This paper is published as part of a PCCP themed issue on recent developments in X-ray absorption spectroscopy Guest Editor: Jeroen Anton van Bokhoven

Editorial

Recent developments in X-ray absorption spectroscopy J. A. van Bokhoven, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c0cp90010a

Perspectives

Parameter-free calculations of X-ray spectra with FEFF9 John J. Rehr, Joshua J. Kas, Fernando D. Vila, Micah P. Prange and Kevin Jorissen, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926434e

The atomic AXAFS and Δμ XANES techniques as applied to heterogeneous catalysis and electrocatalysis D. E. Ramaker and D. C. Koningsberger, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b927120c

Advances in high brilliance energy dispersive X-ray absorption spectroscopy Sakura Pascarelli and Olivier Mathon, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926509k

Communication

μ-XANES mapping of buried interfaces: pushing microbeam techniques to the nanoscale Paolo Ghigna, Sonia Pin, Giorgio Spinolo, Mark A. Newton, Michele Zema, Serena C. Tarantino, Giancarlo Capitani and Francesco Tatti, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c000195c

Papers

L-edge XANES analysis of photoexcited metal complexes in solution Renske M. van der Veen, Joshua J. Kas, Christopher J. Milne, Van-Thai Pham, Amal El Nahhas, Frederico A. Lima, Dimali A. Vithanage, John J. Rehr, Rafael Abela and Majed Chergui, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b927033g

EXAFS as a tool to interrogate the size and shape of mono and bimetallic catalyst nanoparticles Andrew M. Beale and Bert M. Weckhuysen, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b925206a

X-Ray absorption in homogeneous catalysis research: the iron-catalyzed Michael addition reaction by XAS, RIXS and multi-dimensional spectroscopy Matthias Bauer and Christoph Gastl, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926385c

Combined TPRx, in situ GISAXS and GIXAS studies of model semiconductor-supported platinum catalysts in the hydrogenation of ethene Sonja A. Wyrzgol, Susanne Schäfer, Sungsik Lee, Byeongdu Lee, Marcel Di Vece, Xuebing Li, Sönke Seifert, Randall E. Winans, Martin Stutzmann, Johannes A. Lercher and Stefan Vajda, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926493k

Near sulfur L-edge X-ray absorption spectra of methanethiol in isolation and adsorbed on a Au(111) surface: a theoretical study using the four-component static exchange approximation Sebastien Villaume, Ulf Ekström, Henrik Ottosson and Patrick Norman, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926109e

Influence of additives in defining the active phase of the ethylene oxychlorination catalyst N. B. Muddada, U. Olsbye, L. Caccialupi, F. Cavani, G. Leofanti, D. Gianolio, S. Bordiga and C. Lamberti, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926502n

First-principles calculations of X-ray absorption spectra at the K-edge of 3d transition metals: an electronic structure analysis of the pre-edge Delphine Cabaret, Amélie Bordage, Amélie Juhin, Mounir Arfaoui and Emilie Gaudry, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926499j

First steps in combining modulation excitation spectroscopy with synchronous dispersive EXAFS/DRIFTS/mass spectrometry for in situ time resolved study of heterogeneous catalysts Davide Ferri, M. Santosh Kumar, Ronny Wirz, Arnim Eyssler, Oxana Korsak, Paul Hug, Anke Weidenkaff and Mark A. Newton, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926886c

Novel opportunities for time-resolved absorption spectroscopy at the X-ray free electron laser B. D. Patterson and R. Abela, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c003406a

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Spatially resolved 3D micro-XANES by a confocal detection scheme Geert Silversmit, Bart Vekemans, Sergey Nikitenko, Sylvia Schmitz, Tom Schoonjans, Frank E. Brenker and Laszlo Vincze, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c004103n

Wavelet transform EXAFS analysis of mono- and dimolybdate model compounds and a Mo/HZSM-5 dehydroaromatization catalyst Robert O. Savinelli and Susannah L. Scott, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926474d

Electronic structure of alumina-supported monometallic Pt and bimetallic PtSn catalysts under hydrogen and carbon monoxide environment Jagdeep Singh, Ryan C. Nelson, Brian C. Vicente, Susannah L. Scott and Jeroen A. van Bokhoven, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c000403k

Determination of CO, H2O and H2 coverage by XANES and EXAFS on Pt and Au during water gas shift reaction Neng Guo, Bradley R. Fingland, W. Damion Williams, Vincent F. Kispersky, Jelena Jelic, W. Nicholas Delgass, Fabio H. Ribeiro, Randall J. Meyer and Jeffrey T. Miller, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c000240m

Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts Toyli Anniyev, Hirohito Ogasawara, Mathias P. Ljungberg, Kjartan T. Wikfeldt, Janay B. MacNaughton, Lars-Åke Näslund, Uwe Bergmann, Shirlaine Koh, Peter Strasser, Lars G.M. Pettersson and Anders Nilsson, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926414k

In situ time-resolved DXAFS for the determination of kinetics of structural changes of H-ZSM-5-supported active Re-cluster catalyst in the direct phenol synthesis from benzene and O2 Mizuki Tada, Yohei Uemura, Rajaram Bal, Yasuhiro Inada, Masaharu Nomura and Yasuhiro Iwasawa, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c000843p

Sulfur poisoning mechanism of steam reforming catalysts: an X-ray absorption near edge structure (XANES) spectroscopic study Yongsheng Chen, Chao Xie, Yan Li, Chunshan Song and Trudy B. Bolin, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b925910b

Peroxide-like intermediate observed at hydrogen rich condition on Pt(111) after interaction with oxygen Janay B. MacNaughton, Lars-Åke Näslund, Toyli Anniyev, Hirohito Ogasawara and Anders Nilsson, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b926409b

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Complementarity between high-energy photoelectron and L-edge

spectroscopy for probing the electronic structure of 5d transition

metal catalysts

Toyli Anniyev,ab

Hirohito Ogasawara,bMathias P. Ljungberg,

c

Kjartan T. Wikfeldt,cJanay B. MacNaughton,

abLars-Ake Naslund,

ab

Uwe Bergmann,bShirlaine Koh,

dPeter Strasser,

eLars G.M. Pettersson

cand

Anders Nilsson*abc

Received 14th December 2009, Accepted 4th May 2010

First published as an Advance Article on the web 5th May 2010

DOI: 10.1039/b926414k

We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for

selectively probing the platinum partial d-density of states (DOS) in a Pt–Cu nanoparticle

catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR).

The information about occupied Pt d-band states was complemented by Pt L2-edge X-ray

absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found

a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to

higher binding energy compared to pure platinum. The effect of this electronic structure

perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the

d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy

and HAXPES allows for an experimental derivation of the valence electronic structure in an

element-specific way for 5d metal catalysts.

1. Introduction

Catalysis facilitates chemical transformations in the chemical

industry providing many products of importance for daily life,

from fertilizers and plastics to pharmaceuticals.1,2 Catalysis is

vital in the current energy sector related to crude-oil processing

and Fischer–Tropsch synthesis of gasoline. It will also play a

major role in the development of future sustainable techno-

logies, such as photocatalytic CO2 reduction to synthetic fuels

and electrochemical energy-conversion devices for the trans-

portation sector.

As early as in the 1930s scientists identified the electronic

structure3,4 as a major factor that controls the activity of solids

towards heterogeneous catalytic reactions. The understanding

of the role of the electronic structure in determining the

reactivity of the catalyst evolved into a simple d-band model.5

It has been successful in relating the adsorption properties

of rate-limiting intermediates in catalytic processes to the electronic

structure of the catalyst based on the energy position of the

center of the d-band.5,6 To fully take advantage of this

development it will be essential to develop experimental tools

that can probe the d-band of catalytic nanoparticles in both

the occupied and unoccupied regions of the electronic struc-

ture. We can then establish an experimental relationship

between electronic structure and catalytic reactivity which

can be compared with theory.

There are several spectroscopic techniques available to

probe the electronic structure of solids, the most widely used

being X-ray photoelectron spectroscopy (XPS), ultra-violet

photoemission spectroscopy (UPS), X-ray absorption spectro-

scopy (XAS) and X-ray emission spectroscopy (XES).7,8 XPS

is a powerful non-destructive probe of the occupied electronic

levels of solids. In the UV regime both the surface sensitivity

and the photoionization cross section of the valence shell are

the highest.9 Furthermore, the low-energy region has provided

high momentum resolution using angular resolved measure-

ments of single-crystal substrates allowing for a detailed

determination of the band structure.9

In order to probe specifically the electronic structure of

transition metal nanoparticle catalysts using XPS severe

challenges must be overcome. In particular, strong spectral

contributions from the catalyst support (oxides, amorphous

carbon) and residual contaminations could dominate the

valence band spectra. However, this hurdle can be overcome

by utilizing the difference in photon energy dependence of

photoionization cross sections of the active catalyst and its

support. By tuning the photon energy, the contribution from

the support can be selectively suppressed and the catalyst

density of states in the valence shell extracted. This is particularly

effective for probing the electronic structure of bimetallic

alloys where the components have different excitation energy

a Stanford Institute for Materials and Energy Sciences,SLAC National Accelerator Laboratory, 2575 Sand Hill Rd,Menlo Park, CA-94025, USA. E-mail: nilsson@slac.stanford.edu

