Key Reaction Intermediates Research Articles

Customizing catalyst surface/interface structures for electrochemical CO2 reduction.

Electrochemical CO2 reduction reaction (CO2RR) provides a promising route to converting CO2 into value-added chemicals and to neutralizing the greenhouse gas emission. For the industrial application of CO2RR, high-performance electrocatalysts featuring high activities and selectivities are essential. It has been demonstrated that customizing the catalyst surface/interface structures allows for high-precision control over the microenvironment for catalysis as well as the adsorption/desorption behaviors of key reaction intermediates in CO2RR, thereby elevating the activity, selectivity and stability of the electrocatalysts. In this paper, we review the progress in customizing the surface/interface structures for CO2RR electrocatalysts (including atomic-site catalysts, metal catalysts, and metal/oxide catalysts). From the perspectives of coordination engineering, atomic interface design, surface modification, and hetero-interface construction, we delineate the resulting specific alterations in surface/interface structures, and their effect on the CO2RR process. At the end of this review, we present a brief discussion and outlook on the current challenges and future directions for achieving high-efficiency CO2RR via surface/interface engineering.

Preparation and Photochemistry of Parent Triplet Vinylarsinidene.

Vinyl pnictinidenes are an elusive family of molecules that have been suggested as key intermediates in multiple chemical reactions and commonly display a predisposition toward open-shell electronic ground states (as is evident from quantum chemical computations). However, owing to their expected extremely high reactivity, no vinyl pnictinidene has ever been isolated and characterized spectroscopically. Here, we report the synthesis and spectroscopic characterization of vinylarsinidene, a higher congener of vinylnitrene. As we demonstrate, triplet vinylarsinidene can be prepared through the low-temperature photolysis of diazidovinylarsine at 10 K in an argon matrix. The title compound can also be generated through high-vacuum flash pyrolysis of the diazide at 700 °C and trapped analogously. Triplet vinylarsinidene was characterized by IR and UV/vis spectroscopy and displayed remarkably rich unimolecular photochemistry. Upon selective photoirradiation, it rearranges to vinylidenearsine, 2H-arsirene, triplet ethynylarsinidene or an arsinidene (H-As) acetylene complex. The formation mechanisms of these products were rationalized with DFT and CASPT2 computations.

The Impact of Operando Analysis in the Understanding of Oxygen Evolution Reaction Ranging from Intrinsic to Technical Scales

The development of state of the art and cost-effective catalyst materials for the sluggish oxygen evolution reaction (OER) is of paramount importance for realizing practical water electrolysis for green hydrogen production. Crucial for this endeavor is the determination of the OER mechanism and a better understanding of all these background processes that affect catalyst activity and stability. Thus, to further elucidate all these processes operando characterization techniques performed dynamically during OER are used as powerful tools which allow monitoring of key reaction intermediates, active sites, charge transfer and material transformation coupled processes etc. In this work we summarize our recent investigations covering noble and 3d transition metal-based OER electrocatalysts by different operando techniques and we show the importance of such techniques in the unraveling catalyst activation/deactivation mechanisms. Thus, our efforts are ranging from half-cell characterization of electrocatalyst intrinsic properties to single-cell analysis of phenomena occurring during their technical operation.Operando techniques like eQCM, XAS, ICP-OES and single cell testing will be presented aiming to emphasize on the critical role of operando in the understanding of fundamental aspects of electrochemistry [1], like intrinsic catalyst properties (Fig. 1a), the effect of electrolyte impurities such as Fe (Fig. 1b-d) on Ni-based electrocatalysts in alkaline environment [2] as well as mechanistic phenomena taking place during catalyst activation (Fig. 1e-f) leading to the formation of the active sites for OER in acidic environment [3].

A Formal γ-C-H Functionalization of Carboxylic Acids Guided by Metal-Nitrenoids as an Unprecedented Mechanistic Motif.

