Unraveling Hydrogen Atom Transfer Mechanisms with Voltammetry: Oxidative Formation and Reactivity of Cobalt Hydride.

The utility of transition metal hydride catalyzed hydrogen atom transfer (MHAT) has been widely demonstrated in organic transformations such as alkene isomerization and hydrofunctionalization reactions. However, the highly reactive nature of the hydride and radical intermediates has hindered mechanistic insight into this pivotal reaction. Recent advances in electrochemical MHAT have opened up the possibility for new analytical approaches for mechanistic diagnosis. Here, we report a voltammetric interrogation of Co-based MHAT reactivity, describing in detail the oxidative formation and reactivity of the key Co-H intermediate and its reaction with aryl alkenes. Insights from cyclic voltammetry and finite element simulations help elucidate the rate-limiting step as metal hydride formation, which we show to be widely tunable based on ligand design. Voltammetry is also suggestive of the formation of Co-alkyl intermediates and a dynamic equilibrium with the reactive neutral radical. These mechanistic studies provide information for the design of future hydrofunctionalization reactions, such as catalyst and silane choice, the relative stability of metal-alkyl species, and how hydrofunctionalization reactions utilize Co-alkyl intermediates. In summary, these studies establish an important template for studying MHAT reactions from the perspective of electrochemical kinetic frameworks.

Oxidation by Reduction: Efficient and Selective Oxidation of Alcohols by the Electrocatalytic Reduction of Peroxydisulfate.

Alcohol oxidation is an important class of reaction that is traditionally performed under harsh conditions and most often requires the use of organometallic compounds or transition metal complexes as catalysts. Here, we introduce a new electrochemical synthetic method, referred to as reductive oxidation, in which alcohol oxidation is initiated by the redox-mediated electrocatalytic reduction of peroxydisulfate to generate the highly oxidizing sulfate radical anion. Thus, and counter-intuitively, alcohol oxidation occurs as a result of an electrochemical reduction reaction. This approach provides a selective synthetic route for the oxidation of alcohols carried out under mild conditions to aldehydes, ketones, and carboxylic acids with up to 99% conversion yields. First-principles density functional theory calculations, ab initio molecular dynamics simulations, cyclic voltammetry, and finite difference simulations are presented that support and provide additional insights into the S2O82--mediated oxidation of benzyl alcohol to benzaldehyde.

Correlated Optical-Electrochemical Measurements Reveal Bidirectional Current Steps for Graphene Nanoplatelet Collisions at Ultramicroelectrodes.

Single-entity electrochemistry has emerged as a powerful tool to study the adsorption behavior of single nanoscale entities one-at-a-time on an ultramicroelectrode surface. Classical single-entity collision studies have focused on the behavior of spherical nanoparticles or entities where the orientation of the colliding entity does not impact the electrochemical response. Here, we report a detailed study of the collision of asymmetric single graphene nanoplatelets onto ultramicroelectrodes. The collision of conductive graphene nanoplatelets on biased ultramicroelectrode surfaces can be observed in an amperometric i-t trace, revealing a variety of current transients (both positive and negative steps). To elucidate the dynamics of nanoplatelet adsorption processes and probe response heterogeneity, we correlated the collision events with optical microscopy. We show that positive steps are due to nanoplatelets coming into contact with the ultramicroelectrode, making an electrical connection, and adsorbing partly on the glass surrounding the ultramicroelectrode. Negative steps occur when nanoplatelets adsorb onto the glass without an electrical connection, effectively blocking flux of ferrocenemethanol to the ultramicroelectrode surface. These measurements allow rigorous quantification of current transients and detailed insights into the adsorption dynamics of asymmetric objects at the nanoscale.

Revealing Dynamic Rotation of Single Graphene Nanoplatelets on Electrified Microinterfaces.

Nanoparticles interact with a variety of interfaces, from cell walls for medicinal applications to conductive interfaces for energy storage and conversion applications. Unfortunately, quantifying dynamic changes of nanoparticles near interfaces is difficult. While optical techniques exist to study nanoparticle dynamics, motions smaller than the diffraction limit are difficult to quantify. Single-entity electrochemistry has high sensitivity, but the technique suffers from ambiguity in the entity's size, morphology, and collision location. Here, we combine optical microscopy, single-entity electrochemistry, and numerical simulations to elucidate the dynamic motion of graphene nanoplatelets at a gold ultramicroelectrode (radius ∼5 µm). The approach of conductive graphene nanoplatelets, suspended in 10 µM NaOH, to an ultramicroelectrode surface was tracked optically during the continuous oxidation of ferrocenemethanol. Optical microscopy confirmed the nanoplatelet size, morphology, and collision location on the ultramicroelectrode. Nanoplatelets collided on the ultramicroelectrode at an angle, θ, enhancing the electroactive area, resulting in a sharp increase in current. After the collision, the nanoplatelets reoriented to lay flat on the electrode surface, which manifested as a return to the baseline current in the amperometric current-time response. Through correlated finite element simulations, we extracted single nanoplatelet angular velocities on the order of 0.5-2°/ms. These results are a necessary step forward in understanding nanoparticle dynamics at the nanoscale.

Publisher Correction: Electrosynthesis of high-entropy metallic glass nanoparticles for designer, multi-functional electrocatalysis.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Electrosynthesis of high-entropy metallic glass nanoparticles for designer, multi-functional electrocatalysis.

Creative approaches to the design of catalytic nanomaterials are necessary in achieving environmentally sustainable energy sources. Integrating dissimilar metals into a single nanoparticle (NP) offers a unique avenue for customizing catalytic activity and maximizing surface area. Alloys containing five or more equimolar components with a disordered, amorphous microstructure, referred to as High-Entropy Metallic Glasses (HEMGs), provide tunable catalytic performance based on the individual properties of incorporated metals. Here, we present a generalized strategy to electrosynthesize HEMG-NPs with up to eight equimolar components by confining multiple metal salt precursors to water nanodroplets emulsified in dichloroethane. Upon collision with an electrode, alloy NPs are electrodeposited into a disordered microstructure, where dissimilar metal atoms are proximally arranged. We also demonstrate precise control over metal stoichiometry by tuning the concentration of metal salt dissolved in the nanodroplet. The application of HEMG-NPs to energy conversion is highlighted with electrocatalytic water splitting on CoFeLaNiPt HEMG-NPs.