Mechanism and Selectivity of Electrochemical Reduction of CO2 on Metalloporphyrin Catalysts from DFT Studies.

Electrochemical reduction of CO2 to value-added chemicals has been hindered by poor product selectivity and competition from hydrogen evolution reactions. This study aims to unravel the origin of the product selectivity and competitive hydrogen evolution reaction on [MP]0 catalysts (M = Fe, Co, Rh and Ir; P is porphyrin ligand) by analyzing the mechanism of CO2 reduction and H2 formation based on the results of density functional theory calculations. Reduction of CO2 to CO and HCOO- proceeds via the formation of carboxylate adduct ([MP-COOH]0 and ([MP-COOH]-) and metal-hydride [MP-H]-, respectively. Competing proton reduction to gaseous hydrogen shares the [MP-H]- intermediate. Our results show that the pKa of [MP-H]0 can be used as an indicator of the CO or HCOO-/H2 preference. Furthermore, an ergoneutral pH has been determined and used to determine the minimum pH at which selective CO2 reduction to HCOO- becomes favorable over the H2 production. These analyses allow us to understand the product selectivity of CO2 reduction on [FeP]0, [CoP]0, [RhP]0 and [IrP]0; [FeP]0 and [CoP]0 are selective for CO whereas [RhP]0 and [IrP]0 are selective for HCOO- while suppressing H2 formation. These descriptors should be applicable to other catalysts in an aqueous medium.

Stable Cuprous Hydroxide Nanostructures by Organic Ligand Functionalization.

Copper compounds have been extensively investigated for diverse applications. However, studies of cuprous hydroxide (CuOH) have been scarce due to structural metastability. Herein, a facile, wet-chemistry procedure is reported for the preparation of stable CuOH nanostructures via deliberate functionalization with select organic ligands, such as acetylene and mercapto derivatives. The resulting nanostructures are found to exhibit a nanoribbon morphology consisting of small nanocrystals embedded within a largely amorphous nanosheet-like scaffold. The acetylene derivatives are found to anchor onto the CuOH forming CuC linkages, whereas CuS interfacial bonds are formed with the mercapto ligands. Effective electronic coupling occurs at the ligand-core interface in the former, in contrast to mostly non-conjugated interfacial bonds in the latter, as manifested in spectroscopic measurements and confirmed in theoretical studies based on first principles calculations. Notably, the acetylene-capped CuOH nanostructures exhibit markedly enhanced photodynamic activity in the inhibition of bacteria growth, as compared to the mercapto-capped counterparts due to a reduced material bandgap and effective photocatalytic generation of reactive oxygen species. Results from this study demonstrate that deliberate structural engineering with select organic ligands is an effective strategy in the stabilization and functionalization of CuOH nanostructures, a critical first step in exploring their diverse applications.

Ultrafast Preparation of Nonequilibrium FeNi Spinels by Magnetic Induction Heating for Unprecedented Oxygen Evolution Electrocatalysis.

Carbon-supported nanocomposites are attracting particular attention as high-performance, low-cost electrocatalysts for electrochemical water splitting. These are mostly prepared by pyrolysis and hydrothermal procedures that are time-consuming (from hours to days) and typically difficult to produce a nonequilibrium phase. Herein, for the first time ever, we exploit magnetic induction heating-quenching for ultrafast production of carbon-FeNi spinel oxide nanocomposites (within seconds), which exhibit an unprecedentedly high performance towards oxygen evolution reaction (OER), with an ultralow overpotential of only +260 mV to reach the high current density of 100 mA cm-2. Experimental and theoretical studies show that the rapid heating and quenching process (ca. 103 K s-1) impedes the Ni and Fe phase segregation and produces a Cl-rich surface, both contributing to the remarkable catalytic activity. Results from this study highlight the unique advantage of ultrafast heating/quenching in the structural engineering of functional nanocomposites to achieve high electrocatalytic performance towards important electrochemical reactions.

Electrochemical reduction of CO2 to CO and HCOO- using metal-cyclam complex catalysts: predicting selectivity and limiting potential from DFT.

Sustainable fuel production from CO2 through electrocatalytic reduction is promising but challenging due to high overpotential and poor product selectivity. Herein, we computed the reaction free energies of electrocatalytic reduction of CO2 to CO and HCOO- using the density functional theory method and screened transition metal(M)-cyclam(L) complexes as molecular catalysts for CO2 reduction. Our results showed that pKa of the proton adduct formed by the protonation of the reduced metal center can be used as a descriptor to select the operating pH of the solution to steer the reaction toward either the CO or hydride cycle. Among the complexes, [LNi]2+ and [LPd]2+ catalyze the reactions by following the CO cycle and are the CO selective catalysts in the pH ranges 1.81-7.31 and 6.10 and higher, respectively. Among the complexes that catalyze the reactions by following the hydride cycle, [LMo]2+ and [LW]3+ are HCOO- selective catalysts and have low limiting potentials of -1.33 V and -1.54 V, respectively. Other complexes, including [LRh]2+, [LIr]2+, [LW]2+, [LCo]2+, and [LTc]2+ catalyze the reactions resulting in either HCOO- from CO2 reduction or H2 from proton reduction; however, HCOO- formation is always thermodynamically more favorable. Notably, [LMo]2+, [LW]3+, [LW]2+ and [LCo]2+ have limiting potentials less negative than -1.6 V and are based on Earth-abundant elements, making them attractive for practical application.