www.asiachem.news December 2021 | 81 heavy materials or a larger number of people by trucks or buses is not a realistic target for the BEVs but is considered achievable by the FCEVs. Moreover, the time required to refuel the FCEVs is short enough and comparable to the gasoline-powered vehicles, representing their high advantages in comparison with the BEVs. In addition, the fuel-to-electricity conversion efficiency over 60% already achieved by the hydrogen fuel cells3 makes them promising technologies towards the development of a hydrogen energy society. How we separate the fuels yielded in artificial photosynthesis? Currently, the mid-term direction rather concentrates on the hydrogen production based on natural gas reforming (i.e., steam methane reforming) together with the innovative technologies permitting the higher energy conversion efficiency as well as the capture, utilization and storage of the CO2 evolved in the reforming process.4,5 However, the long-term direction must be focused on the truly renewable energy cycles based on the storable fuels given by reduction of either water or CO2. For some simplest renewable fuels, the combustion energy averaged for storing two reductive equivalents (i.e., via 2-electron reduction) decreases in the order of H2 (he) > CO (0.99he), HCHO (0.99he) > HCOOH (0.89he) > CH3OH (0.85he) > CH4 (0.78he), where he denotes the combustion energy of H2 (286 kcal/mol). Only formic acid and methanol are liquid and possess superior characteristics from a viewpoint of energy density together with the feasibility in refueling and transportation in ambient conditions. Furthermore, formic acid has a remarkable potential as a source of H2 because of its reversible conversion capability: HCOOH ↔ H2 + CO2 (DG = -11.6 kcal/mol). 6 There are several ways of converting solar energy into H2, CO, and HCOOH based on the simple 2-electron reduction (Figure 1, a-c). For all cases, the source of electrons and protons can be produced in the artificial photosynthetic water oxidation process (2H2O O2 + 4H+ + 4e-). Without saying, it is important to separately develop some highly efficient water oxidation catalysts (WOCs).7 What are the forms of products in each case? In the gaseous fuel production, flammable or explosive gaseous mixture, {2H2+O2} or {2CO+O2}, is yielded, inevitably requiring the extra costs and energy in the gas separation processes.8 In this context, a two-phase gas evolution system, discussed below, has a great advantage. The photosynthetic production of the {O2+2HCOOH} mixture (Figure 1c) is also advantageous owing to the spontaneous separation of the two products into the gas and aqueous phases. Moreover, substantial efforts have been made to produce high pressure hydrogen based on the catalytic conversion of HCOOH into the {high-pressure H2 + supercritical CO2} mixture within a pressure-resistant vessel having a limited volume (Figure 1d).9,10 This is a promising way to avoid the use of a mechanical compressor which consumes electrical energy during its operation (Figure 1f). In addition, desirable methods of handling the reverse process, i.e., the catalytic hydrogenation of CO2 into HCOOH (Figure 1e), must be advanced in order to facilitate the large-scale transportation of hydrogen energy in a liquid form. The above fuel generation processes combined with water oxidation catalysis may be driven using sustainable energy sources, such as solar, hydroelectric, oceanic, geothermal, wind and so forth. If we limit our discussion on the solar-driven artificial photosynthesis, oxidative and reductive equivalents required to drive the two catalytic processes must be generated via the light-harvesting of molecular and/or semiconductor systems.11-16 Molecular platinum(II)-based complexes as catalysts for hydrogen evolution reaction. One of our interests over the last two decades has concentrated on the basic and applied chemistry of molecular hydrogen evolution reaction (HER) catalyzed by various platinum(II) complexes.17 The study was originally evoked in the late 1980s by finding the HER catalyzed by several cis-diammineplatinum(II) dimers doubly bridged by amidate ligands, [Pt(II)2(NH3)4(a-amidate)2] 2+ (amidate = a-pyrrolidonate, a-pyridonate, acetamidate, 2-fluoroacetamidate, etc.) (Figure 2).18-20 In the earlier studies, their catalytic activity was scrutinized using a multi-component system comprising of [Ru(bpy)3] 2+ (bpy = 2,2’-bipyridine) as a photosensitizer (PS), methylviologen (N,N’- dimethyl-4,4’-bipyridinium; MV2+) as an electron relay (Accepter), a platinum(II) complex as a water reduction catalyst (WRC), and EDTA·2Na (ethylenediaminetetraaceticacid disodium salt) as a sacrificial electron donor (Donor) (Figure 2). For many years, we insisted in studying all catalysts under a common aqueous acetate buffer condition (pH=5.0) in which the driving force for the HER driven by MV+• is only 150 meV. One of the most highly active catalysts (i.e., colloidal platinum) indeed works well so that the exploration of molecular catalysts active with this condition was believed to be the ideal target. The environmentally friendly aqueous conditions free on organic solvents were also considered to be the most suitable conditions when it happens to transfer the technology to the practical applications. Consequently, the Pt(II)-based catalysts had been for a long time the sole family of catalysts active under this condition until we reported on the second and third family of catalysts in 2010s, i.e., carboxylate-bridged dirhodium(II) catalysts21 and a cobalt-NHC catalyst.22 The specific features of the Pt(II)2 dimers are represented by the short bridged Pt(II)-Pt(II) distance (ca. 2.8-3.0 Å) together with the air-oxidizable metal centers displaying a quasi-reversible two-electron one-step Pt(II)2/Pt(III)2 redox couple at ca. 0.4-0.6 V vs. SCE,23 which can also be correlated with the blue and red chromophores in the mixed-valence tetranuclear Pt(2.25+)4 and Pt(2.5+)4 systems given by the stack of dimers. The subsequent studies on various mono- and dinuclear complexes suggested that the metal-metal interaction plays a Figure 1. Handling the 2-electron-reduced fuels towards applications in renewable energy cycles.
RkJQdWJsaXNoZXIy NDU2MA==