b Stanford Synchrotron Radiation Lightsource, SLAC NationalAccelerator Laboratory, 2575 Sand Hill Rd, Menlo Park,CA-94025, USA

c FYSIKUM, Albanova University Center, Stockholm University,S-106 91 Stockholm, Sweden

dDepartment of Chemical and Biomolecular Engineering,University of Houston, Houston, Texas 77204

eDepartment of Chemistry, Chemical Engineering Division,Technical University Berlin, 10623 Berlin, Germany

5694 | Phys. Chem. Chem. Phys., 2010, 12, 5694–5700 This journal is �c the Owner Societies 2010

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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dependence of their valence shell cross-sections. A good

example would be Pt-3d bimetallic alloy catalysts which are

important catalysts for the oxygen reduction reaction (ORR)

in polymer electrolyte membrane fuel cells.

Valence band spectra measured at low photon energies

are dominated by the contribution from the 3d metal valence

band; the situation is reversed for high photon energies, i.e. at

hn= 8 keV the calculated Pt 5d cross-section (3.1 � 102 barns)

is at least an order of magnitude larger than the 3d cross-

section of any 3d transition metal (the smallest is for Sc

3d= 4.77 � 10�2 barns, the largest for Zn 3d= 15.7 barns).10

Thus, the extension of XPS towards hard X-rays is invaluable

for this application. A pioneering experiment,11 done at 8 keV,

demonstrated the feasibility of hard X-ray photoelectron

spectroscopy (HAXPES) measurements. However, the very

low count rate attainable at the time rendered the technique

useless for any practical applications. It was not until the

development of high-brilliance third-generation synchrotron

radiation facilities that successful measurements of core levels12,13

and valence band14,15 using hard X-rays became possible.

Now, however, HAXPES has become one of the most power-

ful techniques with a wide range of applications, from bulk

electronic structure of strongly correlated materials and solar

cells, to multilayer structures; recent reviews of the technique

are available.16,17

While XPS is used to probe the occupied electronic states,

XAS is used to probe the unoccupied valence states. XAS at

the Pt L-edge probes 2p-to-5d transitions. The first intense

spectral feature in the X-ray absorption near-edge struc-

ture (XANES) is called white line (WL) and represents the

unoccupied DOS in the 5d band.18–20 Conventional XANES

spectra are broadened by the finite lifetime of the core hole

created in the process of absorption. To overcome this

fundamental limitation a high energy-resolution fluorescence

detection (HERFD) X-ray spectroscopy was proposed by

Hamalainen and colleagues,21 who measured sub-lifetime

resolved XANES bymonitoring characteristic fluorescence with

resolution better than the natural line width of the core hole. De

Groot et al.22 measured HERFD spectra of the Pt 5d band

using resonant inelastic X-ray scattering (RIXS) and observed

sharpening of spectral features with respect to conventional

XAS. Safonova et al.23 were able to observe changes in the

unoccupied Pt 5d states upon adsorption of COmolecules using

HERFD. Thus this method allows fine-structure in the spectra

to be resolved and is effective for determining small differences

in the unoccupied DOS for, e.g., Pt-based catalytic materials.

The first measurements of the L2 and L3 edges of Pt were

performed by Qi and colleagues.24 These absorption edges of Pt

are dominated by the dipole allowed transitions 2p1/2 to 5d3/2(L2) and 2p3/2 to 5d3/2,5/2 (L3). Compared to the L3-edge which

has an intense white line, the L2 edge of Pt only shows small

structures. The white line in the L2 edge was even thought to be

absent based on conventional XANES. This was attributed to

the fact that the L2 edge of Pt only makes transitions to the 5d3/2band that is almost filled. However, de Groot et al.22 showed

that when measured using HERFD the L2 absorption edge of

Pt has nonzero white line intensity.

