Reactivity of atomically dispersed Pt 2 + species towards H 2 : model Pt – CeO 2 fuel cell catalyst †

The reactivity of atomically dispersed Pt species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt sites are found to be inactive towards H2 dissociation due to high activation energy required for H–H bond scission. Trace amounts of metallic Pt are necessary to initiate


Introduction
2][3] The high demand for noble metals is among the major challenges associated with low-temperature FC technology.[10] The mechanism of H 2 activation on nanostructured CeO 2 containing ionic Pt 2+ , however, remains controversial.2][13] For instance, most of the studies employed Pt-doped CeO 2 materials in the form of solid solutions, Ce 1Àx Pt x O 2Àd , prepared by the solution combustion method. 11,12This preparation method yields oxygen vacancies as the result of charge compensation upon substitution of Ce 4+ ions by Pt 2+ in the Ce 1Àx Pt x O 2Àd solid solution. 12,14In the most stable configuration, the Pt 2+ is displaced from the Ce 4+ site to adopt a square planar arrangement with four lattice oxygen ions and one oxygen vacancy is formed per Pt 2+ site. 15In Ce 1Àx Pt x O 2Àd catalysts, the charge compensating oxygen vacancy is located within the square planar arrangement of Pt 2+ ions forming a Pt-O 3 moiety. 5,12In the corresponding configuration, Pt 2+ has three Pt-O bonds with a length of about 200 pm. 5,12n contrast to the combustion solution method, physical vapor deposition (PVD) and radio frequency magnetron sputtering yield a Pt-O 4 moiety without the oxygen vacancies. 8It was found that Pt 2+ is anchored on the CeO 2 support at {100} nanofacets, which form so-called O 4 nanopockets, abundant at the surface of nanostructured cerium oxide. 8Here, the oxide support serves as a polydentate ligand for Pt 2+ species and their binding can be described similarly to that in classical coordination compounds. 16he formation of the Pt-O 4 moiety is not accompanied by creation of the charge compensating oxygen vacancy.Instead, two Ce 4+ cations are reduced to Ce 3+ per one Pt 2+ site.The corresponding Ce 3+ ions can be re-oxidized to Ce 4+ in an oxygen atmosphere during the catalyst preparation.The exceptionally high stability of the Pt-O 4 moiety 8,17 is likely to be responsible for the excellent durability of the Pt-CeO 2 based anode catalyst even under dynamic operation conditions. 9,10n the present study we provide new insights into the mechanism of hydrogen activation on Pt 2+ -CeO 2 in the absence of oxygen vacancies and identify the active sites for hydrogen dissociation.Our model catalyst approach allows us to isolate Pt 2+ sites as well as to control the amount of metallic Pt 0 in the Pt-CeO 2 films.We employed high-resolution synchrotron photoelectron spectroscopy (SRPES) and resonant photoemission spectroscopy (RPES) to monitor the oxidation states of Pt 2+ and Ce 4+ ions with ultimate surface sensitivity.

