ORIGINAL PAPER

Atomic Ordering and Sn Segregation in Pt–Sn NanoalloysSupported on CeO2 Thin Films

Armin Neitzel1 • Gabor Kovacs2• Yaroslava Lykhach1

• Sergey M. Kozlov2•

Nataliya Tsud3• Tomas Skala3

• Mykhailo Vorokhta3• Vladimır Matolın3

•

Konstantin M. Neyman2,4• Jorg Libuda1,5

Published online: 14 September 2016

� Springer Science+Business Media New York 2016

Abstract The stability and atomic ordering in Pt–Sn

nanoalloys supported on CeO2 thin films have been studied

by means of synchrotron radiation photoelectron spec-

troscopy and density functional calculations. Using CO

molecules as a probe, we explored the development of the

surface structure of supported Pt–Sn nanoalloys with respect

to a reference Pt/CeO2 model system. We found a significant

decrease in the density of CO adsorption sites on supported

Pt–Sn nanoalloys caused by Sn segregation to the surface

upon annealing. Additionally, we found that atomic ordering

in Pt–Sn nanoalloys is driven by the balance between the

surface segregation energy of Sn atoms and the energy of

heteroatomic bond formation. Our calculations demonstrate

a clear tendency for Sn segregation to the nanoalloy surface.

For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a

surface stoichiometry of Pt2Sn which is only slightly

dependent on temperature in thermodynamic equilibrium.

The analysis of Bader charges in Pt–Sn nanoalloys revealed a

strong correlation between the charge and the coordination

number of Sn atoms with respect to Pt neighbors. In partic-

ular, the magnitude of the charge transfer from Sn to Pt

increases as a function of the Sn coordination number.

Keywords Pt–Sn nanoalloy � CO tolerant catalyst � Model

catalyst � Synchrotron radiation photoelectron

spectroscopy � Density functional theory

1 Introduction

Pt-based alloy nanoparticles, nanoalloys, attract significant

attention in the fields of heterogeneous catalysis, energy

conversion, and energy storage as potential catalytic

materials with tailored properties [1–5]. In particular, the

reactivity and selectivity of bimetallic nanoalloys can be

modified through electronic and self-assembly effects

[1–3]. In supported nanoalloys, further enhancement of the

catalytic activity can be achieved through electronic metal-

support interactions [6–9] and synergistic phenomena

[10–12]. For instance, ceria-based supports provide a cat-

alyst with oxygen storage capacity (OSC) [13] and self-

cleaning functionality [10] during the oxidation of carbon

monoxide or hydrocarbons and the water–gas-shift reaction

[14, 15]. Among a variety of bimetallic Pt-based alloys

[1–5], Pt–Sn alloys are of special interest due to their high

CO tolerance under ultra-high vacuum [16–18], high

pressure [18, 19], and electrochemical [20–22] conditions.

The chemisorption of CO has been investigated in great

detail by both density functional calculations and experi-

mental techniques on two stable surface alloys, Pt3Sn and

Pt2Sn, with (111) [17–19, 23, 24], (110) [16, 24], (001)

& Yaroslava Lykhach

yaroslava.lykhach@fau.de

& Jorg Libuda

joerg.libuda@fau.de

1 Lehrstuhl fur Physikalische Chemie II, Friedrich-Alexander-

Universitat Erlangen-Nurnberg, Egerlandstrasse 3,

91058 Erlangen, Germany

2 Departament de Ciencia de Materials i Quımica Fısica and

Institut de Quimica Teorica i Computacional, Universitat de

Barcelona, c/Martı i Franques 1, 08028 Barcelona, Spain

3 Department of Surface and Plasma Science, Faculty of

Mathematics and Physics, Charles University, V

Holesovickach 2, 18000 Prague, Czech Republic

4 ICREA (Institucio Catalana de Recerca i Estudis Avancats),

Pg. Lluıs Companys 23, 08010 Barcelona, Spain

5 Erlangen Catalysis Resource Center, Friedrich-Alexander-

Universitat Erlangen-Nurnberg, Egerlandstrasse 3,

91058 Erlangen, Germany

123

Top Catal (2017) 60:522–532

DOI 10.1007/s11244-016-0709-5

[24], and (102) [25] terminations. At low temperatures and

saturation coverage, CO was found to chemisorb at on-top

and bridge sites on Pt(111) with the mutual ratio near unity

[26–28]. The presence of Sn on the surface of Pt(111)

results in a decrease of the number of adsorbed CO

molecules [17]. In particular, the number of occupied

bridge sites decreases faster with respect to on-top sites. On

stoichiometric Pt3Sn(111) and Pt2Sn(111) surfaces, the

ratios of occupied bridge to on-top sites of 0.34 and 0.22,

respectively, were derived by Paffet et al. [17] based on

high-resolution electron energy loss spectroscopy. In a

more recent study, Hightower et al. [18] found CO to

adsorb exclusively at on-top sites at saturation coverages

on both Pt3Sn(111) and Pt2Sn(111) surfaces. The exception

was a small amount of adsorbed CO at the bridge sites at

low CO coverage on Pt3Sn(111). The low number of

occupied bridge sites on Pt3Sn(111) has been rationalized

in terms of a charge transfer between Pt and Sn that may

favor on-top coordination of adsorbed CO molecules [18].

