Hydrogen generation from water is a clean energy source to address the challenges of climate change and sustainable energy solution. Water splitting consists of water oxidation to oxygen and proton, whereby proton can be reduced to clean hydrogen. The water oxidation reaction (OER), catalyzed by a simple mononuclear cobalt(II)-aqua complex, entrapped in the confined space of a metal organic framework (MOF) in a ‘ship in a bottle’ manner will be discussed first. When we place [Co(H2O)6]2+ coordination complex inside the zeolite-Y cages and deposit this zeolite-Y encapsulating hexa-aqua-Co(II) complex on a working electrode surface, this modified electrode on dipping in an aqueous alkaline solution forms nanofilm of α-Co(OH)2 on zeolite-Y surface, that acts an efficient catalyst for electrochemical water oxidation. The central element Co(II) of a Keggin anion [CoIIW12O40]6– is, as such, not catalytically active. This becomes an efficient electrocatalyst for water oxidation, when the relevant Keggin anion is encapsulated inside the well-defined confined space of the framework material ZIF-8. The same Keggin anion [CoIIW12O40]6– can act as support (host) to attach a {Ni(bipy)2(H2O)}2+ complex (guest) on its surface through coordinate covalent bond via one of its terminal oxo groups and becomes an efficient (heterogeneous) water oxidation catalyst. The well-known coordination complex, [FeIII(salen)Cl] (an well known catalyst for organic conversions) undergoes decomposition at a moderate anodic potential, when it is used as an electrocatalyst for water oxidation. But this versatile complex, upon its encapsulation in a confined space of the framework material ZIF-8, turns into stable active site for electrocatalytic water oxidation. Likewise, a Co(II)-porphyrin complex, CoTMPP usually acts as an ORR (oxygen reduction reaction) catalyst, not as an OER catalyst. Interestingly, when it is encapsulated inside ZIF-8, the resulting host-guest material becomes an efficient electrocatalyst for water oxidation. The second part of this talk will be based on a fully reduced {V15}-type polyoxovanadate (POV) cluster system, which can capture aerial CO2 in its cavity in the form of carbonate. The entrapped carbonate can be excluded from the cavity of this POV cluster as CO2 gas in a gas-solid interface reaction using HCl vapor and the excluded CO2 gas can be reacted with Grignard reagent PhMgBr to produce benzoic acid and triphenyl carbinol (value added organic products).

Host-guest composite materials: Elelctrocatalytic water splitting and aerial carbon dioxide capture

ACS Spring Meeting 2021

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