Here, we report on the valence band electronic structure of a

Pt–Cu nanoparticle catalyst studied with high resolution, high

photon energy XPS and on XAS at the Pt L2-edge using

a novel inelastic scattering concept where the life time

broadening of the L-edge can be reduced. In particular we

demonstrate the complementarity of high-energy photo-

electron and L-edge spectroscopies for probing the electronic

structure of 5d transition metal catalysts.

2. Experimental details

The hard X-ray photoelectron spectra (HAXPES) were measured

at beamline BL47XU of the Spring-8 synchrotron radiation

facility. The UHV system was equipped with a Gammadata

Scienta R4000 spectrometer with a total resolution better than

250 meV. The soft X-ray photoelectron measurements were

performed under ultra high vacuum (UHV) conditions with a

base pressure better than 10�10 torr at the undulator beamline

5–1 of the Stanford Synchrotron Radiation Lightsource

(SSRL). The UHV system was equipped with an electron

energy analyzer (VG-Scienta SES-100) for XPS measurements

with total energy resolution better than 0.4 eV for 1480 eV and

0.2 eV for 150 eV. The incident photon energy was varied

between 150 eV and 1480 eV. The Pt L2-edge XAS spectra

were measured at beamline 6–2 at SSRL in RIXS mode

following the Lg1 emission channel.25

The experiments were performed on polycrystalline Pt and

Cu3Pt foil samples and a dealloyed Pt–Cu bimetallic nano-

particle electrocatalyst. The synthesis of the dealloyed nano-

particle electrocatalyst is a two-step process involving the

preparation of the carbon-supported Cu-rich precursor alloy

nanoparticles (Cu3Pt) followed by partial electrochemical

surface dissolution of Cu (dealloying). The Cu-rich catalyst

precursors were prepared by a conventional impregnation-

reductive annealing method involving liquid Pt and Cu salts

and a high surface area carbon support (the details of the

synthesis are given elsewhere26). We compare the electronic

structure of the dealloyed Pt–Cu nanoparticle catalyst to that

of polycrystalline Pt and Cu3Pt foil samples. No significant

core level shift in the Pt 4f level was observed compared to foil

samples indicating that the Pt atoms are in the metallic state.

We computed the d-band density of states (d-DOS) for bulk

platinum and for the Cu3Pt alloy with the GPAW code27

(https://wiki.fysik.dtu.dk/gpaw/). The LDA functional was

used with default PAW (Projector Augmented Wave)28 setups

and grid spacing. The d-DOS was computed by summing the

local density of states of the Pt atomic d-orbitals.

For bulk platinum the smallest orthorhombic unit cell was

used (four atoms in the cell) with the experimental lattice

constant of 3.92 A. A Monkhorst–Pack grid of 20 � 20 � 20

k-points was used and was seen to be well converged with

respect to the d-DOS. For the alloy, the unit cell, containing

one Pt and 3 Cu atoms, was first optimized, giving a lattice

parameter of 3.682 A, then a 3 � 3 � 3 FCC supercell

containing 108 atoms was created by putting the 27 Pt atoms

and 81 Cu atoms randomly at the lattice positions. Finally, the

atomic positions were relaxed using the EMT potential.29 The

d-DOS was in this case calculated using 2 � 2 � 2 k-points

which can be assumed to be reasonably well converged due to

the disorder in the system and the size of the supercell.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 5694–5700 | 5695

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Contributions from all 27 Pt atoms were summed to give the

configurationally averaged d-DOS.

The Pt L2-edge XAS calculations were performed with the

real-space multiple-scattering FEFF8.430 code. Self-consistent

potentials were calculated out to 6 A around the absorbing

atom, which provides converged results; the convergence with

respect to the full multiple-scattering (FMS) radius was

probed by varying it between 5 A and up to 11 A in the case

of bulk Pt. All spectra showed good convergence, except for a

remaining sensitivity even for the very largest atomic clusters

in the bulk Pt white-line intensity and in the small peak around

13.277 keV. The Hedin-Lundqvist model was used for the self-

energy and the scattering potential of the absorbing atom was

calculated without a core-hole, corresponding to a completely

screened core-hole in the final state. To simulate sub-lifetime

broadening in the experiments, the imaginary component of

the self-energy was reduced by 2 eV.

3. Results and discussion

In order to obtain the Pt 5d contribution to the valence band

spectra, we utilize the different dependence on photon energy

for photoelectron emission from the Pt 5d and Cu 3d bands.

