles synthesized by using ionic liquids present important interactions at the surface region with the anion of the ionic liquid, then certainly changing the energy configuration for this system.53 A more detailed theoretical calculation is needed for comparison purposes. Concerning experimental studies, in the literature is possible to found Cu-Ni nanoparticles synthesized with a Cu-rich surface,54 Ni-rich surface or both cases.55,56 There is a strong dependence of the surface atomic population on the synthesis method employed. For the method employed in this work, using ionic liquids, it is observed a Ni enrichment at the surface of the bimetallic nanoparticles. Moreover, by examining the nanoparticles atomic structures, the observed Cu (Ni) concentration gradient is different from sample to sample, depending on the x value.


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The exposure of the samples to 1 mbar H2 atmosphere at 500 oC induces an increase of the Cu/Ni normalized ratio at the surface region of all samples in comparison to the as prepared case (Table 1). It means there is a higher amount of Cu atoms at the surface after reduction treatment than before the reduction treatment, showing a clear migration of Cu atoms towards the surface of nanoparticles under H2 exposition at 500 oC. Furthermore, the comparison of the Cu/Ni normalized ratio during reduction treatment for different probed depths shows different results, depending on the amount of Cu in the nanoparticle synthesized. Table 1 shows an increase, decrease or a constant ratio when increasing the probed depth from the surface to the inner region of the bimetallic nanoparticles during reduction treatment for the different samples studied. This occurs probably because the time used for reduction treatment (50 min.) was not long enough to ensure a complete migration of Cu atoms towards the surface of the nanoparticles. Since the kinetics of Cu migration to the surface depends on the Cu amount, the change of Cu/Ni normalized ratio depends as well on this factor. The Cu enrichment at the surface of the bimetallic nanoparticles during H2 exposition is in accordance to that predicted theoretically. The Cu-Ni bimetallic clusters present smaller surface free energy with Cu atoms at the surface instead of Ni atoms.57 However, while there are few studies in the literature concerning this issue, the opposite behavior is usually reported.58-61 Beaumont et al.58 studied the surface segregation of atomic species in Cu0.5Ni0.5 and Co0.5Ni0.5 bimetallic nanoparticles supported on SiO2 and TiO2 and exposed to a H2 (at 450 °C), CO (at 200 °C) or O2(at 350 °C) atmosphere by NAP-XPS and Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy (STEM-EDS) techniques. The authors found that the H2 atmosphere induces a Ni migration to the surface of Cu0.5Ni0.5 bimetallic nanoparticles. The authors explain the difference found in comparison to the theoretical prediction by the fact that H2 can dissociate easily on Ni


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surfaces and then the efficient chemisorption of H2 molecules may induce the Ni segregation to the surface. In spite of this the authors affirm the need of a more detailed study about this system. On the other hand, Seemala et al studied the atomic arrangement in Cu-Ni bimetallic nanoparticles supported on TiO2 and Al2O3 oxides after being exposed to reduction and calcination processes.60,61 The authors found a Cu-rich surface in both treatments for those nanoparticles supported on TiO2. However, no specific atomic enrichment at the surface was observed for the nanoparticles supported on Al2O3. One important difference between the present work and those found in the literature is the support used. The cited works study Cu-Ni/SiO2, Cu-Ni/TiO2 and Cu-Ni/Al2O3, but not Cu-Ni/CeO2 nanoparticles. It is known that the support may have a strong influence on the atomic arrangement of bimetallic nanoparticles, as demonstrated previously.13 This may account for the differences found in the works. The Cu migration to the surface during H2 treatment also explains the result found in Table S2 of the SI that Ni atoms are relatively more oxidized than Cu atoms during H2 treatment. Since Cu atoms migrate to the surface they are reduced easier than Ni atoms that migrate to the inner region of the nanoparticle. The same analysis in Table 1 can be done for the NAP-XPS measurements after replacement of H2 by the CO2 atmosphere at 500 oC. In this case, there is a decrease of the Cu/Ni normalized ratio at the surface region in comparison to that obtained during H2 exposition. The decrease on the ratio can be interpreted as Ni migration to the surface region of the nanoparticles, thus increasing the Ni atomic population at the surface and, consequently, decreasing the Cu/Ni normalized ratio. Finally, the comparison of the Cu/Ni normalized ratio during CO2 exposition for different probed depths shows an increase of the ratio for the inner region for all the nanoparticles, which is interpreted as the existence of a Cu-rich core and a Ni-rich shell region.


