Hongliang Xin, Neil Schweitzer, Eranda Nikolla, Suljo Linic *
Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA
A.
Synthesis of Pt Alloy Catalysts
The catalysts were synthesized using a dry impregnation technique. γ-Al
2
O
3
powder from Alpha
Aesar (3 micron, BET S.A. ~78 m
2
/g, 99.97% metal basis) was used as the support for small particles of pure Pt or Pt alloys. To remove impurities from the support, the powder was first stirred in deionized water. Ammonium hydroxide was then added to the solution until the pH reached 10. The powder was then rinsed several times with deionized water, dried under vacuum overnight, and calcined in air at 773
K for 3 hrs.
A 10% Pt/Al
2
O
3
sample was prepared by dissolving the appropriate amount of H
2
PtCl
6
·6H
2
O
(99.95% metals basis, Alpha Aesar) in just enough volume of water to fill the micro pore structure of the
Al
2
O
3
support, as measured by BET analysis (0.366 mL/g). The solution was then added dropwise to the
Al
2
O
3
support. To ensure even coverage of the solution on the support, the powder was shaken vigorously throughout the addition of the solution, and the powder was crushed with a mortar and pestle.
To synthesize the alloy catalysts, an impregnation solution consisting of half the water and half the
H
2
PtCl
6
·6H
2
O for the 10% Pt/Al
2
O
3 catalyst was prepared (this corresponds to a 5% Pt/Al
2
O
3
mixture).
Another impregnation solution was made containing the same amount of water and enough of the appropriate alloy metal salt (CuCl
2
, RuCl
3
, or SnCl
2
) for a Pt/M molar ratio of one. The Pt and alloy metal solutions were then dispersed on the Al
2
O
3
support sequentially, as described above. All samples were then dried overnight under vacuum, and reduced in hydrogen (Cryogenic gases, pre-purified, 200 mL/min) at 573 K for 3 hrs.
1

The PtCu
3
alloy was synthesized using a colloidal, ion exchange method performed at room temperature. 0.1g of Cu(NO
3
)
2
was dissolved in ethylene glycol in a round bottom flask. The flask was sealed with a septum, vented, stirred with a stir bar, and purged with Ar gas. Ar gas was used to purge the vessel throughout the entire synthesis process. Next, just enough aqueous solution of 1.8 M NaOH/
0.5 M NaBH
4
was added to the Cu solution until the solution turned a deep red/brown. This step forms a
Cu colloid. After resting for an hour, an ethylene glycol solution of H
2
PtCl
6
was added to the copper colloid solution (enough Pt precursor for a 3:1 Cu:Pt molar ratio). In this step, the metallic copper in solution reduces the Pt precursor solution, depositing Pt on the Cu colloid particles, turning the solution black. After letting the solution stir for 1 hr, the final colloid solution was then centrifuged and washed several times with isopropyl alcohol to remove and residual sodium. The pure colloid solutions were then mixed with the Al
2
O
3
powder and dried in a heated sonicator (~60°C) until all the solvent had evaporated.
B.
Synthesis of Ni Alloy Catalysts
The Ag/Ni,
Au/Ni and Sn/Ni alloy materials were supported on (8 mol%) yttria-stabilized zirconia (YSZ). YSZ was prepared via a standard co-precipitation method. A mixture of yttrium nitrate
(Y(NO
3
)
3
·6H
2
O) and zirconyl chloride (ZrOCl
2
·2H
2
O) in deionized water was precipitated using a solution of ammonium hydroxide. After filtration and drying over night, the precipitate was calcined at
1073 K for 2 hrs. Ni was introduced to the YSZ support by impregnating Ni nitrate via the incipient wetness technique. The Ni loading in YSZ was about 15 wt% with respect to the total catalyst. After impregnation the catalyst was calcined at 873 K for 2 hrs in air in order to convert the Ni nitrate to Ni oxide. The catalyst was then reduced at 1173 K for 3 hrs using 30% H
2
/N
2
. The Ag/Ni/YSZ, Au/Ni/YSZ and Sn/Ni/YSZ catalysts were synthesized by impregnating AgNO
3
, HAuCl
4
· x H
2
O and SnCl
2
·4H
2
O respectively on the NiO/YSZ catalyst via the incipient wetness technique. The nominal Ag, Au and Sn
2

