Catalytic Solar Water Splitting Inspired by
Photosynthesis. Homogeneous
g
Catalysts
with a Mechanical (“Machine-Like”) Action
Gerry Swiegers
Australian Research Council Centre off Excellence for
f
Electromaterials Science and Intelligent Polymer Research
Institute
University of Wollongong
Wollongong, NSW 2522
Australia
Sub-theme: Molecular Machines
ƒ Much interest in developing
p g “molecular machines”
that drive chemical reactions
ƒ But, what is the “mechanical action” that must
occur within a molecular catalyst to turn it into a
molecular
l l machine
hi ?
Two General Methods of Inducing Change
ƒ Energy Gradient (“Thermodynamics”)
Change driven by an overall release of energy
e.g. a ball falling to Earth under gravity
ƒ Mechanical Interaction (“Mechanics”)
Ch
Change
driven
di
by
b a physical
h i l collision
lli i ((action-reaction
i
i sequence that
h
plays out over time)
e.g. two billiard balls physically colliding
Chemistry:
y Collision Theory
y
STEP (1)
STEP (2)
Reactant collision controlled by the
Product formation controlled by the
“collision frequency” (A)
“activation energy” (Ea)
“mechanics”
“thermodynamics”
A
products
transition
state
B
“Mechanical”
Mechanical reaction:
H+
+ OH-
H 2O
k = 10-10 s
Potential Energ
gy
reactants
Ea
reactants
products
Reaction Coordinate
What Happens
pp
in Catalysis?
y
STEP (1)
STEP (2)
Reactant collision controlled by the
Product formation controlled by the
“collision frequency” (A)
“activation energy” (Ea)
“mechanics”
catalyst
y
reactants
energy:
Ea
catalyst transition
state
binds &
activates formed
reactants
t t
products products
p
released
bind
catalyst
Pottential Energgy
time:
tTS
A
B
“thermodynamics”
uncatalyzed
Ea
catalyzed
Ea
reactants
products
Reaction Coordinate
“Mechanical” Catalysts:
y
H2 g
generation
Dynamic proton binding
+2H+
+ Fe
. .
H H
Fe+
-2H+
Fe
Fe
1
. .
2
Fe H H Fe +
+
2 e-
[2]TSC
Fe +
+ Fe
3
+ Fe
H H Fe +
H2(g)
produces 5 molecules H2 s-1 catalyst-1 over
at least 5 days of continuous operation
-Two dynamic
d
processes ((catalyst
l
fl
flexing and
d proton b
binding)
d
)
which are only synchronized if the catalyst flexes rapidly about a
structure that complements the transition state
Chem Commun 2004, 308
Chem Eur J 2009, 15, 4746
“Mechanical” Catalysts:
y
O2 reduction
N
N
Co N
N
N
N
Co N
N
3
N
N
Co N
N
N
N
Co N
N
“Pac-Man” Catalyst
4
3
O2
H2O
(4 e- process)
H2O
(4 e- process)
4
O2
Chem Commun 2007, 3352
Chem Eur J 2009, 15, 4746
Some Common Features of “Mechanical”
Molecular Catalysts
(1) Reaction controlled by the Catalyst-Mediated Collision Frequency (low
Activation Energy)
(2) The maximum catalytic rate depends on the rate of conformational
flexing (conformational flexing = the “mechanical impetus”)
(3) Catalyst typically flexes rapidly about a shape that complements the
transition
ii
state
(4) Highly efficient and selective form of catalysis (like a machine)
(5) Michaelis-Menten
Michaelis Menten kinetics
Chem Eur J 2009, 15, 4746
• These features also found in many enzymes
QUESTION: Are enzymes mechanical catalysts?
Mechanical Catalysis: Methods of Enzymatic,
Homogeneous, and Heterogeneous Catalysis
S i
Swiegers,
G
G. F
F.
