Key Elementary Steps Research Articles

First-principles study of selective CO2 activation to methanol on PdZn/TiO2: Unveiling Zn/Pd ratio on catalysis performance

In this study, a series of Pd(8-n)Znn/TiO2(n=0–8) were investigated via density functional theory (DFT) to understand the influence of Zn/Pd ratio on their catalysis performance of conversion CO2 to methanol. It is revealed that Zn prefers to replace the bottom-layered Pd when its molar concentration in composition increases. For all surface models, interface between cluster and support plays a key role in CO2 stabilization and activation. Increasing Zn/Pd ratio could weaken the binding strength of CO2 over catalyst, which further hinders the “RWGS” pathway while promotes the “Formate” pathway. Based on the calculation results, a Brønsted–Evans–Polanyi (BEP) relation between the activation barrier (Ea) of key elementary steps from both mechanisms and the binding strength of key intermediates has been established, from which the optimum Zn/Pd ratio leading to the best catalysis performance has been determined. Moreover, it is found that water inclusion in the system does not change the Ea of rate-determining step much even though it has a promotion effect on OH formation steps. Overall, this work provides some meaningful insight into the influence of cluster composition on the catalysis performance of PdZn-based catalyst and provides valuable information about rational design of the supported bimetallic catalyst.

Remote Functionalization by Pd-Catalyzed Isomerization of Alkynyl Alcohols.

In recent years, progress has been made in the development of catalytic methods that allow remote functionalizations based on alkene isomerization. In contrast, protocols based on alkyne isomerization are comparatively rare. Herein, we report a general Pd-catalyzed long-range isomerization of alkynyl alcohols. Starting from aryl-, heteroaryl-, or alkyl-substituted precursors, the optimized system provides access preferentially to the thermodynamically more stable α,β-unsaturated aldehydes and is compatible with potentially sensitive functional groups. We showed that the migration of both π-components of the carbon-carbon triple bond can be sustained over several methylene units. Computational investigations served to shed light on the key elementary steps responsible for the reactivity and selectivity. These include an unorthodox phosphine-assisted deprotonation rather than a more conventional β-hydride elimination in the final tautomerization event.

Elucidating the Reactivity of Oxygenates on Single-Atom Alloy Catalysts.

Doping isolated transition metal atoms into the surface of coinage-metal hosts to form single-atom alloys (SAAs) can significantly improve the catalytic activity and selectivity of their monometallic counterparts. These atomically dispersed dopant metals on the SAA surface act as highly active sites for various bond coupling and activation reactions. In this study, we investigate the catalytic properties of SAAs with different bimetallic combinations [Ni-, Pd-, Pt-, and Rh-doped Cu(111), Ag(111), and Au(111)] for chemistries involving oxygenates relevant to biomass reforming. Density functional theory is employed to calculate and compare the formation energies of species such as methoxy (CH3O), methanol (CH3OH), and hydroxymethyl (CH2OH), thereby understanding the stability of these adsorbates on SAAs. Activation energies and reaction energies of C-O coupling, C-H activation, and O-H activation on these oxygenates are then computed. Analysis of the data in terms of thermochemical linear scaling and Bro̷nsted-Evans-Polanyi relationship shows that some SAAs have the potential to combine weak binding with low activation energies, thereby exhibiting enhanced catalytic behavior over their monometallic counterparts for key elementary steps of oxygenate conversion. This work contributes to the discovery and development of SAA catalysts toward greener technologies, having potential applications in the transition from fossil to renewable fuels and chemicals.

Molecular Bio-inspired Strategies for the Design of Electrocatalytic Systems.

This perspective article delves into the realm of bio-inspired catalysis, highlighting the valuable insights gleaned from enzymatic systems for the design of advanced electrocatalysts. We focus here on three key aspects to mimic: the structure of enzymatic active sites, the essential functions to enable catalytic activity as well as key elementary steps of reaction mechanisms employed by enzymes to ensure maximum efficiency and selectivity. Our research group's contributions to these areas are highlighted, including the synthesis and catalytic activity of cobalt(III) pyridinethiolate complexes, the exploration of bimetallic sites mimicking biological carbon dioxide reductases and of all-ferrous Fe4S4 clusters as mimics of FeP active sites, and the integration of concerted proton-electron transfer (CPET) mediators for the generation of metal hydride species. We emphasize the potential of these bio-inspired approaches in advancing electrocatalyst design and their relevance to molecular catalysis.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation.

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Informing air-carbon ablation modeling with theoretical calculations of atomic oxygen and nitrogen interacting with carbon surfaces.

To understand the gas-surface chemistry above the thermal protection system of a hypersonic vehicle, it is necessary to map out the kinetics of key elementary reaction steps. In this work, extensive periodic density functional theory (DFT) calculations are performed to elucidate the interaction of atomic oxygen and nitrogen with both the basal plane and edge sites of highly oriented pyrolytic graphite (HOPG). Reaction energies and barriers are determined for adsorption, desorption, diffusion, recombination, and several reactions. These DFT results are compared with the most recent finite-rate model for air-carbon ablation. Our DFT results corroborated some of the parameters used in the model but suggest that further refinement may be necessary for others. The calculations reported here will help to establish a predictive kinetic model for the complex reaction network present under hypersonic flight conditions.

