[1]

CO and NO-induced disintegration of Rh, Pd, and Pt nanoparticles on TiO2(110): A first principles study Bryan Goldsmith,1 Evan Sanderson,2 Runhai Ouyang,3 Wei-Xue Li3

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

Introduction

Free energy of reactant-induced NP disintegration

Computational Methodology

Adatom complex formation and particle energetics

adatom/support support bulkformationE E E E= − −

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

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.

If ΔGdisintegration is negative, then the NP should be expected to disintegrate under equilibrium conditions.

Disintegration of supported nanoparticles (NPs) in the presence of reactants can lead to catalyst deactivation or be exploited to redisperse sintered catalysts.

Nanoparticle disintegration can cause catalyst deactivation

Rh/SiO2

Disintegration

Most active size

Density Functional Theory modeling using Vienna Ab-initio Simulation Package

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

Vacuum layer thickness of 15 Å Bottom two tri-layers are fixed in positions.

(4x2) Rutile TiO2(110)

Periodic model

00

0

3( , , ) ( , ) xf x B

pG R T p E n T p k TLn TSR p

µ Ω

∆ = − − + −

medisintegration

γ

Formation energy of adatom complex

Standard gas phase chemical potential

Configurational entropy of complexes

Nanoparticle energy

0G∆ <disintegration

Disintegration can be modeled by the Gibbs Free Energy

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.

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.

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.

[1]

Rh(CO)2 and Rh(NO)2 have more favorable formation energies than Rh(CO) and Rh(NO)

Formation energy = 0.75 eV -1.35 eV

-0.02 eV -1.58 eV

C O

Rh

N

Rh(CO)2

Rh(NO)2

Rh(CO)

Rh(NO)

Rh Pt

Pd

NO binding on (111) facet

NO chemical potential (eV)

Acknowledgments References

Pt, Pd, Rh NP/TiO2(110) disintegration

Conclusions 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

Influence of CO and NO adsorption on Pt, Pd, or Rh nanoparticle surface energetics 𝜸𝒎𝒎

Reactant environment and particle size effects on supported nanoparticle energy

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.

[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

The interaction of CO and NO with Rh adatom is greater than for Pd and Pt adatoms

Formation energy

Rh Pd Pt

Metal(CO) 0.75 0.26 0.09 Metal(CO)2 -1.35 -0.54 -0.69 Metal(NO) -0.02 -0.05 -0.10 Metal(NO)2 -1.58 -0.67 -0.68 Energies are in eV.

Exothermic formation energy promotes particle disintegration

CO and NO bind strongest to Rh metal compared to Pd and Pt metals

111 111( , ) [ ( , )] ( , )me i i i mei

meT P f T PT Pγ γ γ γ γ∆= + ∆ ≈ +∑

Rh Pt Pd

CO binding on (111) facet

CO chemical potential (eV)

Redu

ctio

n in

su

rfac

e en

ergy

(eV/

Å2 )

Temperature ↓, Pressure ↑, Coverage ↑

Redu

ctio

n in

su

rfac

e en

ergy

(eV/

Å2 )

NPE∆

LESS STABLE MO

RE S

TABL

E

(111)NP

3 γΔE =

RΩ

NPE∆

contours (Ω is molar volume)

In the presence of CO In the presence of NO

eV

eV

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.

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.

diameter = 20 Å

In agreement with experiment, Rh(CO)2 predicted to form but not Rh(CO)

Rh(CO)2 Exp.

Detected

not detected

Pd and Pt not predicted to form adatom complexes

In the presence of CO

CO chemical potential (eV)

Gibb

s fre

e en

ergy

of

disi

nteg

ratio

n (e

V)

Exp. Suggested

In agreement with experimental suggestions, Rh(NO)2 predicted to form

Pd and Pt not predicted to form adatom complexes (Under typical conditions)

In the presence of NO

Gibb

s fre

e en

ergy

of

disi

nteg

ratio

n (e

V)

NO chemical potential (eV)

diameter = 20 Å

Include gas phase disintegration Consider adatom translation Particle size-dependent binding

energies

Future work

Reduction in surface energy due to reactant binding

At a specified gas phase chemical potential, the adatom complex that is most likely to form has the most negative disintegration free energy.

Pd(CO)2 (g)