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The Ni migration towards the nanoparticles surface is probably due to the stronger interactions with CO2 as compared to those presented by Cu.62 Again, this fact can be used to explain the observations summarized in Table S3 of the SI that show that the CO2 atmosphere gives an increase in the Ni(II)-O fraction but not in the Cu(II)-O fraction. Since Ni atoms migrate towards the surface, they become more oxidized by interacting with CO2 molecules. Similarly, the changes on the metal/support intensity ratio were investigated by using the intensities of the Cu 2p3/2, Ni 2p3/2 and Ce 3d NAP-XPS regions. The comparison is performed by the (Cu 2p3/2 + Ni 2p3/2)/(Ce 3d) ratio normalized by the corresponding differential cross section. Since the Ce 3d binding energy is close to the Cu 2p3/2 and Ni 2p3/2 ones, the kinetic energy of photoelectrons coming from the Ce 3d electronic level is essentially the same than those coming from the Cu 2p3/2 and Ni 2p3/2 electronic levels. Consequently, the probed depth is almost the same and the same photon energies of the Cu/Ni ratio investigation (1250 eV and 2000 eV) were used. Table 2 shows the metal/support ratio obtained from the NAP-XPS measurements in H2 and CO2 atmosphere for different probed depths as a function of the sample synthesized. The results reveal an increase of the metal/support ratio with the increase of the probed depth only for the Cu/CeO2 and Cu0.60Ni0.40/CeO2 samples. It means there is a higher amount of Cu and Ni atoms at the inner region than at the surface region of the sample. This is consistent with the existence of a capping layer of metal oxide from the support surrounding the metallic nanoparticles. Furthermore, the exposure to an oxidant atmosphere (CO2) gives the same metal/support ratio for the different probed depths. These experimental evidences are consistent with the existence of the SMSI effect since it is well known that an oxidant atmosphere removes the capping layer,20 which results in the same metal/support ratio for the different probed depths. HRTEM images were acquired to confirm this interpretation. Figure 9 presents typical HRTEM


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images of selected samples, namely the Cu/CeO2 nanoparticles after (a) H2 reduction and (b) CO2 oxidation treatment and the Cu0.25Ni0.75/CeO2 nanoparticles after (c) H2 reduction treatment. It is observed a capping layer of the support covering the surface of the Cu metallic nanoparticles after reduction treatment. However, after exposure to an oxidant atmosphere the capping layer is no more present at the surface of the Cu nanoparticles. These observations are in accordance to the conclusions obtained by means of NAP-XPS measurements. Moreover, the Cu0.25Ni0.75/CeO2 nanoparticles do not present the capping layer covering the bimetallic nanoparticles, even after reduction treatment. Again this behavior is expected from the NAP-XPS analysis, where is observed a threshold in the Cu concentration for the occurrence of the SMSI effect, as confirmed by HRTEM measurements.


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Figure 9. Typical HRTEM image of (a) Cu/CeO2 nanoparticles after H2 reduction treatment, (b) Cu/CeO2 nanoparticles after CO2 oxidation treatment, and (c) Cu0.25Ni0.75/CeO2 nanoparticles after H2 reduction treatment.

It is worth noting that the geometrical factor of the SMSI effect occurs only for those samples that presented an energy shift in some peak positions in the fitting of the Ce 3d NAP-XPS region. Then, there is a strong evidence for the elucidation of the nature of the SMSI effect, that occurs through the interaction of the Ce3d10O2p6Ce4f1 (Cu/CeO2 nanoparticles) and Ce3d10O2p6Ce4f0 (Cu0.60Ni0.40/CeO2 nanoparticles) initial states with the neighborhood composed by Cu and Ni atoms. Moreover, the changes observed in the Ce(III) fraction in H2 atmosphere between samples can be explained by the nature of the SMSI effect too. For the case of Cu0.60Ni0.40/CeO2 nanoparticles, the SMSI effect occurs through interaction of the nanoparticles with Ce atoms in the Ce3d10O2p6Ce4f0 initial state, associated to a Ce(IV) oxidation state. This sample has the smaller Ce(III) fraction (higher Ce(IV) fraction) at the surface between all the cases studied (Table S4 of the SI). On the other hand, the Cu/CeO2 nanoparticles present the SMSI effect occurring via interaction of the nanoparticles with the Ce atoms in the Ce3d10O2p6Ce4f1 initial state, associated to the Ce(III) oxidation state and, in fact, this sample has the highest Ce(III) fraction observed in Table S4 of the SI. It indicates that the interaction of the capping layer with the surface of the nanoparticles drives the cerium oxide capping layer reduction behavior. Even considering the SMSI effect is greatly investigated nowadays,19,63 for the best of our knowledge this is the first time that its origin is discussed, then elucidating the nature of the SMSI effect.


