46 Search Results

Molecularly modified electrodes (MMEs) are potent electrocatalysts, but few principles exist for their rational design. Electrocatalysis by soluble molecules depends strongly on substituents that tune the catalyst redox potential (E_1/2), but it is unclear if this parameter similarly impacts MME catalysis. Herein, we employ the hydrogen evolution reaction (HER) as a test case for comparing carbon-adsorbed cobalt phthalocyanine (CoPc/C) and cobalt hexadecafluoro-phthalocyanine (CoFPc/C). By correlating HER activity and voltammetric data to total Co surface concentration across a wide range of catalyst loadings, we find that only 5–25% of adsorbed Co sites contribute to the Co(II/I) redox wave and that thismore » subpopulation poorly correlates with catalytic activity. Instead, in the low-loading limit, catalytic activity correlates linearly with the majority Co(II/I)-silent Co population, revealing per-site turnover frequency (TOF) values for HER. Despite a 230 mV difference in Co(II/I) redox potentials, CoPc/C and CoFPc/C display TOF values differing by less than a factor of 3 when compared over a wide potential range. Mechanistic studies point to an inner-sphere concerted proton–electron transfer step as rate-determining, suggesting that the Co–H bond dissociation free energy (BDFE) rather than the Co(II/I) E_1/2 is thermodynamically relevant. Computational studies indicate that the fluoro-substituents lead to compensatory changes in Co(II/I) E_1/2 and Co(I) basicity, leaving the Co–H BDFE largely unchanged between CoPc and CoFPc and thereby manifesting in similar catalytic rates. Furthermore, these results highlight the limited effect of E_1/2-tuning on MME catalytic activity and motivate the development of methods to directly alter active site–substrate BDFE.« less

Heterogeneous microstructures in polycrystalline metals can enhance the strength and ductility, outperforming homogeneous structures of similar composition. This study investigates deformed cobalt via friction stir spot processing with varying dwell times to uncover the effects of plastic deformation and heat generation on the formation of morphological, phase, and grain boundary character gradients. A new approach to quantify the morphological gradients in materials, which describes grain morphology in terms of density followed by parametric regression, enables direct quantification of processing depth and gradient sharpness. Results show that longer processing times increase the steepness of morphological gradients and reduce the deformation depthmore » for friction stir spot processing with low plunge depths and high tool rotational speeds. The amount of retained FCC is increased in the shorter processing conditions, primarily due to refined grain size, increased defect content, and reduced heat generation. Crystallographic texture analysis of the HCP phase indicated a dominant B-fiber described by (0001) ∥ shear plane normal in the extreme processing conditions and the formation of a P-fiber, shear direction ∥ ⟨11$$\bar2$$0⟩ for intermediate dwell times. The texture of the FCC phase for low processing times was a C texture {100}⟨011⟩ where longer processing times were dominated by a [001] fiber texture with a main {110}⟨100⟩ orientation and emergence of a slight [111] fiber in the longest processing condition. The approaches outlined in this work give insight into quantifying gradients and improve the understanding of highly deformed cobalt.« less