Harnessing the key intermediates in metal-catalyzed reactions is one of the most essential strategies in the development of selective organic transformations. The nitrogen group transfer reactivity of metal-nitrenoids to ubiquitous C-H bonds allows for diverse C-N bond formation to furnish synthetically valuable aminated products. In this study, we present an unprecedented reactivity of iridium and ruthenium nitrenoids to generate remote carbocation intermediates, which subsequently undergo nucleophile incorporation, thus developing a formal γ-C-H functionalization of carboxylic acids. Mechanistic investigations elucidated a unique singlet metal-nitrenoid reactivity to initiate an abstraction of γ-hydride to form the carbocation intermediate that eventually reacts with a broad range of carbon, nitrogen, and oxygen nucleophiles, as well as biorelevant molecules. Alternatively, the same intermediate can lead to deprotonation to afford β,γ-unsaturated amides in a less nucleophilic solvent.

Hydroxyl-Bonded Ru on Metallic TiN Surface Catalyzing CO2 Reduction with H2O by Infrared Light.

Synchronized conversion of CO2 and H2O into hydrocarbons and oxygen via infrared-ignited photocatalysis remains a challenge. Herein, the hydroxyl-coordinated single-site Ru is anchored precisely on the metallic TiN surface by a NaBH4/NaOH reforming method to construct an infrared-responsive HO-Ru/TiN photocatalyst. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (ac-HAADF-STEM) and X-ray absorption spectroscopy (XAS) confirm the atomic distribution of the Ru species. XAS and density functional theory (DFT) calculations unveil the formation of surface HO-RuN5-Ti Lewis pair sites, which achieves efficient CO2 polarization/activation via dual coordination with the C and O atoms of CO2 on HO-Ru/TiN. Also, implanting the Ru species on the TiN surface powerfully boosts the separation and transfer of photoinduced charges. Under infrared irradiation, the HO-Ru/TiN catalyst shows a superior CO2-to-CO transformation activity coupled with H2O oxidation to release O2, and the CO2 reduction rate can further be promoted by about 3-fold under simulated sunlight. With the key reaction intermediates determined by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and predicted by DFT simulations, a possible photoredox mechanism of the CO2 reduction system is proposed.

Probing the Nature of Surface Hydrides by Deuterium Quadrupolar Parameters: A Case Study on Silica‐Supported Zirconium Hydrides

AbstractSupported metal hydrides are key reactive intermediates in various catalytic processes, such as hydrogenation and dehydrogenation, but are often challenging to characterize spectroscopically. Here, deuterium solid state nuclear magnetic resonance spectroscopy is used to understand the structure of the corresponding silica‐supported zirconium hydrides after H/D exchange as an illustrative example of supported metal hydrides, which have been shown to display notable reactivity towards small molecules (e. g., CO2 and N2O) and to activate both C−H and C−C bonds, hence their use in to the conversion of hydrocarbons (alkanes, polyolefins etc.)

Microkinetic Modeling of CO2 Hydrogenation to Formic Acid on Transition Metal Doped Ni(111) Surface

The direct conversion of carbon dioxide (CO2) into valuable chemicals plays a crucial role in mitigating climate change and fostering a sustainable future. To facilitate this process, identifying an effective catalyst is of utmost importance. This study focuses on investigating the potential of nickel-based surfaces as catalysts for the direct hydrogenation of CO2 into formic acid (HCOOH), a valuable chemical with applications as a fuel and hydrogen storage. Several nickel surface models were considered, including pristine Ni(111) and Ni(111) surfaces doped with Cu, Pd, Pt, and Rh metals. Through a microkinetic investigation, we examined the reaction kinetics and determined the rate-determining steps on the Ni(111) surfaces. By elucidating the reaction pathways and characterizing the reaction intermediates, we gained comprehensive insights into the fundamental processes involved in CO2 hydrogenation to HCOOH. This knowledge is crucial for the rational design and optimization of catalysts, enabling the development of highly active catalysts for CO2 conversion. Our study revealed insights into the effect of transition metal doping on the catalytic activity of Ni(111) surfaces. Among the investigated dopants, we found that the addition of Pt in the first layer of Ni(111) surface yielded the best turnover frequency (TOF) for the CO2 conversion to HCOOH. The Pt dopants induced unique electronic properties in the Ni(111) surface, resulting in a reduced activation barrier for key reaction intermediates. This effect contributed to a more efficient conversion of CO2 to HCOOH.

