Homogeneous and heterogenized iridium water oxidation catalysts

Abstract

The development of an efficient catalyst for the oxidative splitting of water into molecular oxygen, protons and electrons is of key importance for producing solar fuels through artificial photosynthesis. We are facing the problem by means of a rational approach aimed at understanding how catalytic performance may be optimized by the knowledge of the reaction mechanism of water oxidation and the fate of the catalytic site under the inevitably harsh oxidative conditions. For the purposes of our study we selected iridium water oxidation catalysts, exhibiting remarkable performance (TOF > 5 s -1 and TON > 20000). In particular, we recently focused our attention on [Cp*Ir(N,O)X] (N,O = 2-pyridincarboxylate; X = Cl or NO 3 ) and [IrCl(Hedta)]Na water oxidation catalysts. The former exhibited a remarkable TOF whereas the latter showed a very high TON. Furthermore, [IrCl(Hedta)]Na was heterogenized onto TiO 2 taking advantage of the presence of a dandling -COOH functionality. The heterogenized catalyst maintained approximately the same catalytic activity of the homogeneous analogous with the advantage that could be reused many times. Mechanistic studies were performed in order to shed some light on the rate-determining step and the transformation of catalysts when exposed to "oxidative stress". It was found that the last oxidative step, preceding oxygen liberation, is the rate-determining step when a small excess of sacrificial oxidant is used. In addition, several intermediates of the oxidative transformation of the catalyst were intercepted and characterized by NMR, X-Ray diffractometry and ESI-MS.

Heterogenized Water Oxidation Catalysts Prepared by Immobilizing Kläui‐Type Organometallic Precursors

“…Molecular [1] and material [2] water oxidation catalysts (WOCs) with remarkable performances have been reported. Furthermore, it has been recently shown that anchoring a properly tailored homogeneous catalyst onto solid supports, thus obtaining heterogenized WOCs, is a promising strategy to combine the best of two worlds; [5][6][7][8] this allows to finely modulating electron density and geometry of WOCs, whereas grafting the latter on solid supports leads to enhanced stability and facilitates catalyst recovering. [5] At the same time, heterogenized WOCs allow the exploitation of metal to be minimized due to the elevate percentage of active centers, in principle, not so different than that in homogeneous catalysis.…”