Dispersed metal species in siliceous zeolites have been actively studied for non-oxidative dehydrogenation of ethane (NDE). Fundamental insights into the dynamics of metal species in zeolites under reaction conditions have rarely been explored. Herein, we report an atomic level understanding of the dynamics and activity of cobalt (Co) sites in dealuminated Beta zeolite (DeAl-BEA) for NDE during induction and reaction conditions with extensive characterization techniques such as diffuse reflectance UV–vis, solid state nuclear magnetic resonance and X-ray photoelectron, X-ray diffraction along with in situ Fourier transform infrared and X-ray absorption spectroscopy. For a catalyst with 0.5 mass % Co loading,more » tetrahedral Co²⁺ mononuclear sites, di-coordinated to the zeolite framework and with two silanol groups in vicinity (i.e., (≡SiO)₂Co(HO–Si≡)₂), form upon exposure to hydrogen during induction and persist through the NDE reaction. Increasing the Co loading to 3.0 mass % yielded Co sites with similar electronic and coordination structures but slightly elongated Co–O bonds. Upon cooling to room temperature, the Co sites persisted in the same coordination environment, though the disappearance of a feature in the Co K-edge near-edge region revealed changes in the active site’s electronic structure coinciding with modest shifts in bond lengths. The electronic structure and activity of (≡SiO)₂Co(HO–Si≡)₂ sites were studied comparatively to a few other hypothetical Co²⁺ coordination structures, using electronic structure calculations and microkinetic simulations. The simulations showed that NDE is controlled by β-hydride elimination following C–H bond activation and that Co-sites possessing flexibility because of neighboring silanol defects are more active. Interestingly, dinuclear Co–O–Co sites (i.e., (≡SiO)Co(HO–Si≡)₂–O–(HO–Si≡)₂Co(≡SiO)) were more active than the mononuclear (≡SiO)₂Co(HO–Si≡)₂ sites because of favorable hydrogen bonding with the vicinal silanol groups. In conclusion, the present study bridges the gap between the knowledge acquired by ex-situ characterizations and the active sites under the reaction conditions in alkane dehydrogenation chemistry.« less

Here, we report the role of pendant amine basicity on the proton-coupled electron transfer (PCET) reactivity for the conversion of [Co^IIICp(P^Ph₂N^R₂)(CH₃CN)]²⁺ complexes to [HCo^IIICp(P^Ph₂N^R₂)]⁺, which is a key transformation involved in catalytic CO₂ conversion to formate and in H₂ evolution. Three complexes were studied, where the amine substituent (R) varies from benzyl, methoxyphenyl, or phenyl. In previous work on the benzyl system, we showed that the amine on the P^Ph₂N^Bn₂ ligand serves as a kinetically accessible protonation site and enables three participating hydride formation mechanisms. In this work, a combination of electrochemical measurements and theoretical calculations were used to showmore » that the electronic donation at the pendant amine influences the accessible PCET mechanism and proton transfer kinetics related to cobalt hydride formation under analogous reaction conditions. Notably, the amine with the most electron-donating substituent correlates to the lowest barrier for amine protonation, and specific cobalt hydride formation mechanisms can be shut off for the amine with the least electron-donating substituent. The mechanistic and kinetic changes upon modulation of the amine substituent have great implications for overall catalytic efficiency and selectivity, especially to generate the cobalt hydride intermediate involved in selective CO₂ reduction to formate. This work shows how to exploit kinetic basicity using ligand-cooperative design to facilitate PCET reactions involved in energy related transformations.« less

Graphitic carbon nitride (C₃N₄) has been widely explored as a photoactive support for single-atom catalysts (SACs). In this paper, three different π-rich aromatic dianhydrides are employed to introduce molecular quantized trap states into the conduction band of C₃N₄ to facilitate efficient charge-separation and to promote an enhanced photocatalytic CO₂ reduction response. These modified C₃N₄ materials have been characterized structurally and spectroscopically for use as the support for Co-based SACs. In photocatalytic CO₂ reduction studies, the Co SAC on naphthalene dianhydride doped C₃N₄ exhibited the highest activity and selectivity towards CO production among the Co SACs tested in this work. Themore » observed improvement in photocatalytic CO₂-to-CO conversion activity correlates with trends in photoinduced charge separation, as revealed by photoluminescence spectroscopy.« less