Copper‐Catalyzed Remote Enantioselective Sulfonylation of Yne‐Allylic Esters with Sodium Sulfinates

AbstractImpressive progress has been made in the copper‐catalyzed asymmetric propargylic substitution (APS) reaction, but its use in remote asymmetric yne‐allylic substitution remains a challenging topic. Herein, we report the first remote enantioselective copper‐catalyzed sulfonylation of yne‐allylic esters with sodium sulfinates. The reaction is assumed to occur via a copper‐vinylvinylidene species as the key reactive intermediate. The use of readily available starting materials, the mild reaction conditions, and the excellent regio‐, enantio‐ and stereoselectivity, as well as broad substrate scope (>70 examples), show the practicality and attractiveness of this method.

Copper-Catalyzed Remote Enantioselective Sulfonylation of Yne-Allylic Esters with Sodium Sulfinates.

Impressive progress has been made in the copper-catalyzed asymmetric propargylic substitution (APS) reaction, but its use in remote asymmetric yne-allylic substitution remains a challenging topic. Herein, we report the first remote enantioselective copper-catalyzed sulfonylation of yne-allylic esters with sodium sulfinates. The reaction is assumed to occur via a copper-vinylvinylidene species as the key reactive intermediate. The use of readily available starting materials, the mild reaction conditions, and the excellent regio-, enantio- and stereoselectivity, as well as broad substrate scope (>70 examples), show the practicality and attractiveness of this method.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation.

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Fabrication of NiZn bimetallic oxides catalysts supported on carbon black waste for catalytic pyrolysis of fatty acid

Herein, NiZn bimetallic oxides derived from calcination treatment of metal-organic frameworks (MOFs) were successfully loaded onto the carbon black waste as support obtained from the oil refinery. The resulting hybrids could serve as environmentally friendly and cost-effective catalysts for the deoxygenation of biomass-derived fatty acids under an inert atmosphere. An online pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system was employed to evaluate their deoxygenation performance. The optimized catalysts demonstrated remarkably high stearic acid conversion of 99.6% and selectivity towards C8-C17 olefins of 57.2%. Furthermore, these catalysts with varying metal loadings also exhibited excellent efficiency in deoxygenation reactions of other types of fatty acids, such as myristic acid and palmitic acid, achieving conversion exceeding 85% and selectivities towards olefins of ca. 50%. When converting coconut oil, the supported NiZn bimetallic oxides showed excellent selectivity towards olefins (>60%). Furthermore, in-situ DRIFTS results suggest that fatty alcohols might be the key intermediates in the deoxidation reactions of fatty acid. This study not only demonstrates the great potential for obtaining valuable chemicals from biomass but also provides some insights into the fabrication of catalyst materials by using industrial solid waste.

Ag+ -Doped InSe Nanosheets for Membrane Electrode Assembly Electrolyzer toward Large-Current Electroreduction of CO2 to Ethanol.

It is an appealing approach to CO2 utilization through CO2 electroreduction (CO2 ER) to ethanol at high current density; however, the commonly used Cu-based catalysts cannot sustain large current during CO2 ER despite their capability for ethanol production. Herein, we report that Ag+ -doped InSe nanosheets with Se vacancies can address this grand challenge in a membrane electrode assembly (MEA) electrolyzer. As revealed by our experimental characterization and theoretical calculation, the Ag+ doping, which can tailor the electronic structure of InSe while diversifying catalytically active sites, enables the formation of key reaction intermediates and their sequential evolution into ethanol. More importantly, such a material can well work for large-current conditions in MEA electrolyzers with In2+ species stabilized via electron transfer from Ag to Se. Remarkably, in an MEA electrolyzer by coupling cathodic CO2 ER with anodic oxygen evolution reaction (OER), the optimal catalyst exhibits an ethanol Faradaic efficiency of 68.7 % and a partial current density of 186.6 mA cm-2 on the cathode with a full-cell ethanol energy efficiency of 26.1 % at 3.0 V. This work opens an avenue for large-current production of ethanol from CO2 with high selectivity and energy efficiency by rationally designing electrocatalysts.