Experimental details: SRPES and RPES
High-resolution SRPES experiments were performed at the Materials Science Beamline (MSB), Elettra synchrotron light facility in Trieste, Italy.The MSB, with a bending magnet source provides synchrotron light in the energy range of 21-1000 eV.The ultrahigh vacuum (UHV) end-station (base pressure 1 Â 10 À10 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 Pt-CeO 2 oxide films were prepared on a well-ordered CeO 2 (111) buffer layer grown on a single crystal Cu(111) substrate.Cu(111) (MaTecK GmbH, 99.999%) was cleaned by several cycles of Ar + sputtering (300 K, 60 min) and annealing (723 K, 5 min) until no traces of carbon or any other contaminant were found in the photoelectron spectra.Then, an epitaxial CeO 2 (111) layer was deposited onto the clean Cu(111) substrate by PVD of Ce metal (Goodfellow, 99.99%) in an oxygen atmosphere (p O 2 = 5 Â 10 À5 Pa, Linde, 99.999%) at 523 K.This preparation method 18 yielded a continuous, stoichiometric CeO 2 (111) film with a thickness of 1.5 nm as determined from the attenuation of the Cu 2p 3/2 intensity.In the next step, the mixed Pt-CeO 2 films were prepared by means of simultaneous PVD of Ce and Pt (Goodfellow, 99.99%) metals in an oxygen atmosphere (p O 2 = 5 Â 10 À5 Pa) onto the CeO 2 (111)/Cu(111) at 110 K.The low deposition temperature results in a growth of Pt-CeO 2 nanoparticles on the CeO 2 (111) film. 8he Pt concentration of 5%, 15% and 18% per volume of the mixed layers was achieved by using different Pt/Ce deposition rate ratios.The deposition rate ratios were determined from the ratios of nominal thicknesses of Pt to CeO 2 deposited during the same period of time.All Pt-CeO 2 /CeO 2 (111) were deposited onto a 1.5 nm thick CeO 2 (111) buffer layer at 110 K and maintained a nominal thickness of Pt-CeO 2 film of 0.3 nm.
During the course of experiments, all Pt-CeO 2 /CeO 2 (111) samples were briefly annealed at 700 K followed by cooling to 110 K.This treatment eliminated the less stable Pt 4+ species yielding exclusively Pt 2+ for 5% Pt-CeO 2 film.For 15% and 18% Pt-CeO 2 , Pt 2+ is formed together with metallic Pt 0 particles of different size (see ref. 8 for details).Then, the Pt-CeO 2 / CeO 2 (111) samples were exposed to 50 L of H 2 (2.67 Â 10 À5 Pa, 250 s) at different temperatures between 110 and 700 K. Hydrogen (Linde 99.999%) was dosed by backfilling the UHV chamber.
The Pt 4f spectra were acquired with photon energies of 180 eV.The binding energies in the spectra acquired with synchrotron radiation were calibrated with respect to the Fermi level.Additionally, Al Ka radiation (1486.6 eV) was used to measure O 1s, Ce 3d, Pt 4f, and Cu 2p 3/2 core levels.All spectra were acquired at constant pass energy and at an emission angle for the photoelectrons of 201 or 01 with respect to the sample normal, while using the X-ray source or synchrotron radiation, respectively.All spectral components were fitted with a Voigt profile.Pt 4f spectra were fitted by doublet peaks with a spinorbit splitting of 3.30 eV and a fixed branching ratio of 1.33.Cu 3p spectra were fitted by a doublet with a fixed spin-orbit splitting of 2.42 eV and a fixed branching ratio of 2. A combination of a linear and Shirley background was subtracted from Pt 4f spectra.