Supported cobalt phthalocyanines (CoPc) are promising catalysts for CO₂ reduction, a critical process for mitigating greenhouse gas emissions. Enhancing the catalytic performance of CoPc involves modifying the interaction between the cobalt center and intermediate species. This study focuses on the effects of tert-butyl substitution on CoPc using (tert-butyl)₄CoPc, where the substitution can both directly alter the molecule’s intramolecular electronic structure and indirectly alter it by the bulky group weakening the interaction with the support. Toward this end, we investigated the structural and chemical properties of (tert-butyl)₄CoPc on a Ag(111) surface at the single-molecule level using three-dimensional atomic force microscopy (AFM)more » with a CO-terminated tip and discussed them in comparison with data for unmodified CoPc and amino-substituted CoPc. Notably, distance-dependent force measurements revealed anomalies in the tert-butyl groups’ force curves, attributed to their rotational flexibility. The tert-butyl (t-butyl) groups were also observed to increase the attraction of the central Co atom to CO, but this effect was attributed largely to enhanced interactions of the back of the tip with the peripheral t-butyl groups. In conclusion, while this longer-range interaction would not be expected to impact the interaction of small molecules with the catalytic center, the results reveal the ability of AFM to characterize longer range environmental interactions that can enhance adsorption and subsequent reactions of larger molecules, as well as the role side chains that offer configurational adaptability may play in these interactions.« less

Cobalt, a critical metal, is anticipated to increase in market demand in the next couple of decades, particularly as a battery material used in electric vehicle application. To boost the domestic production of cobalt in the United States, an electrochemical process has been developed to recover cobalt from a cobaltite-rich concentrate and produce cobalt- and arsenic-rich leachate. The leaching efficiency of cobalt was optimized with a response surface methodology by modifying the electrochemical parameters. A series of experiments based on the Box-Behnken design of experiments were carried out using ferric iron as an electrochemically generated oxidant to leach metals frommore » the concentrate. Operating parameters, such as electrochemical cell current, iron/arsenic molar ratio, and anolyte acidity, were optimized for maximum cobalt recovery. A predicted 73% cobalt extraction efficiency can be achieved with the electrochemically assisted leaching method within 24 h. Compared to other leaching methods, such as bioleaching, electrochemically assisted leaching shows a promising alternative for extracting metals from mining concentrates, showing higher efficiency in less time and under mild conditions.« less

Molten metal chloride salts are promising candidates for advanced heat transfer fluids in generation IV nuclear reactors and beyond. However, research on the magnetic properties of suspended metallic nanoparticles at elevated temperatures is scarce, and there have been no attempts to develop a high-temperature, molten salt based ferrofluid. Such a ferrofluid would enable in situ manipulation of both the physical and thermal properties of the fluid via an external magnetic field. This study examines molten salt based ferrofluids comprised of metallic cobalt nanoparticles dispersed in molten metal chloride salts. High-temperature magnetometry and magneto-thermogalvometric measurements were conducted to evaluate the magneticmore » characteristics of the ferrofluid. Further analysis was conducted through a combination of microscopy, spectroscopy, solubility tests, and diffraction measurements. Furthermore, the metallic cobalt nanoparticles exhibited a notable degree of chemical stability and sustained magnetism from 25 to 800 °C when suspended in NaCl–MgCl₂ and KCl–ZnCl₂ mixtures.« less

We report the synthesis and characterization of a series of high- and low-spin dicobalt complexes of the ^tBuPNNP expanded pincer ligand. Reacting this dinucleating ligand in its neutral form with two equiv of CoCl₂(tetrahydrofuran)_1.5 yields a high-spin dicobalt complex featuring one Co inside and one Co outside of the dinucleating pocket. Performing the same reaction in the presence of two equivalents of KOtBu provides access to a high-spin dicobalt complex wherein both Co centers are bound within the PNNP pocket, and this complex also features a bridging OtBu ligand. Reacting either of the high-spin complexes with excess diethyl silane affordsmore » a low-spin dicobalt complex containing two unusual bridging Si-based ligands. These complexes were investigated using NMR spectroscopy, XAS, single crystal X-ray structure determination, and computational methods, showing that the Si-based ligands are best described as base-stabilized silylenes.« less

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [Co^IIICp(P^Ph₂N^Bn₂)(CH₃CN)]²⁺ to [HCo^IIICp(P^Ph₂NBn₂)]⁺. This complex catalytically converts CO₂ to formate under CO₂ reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the P^Ph₂N^Bn₂ ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the P^Ph₂N^Bn₂ ligand serves as a kinetically accessible protonation site en route to themore » thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pK_a values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. Furthermore, this work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.« less