Table 1 summarizes the values of atomic Cu 3d and Pt 5d

photoionization cross-sections calculated by Scofield10 and

Band et al.31 At low photon energies, the Cu 3d cross-section

is more than 20 times larger than that of Pt 5d, while at high

photon energies the Cu 3d cross-section drops much faster

than that for Pt 5d. To illustrate how we are able to suppress

the Cu contribution by varying the incident photon energy, it

is useful to examine the hn dependence of valence band spectra

of the polycrystalline Cu3Pt sample. Fig. 1 shows valence band

spectra measured at several photon energies. At hn = 150 eV

(UPS regime), at which the Cooper minimum32–35 occurs

providing low Pt 5d cross-section, the peak shape is represen-

tative of Cu 3d states and agrees well with previous studies.36–38

At this photon energy the valence band spectra are dominated

by contributions from the Cu and, as can be seen, Pt derived

states are not observable. At hn = 1480 eV (XPS regime),

where the Pt 5d and Cu 3d cross-sections are comparable, Pt

5d-derived states start to appear at�5 eV,�1.5 eV and�0.5 eV.Finally at 8 keV (HAXPES regime), where the Pt 5d to Cu 3d

cross-section ratio approaches 30, we observe features represen-

tative of Pt 5d states with negligible contribution from the Cu 3d

band. Thus, the favorable Pt 5d/Cu 3d cross-section ratio at

high energies allows one to selectively follow the changes in the

Pt 5d derived states in the valence band upon alloying.

Fig. 2a compares the background39 subtracted experimental

valence band spectra of polycrystalline Pt and Cu3Pt alloy

samples. At hn = 8 keV, due to the favorable Pt 5d/Cu 3d

cross section ratio, the valence band spectra are dominated by

Pt 5d-states with only negligible contributions from copper.

The valence electronic structure of pure Pt is characterized

by a broad partially occupied d-band and agrees well with

previous studies.40,41 It can be clearly seen that the Pt electronic

structure undergoes significant changes upon alloying with

Cu. In particular, Fig. 2a reveals that the width of the Pt

projected d-DOS decreases relative to that of pure Pt. Another

noticeable change is the decrease for Cu3Pt of the occupied

d-DOS in the region between 1 eV and the Fermi level. The

theoretical d-DOS (Fig. 2b) calculated for Pt and Cu3Pt using

GPAW are in good agreement with the experimental results.

For pure Pt all features observed in the experimental HAXPES

spectra are reproduced very well in the DOS calculations.

Pt L2-edge XANES spectra for Cu3Pt and pure Pt are

shown in Fig. 3a. The spectra were normalized to the same

edge jump across the L2-edge. The Pt XANES results show

effects for the unoccupied d-DOS consistent with the

HAXPES observations. In the XANES, the white line inten-

sity, which measures the number of d-holes, was found to be

reduced for Cu3Pt compared to pure Pt. This is in line with

the decrease in occupied d-DOS near the Fermi level upon

alloying, observed in the measured HAXPES valence band

spectra. Fig. 3b shows full multiple-scattering calculations for

Pt L2 edge absorption. Both the white line and the peak at

13.285 keV are reproduced well in the theoretical calculations.

In addition to these features, the calculations show an addi-

tional peak around 7 eV above the edge that seems to be

present also in the experiment, although completely smeared

out. To check the relative importance of s- and d-like partial

DOS we obtained a decomposition of the total L2-edge

absorption into p-to-s and p-to-d contributions (not shown),

which revealed negligible contribution of the s-absorption

channel. This is in agreement with earlier L2,3 XAS of Pt

which found that the L2 edge is dominated by the 2p1/2 to 5d3/2transition.22 This is mainly due to smaller p-s matrix elements

compared to the p-d matrix elements. The reason for the p-s

matrix elements being small has been attributed to the fact

that the 2p and 5d radial wave functions are nodeless, while 6s

has three radial nodes.42

Both theoretical results clearly reproduce correctly the changes

observed in the Pt projected d-DOS upon alloying. In the

occupied d-DOS, the Pt projected part of the d-band for Cu3Pt

alloy is narrowed and the density of states at the Fermi level is

greatly reduced. In the unoccupied d-DOS, a decrease in the

intensity of the white line of the alloy compared to pure Pt is

observed. The favorable comparison between our experimental

Fig. 1 Background subtracted valence band XPS spectra of the

Cu3Pt foil measured at hn = 150 eV (open triangles), hn = 1480 eV

(open squares) and at hn = 8 keV (open circles). Spectra have been

scaled to have equal area.

