Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts

Article pubs.acs.org/JPCC

Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts: Principal Differences in Stability and Reactivity Armin Neitzel,† Alberto Figueroba,‡ Yaroslava Lykhach,*,† Tomás ̌ Skála,§ Mykhailo Vorokhta,§ Nataliya Tsud,§ Sascha Mehl,† Klára Ševčíková,§ Kevin C. Prince,∥ Konstantin M. Neyman,‡,⊥ Vladimír Matolín,§ and Jörg Libuda†,# †

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Departament de Ciència de Materials i Química Física and Institut de Quimica Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1, 08028 Barcelona, Spain § Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic ∥ Elettra-Sincrotrone Trieste SCpA and IOM, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy ⊥ Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain # Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: We have investigated the stability and the reactivity of atomically dispersed Pt, Pd, and Ni species on nanostructured CeO2 films by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. All three metals reveal specific similarities associated with the high adsorption energy of atomically dispersed Pt2+, Pd2+, and Ni2+ species that exceeds the corresponding cohesive energies of the bulk metals. The corresponding Pt−CeO2, Pd− CeO2, and Ni−CeO2 model catalysts have been prepared in the form of thin films on CeO2(111)/Cu(111) substrates and investigated experimentally under ultrahigh vacuum conditions. The atomically dispersed Pt2+, Pd2+, and Ni2+ species were formed exclusively at low concentrations of the corresponding metals. High concentrations resulted in the presence of additional metal oxide phases and emergence of metallic particles. We found that under the employed experimental conditions the Pd−CeO2 films closely resemble the Pt−CeO2 system with respect to the redox behavior upon reaction with hydrogen. Unlike Pt−CeO2, the Pd−CeO2 system shows a strong tendency to stabilize Pd2+ not only at the surface but also in the ceria bulk. In sharp contrast to both Pt−CeO2 and Pd−CeO2, the Ni−CeO2 system does not exhibit the redox functionality required for hydrogen activation due to the remarkably high stability of Ni2+ species.

1. INTRODUCTION

be achieved if atomically dispersed Pt is stabilized on the surface of the catalytic material.8 We found that dispersed Pt2+ species can be anchored on nanostructured cerium oxide, specifically at {100} nanofacets.8,10 The remarkably strong adsorption of Pt2+ cations at these sites results from specific coordination, with Pt2+ residing in the center of a planar quadratic arrangement of four surface oxygen ions, i.e. a square oxygen pocket. In comparison with commercial Pt−Ru catalyst, Pt−CeO2 anode catalyst yielded 103 times higher specific power values (i.e., power density per Pt weight).8,11

Fuel cells (FCs) are considered the next generation power sources for portable, mobile, and stationary applications.1−3 For proton exchange membrane (PEM) FC technology, platinum is the essential catalytic element at both the anode and the cathode sides. However, the high cost of platinum is the main factor limiting large-scale application of fuel cell technology.4 Therefore, great efforts are dedicated to the development of catalytic materials for PEM FCs without or with very low demand for the noble metal.5−8 The key to high noble metal efficiency is to maintain the stability and high dispersion of the noble metal during operation, especially under dynamic operation conditions.9 In this respect, we have recently demonstrated that the maximum noble metal efficiency can © XXXX American Chemical Society

Received: March 3, 2016 Revised: April 11, 2016

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DOI: 10.1021/acs.jpcc.6b02264 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Following the idea that the coordination of Pt2+ in the square oxygen pockets is the reason for the high stability of the Pt− CeO2 films, we assume that the same concept could be applied to other transition metals as well. In particular, the replacement of Pt with less-expensive metals would be of great interest, provided that those could also be stabilized in a similar coordination environment. Here, the other group 10 metals, Pd and Ni, are of special interest. In the oxidation state II they constitute d8 systems that form stable complexes with planar quadratic coordination. Cerium oxide doped with both Pd and Ni has been considered for various applications in catalysis.12−21 In many cases, the active sites have been identified as Pd2+ and Ni2+ ions,19,21 and in some cases it was reported that Pd−CeO2 has similar or even higher catalytic activity than Pt− CeO2 films.12,13 It turned out that the active Pd2+ site is characterized by the specific coordination of Pd2+ in the fcc site of the cerium oxide lattice.18 This structure is very similar to the coordination of Pt2+ in the square oxygen pocket sites on Pt− CeO2.8 In the present study, we investigate the stability of Pd−CeO2 and Ni−CeO2 films in ultrahigh vacuum (UHV) and under hydrogen exposure by means of synchrotron radiation photoelectron spectroscopy (SRPES) and resonant photoemission spectroscopy (RPES). We compare the experimental findings to the energetics and stabilities of atomically dispersed Pt, Pd, and Ni ions predicted by density functional (DF) calculations. In combination with our previous studies on Pt− CeO2, the detailed comparison with Pd−CeO2 and Ni−CeO2 systems provides a consistent picture of the stability and reactivity of ceria-based catalysts with atomically dispersed metals. We identify similarities and differences between the three metals that may help to develop new, more cost-efficient catalytic materials for fuel cell technology and other applications.

2.2. Experimental Methods. High-resolution SRPES and RPES were performed at the Materials Science Beamline (MSB), Elettra Synchrotron Light Facility in Trieste, Italy. With a bending magnet source, the MSB provides synchrotron light in the energy range of 21−1000 eV. The UHV end-station (base pressure 1 × 10−10 mbar) is equipped with a multichannel electron energy analyzer (Specs Phoibos 150), a rear view low energy electron diffraction (LEED) optics, a sputter gun (Ar), and a gas inlet system. The basic setup of the chamber includes a dual Mg/Al X-ray source. The model Pt−CeO2, Pd−CeO2, and Ni−CeO2 mixed oxides were deposited at 110 K onto a well-ordered 1.5 nm thick CeO2(111) buffer layer grown on Cu(111). The preparation procedure follows the recipe developed earlier for the Pt−CeO2 mixed oxide films.8 First, Cu(111) (MaTecK GmbH, 99.999%) was cleaned by several cycles of Ar+ sputtering (300 K, 60 min; Siad Gas Campione, 99.9999%) and annealing (723 K, 5 min) until no traces of carbon or any other contaminant were found in the photoelectron spectra. Then, a well-ordered CeO2(111) buffer layer was deposited onto the clean Cu(111) substrate by physical vapor deposition (PVD) of Ce metal (Goodfellow, 99.99%) in an oxygen atmosphere (pO2 = 5 × 10−5 Pa, Siad Gas Campione, 99.999%) at 523 K. This preparation procedure30 yielded a continuous, stoichiometric CeO2(111) film with a thickness of 1.5 nm as determined from the attenuation of the Cu 2p3/2 intensity. Finally, Pt−CeO2, Pd−CeO2, Ni−CeO2 films were prepared by means of simultaneous PVD of Ce and the corresponding metal, i.e., Pt (Goodfellow, 99.98%), Pd (Goodfellow, 99.98%), or Ni (Goodfellow, 99.98%), in an oxygen atmosphere (pO2 = 5 × 10−5 Pa) onto the CeO2(111)/ Cu(111) buffer layer at 110 K. The concentrations of Pt, Pd, and Ni per volume of the mixed oxide film were determined from the deposition rates of the corresponding metals and Ce. Accordingly, three types of model systems with different concentrations of dopant metals were prepared: Pt−CeO2 (5%, 12%, 22%), Pd−CeO2 (7%, 16%, 20%), and Ni−CeO2 (13%, 42%). The nominal thicknesses of the deposited Pt−CeO2, Pd−CeO2, Ni−CeO2 mixed oxide films were 0.3 and 1.5 nm (data obtained with 1.5 nm thick films are discussed in the Supporting Information). During the course of the experiment, the Pt−CeO2, Pd− CeO2, Ni−CeO2 films were annealed at different temperatures between 110 and 750 K in UHV. In addition, the reaction of Pt−CeO2, Pd−CeO2, and Ni−CeO2 films with hydrogen was studied by exposing the samples to 50 L of H2 (2.67 × 10−5 Pa, 250 s) at different temperatures. Prior to the reaction with hydrogen, the samples were briefly annealed at 700 K (Pt− CeO2, Ni−CeO2) or 600 K (Pd−CeO2). Hydrogen (H2) (Linde, 99.999%) was dosed by backfilling the UHV chamber. During the experiment, the sample temperature was controlled by a dc power supply passing a current through Ta wires holding the sample. Temperatures were monitored by a K-type thermocouple attached to the back of the sample. The Ni 2p, O 1s, Pd 3d, and Pt 4f spectra were acquired with photon energies of 1000, 650, 405, and 180 eV, respectively. The binding energies in the spectra acquired with synchrotron radiation were calibrated with respect to the Fermi level. Additionally, Al Kα radiation (1486.6 eV) was used to measure O 1s, C 1s, Ce 3d, Pt 4f, Pd 3d, Ni 2p, and Cu 2p3/2 core levels. The spectra were acquired at a constant pass energy and at an emission angle for the photoelectrons of 20° and 60°, or 0°

