The electronic character of porphyrin active sites for electrocatalytic reduction of CO2 to CO in a two-dimensional covalent organic framework (COF) was tuned by modification of the reticular ...structure. Efficient charge transport along the COF backbone promotes electronic connectivity between remote functional groups and the active sites and enables the modulation of the catalytic properties of the system. A series of oriented thin films of these COFs was found to reduce CO2 to CO at low overpotential (550 mV) with high selectivity (faradaic efficiency of 87%) and at high current densities (65 mA/mg), a performance well beyond related molecular catalysts in regard to selectivity and efficiency. The catalysts are stable for more than 12 h without any loss in reactivity. X-ray absorption measurements on the cobalt L-edge for the modified COFs enable correlations between the inductive effects of the appended functionality and the electronic character of the reticulated molecular active sites.
Conspectus Increasing demand for sustainable energy sources continues to motivate the development of new catalytic processes that store intermittent energy in the form of chemical bonds. In this ...context, photosynthetic organisms harvest light to drive dark reactions reducing carbon dioxide, an abundant and accessible carbon source, to store solar energy in the form of glucose and other biomass feedstocks. Inspired by this biological process, the field of artificial photosynthesis aims to store renewable energy in chemical bonds spanning fuels, foods, medicines, and materials using light, water, and CO2 as the primary chemical feedstocks, with the added benefit of mitigating the accumulation of CO2 as a greenhouse gas in the atmosphere. As such, devising new catalyst platforms for transforming CO2 into value-added chemical products is of importance. Historically, catalyst design for artificial photosynthesis has been approached from the three traditional fields of catalysis: molecular, materials, and biological. In this Account, we show progress from our laboratory in constructing new hybrid catalysts for artificial photosynthesis that draw upon design concepts from all three of these traditional fields of catalysis and blur the boundaries between them. Starting with molecular catalysis, we incorporated biological design elements that are prevalent in enzymes into synthetic systems. Specifically, we demonstrated that proper positioning of intramolecular hydrogen bond donors or addition of intermolecular multipoint hydrogen bond donors with classic iron porphyrin and nickel cyclam platforms can substantially increase rates of CO2 reduction and break electronic scaling relationships. In parallel, we incorporated a key materials design element, namely, high surface area and porosity for maximizing active site exposure, into molecular systems. A supramolecular porous organic cage molecule was synthesized with iron porphyrin building blocks, and the porosity was observed to facilitate substrate and charge transport through the catalyst film. In turn, molecular design elements can be incorporated into materials catalysts for CO2 reduction. First, we utilized molecular synthons in a bottom-up reticular approach to drive polymerization/assembly into a bulk framework material. Second, we established an organometallic approach in which molecular ligands, including chelating ones, are adsorbed onto a bulk inorganic solid to create and tune new active sites on surfaces. Finally, we describe two examples in which molecular, materials, and biological design elements are all integrated to catalyze the reduction of CO2 into CH4 using a hybrid biological-materials interface with sustainably generated H2 as the reductant or to reduce CO into value-added C2 products acetate and ethanol using a hybrid molecular-materials interface to construct a biomimetic, bimetallic active site. Taken together, our program in catalysis for energy and sustainability has revealed that combining more conventional design strategies in synergistic ways can lead to advances in artificial photosynthesis.
Conversion of the greenhouse gas carbon dioxide (CO2) to value-added products is an important challenge for sustainable energy research, and nanomaterials offer a broad class of heterogeneous ...catalysts for such transformations. Here we report a molecular surface functionalization approach to tuning gold nanoparticle (Au NP) electrocatalysts for reduction of CO2 to CO. The N-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved faradaic efficiency (FE = 83%) for reduction of CO2 to CO in water at neutral pH at an overpotential of 0.46 V with a 7.6-fold increase in current density compared to that of the parent Au NP (FE = 53%). Tafel plots of the NHC carbene-functionalized Au NP (72 mV/decade) vs parent Au NP (138 mV/decade) systems further show that the molecular ligand influences mechanistic pathways for CO2 reduction. The results establish molecular surface functionalization as a complementary approach to size, shape, composition, and defect control for nanoparticle catalyst design.
The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy ...input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO2 reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO2 reduction.
Direct solar-powered production of value-added chemicals from CO2 and H2O, a process that mimics natural photosynthesis, is of fundamental and practical interest. In natural photosynthesis, CO2 is ...first reduced to common biochemical building blocks using solar energy, which are subsequently used for the synthesis of the complex mixture of molecular products that form biomass. Here we report an artificial photosynthetic scheme that functions via a similar two-step process by developing a biocompatible light-capturing nanowire array that enables a direct interface with microbial systems. As a proof of principle, we demonstrate that a hybrid semiconductor nanowire–bacteria system can reduce CO2 at neutral pH to a wide array of chemical targets, such as fuels, polymers, and complex pharmaceutical precursors, using only solar energy input. The high-surface-area silicon nanowire array harvests light energy to provide reducing equivalents to the anaerobic bacterium, Sporomusa ovata, for the photoelectrochemical production of acetic acid under aerobic conditions (21% O2) with low overpotential (η < 200 mV), high Faradaic efficiency (up to 90%), and long-term stability (up to 200 h). The resulting acetate (∼6 g/L) can be activated to acetyl coenzyme A (acetyl-CoA) by genetically engineered Escherichia coli and used as a building block for a variety of value-added chemicals, such as n-butanol, polyhydroxybutyrate (PHB) polymer, and three different isoprenoid natural products. As such, interfacing biocompatible solid-state nanodevices with living systems provides a starting point for developing a programmable system of chemical synthesis entirely powered by sunlight.
Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent ...organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour(-1)) at pH 7 with an overpotential of -0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.
Natural photosynthesis harnesses solar energy to convert CO₂ and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion ...in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO₂ to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts andMethanosarcina barkerias a biocatalyst for CO₂ fixation, we demonstrate robust and efficient electrochemical CO₂ to CH₄ conversion at up to 86% overall Faradaic efficiency for ≥7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO₂, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.
In molecular catalysts, protic functional groups in the secondary coordination sphere (SCS) work in conjunction with an exogenous acid to relay protons to the active site of electrochemical CO2 ...reduction; however, it is not well understood how the acidity of the SCS and exogenous acid together determine the kinetics of catalytic turnover. To evaluate the relative contributions of proton transfer driving forces, we synthesized a series of modular iron tetraphenylporphyrin electrocatalysts bearing SCS amides of tunable pK a (17.6 to 20.0 in dimethyl sulfoxide (DMSO)) and employed phenols of variable acidity (15.3 to 19.1) as exogenous acids. This system allowed us to (1) evaluate contributions from proton transfer driving forces associated with either the SCS or exogenous acid and (2) obtain mechanistic insights into CO2 reduction as a function of pK a. A series of linear free-energy relationships show that kinetics become increasingly sensitive to variations in SCS pK a when more acidic exogenous acids are used (0.82 ≥ Brønsted α ≥ 0.13), as well as to variations in exogenous acid pK a when SCS acidity is increased (0.62 ≥ Brønsted α ≥ 0.32). An Eyring analysis suggests that the rate-determining transition state becomes more ordered with decreasing SCS acidity, which is consistent with the proposal that SCS acidity modulates charge accumulation and solvation at the rate-limiting transition state. Together, these insights enable the optimization of activation barriers as a function of both SCS and exogenous acid pK a and can further guide the rational design of electrocatalytic systems wherein contributions from all participants in a proton relay are considered.
Reported here is the chelate effect as a design principle for tuning heterogeneous catalysts for electrochemical CO2 reduction. Palladium functionalized with a chelating tris‐N‐heterocyclic carbene ...(NHC) ligand (Pd‐timtmbMe) exhibits a 32‐fold increase in activity for electrochemical reduction of CO2 to C1 products with high Faradaic efficiency (FEC1=86 %) compared to the parent unfunctionalized Pd foil (FE=23 %), and with sustained activity relative to a monodentate NHC‐ligated Pd electrode (Pd‐mimtmbMe). The results highlight the contributions of the chelate effect for tailoring and maintaining reactivity at molecular‐materials interfaces enabled by surface organometallic chemistry.
On the surface: Reported is the chelate effect as a design principle for tuning heterogeneous catalysts for electrochemical CO2 reduction. Palladium functionalized with a chelating tris‐N‐heterocyclic carbene ligand exhibits a 32‐fold increase in activity for electrochemical reduction of CO2 to C1 products with high Faradaic efficiency (FEC1=86 %) compared to that of the unfunctionalized Pd foil (FE=23 %).
Understanding how applied potentials and electrolyte solution conditions affect interfacial proton (charge) transfers at electrode surfaces is critical for electrochemical technologies. Herein, we ...examine mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on gold using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). Measurements as a function of the applied potential, the electrolyte pD, and the electrolyte concentration determined both the relative surface populations of acidic and basic forms of 4-MBA, as well as the local electric fields at the SAM–solution interface by following the Stark shifts of 4-MBN. The effective acidity of the SAM varied with the applied potential, requiring a 600 mV change to move the pK a by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pK a, ∼90% of the applied potential dropped across the SAM layer. This emphasizes the importance of distinguishing applied potentials from the potential experienced at the interface. We use the measured interfacial electric fields to estimate the experienced potential at the SAM edge. The SAM pK a showed a roughly Nernstian dependence on this estimated experienced potential. An analysis of the combined acid–base equilibria and Stark shifts reveals that the interfacial charge density has significant contributions from both SAM carboxylate headgroups and electrolyte components. Ion pairing and ion penetration into the SAM also influence the observed surface acidity. To our knowledge, this study is the first concurrent examination of both effective acidity and electric fields, and highlights the relevance of experienced potentials and specific ion effects at functionalized electrode surfaces.
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