438 Search Results

The development of efficient CO₂ reduction technologies hinges upon a thorough understanding of the intricate interplay between solution cations and the characteristics of the electrode surface. Recently, ionic liquids (ILs) have emerged as promising electrolytes for the CO₂ reduction reaction. However, the effect of alkali cations on the electrochemical CO₂ reduction (CO₂R) reaction remains unclear in ILs. Here, in this report, we studied alkali cation effects by assessing the electrocatalytic CO₂R activity with the IL 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF₄], in water with alkali metal co-cations (i.e., Li⁺, Na⁺, and K⁺) using a polycrystalline Sn catalyst. Contrary to previous findings in purelymore » aqueous media with inorganic cations, where alkali cations strongly enhance CO₂R via pH modulation and strengthening of interfacial electric fields, alkali cations in electrolytes containing the IL [EMIM][BF4] negatively impact CO₂R activity on Sn electrodes. These results were attributed to the larger radius and higher concentration of the IL organic cation [EMIM]⁺ that mitigates the impact of alkali cations. These findings highlight the complex interplay between IL cations and alkali metals in shaping CO₂R performance.« less

Epoxides are important intermediates in synthetic chemistry. Stoichiometric peroxyacids, such as meta-chloroperoxybenzoic acid (mCPBA), are widely used to convert alkenes to epoxides but show poor compatibility with aromatic heterocycles and present hazards when scaled. Herein, we report a highly practical alkene epoxidation method that uses the commercially available iron porphyrin, Fe(TPFPP)Cl (TPFPP = tetrakis(pentafluorophenyl)porphyrin), as a catalyst (0.05 mol %) and aqueous NaOCl as the oxidant in acetonitrile as the solvent. No additional ligands or additives are needed, and the reactions proceed under ambient conditions. The method shows a broad scope, affording high yields of epoxides in reactions with terminalmore » and internal aromatic and aliphatic alkenes, heterocycle-containing substrates, glycals, and polyenes. The practicality of the method is demonstrated in the 100 g scale epoxidation of tri-O-acetyl-D-glucal, which proceeds to completion in 15 min at room temperature.« less

Nickel (Ni) foam-based electrodes are excellent catalysts for the oxygen evolution reaction; however, we found that the random pore-size distribution of Ni foams contributes to a significant variability in electrochemically active surface area, compromising experimental reproducibility. We provide insights into quantifying this critical material property, verified by four electrochemical laboratories.

Degradation via sintering is an ongoing challenge that impedes the broad commercial success of supported metallic nanoparticle catalysts. To mitigate degradation via informed catalyst design and process operations, here we aim to disambiguate the underlying mechanisms of sintering by combining theory and experiment in a quantitative framework. While mechanistic sintering models exist, they only model a single sintering pathway, even though multiple sintering mechanisms can occur simultaneously or dominate at different stages of the process. Data-driven machine learning models have emerged as a means to represent complex processes through data regression. However, machine learning models have very large data needsmore » and lack mechanistic insights due to their black-box encoding. To develop an interpretive model of catalyst degradation via sintering, we constructed a hybrid model combining mechanistic “physics-based” models and data-driven methods to obtain both reliable predictions and mechanistic insights regarding experimentally observed sintering phenomena. Focusing on nanoparticle sintering in the Rh–TiO₂ catalyst for the reverse water–gas shift (RWGS) reaction, the hybrid model couples a mechanistic term for Ostwald ripening with energy values calculated via density functional theory (DFT) with a parametric, data-driven discrepancy function term for unmodeled mechanisms. The hybrid model is trained using Bayesian inference with data collected from small-angle X-ray scattering (SAXS) in situ experiments wherein average nanoparticle diameter versus time was measured at three relevant operating temperatures. The calibrated hybrid model results show that an Ostwald ripening-only model parameterized with fixed DFT energies does not fully capture the time and temperature dependence of the SAXS-observed sintering kinetics, and that an additional functional contribution, or DFT energy calibration, is required to reconcile simulation and experiment. Analysis of the hybrid-model error confirms that the hybrid model outperforms both the purely mechanistic and purely data-driven alternatives in terms of expected predictive accuracy for time-evolving average particle sizes. Furthermore, the results support the hypothesis that the Ostwald ripening mechanism is less important for explaining the sintering phenomena as operating temperature increases under an assumed fixed DFT parameterization. This could be explained in one of two ways: either latent, unmodeled sintering mechanisms dominate at higher temperatures, or the DFT uncertainty increases with temperature. The proposed modeling approach directly links theory to experiments and simulations via a statistical hybrid modeling framework and can be extended to other catalytic systems to improve predictive models and mechanistic understanding.« less