Density functional modeling predicted a dramatic decrease

in the stability of adsorbed CO in the presence of Sn near

the Pt adsorption site. This substantially reduces the

number of possible adsorption sites and configurations

[24, 25]. In agreement with experimental findings [18], the

authors [24, 25] verified that CO does not adsorb on top of

Sn atom. Interestingly, Sn located beneath the adsorption

site was found to strengthen CO adsorption, whereas Sn on

the surface has a destabilizing effect on all low-index Pt3Sn

surfaces [24, 25].

A more complex situation is anticipated for nanoalloys

due to the presence of low coordinated sites, formation of

core–shell structures, segregation patterns, and random

alloy assemblies [2–4, 20, 29–31]. Under these circum-

stances, detailed characterization of the surface structure

and composition of supported nanoalloys is essential for

the rational design of CO resistant catalysts. In the present

study we use CO molecules as a probe [32] to investigate

the nature and the structure of surface sites on model Pt–Sn

nanoalloys supported on CeO2 thin films. We employ

synchrotron radiation photoelectron spectroscopy (SRPES)

in combination with density functional modeling to obtain

comprehensive insights into the stability and thermody-

namically driven atomic ordering in Pt–Sn nanoalloys.

2 Materials and Methods

2.1 Synchrotron Radiation Photoelectron

Spectroscopy

High-resolution SRPES was performed at the Materials

Science Beamline, Elettra synchrotron light facility in

Trieste, Italy. The bending magnet source provides

synchrotron light in the energy range of 21–1000 eV.

The UHV end station (base pressure 1 9 10-10 mbar) is

equipped with a multichannel electron energy analyzer

(Specs Phoibos 150), a rear view LEED optics, an argon

sputter gun, and a gas inlet system. The basic setup of

the chamber includes a dual Mg/Al X-ray source.

Pt/CeO2 and Pt–Sn/CeO2 samples were prepared by

physical vapor deposition (PVD) of metals on well-

ordered CeO2 films. First, epitaxial CeO2(111) films

were grown on clean Cu(111) (MaTecK GmbH,

99.999 %) by PVD of Ce metal (Goodfellow, 99.99 %)

in oxygen atmosphere (5 9 10-7 mbar, Linde,

99.999 %) at 523 K, followed by annealing of the films

at 523 K in oxygen atmosphere at the same pressure for

5 min. This procedure [33–35] yielded a continuous and

stoichiometric CeO2(111) film with a thickness of about

2.0 nm. PVD of Sn in UHV at 523 K on CeO2(111)

films yielded Sn–Ce mixed oxide [36–38]. The Sn

concentration in the volume of CeO2(111) film deter-

mined by X-ray photoelectron spectroscopy (XPS) was

about 18 %. For comparison, this concentration corre-

sponds to the deposition of 0.4 nm thick Sn film. Pt

was deposited by means of PVD from a Pt wire

(Goodfellow, 99.99 %) either onto CeO2(111) film (Pt/

CeO2) or Sn–Ce mixed oxide film (Pt–Sn/CeO2) at

300 K in UHV. The nominal thickness of the deposited

Pt layers determined by XPS was 0.7 nm (Pt/CeO2) and

1.5 nm (Pt–Sn/CeO2). In the case of Pt/CeO2, the

deposited thickness of the Pt film corresponds to the

growth of Pt nanoparticles with an average diameter of

3.4 nm [6]. The size of the supported Pt–Sn nanopar-

ticles cannot be determined from the present study. For

comparison, the deposition of the same amount of Pt on

CeO2 film would yield particles with a diameter of

more than 4 nm.

During the course of experiments, the samples were

briefly annealed at different temperatures followed by

cooling to 250 K (Pt/CeO2) or 120 K (Pt–Sn/CeO2). Note

that the choice of different CO adsorption temperatures has

hardly any influence on the occupancy of the adsorption

sites on Pt in the temperature range of 120–250 K. In

particular, the mutual ratios between on-top and bridge

adsorption sites at saturation CO coverages formed at the

temperatures below the onset of CO desorption are similar

[28, 39, 40]. After each annealing/cooling step, if not stated

otherwise, the samples were exposed to 50 L (1

L = 1.33 9 10-6 mbar 9 s) of CO at low temperature.

CO (Linde 99.98 %) was dosed by backfilling the UHV

chamber.

