128 Search Results

Electrochemical polarization is now known to play a key role in thermochemical catalysis at solid–liquid interfaces. However, existing frameworks cannot account for why even nonpolar, nonfaradaic reactions are sensitive to interfacial polarization. In order to uncover the molecular basis of this phenomenon, we herein study the potential-dependent reaction kinetics of ethylene and trans-2-butene hydrogenation at Pt–liquid interfaces. Measurements were performed in aqueous and ortho-difluorobenzene (o-DFB) solutions, spontaneously polarizing the Pt–liquid interfaces by, respectively, varying the pH or dissolving distinct metallocene redox buffers into solution. Here, we find that at comparable mechanistic regimes, the rates of both ethylene and trans-2-butene hydrogenationmore » are maximized near the same electrochemical potential, E. Moreover, the potential-dependence, defined as $$\frac{∂ln 𝑟}{∂𝐸}$$, of trans-2-butene hydrogenation is approximately 2.2× greater than that of ethylene hydrogenation across the full potential range studied. These observations are all consistent with a model in which polarization of the Pt surface away from the local potential of zero free charge (E_PZFC) induces electrostatic organization of the polar solvent and charged ions near the interface, which impedes olefin adsorption and surface reaction because these surface reactions induce electrolyte displacement. Accordingly, interfacial polarization alters the free energy landscape and thus the rate of nonpolar heterogeneous catalysis by controlling the degree of electrostatic organization of polar and charged spectators at the interface, which do not in general need to be specifically chemisorbed onto the surface but could simply be close enough to the surface to be perturbed by the olefin adsorption. These results point toward electrochemical design handles, namely, the electrolyte, catalyst potential, and local E_PZFC of the catalyst, with which to tune interfacial catalysis of thermochemical organic transformations.« less

Addition of Ph₂SiH₂ to [(depe)₂Mo(N)][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane, BAr^F₄ = B(3,5-(CF₃)₂C₆H₃)₄) at 60 °C generated the silyl imido molybdenum hydride complex, trans- [(depe)₂Mo(NSiHPh₂)H][BAr^F₄], a surrogate for a proposed intermediate complex in the photodriven hydrogenation to free ammonia. Irradiation of a THF solution of trans-[(depe)₂Mo(NSiHPh₂)H]- [BAr^F₄] with blue light under H₂ produced free amine along with [(depe)₂MoH₅][BAr^F₄] in 76% yield. This transformation occurred in the absence of a precious metal photocatalyst, suggesting that it was needed only for the initial addition of H₂ to the molybdenum nitride during the first N−H bond-forming step in the photodriven hydrogenation. Deuterium labeling and crossovermore » studies support concerted Si−H bond addition across the Mo≡N bond, enabled by the nucleophilicity of the nitride. Subsequent hydrogenation involves an intramolecular H migration from Mo to the imido ligand, as supported by electronic absorption spectroscopy, transient absorption spectroscopy, initial rate measurements, and deuterium kinetic isotope effect measurements. These findings provide insights into the photodriven hydrogenation of [(depe)₂Mo(N)][BAr^F₄] to ammonia and the role of the photocatalyst in this transformation.« less

Intermetallic compounds serve as model catalysts for selective hydrogenation reactions, offering precise control over the active site composition(s), geometric and electronic structure. The addition of a third element to form a ternary intermetallic alters the exposed crystal facet(s), demonstrating a strategy to impart improved catalytic behavior in intermetallic catalysts. The site-specific substitution of a small fraction of Pd atoms with Au in pyrite-type PdSb₂ results in the preferential exposure of the (100) facet over the (111) facet. Electron back scattered diffraction and density functional theory calculations confirm the facet change upon the substitution of Pd with Au to form themore » ternary Pd_1−xAu_xSb₂ (0.075 ≤ x ≤ 0.25). The (100) facet demonstrates higher net alkene selectivity due to significantly weaker alkene binding compared to the (111) facet. Distinct from our prior work on chemical substitution to directly alter the active site composition, this work demonstrates the indirect modification of active sites via preferential facet exposure.« less