Cobalt-Catalyzed Asymmetric Reductive Dicarbofunctionalization of 1,3-Dienes with o-Bromoaryl Imines as a Bis-Electrophile

Herein, we report a cobalt-catalyzed asymmetric reductive 1,2-dicarbofunctionalization of 1,3-dienes employing o-bromoaryl imines as a bis-electrophilic coupling partner. This method provides an entry to prepare disubstituted cis-indanes in high diastereo- and enantioselectivities under mild reaction conditions. The proposed reaction mechanism consists of enantioselective intermolecular migratory insertion and diastereoselective intramolecular allylation as the key elementary steps.

Mechanistic Studies of Carbonyl Allylation Mediated by (NHC)CuH: Isoprene Insertion, Allylation, and β-Hydride Elimination.

The ability of Cu-H complexes to undergo selective insertion of unsaturated hydrocarbons under mild conditions has rendered them valuable, versatile catalysts. The direct formation of Cu allyl intermediates from unfunctionalized 1,3-dienes and transient Cu hydrides is an appealing strategy for upgrading conjugated diene feedstocks. However, empirical mechanistic studies of the underlying elementary steps and characterization of key intermediates in Cu-H catalysis are sparse. Using [(NHC)CuH]2 (NHC = N-heterocyclic carbene), we examined the steric effects of NHC ligands on two key elementary steps of CuH-catalyzed carbonyl allylation: the insertion of a diene into the Cu-H bond to produce a Cu-allyl complex, and the formation of C-C bonds from stoichiometric allylations of ketones and aldehydes. The resulting allyl and homoallylic alkoxide complexes have been characterized by NMR spectroscopy and single-crystal X-ray diffraction. Employing isolable (NHC)Cu-allyl complexes, we further evaluated the roles of the ligand size, electronic properties of carbonyl substrates, coordinating groups within the substrate, and solvent on the regioselectivity, diastereoselectivity, and relative rate of the C-C bond formation step. In contrast to the clean allylation of ketones, allylation of aldehydes provided a rare example of a formal β-hydride elimination reaction from a secondary homoallylic alkoxide species. Mechanistic studies of key elementary steps provide insights for a range of catalytic reactions of dienes mediated by hydride complexes.

Potential dependence of OER/EOP performance on heteroatom-doped carbon materials by grand canonical density functional theory.

Revealing the effect of external applied potential on the reaction mechanism and product selectivity is of great significance in electrochemical studies. In this work, the grand canonical density functional theory method was applied to simulate the explicit electrocatalytic process of oxygen evolution reaction and electrochemical ozone production due to the O3 product sensitivity toward the applied potential. Over the Pt/Pd single atom embedded on B/N co-doped graphene (Pt/Pd-BNC) surface, crossover points of O2/O3 selectivity inversion were predicted to be 1.33 and 0.89V vs standard hydrogen electrode, which were also consistent with the previous experimental results. An in-depth analysis of the energetic terms in the reaction free energies also found the considerable impact of the applied potential on the Helmholtz free energy term, with optimal potential predicted for the key elementary steps, and linear correlations between electrode potential (U) and reaction free energy were found for each elementary step. This study offers extensive knowledge on the potential effect on the O2/O3 selective formation on two-dimensional anode surfaces and provides new insights for investigating the reactivity/selectivity on electrode surfaces in real reaction conditions.

Zincate‐Mediated Remote Functionalisation of p‐Iodobenzyl Derivatives Through Metallotropy in 2‐Methyltetrahydrofuran as Key Solvent

Abstract2‐MeTHF (2‐methyltetrahydrofuran) promotes the zincate‐mediated remote functionalisation of p‐iodobenzyl derivatives through metallotropy with a broad scope. This biosourced solvent remarkably impacted all the key elementary steps involved in the tandem reaction. The method tolerates a wide range of sensitive functionalities within the substrates as well as enolisable and activated or deactivated aromatic aldehydes as reaction partners. A challenging benzyl naphthalene derivative was even reacted through an original dearomatisation/rearomatisation/metallotropy chain.magnified image

The Homogeneous Gas-Phase Formation Mechanism of PCNs from Cross-Condensation of Phenoxy Radical with 2-CPR and 3-CPR: A Theoretical Mechanistic and Kinetic Study.