The core level spectra of C 1s, Pt 4f, and Sn 4d

were acquired at photon energies of 410, 180, and

60 eV, respectively. The photon energies have been

selected to achieve high photoionization cross sections

Top Catal (2017) 60:522–532 523

123

of the corresponding core levels and high surface

sensitivity. The choice of photon energies yielded a

difference in the information depths of about 22 % for

Pt 4f and Sn 4d core levels. The binding energies in

the spectra were calibrated with respect to the Fermi

level. Additionally, Al Ka radiation (1486.6 eV) was

used to measure O 1s, C 1s, Ce 3d, Sn 3d, Pt 4f, and

Cu 2p3/2 core levels. All spectra were acquired at

constant pass energy and at an emission angle for the

photoelectrons of 20� or 0� with respect to the sample

normal, while using the X-ray source or synchrotron

radiation, respectively. The values of total spectral

resolution were 1 eV (Al Ka), 150 meV (hm = 60 eV),

200 meV (hm = 115–180 eV), 400 meV (hm =

410 eV), and 650 meV (hm = 650 eV). All SRPES

data were processed using the KolXPD fitting software

[41]. Details of the fitting procedure for Sn 4d spectra

can be found elsewhere [37].

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.

2.2 Density Functional Calculations

Density functional (DF) calculations were performed

with the periodic plane-wave code VASP [42]. We used

the PBE [43] exchange–correlation functional considered

to be one of the most appropriate common functionals

for the description of transition metals [44, 45]. The

interaction between valence and core electrons was

treated with the projector augmented wave approach. To

moderate computational expenditures we used the default

cut-off value 241.1 eV defined by core potentials of Pt

and Sn with 10 and 14 valence electrons, respectively.

This approach was previously shown to yield results very

close to those obtained with the cut-off 415 eV [46], and

our calculations with Pt–Sn NPs showed that the appli-

cation of the larger cut-off never resulted in an energy

change higher than 0.07 eV, that is, 0.5 meV/atom. The

one-electron levels were smeared by 0.1 eV using the

first-order method of Methfessel and Paxton [47], finally,

converged energies were extrapolated to the zero

smearing. Calculations were performed only at the C-

point in the reciprocal space. All atoms were allowed to

move (relax) during the geometry optimization until the

largest component of the forces acting on them became

less than 0.2 eV/nm. The separation between nanoparti-

cles (NPs) exceeded 0.7 nm, at which the interaction

between adjacent particles was shown to be negligible

[48].

3 Results and Discussion

3.1 Preparation of Model Pt–Sn Nanoalloys

Supported on CeO2 Film

The supported Pt–Sn nanoalloys have been prepared fol-

lowing a procedure described earlier [37]. This procedure

involves the preparation of a mixed Sn–Ce oxide by the

deposition of Sn on a CeO2(111) film in UHV at 523 K

[36–38] followed by Pt deposition in UHV at 300 K. Based

on thermodynamic considerations [49], the formation of

Pt–Sn alloy is driven by the formation of Pt–Sn bonds. This

is reflected by the high heat of formation of the inter-

metallic compounds, which are by 30–80 kJ/mol atoms

more favorable than the coexistence of monometallic Pt

and Sn phases. The corresponding Sn 4d spectra are shown

in Fig. 1 along with the schematic representations of the

prepared surfaces. The formation of Sn–Ce mixed oxide is

indicated by the emergence of one doublet in Sn 4d spec-

trum at the binding energy of 25.5 eV (Sn 4d5/2) associated

with Sn2? ions. Pt deposition yielded the reduction of Sn2?

to Sn0 accompanied by the formation of Pt–Sn alloy

nanoparticles [37]. During this process two Ce3? ions are

re-oxidized per one Sn0 atom formed, thus fully recovering

the CeO2 stoichiometry. Similar behavior has also been

Fig. 1 Sn 4d spectra obtained from CeO2(111) (bottom), Sn–CeO2

(middle), and Pt–Sn/CeO2(111) (top). In the corresponding ball

models, red, ivory, gray, and blue balls represent oxygen, cerium, tin,

and platinum, respectively. Sn 4d spectra were acquired with

hm = 60 eV. The intensity of Sn 4d spectrum obtained from Sn–

CeO2 film (middle) is divided by a factor of 5

524 Top Catal (2017) 60:522–532

123

observed upon deposition of Pd onto Sn–Ce mixed oxide

[50]. Two doublets arise in the corresponding Sn 4d

spectrum at 23.8 and 24.2 eV associated with the surface

and bulk Pt–Sn contributions, respectively. In the present

work, the deposition of a sufficient amount of Pt resulted in

complete reduction of Sn2? yielding Pt–Sn alloy

nanoparticles supported on the CeO2 film. We attempted to

estimate the bulk stoichiometry of supported Pt–Sn

nanoalloys by means of XPS. Assuming a homogeneous

distribution of Sn atoms in the as-prepared Pt–Sn

nanoparticles, the Pt/Sn concentration ratio can be deter-

mined from the integrated Pt 4f and Sn 3d intensities

normalized with respect to the corresponding sensitivity

factors and the inelastic mean free paths (IMFPs) of the

photoelectrons [51]. In this way, we obtained an average

Pt/Sn bulk ratio of about 4.0 ± 0.3 as determined from the

Pt 4f and Sn 3d spectra acquired at photoemission angles of

20� and 60� with respect to the sample normal. Based on

the Pt–Sn equilibrium phase diagram [49, 52], only Pt and

Pt3Sn phases are favorable at this Pt/Sn concentration ratio.