loadings were about 2 wt% with respect to the Ni content in the catalyst. The Ag/Ni/YSZ, Au/Ni/YSZ and Sn/Ni/YSZ catalysts were reduced at 1173 K for 3 hrs using 30% H
2
/N
2
. We showed previously that this synthesis procedure yields surface alloy particles, characterized by pure Ni core and Ni/M (M = Ag,
Au, Sn) shell.
X-ray absorption spectroscopy (XAS) studies were performed at Argonne National Laboratory
Advanced Photo Source (APS) beamline MR-CAT 10-ID-B. Scans of the Pt L
3
-edge (~11564eV) were collected in transmission mode using Ar ion chambers for detection [1]. Data was simultaneously recorded for Pt foil. The Pt is usually used as the Pt standard for energy calibration. The pure Pt and Pt alloy samples were packed into wafers supported in a steel sample tube, which could accommodate six separate samples. The steel sample holder was then placed in a glass in-situ reactor. The samples were re-reduced in 4% H
2
/N
2 at 573 K for 1 hrs. The samples were then flushed in N
2
at 573 K to remove any residual hydrogen. Scans were performed at room temperature in nitrogen.
XAS results were analyzed using publicly available software Athena and Artemis following standard analysis procedures [2]. First, the curves were calibrated by setting the point of the Pt foil data with the maximum slope to the theoretical Pt L
3
-edge onset, 11564 eV. The background data of each curve was removed by fitting the pre-edge data to a linear function and subtracting out the result. The curves were normalized by fitting the post-edge data to a third degree polynomial and dividing the data by the extrapolated value at E
0
(the edge onset). Phase and amplitude for each first nearest neighbor scattering path (Pt-Pt and Pt-M where M is Cu, Ru, or Sn) were calculated using FEFF [2]. The Pt L
3
edge EXAFS spectra for different materials are shown in Fig. S1 through Fig. S4. Fitting parameters can be viewed in Table S1.
3

To probe the electronic structure of Ni alloys, we used electron energy loss spectroscopy (EELS), which gives identical information as XANES, i.e., it maps out the states above the Fermi level localized on an element in a material. We used EELS to study Ni alloys since it offers better spatial resolution than XANES and we could easily focus the electron beam on the shell regions of the Ni alloy particles, which are characterized by the formation of the alloy phase. The EELS experiments were carried out in a JEOL 2010F field-emission electron microscope. The instrument was operated at 200 kV under a pressure of 1.5

10
-7
Torr in scanning transmission electron mode. A Gatan imaging filter was utilized to collect the low-angle inelastically scattered electrons in order to obtain the electron energy-loss nearedge structure (ELNES) spectra, which were analyzed using Digital Micrograph. The lens conditions during operation were defined for a probe size of ~0.2 nm, with a convergence angle of 13 mrad and a collection angle of 40 mrad. The energy resolution of the ELNES spectra (defined by the full width at half-maximum of the zero-loss peak) was 1.4 eV at a dispersion of 0.3 eV per pixel.
The specimens for the electron microscopy experiments were prepared by directly placing the pre-reduced catalyst powders on holey carbon-coated copper grids. Once inside the electron microscope, the samples were heated under vacuum to 773 K to desorb any impurities. The size of the particles was measured using a number of techniques, including the analysis of X-ray diffraction spectra, Brunauer-
Emmett-Teller (BET) surface area measurements, and TEM. The diameter of the analyzed particles was
~20 nm.
The ELNES spectra were normalized with respect to the atomic cross section, which was calculated by integrating the area under the energy-loss spectra between 30 and 40 eV above the onset of the Ni L
3
-edge [3,4]. The background effects were subtracted using a standard power-law method [5].
4

