Efficient Chemical and Visible‐Light‐Driven Water Oxidation using Nickel Complexes and Salts as Precatalysts

Abstract

Chemical and visible-light-driven water oxidation catalyzed by a number of Ni complexes and salts have been investigated at pH 7-9 in borate buffer. For chemical oxidation, [Ru(bpy)3](3+) (bpy = 2,2'-bipyridine) was used as the oxidant, with turnover numbers (TONs) >65 and a maximum turnover frequency (TOFmax) >0.9 s(-1). Notably, simple Ni salts such as Ni(NO3 )2 are more active than Ni complexes that bear multidentate N-donor ligands. The Ni complexes and salts are also active catalysts for visible-light-driven water oxidation that uses [Ru(bpy)3](2+) as the photosensitizer and S2 O8 (2-) as the sacrificial oxidant; a TON>1200 was obtained at pH 8.5 by using Ni(NO3)2 as the catalyst. Dynamic light scattering measurements revealed the formation of nanoparticles in chemical and visible-light-driven water oxidation by the Ni catalysts. These nanoparticles aggregated during water oxidation to form submicron particles that were isolated and shown to be partially reduced β-NiOOH by various techniques, which include SEM, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, XRD, and IR spectroscopy. These results suggest that the Ni complexes and salts act as precatalysts that decompose under oxidative conditions to form an active nickel oxide catalyst. The nature of this active oxide catalyst is discussed.

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

“…11,12,13 In particular, Ru-aqua complexes with polypyridylic ligands 14,15,16,17 are robust catalysts that allow for the analysis of mechanisms much more accessible than those of first row transition metals with labile ligands. 18,19,20,21,22 When combined with the theoretical analysis, via density functional theory (DFT) calculations, spectroscopic and electrochemical measurements provide detailed information on the nature of reaction intermediates and activation free energies along the catalytic cycle of water oxidation. 23,24,25,26,27,28,29,30 Strong sigma donation groups like carboxylate ligands in 2,2'-bipyridine-6,6'-dicarboxylic acid (H 2 bda; see Chart 1 for a drawing), together with seven coordination have allowed easy access to reactive species in high oxidation states such as [Ru V (O)(bda)(pic) 2 ] + (pic is 4-picoline) where the metal center is at formal oxidation state of V. 31 Additional tuning of the activation energy barriers can result from supramolecular interactions, based on π-π stacking of ligands with π -extended conjugation such as isoquinoline and its derivatives favoring formation of dinuclear peroxo intermediates.…”

Cobalt–Salen Complexes as Catalyst Precursors for Electrocatalytic Water Oxidation at Low Overpotential

“…21 As a consequence, developing cheap and abundant materials as WOCs with low overpotential and high efficiency is highly desirable. 22 In the literature, various homogeneous and heterogeneous systems based on other first-row transition metals have attracted much attention for the oxygen evolution reaction (OER), such as cobalt (Co), [23][24][25] nickel (Ni), [26][27][28][29] copper (Cu), [30][31][32] and iron (Fe). [33][34][35] More recently, it has been found that active WOCs can be prepared by electrodeposition or photodeposition from organic metal complexes.…”

Behavior of the Ru-bda Water Oxidation Catalyst Covalently Anchored on Glassy Carbon Electrodes

“…The nature of the transition metal complex as well as the oxide formation protocol strongly influence catalytic performance. 28,6,29,30 RuO 2 has long been known to be an effective electrocatalytic material for water oxidation to molecular dioxygen. 31,32 Recent work has focused on the relationship of particle size and shape with catalytic water oxidation performance, at different pHs, including catalysts immobilized on different electrode surfaces.…”