As anion exchange membrane systems have emerged as a competitive low temperature electrolysis technology, research has expanded to other components and device integration. In this study, nickel (Ni) and stainless steel (SS)-based porous transport layers (PTLs) are investigated in membrane electrode assemblies (MEAs). Compared to MEAs using Ni, the SS PTL shows higher performance due to less kinetics and residual loss and possibly due to a combination of iron mobility improving oxygen evolution reactivity and electron conduction pathways, as well as higher porosity increasing site access. Voltage decay rates of approximately 144 and 115 μV/h, respectively, for the Ni andmore » SS PTLs are found, although the long-term durability and lifetime implications are convoluted. Voltage breakdown analysis confirms that both PTLs saw significant increases in residual loss possibly due to catalyst/PTL property changes that affected electronic, ionic, and mass transport pathways. For the Ni PTL, a higher proportion of the losses were due to cell kinetics; comparatively, more of the SS PTL losses were due to increases in the high frequency resistance. The experimental findings presented here provide insights on the impact of the PTL materials and their properties.« less

Pt-based electrocatalysts are effective for the hydrogen evolution reaction (HER); however, their limited ability to facilitate water dissociation and suboptimal hydrogen binding energy (H_BE) in alkaline electrolytes result in slow reaction kinetics, which hinders their cost-efficiency and practical applications. This study reports the synthesis of Co₂P-Pt heterostructure nanorods using a seed-mediated growth method, producing a high density of Co₂P-Pt interfacial sites. Density functional theory (DFT) calculations indicate that electronic interactions at these interfaces optimize H_BE on Pt, while the interfacial sites promote water dissociation. The Co₂P-Pt nanorods demonstrate an overpotential of 14 mV at 10 mA cm⁻² for the HER,more » highlighting the potential of precisely engineered metal-metal phosphide interfaces for enhancing electrocatalytic efficiency.« less

The conversion of C–H bonds into amines by nitrene insertion is an attractive transformation since it is both atom- and step-economical, and provides a direct route to functionalizing hydrocarbons. Using an iron catalyst [{(^tBupyrr)₂pyr}Fe(OEt₂)] (1-OEt₂) ((^tBupyrr)₂pyr^2– = 3,5-^tBu₂-bis(pyrrolyl)pyridine), we recently demonstrated the catalytic conversion of weak C–H bonds into secondary amines using aryl azides as the nitrene source [Zars, E.; Angew. Chem., Int. Ed. 2023, 62, e202311749]. Here, we describe detailed mechanistic studies of this intermolecular C–H amination reaction under catalytic conditions. We find by Variable Time Normalization Analysis (VTNA) that the conversion of xanthene (2-H₂) and 2,4,6-trimethyl-phenyl azide (^Me3)more » catalyzed by 1-OEt₂ is an overall 3/2 order process, being 1^st order in 2-H₂ and half order in ^Me3. A kinetic isotope effect study (KIE) using 2-d₂ results in a significant decrease in the rate (KIE = 61(15)), which clearly implicates the C–H insertion step as rate-determining. Furthermore, treatment of 1-OEt₂ with one equivalent of N₃-2,6-ⁱPr₂–C₆H₃ yields the mixed-valence C–N coupled product [(^tBupyrr)₂pyrFe-N═C(2,6ⁱPr₂–Ph)═N-(2,6ⁱPr₂–Ph))Fe^tBupyrrpyr(2-H-pyrr)] (5^iPr). Quantum chemical calculations confirm the electronic structure of the mixed-valence dimer in 5^iPr and rationalize the Hammett correlation by a delicate balance in the dinuclearization of the catalytically active monomers. Calculations further indicate significant tunneling for the pivotal H atom abstraction by the iron-imidyl complex. Combining all these results allows us to propose a mechanism consisting of imido formation in equilibrium with a radical-coupled diiron system, followed by stepwise C–H insertion via a linear H atom abstraction transition state and subsequent radical rebound.« less

