tailored mesoscale gold alloy materials for energy- and...
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Tailored Mesoscale Gold Alloy Materials for
Energy- and Resource-Efficient Catalysis
Michelle Personick, Branko Zugic, Robert Madix, Cynthia Friend
Harvard University
ACS Spring Meeting
March 25, 2015
HARVARDUNIVERSITY
Selective Oxidation Using Au-based Catalysts—
Toward Efficient Chemical Production
“The Outlook for Energy: A View to 2040” (ExxonMobil, 2014)
Goal: Develop design principles
for increasing selectivity and
lowering operating temperatures
to reduce energy expenditure.
Au catalysts are highly
selective and operate at low
temperatures.
Materials that blend metal
compositions and bridge
multiple length scales open up
a wealth of opportunities for
designing efficient catalysts.
Growing role of chemical production
in industrial energy demand
HARVARDUNIVERSITY
Au Alloy Materials that are Dilute in Ag Dissociate
O2 Efficiently While Maintaining Selectivity of Au
Approach:
Investigate materials with a minority active component (Ag) for O2
dissociation; migration (spillover) to Au leads to selective reactivity.
Catalyst:
Nanoporous gold (npAu) is a dilute alloy (1-3 at.% Ag) prepared by selectively etching Ag from a bulk Ag70Au30 alloy precursor.
Gold
O2
Gold
O
Gold
HARVARDUNIVERSITY
Unsupported npAu Catalysts can be Prepared
with a Variety of Architectures
Ingots Foils Shells
Thickness: 300 µm 100 nm 400 nm
Ligament width: ~50 nm ~30 nm ~75 nm
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Ingots used in this work were provided by Monika Biener and Juergen Biener at LLNL.
1-3% Ag
HARVARDUNIVERSITY
Mechanistic Principles from Ultra-high Vacuum Studies
Predict Behavior Under Atmospheric Conditions
This correspondence is used to address key challenges:
(1) catalyst activation
(2) controlling and maintaining selectivity and activity
Model: O on Au(111)—
UHVCatalyst: Nanoporous Au—
UHV or 1 atm
HARVARDUNIVERSITY
Methanol Oxidation:
Selective Self-Coupling to Methyl Formate
Xu et al. Angew. Chem., Int. Ed 2009, 48, 4206.
Xu et al. J. Phys. Chem. C. 2011, 115, 3703.
Methoxy identified using
vibrational spectroscopy
& isotopic labeling
Au (111)
O3
HARVARDUNIVERSITY
npAu/O2
Oxidation of Methanol on O/npAu Under
Well-Controlled Conditions at Low Pressure
Stowers et al. J. Catal. 2013, 308, 131.
CH3OH
CH3OC(=O)H
CO2 + H2O
CH2=O
O/npAu
Atomic oxygen is required for any reactivity on npAu.
O2 does not dissociate unless npAu is first cleaned using ozone doses.
O2
HARVARDUNIVERSITY
Catalyst Material Fundamentally Changed by Activation
Procedure Derived From Fundamental Studies
Ozone pre-treatment reproducibly activates npAu catalysts (ingots,
foils, and shells) for the sustained oxidation of alcohols.
Procedure:
1. Pre-treatment in flowing ozone (3% O3 in He) at 150 °C for 1
hour, followed by cooling to room temperature in He.
2. Heating to 150 °C in a stream of 10% methanol and 20% O2
in He.
These ozone-activated npAu catalysts are a fundamentally
different catalytic material.
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Stowers et al. J. Catal. 2013, 308, 131.
50 mL/min
HARVARDUNIVERSITY
Second Step of Activation Procedure Takes Place
Under Reaction Conditions for Methanol Oxidation
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Methyl formate (initial activation)
Methyl formate (reactivation)
CO2 (initial activation)
Conditions: 10% methanol and 20% O2 in He,
150°C, 50 mL/min, 10 mg npAu shells CO2 is only
formed during the
initial activation
period following
ozone treatment.
Previously active
catalyst materials
reactivate after
exposure to air
for four months
without additional
ozone treatment.
HARVARDUNIVERSITY
Activity of npAu Catalyst Materials is Stable Over Time
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Stable conversion of
methanol has been
observed for one month
after an initial 24 hour
stabilization period.
Conditions: 10% methanol and 20% O2 in He,
150°C, 50 mL/min, 50 mg npAu shell catalyst
HARVARDUNIVERSITY
New Procedure Activates All Three npAu Architectures
Catalyst
material
Rate of conversion
of methanol
Selectivity to
methyl formate
Ingots 0.017 mmol s-1 g-1 > 99%
Foils 0.091 mmol s-1 g-1 > 99%
Shells 0.083 mmol s-1 g-1 > 99%
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Higher rates of methanol conversion for the
foils and shells indicate that these materials
overcome some of the mass transport
limitations of the ingots.
