18 Search Results

AlMe3-treatment of the borylation precatalyst La(BH₄)₂(THF)_2.5^-Ph₃Si-HY₃₀ affords La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀, which is the superior lanthanum-based precatalyst for benzene borylation with pinacolborane (HBpin), giving higher turnovers (>285) and improved yields (up to 42%) of phenylpinacolborane (PhBpin). Solid-state NMR spectroscopy, X-ray adsorption spectroscopy, and theoretical studies characterized the precatalytic sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ as κ²-O,O-{≡SiO(=Al)≡SiO}La(BH₄)₂(AlMe₃), revealing that AlMe₃ had displaced the THF ligands. In contrast to the expected high reactivity of THF-free organolanthanum, the turnover frequency (TOF) for PhBpin formation catalyzed by La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ (2.7 h^–1) is slightly lower than that of untreated La(BH₄)₂(THF)_2.5^-Ph₃Si-HY₃₀ (3.6 h^–1), implying that AlMe₃ is a stronger inhibitor than THF formore » lanthanum. On the other hand, AlMe₃-treatment of inactive La(BH₄)₂(THF)_2.2^-SiO₂ generates an active benzene borylation catalyst. AlMe₃ also desorbs surface–O-B_xH_y species and quenches residual Brønsted acid sites (BAS) and silanols. Alumination of the BAS inhibits HBpin degradation, while alumination of silanols creates sites for that reaction. The 8-fold inhibition of the BAS-catalyzed HBpin decomposition rate by AlMe₃ treatment gives a kinetic advantage to the lanthanum-catalyzed C–H borylation, leading to increased yields and turnovers. Knowledge of the competing roles of sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ and the catalytic rate law law enables identification of favorable conditions of low [HBpin] to maximize turnovers or PhBpin yield. Sterics affect the selectivity in borylation of substituted arenes and heteroarenes, which can proceed without the precoordination of a donor. These steric effects, as well as the AlMe₃ treatment having an opposite effect on the activity of lanthanum in HY vs SiO₂, point to confinement-activated sites.« less

Designing Li-ion battery cathodes free from critical raw materials such as Co and Ni has a huge technological and societal impact. Though anion redox-based Li-rich oxide cathodes allow designing Co and Ni free cathode compositions, the Li-rich oxides witnessed voltage fade, voltage hysteresis, and irreversible oxygen release despite their high capacity. Conversely, anion redox through highly covalent chalcogenides (S/Se) is emerging due to the improved covalency between metal d and ligand p bands. Here, we investigate the tuning of multi-chalcogen (S/Se) p-band and redox-active metal d-band in a model Li-rich chalcogen composition Li_1.13Ti_0.57Fe_0.3S_2-ySe_y (y = 0 - 1) through in-depthmore » electrochemical, X-ray spectroscopy, and DFT-based electronic structure investigations. Introducing the appropriate amount of Se p band character in anion redox sulfides increases interlayer distance and metal - ligand covalency without modifying the original crystal structure, promoting significant electrochemical reversibility through mixed anionic (Se^2-/Se^n-, S^2-/S^n-, wherein n<2) and cationic (Fe²⁺/Fe³⁺) redox reactions. Here we show the detailed Fe, S, and Se redox contributions during Li insertion/extraction through X-ray Absorption (XAS) and Hard X-ray Photoemission Spectroscopy (HAXPES) measurements. The orbital tuning approach improves rate capability for more than 10 C charge-discharge rate, exhibiting more than 50% of its original capacity obtained at C/20 rate. The buffer cation in the lattice (Ti⁴⁺) remains electrochemically inactive even after significant Se p-band introduction in the sulfide framework. Overall, this work takes advantage of multi-anion redox chemistry to uncover practically demanding fast charging-discharging characteristics in intercalation cathodes. The obtained knowledge of this design can be extended to other oxide and chalcogen cathodes for high performance Li-ion batteries.« less