Ag+‐Doped InSe Nanosheets for Membrane Electrode Assembly Electrolyzer toward Large‐Current Electroreduction of CO2to Ethanol

AbstractIt is an appealing approach to CO2utilization through CO2electroreduction (CO2ER) to ethanol at high current density; however, the commonly used Cu‐based catalysts cannot sustain large current during CO2ER despite their capability for ethanol production. Herein, we report that Ag+‐doped InSe nanosheets with Se vacancies can address this grand challenge in a membrane electrode assembly (MEA) electrolyzer. As revealed by our experimental characterization and theoretical calculation, the Ag+doping, which can tailor the electronic structure of InSe while diversifying catalytically active sites, enables the formation of key reaction intermediates and their sequential evolution into ethanol. More importantly, such a material can well work for large‐current conditions in MEA electrolyzers with In2+species stabilized via electron transfer from Ag to Se. Remarkably, in an MEA electrolyzer by coupling cathodic CO2ER with anodic oxygen evolution reaction (OER), the optimal catalyst exhibits an ethanol Faradaic efficiency of 68.7 % and a partial current density of 186.6 mA cm−2on the cathode with a full‐cell ethanol energy efficiency of 26.1 % at 3.0 V. This work opens an avenue for large‐current production of ethanol from CO2with high selectivity and energy efficiency by rationally designing electrocatalysts.

Trapping of Arynes with In Situ Generated Aryltrifluoromethylnitrones Enables Access to Trifluoromethylated Benzoxazolines.

Herein, we report a rare example of trapping arynes using in situ generated aryltrifluoromethylnitrone to access an important class of trifluoromethylated benzoxazolines. This three-component strategy involves a nitrone formation/[3 + 2] cycloaddition/thermal rearrangement cascade and furnishes trifluoromethylated benzoxazolines in high yields. The scope of the reaction is quite broad with respect to aryltrifluorodiazoethanes, nitrosoarenes, and arynes. The proposed reaction pathway is supported through the isolation and characterization of the key reaction intermediates phenyltrifluoromethylnitrone and benzisoxazoline.

Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity.

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C-C and one C-N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals. In addition, the macrocyclic distortion also favours a rare intramolecular ferromagnetic interaction between the CuII and π-cation radical spins to form a triplet state. The structural and electronic perturbation in the unconjugated dication diradical possibly enables the bridging pyrrolic nitrogen to undergo a nucleophilic attack at the nearby β-carbon of the porphyrin π-cation radical with a computed free energy barrier of >20 kcal mol-1 which was supplied in the form of reflux condition to initiate such a rearrangement process. UV-vis, EPR and ESI-MS spectroscopies were used to monitor the rearrangement process in situ in order to identify the key reactive intermediates leading to such an unusual transformation.

CO2 photoreduction with high selectivity of C2H4 production on conjugated microporous polymer with Cu single atoms

Selective conversion of CO2 to fuels and chemicals has been considered one of the key challenges in the photocatalytic CO2 reduction reaction. The multi-electron reduction reaction requires the design of photocatalysts that can adsorb CO2 and transfer electrons. Herein, Cu single atoms are confined in a conjugated microporous polymer (CMP) to construct Cun@CMP photocatalyst for CO2 reduction to produce C2H4 and CH4. By adjusting Cu loading, Cu0.012@CMP exhibited a high performance with production yields of 236.81 μmol·g−1·h−1 for C2H4, and 78.2 % selectivity for C2H4 formation. DFT calculations displayed the band structure and confirmed Cu–N together with characterization. The in situ DRIFTS revealed *CH2 and *C2H4 as key reaction intermediates to form CH4 and C2H4, respectively. The synergy effect in Cu single-atoms and CMP contributed greatly to C–C coupling and interfacial charge transfer, thus achieving excellent activity for conversion of CO2 to C1/C2. This work highlights the potential for constructing Cu single atoms on CMP to produce highly selective C2H4.

Cyclization of 1-aryl-4,4,4-trichlorobut-2-en-1-ones into 3-trichloromethylindan-1-ones in triflic acid.