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Moreover, the proposed interaction between nanoparticles and support is evidenced by DOS calculations. The comparison between the DOS of CeO2 bulk (Figure S5) and (100) surface (Figure 8(a)) shows more localized states in the valence and conduction bands of CeO2 surface. This localization evidences the high reactivity of the CeO2 (100) surface, and can be associated to the formation of surface states resultant from the break of symmetry of the crystal by the O termination at the surface. When including the Cu2Ni2 cluster (Figure 8(b)), the presence of Cu 4p and Ni 3d states splits the Ce 4f state. The valence band of CeO2 was shifted towards the Fermi level, then enabling the contact of the O 2p state with the Ce 4f state. Besides that, the fact that the Cu and Ni electronic states with higher DOS are those of the valence band, associated with the decrease of O 2p DOS, evidences electronic transferring from the support to the metal. A further indicative of this charge transfer is obtained by calculating the charge distribution around the atoms. Figure S6 of the SI shows the asymmetrical distribution of charges around the surface O atoms, directed to the Cu2Ni2 cluster. These evidences are in accordance to the energy shift towards smaller binding energies of some components of the Ce 3d NAP-XPS measurements, which are associated to an O-Ce hybridized state. Table 2 also shows a dependence of the existence of the SMSI effect with the Cu amount in the nanoparticles. The SMSI effect occurs only for those samples with higher Cu concentrations, namely x = 1.0 and x = 0.60, and there is a threshold for the beginning of the SMSI effect. However, in a previous work, Matte et al. studied the reduction process of Cu/CeO2 and Ni/CeO2 monometallic nanoparticles exposed to a H2 atmosphere at 500 °C by XPS, in situ XAS and in situ time-resolved XAS measurements, and the SMSI effect was observed only for the Ni/CeO2 nanoparticles.12 The difference between both works lays on the chemical composition of the nanoparticles surface before the reduction treatment in H2 atmosphere. The nanoparticles studied


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here presented a surface rich in Cu-Cl and Ni-O chemical states, while the nanoparticles studied by Matte et al. presented Cu-O and Ni-Cl rich surfaces.12 These differences may explain the opposite observations, since Cl atoms have a strong influence on the SMSI occurrence as reported earlier.21,64-66 In fact, there are several works in the literature concerning the SMSI effect in Cu-Ni nanoparticles showing different results depending on the specific characteristics of the samples.12,67,68

Table 2. NAP-XPS (Cu 2p3/2 + Ni 2p3/2)/Ce 3d ratio normalized by the differential cross section for incident photon energy of 1250 eV (𝜆IMFP ≈ 8 Å) and 2000 eV (𝜆IMFP ≈ 17 Å) during 1 mbar H2 treatment and 1 mbar CO2 treatment at 500 °C in CuxNi1-x/CeO2 (x = 0.25, 0.35, 0.60, 1.00) nanoparticles. (Cu 2p3/2 + Ni 2p3/2)/Ce 3d normalized ratio H2 atmosphere

CO2 atmosphere

𝜆IMFP ≈ 8 Å

𝜆IMFP ≈ 17 Å

𝜆IMFP ≈ 8 Å

𝜆IMFP ≈ 17 Å

Cu/CeO2 nanoparticles

0.2

0.4

0.3

0.3

Cu0.60Ni0.40/CeO2 nanoparticles

0.3

0.4

0.5

0.4


0.8

0.8

0.7

0.7


2.4

2.2

2.8

2.2

Sample

Figure 10 shows a schematic representation of the atomic phenomena occurring during H2 and CO2 treatment in the CuxNi1-x/CeO2 (0 < x < 1) nanoparticles. The higher the Cu concentration, the higher the probability of occurrence of the SMSI effect, and the stronger the Cu (Ni) segregation to the surface of the nanoparticles during H2 (CO2) treatment.