According to DeSario and DiSalvo [52], Pt3Sn nanoparti-

cles adopt face-centered-cubic structure at 300 K and order

into a Cu3Au-like structure upon annealing to 473 K.

3.2 Probing Surface Sites on Pt/CeO2 by Means

of CO Adsorption

In order to establish the role of Sn, we first investigated the

temperature induced changes in the surface structure of

monometallic Pt nanoparticles supported on the CeO2(111)

film by probing with CO. Earlier, three types of surface

sites associated with CO adsorption in bridge configuration

on terraces (terrace-bridge), on-top configuration on ter-

races (terrace-on-top) and on-top at low-coordinated sites

(step-on-top) have been identified by IR spectroscopy [32].

The relative abundancy of these sites can be estimated by

means of SRPES. Typical C 1s binding energies associated

with CO adsorption at terrace-bridge, terrace-on-top, and

step-on-top sites observed on Pt(111) [26–28] and stepped

Pt surfaces [26, 27, 53, 54] are in the ranges of 285.91–

286.27 eV, 286.60–286.80 eV, and 286.24–286.43 eV,

respectively. The corresponding binding energy separa-

tions between the CO species adsorbed on the same sur-

faces are 0.64–0.73 eV (between terrace-bridge and

terrace-on-top sites) and 0.33–0.36 eV (between step-on-

top and terrace-on-top sites). Accordingly, the differences

in the relative binding energy separations between different

species are much smaller than the differences in the bind-

ing energies of the same species on different Pt surfaces.

This allowed us to introduce restraints on the fitting

parameters of the C 1s spectra obtained from Pt/CeO2. In

particular, we assembled an envelope containing the peaks

with fixed mutual binding energy separations. In this

envelope, only the binding energy of the peak associated

with the terrace-on-top sites was allowed to change freely

while the binding energies of the rest of the peaks were

restraint by constant binding energy separations of 0.6 eV

(between terrace-bridge and terrace-on-top sites) and

0.3 eV (between step-on-top and terrace-on-top sites). In

view of the low spectral resolution of the C 1s spectra, this

approach yielded consistent fitting results and the lowest

fitting error.

Selected C 1s spectra obtained after adsorption of 50 L

CO on Pt/CeO2(111) film at 250 K are shown in Fig. 2a.

Three peaks associated with CO adsorption at terrace-on-

top, step-on-top, and terrace-bridge sites can be resolved in

the spectra at 286.80, 286.50, and 286.20 eV, respectively.

A small peak at 287.50 eV represents a satellite associated

with CO adsorption in on-top geometry at high coverage

[55]. This peak was also included into the fitting envelope

with fixed binding energy separation with respect to the

peak associated with terrace-on-top site. Additional minor

contributions from carbonate and carboxylate species

formed upon CO adsorption on CeO2 emerge at

289.1–289.6 and 288.0–288.4 eV, respectively [56]. Since

the binding energies of these species do not overlap with

those associated with CO adsorbed on Pt, we will no longer

focus on their presence in the C 1s spectra of the investi-

gated films. The integrated C 1s intensities associated with

CO adsorption at different adsorption sites are plotted in

Fig. 2b. Additionally, the intensities of the CO species

adsorbed at terrace-bridge and step-on-top sites normalized

to the intensity of CO species at terrace-on-top sites are

plotted in Fig. 2c. The ratio of the terrace-bridge to terrace-

on-top sites on supported Pt nanoparticles is considerably

lower than the ratio of unity reported on Pt(111) [28].

During the course of experiment, we briefly annealed the

Pt/CeO2 film to different temperatures followed by cooling

and CO adsorption at 250 K after each annealing step (see

Sect. 2.1 for details). With increasing annealing tempera-

ture of the Pt/CeO2 film, we observed a gradual decrease in

the number of terrace-on-top and step-on-top sites

accompanied by an increase of the number of terrace-

bridge sites (Fig. 2b, c). The observed behaviour is con-

sistent with faceting of Pt particles and the growth of larger

terraces. A general decrease of the total C 1s intensity

(Fig. 2b) indicates a decrease of the surface area of Pt

particles due to sintering and coalescence [57].

3.3 Probing Surface Sites on Pt–Sn/CeO2 by Means

of CO Adsorption

Selected C 1s spectra obtained after adsorption of 50 L CO

on as-prepared Pt–Sn/CeO2 at 120 K are shown in Fig. 3a.

