Owrutsky (for Berman) - Molecular Dynamics and Theoretical Chemistry - Spring Review 2012

15 February 2012

Integrity Service Excellence

DISTRIBUTION A: Approved for public release; distribution is unlimited.

Jeffrey C. Owrutsky /

Acting for Michael Berman

Program Manager

AFOSR / RSA

Air Force Research Laboratory

MOLECULAR DYNAMICS

& THEORETICAL

CHEMISTRY

8 MAR 2012

2 DISTRIBUTION A: Approved for public release; distribution is unlimited.

2012 AFOSR SPRING REVIEW

NAME: Jeff Owrutsky / Michael Berman BRIEF DESCRIPTION OF PORTFOLIO: Research on understanding and exploiting chemical reactivity and energy flow in molecules to improve Air Force systems, processes, and materials. Understanding and exploiting chemical reactivity and catalysis for improved storage and utilization of energy LIST SUB-AREAS IN PORTFOLIO: Molecular Dynamics Theoretical Chemistry Atmospheric & Space, Energetics, Nanostructures and Catalysis


Challenges in Molecular Dynamics Molecular Dynamics, Theoretical Chemistry, Nanoenergetics

•Energetic Materials (Rocket propellants, explosives)

– Energetic ionic liquids CHNO limit; new approaches

– Energetic nanostructures Sensitivity, mechanisms

– Catalytic enhancement Safer, penetrating munitions

• Nanostructures/Sensors (Energy, catalysis, sensing)

– Nanostructures for catalysis Atomic scale imaging and control

– Photoelectrochemical materials Activity and stability

– Plasmonics Size- and shape-mediated properties

•Atm/Space Chemistry (Signatures, surveillance)

– Upper atmosphere, space Hypersonic propulsion, gas/surf interact.

– Signatures & backgrounds Rates/mech. of ion-molecule reactions

– Ion & plasma processes Predictive codes, communication

• Lasers and Diagnostics (Infrared lasers, missile defense)

– High-Power Gas Lasers Efficient pumping, energy transfer

– Novel analytical tools/methods Relaxation processes


Scientific Challenges

• Imaging and Control of Catalysis

– Understanding control of mechanisms :

– Comprehensive approach using emerging methods in

• Synthesis - Prepare - Make

• Simulation - Predict - Model

• Sensing - Probe - Measure

reactions: properties, interactions & mechanisms

nanostructures to promote activity and stability

– Catalysis is key to energy storage, fuel production and utilization

– Important practical military and industrial impacts

– Co-catalysts, promoters, substrates, new materials, …

• Energetics

– Enhance and improve energy density, impulse, stability


Transformational Opportunities

Endothermic Fuels for

cooling high-speed vehicles

Mission enabled by catalysis

vasst.info/

Secure Energy and Power

– Alternatives fuels

– Efficient generation

Propellants & Energetic Materials

Hypergolic ionic liquids


Related Work in Other Agencies

• NSF

– Solar Energy Initiative (SOLAR)

– Center Powering the Planet

– Chemistry Catalysis, Materials & Nanoscience Centers

• DOE

– Energy Frontier Research Centers, Solar Fuels Hub, JCAP

– X-ray, electron, laser Facilities (Argonne, SLAC, ALS)

• DOD

– AFOSR fuel production complements ONR fuel utilization

– Cooperation on energetic materials

– AFOSR: physical chemistry oriented, molecular & mechanistic


AFOSR Molecular Dynamics

Program Strategy

• Molecular / Chemical Physics Emphasis

– Build from gas phase / small molecule

– State selective energy, charge transfer & reactions

– Connect to condensed phase – surfaces for catalysis

– Model to practical system

– Clusters & nanomaterials – unique behavior

• post-ato-molecular and pre-bulk

• Comprehensive & coordinated

– Theory experiment

– Systematic and probing

• design rules - understanding for control

• mechanisms for working systems / effects


Program Trends

• Catalysis – Networked, Actuated, Novel Probes

• Sustainable Energy

• Small Molecule Activation

• Ionic Liquid Propellants

• Plasmonics

• Plasma / Ion Chemistry/ Interfaces

• Hybrid Chemical Lasers

• Sensors for Trace Detection


Transition: Mass Spectrometry and

Ion Mobility Spectrometry

Chemical Detection using Portable Instrumentation

Miniaturized mass spectrometers (MSs)

• real-time, in-field

• atmosphere sampling

• differential mobility spectrometry.

