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!