2. MATERIALS AND METHODS 2.1. Density Functional Computations. Spin-polarized calculations22 have been performed using a plane-wave densityfunctional method with the PW91 exchange−correlation functional 23 of a generalized-gradient (GGA) type as implemented in the code VASP.24 A consistent description of stoichiometric and partially reduced ceria can be achieved within the so-called GGA+U approach, in which the on-site Coulombic interaction for Ce 4f electrons is included.25,26 We applied a GGA+U scheme27 with U = 4 eV.25 The effect of core electrons on valence electron density was taken into account via the projector-augmented wave method.28 Plane-wave basis functions with kinetic energy up to 415 eV were used. The cerium oxide support was modeled as a cuboctahedral Ce40O80 nanoparticle (NP), which retains the characteristic cubic fluorite type crystal structure (Fm3m) of CeO2 bulk and exposes small O-terminated {111} and {100} facets.25,29 The choice of this CeO2 NP is justified by the weak dependence of the adsorption energy of Pt2+ species on the size of the NPs bigger than Ce40O80.8 This allowed us to model the experimental Pt−CeO2 system consisting of particles 3 nm in size8 with smaller Ce40O80 NP models (about 1.5 nm). The size of the unit cells was large enough to ensure at least 1 nm separation between atoms in neighboring unit cells. Calculations of the finite models included only the Γ-point. Geometry optimization of all atoms in the NP models has been carried out. More computational details can be found elsewhere.8,22,25,29 B


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Figure 1. Adsorption energies (kJ/mol) of (a) Pt2+, (b) Pd2+, and (c) Ni2+ species (with respect to free neutral atoms) in square planar coordination on the Ce40O80 nanoparticles.22 Yellow, brown, and red spheres represent Ce4+ cations, Ce3+ cations, and O2− anions, respectively.

study, we have shown that Pt atoms interact very strongly with the small {100} nanofacets.8 The four oxygen atoms bind to Pt, adopting a square-planar coordination with Pt−O distances of 205 pm (see Figure 1a). Two electrons are donated by Pt atoms, giving rise to the formation of a Pt2+ cation and two Ce3+ centers in the Ce40O80 NP. The specific square-planar coordination of Pt2+ sites at the {100} nanofacets of nanostructured cerium oxide8 resulted in a very high adsorption energy of −678 kJ/mol that substantially exceeded the cohesive energy of Pt metal (−564 kJ/mol).35 We assembled similar structures by anchoring Pd (Figure 1b) and Ni (Figure 1c) atoms at the same {100} sites of the Ce40O80 NP. The interaction with these adsorption sites yielded Pd2+ and Ni2+ states. Similarly to the case of Pt, the transfer of two electrons from atomic Pd and Ni to the support resulted in the reduction of two Ce4+ ions to Ce3+. These Ce3+ ions are localized in corner positions of the NP, which are the most prone to reduction due to their low coordination.29 The comparison of the structural parameters revealed almost identical Pt−O and Pd−O distances (205 pm), while Ni−O distances were shorter (189 pm) due to the smaller size of Ni2+ ions. Similar to Pt2+ species, the calculated adsorption energies of Pd2+ (−504 kJ/mol) and Ni2+ (−678 kJ/mol) are larger in magnitude than the corresponding bulk cohesive energies of Pd (−376 kJ/mol)35 and Ni (−428 kJ/mol)35 metals. The high stability of all three metal atoms in a square-planar coordination at {100} surface sites of the Ce40O80 NP is typical for the situation in classical coordination compounds of transition metals, with the difference being that here the entire NP acts as a polydentate ligand.36 It is interesting to analyze the trends in energetic stabilities of the three adsorbed metal atoms in the light of the differences between the bulk metal cohesive energy and the heat of formation of the metal oxides. The respective data are summarized in Table 1.

with respect to the sample normal, while using the X-ray source or synchrotron radiation, respectively. The total spectral resolutions were 1 eV (Al Kα), 200 meV (hν = 115−180 eV), 410 meV (hν = 405 eV), and 650 meV (hν = 650 eV). All spectral components were fitted with a Voigt profile. A Shirley background (Pd 3d) and a linear background (Ni 2p3/2) were subtracted from the spectra before fitting. A combination of a linear and Shirley background was subtracted from the Pt 4f spectra. The broad Cu 3p doublet which evolves during the coarsening of the Pt−CeO2 mixed oxide thin films upon annealing in UHV was subtracted from the Pt 4f spectra for clarity. The Pt 4f spectra were fitted with a Voigt doublet profile with a spin−orbit splitting of 3.3 eV and a fixed branching ratio of 1.33. Pd 3d spectra were fitted with a Voigt doublet profile with a spin−orbit splitting of 5.3 eV. Due to the arching background, spin−orbit branching ratios of the Pd 3d were fixed to 2 and 1.7 for the doublets associated with Pd in the solid solution and the PdO particles and metallic Pd, respectively. Integrated intensities were recalculated with respect to a statistical branching ratio of 1.5. Valence band spectra were acquired at three different photon energies, 121.4, 124.8, and 115.0 eV, that correspond to the resonant enhancements in Ce3+ and Ce4+ ions and to offresonance condition, respectively. Analysis of the spectra obtained with these photon energies forms the basis of RPES.31,32 The Ce3+ resonance at a photon energy of 121.4 eV is caused by a super Coster−Kronig decay involving electron emission from Ce 4f states located about 1.4 eV below the Fermi edge. The Ce4+ resonance at a photon energy of 124.8 eV involves emission of O 2p electrons (hybridized with Ce states) from the valence band around 4.0 eV. The valence band spectrum measured at a photon energy of 115 eV is used as a background for the calculation of the intensity difference of the feature’s on- and off-resonance, denoted as the resonant enhancements for Ce3+ [D(Ce3+)] and for Ce4+ [D(Ce4+)], respectively. The resonant enhancement ratio (RER), calculated as D(Ce3+)/D(Ce4+), is the direct measure of the degree of reduction of cerium oxide. Additionally, we determined the relative concentration of Ce3+ ions at the surface based on the calibration curve published earlier.33 All SRPES data were processed using KolXPD fitting software.34