Inte

nsi

ty

• High-resolution IMS separation of complex samples for isotopic distribution analysis. • To reduce sample analysis times from weeks (currently) to minutes/hours.

Rapid Isotopic Distribution Analysis for Nuclear Forensics and Attribution using IMS/MS


• Two Tracks for CO2 reduction studies

‒ Systematic • understand geometric and energetic

factors to promote reduction

• sequential reaction

• identify barrier structures, reactions pathways

• determine mechanism of demonstrated system

‒ Understand Effective Systems Bocarsly organic reduction catalyst

• Pyrdine on p-GaP

• Homogeneous or heterogeneous

mechanism?

Solar Fuels Two Tracks – Systematic and Following the Enigmatic

Formic acid

catalyst

H2 + CO2

HCOOH

+ H2

H2CO

+ H2

CH3OH

Formaldehyde catalyst

Methanolcatalyst

Catalysis – “Assembly Lines”

for three step conversion of

CO2 to methanol


CO2 Reduction Motifs

Ni cyclam – structurally

favorable for reduction

Binding and Desorption Kinetics

Activity and Scaling Parameters

• CO2 reduction - identify rate limiting

steps via DFT

• CatApp – suncat.stanford.edu/catapp

online surface specific barriers

Kubiak, UCSD Norskov, Stanford

• Extend to NiP2N2

• Mediated by hydride

transfer energetics


Mechanism for CO2 Reduction?

12 DISTRIBUTION STATEMENT A – Unclassified, Unlimited Distribution

homogeneous mechanism:

= semiconductor/oxide surface

with pyridinyl radicals

heterogeneous mechanism:

Morris, A. J.; McGibbon, R.T.; Bocarsly, A.B. ChemSusChem, 2011, 4, 191-196


Energetics rules out homogeneous mechanism Reaction is surface-catalyzed High endoergicity due to electronic structure of pyridinyl Carter, Princeton

Mechanism for CO2 Reduction?

CO2 reduction studies with related materials Reaction is robust and complicated • Imidazole catalyzes CO2 reduction • MeOH with imidazole & histidine on gold • Formic acid with imidazole on iron pyrite Bocarsly, Princeton


Nanocatalysis for Propulsion

Motivation : Endothermic Fuel for Hypersonic Engine Thermal Control and More Efficiency Objectives: Fundamentals relating to fuel-soluble/dispersible catalysts and precursors Chemistry

• Active sites • Solubility • Active catalyst generation from precursors • Mechanisms • Energetics • Kinetics • Mixing • Droplets/vaporization/NP nucleation • Nanoscale fluid mechanics

Physics


Kaden, Wu, Kunkel, Anderson, Univ. of Utah - Science 326 (2009) 826-9

Electronic Structure Controls

Catalytic Activity

• Nanocatalysts have activity that depends on particle size.

• For the first time, activity was correlated on an atom-by-atom basis with particle electronic structure, and particle size.

• Theorists from VCU and BNL are calculating reasons for variation of electronic properties with size

Cluster Size (Number of Atoms)

0 5 10 15 20 25 De

via

tion

fro

m N

-S C

ha

rge

Sca

ling

(e

V)

-0.4

-0.2

0.0

0.2

Act

ivity

(C

O2 p

er

TP

R x

109)

0

2

4

6

8

10ActivityPd 3d XPS Shift

Single Layer Islands

Growth of 2nd

Layer

Clean TiO2

Comparison of CO oxidation activity with Pd 3d orbital energy (deviation from bulk-like scaling)

Pdn/TiO2(110) model catalyst

Oxygen activation efficiency: highly Pdn size dependent


Combined GISAXS/GIXAS/TPRx

characterization

Stefan Vajda et al , Argonne National Laboratory and Yale University

• clusters are the most active on MgO support

• fluxional ~ 3 nm nanostructure – most reactive

• cooling to RT reduces clusters size (?)

