Intermediates In Catalytic Reactions Research Articles

Advanced Development of In situ Characterization Technique for Electrocatalytic Hydrogen Evolution Reaction

AbstractHydrogen evolution reaction (HER) from water splitting using renewable energy provides a promising solution to the global energy crisis and environmental problems. However, the lack of in‐depth understanding of the reaction mechanism and clear identification of the catalytic active site has hindered the further development of low‐cost, high‐performance, and long‐life efficient electrocatalysts. Through in situ characterization techniques, the activity and stability of catalysts can be monitored in real‐time, track the structural evolution of catalytic reaction intermediates, and obtain a deep understanding of catalytic reaction mechanism, so as to feedback and guide the development and utilization of the catalyst. In this review, advanced development of in situ characterization techniques in electrocatalytic HER in recent years is summarized. In situ spectroscopy can be used to track the behavior of reaction intermediates such as adsorption, desorption, and structural evolution at the molecular level to reveal the electrocatalytic HER mechanism. The imaging techniques can be used to observe the evolution of material structure during HER in situ, which provides valuable information for the understanding of visualization of catalytic hydrogen evolution at atomic level. This review contributes to theadvanced development of in situ characterization technique for electrocatalytic HER.

COF‐Topological Quantum Material Nano‐heterostructure for CO2 to Syngas Production under Visible Light

AbstractEfficient solar‐driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF‐topological quantum material nano‐heterostructure, COF@TI with a newly synthesized donor‐acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron‐hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile‐water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano‐heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Enhanced Reactivities of Iron(IV)-Oxo Porphyrin Species in Oxidation Reactions Promoted by Intramolecular Hydrogen-Bonding.

High-valent iron-oxo species are one of the common intermediates in both biological and biomimetic catalytic oxidation reactions. Recently, hydrogen-bonding (H-bonding) has been proved to be critical in determining the selectivity and reactivity. However, few examples have been established for mechanistic insights into the H-bonding effect. Moreover, intramolecular H-bonding effect on both C-H activation and oxygen atom transfer (OAT) reactions in synthetic porphyrin model system has not been investigated yet. In this study, a series of heme-containing iron(IV)-oxo porphyrin species with or without intramolecular H-bonding are synthesized and characterized. Kinetic studies revealed that intramolecular H-bonding can significantly enhance the reactivity of iron(IV)-oxo species in OAT, C-H activation, and electron-transfer reactions. This unprecedented unified H-bonding effect is elucidated by theoretical calculations, which showed that intramolecular H-bonding interactions lower the energy of the anti-bonding orbital of iron(IV)-oxo porphyrin species, resulting in the enhanced reactivities in oxidation reactions irrespective of the reaction type. To the best of the knowledge, this is the first extensive investigation on the intramolecular H-bonding effect in heme system. The results show that H-bonding interactions have a unified effect with iron(IV)-oxo porphyrin species in all three investigated reactions.

COF-Topological Quantum Material Nano-heterostructure for CO2 to Syngas Production under Visible Light.

Efficient solar-driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF-topological quantum material nano-heterostructure, COF@TI with a newly synthesized donor-acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron-hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile-water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano-heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Electron Delocalization and Transfer Across Polyoxometalates-Based Subnanomaterials in Catalytic Reactions.

Due to their identical building blocks and high surface-to-volume ratio, subnanomaterials exhibit significant properties compared to their bulk nanomaterial counterparts. The interactions between these building blocks can result in either equal or unequal sharing of electrons, leading to electron transfer in heterojunctions or electron delocalization within symmetric structures. Clusters, possessing electronic properties akin to atoms, can serve as reservoirs of electrons to stabilize crucial intermediates in catalytic reactions. This perspective provides a novel understanding of well-defined subnanomaterials with distinct architectures, such as cluster-based constructions and co-assembled heterojunctions, emphasizing the relationship between electronic structures and catalytic properties. The objective is to provide novel perspectives on the realm of subnanomaterials and cluster-based architectures.

Single Atom Alloys Segregation in the Presence of Ligands.