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 Ce 3+ , Ce 4+ ions, and to off-resonance conditions, respectively.Analysis of the spectra obtained with these photon energies forms the basis of RPES. 19,20The ratio between the corresponding resonant intensities, D(Ce 3+ )/D(Ce 4+ ), denoted as a Resonant Enhancement Ratio (RER) is a direct measure of the degree of reduction of cerium oxide and can be used to quantify the concentration of Ce 3+ ions in the films. 21All SRPES data were processed using KolXPD fitting software. 22The values of total spectral resolution were 1 eV (Al Ka), 200 meV (hn = 115-180 eV), 400 meV (hn = 410 eV) and 650 meV (hn = 650 eV).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.

Computational details: model Pt 2+ sites on extended ceria surfaces
The theoretical study of the adsorption and dissociation of H 2 on model Pt sites at extended CeO 2 surfaces was based on the density functional theory (DFT) calculations employing the Perdew-Burke-Ernzerhof (PBE) 23 generalized gradient approximation (GGA) for the exchange and correlation (XC) functional.In line with previous studies, [24][25][26] the ions were described with Vanderbilt's ultrasoft pseudopotentials, 27 while the crystal wavefunction and density were expanded with a plane-wave basis set limited by energy cutoffs of 40 and 320 Ry, respectively.It is well established that the usual local and gradient-corrected approximations of the XC functional cannot describe the correct insulating electronic structure of ceria-based materials. 24,28,29his is because the electron self-interaction effects arising from the high localization of the Ce 4f electrons are not properly cancelled out by these XC approximations.The addition of an on-site Hubbard U term to the GGA functional (GGA+U) has been proven to be a practical and efficient method to simulate ceria-based systems. 24,28,29For the periodic surface-slab calculations we used the GGA+U method as implemented by Cococcioni and  This journal is © the Owner Societies 2016 de Gironcoli, 30 employing a value of the parameter U = 4.5 eV in line with previous works. 24,25,31Integrals in the Brillouin zone were calculated at the G point.These calculations were performed using the Quantum Espresso package. 32he CeO 2 (111) surface was modeled with 3 O-Ce-O tri-layers separated by more than 12 Å of vacuum normal to the surface.The coordinates of the lowermost tri-layer were constrained to their bulk-like optimized positions, while all other atoms were allowed to relax during the structural optimizations.The Pt 6 cluster supported on the CeO 2 (111) surface was modeled as described in ref. 26.The CeO 2 surface step was modeled with a vicinal surface exposing (110) facets between (111) terraces, and was described with monoclinic periodic slabs separated by more than 10 Å of vacuum in the direction perpendicular to the (111) terrace and labeled as step I in the ref.33.The dimensions of the cell were 17.97 Â 11.67 Å along the [1% 12] and [1% 10] directions, where the [1% 10] direction is parallel to the step edge.The vicinal surface slabs were 3 CeO 2 tri-layers thick.It has been shown that this thickness is sufficient to converge the structural and thermodynamic step properties. 33The Pt 2+ ion at this step edge was obtained by adsorbing a Pt atom at the hollow site of the (110) facet of the vicinal surface.During the geometry optimization, the atomic positions of the lowermost CeO 2 tri-layer were constrained as well as those of the Ce atoms in the central tri-layer, except for the Ce atoms below the step edge.All structures were relaxed by minimizing the atomic forces, until the maximum component of the residual forces on each ion was less than 0.02 eV Å À1 .
To compute the minimum energy path for H 2 dissociation we employed the Climbing-Image Nudge-Elastic-Band method 34 using the Broyden scheme.These calculations were performed in the supercells described above including up to 17 replica images.