The Ni L
2,3
-edge ELNES spectra are shown in Fig. S5. Integration of areas under the peaks for pure Ni and Ni alloys showed that the number of states localized on Ni is preserved as shown in Table
S2, i.e., it does not change in response to alloying.
A.
Adsorption Energies Calculations
First-principles density functional theory (DFT) calculations were performed using the ultrasoft pseudopotential [6] plane-wave method with the generalized gradient approximation [7] (GGA-PW91) coded in Dacapo [8]. The wave-functions were expanded in plane-waves with an energy cutoff of 350 eV. The Pt skin alloys were modeled by a four-layer slab separated by 10 Å of vacuum space with subsurface 3 d , 4 d or 5 d metals. An adsorbate was placed at 1/4 ML coverage on one side of the slab. A dipole correction was applied in z -direction to minimize interactions among neighboring cells. The adsorbates and top two layers were relaxed until the force on the atoms was less than 0.05 eV/Å. We used 32 Monkhorst-Pack irreducible k -points in the irreducible Brillouin-zone (IBZ) of the p (1

1) Pt surface unit cell
[9]
. A finite temperature Fermi function ( k
B
T el
= 0.1 eV) was utilized to facilitate the
SCF convergence, and the total energy was extrapolated to k
B
T el = 0 eV. Our calculations have shown that this setup yields converged adsorption energy differences between an adsorbate on different surfaces. The adsorption energies of different adsorbates on Pt skin alloys (3 d , 4 d and 5 d metals) as shown in Fig. 4 and Fig. S6 were calculated and referenced to the corresponding gas phase CO, H
2
O, N
2 and H
2
.
The Bader and Mulliken charge distribution of 3 d -M/Pt skin alloys has been determined to investigate the effect of alloying on the number of electronic charge localized on surface Pt atoms. In order to evaluate the number of localized d -states on surface Pt atoms, we have calculated the projected
5

density of states (PDOS) by projection of the one-electron states onto spherical harmonic atomic d orbitals centered on surface Pt atoms. Since there is an ambiguity of the cutoff radius to partition the d charge, we have calculated the integrated number of s , p , d electrons with different cutoff radii. These results are shown in Fig. S7. We can see that there is a slight charge transfer from the 3 d -M to the surface Pt atoms based on Bader and Mulliken distribution. However, orbital-resolved integrated projected density of states and Mulliken charge distribution indicate that the charge transfer stems from the 3 d -M to the delocalized sp -states of Pt atoms.
B.
Oscillator Strength Calculations (X-ray Absorption Near Edge Structure Calculations)
X-ray absorption near edge structure (XANES) of the Pt L
3
-edge (oscillator strength) was calculated from first-principles using plane-wave pseudopotential method as incorporated in CASTEP code
[10]
. The exchange-correlation was described using PW91 functional of the generalized gradient approximation (GGA) [7]. The wave-functions were expanded in plane-waves with an energy cutoff of
400 eV and the system was modeled by a supercell with the lattice constant from experimental measurement. 12 × 12 × 12 Monkhorst–Pack [9]
set of k -points was used for Brillouin-zone sampling of
1 × 1 × 1 unit cell. The k -points density in k -space was held constant for calculations with larger unit cells. An ultrasoft pseudopotential [6] was generated on the fly while performing the calculations of the oscillator strength with partial occupancy of core states [11] . To test the significance of core-hole effect in the calculations of the oscillator strength, we calculated Pt L
3
-edge XANES spectra of the same system using different populations of the core hole states. The results of these calculations are shown in
Fig. S8. We found that the spectrum calculated with ground state approximation matches the experimental measurement very well, which indicates that the core-hole interaction has only negligible effect for the Pt alloy system. All oscillator strength calculations with partial core holes were carried out using unit cells sufficiently large to eliminate the core-hole interactions between periodic images. The
6