John Wiley & Sons, New York, 2008
Can we Mimic an Enzyme?
y
Mn
O
O
O
Mn
Mn
O
Mn
O
Mn
O
Mn O
CORE:
O Mn
Mn
(p-MeO-C6H4)2PO2
Bio-inspired Mn-oxo Cubane Model Complex
G. C. Dismukes, Princeton University
Water-Oxidizing Complex of
Photosystem II
(PSII-WOC)
(PSII
WOC)
Cubane:
- Shape and structure similar to enzyme active site
- Dynamically self-assembles
- Flexible
- Low activation energy for O2 release (photolytic)
Cubane forms O2 when illuminated
O
Mn
Mn
O
Mn
hν
O
O
O
Mn
Mn
Mn
O
Mn
O
O
Mn
8
O
Mn
+ 1 ligand released
Mn
O
Mn
O
O
Mn
?
+ 1 ligand
+ 2 H 2O
- 4H+- 4 e-
Mn
Mn-Mn distance
lengthens
O
Mn
O
Mn
O
O
O
Mn
Mn
O
O
Mn
O-O distances shorten
Corner O's collide
Mn
O
Mn
Mn
2-
O
O
Mn
Peroxo (O2 ) forms
O
Mn
O
Mn
Mn
Mn
Mn
O
Superoxo (O2-) forms
O
Mn
"butterfly"
9
DeAngelis, Carr
+
O2
Cubane in Nafion layer
y on GC electrode
Electrode biased at 1.0 V (vs. Ag/AgCl)
Ill
Illuminated
i t d att 250-750
250 750 nm
2.0
Current (µA)
15
1.5
1.0
0.5
0.0
Currrent (µA)
A
0
40
80
Time (sec)
120
160
3
2
1
0
>295nm >395nm >455nm >495nm >570nm >275nm
B
100
150
200
250
300
Time (sec)
350
400
Peak turnover frequency:
270 molecules O2 h-1 catalyst-1
Total turnovers:
>1000 readily achieved
Angew Chem Int Ed Engl 2008, 47, 7335
Cubane+ ion-exchanged
g
into Nafion: CV
Mn2+ in
M
i
Nafion
8+ in
Nafion
10
60
40
Current ((µA)
8
20
6
0
4
2
0
-2
0.4
0.6
0.8
1.0
Potential (Vvs Ag/AgCl)
Potential (V vs Ag/AgCl)
1.2
Proposed
p
Mechanism
Catalyst
y dis-assembles and re-assembles under turnover conditions
Phys Chem Chem Phys 2009, 11, 6441
Inorg Chem 2009, 48, 7269
CV
2+/3+
N
N
-O O
Ru N
N
N
N
O
O
N
N
Ru N
N
N
N
Currrent (µA)
8+ in Nafion
A
0.6
0.8
1 .0
Potential V (vs Ag/AgCl)
Ru(bpy)32+
hν
1.2
Ru(bpy)33+ +
2+/3+
e-
Water-Splitting
p
g “Graetzel” Cell
L. Spiccia, Monash University
Water-Splitting
p
g “Graetzel” Cell
Uses H2O as electrolyte – even impure H2O (seawater).
(seawater)
Eliminates the need for acetonitrile electrolyte which
bedevils existing Graetzel cells
J Am
A Chem
Ch
Soc
S 2010,
2010 132,
132 2892
• System splits seawater into pure oxygen (no chlorine formation)
p
company:
p y “Cube Catalytics
y
LLC”
• Spin-off
(funded by New Energy Ventures, New Jersey)
Acknowledgements
Princeton University / Rutgers University, USA
Prof Chuck Dismukes
Dr Greg Felton
Monash University, Australia
Prof Leone Spiccia
Dr Rob Brimblecombe
Dr Rosalie Hocking
Dr Annette Koo
University
i
i off Wollongong, Australia
A
i
Prof Gordon Wallace
Dr Jun Chen
Dr Chee Too
CSIRO Molecular and Health Technologies, Australia
Dr Junhua Huang
Funding
Australian Research Council
Australian Academy of Science
University of Wollongong
CSIRO