Table 1. Adsorption Energies of Pt2+, Pd2+, and Ni2+ at the Square Oxygen Pocket Site on the Ce40O80 Nanoparticle Compared to the Bulk Cohesive Energies of Pt, Pd, and Ni and the Heats of Formation of the Most Stable Metal Oxides PtO2, PdO, and NiO

3. RESULTS AND DISCUSSION 3.1. Stability of Pt, Pd, and Ni Atoms on CeO2 Predicted by Density Functional Calculations. We start by addressing the relative stability of atomically dispersed Pt, Pd, and Ni atoms on ceria NPs by DF calculations.22 To this end, we follow the strategy applied for Pt adsorption on nanostructured CeO2.8 Briefly, the oxide support is modeled using a cuboctahedral Ce40O80 NP, which exposes Oterminated {111} and small {100} nanofacets.8 In our recent

energies, kJ/mol adsorption energy at square oxygen pocket site on Ce40O80 bulk cohesive energy oxide heat of formation

Pt

Pd

Ni

−678a

−504f

−678f

−564b −80c (PtO2)g

−376b −118.6d (PdO)

−428b −244.3e (NiO)

a

From ref 8. bFrom ref 35. cFrom ref 37. dFrom ref 38. eFrom ref 39. From ref 22. gDue to the lack of stability of PtO, the heat of formation is given for PtO2.

f

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Figure 2. Stability of 12% Pt−CeO2 (a−c), 7% Pd−CeO2 (d−f), and 13% Ni−CeO2 (g−i) upon annealing in UHV. Pt 4f (a), Pd 3d (d), and Ni 2p3/2 (g) spectra obtained with hν =180, 410, and 1000 eV, respectively. The integrated intensities of the surface species (b, e, h) and RER (c, f, i) on 12% Pt−CeO2 (b, c), 7% Pd−CeO2 (e, f), and 13% Ni−CeO2 (h, i) as a function of temperature. The data in (a, b) are adapted from ref 8 by permission (Copyright 2014 John Wiley & Sons, Inc.).

the situation is somewhat different. Whereas the adsorption energy at the square oxygen pocket sites is identical to that of Pt (−678 kJ/mol), the bulk cohesive energy value of metallic Ni (−428 kJ/mol)35 is much lower than that of Pt (−564 kJ/ mol).35 We, therefore, expect the formation of metallic Ni particles from the adsorbed Ni2+ species to be strongly disfavored. In addition, the larger heat of formation for NiO in comparison to that for PtO2 and PdO (see Table 1) should generally favor the formation of oxidic Ni phases even on strongly reduced catalyst films. 3.2. Stability of Pt−CeO2, Pd−CeO2, and Ni−CeO2 Mixed Oxides: Low Dopant Concentration. In order to experimentally verify the above predictions obtained from theory, we have investigated the stability of Pt−CeO2, Pd− CeO2, and Ni−CeO2 thin films at different concentrations of

For all three metals the adsorption energies of the atomic species exceed in magnitude the bulk cohesive energies. This implies that, once anchored at the square oxygen pocket sites, all metal atoms are stable with respect to the formation of metallic NPs. Upon reduction of the CeO2 NP, the adsorption energy value at the square oxygen pocket sites decreases and the formation of metallic NP may eventually become favorable.8,40 In this respect it is noteworthy that the calculated adsorption energy value for Pd at the square oxygen pocket sites is 174 kJ/mol lower than that for Pt (see Table 1). At a first glance we may, therefore, expect a weaker stabilization of ionic Pd. However, this difference is compensated by a similar difference of about 190 kJ/mol in the bulk cohesive energies of Pt and Pd. Thus, we expect the atomically dispersed Pt and Pd to show similar stability with respect to agglomeration. For Ni, D


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difference between the two metals is the lower thermal stability of Pd2+ sites compared to Pt2+. We associate this difference with the fact that Pt2+ is preferentially stabilized at the square oxygen pocket sites at the surface, whereas Pd can be stabilized both at the surface and in the form of a solid solution in the bulk.18 In contrast to the Pt−CeO2,8 the Pd−CeO2 films maintain a considerably higher degree of reduction at even lower dopant concentration. The corresponding RER is plotted in Figure 2f. The higher degree of reduction of the Pd−CeO2 films is likely caused by the formation of the PdO phase. Strong redox interaction between Pd and cerium oxide has been reported earlier.19,31,46 The charge transfer between Pd and Ce4+ is accompanied by the withdrawal of lattice oxygen and growth of PdO particles. Consequently, the decomposition of PdO during annealing results in reoxidation of Ce3+. Finally, we turn to the Ni−CeO2 film. Ni 2p3/2 spectra obtained from 13% Ni−CeO2 are shown in Figure 2g. The main peak at 855.0−855.3 eV is associated with Ni2+ ions. Additionally, two satellite features I and II emerge at 857.0 and 861.1 eV, respectively. Satellite I arises from the nonlocal screening effects47 and, therefore, is sensitive to the degree of Ni dispersion48 in CeO2. In particular, the low intensity of the satellite I is consistent with high Ni dispersion in CeO2 film. Satellite II emerges due to a shakeup process associated with the O 2p → Ni 3p charge-transfer multielectron excitation during ionization of Ni 2p core level.47 Annealing of Ni−CeO2 film results in a decrease of both satellites I and II, which is consistent with enhanced diffusion of Ni2+ toward the bulk. Noteworthy, the Ni−CeO2 films retain a low degree of reduction, similarly to that detected in the Pt−CeO2 mixed oxide.8 Unlike on Pd−CeO2, we identified only one phase associated with Ni2+ in CeO2. According to Barrio et al.,21 however, formation of Ni−CeO2 solid solution is often accompanied by a segregated NiO phase. The authors reported that the solubility limit of Ni in CeO2 is 12%. The presence of satellites I and II likely indicates the existence of a small amount of a NiO phase on 13% Ni−CeO2 film. The disappearance of the satellites I and II from the Ni 2p spectra suggests diffusion of Ni2+ into the substrate and, most likely, decomposition of the NiO phase during annealing in UHV. This observation suggests that a single phase associated with a solid solution of Ni2+ is formed for 13% Ni−CeO2 above 600 K. It could be speculated that the as-prepared 13% Ni−CeO2 film may contain a small amount of Ni3+. The spectral contribution from Ni3+ overlaps with the satellite I and therefore cannot be ruled out.49 We assume that in the presence of Ni3+ its reduction to Ni2+, as indicated by the decrease of the intensity of satellite I, should be accompanied by the decrease of the RER. We observe, however, that the RER decreases in the region where the intensity of the satellite I is still constant (compare parts h and i of Figure 2). This observation suggests that the decrease of the RER is not related to the intensity of satellite I. Therefore, we believe that the intensity of satellite I does not contain any contribution from Ni3+; i.e., the 13% Ni−CeO2 film does not contain Ni3+. The decrease of the RER below 200 K is most likely caused by desorption of traces of water accumulated on the surface at low temperature. Earlier we found that water adsorption on CeO2(111) leads to slight changes in the RER.50 To summarize, in the limit of low concentrations, Pt2+, Pd2+, and Ni2+ are stabilized in atomically dispersed form on the surface of CeO2. The high stability of the ionic species is in agreement with our DF calculations. However, the thermal

