Supporting Information
The Origin of Rh and Pd Agglomeration on the
CeO2(111) Surface
Baihai Li,1,2 Obiefune K. Ezekoye,2 Qiuju Zhang,1 Liang Chen,1,* Ping
Cui,1 George Graham,2,* Xiaoqing Pan2,*
1
Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo 315201, P. R. China
2Department
of Materials Science and Engineering, University of Michigan, Ann
Arbor, Michigan 48109
Energetics and electronic structures of Pd (Rh) atoms and monolayer on
CeO2(111)
As labeled in the inset of Figure S1, five binding sites on the CeO2(111) surface
are considered in our calculations: (1) three-fold OS site above the subsurface O atoms,
(2) two-fold OFB site on the surface O bridge, (3) Ce site on top of Ce atoms, (4)
two-fold OSB site above the subsurface O bridge and (5) one-fold OF site on the
surface O atoms. The binding energy (EB) for Pd and Rh deposited on CeO2(111) is
defined as:
EB    EM  S   N  EM  ESopt  N


opt
(1)
where EM , ESopt and EMopt S represent the energies of a single Pd or Rh atom, the
clean CeO2(111) substrate and the metal/substrate complex, respectively; N is the
number of metal atoms. For the clusters (e.g., monolayer, tetrahedral and icosahedral)
deposited on the substrate, the overall binding energy includes two contributions:
metal-substrate and metal-metal interactions. The metal-metal cohesive energy (Ecoh)
is defined as:

Ecoh   EM isolated  N  EM
N

N
Then the metal-substrate adhesion energy (△EMS) can be evaluated by:
(2)