The complicated dynamics of active sites on single-atom catalysts under reducing conditions limits their applications in hydrogenation reactions and mechanistic understanding. Herein, we report that on Rh₁/TiO₂, *H spillover during room-temperature ethylene hydrogenation hydroxylates and reduces TiO₂, enhancing the intrinsic activity of Rh₁ by 9-fold. Spectroscopic and kinetic evidence suggests that the spillover of *H is suppressed by their facile reaction with C₂H₄, most of the spilled *H are nonreactive spectators, and >99% turnovers occur on a small subset (<20%) of exposed “active Rh₁”. Steady-state kinetics indicates competitive adsorption between H and C₂H₄, H₂ dissociation is the rate-determining step, andmore » the apparent activation barrier (E_a,app) of the reaction is ~48 kJ/mol. The evolution of Rh₁ under H₂ was further tracked by spectroscopic and microscopic techniques at elevated temperatures. At 200 °C, more Rh₁ are exposed, but these Rh₁ are at least 5-fold less active than that of the “active Rh₁”. At 300 °C, Rh clusters derived from Rh₁ become the main active sites, shifting E_a,app to 62 kJ/mol, characteristic of Rh nanoparticles. At ≥400 °C, larger and more active Rh particles in the strong metal–support interaction state are created. In conclusion, this work revealed the unexpected regulation effects of *H spillover on M₁ active sites under ambient conditions, differentiated the minority active M₁ sites, and demonstrated how the stability of M₁ under reducing atmospheres affects hydrogenation catalysis.« less

Improving the kinetics and selectivity of CO₂/CO electroreduction to valuable multi-carbon products is a challenge for science and is a requirement for practical relevance. Here we develop a thiol-modified surface ligand strategy that promotes electrochemical CO-to-acetate. We explore a picture wherein nucleophilic interaction between the lone pairs of sulfur and the empty orbitals of reaction intermediates contributes to making the acetate pathway more energetically accessible. Density functional theory calculations and Raman spectroscopy suggest a mechanism where the nucleophilic interaction increases the sp² hybridization of CO_(ad), facilitating the rate-determining step, CO* to (CHO)*. We find that the ligands stabilize the (HOOC–CH₂)*more » intermediate, a key intermediate in the acetate pathway. In-situ Raman spectroscopy shows shifts in C–O, Cu–C, and C–S vibrational frequencies that agree with a picture of surface ligand-intermediate interactions. A Faradaic efficiency of 70% is obtained on optimized thiol-capped Cu catalysts, with onset potentials 100 mV lower than in the case of reference Cu catalysts.« less

Zeolites containing Rh single sites stabilized by phosphorous were prepared through a one-pot synthesis method and are shown to have superior activity and selectivity for ethylene hydroformylation at low temperature (50°C). Catalytic activity is ascribed to confined Rh₂O₃ clusters in the zeolite which evolve under reaction conditions into single Rh³⁺ sites. These Rh³⁺ sites are effectively stabilized in a Rh-(O)-P structure by using tetraethylphosphonium hydroxide as a template, which generates in situ phosphate species after H₂ activation. In contrast to Rh₂O₃, confined Rh0 clusters appear less active in propanal production and ultimately transform into Rh(I)(CO)₂ under similar reaction conditions. Asmore » a result, we show that it is possible to reduce the temperature of ethylene hydroformylation with a solid catalyst down to 50°C, with good activity and high selectivity, by controlling the electronic and morphological properties of Rh species and the reaction conditions.« less

Multiferroics have tremendous potential to revolutionize logic and memory devices through new functionalities and energy efficiencies. To reach their optimal capabilities will require better understanding and enhancement of the ferroic orders and couplings. In this work, we use ϵ-Fe₂O₃ as a model system with a simplifying single magnetic ion. Using 15, 20, and 30 nm nanoparticles, we identify that a modified and size-dependent Fe–O hybridization changes the spin–orbit coupling and strengthens it via longer octahedra chains. Fe–O hybridization is modified through the incommensurate phase, with a unique two-step rearrangement of the electronic environment through this transition with attraction and thenmore » repulsion of electrons around tetrahedral Fe. Interestingly, size effects disappear in the high-temperature phase where the strongest Fe–O hybridization occurs. By manipulating this hybridization, we tune and control the multiferroic properties.« less

Nb₃Sn is a promising thin film material for superconducting radio frequency (SRF) applications. Although surface resistivity, critical temperature, and critical field are advantageous in comparison to pure Nb, currently the performance of Nb₃Sn is lacking behind due to its complex defect structure and phase inhomogeneities. In this work, the influence of the kinetic energy of the deposited particles on the defect structure in Nb₃Sn thin films synthesized at low temperatures is investigated. A combination of extended x-ray absorption fine structure analysis, x-ray absorption spectroscopy mapping, and transmission electron microscopy reveals an improved local order and elemental homogeneity of the Nb₃Snmore » films induced by higher kinetic energies of the elemental species during deposition. Even more, these process conditions lead to suppressed local inhomogeneities at grain boundaries, which can be one of the causes of critically reduced superconducting properties of low temperature sputter-coated Nb₃Sn thin films. Finally, we show that the magnetic field-induced reduction of critical currents across weak-links formed at grain boundaries can be eliminated by the suggested materials’ synthesis.« less