Selective ring-opening polymerization (ROP) of chiral lactones enables access to biodegradable polyesters with precisely controlled microstructures. Here, combined DFT modeling and experimental validation elucidate how fine-tuning of enantiopure SalBinam aluminum catalysts (through introduction of bromine atoms and tert-butyl groups, respectively, in ortho- and para-positions) modulates regio- and stereocontrol in the ROP of methyl glycolide (MeG) and lactide (LA). Computations reveal that regioselectivity in MeG polymerization arises mainly from steric repulsion, with a small contribution from weak stabilizing C-H···Br interactions that favor ring-opening at the glycolic site, consistent with the experimentally enhanced regioselectivity for (R)-MeG. In contrast, the same steric congestionmore » at the ligand’s ortho positions destabilizes the key transition states in rac-LA polymerization, reducing the calculated stereoselectivity. Experiments confirm the predicted loss of stereocontrol, yielding nearly atactic PLA under standard conditions. Extension of the computational framework to rac-MeG polymerization promoted by racemic catalyst identified a low-barrier, stepwise polymer chain exchange pathway that rationalizes the experimentally observed syndiotacticity of poly(lactic-co-glycolic acid).« less

The development of molecularly modified metal− organic frameworks (MOFs) for electrochemical water oxidation has emerged as a promising strategy for efficient artificial photosynthesis. In this study, a ruthenium-based water oxidation catalyst (WOC) was incorporated into the UiO-67 framework, forming RuM−UiO-67 films grown directly on conductive FTO substrates. These modified films demonstrate efficient water oxidation activity at near-neutral pH (pH 6), operating at a low overpotential of ∼600 mV. The catalyst exhibits a turnover frequency (TOF) of (0.32 ± 0.02) s⁻¹ at 1.5 V versus the normal hydrogen electrode (NHE) in buffered solution (pH 6) for the oxygen evolution reaction. Notably,more » incorporation into the MOF results in a 12-fold increase in electroactive surface coverage compared to a monolayer of the same catalyst on bare FTO. Faradaic efficiency analysis revealed incomplete conversion to O₂, and follow-up iodometric analysis confirmed the formation of H₂O₂ as a competing two-electron oxidation product during electrocatalysis. These results highlight the utility of MOF-based architectures for maximizing the catalyst accessibility and stability under electrochemical water oxidation conditions.« less

The electrochemical reduction of CO₂ (CO₂RR) is a potentially scalable approach for converting captured carbon dioxide into value-added products. Conventional gas-phase electrolysis systems can suffer from carbonate crossover, which limits the efficiency of the system. Liquid-phase (bi)carbonate electrolysis using bipolar membrane electrode assemblies (BPMMEA) has emerged as a promising alternative. The interposer layer, a porous mass-transport material between the BPM and the catalyst, is an essential component of the MEA, as it allows evolved CO₂ to reach the catalyst surface for reaction. In the absence of this layer, evolved CO₂ generated by the pH swing process at the BPM canmore » be converted back into (bi)carbonate (CO₂ recapture) due to the high bulk pH. Thus, clear design guidelines are needed to maximize CO₂ conversion, minimize CO₂ recapture in the catholyte, and improve energy efficiency. Here, the transport properties of the interposer are systematically characterized by X-ray tomography and symmetric-cell impedance spectroscopy to quantify porosity, tortuosity, and the resulting MacMullin number. We then examine the correlation between these material properties and the electrolyzer performance. We focus on characterizing two commercial porous membrane filters, mixed cellulose ester (MCE) and poly(ether sulfone) (PES).« less