Single atom alloys (SAAs) have gained remarkable attention due to their tunable properties leading to enhanced catalytic performance, such as high activity and selectivity. The stability of SAAs is dictated by surface segregation, which can be affected by the presence of surface adsorbates. Research efforts have primarily focused on the effect of commonly found catalytic reaction intermediates, such as CO and H, on the stability of SAAs. However, there is a knowledge gap in understanding the effect of ligands from colloidal nanoparticle (NP) synthesis on surface segregation. Herein, we combine density functional theory (DFT) and machine learning to investigate the effect of thiol and amine ligands on the stability of colloidal SAAs. DFT calculations revealed rich segregation energy (Eseg) data of SAAs with d8 (Pt, Pd, Ni) and d9 (Ag, Au, Cu) metals exposing (111) and (100) surfaces, in the presence and absence of ligands. Using these data, we developed an accurate four-feature neural network using a multilayer perceptron regression (NN MLP) model. The model captures the underlying physics behind surface segregation in the presence of adsorbed ligands by incorporating features representing the thermodynamic stability of metals through the bulk cohesive energy, structural effects using the coordination number of the dopant and the ligands, the binding strength of the adsorbate to the metals, strain effects using the Wigner-Seitz radius, and electronic effects through electron affinities. We found that the presence of ligands makes the thermodynamics of segregation milder compared to the bare (nonligated) SAA surfaces. Importantly, the adsorption configuration (e.g., top vs bridge) and the binding strength of the ligand to the SAA surface (e.g., amines vs thiols) play an important role in altering the Eseg trends compared to the bare surface. We also developed an accurate NN MLP model that predicts Eseg in the presence of ligands to find thermodynamically stable SAAs, leading to the rapid and efficient screening of colloidal SAAs. Our model captures several experimental observations and elucidates complex physics governing segregation at nanoscale interfaces.

Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate.

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)2FeII]2- leads to the octahedral [(TSMP)2FeIII]-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)2FeIII]- can be reversibly oxidized to the stable high-valent [(TSMP)2FeIV]0 complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (giso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Simple purification of small-molecule-labelled peptides via palladium enolate formation from β-ketoamide tags.

Palladium enolates derived from β-ketocarbonyl compounds serve as key intermediates in various catalytic asymmetric reactions. We found that the palladium enolate formed from β-ketoamide is stable in air and moisture and we applied this property to develop a peptide purification system using β-ketoamide as a small affinity tag in aqueous media. A solid-supported palladium complex successfully captured β-ketoamide-tagged molecules as palladium enolates and released them in high yield upon acid treatment. Optimum conditions for the catch and release of tagged peptides from a mixture of untagged peptides were established. To demonstrate the value of this methodology in identifying the binding site of a ligand to its target protein, we purified and identified a peptide containing the ligand-binding site from the tryptic digest of cathepsin B labelled with a covalent cathepsin B inhibitor containing a β-ketoamide tag.

The Use of Iron Ore as a Catalyst in Fischer–Tropsch Synthesis—A Review

The use of iron ore as an alternative to conventional Fischer–Tropsch synthesis (FTS) iron catalyst has been identified as a way to achieve a cost-effective catalyst. In recent times, considerable progress has been made to build a strong case for iron ore as a viable alternative to traditional iron catalysts. Nevertheless, there are still opportunities to enhance the current iron ore low-temperature Fischer–Tropsch (LTFT) catalysts and pave the way for optimal performing catalysts. In this study, we thoroughly examined the various publications on iron ore catalysts used for FTS and highlighted the research gaps in the studies. The study identified the progress made so far, opportunities, and challenges regarding the use of iron ore as a catalyst in FTS. One of the critical areas that needs to be addressed from the review is establishing the deactivation pathways of these catalyst systems. The application of advanced spectroscopic and computational methods is also suggested to elucidate the relationship between the synthesis conditions, active catalytic sites, reaction intermediates, and catalytic performance to fabricate optimized iron ore LTFT catalysts.

Facet-Controlled PdP Nanosheets for Ethanol Electrooxidation

Understanding the contribution of crystal facets to the adsorption/desorption behavior of catalytic reaction intermediates is crucial for the design of high-performance electrocatalysts. In this work, we designed a synthesis strategy for ultrathin PdP nanosheets enclosed by different low-index facets, for example, {100}, {110}, and {111}, and studied their performance for electrocatalytic ethanol oxidation reaction (EOR). The results showed that the mass activity of the PdP nanosheets enclosed by the {100} facet was 4.07 A/mg, which was 1.43, 2.56, and 5.58 times as high as that of the PdP {110}, PdP {111}, and commercial Pd/C, respectively. Additionally, the PdP {100} displayed the best stability and the lowest activation energy (Ea), which further proved that the atomic configuration on the crystallographic {100} facet was greatly beneficial for EOR. The universality of this synthesis strategy was shown by producing nanocrystals with various amounts of P doping and elemental compositions (e.g., {100}-exposed PdAgP, PdAuP, PdCuP, and PdPtP).

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide.