5696 | Phys. Chem. Chem. Phys., 2010, 12, 5694–5700 This journal is �c the Owner Societies 2010

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and theoretical results suggests that theory can be used success-

fully to analyze the trends seen in both the occupied and

unoccupied Pt derived d-DOS upon alloying.

The observed changes in d-band states are explained by the

local hybridization picture. The d–d hybridization, and there-

fore the d bandwidth, is expected to change when metals with

different d-level binding energies are alloyed. The decreased Pt

projected d density of states near the Fermi level is similar to

that observed previously in a study of the electronic structure

of different Pt–Cu alloy surfaces using soft X-ray photoelectron

spectroscopy;43 the focus was mainly on the variations in the

Cu projected d states upon alloying. Utilizing the Cooper

minimum of the Pt 5d band at 150 eV the authors reported

significant hybridization of the Cu derived d-DOS between

�1.3 eV and EF. Using a photon energy at which the Pt 5d

cross section is high (hn = 80 eV), they observed, similar to

our results, a decrease in the density of Pt projected d states

near the Fermi level which was attributed to significant

hybridization between Cu 3d and Pt 5d bands. Nahm and

colleagues44 developed an iterative method for extracting the

Pt 5d partial DOS, again utilizing the difference in Pt 5d and

Cu 3d cross sections at two different energies (hn= 60 eV and

hn = 160 eV) and confirmed the earlier observations. The

unoccupied part of the Pt projected d-DOS, was measured by

Lee and colleagues45 using Pt L2,3-edge XAS of Cu–Pt alloy

foils. Their results indicate that upon alloying the WL intensity

Table 1 Calculated Pt 5d and Cu 3d cross-sections at selected photon energies (Mbarns/shell)

Photon energy/eV 150 eV 1480 eV 8 keV

Pt 5d 1.94 � 10�1 a 2 � 10�2 a 1.98 � 10�2 b 3.16 � 10�4 b

Cu 3d 4.4a 8.3 � 10�3 a 7.8 � 10�3 b 1.08 � 10�5 b

Ratio Pt 5d/Cu 3d 0.044 2.44 2.54 29.3

a From Band et al.31 b From Scofield.10

Fig. 2 (a) Valence band XPS spectra (hn = 8 keV) with comparison

of Pt (solid line) and Cu3Pt (open circles) foils. Spectra are normalized

to have equal area and the inelastic background was removed. (b)

Theoretical Pt density of d-states (d-DOS) computed for Cu3Pt alloy

(open triangles) and Pt (solid line).

Fig. 3 (a) Pt L2-edge XAS of Pt (solid line) and Cu3Pt (open circles)

foils. The spectra were normalized well above the absorption edge.

(b) Theoretical X-ray absorption coefficient for the platinum L2-edge

of Pt (solid line) and Cu3Pt (open triangles) normalized to match the

experiment.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 5694–5700 | 5697

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in the L3-edge decreases, consistent with the decrease in

number of unoccupied d-states. However their L2-edge XAS

results show an increased WL intensity for the Cu–Pt alloys

compared to pure Pt, contrary to our findings. We explain the

contradiction by the lack of adequate resolution in their

spectra due to lifetime broadening inherently present in con-

ventional XANES. As a consequence of this limitation the

L2-edgeWL is absent and, therefore, a broad feature at 13280 eV

instead was regarded as a WL.

Finally, we show the results on the carbon-supported

Pt–Cu alloy nanoparticle catalyst prepared by electrochemical

dealloying. A family of Pt–Cu nanoparticle catalysts prepared

in the same manner exhibit enhanced ORR activities of up to

5–6 times compared to pure Pt catalysts.26

The average size of the Pt–Cu nanoparticle catalysts was

B3.4 nm.46 Table 2 summarizes the compositional analysis of

the nanoparticle catalyst.w The derived concentration profile

of the dealloyed nanoparticles suggests a core–shell structure

where the shell consists of Pt atoms while the core is

50%–50% in composition. This structure is interesting since

the Pt-shell can assume the lattice constant of the core, which

is smaller than the bulk lattice constant of Pt. This will result

in a strained shell46 which will change the electronic structure

and, as consequence, the reactivity of the surface Pt atoms. On

the other hand there may also be a non-negligible chemical

interaction between the core Cu atoms and the surface Pt

atoms. These two effects combined are likely underlying causes

of the enhanced reactivity observed for the thus prepared

nanoparticle catalyst.