GISAXS: Evolution of a fluxional

nano-assembly from

sub-nm Co clusters on MgO support

Dehydrogenation of Cyclohexene on Supported sub-nm Con Clusters

• size–selected nanoparticle deposition

• in situ X-ray characterization under realistic

reaction conditions

• combined with catalyst tests


Ultrafast Dynamics of Surface

Functionalized Heterogeneous Catalyst

Surface – dry 150 ps (surface layer in air)

Surface – wet 50 ps (surface layer in CHCl3)

Solution 5 ps (head group in bulk CHCl3)

Fayer (Stanford U.) and co-workers, Science 334, 634 (2011).

kecho =

k2+k3-k1k1

k2k3

k1

k2

k3

sample

monochromator

loca

l osc

illa

tor

beam combiner

MCT

array

vibrationalecho

1 2t t3Tw

t – coherence periods; Tw – population period

vibrationalecho

2D IR vibrational echo spectroscopy on surfaces .


Plasmon-enhanced Photocatalytic

Activity of Iron Oxide on Au Nanopillars

• Enhanced (up to 50% over solar spectrum) photocurrent in a thin-film iron oxide photoanode coated on arrays of Au nanopillars.

• Attributed primarily to the increased optical absorption from both surface plasmon resonances and photonic-mode light trapping in the nanosctructured topography.

• The resonances can be tuned to a desirable wavelength by varying the thickness of the

iron oxide layer.

P. Yang (UC Berkeley) ACS Nano, 2011


Active optical nanoantennas

Halas (Rice) and coworkers Science 332, 702-4 (2011)

B

C

200 nm

A

ITO

SiO2

Au

Silicon

(n-type)

energy band diagram

metal Si (n-type)

e-

B

indium

contactITO

contact

EC

EV

laser

A

EF

Ti

B

C

200 nm

A

ITO

SiO2

Au

Silicon

(n-type)

energy band diagram

metal Si (n-type)

e-

B

indium

contactITO

contact

EC

EV

laser

A

EF

Ti

Plasmonic

antenna

200 nm

0

25

50

75

100

0

25

50

75

100 270

90

0

Cu

rren

t (%

max)

180

1300 1450 1600

Ph

oto

cu

rren

t sp

ectr

a (

a.u

.)

Wavelength (nm)

1300 1450 1600

122 nm

116 nm

Ab

so

rpti

on

(a.u

.)

Wavelength (nm)

110 nm

134 nm

128 nm

146 nm

140 nm

158 nm

152 nm

200 nm

• Hot electrons originating from the decay of surface plasmons, known to mediate chemical reactions,can also be harvested in a device geometry

• Nanorod antennas - wavelength-dependent resonant response inject hot electrons across metal-semiconductor interface: a “nanoantenna-diode”

• Wavelength & polarization - specific photodetection

• Photodetection below the bandgap of the semiconductor enables new materials for infrared photosensitivity


Another Dimension: Networked or Actuated Catalysts

Field Mediated Nanoscale Dynamics

• Time-, energy- and polarization-resolved magneto-optical spectroscopy to study the optical properties of semiconducting nanostructure arrays.

• Magnetic field to control of the exciton fine structure populations of CdSe nanocrystals.

Knappenberger (FSU) and coworkers, J. Phys. Chem. C 115, 14517 (2011); FA9550-10-1-0300

B

• Expand capabilities expanded dimensions in structure or processes – modulated catalysts

• Networked / building blocks – linking to exploit coupling between units

• Actuate/sequential – remediate to overcome poisoning for active catalysts

• Field or energized particle modification

• Plasma – catalyst hybrids


Electric Field Control of a Metal Oxide-Catalyzed Reaction

• Up to 63-fold change in product ratio induced by the voltage-controlled interfacial electric field

• Field–dipole differentiation of transition states implicated

Kanan (Stanford) and coworkers


Directing the Motion of a Polymerization Motor Via Substrate Gradient

• Polymerization motor for Janus nanoparticles collect at the gel edge over time in a gradient of norbornene

• Control experiments with non-motor particles and non-polymerizable “fuel” showed no increase over time

• Chemotaxis phenomenon – potential for system repair by directing the motor motion to a damaged spot

Pavlick, Sengupta, Mcfadden, Zhang, Sen (Penn State) Chem. Int. Ed. 50, 9374 (2011) Angew.