Reactive capture and conversion (RCC) is a process intensification approach that integrates CO₂ capture and hydrogenation within a single unit, removing the CO₂ purification and storage steps of traditional process flow schemes. This alters the catalytic step from a traditional steady-state (SS) flow process to a transient capture and conversion cycle, which could lead to product distributions distinct from those observed in conventional SS experiments. Such differences are investigated in the combined capture and hydrogenation of carbon dioxide to methanol over a ZnZrO₂ catalyst and a ZnZrO₂ + NaNO₃/Mg₃AlO_x catalytic sorbent (CS) using fixed-bed kinetic measurements, in situ diffuse reflectancemore » infrared Fourier transform spectroscopy (DRIFTS), and steady-state isotopic transient kinetic analysis-DRIFTS (SSITKA-DRIFTS). Under SS conditions, ZnZrO₂ produced methanol through sequential hydrogenation of HCOO* and CH₃O* intermediates. On the contrary, CO was attributed primarily to CO₂ dissociation at oxygen vacancies, as supported by isotopic shifts and measured reaction orders. For the CS, isotopic switching experiments suggested that monodentate carbonate species (CO₃²⁻, abbreviated as m-CO₃²⁻) act as active intermediates that can be hydrogenated to HCOO* and subsequently to CH₃O. Under RCC conditions, in situ DRIFTS and isotopic experiments reveal that m-CO₃²⁻ species formed during the CO₂ capture step follow two competing routes upon H₂ exposure: (i) direct hydrogenation to methane on the sorbent domain or (ii) migration of m-CO₃²⁻ to the ZnZrO₂ domain, where they are hydrogenated to methanol through the HCOO pathway. Overall, RCC enables carbonate hydrogenation routes not observed under SS cofeed conditions. Thus, the reaction pathways and rates during RCC can be different from operation under conventional SS conditions, and the product distribution is determined here by competition between carbonate hydrogenation on sorbent sites and migration to ZnZrO₂ for methanol synthesis.« less

Experiments of in-situ DRIFTS combined with modulation excitation (ME) spectroscopy showed a rich surface chemistry associated with the adsorption of CO₂ on nanocubes and nanospheres of ceria. The nanocubes exposed faces with a (100) orientation, with the edges and corners displaying (110) and (111) orientations, respectively. Here, the nanospheres mainly contained ceria (111) and (110) planes. DFT calculations showed that CO₂ is a multidentate adsorbate on ceria that can undergo changes in its bonding configuration depending on the chemical environment. At 250 °C, a temperature typically used for the conversion of CO₂ into oxygenates, alkanes and olefins, CO₂ reacted withmore » O centers or OH groups present on the nanocubes and nanospheres to yield bi- and tri-dentate carbonates, hydroxycarbonates, and formates. Both nanostructures were highly reactive and a dynamic equilibrium was established: carbonate species were rapidly generated upon the injection of CO₂ and they decomposed upon the removal of CO₂ from the gas phase. In the case of the ceria nanocubes, the adsorption/desorption processes were essentially reversible, opening the door to catalytic transformations. A larger concentration of defects in the ceria nanospheres led to strongly bound carbonates and formates that may be spectators, site blockers, or surface modifiers in catalytic processes. In the ME studies, additional intermediates were detected, and it was clear that the response of surface species to the presence/absence of CO₂ was highly dependent on the morphology of the ceria nanostructures.« less

Liquid organic hydrogen carriers (LOHCs) are an attractive fuel source due to their compatibility with existing transportation methods and ease of use. However, they suffer from sluggish (de)hydrogenation kinetics. One promising platform for developing next-generation catalysts is metal–organic frameworks (MOFs), which can enable systematic interrogation into the influence of metal identity and spatial arrangement. In this study, the effect of the coordination environment was investigated using Ni- and Co-based azolate MOFs: MFU-4l-OH (M_xZn_5–x(OH)₄(BTDD)₃; x = 4 for M = Co and x = 3 for M = Ni, H₂BTDD = bis(1H-1,2,3-triazolo[4,5-b][4′,5′-i])dibenzo[1,4]dioxin), composed of single-site nodes, and M(OH)₂BBTA (M = Ni,more » Co; H₂BBTA = 1H,5H-benzo(1,2-d:4,5-d’)bistriazole), composed of extended chain-type nodes. The catalysts were characterized by isotherms, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS) analysis. Acetylene hydrogenation activity under steady state conditions (150 °C, 1:1 C₂H₂:H₂) revealed higher turnover frequencies (TOFs) up 1.17 × 10^–2 mol C₂H₂ mol⁻¹ Ni min⁻¹ and 1.98 × 10^–3 mol C₂H₂ mol⁻¹ Co min⁻¹ for Ni-MFU-4l-OH and Co-MFU-4l-OH, respectively, compared to their BBTA analogues. However, the Co-based MOFs, particularly Co₂(OH)₂-BBTA, exhibited greater selectivity (up to 19%) for the fully hydrogenated ethane product. Isosteric heat of adsorption (Q_st) measurements for ethylene and ethane revealed that the BBTA framework had stronger binding to the products than MFU-4l. Furthermore, these findings demonstrate that metal identity and coordination environment may modulate acetylene hydrogenation performance, leading to design principles for tuning LOHC hydrogenation catalysts.« less

