Tailored Mesoscale Gold Alloy Materials for Energy- and ...efrc.harvard.edu/files/imasc/files/tailored_mesoscale_gold_alloy_materials_for_energy...Methyl formate (initial activation)

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

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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.


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


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


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%


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%


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.


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

HARVARDUNIVERSITY

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

HARVARDUNIVERSITY

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.


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


HARVARDUNIVERSITY

Nanoporous Structure Forms After Etching

Calcined in air at 450°C for 1 hour

Etched in conc. nitric acid for 24 hours


200 nm1 µm

Average ligament size = 75 nm

HARVARDUNIVERSITY

Catalyst Material is not Uniformly Spherical

Hierarchical porosity still facilitates reagent flow.

10 µm


HARVARDUNIVERSITY

Ligaments Coarsen During Reaction

Fresh ~ 1 month of reaction

1 µm 1 µm

200 nm


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.

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Tailored Mesoscale Gold Alloy Materials for Energy- and ...efrc.harvard.edu/files/imasc/files/tailored_mesoscale_gold_alloy_materials_for_energy...Methyl formate (initial activation)