Similarly to Pt/CeO2(111), we identified three components

in the C 1s spectra associated with CO adsorption at

Top Catal (2017) 60:522–532 525

123

terrace-on-top, step-on-top, and terrace-bridge sites. The

corresponding components emerge at 286.65, 286.35, and

286.05 eV. In comparison to the Pt/CeO2 system, the

binding energies of the corresponding species are by

0.15 eV lower due to particle size effects [58]. A satellite

associated with CO adsorbed at on-top sites emerges at

287.35 eV. Integrated C 1s intensities associated with CO

adsorption at different sites are plotted in Fig. 3b. Addi-

tionally, the intensities of the CO species adsorbed at ter-

race-bridge and step-on-top sites normalized to the

intensity of CO species adsorbed at terrace-on-top sites are

plotted in Fig. 3c. In contrast to Pt/CeO2(111), we

observed a rapid increase in the number of low-coordinated

step-on-top sites at the expense of terrace-on-top sites at

Fig. 2 a C 1s spectra obtained from a Pt/CeO2(111) model catalyst

annealed at different temperatures followed by cooling and exposure

to 50 L of CO at 250 K, b Integrated C 1s intensities associated with

CO adsorption at on-top terrace (blue squares), bridge terrace (green

triangles), on-top low-coordinated sites (red circles), and total C 1s

intensity (black triangles) as a function of temperature, c ratio of

integrated C 1s intensities associated with low-coordinated on-top

(red circles) and bridge terrace (green triangles) sites with respect to

on-top terrace sites as a function of temperature. C 1s spectra were

acquired with hm = 410 eV

Fig. 3 a C 1s spectra obtained from a Pt–Sn/CeO2 model catalyst

annealed at different temperatures followed by cooling and exposure

to 50 L of CO at 120 K, b Integrated C 1s intensities associated with

CO adsorption at on-top terrace (blue squares), bridge terrace (green

triangles), on-top low-coordinated sites (red circles), and total C 1s

intensity (black triangles) as a function of temperature, c ratio of

integrated C 1s intensities associated with low-coordinated on-top

(red circles) and bridge terrace (green triangles) sites with respect to

on-top terrace sites as a function of temperature. C 1s spectra were

acquired with hm = 410 eV

526 Top Catal (2017) 60:522–532

123

350 K followed by a rapid decrease of the total C 1s

intensity. A spike in the ratio of step-on-top to terrace-on-

top sites at 350 K is clearly visible in Fig. 3c. Rapid

increase in the number of the low-coordinated sites is

consistent with a considerable roughening of the terraces

on supported Pt–Sn nanoalloys. Additionally, we observed

a strong preference of adsorbed CO molecules for step-on-

top sites. In particular, we revealed migration of CO spe-

cies adsorbed at 300 K at terrace-on-top and terrace-bridge

sites to the step-on-top sites upon annealing of CO-exposed

Pt–Sn nanoalloys to 350 K (data not shown). In strong

contrast to Pt/CeO2(111), the ratio of terrace-bridge to

terrace-on-top sites decreases until it vanishes at 600 K.

The subsequent annealing above 700 K removes low-co-

ordinated adsorption sites leaving only a small number of

terrace-on-top sites behind.

An additional peak in C 1s spectra of as-prepared Pt–Sn/

CeO2 film at 284.0 eV is associated with a carbon con-

tamination. CO adsorption has no influence on the intensity

of this peak. The contamination is gradually removed from

the surface upon annealing above 500 K due to the reaction

with oxygen provided via the reverse spillover from the

support [37].

3.4 Pt–Sn Interaction Upon CO Adsorption

on Pt–Sn/CeO2

In order to understand the effect of annealing on the distri-

bution and abundance of CO adsorption sites on the Pt–Sn/

CeO2 system discussed above, we examined the corre-

sponding changes in the Sn 4d spectra. To disentangle the

changes caused by annealing and CO adsorption we prepared

a reference Pt–Sn/CeO2 sample of similar composition. Sn 4d

spectra obtained from the reference system annealed at dif-

ferent temperatures in UHV are compared with Sn 4d spectra

obtained from Pt–Sn/CeO2 samples probed by CO in Fig. 4a

and b, respectively. Sn 4d spectra obtained from both as-

prepared Pt–Sn/CeO2 samples (300 K, Fig. 4a and b, top

spectra) contain two components associated with the surface

and bulk contributions from the Pt–Sn alloy as discused above

[37]. The corresponding 4d5/2 peaks emerge at 23.8 and

24.2 eV. Total integrated intensities of Sn 4d and Pt 4f

obtained from the reference Pt–Sn/CeO2 film are plotted in

Fig. 4c as a function of temperature. The ratio of the inte-

grated intensities of Sn 4d to Pt 4f, I(Sn 4d)/I(Pt 4f), is plotted

in Fig. 4d as a function of the annealing temperature. The

increase of the I(Sn 4d)/I(Pt 4f) ratio indicates segregation of

Sn onto the surface during annealing.

We found out that at 350 K, CO adsorption has the

largest impact on the Sn spectra obtained from Pt–Sn/CeO2

(compare spectra before and after CO adsorption, Fig. 4b).

In particular, the surface Pt–Sn component is attenuated

and shifted to higher binding energy where it overlaps with

the bulk Pt–Sn component. We assume that attenuation and

shift of the surface Pt–Sn component is connected to a

strong charge transfer from the surface Sn atoms. The

charge transfer between Sn and Pt in Pt–Sn alloys has been

discussed in the literature [59, 60]. However, the magni-

tude of this effect is the largest at 350 K. We assume that a

specific mutual assembly of Pt and Sn atoms in the near-

surface region increases the Pt–Sn interaction and

strengthens the CO adsorption at the corresponding Pt sites.