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or iPrPPHP) is used to react with FeCl2 under a carbon monoxide (CO) atmosphere to yield trans-(iPrPPHP)Fe(CO)Cl2. A subsequent reaction with NaBH4 produces syn/anti-(iPrPPHP)FeH(CO)Cl or cis,anti-(iPrPPHP)Fe(CO)H2, depending on the amount of NaBH4 employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH2OH (under H2) or PhCO2CH2Ph (under Ar). It also catalyzes the dehydrogenation of PhCH2OH to PhCHO and PhCO2CH2Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO2), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H2 while forming anti-(iPrPPHP)FeH(CO)(OCHO)·HCO2H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO2 under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Structure and Reactivity of Nonporphyrinic Terminal Manganese(IV)–Hydroxide Complexes in the Oxidative Electrophilic Reaction

High-valent transition metal-hydroxide complexes have been proposed as essential intermediates in biological and synthetic catalytic reactions. In this work, we report the single-crystal X-ray structure and spectroscopic characteristics of a mononuclear nonporphyrinic MnIV-(OH) complex, [MnIV(Me3-TPADP)(OH)(OCH2CH3)]2+ (2), using various physicochemical methods. Likewise, [MnIV(Me3-TPADP)(OH)(OCH2CF3)]2+ (3), which is thermally stable at room temperature, was also synthesized and characterized spectroscopically. The MnIV-(OH) adducts are capable of performing oxidation reactions with external organic substrates such as C-H bond activation, sulfoxidation, and epoxidation. Kinetic studies, involving the Hammett correlation and kinetic isotope effect, and product analyses indicate that 2 and 3 exhibit electrophilic oxidative reactivity toward hydrocarbons. Density functional theory calculations support the assigned electronic structure and show that direct C-H bond activation of the MnIV-(OH) species is indeed possible.

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst.

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.

Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors.

Reactive carbenes generated from diazo compounds are key intermediates for a range of organic reactions to afford synthetically useful organic compounds. The majority of these reactions have been carried out using transition metal catalysts. However, the formation of carbene intermediates using main group elements has not been widely investigated for synthetic purposes. Recent studies have demonstrated that triarylboranes can be used for the in situ generation of reactive carbene intermediates in both stoichiometric and catalytic reactions. These new reactivities of triarylboranes have gained significant attention in synthetic chemistry particularly in catalytic studies. The range of organic compounds that have been synthesized through these reactions are important as pharmaceuticals or agrochemicals. In this perspective, we highlight the recent progress and ongoing challenges of carbene transfer reactions generated from their corresponding diazo precursors using triarylboranes as catalysts. We also highlight the stoichiometric use of triarylboranes in which the boranes not only activate the diazo functionality to afford a carbene intermediate but also actively participate in the reactions as a reagent. The different mechanisms for activation and carbene transfer are described along with the mechanistic and computational studies that have aided the elucidation of these reaction pathways. Potential opportunities for the use of boranes as a catalyst toward different carbene transfer reactions and their future prospects are discussed.

Hydrogen Bonds Dominate Brønsted Acid Sites in Zeolite SSZ-42: A Classification of Their Diversity.

The zeolite catalyst SSZ‐42 shows a remarkable high abundance (≈80 %) of hydrogen‐bonded Brønsted acid sites (BAS), which are deshielded from the 1H chemical shift of unperturbed BAS at typically 4 ppm. This is due to their interaction with neighboring oxygen atoms in the zeolite framework when oxygen alignments are suitable. The classification and diversity of hydrogen bonding is assessed by DFT calculations, showing that oval‐shaped 6‐rings and 5‐rings allow for a stronger hydrogen bond to oxygen atoms on the opposite ring side, yielding higher experimental chemical shifts (δ (1H)=6.4 ppm), than circular 6‐rings (δ(1H)=5.2 ppm). Cage‐like structures and intra‐tetrahedral interactions can also form hydrogen bonds. The alignment of oxygen atoms is expected to impact their role in the stabilization of intermediates in catalytic reactions, such as surface alkoxy groups and possibly transition states.