Computational details: model Pt 2+ sites on ceria nanoparticles
6][37] In line with previous studies, 19,[38][39][40] present calculations of ceria NP systems have been performed using the PW91 41 XC potential corrected with a value of U = 4 eV.The valence states were represented using plane-wave basis set with a cut-off of 415 eV for the kinetic energy.The core-valence electrons interaction has been described through the projector augmented wave method. 42nly the G-point has been used to sample the reciprocal space.The electron density was self-consistently converged with a 10 À4 eV total energy threshold and structures were optimized until forces acting on all atoms were smaller than 0. Whereas ceria NPs of many different shapes and sizes coexist in experiments, simulated models have to be restricted to a model selected from a variety of low-energy nanoparticulate systems. 39,40,43e have chosen the NP Ce 40 O 80 to represent in a qualitative fashion properties of the experimentally treated nanostructures.Our benchmark calculations have shown, for instance, that increasing the NP to Ce 80 O 160 insignificantly affects the adsorption of metal atoms. 8Note also that the increase of the U-value to 4.5 eV (the value used in the calculations of the extended ceria surfaces, see Section 2.2) will strengthen the binding of Pt atom to the O 4 site on the NP Ce 40 O 80 by only 4%. 8

Results and discussion
3.1.Reactivity of Pt-CeO 2 films towards molecular hydrogen Pt 4f spectra obtained from 5%, 15%, and 18% Pt-CeO 2 films prepared by physical vapor co-deposition of Pt and Ce metals in an oxygen atmosphere onto CeO 2 /Cu(111) buffer layer followed by a brief annealing to 700 K in UHV are shown in Fig. 1 (top traces).
The Pt 4f spectrum obtained from 5% Pt-CeO 2 film contains a single spin-orbit doublet at 73.0 eV (Pt 4f 7/2 ) associated with Pt 2+ species in a square planar coordination, i.e.Pt-O 4 moiety. 8 second broad spin-orbit doublet arises from the underlying Cu(111) substrate at 75.1 eV (Cu 3p 3/2 ).Earlier we reported that Pt 4+ ions formed in small amounts on Pt-CeO 2 films prepared by PVD at 110 K are largely eliminated by annealing above 300 K in UHV. 8,44Besides the major contribution from Pt 2+ species, two additional spin-orbit doublets emerge in the Pt 4f spectra obtained from 15% and 18% Pt-CeO 2 films at 71.1-71.2(Pt 4f 7/2 ) and 72.0 eV (Pt 4f 7/2 ).We associate the corresponding peaks with the presence of metallic (Pt 0 ) and oxidized Pt particles (Pt*).We assume that the Pt 0 particles on the samples with 15% and 18% Pt are partially covered by atomic oxygen due to the reverse oxygen spillover triggered by annealing. 19,26The same phenomenon likely leads to oxidation of smaller Pt particles resulting in the Pt* component in the spectra of both 15% and 18% Pt-CeO 2 films. 45Earlier a similar component was detected by Hatanaka et al. 46 and Wu et al. 47 at 72.1 eV in the spectra of small oxidized Pt clusters supported on CeO 2 .
][50][51] The dissociation of H 2 is associated with a high activation barrier (B1.0 eV) [48][49][50] and therefore requires high pressures (B10 5 Pa) [52][53][54][55][56] and/or elevated temperatures.In our previous studies, however, we did not observe any activity of noble-metal-free CeO 2 (111) towards H 2 dissociation under the conditions of a UHV model experiment (o10 À4 Pa). 57This observation suggests that the reaction rate of hydrogen dissociation on pure CeO 2 (111) under the UHV conditions is too low to yield measurable changes in the oxidation state of cerium cations.
In order to detect whether H 2 dissociation is possible on the Pt-CeO 2 films, we monitored the oxidation states of both Pt and Ce cations.In particular, we employed resonant photoemission spectroscopy 20 to quantify the oxidation state of Ce cations with high surface sensitivity (see Methods).The Pt 4f spectra obtained from 5%, 15%, and 18% Pt-CeO 2 films exposed to hydrogen at different temperatures are shown in Fig. 1.The corresponding development of the relative concentrations of Pt 2+ , Pt 0 , and Pt* along with Ce 3+ are plotted in Fig. 2. We found that interaction of H 2 with the 5% Pt-CeO 2 film does not cause reduction of Pt 2+ or Ce 4+ (Fig. 2a and d).This observation strongly suggests that the Pt 2+ sites do not facilitate dissociation of molecular hydrogen under the experimental conditions employed.
To clarify this experimental finding we performed DFT calculations to compare the reaction pathways and the energetics for H 2 dissociation on both the Pt 2+ (Fig. 3, red line) and the CeO 2 (111) surface sites (Fig. 3, black line).The Pt-CeO 2 system was modeled as a low-energy vicinal surface exposing CeO 2 (111) terraces and step edges along the [1% 10] direction. 33We found  This journal is © the Owner Societies 2016 that Pt atoms bind at step facets forming planar Pt-O 4 moieties that have structural and electronic properties very similar to Pt-O 4 species identified on ceria nanoparticles. 8Our present calculations show that the activation energy for H 2 dissociation at the Pt 2+ sites is even larger than the barrier on the pristine CeO 2 (111) surface.
Compared to the value of 1.0 eV calculated for the pristine CeO 2 (111) surface, 51 the barrier increases to 1.2 eV in the presence of the Pt 2+ sites at the steps.The lower propensity towards H 2 adsorption on these Pt 2+ sites can be explained by electrostatic arguments as proposed by Garcı ´a-Melchor and Lo ´pez. 48The higher activation energy calculated for the Pt 2+ sites implies that H 2 desorption is strongly favored against dissociation, in accordance with the experiment showing no activity for the 5% Pt-CeO 2 system.
In sharp contrast to the 5% Pt-CeO 2 , we detected facile reduction of Pt 2+ species coupled with the reduction of Ce 4+ on both 15% and 18% Pt-CeO 2 samples upon reaction with H 2 at elevated temperatures (see Fig. 1b, c, 2b, c, e and f).Eventually, all Pt 2+ species are reduced quantitatively to metallic Pt 0 .Remarkably, very small amounts of Pt 0 turned out to be sufficient to initiate the reduction process (see Fig. 1b, 2b  and e).Note that such small amounts of metallic Pt 0 as are present on the 15% Pt-CeO 2 film (corresponding to a nominal coverage of less than 0.05 Å Pt) could be hardly detected by conventional X-ray photoelectron spectroscopy.Quite consistently, our DFT simulations show that sub-nm sized Pt clusters supported on ceria are, indeed, highly active for H 2 dissociation.
For instance, we calculated an activation energy of B0.3 eV (Fig. 3, green line) for Pt 6 particle supported on CeO 2 (111), which can be easily overcome even at low temperature.