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Figure 10. Schematic representation of the atomic arrangement and cerium oxide capping layer formation (SMSI effect) during H2 reduction treatment as a function of the Cu concentration in CuxNi1-x/CeO2 (0 < x < 1) nanoparticles. The CO2 atmosphere recovers the surface of the nanoparticles to the state without the capping layer presence.

In the NAP-XPS analysis of the Cu 2p3/2 region during 1 mbar H2 treatment at 500 °C (Table S2 of the SI) an increase in the Cu(II)-O fraction for increasing probed depths in the bimetallic nanoparticles was observed. This can also be explained considering the SMSI effect. The effect should be understood as a statistical process that happens significantly for Cu amounts higher than a given threshold value. Then, when probing the surface region of the nanoparticles it is being probed the surface region of the clean surface of bimetallic nanoparticles and the capping layer of the bimetallic nanoparticles that suffered the SMSI effect. When probing a higher depth it is being probed the inner region of the clean surface of bimetallic nanoparticles but also the surface region of bimetallic nanoparticles covered by the capping layer, then probing oxidized


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states. The change of Cu(II)-O fraction with the probing depth depends on the mean shell thickness value of the capping layer, which may depend on the bimetallic nanoparticles nature. The higher surface fraction of Cu(II)-O and Ni(II)-O oxidized states for higher amounts of Cu concentration during H2 reduction treatment (Table S2 of the SI) can now be also explained considering the SMSI effect. The higher Cu amount in bimetallic nanoparticles gives a more evident SMSI effect which induces the existence of Cu(II)-O and Ni(II)-O oxidized states, due to the interaction of Cu and Ni atoms with the CeO2-x capping layer. However, the monometallic Cu/CeO2 nanoparticles present no Cu(II)-O component although the SMSI effect is also detected for this system, which should be related to the different interaction of the capping layers with Cu atoms in this case, that happens via Ce(III) entities (Ce3d10O2p6Ce4f1 initial state). In fact, the surface sample with the smaller Cu(II)-O fraction presents the higher Ce(III) fraction (SMSI effect occurring through Ce(III) entities - Ce3d10O2p6Ce4f1initial state) and the surface sample with the higher Cu(II)-O fraction presents the smaller Ce(III) fraction (SMSI effect occurring through Ce(IV) entities - Ce3d10O2p6Ce4f0 initial state), as observed in Tables S2 and S4 of the SI. The kinetics of reduction of the CuxNi1-x/CeO2 (0 < x < 1) nanoparticles were studied using in situ time-resolved XANES measurements. The results presented in Figure 6 reveal that the Cu reduction occurs through a well-known two-stage process that begins with the cleavage of O bonds, transforming CuO into Cu2O. Later, it takes place a second stage associated to the transformation of Cu2O into Cu(0).69 When reaching 400 °C, all the samples are essentially metallic already. The reduction mechanism of the bimetallic nanoparticles starts at different temperatures, depending on the Cu amount. The Cu0.60Ni0.40/CeO2 sample is the first bimetallic sample that