the group 10 metals. Three samples were prepared by deposition of 0.3 nm thick Pt−CeO2, Pd−CeO2, and Ni− CeO2 films onto 1.5 nm buffer layer of CeO2(111) on Cu(111) (see section 2.2 for details). Earlier, we investigated the structure of Pt−CeO2 film in great detail.8 Briefly, the Pt−CeO2 film consists of CeO2 nanoparticles, grown on the CeO2(111) buffer layer, with Pt atoms anchored at the {100} facets in the form of Pt2+ species. We start considering the ceria films with low Pt, Pd, and Ni concentrations. The Pt 4f, Pd 3d, and Ni 2p3/2 core level spectra obtained from 12% Pt−CeO2, 7% Pd−CeO2, and 13% Ni−CeO2 films deposited at 110 K, respectively, are shown in Figure 2. Two spin−orbit doublets associated with Pt2+ and Pt4+ cations emerge in the Pt 4f spectra at 73.0 (4f7/2) and 74.0 eV (4f7/2), respectively (see Figure 2a). Earlier, we associated the peak at 73.0 eV with Pt2+ in square oxygen pocket sites at the surface of the ceria film.8,41 The peak at 74.0 eV has been assigned to Pt4+ species in the bulk of the film.42 The integrated intensities of the spectral contributions from Pt2+ and Pt4+ species are plotted in Figure 2b as a function of temperature. We note that below 350 K the Pt2+ intensity increases upon annealing at the expense of Pt4+. This behavior results from the low stability of the Pt4+ species. Migration of Pt4+ species toward the surface is accompanied by its reduction to Pt2+, leading to a slight decrease of the Ce3+ concentration in the film (see Figure 2c). During the subsequent annealing, the Pt 4f intensity associated with the Pt2+ species remains constant up to 750 K. This high stability is in line with the calculated high adsorption energy of Pt2+ species in the square oxygen pocket sites (see Table 1). For the Pd−CeO2 film, two spin−orbit split doublets emerge in the Pd 3d spectra at 338.0 eV (3d5/2) and at 336.4 eV (3d5/2) (see Figure 2d). According to the binding energies of these peaks, we assign both species to Pd2+ in two different chemical environments, namely, in a Pd−CeO2 solid solution18 and in PdO particles.43 According to Gulyaev et al.,18 the Pd−CeO2 solid solution is characterized by a specific structure element: Pd2+ in a face-centered cubic site of the CeO2 lattice. The corresponding arrangement of Pd2+ species on nanostructured CeO2 is similar to that identified on Pt−CeO2 films.8 The integrated intensities of the spectral contributions from Pd2+ and PdO species are plotted in Figure 2e as a function of temperature. With increasing temperature, the PdO signal decreases steadily. We note that, unlike Pt2+, the concentration of Pd2+ ions in the solid solution does not increase at the expense of PdO. This implies that decomposition of PdO occurs due to diffusion of Pd ions into the bulk. As a result, the surface concentration of Pd2+ in the solid solution remains constant up to 600 K. Above 600 K, rapid diffusion of Pd2+ into the bulk in the absence of PdO leads to complete disappearance of Pd from the surface region at temperatures below 700 K (see Figure 2d). Diffusion of Pd into the bulk was verified by means of angle-resolved XPS. In particular, we found that the migration of Pd2+ into the bulk is accompanied by the formation of a Pd−Cu alloy at the interface between the Cu(111) substrate and CeO2 buffer layer (see Supporting Information, Figure S1). It could be speculated that the kinetic barrier associated with the size of the metal ions may prevent the diffusion of Pt2+ species into the bulk. However, compared to the ionic radius of Pt2+ (80 pm),44,45 the ionic radius of Pd2+ in the same coordination environment is similar (80 pm)44 or even larger (86 pm).45 Therefore, we conclude that the characteristic E


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Figure 3. Stability of 22% Pt−CeO2 (a−c), 20% Pd−CeO2 (d−f), and 42% Ni−CeO2 (g−i) upon annealing in UHV. Pt 4f (a), Pd 3d (d), and Ni 2p3/2 (g) spectra obtained with hν = 180, 410, and 1000 eV, respectively. The integrated intensities of the surface species (b, e, h) and RER (c, f, i) on 22% Pt−CeO2 (b, c), 20% Pd−CeO2 (e, f), and 42% Ni−CeO2 (h, i) as a function of temperature.

stabilities of the Pd−CeO2 and Ni−CeO2 films are compromised by the diffusion of Pd2+ and Ni2+ into the bulk upon annealing above 600 K. This differs from the thermal behavior of the Pt−CeO2 system, which is stable under similar conditions. Additionally, we address the stability of 1.5 nm thick 12% Pt−CeO2, 7% Pd−CeO2, and 13% Ni−CeO2 films deposited on a 1.5 nm buffer layer of CeO2(111) on Cu(111) in the Supporting Information (see Figure S2). Briefly, we found that the thickness of the mixed oxide films has a significant impact on the stabilities of Pt−CeO2 and Pd−CeO2 films prepared with low dopant concentrations. In particular, we observed the partial reduction of Pt2+ and Pd2+ species accompanied by formation of metallic Pt0 and Pd0 particles upon annealing in UHV. 3.3. Stability of Pt−CeO2, Pd−CeO2, and Ni−CeO2 Mixed Oxides: High Dopant Concentration. In the next step, we investigate the stability of the group 10 metals on CeO2 films at high dopant concentrations. The corresponding

Pt 4f, Pd 3d, and Ni 2p3/2 spectra obtained from 0.3 nm thick 22% Pt−CeO2, 20% Pd−CeO2, and 42% Ni−CeO2 films deposited onto 1.5 nm thick CeO2(111) buffer layers are shown in Figure 3. The spectral features observed on the as-prepared 22% Pt− CeO2 film are similar to those identified on the 12% Pt−CeO2 film (see section 3.2). Accordingly, we assign the two peaks in the Pt 4f spectra to Pt2+ and Pt4+, respectively (Figure 3a). The Pt4+ species are converted into Pt2+ upon annealing at temperatures below 350 K (see Figure 3b). In comparison to the 12% Pt−CeO2, the Pt2+ ions in the 22% Pt−CeO2 film show much lower stability, however. Annealing of the film above 300 K already triggers loss of Pt2+. Simultaneously, two new components emerge in the Pt 4f spectra at 72.0 (4f7/2) and 71.0 eV (4f7/2). According to their binding energy, we assign the peak at 71.0 eV to metallic Pt0. The emergence and growth of this peak indicate formation of metallic Pt particles. Reduction of Pt2+ to Pt0 is most likely also associated with F


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Figure 4. Reactivity of 5% Pt−CeO2 (a−c), 7% Pd−CeO2 (d−f), and 13% Ni−CeO2 (g−i) films toward hydrogen activation. The samples were briefly annealed at 700 K (5% Pt−CeO2, 13% Ni−CeO2) or 600 K (7% Pd−CeO2). Pt 4f (a), Pd 3d (d), and Ni 2p3/2 (g) spectra obtained with hν = 180, 410, and 1000 eV, respectively. The integrated intensities of the surface species (b, e, h) and RER (c, f, i) on 5% Pt−CeO2 (b, c), 7% Pd−CeO2 (e, f), and 13% Ni−CeO2 (h, i) following exposure to 50 L of H2 as a function of temperature. The data points obtained prior to the hydrogen exposure are circled. Data in parts a−c are reproduced from ref 40 with permission (Copyright 2016 Royal Society of Chemistry).

