Bryan Goldsmith,1 Evan Sanderson,2 Runhai Ouyang,3 Wei

Bryan Goldsmith,1 Evan Sanderson,2 Runhai Ouyang,3 Wei

CO and NO-induced disintegration of Rh, Pd, and Pt nanoparticles on TiO2(110): A first principles study
Bryan
1
Goldsmith,
Evan
2
Sanderson,
Runhai
3
Ouyang,
Wei-Xue
3
Li
1. Department of Chemical Engineering, University of California, Santa Barbara, CA, 93106-5080
2. Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106-9510
3. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Free energy of reactantinduced NP disintegration
Density functional theory allows the rapid calculation of the
effects of reactant partial pressure (p), temperature (T), NP radius
(R), as well as particle morphology on the stability of supported
NPs toward disintegration into adatom complexes.
Nanoparticle disintegration
can cause catalyst deactivation
If ΔGdisintegration is negative,
then the NP should be expected to
disintegrate under equilibrium
conditions.
Most active size
[1]
NP disintegration in the presence of reactants is common
e.g. Rh/TiO2, Rh/Al2O3, Pd/Fe2O3,
Pt/SiO2, Ir/Al2O3, Ni/TiO2, Ru/TiO2
We address the following questions:
 Between supported Rh, Pd and Pt catalysts, which one is
more susceptible to the disintegration.
 Among NO and CO, which one is more efficient for the
redispersion of the low surface area catalyst.
 How sensitive do these results depend on the particle size,
temperature and partial pressure.
 0
3Ωγ me
px 
Ef
∆Gdisintegration ( R, T , p ) =−
− n  µ x (T , p0 ) + k BTLn  − TS
R
p0 

Nanoparticle
energy
Standard gas phase
chemical potential
An exothermic adatom complex formation energy and a small
particle radius will promote NP disintegration.
Increasing the reactant species chemical potential, by raising
pressure or lowering temperature, will also enhance disintegration.
Adatom complex formation
and particle energetics
CO binding on (111) facet
Rh Pt Pd




Projector Augmented Wave method
RPBE Functional + Spin polarization
Plane wave kinetic energy cutoff = 400 eV
Forces converged to 0.03 eV/Å via conjugate gradient
(4x2) Rutile TiO2(110)
Periodic
model
C
Rh
Rh(CO)
Rh(NO)
diameter = 20 Å
Rh(CO)2
Exp.
Detected
Pd and Pt not predicted
to form adatom complexes
CO chemical potential (eV)
In agreement with
experimental suggestions,
Rh(NO)2 predicted to form
diameter = 20 Å
Exp.
Suggested
Pd and Pt not predicted
to form adatom complexes
(Under typical conditions)
NO chemical potential (eV)
Rh NPs are more susceptible to CO and NO-induced disintegration
compared to both Pd and Pt NPs, and NO is a greater promoter of
NP disintegration than CO. The facile disintegration of Rh NPs is due
to the large and exothermic formation energy of the Rh adatom
complexes. These findings provide insights for how to either prevent
or promote reactant-induced NP disintegration.
contours (Ω is molar volume)
R
In the presence of NO
∆ E NP
eV
eV
Rh(CO)2
-1.35 eV
These trends, which are manifested by the interplay between
coverage dependent adsorption energies, reactant-induced
surface stabilization, and NP size, give insight into the variability
of particle stability over a broad range of reaction conditions.
 The interaction between CO and NO with the Rh adatom is
greater than for the Pd and Pt adatoms.
 The Rh/TiO2(110) NPs are less resistant to CO and NO-induced
disintegration than the Pd/TiO2(110) and Pt/TiO2(110) NPs
 NO is a more efficient reactant for particle redispersion than CO
Future work
 Include gas phase disintegration
 Consider adatom translation
 Particle size-dependent binding
energies
Pd(CO)2 (g)
Rh(NO)2
Vacuum layer
thickness of 15 Å
Bottom two tri-layers
are fixed in positions.
Pd
NO chemical potential (eV)
LESS STABLE
Formation energy = 0.75 eV
N
Rh
Pt
Reactant environment and particle size
effects on supported nanoparticle energy
ΔE NP =
not
detected
Conclusions
Most stable adatom positions on TiO2(110), corresponding to formation
energies of 2.88, 2.01 and 3.12 eV for Rh, Pd, and Pt, respectively.
O
NO binding on (111) facet
Temperature ↓, Pressure ↑, Coverage ↑
3Ω γ(111)
In agreement with
experiment,
Rh(CO)2 predicted to
form but not Rh(CO)
In the presence of NO
CO chemical potential (eV)
In the presence of CO
E formation
= Eadatom/support − Esupport − Ebulk
Reduction in surface
energy due to reactant
binding
CO and NO bind strongest to Rh metal
compared to Pd and Pt metals
MORE STABLE
Density Functional Theory modeling using Vienna Ab-initio
Simulation Package
At a specified gas phase chemical potential, the adatom
complex that is most likely to form has the most negative
disintegration free energy.
In the presence of CO
111
111
[
(
,
)]
f
T
P
γ
γ
γ
γ
+
∆
≈
+
∆
∑i i i
me
me (T , P )
∆ E NP
Computational
Methodology
0.09
-0.69
-0.10
-0.68
i
Reduction in
surface energy (eV/Å2)
Configurational
entropy of complexes
Formation energy of
adatom complex
0.26
-0.54
-0.05
-0.67
Exothermic formation
energy promotes
particle disintegration
Influence of CO and NO adsorption on
�𝒎𝒎
Pt, Pd, or Rh nanoparticle surface energetics 𝜸
(T , P)
γ me =
[1]
0.75
-1.35
-0.02
-1.58
Pt
Gibbs free energy
of disintegration (eV)
∆Gdisintegration < 0
Disintegration
Pd
Energies are in eV.
Disintegration can be modeled
by the Gibbs Free Energy
Rh/SiO2
Rh
Gibbs free energy
of disintegration (eV)
Disintegration of supported nanoparticles (NPs) in the presence
of reactants can lead to catalyst deactivation or be exploited to
redisperse sintered catalysts.
Formation
energy
Metal(CO)
Metal(CO)2
Metal(NO)
Metal(NO)2
Reduction in
surface energy (eV/Å2)
Introduction
Pt, Pd, Rh NP/TiO2(110)
disintegration
The interaction of CO and NO with Rh adatom
is greater than for Pd and Pt adatoms
-1.58 eV
-0.02 eV
Rh(CO)2 and Rh(NO)2 have more favorable
formation energies than Rh(CO) and Rh(NO)
Acknowledgments
References
This work was funded by the NSFC (21173210, 21225315), the 973
Program (2013CB834603), and the NSF-OISE 0530268. B.R.G.
acknowledges the PIRE-ECCI program for a graduate fellowship. E.D.S.
thanks IRES-ECCI for an undergraduate summer research fellowship.
[1] McClure, S. M.; Lundwall, M. J.; Goodman, D. W. Proc. Natl. Acad.
Sci. 2011, 108, 931
[2] Ouyang, R.; Liu, J.-X.; Li, W.-X. J. Am. Chem. Soc. 2012, 135, 1760