Effects of Solvents on Adsorption Energies: A General Bond-Additivity Model

While a vast body of knowledge exists about adsorption energies of catalytic reaction intermediates on solid surfaces in gas or vacuum conditions based on experimental studies and calculations using quantum mechanics, much less is known about adsorption energies in the presence of liquid solvents. We present here a method for estimating adsorption energies in the liquid phase based on the gas-phase adsorption energy, the solvent’s adhesion energy to the solid surface, and the gas-phase adsorbate’s solvation energy. A simple bond-additivity model was recently developed for approximating the change in adsorption energy (relative to gas phase) due to the additional presence of liquid solvents using the solvent’s adhesion energy and the gaseous adsorbate’s solvation energy, but that model was limited to adsorbates whose thickness is much smaller than its lateral dimension (parallel to the surface). Here we present a simple extension of that model to adsorbates of finite thickness and general shape. We propose a model to convert the experimental solvation energy of a gaseous molecule into a molecule–solvent adhesion energy by assuming isotropic interaction of the molecule with the solvent. This adhesion energy allows us to estimate the fraction of this solvation energy that is retained when the molecule is adsorbed, based on the molecule’s shape, size, and adsorption geometry. As in the earlier bond-additivity model, adsorption energies in solvent are lower in magnitude than in the gas phase by an amount approximately equal to the adhesion energy of the solvent to the surface times the surface area of the solvent molecules displaced upon adsorption. We also report the predicted effects of different solvents for molecules on metal surfaces where solvation energies, gas-phase adsorption energies, and solvent/surface adhesion energies are available in the literature.

Elucidating molecular mechanisms of two-dimensional chemical reactions

Elucidating molecular mechanisms of two-dimensional chemical reactions

Pair sites on Al3O nodes of the metal-organic framework MIL-100: Cooperative roles of defect and structural vacancy sites in methanol dehydration catalysis

The metal–organic framework (MOF) MIL-100, which incorporates vacancy sites on Al3O nodes, was synthesized with various densities of missing-benzene-1,3,5-tricarboxylate (BTC3−)-linker defect sites by using HNO3 or acetic acid as a modulator. We report data that resolve the effects on reactivity of the node vacancies and defects that are neighbors of each other. For example, extra-framework BTC3− ligands were bonded to one vacancy site per node, and modulator-derived NO3− and acetate ligands were bonded in various ratios to defect sites. In reactions with formic acid, these ligands were replaced with monodentate and bidentate formates. The ligands on defects were found to be more reactive than those on vacancies. The MOF was used to catalyze methanol dehydration at 220 °C and 1 bar in a flow reactor, and the catalytic activity increased with increasing numbers of defects and with replacement of extra-framework BTC3- with formate. Catalysis started as methanol reacted with formate ligands to form methyl formate, which desorbed, leaving methoxy groups on the nodes—and these became catalytic reaction intermediates. IR spectra and catalyst performance data show that that catalysis proceeds as bidentate methoxy bonded to a defect site reacts with monodentate methoxy bonded to a neighboring vacancy site. Having a high density of these paired sites, MIL-100(Al) is markedly more active catalytically than the MOF MIL-53, in which these essential sites are mostly isolated. Deactivation of MIL-100 during catalysis resulted from self-inhibition, as bidentate methoxy ligands were replaced with μ2-OH ligands formed from water produced in the dehydration reaction. The results highlight the role of pair sites on MOF nodes for catalyzing a reaction that involves a bimolecular step, indicating how to control the catalytic sites. We thus foresee opportunities to tailor MOF catalytic properties for reactions involving various numbers of catalytic sites on the nodes.

Visualization of the intermediates in organic catalytic reaction by single-molecule electrical spectroscopy

The capture and characterization of highly reactive and short-lived intermediates in organic catalytic reactions are challenging yet are very important for mechanism elucidation. Recently in <i>Matter</i>, Guo and co-workers employed single-molecule electrical spectroscopy (SMES) to detect the key intermediate and analyze the mechanism of <i>N</i>-heterocyclic carbene (NHC) catalyzed benzoin reactions.

Au@ZIF-8 Core–Shell Nanoparticles as a SERS Substrate for Volatile Organic Compound Gas Detection

Surface-enhanced Raman spectroscopy (SERS) is a promising ultrasensitive analysis technology due to outstanding molecular fingerprint identification. However, the measured molecules generally need to be adsorbed on a SERS substrate, which makes it difficult to detect weakly adsorbed molecules, for example, the volatile organic compound (VOC) molecules. Herein, we developed a kind of a SERS detection method for weak adsorption molecules with Au@ZIF-8 core-shell nanoparticles (NPs). The well-uniformed single- and multicore-shell NPs can be synthesized controllably, and the shell thickness of the ZIF-8 was able to be precisely controlled (from 3 to 50 nm) to adjust the distance and electromagnetic fields between metal nanoparticles. After analyzing the chemical and physical characterization, Au@ZIF-8 core-shell NPs were employed to detect VOC gas by SERS. In contrast with multicore or thicker-shell nanoparticles, Au@ZIF-8 with a shell thickness of 3 nm could efficiently probe various VOC gas molecules, such as toluene, ethylbenzene, and chlorobenzene. Besides, we were capable of observing the process of toluene gas adsorption and desorption using real-time SERS technology. As observed from the experimental results, this core-shell nanostructure has a promising prospect in diverse gas detection and is expected to be applied to the specific identification of intermediates in catalytic reactions.