Chemical Reactivity of Sulfur Oxides on Pt111

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Chemical Reactivity of Sulfur Oxides on Pt(111)
Xi Lin
Atomistic Modeling And Simulation Seminars June
2003, MIT
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Outline

• Overview of sulfur chemistry
• Next-generation automotive catalytic converter
  design sulfur poisoning
• Thermodynamics should a chemical reaction occur
  or not
• Chemical kinetics how quick does a chemical
  reaction occur
• Conclusions

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Sulfur Is A Unique Link
S

Molecules of first-row elements gas-phase
molecules, organic molecules, conducting
polymers, and bio-molecules
Transition Metals solid surfaces, catalyst
particles, electrodes, and metal contacts
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Single-Molecule Electronics
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Molecular Actuator Artificial Muscle
courtesy of the Swager Group the Smela group
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Signal Transduction Pathway Regulator
Van Montfort et al. Nature 423, 773, 2003
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Van Montfort et al. Nature 423, 773, 2003
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Automotive Catalytic Converter
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Three-way Catalysts for CO, HC and NOx
Oxidation
Reduction
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Lean NOx Trapping Sulfur Poisoning
lean
A/F
14.6
rich
H2O
Time (s)
30
60
90
CO2
rich
CO
CO
lean
H2
SO42-
H2
O2
N2
SO2
HC
SO3
NO3-
NO2
NO2
HC
BaO
NO
lean
rich
Pt
Pt
SO3 severely poisons BaO
Al2O3
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Objectives

• Construct and test computational models against
  existing experiment data
• Resolve existing experimental difficulties
• Provide insight into chemical reactivity
• Understand sulfur chemistry as a step towards
  design of sulfur resistant materials

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Approach First-Principles Density Functional
Theory Computation

• Periodic boundary condition (three-layer slab)
  with plane-wave basis sets
• Ultra-soft pseudopotentials (25 Ry cutoff)
• Exchange-correlation functional by generalized
  gradient corrections
• Dipole correction
• Vibrational analysis by harmonic oscillators
• Nudged elastic band for the minimum energy paths
• DACAPO/CPMD/VASP

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Typical change in binding energy 18 kJ/mol
compared to the four-layer model
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What Are Known from Experiments

• Experimental designed system gas-phase SO2,
  without or with O2, on the single-crystal Pt(111)
  surface
• At low temperatures (lt 130K) without O2
  molecularly adsorbed SO2
• Perpendicular configuration bound via S and one
  O
• Parallel configuration no details
• At raised temperatures (190K 270 K) without O2
  conflicting observations
• Two new vibrational features appear no details
• Nothing new other than signal intensity changes
• Several new vibrational features appear when O2
  is present no details

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Summary of Experimentally Measured Vibrational
Frequencies
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Most Strongly Bound Configurations of SO2
fcc ?2-Sb,Oa 117.6 kJ/mol
fcc ?2-Sa,Oa,Oa 107.2 kJ/mol
XPS HREELS, Y. M. Sun, D. Sloan, D. J.
Alberas, M. Kovar, Z. J. Sun and J. M. White,
Surf. Sci., 319, 34 (1994).
XPS NEXAFS, M. Polcik, L. Wilde, J. Hasse, B.
Brena, G. Comelli, and G. Paolucci, Surf. Sci.,
381, L568 (1997).
100150 kJ/mol TPD, St. Astegger and E.
Bechtold, Surf. Sci., 122, 491 (1982).
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Assignment of Experimentally Measured
Frequencies SO2 at 130K
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Assignment of Experimentally Measured
Frequencies SO2 at 190K
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Assignment of Experimentally Measured
Frequencies SO2O2 at 160K
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Universal Lateral Dipole Rule for Sulfur Oxides
on Pt(111)
31.9 kJ/mol
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What makes a strong binding configuration?