Conditions: 10% methanol and 20% O2
in He, 150°C, 50 mL/min
HARVARDUNIVERSITY
NpAu Catalysts Readily and Selectively Couple
Higher Alcohols to Form Esters
Catalyst
material
Ethanol self-coupling
selectivity to ester*
1-Butanol self-coupling
selectivity to ester*
Ingots 36.1% 20.6%
Foils 22.1% 11.9%
Shells 20.2% 16.4%
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Stowers, Friend, Madix, Biener, Biener, Submitted.
Aldehyde (acetaldehyde or
butyraldehyde) is the only
other product.
No CO2 detected.
Conditions: 5% alcohol and 20% O2 in He, 150°C, 50 mL/min
HARVARDUNIVERSITY
NpAu Activated by New Procedure is a
Fundamentally Different Catalytic Material
Distinct from previously reported npAu materials,
ozone pre-treated npAu catalysts:
Activate reproducibly and recover their activity after
being exposed to air for four months, without requiring
a second ozone treatment.
Operate stably at or above 150 °C, and are inactive in
the 20-80 °C temperature range previously reported.
Catalyze the self-coupling of 1-butanol to form butyl
butyrate and stably catalyze the self-coupling of
ethanol to yield ethyl acetate.
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Guided Catalyst Design at the Interface of UHV Models,
Catalytic Conditions, and Computational Studies
UHVFundamentals
From
Model
Studies:
Au(111)
Adding Materials
Complexity: npAu
(AgAu alloy)
Mapping to
Catalytic Conditions:
continuous flow, 1 atm.
Optimization of
Geometry:
shells and foils
Computation:
O2 dissociation
surface interactions
reactant flow
Feedback to
Model Studies:
new alloy compositions
mechanistic studies
Identification of Reactivity
Acknowledgements
Prof. Cynthia Friend
Prof. Robert Madix
Dr. Branko Zugic
Dr. Kara Stowers
Dr. Lu-Cun Wang
Nare Janvelyan
Dr. Juergen Biener (LLNL)
Dr. Monika Biener (LLNL)
Friend Research Group
This work was supported as part of the Integrated Mesoscale Architectures for
Sustainable Catalysis, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Basic Energy Sciences.
HARVARDUNIVERSITY
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Ligament Morphology Changes During Activation
(A) After dealloying in nitric acid
(B) After ozone treatment (70 mL/min,
20 g/Nm3) at 150 °C for 1 h
(C) After exposure of ozone-treated
foil to 6.5% MeOH-20% O2 at
room temperature for 30 min
(D) After exposure to reaction
conditions of 6.5% MeOH-20% O2
at 150 °C.
12K gold foil
Zugic, Personick, Janvelyan, Madix, Friend, In Preparation.
HARVARDUNIVERSITY
Diffusion Limitation of Ingots in Flowing Reactants
Reagent Flow
Diffusion of gases
partway into ingot
Bypass and surface
interactions
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1 μm
200 nm
O/Au(111) Surface npAu Ingot
npAu Shells
Catalytic Performance of npAu vs O/Au(111) is Similar
Even for Complex Environments
5-10% Alc; 20% O2; 150°C
10% Alc; 20% O2; 150°C
Xu et al. J. Am. Chem. Soc. 2010, 132, 16571.
Min et al. J Phys. Chem. B 2006, 110, 19833.
Wang, Stowers, Zugic, Personick, Biener, Biener, Friend, Madix, Submitted.
HARVARDUNIVERSITY
Synthesize Au nanoparticles
(Au NPs, ~13 nm)
Coat polystyrene (PS) microspheres with Au NPs
Deposit Au on Au NP-coated PS spheres
SEMEDX
Adapted from: Nyce et al. Chem. Mater. 2007, 19, 344.
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
Deposit Ag on Au-coated
PS spheres
Calcine to anneal Ag and Au and to
remove PS core
Etch in nitric acid to remove Ag
Synthesis of Hollow Nanoporous Au Microspheres
HARVARDUNIVERSITY
Synthesis of Hollow Nanoporous Au Microspheres
Au-coated Au/Ag-coated Annealed Dealloyed
85% Ag 15% Au 1% Ag 99% Au
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
HARVARDUNIVERSITY
Nanoporous Structure Forms After Etching
Calcined in air at 450°C for 1 hour
Etched in conc. nitric acid for 24 hours
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
200 nm1 µm
Average ligament size = 75 nm
HARVARDUNIVERSITY
Catalyst Material is not Uniformly Spherical
Hierarchical porosity still facilitates reagent flow.
10 µm
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
HARVARDUNIVERSITY
Ligaments Coarsen During Reaction
Fresh ~ 1 month of reaction
1 µm 1 µm
200 nm
Personick, Zugic, Biener, Biener, Madix, Friend, Submitted.
HARVARDUNIVERSITY
Synthesis Method is Adaptable to Other Materials,
Cores, and Particle Morphologies
1 µm
Templated solution phase deposition is amenable
to other metals.
Core material composition and size can be varied.
Morphology is tailorable.
Unpublished work.