Trichloromethyl-substituted enones (1-aryl-4,4,4-trichlorobut-2-en-1-ones, ArCOCH=CHCCl3, CCl3-enones) undergo intramolecular transformation into 3-trichloromethylindan-1-ones (CCl3-indanones) in Brønsted superacid CF3SO3H (triflic acid, TfOH) at 80 °C within 2-10 h in yields up to 92%. Protonation of the carbonyl oxygen of the starting CCl3-enones by TfOH affords the key reactive intermediates, the O-protonated forms ArC(=OH+)CH=CHCCl3, which are then cyclized into the target CCl3-indanones. These cations have been studied experimentally by means of NMR spectroscopy in TfOH and theoretically by DFT calculations. Under the same superacidic conditions in TfOH, CCl3-hydroxy ketones (1-aryl-4,4,4-trichloro-3-hydroxybutan-1-ones; ArCOCH2CH(OH)CCl3) undergo dehydration to the corresponding CCl3-enones, which are further cyclized into CCl3-indanones. The yields of CCl3-indanones starting from CCl3-hydroxy ketones are up to 86% in TfOH at 80 °C within 3-18 h.

(Invited) Electrocatalysis and Surface Chemistry in Lithium Sulfur Batteries

This talk will focus on the application of electrocatalysis and surface/interface chemistry to solve the cycling stability problem of lithium sulfur batteries, which are a type of future high energy rechargeable battery that can deliver a few times more energy than the prevalent lithium ion batteries of the same mass. One critical problem is the adsorption of lithium polysulfides, the key reaction intermediates that control the electrochemical performance of the sulfur cathode. We find that 3d transition metal oxides effectively bind polysulfides via oxygen-lithium or metal-sulfur bonding, whereas metal phosphides and chalcogenides only do so with their oxidized surfaces. When the electrolyte/sulfur ratio is lowered for even higher energy density (lean electrolyte condition), we discover that catalysis is needed to overcome the potential limiting reaction barrier associated with the conversion of reaction intermediates. With this understanding, we have developed high capacity sulfur cathode materials achieving hundreds of stable charging-discharging cycles. This surface/interface chemistry approach also enables us to analyze the cycling behavior of the lithium/sodium metal anode and to develop electrode/electrolyte interphases for improving the charging-discharging efficiency and cycling stability, including lithium metal electrodes working with solid-state electrolyte.

Role of Electrolyte-Adsorbates Interaction in Determining CO2 Reduction Reaction Pathways

Electrochemical CO2 reduction reaction (CO2RR) is a promising strategy to convert atmospheric CO2 into valuable fuels and chemicals. Although the optimization of the catalyst-adsorbates interaction via tuning the binding energies of electrocatalysts has been extensively studied, activity and product selectivity remain a challenge. Recently, there has been a growing recognition that cations in electrolytes influence the activity of surface electrocatalytic reactions.1 A recent density functional theory study suggested that partially desolvated metal cations stabilize CO2RR intermediates via electrostatic interactions.2 Therefore, an experimental understanding of how the cations (and surrounding water molecules) control the intermediates present on the surface and govern the reaction pathway is critical to establish further insights into the design of electrolytes for CO2RR.In this talk, the role of alkali metal cations and water molecules on the CO2RR is revealed by tracking the surface-bound intermediates using operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). We will also integrate the operando surface analysis with final product selectivity, further strengthening the understanding of the reaction mechanism. It will also be demonstrated that the holistic information about the electrolyte-adsorbates interaction can provide additional knobs to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for the CO2RR.[Reference]1 Ringe, S. et al., Energy Environ. Sci. 2019, 12, 3001–3014.2 Monteiro, MCO. et al., Nat. Catal. 2021, 4, 654–662.

Synthesis of 2‐Trifluoromethyl‐4‐aminoquinolines via Heating‐Promoted Multi‐component Reaction of CF3‐Imidoyl Sulfoxonium Ylides and Amines with Difluorocarbene as a C1 Synthon†

Comprehensive SummaryA thermally‐induced multi‐component reaction of CF3‐substituted imidoyl sulfoxonium ylides (TFISYs), amines and (triphenylphosphonio)difluoroacetate (PDFA) has been developed, allowing a facile access to 2‐trifluoromethyl‐4‐aminoquinolines in high yields. The reaction proceeds smoothly with or without the addition of sulfur and utilizes difluorocarbene as a C1 synthon under simply heating conditions. Mechanistic study reveals that in‐situ generated thiocarbonyl fluoride, isothiocyanate or gem‐difluoroalkene might act as the key reaction intermediates.