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CONCEPTS ON THE ACTIVATION OF CARBON DIOXIDE
B. Viswanathan
Calculated bond lengths Re, harmonic force constants ke,bond
angle, Θe and bending force constants, kθ
Molecule
State
Re (Ao)
Ke
(mdyne/Ao)
Θe (deg)
Kθ
(mdyne/A
CO2
X1Σg+
1.14(1.16)
13.68(16.02)
180(180)
1.33
(0.393)
CO2
1B
2
1.24
7.99
115(122)
1.39
o)
Possible modes of activation of carbon dioxide
γ-radiation
Radiochemical
CO2
• Chemical reduction
→
HCOOH, HCHO
2Mg + CO2 → 2MgO + C
Sn + 2CO2 → SnO2 + 2CO
2Na + 2CO2 → Na2C2O4
• Thermo chemical
CO2
• Photo chemical
• Electrochemical
Ce4+
→
CO + ½O2
T>900°C
hν
CO2 → CO, HCHO, HCOOH
eV
CO2 + xe- + xH+ → CO, HCOOH, (COOH)2
• Biochemical bacteria
CO2 + 4H2 → CH4 + 2H2O
• Biophotochemical
hν
CO2 + oxoglutaric acid → isocitric acid
• Photo electrochemical
hν
CO2 + 2e- + 2H+ → CO + H2O
eV, semicond
• Bioelectrochemical
enzyme
CO2 + oxoglutaric acid
→
isocitric acid
eV, methylviologen
• Biophotoelectrochemical
hν. enzyme, p-1nP
CO2 →
HCOOH
eV, methylviologen
• M. A. Scibioh & B. Viswanathan, Proc. Indn. Natl. Acad. Sci., 70 A (3), 2004.
Why only electrochemical and photo-electrochemical activation
of carbon dioxide?
• One need to consider the bonding scheme of carbon
dioxide and evaluate which mode of activation is
feasible and possible
• Which site in the molecule has to be activated and
why?
• What selectivity or product slate one wants?
• What is the basic difference between electrochemical
activation and other modes of activation – if it exists?
• Do we need reduction alone or both reduction and
decomposition if so how?
The energy level diagram for carbon dioxide
The points of relevance are:
The 2s orbital of oxygen with binding energy
-32.4 eV does not participate in bonding and
still are lone pair states in oxygen of CO2.
The B E or C 2s and 2p states are -10.7 and
-19.4 eV while that of oxygen 2p states are
at-15.9 eV.
This gives rise to 2σ and 2π bonds with
non bonding states. Only considering the
bonding states and designating them
as 1σg, 1σu, (-38 eV) 2 σg, (-19.5) 2 σu,(18)
1πu,(17.5) 1πg (14).
In this scheme the bonding orbitals are
1σg, 1σu, and 1πu. The non bonding states are
2 σg,, 2 σu, and 1πg.
It is to be noted that the HOMO is a non bonding
π orbital. The bonding σ orbitals are deep lying.
Mostly the non bonding orbitals are frontier
orbitals. Note the energy positions of each of the
orbitals. The 2s orbital of oxygen is at -32.4 eV and
hence deep down.
THE MOLECULAR ORBITAL CONTOUR DIAGRAM FOR CARBON DIXOIDE
Selective Properties and Energy-Level Diagram for CO2
Selected properties of CO2
Point Group
Dαh
Ground state
1Σg+
Boiling point(0C)
-78.5
HOMO
1πg
LUMO
2πu
Bond length (Å)
1.16 (C-O)
Bond energy (eV)
5.453
Ionization potential (eV)
13.78
Electron affinity
-0.6
IR data (cm-1)
1320, 235, 668
MO diagram for CO2
M. A. Scibioh & B. Viswanathan, Proc. Indn. Natl. Acad. Sci., 70 A
(3), 2004. 407-462.
Why combined mode of activation?
In the photochemical activation, one has to
consider the optical excitation and the
absorbance characteristics of the molecule and it
is not favourble and hence it is directly may not
be a solution at this time. Only energy transfer
mechanism may be useful in this case and hence
electrochemical means have to be combined with
other means of activation.
What are the options of electrochemical
activation
1. Electrochemical activation implies another variable
namely potential. The application of potential( or in
reality the field strength) can distort the PE diagram of
the molecule and hence activation is possible.
2. The simultaneous presence of species like Hydroxyl can
give rise to reduction, essentially the reduction process is
favourable in electrochemical sense and hence the issue
of selectivity does not arise.
