Stability of Catalytic Centres in Light‐Driven Hydrogen Evolution by Di‐ and Oligonuclear Photocatalysts

Abstract

A review. In recent decades, mimicking natural photosynthesis by artificial photocatalysis represented a major research direction with the ultimate goal of reducing fossil fuel consumption through efficient solar energy harvesting. To transfer molecular photocatalysis from the lab scale to an industrially relevant process, it is important to overcome instability problems of the catalysts during light‐driven operation. As it is well‐known that many of the typically utilized noble metal‐based catalytic centres (e. g. Pt and Pd) undergo particle formation during (photo)catalysis and thus switch the whole process from a homogeneous into a heterogeneous one, an understanding of the factors governing particle formation is crucially needed. The review therefore focuses on di‐ and oligonuclear photocatalysts bearing a range of different bridging ligand architectures for drawing structure‐catalyst‐stability relationships in light‐driven intramolecular reductive catalysis. In addition, ligand effects at the catalytic centre and their implications for catalytic activity in intermolecular systems will be discussed, as will important insights into the future design of operationally stable catalysts.

Bridging Platinum and Palladium to Bipyridine-Annulated Perylene for Light-Driven Hydrogen Evolution

“…P-Pd appeared to be less active, with an overall TON of 50 over 48 h of irradiation, which is essentially one catalytic turnover per hour (Figure ). Since these catalytic motifs of the structure MN^NCl 2 (M = Pt, Pd) are known to also form catalytically active nanoparticles upon irradiation, − we performed mercury poisoning of the catalytic samples. , Elemental mercury is well known to adsorb both platinum and palladium nanoparticles, which disables their proton reduction ability. Upon addition of ∼1000-fold excess of mercury, hydrogen evolution remained stable for P-Pt .…”

“…At the molecular level, the Pd nanoparticle formation in P-Pd stems from the population of low-lying metal-centered states ( 3 MC). In square planar d 8 coordination compounds, these states feature a decreased sigma bond order between the central metal and all ligands due to the population of the d x 2 – y 2 orbital (or rather the σ x 2 − y 2 * molecular orbital). − In case of P-Pd , time-dependent density functional theory (TDDFT) calculations reveal that the five lowest energy triplet states in the singlet ground state structure are of 3 MC character (Table S2). Therefore, population of these states leads in consequence to dissociation at the catalytic center.…”

Efficient Energy Conversion using Tri-nuclear M-Ru(II)-M[M= Pt(II), Re(I)] Catalysts

“…[6][7][8][9][10][11] More specifically, ruthenium(II)-polypyridyl based light-absorbing unit, along with (or bonded through a bridging unit) Pt/Pd/Rh/Re or Co metal based catalyst have shown efficient light-induced electron transfer. [12][13][14][15][16][17] The success of the catalytic cycle relies on electron-donation capacity of the species and these systems holds promise for catalytic processes like carbon dioxide reduction and water splitting. Ruthenium bipyridine complexes, with saturated/non-conjugated bridge particularly with two-carbon spacer show higher yields.…”

Rationalizing In Situ Active Repair in Hydrogen Evolution Photocatalysis via Non‐Invasive Raman Spectroscopy

“…Deterioration and degradation processes are an inherent limitation of catalytic systems [12] triggering research into methods to repair catalytic centers and revitalize catalytic activity, [13–15] as recently reviewed [16] …”

“…[6] Hence, natural photosynthesis remains the inspiration for the development of efficient artificial light-tochemical energy conversion strategies. [7][8][9][10][11] Deterioration and degradation processes are an inherent limitation of catalytic systems [12] triggering research into methods to repair catalytic centers and revitalize catalytic activity, [13][14][15] as recently reviewed. [16] Pfeffer et al have discovered an active repair strategy for their dinuclear photocatalyst ([(tbbpy) 2 Ru(tpphz)PtI 2 ]-(PF 6 ) 2 (RuPtI 2 , with tbbpy = 4,4'-di-tert.-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:2'',3''-h:2''',3'''j]phenazine and PF 6 > = hexafluorophosphate anion)).…”