Fig. 4 compares the valence band HAXPES and Pt L2-edge

XAS spectra of pure Pt foil and the carbon-supported Pt–Cu

nanoparticle catalyst before and after dealloying. As mentioned

earlier, using HAXPES allowed us to suppress the contri-

bution from Cu and the carbon support while enhancing the

contribution from Pt sites, which are active for the ORR. The

overall appearance of the valence band of the Cu3Pt precursor

(before the dealloying process) nanoparticles is similar to the

Cu3Pt alloy foil shown in Fig. 2a. The spectrum for the nano-

particle precursor shows some broadening of the individual

peaks compared to the Cu3Pt alloy foil, which is explained by

the structure disorder of the nanoparticle and a non-uniform

size distribution. Pt L2-edge XAS results show a significant

reduction in WL intensity for Pt–Cu alloy precursor nano-

particles similar to what was observed for Cu3Pt alloy foil.

Upon dealloying we see broadening of the Pt projected d-DOS

in the valence band HAXPES. The d-DOS near the Fermi level

is increased compared to the Cu3Pt precursor nanoparticles. In

L2-edge XAS this results in an increase in the WL intensity,

implying an increase in the number of d-holes. When com-

paring the nanoparticle catalyst before and after dealloying, one

has to remember that both HAXPES and Pt L2-edge XAS are

bulk sensitive techniques and that spectra are averaged over the

whole nanoparticle volume. For dealloyed Pt–Cu catalyst this

means that the spectra represent the average of the alloy core

and Pt-rich shell. This explains why observed changes upon

dealloying are small. When compared to pure Pt, there is

still a significant modification of the electronic structure of the

dealloyed Pt–Cu nanoparticle catalyst. A narrowing of the Pt

projected d-band and a decrease in the Pt projected d-DOS near

the Fermi level are observed for the Pt–Cu nanoparticle

catalyst. Pt L2-edge XAS results of the Pt–Cu catalyst show a

decreased WL intensity compared to pure Pt. These observa-

tions are consistent with the downward shift of the center-of-

mass of the Pt projected part of the d-band.

The effect of the modified electronic structure of the catalyst

on its catalytic reactivity can be readily explained within the

framework of the d-band model developed by Nørskov and

colleagues.5 The d-band model has been successful in relating

adsorption properties of rate-limiting intermediates to the

electronic structure of the catalyst.47,48 When simple adsorbates

like O, C and N adsorb on a metal surface, as a result of the

metal–adsorbate interaction, the adsorbate valence 2p-level

forms bonding and antibonding states with the d-band of the

metal.48 Population of the oxygen–metal antibonding states

leads to more Pauli repulsion and thus to weakening of the

metal–adsorbate bond.5 Recently,46 we have shown that when

platinum is strained the Pt d-band experiences a downward

shift similar to the shift observed for the Pt projected d-DOS of

the Pt–Cu nanocatalyst. Using oxygen K-edge X-ray emission

(XE) and absorption (XA) spectroscopies we studied changes

in the oxygen 2p and Pt 5d antibonding states projected onto

Table 2 Compositional depth profile of the Pt–Cu nanoparticlecatalyst before and after dealloying. The chemical composition isgiven in Pt at%

Photon energy/eV 260 620 8000

Cu3Pt precursor 8% 12% 31%Cu3Pt dealloyed 84% 68% 59%

Fig. 4 Valence band XPS and Pt L2-edge XAS spectra of the carbon-

supported dealloyed Pt–Cu nanoparticle catalyst (solid line), Cu3Pt

alloy nanoparticle precursor (dotted line) and Pt foil (dashed line).

w We obtained compositional depth profiles of the catalyst by measuringthe ratio of the Pt 4f to Cu 3p XPS intensities normalized to theirrespective subshell photoionization cross sections50 at various photonenergies corresponding to different probing depths. The kinetic energyof the photoelectron defines the inelastic mean free path (IMFP) andthe probing depth of the analysis. We varied the photoelectron kineticenergy by changing the incident photon energy to obtain the composi-tion at different probing depths. For any given incident photon energy,the Pt 4f (binding energy EB = 71 eV for f7/2) and Cu 3p (bindingenergy EB = 75 eV for p3/2) photoelectrons with similar kineticenergies are collected from the same probing depth. The ratio of Pt4f and Cu 3p XPS intensities (normalized to cross sections) can thus beused to estimate the Pt to Cu ratio averaged within the given probingdepth. Estimated (using the TPP2M formula51) probing depths atphoton energies of 250 eV, 620 eV and 8000 eV are, respectively, 0.6 nm,1 nm and 7 nm, where the latter is comparable to the particle size.

