Eva M. Nichols

Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water

Improving cobalt catalysts Tethering molecular catalysts together is a tried and trusted method for making them easier to purify and reuse. Lin et al. now show that the assembly of a covalent organic framework (COF) structure can also improve fundamental catalytic performance. They used cobalt porphyrin complexes as building blocks for a COF. The resulting material showed greatly enhanced activity for the aqueous electrochemical reduction of CO2 to CO. Science, this issue p. 1208 A covalent lattice enhances the activity of a catalyst for electrochemical conversion of carbon dioxideto carbon monoxide. 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.

Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals

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.

A Molecular Surface Functionalization Approach to Tuning Nanoparticle Electrocatalysts for Carbon Dioxide Reduction

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.

Hybrid bioinorganic approach to solar-to-chemical conversion

Significance Natural photosynthesis, a process of solar-to-chemical conversion, uses light, water, and carbon dioxide to generate the chemical products needed to sustain life. Here we report a strategy inspired by photosynthesis in which compatible inorganic and biological components are used to transform light, water, and carbon dioxide to the value-added product methane. Specifically, this solar-to-chemical conversion platform interfaces photoactive inorganic materials that produce hydrogen from water and sunlight with microorganisms that consume this sustainably derived hydrogen to drive the transformation of carbon dioxide to methane with high efficiency. This system establishes a starting point for a broader materials biology approach to the synthesis of more complex chemical products from carbon dioxide and water. 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 and Methanosarcina barkeri as 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.

Reticular Electronic Tuning of Porphyrin Active Sites in Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction

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.

Supramolecular Porphyrin Cages Assembled at Molecular-Materials Interfaces for Electrocatalytic CO Reduction

Conversion of carbon monoxide (CO), a major one-carbon product of carbon dioxide (CO2) reduction, into value-added multicarbon species is a challenge to addressing global energy demands and climate change. Here we report a modular synthetic approach for aqueous electrochemical CO reduction to carbon–carbon coupled products via self-assembly of supramolecular cages at molecular–materials interfaces. Heterobimetallic cavities formed by face-to-face coordination of thiol-terminated metalloporphyrins to copper electrodes through varying organic struts convert CO to C2 products with high faradaic efficiency (FE = 83% total with 57% to ethanol) and current density (1.34 mA/cm2) at a potential of −0.40 V vs RHE. The cage-functionalized electrodes offer an order of magnitude improvement in both selectivity and activity for electrocatalytic carbon fixation compared to parent copper surfaces or copper functionalized with porphyrins in an edge-on orientation.

Chelating N-Heterocyclic Carbene Ligands Enable Tuning of Electrocatalytic CO2 Reduction to Formate and Carbon Monoxide: Surface Organometallic Chemistry

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.