calculated spectra for all alloys were adjusted (shifted in energy by a constant value) so that the calculated edge onset energy for pure Pt corresponds to the experimentally measured edge onset energy of pure Pt L
3
-edge. Gaussian broadening with 3.0 eV, which encompasses instrumental broadening and the lifetime of excited states, was applied in order to compare calculated spectra to experimental measurements.
The Newns-Anderson (N-A) model of chemisorption was used to investigate the effect of the sp band occupancy on the adsorption energies of various adsorbates (atomic oxygen as an example) on Pt and Pt skin alloys. We note that the N-A model is used to describe the coupling between an adsorbate state and a large number of electronic states of a surface. For details about the algorithm, see the original contributions [12,13] and the following publications
[14,15]
.
To approximate the sp electronic band of the Pt and Pt alloy substrates, we used a semi-elliptic function fitted to the DFT-calculated sp projected density of states on the surface Pt atoms. The band occupancy, depending on the constituent metal elements in alloys, was consistent with the band filling obtained from DFT calculations.
The weighted sp density of states of a substrate can be written as Eq. (1.1).

(

)
 
V sp
2

(

), (1.1) where

(

) is a semi-elliptic function representing the sp projected density of state on surface Pt atoms and V sp
is the coupling matrix element between the adsorbate valence state with the electronic band of substrates [16] . As the adsorbate is brought close to the surface, the two sets of states will be coupled by the matrix element, V sp
. The projected density of states on the adsorbate and therefore the hybridization
(adsorption) energy due to the interaction can be calculated from the N-A model [12,13]. In Fig. S9, we show the hybridization energy of the atomic oxygen interacting with the sp electronic bands of different
7

3 d -M/Pt skin alloys (the different alloys are characterized by different occupancies of the sp band) as a function of the coupling matrix element, V sp
2
. Assuming that the coupling matrix element between the adsorbate state and the sp -band is similar for the different 3 d -M/Pt skin alloys (this should be the case since it is Pt surface atoms that are interacting with the adsorbate for all skin alloys), we can see in Fig.
S9 that the hybridization energy is not sensitive to the change in the occupancy of the sp -band. This leads us to conclude that the difference of the binding energy of adsorbates on Pt skin alloys is mainly due to the change of the d -band.
It is worth noting that the reasonable value for V sp
2 is ~ 0.35 eV
2
. This can be estimated by realizing that the hybridization energy for atomic oxygen interacting with the sp -band of metal surfaces is around -5.0 eV [16] [17].
8

[1] http://mrcat.iit.edu/mrcat_instrumentation.html.
[2] B. Ravel and M. Newville, Journal of Synchrotron Radiation 12 , 537-541 (2005).
[3] V. J. Keast, A. J. Scott, R. Brydson, D. B. Williams, and J. Bruley, Journal of Microscopy 203 , 135-175 (2001).
[4] G. A. Botton, G. Y. Guo, W. M. Temmerman, and C. J. Humphreys, Phys. Rev. B 54 , 1682 (1996).
[5] R. F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope , 2nd ed. (Springer, 1996).
[6] D. Vanderbilt, Phys. Rev. B 41 , 7892 (1990).
[7] J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson, D. J. Singh, and C. Fiolhais, Phys. Rev. B
46 , 6671 (1992).
[8] https://wiki.fysik.dtu.dk/dacapo/Dacapo.
[9] H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13 , 5188 (1976).
[10] M. D. Segall, Journal of Physics: Condensed Matter 14 , 2957-2973 (2002).
[11] S. Gao, C. J. Pickard, A. Perlov, and V. Milman, J. Phys.: Condens. Matter 21 , 104203 (2009).
[12] D. M. Newns, Phys. Rev. 178 , 1123 (1969).
[13] P. W. Anderson, Phys. Rev. 124 , 41 (1961).
[14] J. P. Muscat and D. M. Newns, Progress in Surface Science 9 , 1-43 (1978).
[15] S. K. Lyo and R. Gomer, Phys. Rev. B 10 , 4161 (1974).
[16] R. M. Lambert and G. Pacchioni, Chemisorption and Reactivity on Supported Clusters and Thin Films (Springer,
1997).
[17] D. R. Lide, CRC Handbook of Chemistry and Physics, 90th Edition , 90th ed. (CRC, 2009).
9