Also for the Pd−CeO2 film we find a qualitatively similar behavior at 20% Pd compared to 7% Pd (see section 3.2). The formation of a Pd−CeO2 solid solution is again accompanied by the formation of PdO. However, we detected a much higher amount of PdO on the 20% Pd−CeO2 (Figure 3d,e) along with a considerably higher concentration of Ce3+ (see Figure 3f). Stepwise annealing in UHV results in a decrease of Pd2+ concentration in the solid solution between 110 and 600 K until it vanished at 650 K. Similar to the 22% Pt−CeO2 film, the decrease of Pd2+ concentration gives rise to an increase of the PdO amount prior to the emergence of the metallic Pd0 particles at 250 K. Following reduction of Pd 2+ and decomposition of PdO, formation of metallic Pd particles is accompanied by reoxidation of ceria, as indicated by a decreasing RER (Figure 3f). Above 600 K, the signal from metallic Pd finally decreases, which may be due to sintering or loss of Pd by diffusion of Pd2+ through the ceria film. Finally, we turn to the Ni−CeO 2 film at high Ni concentration. For the 42% Ni−CeO2 film, we again find the

the emergence of intermediate species labeled Pt* that give rise to the peak at 72.0 eV. The nature of this intermediate species is not straightforward. According to the binding energy, the feature could be associated either with small Pt particles partially oxidized due to the electronic metal−support interaction 33 or PtO x species.51−53 For instance, PtOx species have been previously observed upon reversible changing of the Pt oxidation state in ceria-based systems52 and upon redispersion of Pt particles supported on well-defined CeO2(100) nanostructures.53 Emergence of this component prior to the metallic Pt0 may suggest that reduction of Pt2+ involves an intermediate Pt* species. As shown in Figure 3c, reduction of Pt2+ is accompanied by slight changes in the degree of reduction of cerium oxide. Particularly, the emergence of PtOx coincides with an increase of the RER while the emergence of metallic Pt causes a decrease of the RER (this behavior is more prominent on the 1.5 nm thick 22% Pt−CeO2; see Figure S3, Supporting Information). G


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Figure 5. Reactivity of 18% Pt−CeO2 (a−c), 16% Pd−CeO2 (d−f), and 42% Ni−CeO2 (g−i) films toward hydrogen activation. The samples were briefly annealed at 700 K (5% Pt−CeO2, 42% Ni−CeO2) or 600 K (7% Pd−CeO2). Pt 4f (a), Pd 3d (d), and Ni 2p3/2 (g) spectra obtained with hν = 180, 410, and 1000 eV, respectively. The integrated intensities of the surface species (b, e, h) and RER (c, f, i) on 5% Pt−CeO2 (b, c), 16% Pd− CeO2 (e, f), and 42% Ni−CeO2 (h, i) following exposure to 50 L of H2 as a function of temperature. The data points obtained prior to the hydrogen exposure are circled. Data in parts a−c are adapted from ref 40 with permission (Copyright 2016 Royal Society of Chemistry).

same principal components as for the 13% Ni−CeO2 film. However, the intensities of the satellites I and II accompanying the main peak from Ni2+ are somewhat larger on the 42% Ni− CeO2 film (see Figure 3g,h). Stepwise annealing in UHV leads to a slight increase of satellites I and II followed by their decrease above 400 K. Unlike on the 13% Ni−CeO2 film, neither satellite I nor II vanish at 700 K. This may indicate the presence of NiO aggregates at the surface of the Ni−CeO2 solid solution even after annealing to 700 K.21 In sharp contrast to the Pt−CeO2 and Pd−CeO2 films with high Pt (Pd) concentration, we detect no formation of metallic Ni particles on Ni−CeO2. This observation is in excellent agreement with the theoretical prediction suggesting that the formation of metallic Ni from stabilized ionic Ni2+ is strongly disfavored in comparison to the formation of Pd and Pt. Instead, we observe phase separation and the formation of NiO aggregates. We address the stability of 1.5 nm thick 22% Pt−CeO2, 20% Pd−CeO2, and 42% Ni−CeO2 films deposited on a 1.5 nm

buffer layer of CeO2(111) on Cu(111) in the Supporting Information (see Figure S3). Briefly, we found that the thickness of the mixed oxide films with high dopant concentrations does not cause significant differences in the stabilities of the 1.5 nm thick films with respect to 0.3 thick films. The reduction of Pt2+ and Pd2+ species to metallic Pt0 and Pd0 particles upon annealing of Pt−CeO2 and Pd−CeO2 films proceeds according to a similar scenario, as described above. 3.4. Reactivity of Model Pt−CeO2, Pd−CeO2, and Ni− CeO2 Mixed Oxides toward Hydrogen. Finally, we studied the reactivity of the 0.3 nm thick Pd−CeO2 and Ni−CeO2 films deposited onto 1.5 nm thick CeO2(111) buffer layers toward H2 as a function of dopant concentration. The reactivity of the Pt−CeO2 films was investigated earlier.40 All samples were briefly annealed in UHV (see section 2.2) prior to the reaction with hydrogen. The reactivity of the 5% Pt−CeO2, 7% Pd−CeO2, and 13% Ni−CeO2 films was studied under similar conditions. The H