A Stable, Room Temperature IL Fuel Based on Borohydride Anion: [Al(BH4)4]-

Stefan Schneider, Tom Hawkins, Yonis Ahmed, Michael Rosander, Jeff Mills and Leslie Hudgens , Angew. Chem. Int. Ed. 12 May 2011, DOI: 10.1002/anie.201101752 (AFRZ/RZ)

Ionic liquid propellents & energetic materials • trihexyl-tetradecyl-phosphonium (THTDP) cation - stable with bases and reducing agents*

• THTDP reduces MP & promote liquidus

• [Al(BH4)4]- also promotes liquidus

[THTDP] [BH4] + Al (BH3) → *THTDP+ *Al (BH4) 4]

Combined, the two ions create a low viscosity, hypergolic IL-fuel!

Fuel\Oxidizer 90%H2O2 98%H2O2 N2O4 WFNA

R4P Al(BH4)4 Ignition Ignition Ignition Explosive

Ignition Delay < 30ms < 30ms Vapor ignition -


Hypergolic Ionic Liquids and Metal Nanoparticles

• Milling boron nanoparticles in ILs (Utah) leads to air stable, unoxidized boron nanoparticles - can be used for stable colloids in hypergolic ILs (Alabama).

• Calculations (Edwards) suggest types of interactions between anions and a B80

cluster.

• IL remains hypergolic & boron adds energetic value - longer ignition duration without increasing the ignition delay.

• This leads to the ability to add a variety of metal, reactive nanoparticles into ionic liquids to tune their performance.

Air Stable, unoxidized boron from milling with ILs

A hypergolic IL, [BMIM][DCA] is still

hypergolic even with boron nanoparticles incorporated.

Boron nanoparticles stabilized in [BMIM][DCA]

1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA]

Rogers et al.


Criticality and Vapor-Liquid Equilibrium

of Ionic Liquids

• Computed vapor-liquid phase equilibrium of a series of ionic liquids

− Coexistence densities, vapor pressures, enthalpy and entropy of vaporization

− Deduced the aggregation state of vapor phase

• Provides key information for physical properties pertinent to energetics and fundamental experiments

Maginn (Notre Dame) and Rai; J. Phys. Chem. Lett. 2, 1439 (2011)

26 DISTRIBUTION A: Approved for public release; distribution is unlimited. Distribution A: Approved for public release; distribution unlimited

Ionic Liquid Photoioniziation

VUV Photoionization TOF Mass Spectroscopy • Aerosol, gentle production of IL ion pairs “cooler”, reduced internal energy • ALS soft ionization for low fragmentation • EMIM Br decomposes during evaporation • Hypergolic IL reaction products

EMIM Br

Leone (UC Berkeley) with Boatz, Vaghjiani & Chambreau (AFRL/RZ)

http://pubs.acs.org/action/showImage?doi=10.1021/jp200633b&iName=master.img-000.jpg&type=master


Summary

• Catalysis

– transformational impacts on DoD systems

– critical for efficient Power and Energy generation and utilization

– propulsion / energetic material enhancements

• Chemical dynamics – emerging methods and new insights into catalysis

– intermediates and mechanisms needed to understand and optimize catalysts

– AFOSR leading the way in applying new tools to understand catalytic mechanisms

• Many new areas of opportunity:

– alternative and renewable fuel production

– atomic scale imaging and control of catalysis

– new dimensions in catalyst and energetic material structures and control

Owrutsky (for Berman) - Molecular Dynamics and Theoretical Chemistry - Spring Review 2012

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