The increase in the number of low-coordinated on-top sites

indicates substantial roughening of the nanoalloy surface at

these conditions.

The enrichment of Sn at the surface of Pt–Sn nanopar-

ticle upon annealing is consistent with a decrease in the

total amount of adsorbed CO and the ratio of occupied

terrace-bridge to terrace-on-top sites in particular. The

decrease in the number of step-on-top sites also indicates

the substitution of low-coordinated Pt sites (corners, edges,

steps) with Sn atoms. Upon annealing above 600 K a new

component emerges at 25.3 eV in the Sn 4d spectra

obtained from Pt–Sn/CeO2 probed by CO (Fig. 4b). The

corresponding component was not detected in Sn 4d

spectra obtained from the reference Pt–Sn/CeO2 system

(compare with Fig. 4a). The binding energy of this peak is

characteristic of the Ce–Sn mixed oxide [37]. We associate

the emergence of this component with the partial encap-

sulation of Pt–Sn/CeO2 probed by CO. Formation of the

Ce–Sn mixed oxide on the surface of the supported Pt–Sn

alloy results in attenuation of the surface Pt–Sn alloy

component in the Sn 4d spectra. Indeed, this component

becomes considerably smaller on Pt–Sn/CeO2 probed by

CO in comparison to the reference Pt–Sn/CeO2 system

(compare Figs. 4a and 4b at 750 K). We believe that the

difference in the behavior of the two samples results from

the different extent of CeO2 reduction caused by reverse

oxygen spillover [10, 37] during annealing in UHV (ref-

erence sample) and reaction with CO (probing by CO). In

the case of supported Pt–Sn nanoalloys, oxygen reverse

spillover could lead to Sn oxidation and the formation of a

SnOx film at the surface of the nanoparticles [61]. This

effect appears negligible in the case of the reference

sample. However, considerably higher amounts of oxygen

are removed upon reaction with CO on supported Pt–Sn

nanoalloys. It was reported that under strongly reducing

conditions, oxygen reverse spillover is accompanied by the

migration of Ce3? species leading to the encapsulation of

Pt by the CeOx film [62, 63]. In the case of supported Pt–Sn

nanoalloys, the formation of Ce–Sn mixed oxide at the

surface of the nanoparticles may thus be favored over the

coexistence of two separate CeOx and SnOx phases.

Top Catal (2017) 60:522–532 527

123

3.5 Modeling of Pt–Sn Nanoalloys

In order to establish the factors governing the atomic

arrangement (chemical ordering) in Pt–Sn nanoalloys, we

employed a recently developed computational method [46]

for the global optimization of the chemical ordering in a

NP of a particular shape, size and composition. Briefly, this

method is based on topological energy expressions, which

Fig. 4 a Sn 4d spectra obtained from the reference Pt–Sn/CeO2

system annealed at different temperatures followed by cooling and

b Pt–Sn/CeO2 annealed at different temperatures followed by cooling

and exposure to 50 L of CO at 120 K; The spectra obtained after

annealing/cooling before CO adsorption (black) and after CO

adsorption (green) are shown for comparison, c Integrated total

intensities of Pt 4f (black squares) and Sn 4d (red circles) spectra and

d the ratio of Sn 4d to Pt 4f intensities as a function of temperature on

the reference Pt–Sn/CeO2 film. Sn 4d and Pt 4f spectra were acquired

with hm of 60 and 180 eV, respectively

528 Top Catal (2017) 60:522–532

123

solely depend on the atomic arrangement of the alloying

metals in a Pt–Sn NP. Specifically, the method involves the

use of ei energy parameters (descriptors) associated either

with the surface segregation energy of Sn atoms or the

formation energy of heteroatomic Pt–Sn bonds. The cor-

responding values of ei were acquired via multiple linear

regression fitting [64] for a set of NP structures with dif-

ferent chemical orderings. The topological energy expres-

sions were applied to the global optimization of chemical

ordering via highly efficient Monte-Carlo simulations that

involve swapping of multiple atoms. The combination of

such simulations with the topological expressions ensures

that the global minimum structure for a NP of a certain

shape, size and composition can already be obtained after

DF calculations of only a few dozens of structures. Such

simulations also allowed us to determine average proper-

ties of Pt–Sn NP in thermodynamic equilibrium at a given

temperature.

In the present study, we employ the Pt105Sn35 NP with

truncated octahedral fcc structure. The descriptors gener-

ated for this NP were used to establish the chemical

ordering in larger NPs of the same stoichiometry. The latter

approach was validated in our earlier studies [46, 65].

The analysis of the descriptor values revealed a strong

tendency for the segregation of Sn atoms onto the surface

of the Pt105Sn35 NP. In particular, the energy gains asso-

ciated with the segregation of a Sn atom on 6-coordinated

corner, 7-coordinated edge and 9-coordinated (111) terrace

positions were 1165, 867 and 556 meV, respectively. The

energy of heteroatomic bond formation was calculated to

be -101 meV per Pt–Sn bond. This number is much higher

in magnitude than determined for common nanoalloys of d-

metals, e.g. Pt–Co [65] and Pd–Au [46], but is similar to

that obtained for other intermetallic alloys like Pd–Zn [46].