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The Journal of Physical Chemistry C development of the corresponding Pt 4f, Pd 3d, and Ni 2p3/2 spectra is shown in Figure 4. Upon annealing of both 7% Pd− CeO2 and 13% Ni−CeO2 films in hydrogen, we make the same observation as previously reported for the 5% Pt−CeO2.40 Specifically, we find no reduction of Pd2+ and Ni2+ (see Figure 4d,e,g,h) Also, the RER does not indicate any appreciable reduction of cerium cations (Figure 3f,i) on these samples. Therefore, we conclude that similarly to Pt2+ ions,40 the Pd2+ and Ni2+ sites are not reactive toward H2 under similar conditions. At higher levels of metal doping the situation is different. The results of the corresponding experiments are shown in Figure 5. The 0.3 nm thick 18% Pt−CeO2, 16% Pd−CeO2, and 42% Ni− CeO2 films were prepared on a 1.5 nm thick CeO2(111) buffer layer followed by brief annealing to 700 K in UHV. This procedure yielded metallic Pt0 and PtO on 18% Pt−CeO240 and metallic Pd0 and PdO particles on 16% Pd−CeO2 (Figure 5a,d, top spectra). In contrast, no metallic Ni0 was detected on 42% Ni−CeO2 (Figure 5g, top spectrum). All films were exposed to H2 at temperatures between 110 and 700 K. For the Pd−CeO2 and Pt−CeO2 films, we observe a very similar behavior. Both samples are reduced upon H2 exposure at elevated temperature, leading to conversion of the ionic species to metallic Pt0 and Pd0 particles, respectively. For both metals, reduction starts above 350 K (see Figure 5b,e). The reduction is almost complete at 550 K for the Ptdoped samples and already at around 450 K for the Pd-doped sample. As expected, the reaction is accompanied by an increase of the RER (Figure 5c,f), indicating reduction of the Ce4+ cations to Ce3+. Overall, Pd−CeO2 films show a very similar reactivity to Pt− CeO2 at both low and high concentrations of dopant metal. Therefore, we conclude that the mechanism of hydrogen activation on Pd−CeO2 must be similar to that reported for Pt−CeO2 films.40 Briefly, we suggest that H2 is activated on metallic Pt or Pd followed by its reaction with oxygen provided via the reverse spillover from the support. This reaction channel produces water, which desorbs instantly. Oxygen spillover is activated around 400 K32 and leads to the formation of oxygen vacancies and Ce3+ ions. As discussed for the Pt−CeO2 films,40 the vacancies eventually destabilize the ionic Pt2+ or Pd2+ and promote reduction to metallic particles. In contrast to the Pt−CeO2 and Pd−CeO2 films, we did not observe any reactivity toward H2 on the Ni−CeO2 mixed oxide, regardless of the Ni concentration. We assume that the reason for this difference in reactivity is the high stability of Ni2+ in both the 13% and the 42% Ni−CeO2 films. In order to probe the role of metallic Ni in hydrogen activation, we deposited small amounts of metallic Ni onto Ni−CeO2 films in UHV. The corresponding development of Ni 2p3/2 spectra is shown in Figure 6. Deposition of Ni on 42% Ni−CeO2 at 110 K in UHV resulted in an emergence of a new peak at 852.5 eV associated with metallic Ni0 accompanied by a plasmon loss feature at 858.0 eV.54 However, these peaks disappeared completely after annealing of the film to 700 K, indicating quantitative oxidation of Ni0 to Ni2+. Repeated deposition of Ni in UHV at 110 K again yielded Ni0. The subsequent annealing to 700 K resulted in only partial oxidation and a part of the Ni0 remaining. The annealing of film containing metallic Ni0 in hydrogen at 600 K did not yield a detectable change of the oxidation state of Ni. We conclude that a reaction sequence similar to that for Pd− CeO2 or Pt−CeO2 leading to reduction of cationic species is not possible for Ni−CeO2, even in the presence of metallic Ni.

Figure 6. Ni 2p3/2 spectra obtained from 42% Ni−CeO2 film annealed at 700 K (a) after the first deposition of Ni in UHV at 115 K (b) followed by annealing to 700 K in UHV (c), second deposition of Ni in UHV at 115 K (d) followed by annealing to 700 K in UHV (e), and exposure to 50 L of H2 at 600 K (f). The spectra were obtained with the X-ray source (hν = 1486.6 eV) at a photoemission angle of 60°.

Besides the high stability of Ni2+, a reason for the low reactivity could be that annealing of the Ni particles on Ni−CeO2 triggers formation of a NiO capping layer. Such encapsulation of the metallic Ni particles could prevent the dissociation of hydrogen.

4. CONCLUSIONS We have investigated the thermal stability of Pt−CeO2, Pd− CeO2, and Ni−CeO2 films on CeO2(111)/Cu(111) and their reactivity toward molecular hydrogen. Changes in the surface composition and in the oxidation states of Pt, Pd, and Ni were monitored by means of SRPES, and the oxidation state of cerium was monitored by means of RPES. Our results show that Pd-doped and Pt-doped CeO2 films exhibit a similar redox behavior during the reaction with H2. Pronounced differences are found, however, with respect to the thermal behavior of the two systems. Ni-doped CeO2 films behave differently due to the high stability of Ni2+ and its lower propensity to form metallic particles. The experimental findings are consistent with DF calculations on the stability of ionic group 10 metal species on CeO2 NPs. The most important similarities and differences between the three dopant metals (Pt, Pd, Ni) are as follows: (1) The thermal stability of Pt−CeO2, Pd−CeO2, and Ni− CeO2 films strongly depends on the concentration of the dopant (Pt, Pd, Ni). For all metals the oxidation state 2+ (i.e., Pt2+, Pd2+, and Ni2+) is the most stable on the CeO2 films. (2) In the limit of low dopant concentration, a single phase containing stable Pt2+, Pd2+, and Ni2+ can be formed. For Pt− CeO2, less stable Pt4+ species in the as-prepared film is converted to Pt2+ above 300 K. For Pd−CeO2, dispersed Pd2+ in ceria coexists with a PdO phase. In contrast to Pt2+, dispersed I


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The Journal of Physical Chemistry C Pd2+ ions are stabilized both at the surface and in the bulk. For Ni−CeO2, formation of stable Ni2+ in the solid solution is accompanied by a less stable NiO phase. The latter decomposes upon annealing to 600 K in UHV. (3) Characteristic differences are observed for the thermal stability of Pt2+, Pd2+, and Ni2+ species. In contrast to the high thermal stability of atomically dispersed Pt2+, which is associated with anchoring at {100} nanofacets on CeO2 aggregates, both Pd2+ and Ni2+ diffuse into the CeO2 bulk upon annealing above 600 K. (4) At high dopant concentration, the stabilities of Pt−CeO2 and Pd−CeO2 are similar. Both systems show thermally induced reduction of Pt2+ or Pd2+ and formation of metallic particles. Unlike these, Ni2+ in Ni−CeO2 films does not undergo reduction upon annealing. (5) A similarity between Pd−CeO2 and Ni−CeO2 films is the coexistence of PdO and NiO phases along with the stable Pd2+ and Ni2+ in solid solution. For Pd−CeO2 the formation of PdO leads to reduction of CeO2, whereas the degree of reduction for Ni−CeO2 films is always low. (6) The isolated Pt2+, Pd2+, and Ni2+ sites are not active for hydrogen dissociation under the experimental conditions employed. Metallic particles are required for activation of H2. In the presence of metallic particles, both Pt−CeO2 and Pd− CeO2 show similar reactivity. The reaction involves reverse spillover of lattice oxygen and formation of Ce3+ and is accompanied by the reduction of the dispersed Pt2+ and Pd2+ species. Unlike on Pt−CeO2 and Pd−CeO2, no H2 activation occurs on Ni−CeO2, even in the presence of metallic Ni.

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funding from the European Community’s Seventh Framework Programme (FP7/2007−2013) under grant agreement no. 312284.

