UKCCSC Meeting, 18 - 19 April 2007 Nottingham

Long Term Utilisation

To develop, for the first time, catalysts which allow photocatalytic reduction to be performed in supercritical CO2

Prof M. George / A. Cowan

Strategy for CO2 Reduction

• Reduction of CO2 requires energy

Photon as energy source (Photochem)

Electricity as energy source (Electrochem)

• One electron process is unfavorable

Multi-electron transfer catalysts

CO2 + e- CO2- E = 1.9 V (vs NHE at pH 7)

CO2 + H+ + 2e- HCO2- E = 0.49 V

CO2 + 2H+ + 2e- CO + H2O E = 0.53 V

CO2 + 6H+ + 6e- CH3OH + H2O E = 0.38 VComments Inorg. Chem. 1997, 19, 67Coord. Chem. Rev. 1999, 185, 373

CO2 ReductionIf Nature Can Do It,

Why Can't We?

http://photoscience.la.asu.edu

• Artificial photosynthesis for CO2 reduction typically requires a

photosensitizer, a catalyst and an electron donor

• Products are CO, formate, and H2

Charge separation

Re

N

NCl

CO

CO

CO

Re

N

NCl

CO

CO

CO

+-

Re

N

NCl

CO

CO

CO

-

charge separationh

TEA

TEA+

• Co macrocycles

• Ni macrocycles

• Cobalt and Iron porphyrins,

Phthalocyanines and corroles

• Ru(bpy)2(CO)X

• Re(bpy)(CO)3X

• Ni(bpy)32+

Re

N

NCl

CO

CO

CO

- CO, formate, carbonate are produced

- BUT: low turnover numbers, catalyst poisoning, by-products

George, M. W., et al. (1996) Organometallics 15, 3374-3387

• Reaction of the catalytically active species with CO2 is very slow

• Solvent binds to the “vacant site”

-Low turnover numbers, catalyst poisoning, by-products

Hayashi, Y., Kita, S., Brunschwig, B. S. & Fujita, E. (2003) J. Am. Chem. Soc., 125, 11976-11987.

•CO2 is the solvent and reagent

•Achieve solvent density comparable to conventional solvents

•Tuneable Properties

Why operate in scCO2 ?

Tc = 304 K

Pc= 72.9 atm

• Problem: Most metal carbonyls are insoluble in non-polar solvents!

• Solution:

C9H19 chains

Soluble in non-polar solvents!

N

N

Re

ClOC

OC

OC

Re

N

NCl

CO

CO

CO

Re

N

NCl

CO

CO

CO

Re

N

NCl

CO

CO

CO

8

8

• Investigate photophysics/photochemistry in conventional solvents and scCO2

• Test catalytic ability of new complexes in scCO2

• Feedback into rational catalyst design

PROBE SAMPLE DETECTOR

PUMP

IR

UV

TRIR Spectroscopy

Kinetic studies of mechanisms from picosecond (10-12 s) to seconds

2100 2050 2000 1950 1900 1850

OD = 0.052052

2022

1977

1951

1925

1906

50 ns

Re

N

NCl

CO

CO

CO

X

X

Re

N

NCl

CO

CO

CO

+-

X

X

charge separation

h

In n-heptane similar excited state to un-substituted complex

v

Conclusions• Solvent coordination is a problem in existing

Re-bpy CO2 reduction catalysts

• CO2 soluble analogue may overcome this problem – work in scCO2

• Currently examining mechanism with TRIR spectroscopy

• We will start testing catalytic ability shortly

• Mike George

• Etsuko Fujita / Dave Grills

• John Gavey

Future aims

• A fundamental challenge is the replacement of the “sacrificial” electron donors by species that will lead to useful (or benign) chemicals in their own right

– CO2 + 2H2O CH3OH + O2

– CO2 + CH4 CH3COOH

• If we can do this, then we can do what Nature does