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starts to be reduced, with reduction temperature as low as 200 °C. On the other hand, the reduction temperature for the sample with lower Cu concentration is around 300 °C. Therefore, the reduction temperature of Cu atoms is inversely related to the Cu concentration of the bimetallic nanoparticles. However, the same trend cannot be applied for the monometallic case with 100% of Cu atoms which presents the highest reduction temperature. In fact, the Cu/CeO2 nanoparticles present a lower reduction temperature than the non-supported Cu monometallic case. It is important to stress out also that the initial Cu(0) fraction is overall the same for the different bimetallic nanoparticles, so it has no influence. On the other hand, the smaller is the initial CuO fraction the smaller is the reduction temperature, showing that the CuO fraction is an important factor to be considered too. Based on the Cu reduction process described, small amounts of CuO fraction also help in an earlier reduction of Cu atoms. In Figure 6 the samples presenting the SMSI effect (Cu/CeO2 and Cu0.60Ni0.40/CeO2) have a lower reduction temperature as compared to similar samples without the SMSI effect occurrence (Cu0.35Ni0.65/CeO2, Cu0.25Ni0.75/CeO2, and Cu nanoparticles). This may be an indication of the electronic factor of the SMSI effect that occurs at low temperatures and charge transfer from the support to the nanoparticles,70 helping on the reduction process.12 An analogous conclusion was obtained in a previous study for Ni/CeO2 and Cu/CeO2 monometallic nanoparticles where the Cu nanoparticles reduced at around 300 °C, in accordance to the results found in this study.12 Another possibility is the reduction of the nanoparticles induced by the cerium oxide capping layer surrounding the nanoparticles due the interaction between the capping layer and the surface of the nanoparticles. In this case, the CeO2 support would present a small oxidation during the reduction process.


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In fact, an unexpected phenomenon is observed during the reduction process of the Cu/CeO2 and Cu0.60Ni0.40/CeO2 samples. At around 200 °C, there is a decrease of the Ce(III) fraction (as can be seen in Figure 7), even with the sample being exposed to the H2 reduction atmosphere. This partial oxidation from Ce(III) to Ce(IV), more evident for the Cu0.60Ni0.40/CeO2 nanoparticles than for the Cu/CeO2 ones, may evidence the O donation process from the nanoparticles to the capping layer surrounding it, as proposed earlier. The oxidation temperature of the cerium oxide supports matches perfectly with the Cu reduction temperature (Figure 6) and then evidences that the Cu reduction of Cu0.60Ni0.40/CeO2 and Cu/CeO2 nanoparticles is favored by the CeO2-x capping layer surrounding the nanoparticles. It supports the idea of strong interaction between the capping layer and the surface of the nanoparticles. In fact, it can be used to explain the shift in the reduction temperature of Cu atoms observed for these nanoparticles in comparison to those without the SMSI effect (Figure 6). The capping layer acts as a reducing agent of Cu and Ni atoms, then helping on the Cu reduction process and being oxidized. Considering the Ce(III) decrease during reduction process, it is possible to identify the exact moment in which the geometrical factor of the SMSI effect occurs. Thereby, these data show for the first time evidences for the beginning of the occurrence of the geometrical factor of the SMSI effect and offer an interesting method to identify the SMSI effect based on this fingerprint in the time evolution of the Ce(III) fraction.

5. Conclusions In this work, the SMSI effect was studied in great detail for CuxNi1-x/CeO2 (0 < x < 1) nanoparticles. A threshold on the Cu concentration for the occurrence of the SMSI effect under


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H2 atmosphere was determined. Only nanoparticles with high Cu concentrations show the occurrence of the SMSI effect. In these cases, the geometrical factor of the SMSI effect is evidenced and the interaction between the CeO2-x capping layer and the surface of the nanoparticles gives an earlier reduction of Cu atoms, which is associated to an oxidation of Ce atoms during H2 treatment. The interaction occurs through Ce(III) (Ce3d10O2p6Ce4f1) or Ce(IV) (Ce3d10O2p6Ce4f0) entities, depending on the Cu concentration.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publications website. Quantification of the chemical components and Ce(III) fraction by XPS, NAP-XPS, and XANES measurements, NAP-XPS spectra, energy positions of the peaks used in the Ce 3d NAP-XPS fitting, and typical fitting in the XANES measurement at Cu K edge. (docx)

AUTHOR INFORMATION Corresponding Author * Fabiano Bernardi, e-mail: [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.


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ACKNOWLEDGMENT The authors thank the ALBA, LNLS, LNNano, CESUP-UFRGS, and CMM-UFRGS staff for their assistance, DIMAT/NULAM for the use of Electron Microscopy facility at INMETRO, and Prof. Jairton Dupont (IQ-UFRGS) for providing the CuxNi1-x samples used in this work. W. T. F., G. B. D. M, D. L. B., and F.B. thank CNPq for the research grant. The authors also thank to CAPES.

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Understanding the Strong Metal-Support Interaction (SMSI) Effect in

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