3. Molecular activation of CO2 or bond activation can also
be influenced by the field strength
4. Do we have enough knowledge on the reduction of
carbon dioxide in photosynthesis if so how much one can
mimic the same.
Interaction of CO2 with Transition Metal Centers
Reactive positions of CO2 molecule & electronic properties of a transition metal centre required for complexation
Orbital overlapping & electrostatic interaction
of coordination modes of CO2
Structural types of metal–CO2 complexes
M. A. Scibioh & B. Viswanathan
Proc. Indn. Natl. Acad. Sci., 70 A (3), 2004
Electrochemical Reduction of CO2
M. A. Scibioh & B. Viswanathan, Proc. Indian. Natl. Acad. Sci., 70 A (3), 2004.
CO2 electro-reduction on sp group metal electrodes
M. Jitaru, J. Appl. Elec.Chem 27 (1997) 875
Periodic table for CO2 reduction products
At –2.2 V /SCE in low temperature, 0.05 M KHCO3 solution
Y Hori et al., J Chem Soc Chem Commun (1987) 728
Summary of Metal Cathodes Employed for Electroreduction of CO2
M. A. Scibioh & B. Viswanathan, Proc. Indian. Natl. Acad. Sci., 70 A (3), 2004
Electro-catalytic Reduction of CO2
(a) Molecular electro-catalysts in solution; (b) Cathodic materials modified by surface
deposition of molecular electro-catalysts
M. A. Scibioh & B. Viswanathan, Proc. Indian. Natl.
Acad. Sci., 70 A (3), 2004.
Photoreduction of CO2
Energy band modes of an n-type
semiconductor with a Schottky-type barrier
Pd/RuO2/TiO2 photoreduction of CO2
(a) band–band transition;
(b) surface state population transition. Vs and Vs0,
surface potential difference; CB, conduction band;
VB, valence band; Et, surface state level; EF, Fermi
level.
T.Xie, Mater Chem Phy 70 (2001) 103
Photoreduction of CO2 - Perception
Unsolved Problems!
•
•
•
•
•
•
•
TON (mol reduction product of CO2 / mol catalyst) are still low
Efficiencies of the reactions is unsatisfactory-both the amount of reduction products of
CO2 (usually C1 products) & oxidation products of the sacrificial donor
The tuning of the single components w.r.t. their redox potentials, life times and
selectivity is not well understood.
Necessary to device systems which do not require sacrificial donors light energy is also
used for degradation of sacrificial donors, influencing the energy balance of the reactions
unfavorably
Macrocyclic complexes of transition metal ions- satisfy the requirements of a useful
relay. They may play a dual role as a catalysts and relays
Even with transition metal complexes – Reduction products have not been of great
economic value (usually only C1 products)
Multicomponent systems containing photoactive center, electron relays and/or molecular
electrocatalysts in addition to possible microheterogeneous systems will be discovered
ON SEMICONDUCTORS - CATALYSED BY MOLECULAR SPECIES
Appealing Approach!
An important energy input contribution from light might be expected, thus diminishing
electricity consumption
Principle
An Example
J.P. Collin & J.P. Sauvage, Coord. Chem. Rev. 93 (1989) 245
CO2 Activation by Metal Complexes- Perception
•
Binding of CO2 to a metal centre leads to a net electron transfer from metal to
LUMO of CO2 & thus leads to its activation.
•
Hence, coordinated CO2 undergoes reactions that are impossible for free CO2.
•
Many stoichiometric & most catalytic reactions involving CO2 activation proceed via
formal insertion of CO2 into highly reactive M–E bonds →
formation of new C–E bonds.
•
These reactions might not necessarily require strong coordination of CO2 as in
stable complexes, but are generally initiated by nucleophilic attack of E at Lewis acidic
carbon atom of CO2.
•
Weak interaction between the metal & the lone pairs of one oxygen atom of CO2 may
play a role in supporting the insertion process.
•
Although we are more knowledgeable about CO2 activation, the effective activation of
CO2 by transition metal complexes is still a goal!
Direct photoreduction of CO2
At the surface of semiconducting materials; p-Si, p-CdTe, p-InP, pGaP, n-GaAs
Three principles of photocatalytic cycles of CO2 reduction
Some concepts at this stage!
• Electrochemical activation is possible but being a gas
under normal conditions the yields and selectivity
depend on potential nature of electrode and medium
that can be employed.
• Direct photochemical activation may be possible but
cannot be selective
• Photo-electrochemical can be one of the choices now
on hand to activate carbon dioxide
•
Do you have any challenges to pose!

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