EMS    EB  Ecoh  or EMS   EMopt S  ESopt  EM isolated
N

N
(3)
where the EM isolated is the energy of the isolated metal cluster.
N
Figure S1. (Left) Top view and (Right) Side view of supercell. The white spheres
represent Ce atoms. The O atoms of subsurface layer (OS) are represented as blue
spheres, while the outmost surface O atoms (OF) and other O atoms are highlighted
as red spheres.
The calculated binding energies for a single Pd or Rh atom deposited on each
binding site of CeO2(111) are summarized in Table S1. It is found that the deposition
on the OFB site yields the highest binding energies of 1.87 and 2.86 eV for Pd and Rh,
respectively. The two Pd-coordinated OF atoms move outward by 0.22 Å from the
surface plane, forming two Pd-O bonds of 2.23 Å. Similarly, the Rh adatom drags the
two nearest neighboring OF ions out of the surface by 0.26 Å and forms two shorter
and stronger Rh-OF bonds of 2.07 Å. Correspondingly, the associated Ce-O bonds are
also weakened. The second most stable position is the three-fold OS site, followed by
the two-fold OSB and one-fold OF site.
Table S1. Binding energy (eV) of Pd and Rh single atoms on CeO2 (111).
1 Pd (Rh)/CeO2(111)
Binding Site
OF
OS
Ce
OFB
OSB
Pd-EB
1.58
1.84
1.28
1.87
1.65
Rh-EB
1.77
2.72
1.69
2.86
2.17
Table S2. Binding energy (eV/atom) of Pd(Rh) monolayer deposited on the CeO2
(111) surface according to eqn.(1)
Binding Site
OF
OS
Ce
OFB
Pd-EB
1.94
1.76
1.44
1.88
Rh-EB
2.58
2.58
2.24
2.90
We have shown that the OFB site is identified as the most favorable binding site
for the single Pd or Rh atoms. Instead, the Pd monolayer with each Pd atom deposited
on the OF sites yields the highest binding energies of 1.94 eV/atom (see Table S2), of
which 1.22 eV is contributed from the weakened Pd-CeO2 adhesion energy. Each Pd
atom donates 0.21 to the substrate, which are slightly smaller than in the case of
single atom deposition. Accordingly, each of the nearest Ce ions obtains 0.12
electrons. The deposition on OFB has a slightly lower binding energy of 1.88 eV/atom.
In addition, structural optimizations demonstrate that the Pd monolayer could keep the
rhombus shape on all binding sites, with a cohesive energy of 0.72 eV/atom. On the
other hand, the Rh monolayer with each Rh atom deposited on the OFB sites still
yields the highest binding energy of 2.90 eV/atom, although it is accompanied with
drastic distortion of the substrate. The outmost OF ions are dragged out of the surface
plane by 0.15 Å in the horizontal direction and 0.54 Å in the vertical direction. The
Rh ad-layer drastically loosens the Ce-O bond strength and enhances the activity of
the outmost O ions. Each Rh atom donates 0.33 electrons to the substrate, while each
of the nearest Ce ion gains 0.20 electrons, which is 0.09 electrons more than in the
case of single Rh deposition. Moreover, the Rh rhombus shape is distorted to be a
parallelogram with one set of 0.013 Å longer than the other set, yielding a cohesive
energy of 1.15 eV/atom and the Rh-CeO2 adhesion energy of 1.75 eV/atom. In
contrast, the Rh monolayer can remain the rhombus shape on the OF, OS, and Ce sites
with a slightly lower cohesive energy of 1.01 eV/atom. It’s noteworthy that for both
Pd and Rh on OFB, the calculated binding energies at 1ML coverage are very close to
those in the case of single atom deposition. It may lead to an illusion that the Pd-Pd
and Rh-Rh lateral interactions on OFB are negligible. In fact, the Pd-CeO2 and
Rh-CeO2 adhesion interactions at 1 ML coverage are substantially weakened by 0.72
and 1.15 eV/atom, respectively. However, the loss is fortuitously compensated by the
cohesive energy.
As shown in the prior calculations, the single Rh atom favors the OS site over the
OF site. However, with the deposited coverage increased to full monolayer, the
binding strength of Rh on OF is greatly enhanced so that it yields an equivalent
binding energy with the OS site (2.58 eV/atom). In fact, for both Pd and Rh monolayer
deposition on OF, their interfacial structures undergo the smallest distortion. There is
nearly no difference for the Pd-O and Rh-O bond lengths in the case of single atom
and one monolayer deposition. Each Pd and Rh atom of the monolayer donates 0.10
and 0.16 electrons to the substrate, while each of the closest OF ions merely gains 0.01
and 0.03 electrons, respectively. The rest of charge flows to the next nearest ions (e.g.,
Ce).
Figure. 2. The electron density difference for 1 ML Pd and Rh deposited on OF sites:
(left) Pd/CeO2(111); (Right) Rh/CeO2(111). The contour levels are at 0.01 e/Å3.
To clearly visualize the bonding behaviors and charge transfer from Pd and Rh
monolayer to the substrate, we calculated the charge density differences for Pd and Rh
monolayers on OF. As illustrated in Figure. 2, the overall shapes of Pd-4d and Rh-4d
orbitals remain nearly intact upon deposition. An electron donation and back-donation
process seems to dictate the bonding behavior, which induces the polarization of Pd
and Rh ad-layers. For Pd, electrons are accumulated in the “doughnut” (part of the
Pd-4dz2 orbitals) parallel to the CeO2 surface, while electron deficiency appears in the
two lobes normal to the surface. The general feature is similar to what Alfredsson and
coworkers found.1 In contrast, the whole Rh-4dz2 orbital donate electrons to the
substrate, as represented in broken lines. On the contrary, the four lobes of the Rh-4dyz
orbital are shown to be in solid lines, indicating the electron back-donation from the
substrate. The distinct metal-metal interactions and metal-substrate interactions differ
Pd from Rh arises from their intrinsically different electron configuration. Pd is closed
shelled with all 4d orbitals filled, while Rh has an empty 4dyz orbital.
Note, the Pd and Rh monolayer deposition on the OSB and CeB sites are not stable.
Most Pd and Rh atoms would move out of their initial positions, accompanied with
the significant distortion of the substrate upon structural optimizations.
1
Alfredsson, M.; Catlow, C. R. A. Phys. Chem. Chem. Phys. 2001, 3, 4129.