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(1) Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88, 981−1007. (2) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. A Review of Anode Catalysis in the Direct Methanol Fuel Cell. J. Power Sources 2006, 155, 95−110. (3) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells: Principles, Types, Fuels, and Applications. ChemPhysChem 2000, 1, 162−193. (4) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241−247. (5) Tang, J. M.; Jensen, K.; Waje, M.; Li, W.; Larsen, P.; Pauley, K.; Chen, Z.; Ramesh, P.; Itkis, M. E.; Yan, Y.; et al. High Performance Hydrogen Fuel Cells with Ultralow Pt Loading Carbon Nanotube Thin Film Catalysts. J. Phys. Chem. C 2007, 111, 17901−17904. (6) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324, 71−74. (7) Fiala, R.; Vaclavu, M.; Vorokhta, M.; Khalakhan, I.; Lavková, J.; Potin, V.; Matolínová, I.; Matolín, V. Proton Exchange Membrane Fuel Cell Made of Magnetron Sputtered Pt-CeOx and Pt-Co Thin Film Catalysts. J. Power Sources 2015, 273, 105−109. (8) Bruix, A.; Lykhach, Y.; Matolínová, I.; Neitzel, A.; Skála, T.; Tsud, N.; Vorokhta, M.; Stetsovych, V.; Ševčiková, K.; Mysliveček, J.; et al. Maximum Noble Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum. Angew. Chem., Int. Ed. 2014, 53, 10525− 10530. (9) Lei, M.; Wang, Z. B.; Li, J. S.; Tang, H. L.; Liu, W. J.; Wang, Y. G. CeO2 Nanocubes-Graphene Oxide as Durable and Highly Active Catalyst Support for Proton Exchange Membrane Fuel Cell. Sci. Rep. 2014, 4, 7415. (10) Aleksandrov, H. A.; Neyman, K. M.; Vayssilov, G. N. The Structure and Stability of Reduced and Oxidized Mononuclear Platinum Species on Nanostructured Ceria from Density Functional Modeling. Phys. Chem. Chem. Phys. 2015, 17, 14551−14560. (11) Fiala, R.; Figueroba, A.; Bruix, A.; Vaclavu, M.; Rednyk, A.; Khalakhan, I.; Vorokhta, M.; Lavkova, J.; Illas, F.; Potin, V.; et al. High Efficiency of Pt2+- CeO2 Novel Thin Film Catalyst as Anode for Proton Exchange Membrane Fuel Cells. Appl. Catal., B 2016, DOI: 10.1016/j.apcatb.2016.02.036. (12) Hegde, M. S.; Madras, G.; Patil, K. C. Noble Metal Ionic Catalysts. Acc. Chem. Res. 2009, 42, 704−712. (13) Singh, P.; Hegde, M. S. Sonochemical Synthesis of Thermally Stable Hierarchical Ce1−xMxO2−δ (M = Pt or Pd, 0 ≤ x ≤ 0.10) Nanocrystallites: Redox Properties and Methanol Electro-Oxidation Activity. Cryst. Growth Des. 2010, 10, 2995−3004. (14) Kurnatowska, M.; Kepinski, L.; Mista, W. Structure Evolution of Nanocrystalline Ce1−xPdxO2−y Mixed Oxide in Oxidizing and Reducing Atmosphere: Reduction-Induced Activity in Low-Temperature CO Oxidation. Appl. Catal., B 2012, 117−118, 135−147. (15) Bera, P.; Patil, K. C.; Jayaram, V.; Subbanna, G. N.; Hegde, M. S. Ionic Dispersion of Pt and Pd on CeO2 by Combustion Method: Effect of Metal−Ceria Interaction on Catalytic Activities for NO Reduction and CO and Hydrocarbon Oxidation. J. Catal. 2000, 196, 293−301. (16) Misch, L. M.; Kurzman, J. A.; Derk, A. R.; Kim, Y.-I.; Seshadri, R.; Metiu, H.; McFarland, E. W.; Stucky, G. D. C−H Bond Activation by Pd-Substituted CeO2: Substituted Ions versus Reduced Species. Chem. Mater. 2011, 23, 5432−5439. (17) Meng, L.; Lin, J.-J.; Pu, Z.-Y.; Luo, L.-F.; Jia, A.-P.; Huang, W.X.; Luo, M.-F.; Lu, J.-Q. Identification of Active Sites for CO and CH4

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02264. Additional information about the stability of 1.5 nm thick Pt−CeO2, Pd−CeO2, and Ni−CeO2 films during annealing in UHV (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 9131 8520944. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the European Community (FP7-NMP.2012.1.1-1 project ChipCAT, Reference No. 310191), by the “Deutsche Forschungsgemeinschaft” (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative, by Spanish MINECO (grants CTQ2012-34969 and CTQ201564618-R), by the Generalitat de Catalunya (grants 2014SGR97 and XRQTC), and by the Czech Science Foundation (grant 1506759S). The authors acknowledge support from the COST Action CM1104 “Reducible oxide chemistry, structure and functions”. Computer resources and technical expertise and assistance were provided by the Red Española de Supercomputación. The research leading to these results has received J


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The Journal of Physical Chemistry C Oxidation over PdO/Ce1−xPdxO2−δ Catalysts. Appl. Catal., B 2012, 119−120, 117−122. (18) Gulyaev, R. V.; Kardash, T. Y.; Malykhin, S. E.; Stonkus, O. A.; Ivanova, A. S.; Boronin, A. I. The Local Structure of PdxCe1‑xO2‑x‑d Solid Solutions. Phys. Chem. Chem. Phys. 2014, 16, 13523−13539. (19) Gulyaev, R. V.; Slavinskaya, E. M.; Novopashin, S. A.; Smovzh, D. V.; Zaikovskii, A. V.; Osadchii, D. Y.; Bulavchenko, O. A.; Korenev, S. V.; Boronin, A. I. Highly Active PdCeOx Composite Catalysts for Low-Temperature CO Oxidation, Prepared by Plasma-Arc Synthesis. Appl. Catal., B 2014, 147, 132−143. (20) Boronin, A. I.; Slavinskaya, E. M.; Danilova, I. G.; Gulyaev, R. V.; Amosov, Y. I.; Kuznetsov, P. A.; Polukhina, I. A.; Koscheev, S. V.; Zaikovskii, V. I.; Noskov, A. S. Investigation of Palladium Interaction with Cerium Oxide and Its State in Catalysts for Low-Temperature CO Oxidation. Catal. Today 2009, 144, 201−211. (21) Barrio, L.; Kubacka, A.; Zhou, G.; Estrella, M.; Martinez-Arias, A.; Hanson, J. C.; Fernandez-Garcia, M.; Rodriguez, J. A. Unusual Physical and Chemical Properties of Ni in Ce1‑xNixO2‑y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction. J. Phys. Chem. C 2010, 114, 12689−12697. (22) Figueroba, A.; Kovacs, G.; Bruix, A.; Neyman, K. M. Towards Stable Single-Atom Catalysts: Strong Binding of Atomically Dispersed Transition Metals on the Surface of Nanostructured Ceria. Catal. Sci. Technol. 2016, DOI: 10.1039/C6CY00294C. (23) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. (24) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (25) Migani, A.; Vayssilov, G. N.; Bromley, S. T.; Illas, F.; Neyman, K. M. Greatly Facilitated Oxygen Vacancy Formation in Ceria Nanocrystallites. Chem. Commun. 2010, 46, 5936−5938. (26) Paier, J.; Penschke, C.; Sauer, J. Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment. Chem. Rev. 2013, 113, 3949−3985. (27) Anisimov, V. I.; Aryasetiawan, F.; Lichtenstein, A. I. FirstPrinciples Calculations of the Electronic Structure and Spectra of Strongly Correlated Systems: The LDA+U Method. J. Phys.: Condens. Matter 1997, 9, 767−808. (28) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (29) Migani, A.; Vayssilov, G. N.; Bromley, S. T.; Illas, F.; Neyman, K. M. Dramatic Reduction of the Oxygen Vacancy Formation Energy in Ceria Particles: A Possible Key to Their Remarkable Reactivity at the Nanoscale. J. Mater. Chem. 2010, 20, 10535−10546. (30) Šutara, F.; Cabala, M.; Sedlácě k, L.; Skála, T.; Škoda, M.; Matolín, V.; Prince, K. C.; Cháb, V. Epitaxial Growth of Continuous CeO2(111) Ultra-Thin Films on Cu(111). Thin Solid Films 2008, 516, 6120−6124. (31) Matolin, V.; Matolinova, I.; Sedlacek, L.; Prince, K. C.; Skala, T. A Resonant Photoemission Applied to Cerium Oxide Based Nanocrystals. Nanotechnology 2009, 20, 215706. (32) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skála, T.; Bruix, A.; Illas, F.; Prince, K. C.; et al. Support Nanostructure Boosts Oxygen Transfer to Catalytically Active Platinum Nanoparticles. Nat. Mater. 2011, 10, 310−315. (33) Lykhach, Y.; Kozlov, S. M.; Skála, T.; Tovt, A.; Stetsovych, V.; Tsud, N.; Dvořaḱ , F.; Johánek, V.; Neitzel, A.; Mysliveček, J.; et al. Counting Electrons on Supported Nanoparticles. Nat. Mater. 2016, 15, 284−288. (34) Libra, J. KolXPD: Spectroscopy Data Measurement and Processing; http://www.kolibrik.net/science/kolxpd/ (accessed March 15, 2015). (35) Kittel, C. Introduction to Solid State Physics, 8th ed.; Wiley: New York, 2004. (36) Hu, A. G.; Neyman, K. M.; Staufer, M.; Belling, T.; Gates, B. C.; Rösch, N. A Surface Site as Polydentate Ligand of a Metal Complex:

Density Functional Studies of Rhenium Subcarbonyls Supported on Magnesium Oxide. J. Am. Chem. Soc. 1999, 121, 4522−4523. (37) Nagano, Y. Standard Enthalpy of Formation of Platinum Hydrous Oxide. J. Therm. Anal. Calorim. 2002, 69, 831−839. (38) Warner, J. S. The Free Energy of Formation of Palladium Oxide. J. Electrochem. Soc. 1967, 114, 68−71. (39) Masterton, W. L.; Slowinski, E. J.; Stanitski, C. L. Chemical Principles, 6th ed.; Saunders College Pub.: Philadelphia, PA, 1985; p 946. (40) Lykhach, Y.; Figueroba, A.; Camellone, M. F.; Neitzel, A.; Skála, T.; Negreiros, F. R.; Vorokhta, M.; Tsud, N.; Prince, K. C.; Fabris, S.; et al. Reactivity of Atomically Dispersed Pt2+ Species towards H2: Model Pt−CeO2 Fuel Cell Catalyst. Phys. Chem. Chem. Phys. 2016, 18, 7672−7679. (41) Neitzel, A.; Lykhach, Y.; Skála, T.; Tsud, N.; Vorokhta, M.; Mazur, D.; Prince, K. C.; Matolín, V.; Libuda, J. Surface Sites on Pt− CeO2 Mixed Oxide Catalysts Probed by CO Adsorption: A Synchrotron Radiation Photoelectron Spectroscopy Study. Phys. Chem. Chem. Phys. 2014, 16, 24747−24754. (42) Matolín, V.; Matolínová, I.; Václavů, M.; Khalakhan, I.; Vorokhta, M.; Fiala, R.; Piš, I.; Sofer, Z.; Poltierová-Vejpravová, J.; Mori, T.; et al. Platinum-Doped CeO2 Thin Film Catalysts Prepared by Magnetron Sputtering. Langmuir 2010, 26, 12824−12831. (43) Kibis, L. S.; Stadnichenko, A. I.; Koscheev, S. V.; Zaikovskii, V. I.; Boronin, A. I. Highly Oxidized Palladium Nanoparticles Comprising Pd4+ Species: Spectroscopic and Structural Aspects, Thermal Stability, and Reactivity. J. Phys. Chem. C 2012, 116, 19342−19348. (44) Ahrens, L. H. The Use of Ionization Potentials. Part 1. Ionic Radii of the Elements. Geochim. Cosmochim. Acta 1952, 2, 155−169. (45) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, B25, 925−946. (46) Wilson, E. L.; Grau-Crespo, R.; Pang, C. L.; Cabailh, G.; Chen, Q.; Purton, J. A.; Catlow, C. R. A.; Brown, W. A.; de Leeuw, N. H.; Thornton, G. Redox Behavior of the Model Catalyst Pd/CeO2‑x/ Pt(111). J. Phys. Chem. C 2008, 112, 10918−10922. (47) van Veenendaal, M. A.; Sawatzky, G. A. Nonlocal Screening Effects in 2p X-Ray Photoemission Spectroscopy Core-Level Line Shapes of Transition Metal Compounds. Phys. Rev. Lett. 1993, 70, 2459−2462. (48) Sanz, J. M.; Tyuliev, G. T. An XPS Study of Thin NiO Films Deposited on MgO(100). Surf. Sci. 1996, 367, 196−202. (49) Preda, I.; Mossanek, R. J. O.; Abbate, M.; Alvarez, L.; Méndez, J.; Gutiérrez, A.; Soriano, L. Surface Contributions to the XPS Spectra of Nanostructured NiO Deposited on HOPG. Surf. Sci. 2012, 606, 1426−1430. (50) Lykhach, Y.; Johánek, V.; Aleksandrov, H.; Kozlov, S. M.; Happel, M.; Skála, T.; Petkov, P. S.; Tsud, N.; Vayssilov, G. N.; Prince, K. C.; et al. Water Chemistry on Model Ceria and Pt/Ceria Catalysts. J. Phys. Chem. C 2012, 116, 12103−12113. (51) Miller, D. J.; Ö berg, H.; Kaya, S.; Sanchez Casalongue, H.; Friebel, D.; Anniyev, T.; Ogasawara, H.; Bluhm, H.; Pettersson, L. G. M.; Nilsson, A. Oxidation of Pt(111) under Near-Ambient Conditions. Phys. Rev. Lett. 2011, 107, 195502. (52) Hatanaka, M.; Takahashi, N.; Takahashi, N.; Tanabe, T.; Nagai, Y.; Suda, A.; Shinjoh, H. Reversible Changes in the Pt Oxidation State and Nanostructure on a Ceria-Based Supported Pt. J. Catal. 2009, 266, 182−190. (53) Wu, T.; Pan, X.; Zhang, Y.; Miao, Z.; Zhang, B.; Li, J.; Yang, X. Investigation of the Redispersion of Pt Nanoparticles on Polyhedral Ceria Nanoparticles. J. Phys. Chem. Lett. 2014, 5, 2479−2483. (54) Kim, K. S.; Winograd, N. X-Ray Photoelectron Spectroscopic Studies of Nickel-Oxygen Surfaces Using Oxygen and Argon IonBombardment. Surf. Sci. 1974, 43, 625−643.

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Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts

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