Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.Chemistry and Chemical Biolog

Approaches for Sample Characterization and Lithography with Nanoparticles using Modes of Scanning Probe Microscopy

Measurement and imaging modes of scanning probe microscopy (SPM) have been routinely applied for characterizing systems of nanoparticles; however the evolution of fabrication methods to prepare arrangements of nanoparticles remains a challenge. Reproducible fabrication of surface structures which integrate nanoparticles within ultra-small patterns will require innovative approaches to achieve high throughput and precision. Strategies for nanoscale lithography have been introduced for preparing defined arrangements of nanoparticles on surfaces based on physical or chemical interactions. For example, physisorption was employed for attaching nanoparticles based on colloidal lithography and site-directed assembly. Microfabricated atomic force microscope (AFM) tips with capillary channels have been used to pattern nanoparticles through electrostatic interactions. Specific chemical interactions can be designed for patterning nanoparticles with dip-pen nanolithography and SPM-based fabrication. Studies with nanoparticles are reviewed, which have applied either in situ and ex situ approaches for imaging and measurements using modes of SPM. The imaging principle for contact and tapping modes are described with example studies of nanoparticle patterns. The SPM modes for measuring physical properties (e.g. magnetism, softness, conductance) using force modulation microscopy (FMM), magnetic force microscopy (MFM), magnetic sample modulation (MSM), and conductive probe AFM are described for selected studies of lithography with nanoparticles. Strategies for patterning nanoparticles using lithography modes of nanoshaving, dip-pen nanolithography, and tip-induced oxidation have been reported for a range of nanoparticle systems. Applications for nanotechnology will require the integration of nanoparticles within engineered surface architectures. Stable, organized arrangements of nanoparticles with robust chemical/physical attachment to surfaces will be needed for applications, to fully gain advantages of the characteristic quantum properties of nanoparticles

Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt–terpyridine complexes

International audienceUnderstanding the activity and selectivity of molecular catalysts for CO 2 reduction to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt–terpyridine compounds have been shown to be catalysts for CO 2 reduction to CO while simultaneously producing H 2 from the requisite proton source. To investigate the parameters governing the competition for H + reduction versus CO 2 reduction, the cobalt bisterpyridine class of compounds is first evaluated as H + reduction catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate constants for H + reduction. When this class of compounds is next submitted to CO 2 reduction conditions, a trend is found in which the less active catalysts for H + reduction are the more selective towards CO 2 reduction to CO. This represents the first report of the selectivity of a molecular system for CO 2 reduction being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave analysis and a catalytic Tafel plot is provided

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes

Hexavalent chromium is a contaminant of concern in water. Electrochemical methods are being developed to reduce toxic Cr(vi) to benign Cr(iii) at the point of generation or point of use. The effectiveness of glassy carbon electrodes to detect and reduce Cr(vi) in cyclic voltammetry was recently demonstrated. Herein, we report that the nature of the buffer system used, at a fixed pH, has unexpected impacts on the electrochemical reduction of Cr(vi) in water. At low concentrations of Cr(vi), the buffer influences the PCET step gating Cr(vi) reduction on the timescale of cyclic voltammetry experiments. At higher concentrations of Cr(vi), the effect is more complex. Data suggests impacts on both the chemical steps of Cr(vi) reduction and the nature of the products formed, hypothesized to be due to chelation effects. In particular, evidence of adsorption on the electrode surface is seen through cyclic voltammetry studies in certain buffers. Chronoamperometry studies confirm the adsorption of chromium containing species on the electrode surface during Cr(vi) electroreduction. XPS confirms Cr(iii) as the product. The activity of the electrode is regained after an acid wash step, without the need for re-polishing. This work provides a framework to understand the impact of the presence of small organic acids on Cr(vi) reduction for water purification

Fighting Deactivation: Classical and Emerging Strategies for Efficient Stabilization of Molecular Electrocatalysts

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Development of highly active molecular electrocatalysts for fuel-forming reactions has relied heavily on understanding mechanistic aspects of the electrochemical transformations. Careful fine-tuning of the ligand environment oriented mechanistic pathways towards higher activity and optimal product distribution for several catalysts. Unfortunately, many catalysts deactivate in bulk electrolysis conditions, diminishing the impact of the plethora of highly tuned molecular electrocatalytic systems. This Minireview covers classical and emerging methods developed to circumvent catalyst deactivation and degradation, with an emphasis on successes with molecular electrocatalysts

Reaction parameters influencing cobalt hydride formation kinetics: Implications for benchmarking H\u3csub\u3e2\u3c/sub\u3e-evolution catalysts

© 2016 American Chemical Society. The need for benchmarking hydrogen evolution catalysts has increasingly been recognized. The influence of acid choice on activity is often reduced to the overpotential for catalysis. Through the study of a stable cobalt hydride complex, we demonstrate the influence of acid choice, beyond pKa, on the kinetics of hydride formation. A linear free energy relationship between acid pKa, and second-order rate constants is observed for weaker acids. For stronger acids, however, further increases in pKa, do not correlate to increases in rate constants. Further, steric bulk around the acidic proton is shown to influence rate constants dramatically. Together, these observations reveal the complex factors dictating catalyst performance

Molecular polypyridine-based metal complexes as catalysts for the reduction of CO ₂

International audiencePolypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO2 reduction to date. Initial reports in the 1980s began with an emphasis on 2nd and 3rd row late transition metals, but more recently the focus has shifted towards earlier metals and base metals. Polypyridyl platforms have proven quite versatile and amenable to studying various parameters that govern product distribution for CO2 reduction. However, open questions remain regarding the key mechanistic steps that govern product selectivity and efficiency. Polypyridyl complexes have also been immobilized through a variety of methods to afford active catalytic materials for CO2 reductions. While still an emerging field, materials incorporating molecular catalysts represent a promising strategy for electrochemical and photoelectrochemical devices capable of CO2 reduction. In general, this class of compounds remains the most promising for the continued development of molecular systems for CO2 reduction and an inspiration for the design of related non-polypyridyl catalysts