Insights into the directions to increase turnover frequency and turnover number during photochemical water oxidation with molecular Ru catalysts. Issue 10 (7th September 2022)

• Record Type: Journal Article
• Authors: Pokhrel, Indira; Reddy, Guda Dinneswara; Kwon, Taejin; Choi, Eungyeong; Chun, Yu Sung; Lee, Sang Tak; Sung, Bong June; Lee, Duck Hyung; Bin Oh, Han; Yoon, Kyung Byung
• Is Part Of: Energy & environmental science. Volume 15:Issue 10(2022)
• Page Start: 4259
• Page End: 4288
• Publication Date: 2022-09-07
• Subjects: Energy conversion -- Periodicals; Fuel switching -- Periodicals; Environmental sciences -- Periodicals; Environmental chemistry -- Periodicals; 333.79
• DOI: 10.1039/d2ee01128j
• Languages: English
• ISSNs: 1754-5692

Abstract

Abstract : Four novel and three known mono Ru water oxidation catalysts were prepared and their photocatalytic performances in terms of turnover frequency (TOF) and turnover number (TON) were analyzed, and new directions to increase TOF and TON were presented. Abstract : A series of four mononuclear water oxidation catalysts (WOCs), denoted as [A2 Tpz –Ru II ], in which A2 represents two axial monodentate ligands, pyridine (Pyr), 4-picoline (4-Pic), isoquinoline (Iqn), and N -methyl imidazole (Imd), and Tpz represents a planar tetradentate ligand in the equatorial position (6, 6′-di(1 H -pyrazol-1-yl)-2, 2′-bipyridine) were prepared. A series of four photosensitizers (PSs), [Ru(bpy)3 ] 2+, [Ru(bpy)2 {bpy-(CO2 Et)2 }] 2+, [Ru(bpy){bpy-(CO2 Et)2 }2 ] 2+, and [Ru{bpy-(CO2 Et)2 }3 ] 2+, where bpy and bpy-(CO2 Et)2 denote 2, 2′-bipyridine and diethyl [2, 2′-bipyridine]-4, 4′-dicarboxylate, respectively, denoted as [PS- n ] 2+, where n = 0, 2, 4, and 6, were prepared. The three known WOCs with the related structures were also prepared. The catalytic performances of the above seven WOCs were evaluated in terms of turnover frequencies (TOFs) and turnover numbers (TONs), while varying PS, and their performances were compared. The measured TOFs were much smaller than those chemically measured. The low concentration of auxiliary oxidant, [PS- n ] 3+, arising from low photon flux density of the incident solar simulated light is proposed to be responsible for the large decrease in TOFs. The oxidation power of the oxidized form of [PS- n ] 2+ ([PS- n ] 3+ ), the oscillator strength ( f ) of [PS- n ] 2+ and the lifetime of the [PS- n ] 2+ in the excited state ( τ ps* ) also sensitively affect the catalytic performance of a WOC. The results were also analyzed by the reaction schemes that were widely used for mononuclear Ru-based WOCs. For this, the corresponding energy-minimized structures, the energy levels of highest occupied molecular orbitals of the initially added WOCs and intermediates, and the free energy change (Δ G °) for each reaction step were also derived from DFT calculations. This work introduces a new promising series of WOCs, and gives insights into the directions to increase TONs and TOFs during photochemical evaluations of WOCs.