AtomicScale Insight into Catalytic Promoters

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Atomic-Scale Insight into Catalytic Promoters

• J. G. Kushmerick and P. S. Weiss
• Department of Chemistry
• Pennsylvania State University
• University Park, PA 16802

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Abstract

• Two new roles for catalytic promoters are
  suggested based on scanning tunneling microscopy
  and spectroscopy measurements. Ni promoted
  molybdenum disulfide (MoS2) is an important
  catalyst for removing sulfur from petroleum
  feedstock. We imaged the MoS2 basal plane bare
  and with adsorbed Ni atoms at 298K, 77K and 4K
  using a low temperature ultrahigh vacuum scanning
  tunneling microscope. Ni atoms freely diffuse
  across the surface down to temperatures below
  80K. At 4K, isolated atoms and small clusters of
  Ni are stable and can be imaged. Even at 4K, Ni
  atoms can easily be manipulated with the
  microscope tip. Spectroscopic measurements reveal
  that Ni promoters create favorable electronic
  structures for binding sulfur-containing
  reactants. Our observations suggest two new roles
  for the Ni promoter atoms 1) they increase the
  sticking probability of sulfur-containing
  hydrocarbons by binding them to the unreactive
  basal planes of MoS2 and 2) they transport these
  hydrocarbons as organometallic complexes to the
  active sites for reaction.

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Industrial Oil Processing
Crude Oil

• Initial Separation
• Conversion
• Removal of Impurities
• Light Ends Upgrading

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Chemical Reactions
Idealized combustion. CxHy O2
H2O CO2
Combustion of sulfur-containing
hydrocarbon. HxCy S O2
H2O CO2 SO2
Leads to acid rain production. SO2 ½O2
SO3 H2O SO3 H2O H2SO4
Hydrodesulfurization process. CxHy S 2H2
CxHy2 H2Sremoved
catalyst
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Industrial Catalyst

• MoS2 supported on Al2O3 promoted with Ni or Co
  adatoms.

The active sites are believed to be at MoS2
edges, but their structure is unknown.
van der Waals interactions
MoS2 Sheet
MoS2 Sheet
Al2O3 Support
Active site?
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Thiophene as a Model Reactant

• Thiophene is the most difficult feedstock
  component to desulfurize and thus is studied as a
  model system.

Hydrocarbons H2S
Multiple Steps
H2
Accepted general mechanism neglects role of
promoter.
Mo
Mo ?5 bound to thiophene.
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Atomic Resolution Imaging

• The ability of STM to image surfaces with atomic
  resolution, even close-packed metal surfaces,
  enables probing catalytic activity at the atomic
  level.

MoS2 45Å x 45Å Vs -1V I 0.3nA T77K
Pt(111) 50Å x 50Å Vs -20mV I 8nA T4K
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Electronic Perturbations
The STM can measure the electronic perturbations
caused by surface defects or adsorbates. Here a
point vacancy in the surface of HOPG causes
anisotropic electron scattering.
75Å x 75Å Vs -50mV, I0.5nA T77K
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Ni on MoS2
Ni promoter adatoms remain mobile at 77K, and
remain very easy to move with STM tip at 4K.

• A) 95Å x 30Å Vs -1.5V, I0.1nA, T4K
• B) Tip sample separation for cross section
  between
• the two arrows.

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Ni3 Cluster Electronic Structure on MoS2
2V, 0.1nA
Ni3 cluster enhances empty states at 2V above the
Fermi level, and depletes filled states 2V
below. STM images of Ni3 on MoS2, 60Å x 60Å,
T4K
1.4V, 0.1nA
2V, 0.2nA
Ni3 cluster optimizes binding of nucleophilic
molecules.
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Spectroscopy of Ni3 Cluster

• Complementary information can be acquired with
  spectroscopy.
• Cluster has sharp feature 2V above the Fermi
  level, and depletes states at 1.75V.

I vs. V
(dI/dV)/(I/V) vs. V

• (dI/dV)/(I/V) is proportional to the local
  density of states.

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Probe Chemical Environment
Vibrational spectroscopy of isolated molecules
can give insight into the chemistry of different
sites. Inelastic electron tunneling spectroscopy
(IETS) with the STM can determine the molecular
vibrational modes.
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Vibrational Identification

• Vibrational fingerprint of molecular adsorbates
  will enable chemical identification of
  reactants/products.

Possible Peak Assignments A 183 meV C-H out
of plane bending mode (B2u) B 99 meV
C-H in plane bending mode (A2u) C 33
meV Pt111 phonon?
C
B
A
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New Roles for Catalytic Promoters

• Enhance sticking probability by catching reactant
  molecules.
• Transport molecules to active sites across
  chemically saturated forbidden zone of
  surface.
• We need to verify this with adsorbed
  organometallic complexes.

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Upcoming Experiments

• Create step edges in the MoS2 surface with the
  STM tip or exfoliate MoS2 crystallites onto the
  basal plane.
• Dose the Ni/MoS2 system with thiophene and
  directly determine reaction geometry.
• Stay tuned!

Support
National Science Foundation Graduate Trainee
Fellowship Office of Naval Research Exxon
Education Foundation Petroleum Research Fund
administered by the American Chemical Society
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Support

• National Science Foundation Graduate Trainee
  Fellowship
• Office of Naval Research
• Exxon Education Foundation
• Petroleum Research Fund administered by the
  American Chemical Society