Abstract
The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule–material hybrid systems are organized as “dark” cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond “classical” H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, Cx>1 products from CO2 utilization, and other reductive organic transformations.
Highlights
The development of carbon-based light-harvesting colloids enables the utilization of hydrophobic interactions to interface nonwater-soluble catalysts, whereas dye-sensitized photocatalysis (DSP) systems allow for the implementation of cheap organic chromophores instead of precious-metal-based complexes
In-depth investigations of electron transfer (ET) kinetics, electrochemical behavior of immobilized catalysts, and the nature of catalyst−surface interactions have revealed key information which may aid in catalyst design that is optimized for a specific surface
The low catalyst loadings, background H2 evolution activity of quantum dot (QD), slow electron accumulation and turnover in catalysts leading to charge recombination, general instability of the colloid/catalyst interface, as well as the low driving force of Metal oxide (MOx)-based systems toward CO2 reduction all appear to be of primary importance
Summary
2814 The worldwide reliance on fossil fuels as energy carriers and raw 2816 materials for industrial products presents several challenges for 2817 the coming decades. These archetypal biological systems are vital to advancing our understanding of the fundamental aspects of small molecule activation, which may allow for improved design of catalysts and catalytic processes for fuel production and manufacture of commodities.[1,17] while fuel-producing enzymes can reach high turnover rates with excellent selectivity for specific transformations, they are typically sensitive to operational conditions such as pH, temperature, and the presence of O2, and are restrictively expensive to isolate for large-scale applications.[11,16−19] Solid-state material catalysts are often stable but may exhibit low product selectivity, giving mixtures of carbonbased products from CO2 reduction.[20,21] In contrast, synthetic coordination complex catalysts can achieve excellent product selectivity with acceptable long-term stability and tolerance to experimental conditions.[21−23] They are amenable to mechanistic investigation through in situ spectroscopy, and their catalytic properties can be tailored through variation of the metal, and through rational ligand design.[21,24] Immobilizing molecular catalysts on solid-state supports can provide a system displaying single-site catalysis, enhancing selectivity and simplifying identification of the active species, while facilitating product isolation and catalyst recycling.[21,24,25] use of a suitable semiconductor (SC) support can supply the energy required for substrate transformation directly through absorption of solar light.[20,25]
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