Both the Sn segregation energy and the energy of het-

eroatomic bond formation contribute substantially to the

formation energy of the Pt105Sn35 NP. Therefore, both

factors affect the most stable atomic arrangement in Pt–Sn

NPs. Similar conclusion was reached also for smaller

PtnSnn (n = 1–10) and PtmSn3m (m = 1–5) clusters by

Huang et al. [66].

In Fig. 5, we show two Pt105Sn35 structures with dif-

ferent Pt–Sn ordering. The high-energy structure (a) is

characterized by higher Pt/Sn surface ratio and contains

more low-coordinated Pt atoms in comparison to the

structure (b) (33 vs. 24, respectively, Fig. 5). Therefore, the

structure (a) may be considered an approximation of the as-

prepared Pt–Sn nanoalloys discussed in Sects. 3.1, 3.2, 3.3,

3.4. The structure (b) features the most favorable chemical

ordering, which is 7.3 eV lower in energy than the struc-

ture (a). The structure with the most favorable chemical

ordering has Pt2Sn surface stoichiometry.

Additionally we studied the surface composition of

Pt105Sn35 nanoalloys in thermodynamic equilibrium at

different temperatures up to 1000 K (see Fig. 6). The

surface Pt/Sn ratio of 2.0 remains constant even at 1000 K.

This is related to the limitation that the NP cannot

accommodate more Sn atoms in the interior without sig-

nificant destabilization, neither can more Sn atoms segre-

gate on the surface, since most of them occupy surface

positions already at 0 K. In order to provide a more general

image of the temperature effect on the chemical ordering in

Pt–Sn NPs, we also performed simulations for the larger

Pt1097Sn386 NP. With respect to the total number of atoms

per particle, the Pt1097Sn386 NP represents a close match to

Fig. 5 a Representative structure of the Pt105Sn35 NP with a high-

energy atomic arrangement according to density functional calcula-

tions. We can notice that a, which lies 7.3 eV higher in energy than

the structure b with the most favorable chemical ordering exposes

rough surface, just as we observed experimentally for NPs with higher

Sn content in the NP interior; b The structure of the Pt105Sn35 NP with

the most energetically favorable (globally optimized) chemical

ordering. Color coding of atoms: Pt—dark blue, Sn—grey

Top Catal (2017) 60:522–532 529

123

the experimental Pt–Sn nanoalloys on CeO2. A rough

estimate [6] yielded about 1800 atoms per particle for the

supported Pt–Sn nanoalloys which is only slightly larger

than the total number of atoms per particle used in the

calculations (Pt1097Sn386). In the structure with the most

stable chemical ordering of Pt1097Sn386 (at 0 K) the surface

Pt/Sn ratio is 1.85, which does not change significantly up

to 400 K, and even at 1000 K the ratio decreases to only

1.55 which corresponds to a change of the surface Pt/Sn

ratio by ca. 16 % (Fig. 6).

The changes in the surface composition as a function of

the temperature are more complex than, e.g. in the case of

Pt–Co nanoalloys [65, 67], where in the minimum-energy

structures most Pt atoms are segregated on the surface and

such segregation is basically the only governing factor of

the chemical ordering. In Pt1097Sn386 only ca. 55 % of Sn

atoms are on the surface at 0 K, and the changes in the

surface composition with temperature result from the bal-

ance between several governing factors, that is, the Sn

surface segregation and the preference for heteroatomic Pt–

Sn bond formation.

We made an effort to compare the surface stoichiometry

of supported Pt–Sn nanoalloys with those predicted by the

DF calculation. To this aim we calculated the Pt/Sn surface

ratio from the total integrated intensities of the Pt 4f and

the Sn 4d spectra obtained by SRPES. First, we determined

the empirical sensitivity coefficients which relate the

intensities and the concentrations of Pt and Sn. The ratio of

the coefficient can be obtained from a linear regression of

the Pt 4f intensity as a function of the Sn 4d intensity

assuming that the total number of surface atoms (the sum

of Pt and Sn atoms) is constant. As this analysis procedure

requires constant particle size and shape we considered the

temperature region of 300–550 K only. Note that in this

region the size of the nanoalloy particles is not influenced

by the annealing. In this way, we obtained Pt/Sn surface

concentration ratio of about 8 at 300 K and 1.6 at

700–750 K. Noteworthy the value of 1.6 is in an excellent

agreement with the Pt/Sn ratio of 1.55 in Pt1097Sn386 NP

derived from the DF calculations.

Finally, we investigated the charge transfer in the

Pt105Sn35 NP by means of Bader charge analysis [68]. We

found that Sn atoms donate significant amount of electrons

to neighboring Pt atoms. The low-coordinated Sn atoms (at

corners) can be described with a Bader charge of 0.6–0.8 a.

u., whereas Sn atoms interacting with more Pt atoms (in the

bulk, for instance) can hold a Bader charge of up to 1.26 a.

u. This substantial charge transfer is typical for inter-

metallic systems formed of one d and one sp metal [69, 70].