The mechanism of Pt 2+ reduction in the presence of metallic Pt particles
In view of the high stability of the Pt-O 4 moiety, 8 the observed reduction of Pt 2+ sites upon reaction with hydrogen may appear surprising.However, we reported previously that the adsorption of atomic hydrogen, leading to hydroxylation of the oxygen ions of the Pt-O 4 moiety and reduction of Ce 4+ results in destabilization and, possibly, in reduction of Pt 2+ species. 8In the presence of metallic Pt 0 on Pt-CeO 2 , atomic hydrogen could spillover from Pt particles to the support leading to hydroxylation of the Pt-O 4 moiety.In fact, the calculated reaction pathway on Pt 6 /CeO 2 (Fig. 3, final state bottom panels) suggests the tendency for H to migrate to the cluster periphery and accumulate at the boundary in contact with the oxide support.
On Pt/CeO 2 (111) model catalysts, reversible hydrogen spillover is typically indicated by an increase of the RER between 190 and 260 K by about DRER = 0.3. 57On both the 15% and the 18% Pt-CeO 2 samples (Fig. 2e and f) the corresponding effect is much weaker.Therefore, we assume that there is only little tendency for hydrogen spillover from the metallic Pt particles to the oxide for the Pt-CeO 2 catalysts employed in the present work.This suggests that hydrogen spillover and hydroxylation of Pt-O 4 are not the key steps in Pt 2+ reduction during the reaction with H 2 .
Thus, the mechanism of Pt 2+ reduction is likely to be associated with the formation of oxygen vacancies upon reverse oxygen spillover 19 from Pt-CeO 2 to the Pt particles.Upon exposure to H 2 the spilt-over oxygen is continuously removed from the Pt particles by reaction with hydrogen and formation of water which desorbs immediately.This reaction channel leads to formation of oxygen vacancies accompanied by the reduction of Ce 4+ .The corresponding scenario is consistent with the absence of hydroxyl groups in the O 1s spectra on 15% and 18% Pt-CeO 2 films exposed to H 2 above 350 K (data not shown).The developments of the Pt 2+ (Fig. 2b and c) and the Ce 3+ (Fig. 2e and f) concentrations show that reduction of Ce 4+ slightly precedes the reduction of Pt 2+ .This suggests that the Pt 2+ ions are destabilized in the presence of oxygen vacancies and at sufficiently high vacancy concentration are eventually reduced to Pt 0 .Indeed, our DFT calculations show that the formation of oxygen vacancies notably lowers the adsorption energy of Pt atom in Pt-Ce 40 O 80 and 4Pt-Ce 40 O 80 NPs (see Fig. 4).The stability of Pt 2+ depends on various parameters such as the proximity of the oxygen vacancies, their number, the Pt 2+ concentration, and the distribution of Ce 3+ ions (see ESI † for details).
The criterion for Pt 2+ being stable against formation of metallic particles is that the magnitude of adsorption energy of atomic Pt, E ad , is larger than the bulk cohesive energy value of Pt (À5.85 eV). 58For isolated Pt 2+ in the {100} O 4 nanopocket of stoichiometric CeO 2 this is clearly the case (E ad = À7.02eV).The formation of a single oxygen vacancy in a position outside the {100} nanopocket (Pt-Ce 40 O 79 , vacancy positions +5 or +10, see Fig. 4) is not energetically sufficient to trigger the reduction  4).
The adsorption energy of atomic Pt in the 2+ state falls dramatically below the cohesive energy (to À4.55 or À4.99 eV) if a single vacancy is created inside the square pocket (position +2).Such decrease is a direct consequence of the loss of the square-planar coordination for the Pt 2+ cations.However, the vacancy formation energy is strongly enhanced at the pocket sites themselves (see Tables S1 and S2, ESI †).This suggests that, at least initially, the oxygen vacancies will be formed preferentially outside the Pt-O 4 moiety.If two of such oxygen vacancies are created per Pt 2+ ion, we indeed find configurations in which the Pt 2+ adsorption energy falls well below the cohesive energy.This is the case, for instance, upon creation of eight oxygen vacancies in the 4Pt-Ce 40 O 80 system (4Pt-Ce 40 O 72 , vacancy position AE4, AE5, AE20, AE21 or AE3, AE6, AE22, AE19, see Fig. 4).Thus, DFT calculations indicate that the onset of Pt 2+ reduction occurs when approximately two oxygen vacancies are created per each Pt 2+ site.
Interestingly, a rough estimation of the ratio between the concentrations of oxygen vacancies and the Pt 2+ cations at the onset of Pt 2+ reduction can be obtained from the experimental data (see ESI, † Table S3).The concentration of oxygen vacancies produced by oxygen spillover can be calculated from the temperature dependent change in the Ce 3+ concentration, taking into account that two Ce 3+ ions are produced per oxygen vacancy (note that the Ce 3+ ions present at low temperature originate from the charge transfer from metallic particles without the generation of oxygen vacancies). 19,59As well, a certain number of Ce 3+ ions emerge upon the redox reaction between Pt and CeO 2 during the growth of Pt 2+ -CeO 2 films. 8In addition we have to take into account the consumption of Pt 2+ ions because the reduction of one Pt 2+ ion leads to formation of two additional Ce 4+ centers, which subsequently may be reduced to Ce 3+ by formation of one oxygen vacancy.Taking both contributions into account, we obtain a ratio of 1.5 between the concentration of the oxygen vacancies and the concentration of Pt 2+ ions in the temperature region between 450 and 500 K, where the onset of Pt 2+ reduction is observed (see ESI † for details of the evaluation procedure).It is noteworthy that this value is in fairly good agreement with the results of our DFT calculations.