Insertion reactions play a central role in the catalytic synthesis of ethanol and higher alcohols. X-ray photoelectron and absorption spectroscopies have been used to follow migratory CO insertion and C─C coupling in a cis-[Ru(2,2′-bipyridine)₂(CO)(CH₃)]⁺ complex heated in a vacuum or exposed to CO. Heating of the Ru complex in a vacuum to temperatures above 50 °C induced spontaneous migration of CO into the Ru─CH₃ bond to yield a ─COCH₃ ligand. In conclusion, after adding CO to the background gas, the CO insertion reaction was seen at room temperature, opening the door for the synthesis of ethanol and more energy densemore » liquids.« less

The conversion of CO₂ into value-added chemicals, such as methanol, offers a promising pathway toward a renewable energy future. However, a precise kinetic control and a highly selective catalyst are necessary to overcome the thermodynamic preference for CO₂ hydrogenation to methane. Rhenium-based catalysts, particularly Re/TiO₂, demonstrate high activity and selectivity for methanol under high-pressure conditions. For example, at 100 bar and 200 °C, a methanol selectivity of 97−99% was obtained. Catalysts with 1 wt % Re and 5 wt % Re/ TiO₂ were used to study the effect of cluster sizes. At 250 °C, the 1 wt % catalyst achievesmore » 97% selectivity at 23% conversion, whereas 5 wt % Re/TiO₂ achieves 74% selectivity at 40% conversion, corresponding to a drop in space-time yield from 65 to 16 g_CH3OH·g_Re⁻¹·h⁻¹, respectively. X-ray absorption spectroscopy provided insights into the structure of the active sites, while density functional theory calculations revealed the effects of cluster size on the energy barriers for H₂ activation, CH₃OH dissociation, and CH₃OH desorption, all of which directly influence conversion and selectivity. These results underscore the importance of balancing cluster size for optimal catalyst performance and provide insights into the design of efficient and selective catalysts for renewable methanol production.« less

Oxidation of the Chatt-type tungsten dinitrogen compound, trans-(depe)₂W(N₂)₂ (depe = Et₂PCH₂CH₂PEt₂), with [(η⁵-C₅H₅)₂Fe][BAr^F₄] (BAr^F4 = B(3,5-(CF₃)₂C₆H₃)₄) resulted in isolation of [(depe)₂WN][BAr^F₄], a rare example of a tungsten(IV) nitride prepared from N₂ cleavage. A bimetallic μ-N₂ ditungsten intermediate supported by terminal N₂ ligands was identified, and irradiation with visible light promoted dinitrogen cleavage and formation of [(depe)₂WN][BAr^F₄]. Performing the analogous one-electron oxidation of the related tungsten dinitrogen compound, trans-(dppe)₂W(N₂)₂ (dppe = Ph₂PCH₂CH₂PPh₂), furnished the corresponding cationic, 17-electron tungsten dinitrogen complex, [(dppe)₂W(N₂)₂][BAr^F₄], that was characterized by X-ray diffraction and vibrational and EPR spectroscopies. The generation of [(dppe)₂W(N)][BAr^F₄] was observed in low yieldmore » from the in situ formed mixed N₂-bridged compound, [(N₂)(depe)₂W(μ-N₂)W(dppe)₂(N₂)][BAr^F₄]₂, and was confirmed by independent synthesis using 1-azidoadamantane. Addition of ammonia or water to [(depe)₂WN][BAr^F₄] resulted in formation of the cationic imide and hydroxide complexes, [(depe)₂W(NH)(X)][BAr^F₄] (X = NH₂, OH). Irradiation of [(depe)₂WN][BAr^F₄] with 440 nm visible light in the presence of Ir(ppy)₃ (ppy = 2-phenylpyridine) under 4 atm of dihydrogen resulted in hydrogenation of the tungsten nitride to the cationic tungsten pentahydride, [(depe)₂WH₅][BAr^F₄], with the release of free ammonia in 21% yield, a rare example of ammonia generation from dinitrogen and dihydrogen from a well-defined tungsten nitride.« less

High-entropy oxides (HEOs) represent a frontier in catalyst design via entropy-stabilized solid solution formation. However, their catalytic efficiency is limited by their bulk and dense nature. This work presents a strategic approach to tackle this challenge by fabricating mesoporous amorphous HEO films (MA-HEOF) possessing maximized active site utilization efficiency. The success hinges on the as-developed geometric engineering strategy via controlled deposition–precipitation to confine the amorphous HEO thin film on the surface of mesoporous channels. The unique structure of MA-HEOF was elucidated via microscopy-, X-ray-, and neutron-based techniques, which were manifested by enriched surface-activated lattice oxygen and enhanced redox activity, asmore » confirmed by isotope studies. Besides, the MA-HEOF could stabilize and modulate the properties of integrated noble metal sites, enhancing their redox activity in diverse reactions. In conclusion, the approaches and insights presented herein provide guidance on maximizing the utilization efficiency of high-entropy materials in catalysis and beyond.« less