However, its value is not exceptionally high even for alloys

of two d metals, since for instance in Pt–Ti particles the

largest Bader charge on Ti atoms was calculated to be as

high as 2 a. u. [71].

In Fig. 7 we depicted the calculated Bader charges on

Sn atoms in the global minimum structure (b) as a function

of the coordination number of Sn atoms with respect to the

nearest-neighbor Pt atoms. It can be clearly seen that the

charge transfer from Sn to Pt increases with the increasing

coordination number of Sn atoms. We also mention that in

structure (a) the location of more Sn atoms in the NP

interior enhances interactions between Pt and Sn atoms,

resulting in stronger overall electron depletion in Sn.

Based on this observation we may speculate that the

enhanced Pt–Sn interaction associated with significant

changes in Sn 4d spectra upon CO adsorption on supported

Pt–Sn nanoalloys discussed in Sect. 3.4 indeed results from

the location of Sn beneath the Pt adsorption sites. In such

Fig. 6 Equilibrium Pt/Sn surface concentration ratio as a function of

the temperature in Pt105Sn35 (black) and Pt1097Sn386 (red)

Fig. 7 Calculated Bader charge values on Sn atoms in the global

minimum structure obtained for Pt105Sn35 as a function of the number

of nearest neighbor Pt atoms connected to each Sn atom

530 Top Catal (2017) 60:522–532

123

positions, Sn atoms have larger coordination number than

those on the surface of the nanoalloys, and, therefore,

interact more strongly with Pt. The effect is magnified in

the presence of CO molecules due to coupling with the

charge transfer between Pt d states and CO orbitals [72].

The magnitude of this effect can be qualitatively esti-

mated from the differences between the Sn 4d spectra

obtained before and after CO adsorption (see Fig. 4b). Note

that CO does not adsorb on Sn atoms [18] and, therefore,

the observed effects in the Sn 4d spectra are exclusively

caused by the Pt–Sn charge transfer.

4 Conclusions

The stability and atomic ordering in Pt–Sn nanoalloys were

investigated as a function of temperature by means of

SRPES and density functional modeling. The differences in

the geometric and electronic surface structure of Pt–Sn

nanoalloys supported on the CeO2 film were explored by

CO adsorption in comparison with monometallic Pt parti-

cles. Our findings are summarized below:

1) CO molecules adsorbed at terrace sites in on-top and

bridge positions and at low-coordinated sites in on-

top configuration have been identified on both Pt/

CeO2 and Pt–Sn/CeO2.

2) The relative increase in the number of bridge sites

and the decrease of low-coordinated sites indicated

faceting, i.e. the growth of larger terraces, on the

monometallic Pt nanoparticles supported on CeO2

film upon annealing.

3) In contrast to Pt/CeO2, a relative decrease in the

numbers of bridge and low-coordinated sites, and a

rapid decrease of the total number of adsorption sites

in general, were observed on the supported Pt–Sn

nanoalloy. This behavior is rationalized in terms of

the low stability of Pt-rich surface compositions in

as-prepared supported Pt–Sn nanoalloys, which is

converted upon annealing to a thermodynamically

more stable Pt–Sn ratio through surface segregation

of Sn.

4) In a narrow temperature range between 350 and

400 K, we found a temporary roughening of the

terraces on the supported Pt–Sn nanoparticles asso-

ciated with the formation of low-coordinated sites

favorable for CO adsorption.

5) The atomic ordering in the modelled Pt105Sn35 and

Pt1097Sn386 nanoparticles is driven by the balance

between the surface segregation energy of Sn atoms

and the energy of heteroatomic Pt–Sn bond formation.

The predicted surface stoichiometry in both nanopar-

ticles is * Pt2Sn until high temperatures.

6) The compatibility between the experiment and

density functional modeling has been achieved in

terms of the total atoms per particle in the Pt1097Sn386

NP model and supported Pt–Sn nanoalloys on the

CeO2 film. The corresponding surface Pt/Sn ratios

determined under thermodynamic equilibrium con-

ditions at high temperature in the Pt1097Sn386 NP

model (1.55) and supported Pt–Sn nanoalloys (1.6)

are in an excellent agreement.

7) According to the Bader analysis the magnitude of the

charge transfer from Sn to Pt is increasing with

increasing coordination number of Sn atoms with

respect to Pt atoms.

Acknowledgments This work was funded by the European Com-

munity (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 the Spanish MINECO

(Grants CTQ2012-34969 and CTQ2015-64618-R co-funded by

FEDER), by the Generalitat de Catalunya (Grants 2014SGR97 and

XRQTC), and by the Czech Science Foundation (Grant 13-10396S).

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 Espanola de Supercomputacion. Y.L., A.N., M.V., and N.T.

thank Elettra and Dr. Kevin C. Prince for excellent working condi-

tions and support. The research leading to these results has received

funding from the European Community’s Seventh Framework Pro-

gram (FP7/2007-2013) under Grant Agreement No 312284.

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