Conclusions
We have studied H 2 activation on Pt-CeO 2 model catalysts under UHV conditions by means of SRPES, RPES, and DFT calculations.The isolated Pt 2+ sites on the catalyst surface are found to be inactive towards H 2 dissociation.The activation of H 2 on Pt-CeO 2 requires trace amounts of metallic Pt 0 to be present on the surface.The low reactivity of the Pt 2+ sites is manifested by a high activation barrier for H 2 dissociation which exceeds even the barrier on pristine CeO 2 (111).In the presence of small amounts of metallic Pt 0 , the surface becomes very active towards hydrogen dissociation and Pt 2+ is rapidly reduced.The proposed reduction mechanism involves reverse spillover of oxygen from the Pt-CeO 2 support to the Pt 0 particles followed by the clean-off reaction with hydrogen.Reduction of the ceria support subsequently leads to substantial destabilization of the ionic Pt 2+ species.Once a threshold concentration of oxygen vacancies is reached, Pt 2+ is reduced and metallic Pt particles are formed.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 CTQ2015-64618-R), by the Generalitat de Catalunya (grants 2014SGR97 and XRQTC), and by the Czech Science Foundation (grant 15-06759S).The authors acknowledge a support by the COST Action CM1104 ''Reducible oxide chemistry, structure and functions''.Computer resources, technical expertise and assistance were provided by the Red Espan ˜ola de Supercomputacio ´n and by the ISCRA initiative of the CINECA consortium.Y. L., A. N., M. V., N. T., and K. C. P. thank Elettra for excellent working conditions and support.

Fig. 2
Fig. 2 Relative concentrations of Pt 2+ , Pt 0 , Pt* per total Pt content (a-c) and Ce 3+ per total number of Ce and RERs (d-f) on 5% Pt-CeO 2 (a and d), 15% Pt-CeO 2 (b and e), and 18% Pt-CeO 2 (c and f) films as a function of temperature during annealing under exposure to 50 L H 2 .The circled data points were obtained prior to the hydrogen exposure.

Fig. 1
Fig. 1 Pt 4f spectra obtained from 5% Pt-CeO 2 (a), 15% Pt-CeO 2 (b), and 18% Pt-CeO 2 (c) films prepared by co-deposition of Pt and Ce in an oxygen atmosphere at 110 K on CeO 2 (111)/Cu(111) films followed by a brief annealing to 700 K (top spectra) and subsequently exposed to 50 L of hydrogen at different temperatures.The Cu 3p signal originates from the underlying Cu(111) substrate.The spectra were acquired with photon energy of 180 eV.

Fig. 3
Fig. 3 Minimum energy paths for H 2 dissociation on the pristine CeO 2 (111) surface (black, adapted with permission from ref. 51.Copyright 2015 American Chemical Society), the ionic Pt 2+ sites (red), and the supported Pt 6 cluster (green) -middle panel.The top and bottom panels display the initial (IS), transition (TS), and final (FS) states of H 2 dissociation on the Pt 2+ -CeO 2 and Pt 6 /CeO 2 (111) systems.The activation energies in TS states (open symbols) are given in parentheses.Ce 4+ , Ce 3+ , and O 2À ions are displayed as yellow, brown, and red spheres, respectively.

Fig. 4
Fig. 4 Adsorption energies of atomic Pt in the 2+ state, E ad , without (n = 0) and with n (n 4 0) oxygen vacancies on (a) Pt-Ce 40 O 80Àn and (b) 4Pt-Ce 40 O 80Àn nanoparticle models.E ad values calculated in the absence of oxygen vacancies are shown in bold.The transparent planes cut the nanoparticles into two halves.The vacancies with the signs ''+'' and ''À'' are located in front of and behind the planes, respectively.The sign ''AE''indicates two symmetric oxygen vacancies located in front of and behind the transparent planes.For the 4Pt-Ce 40 O 80Àn nanoparticle (b), the formation of one oxygen vacancy at the position +2 leads to significant decrease of E ad of one Pt 2+ in the proximity of the vacancy (1Pt) while the remaining three Pt 2+ species (3Pt) are influenced less significantly.Formation energies of oxygen vacancies, E vac , in Pt-Ce 40 O 80 model (a) were calculated with respect to the energy of O 2 molecule.