Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmospheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Powering the planet: Chemical challenges in solar energy utilization

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Hydrogen Production by Molecular Photocatalysis

The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable, and efficient systems for the conversion and storage of renewable energy sources, such as solar energy. The production of hydrogen, a fuel with remarkable properties, through sunlight-driven water splitting appears to be a promising and appealing solution. While the active sites of enzymes involved in the overall water-splitting process in natural systems, namely hydrogenases and photosystem II, use iron, nickel, and manganese ions, cobalt has emerged in the past five years as the most versatile non-noble metal for the development of synthetic H(2)- and O(2)-evolving catalysts. Such catalysts can be further coupled with photosensitizers to generate photocatalytic systems for light-induced hydrogen evolution from water.

Splitting water with cobalt.

Light-Emitting Self-Assembled Materials Based on d(8) and d(10) Transition Metal Complexes.

Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed by a transition metal complex. A challenge that has occupied researchers for several decades is to create molecular photocatalysts to promote the production of hydrogen from homogeneous solution. We now report the use of a two-electron mixed-valence dirhodium compound to photocatalyze the reduction of hydrohalic acid to hydrogen. In this cycle, photons break two RhII–X bonds of a LRh0–RhIIX2 core in the presence of a halogen trap to regenerate the active LRh0–Rh0 catalyst, which reacts with hydrohalic acid to produce hydrogen.

https://science.sciencemag.org/content/sci/293/5535/1639.full.pdf

Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst.

The singly bonded dirhodium(II) complex, Rh2(dfpma)3Br4 (dfpma = bis(difluorophosphine)methylamine, CH3N(PF2)2), photoreacts when irradiated with UV or visible light. The mixed-valence LRh0RhΙΙX2 species, Rh2(dfpma)3Br2(L), is obtained quantitatively when THF solutions containing Rh2(dfpma)3Br4 and excess L = dfpma or PR3 are photolyzed (λexc > 436 nm). The photoreaction quantum yield is ∼10-2 over the near-UV absorption manifold, decreasing slightly as the excitation wavelength is extended into the visible region (φp313-405 = 0.022(3), φp436-468 = 0.012(1)). Continued irradiation with excitation wavelengths coincident with the absorption manifold of the LRh0RhΙΙX2 complex results in a second two-electron elimination reaction to give the LRh0Rh0L dimer, Rh2(dfpma)3L2, again in quantitative yield but with an attenuated quantum efficiency (φp313-365 = 0.0035(3), φp405-436 = 0.0017(3)). For photoreactions performed in THF, the bromine photoproduct is HBr, which may be trapped with 2,6-lutidine. NMR experimen...

Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

A novel one-dimensional (1D) heterometallic chain, {[Rh2(dfpma)2(MeCN)4]2[Ag(MeCN)4]}[PF6]5 (1), is afforded from the in situ reaction of [ClRh(cod)]2 with [Ag(MeCN)4][PF6] and dfpma (dfpma = bis(difluorophosphine)methylamine). Dichroic crystals, which are obtained from MeCN/Et2O solutions, crystallize in the monoclinic space group C2/m with a = 13.570(5) A, b = 20.895(9) A, c = 13.810(6) A, β = 104.904(7)°, V = 3784(3) A3, Z = 4. X-ray diffraction studies reveal an asymmetric unit comprising two RhI2 dimers and a square planar AgI cation; this subunit propagates to form a 1D heterometallic chain. Compound 1 displays novel spectroscopic properties in the solid state, including temperature-dependent luminescence.

A luminescent heterometallic dirhodium-silver chain.

Photochemistry of dirhodium(II,II) diphosphazane tetrachloride complexes

Cette invention concerne notamment un procede de production d'hydrogene comprenant une solution protique, un photocatalyseur pouvant effectuer une reduction a deux electrons d'ions hydrogene et un piege de sous-produits. Un mode de realisation comprend un procede consistant a exposer le milieu de reaction a un rayonnement pouvant photo-exciter le photocatalyseur afin que de l'hydrogene soit produit. La solution protique peut comprendre au moins un acide hydrohalique, un silane et de l'eau, lequel acide hydrohalique pouvant etre acide chlorhydrique, bromure d'hydrogene, fluorure d'hydrogene ou iodure d'hydrogene. Cette invention porte egalement sur de nouveaux composes de metal de transition. Des modes de realisation concernent un compose comprenant deux atomes de metal de transition, lesquels atomes de metal de transition se trouvant dans un etat de valence mixte a deux electrons, au moins un metal de transition qui n'est pas le rhodium ; et au moins un ligand pouvant stabiliser l'atome de metal de transition dans un etat de valence mixte a deux electrons.

Procede de production photocatalytique d'hydrogene a partir de solutions protiques utilisant des complexes binucleaires a valence mixte et a deux electrons et complexes de ce type

Alan F. Heyduk

Papers

Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst.

Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

A luminescent heterometallic dirhodium-silver chain.

Photochemistry of dirhodium(II,II) diphosphazane tetrachloride complexes

Procede de production photocatalytique d'hydrogene a partir de solutions protiques utilisant des complexes binucleaires a valence mixte et a deux electrons et complexes de ce type