5698 | Phys. Chem. Chem. Phys., 2010, 12, 5694–5700 This journal is �c the Owner Societies 2010

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the oxygen atom. The XES and XAS results revealed that

when the Pt d band experiences a downward shift it pulls more

of the antibonding states of oxygen below the Fermi level

(antibonding states of oxygen on pure Pt are located above the

Fermi level), which results in more occupation of the anti-

bonding states. This results in weaker bonding of oxygen

species to the strained Pt surface. Combined HAXPES and

Pt L2-edge XAS results of the Pt–Cu nanoparticle catalyst

indicate a downward shift of the Pt projected d-DOS similar to

that observed for the strained Pt surface, so the conclusion can

be transferred directly to the Pt–Cu nanoparticle catalyst, i.e.

the downward shift of the Pt projected d-DOS reduces the

binding energy of oxygen species adsorbed on the surface of

the dealloyed Pt–Cu nanoparticle catalyst. Theoretical results

based on the microkinetic model developed by Hammer and

Nørskov47 for the oxygen reduction reaction49 suggest a

volcano relationship between ORR rate and the binding energy

of the oxygen species. The volcano shape implies that a

downward shift of the Pt d-band enhances the ORR activity

by reducing the binding energy of intermediate oxygen species

and, thus, reducing the activation barriers for proton- and

electron-transfer processes. This interpretation explains con-

sistently observed changes in the Pt projected d-DOS of the

dealloyed Pt–Cu nanoparticle electrocatalyst and its enhanced

ORR reactivity.

4. Conclusion

In summary, the present study demonstrates the feasibility of

hard X-ray photoelectron spectroscopy (HAXPES) to study

the valence band electronic structure of Pt–Cu nanoparticle

catalysts. We found a significant electronic perturbation of the

Pt projected part of the d-DOS: it becomes narrower and shifts

to higher binding energy in the dealloyed Pt–Cu nanoparticles

compared to bulk Pt. The downshift of the d-band is con-

firmed by Pt L2-edge X-ray absorption measurements which

probe the unoccupied valence states. The electronic structure

was used to explain the enhanced activity of the catalysts

towards ORR within the theoretical framework of the d-band

model. We argue, within the d-band model, that the downward

shift of the Pt projected d-DOS leads to a weakening of the

chemical bond between oxygen species and catalyst surface

atoms and thus to enhanced ORR reactivity.

The development of HAXPES gives a new unique addi-

tional capability to investigate the electronic structure of

transition metal catalysts that is complementary to L-edge

spectroscopy since it probes directly the occupied part of the

valence band. The technique will be applicable in general for

most d-band metal catalysts since the cross section of the metal

will be enhanced in comparison to the valence band emission

from C, N and O atoms in the support. This enhancement is

particularly strong for the 5d transition metal series where also

the partial 5d density of states can be projected for alloy

catalysts. It can form the basis to determine the d-band center

and width in real nanoparticle catalysts on supports and allow

a direct comparison with theory. Furthermore, the technique

can be extended to ambient pressures allowing for in situ

monitoring of the catalyst electronic structure under reaction

conditions during gas phase catalytic reactions. This requires a

modification of the energy analyzer to incorporate a differen-

tial pumping section. The technique of HAXPES can thus be

widely applied to transition metal catalysts and complement

conventional L-edge spectroscopy as probe of the electronic

structure.

Acknowledgements

This work was supported by the Division of Materials Sciences

and Engineering, Office of Basic Energy Sciences, US Depart-

ment of Energy, under the auspices of the President’s Hydrogen

Fuel Initiative, and by the Swedish national research council.

This research was partly carried out at the Stanford Synchrotron

Radiation Laboratory, a national user facility operated by

Stanford University on behalf of the US Department of

Energy, Office of Basic Energy Sciences. Portions of this

research were performed at Spring-8 with the approval of Japan

Synchrotron Radiation Research Institute as Nanotechnology

Support Project of the Ministry of Education, Culture, Sports,

Science and Technology (Proposal No. 2007A2005 and

2008A1671/BL-47XU). A generous grant of computer time at

the Swedish NSC national computing center is gratefully

acknowledged. P.S. acknowledges support from UNICAT

cluster of excellence in catalysis funded by the GermanNational

Science Foundation and located at the Technical University

Berlin, Germany.

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