FIG. S1. Pt L
3
-edge EXAFS Spectra for Pure Pt and Pt
3
Sn Alloy in (a) r -space, (b) k -space.
10

FIG. S2. Pt L
3
-edge EXAFS Spectra for Pure Pt and PtRu Alloy in (a) r -space, (b) k -space.
11

FIG. S3. Pt L
3
-edge EXAFS Spectra for Pure Pt and PtCu Alloy in (a) r -space, (b) k -space.
12

FIG. S4. Pt L
3
-edge EXAFS Spectra for Pure Pt and PtCu
3
Alloy in (a) r -space, (b) k -space.
TABLE S1. Structural Parameters from EXAFS Fitting Results for Pt L
3
-edge EXAFS Spectra of Pt Alloys.
(Coordination Numbers CN, Distances R ).
Sample Scatter Path
PtRu
Pt - Pt
Pt - Ru
Pt
3
Sn
PtCu
PtCu
3
Pt - Pt
Pt - Sn
Pt - Pt
Pt - Cu
Pt - Pt
Pt - Cu
CN R (Å)
4.5 ± 0.4 2.75 ± 0.01
1.8 ± 0.4 2.75 ± 0.02
4.8 ± 0.3 2.78 ± 0.01
3.1 ± 0.2 2.74 ± 0.01
7.6 ± 0.8 2.73 ± 0.01
2.5 ± 0.5 2.64 ± 0.02
- -
12.8 ± 1.3 2.59 ± 0.01
R Model (Å) E
0
(eV)
2.73
2.73
7.4 ± 1.4
7.4 ± 3.6
2.83
2.83
4.4 ± 1.0
4.9 ± 0.9
2.71
2.66
7.6 ± 1.5
7.3 ± 5.1
-
2.60
-
5.1 ± 1.1
13

FIG. S5. The Ni L
2,3
-edge ELNES Spectra of (a) Ni, Ag-Ni and Au-Ni Particles, (b) Ni and Sn-Ni
Particles. The Ni L
2,3
-edge ELNES spectra were measured with the electron beam penetrating the perimeter of the alloy particles(probing the surface). It is clear that the Ni L
2, 3
-edge is broader for Ag-
Ni, Au-Ni and Sn-Ni particles compared to pure Ni. Quantitative analysis of the spectra (Table S2) showed that the number of d -holes localized on Ni is identical for both systems suggesting the local charge is preserved.
TABLE S2. Integrated and Normalized (with Respect to Ni Atomic Cross Section) Values of the
Measured Ni L
2,3
-edge ELNES spectra for YSZ-Supported Monometallic Ni, Ag-Ni, Au-Ni and Sn-Ni
Alloys a
Catalysts Ni/YSZ Ag-Ni/YSZ Au-Ni/YSZ Sn-Ni/YSZ
Integrated (a.u.)
Normalized ( e )
1.32 ± 0.07
1.00
1.26 ± 0.07
0.97 ± 0.04
1.45 ± 0.07
1.10± 0.04
1.31 ± 0.07
0.99± 0.04 a
The range of integration for the measured Ni L
2,3
-edge ELNES was -5 to 35 eV from the onset of the Ni L
3
-edge.
It included the near-edge regions of both the Ni L
3
- and L
2
-edge.
14

FIG. S6. Relationship between the Characteristic Length of Metal d -orbitals with the Binding Energies of
Adsorbates on Pt 4 d (a) and 5 d (b) Skin Alloys.
15

FIG. S7. Charge Analysis of 3 d -M/Pt Skin Alloys. (a) Bader and Mulliken Charge Analysis. (b) Integration of d
Electrons from Projected Density of States on the Surface Pt Atom. (c) Integration of s Electrons. ( d ) Integration of p Electrons.
16

FIG. S8. Pt L
3
-edge XANES Spectra Calculated with Different Core-hole Approximations Compared with the
Experimental Measurement.
17

FIG. S9. Newns-Anderson Model Calculated Atomic Oxygen Adsorption Energies on 3 d -M/Pt Skin Alloys as a
Function of Coupling Matrix Elements.
18