• For the strongest bound surface SOx (x 1, 2, 3,
  and 4) configurations, S has a tetrahedral
  binding to its neighboring atoms, including x O
  and 4-x Pt atoms.
• Atomic S takes the highest coordinated surface
  site
• The second strongest bound surface SOx (x 0, 1,
  2, and 3) configurations are obtained by taking
  one unbound O atom from the strongest bound
  surface SOx (x 1, 2, 3, and 4), respectively

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S
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From SO4 to SO3
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From SO3 to SO2
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From SO2 to SO
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From S to SO
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Novel Surface Reconstructions by Sulfur Oxides
6.6 kJ/mol more stable
reference
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Thermodynamic Significant Coverage Effects
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Use Transition State Search Methods to Compute
the Reaction Barrier Heights
Choose minimum energy configurations from
our search of binding configurations
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SO2 Oxidation Reaction

• SO2(g)1/2O2(g)?SO3(g) is energetically
  favorable, but kinetically prohibited in
  gas-phase
• Question how does Pt promote the reaction

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Langmuir-Hinshelwood Mechanism at High Coverage
Limit ? ¼ ML

• (kJ/mol) ?E Ea ?G Ga
• (600K, 1atm)
• ½O2(g)O(a) -89 0 -33 0
• SO2(g)O(a)(OSO2)(a) -11 0 103 0
• (OSO2)(a)SO3(a) -91 30 -84 28
• SO3(a)SO3(g) 113 0 -11 0

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Eley-Rideal Mechanism

• Reactions directly between surface species and
  gas-phase molecules

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Low Coverage Limit ? 0 ML

• Self-diffusion reactions of O, SO2, and SO3 are
  needed for the Langmuir-Hinshelwood mechanism
• Assume further reactions of both LH and ER
  mechanisms are similar to the high coverage case

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O Half Self-diffusion Initial and Final States
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O Half Self-diffusion MEP and TS
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Hard-Sphere Self-diffusing against Static
Hard-Sphere Surface
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SO2 Half Self-diffusion (I) fcc ?2-Sb,Oa hcp
?3-Sa,Oa,Oa
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Identification of Self-Diffusion Reaction
Coordinates
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SO2 Half Self-diffusion (II) fcc ?2-Sb,Oa hcp
?2-Sb,Oa
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SO3 Half Self-diffusion fcc ?3-Sa,Oa,Oa hcp
?3-Sa,Oa,Oa
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Summary of Low Coverage Limit ? 0 ML

• Self-diffusion reactions of O, SO2, and SO3 are
  not the rate-limiting step of the
  Langmuir-Hinshelwood mechanism
• With abundant O2, any small amount of SO2 will be
  oxidized to SO3

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Conclusions I

• Computation verifies experimental identifications
  of chemisorbed sulfur oxides on Pt(111)
• Stable configurations
• Binding energies
• Vibrational modes
• Computation resolves experimental conflicting
  observations
• S centers of all surface sulfur oxide species
  have a strong tetrahedral binding preference
• A general lateral interaction rule is derived for
  the chemisorption energy of sulfur oxides
• Novel surface reconstructions are predicted upon
  the chemisorption of SO4

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Conclusions II

• Thermodynamic analysis indicates that the
  reaction mechanism is strongly coverage dependent
• The most stable surface species are
• Low surface coverage limit atomic S
• High surface coverage limit SO4
• Oxygen-saturated surface limit SO4
• The Langmuir-Hinshelwood mechanism is favorable
  over the Eley-Rideal mechanism at high coverage
  limits
• Self-diffusion reactions are not the
  rate-limiting step for SO2 oxidation reaction
  catalyzed by Pt(111)

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Conclusions III

• Self-diffusivity decreases in the order of SO2 gt
  O gt SO3
• self-diffusion minimum energy path of atomic
  oxygen is a sector of a cycle
• Complementary mechanisms are revealed for
  self-diffusion of molecular SO2
• The chemisorption of SO2 on oxygen pre-covered
  Pt(111) is identified as the overall
  rate-limiting step
• A patch of Pt(111) in automotive catalytic
  converters is sufficient to promote the oxidation
  reaction of SO2 to SO3 under oxygen rich
  conditions

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Acknowledgments

• Bernhardt L. Trout (Ph. D. thesis advisor)
• Ford Motor Company
• National Science Foundation