Chlorophenols (CPs) and phenol are abundant in thermal and combustion procedures, such as stack gas production, industrial incinerators, metal reclamation, etc., which are key precursors for the formation of polychlorinated naphthalenes (PCNs). CPs and phenol can react with H or OH radicals to form chlorophenoxy radicals (CPRs) and phenoxy radical (PhR). The self-condensation of CPRs or cross-condensation of PhR with CPRs is the initial and most important step for PCN formation. In this work, detailed thermodynamic and kinetic calculations were carried out to investigate the PCN formation mechanisms from PhR with 2-CPR/3-CPR. Several energetically advantageous formation pathways were obtained. The rate constants of key elementary steps were calculated over 600~1200 K using the canonical variational transition-state theory (CVT) with the small curvature tunneling (SCT) contribution method. The mechanisms were compared with the experimental observations and our previous works on the PCN formation from the self-condensation of 2-CPRs/3-CPRs. This study shows that naphthalene and 1-monochlorinated naphthalene (1-MCN) are the main PCN products from the cross-condensation of PhR with 2-CPR, and naphthalene and 2-monochlorinated naphthalene (2-MCN) are the main PCN products from the cross-condensation of PhR with 3-CPR. Pathways terminated with Cl elimination are preferred over those terminated with H elimination. PCN formation from the cross-condensation of PhR with 3-CPR can occur much easier than that from the cross-condensation of PhR with 2-CPR. This study, along with the study of PCN formation from the self-condensation 2-CPRs/3-CPRs, can provide reasonable explanations for the experimental observations that the formation potential of naphthalene is larger than that of 1-MCN using 2-CP as a precursor, and an almost equal yield of 1-MCN and 2-MCN can be produced with 3-CP as a precursor.

Potential-Dependent Free Energy Relationship in Interpreting the Electrochemical Performance of CO2 Reduction on Single Atom Catalysts

Acquiring the fundamental understanding of electrochemical processes occurring at the complex electrode–liquid interface is a grand challenge in catalysis. Herein, to gain theoretical insights into the experimentally observed potential-dependent activity and selectivity for the CO2 reduction reaction (CO2RR) on the popular single-iron-atom catalyst, we performed ab initio molecular dynamics (AIMD) simulation, constrained MD sampling, and thermodynamic integration to acquire the free energy profiles for the proton and electron transfer processes of CO2 at different potentials. We have demonstrated that the adsorption of CO2 is significantly coupled with the electron transfer from the substrate while the further protonation does not show distinct charge variation. This strongly suggests that CO2 adsorption is potential-dependent and optimizing the electrode potential is vital to achieve the efficient activated adsorption of CO2. We further identified a linear scaling relationship between the reaction free energy (ΔG) and the potential for key elementary steps of CO2RR and HER, of which the slope is adsorbate-specific and not as simple as 1 eV per volt as suggested by the traditional computational hydrogen electrode (CHE) model. The derived scaling relationship can reproduce the experimental onset potential (Uonset) of CO2RR, potential of the maximal CO2-to-CO Faraday efficiency (FECO), and potential where FECO = FEH2. This suggests that our state-of-the-art model could precisely interpret the activity and selectivity of CO2RR/HER on the Fe-N4-C catalyst under different electrode potentials. In general, our study not only provides an innovative insight into the theoretical explanation of the origin of the solvation effect from the perspective of charge transfer but also emphasizes the critical role of electrode potential in the theoretical consideration of catalytic activity, which offers a profound understanding of the electrochemical environment and bridges the gap between theoretical predictions and experimental results.

Strategies for accessing photosensitizers with extreme redox potentials

Photoredox catalysis has been prominent in many applications, including solar fuels, organic synthesis, and polymer chemistry. Photocatalytic activity directly depends on the photophysical and electrochemical properties of photocatalysts in both the ground state and excited state. Controlling those properties, therefore, is imperative to achieve the desired photocatalytic activity. Redox potential is one important factor that impacts both the thermodynamic and kinetic aspects of key elementary steps in photoredox catalysis. In many challenging reactions in organic synthesis, high redox potentials of the substrates hamper the reaction, leading to slow conversion. Thus, the development of photocatalysts with extreme redox potentials, accompanied by potent reducing or oxidizing power, is required to execute high-yielding thermodynamically demanding reactions. In this review, we will introduce strategies for accessing extreme redox potentials in photocatalytic transformations. These include molecular design strategies for preparing photosensitizers that are exceptionally strong ground-state or excited-state reductants or oxidants, highlighting both organic and metal-based photosensitizers. We also outline methodological approaches for accessing extreme redox potentials, using two-photon activation, or combined electrochemical/photochemical strategies to generate potent redox reagents from precursors that have milder potentials.

A Triple Photoredox/Cobalt/Brønsted Acid Catalysis Enabling Markovnikov Hydroalkoxylation of Unactivated Alkenes.

We demonstrate Markovnikov hydroalkoxylation of unactivated alkenes using alcohols through a triple catalysis consisting of photoredox, cobalt, and Brønsted acid catalysts under visible light irradiation. The triple catalysis realizes three key elementary steps in a single catalytic cycle: (1) Co(III) hydride generation by photochemical reduction of Co(II) followed by protonation, (2) metal hydride hydrogen atom transfer (MHAT) of alkenes by Co(III) hydride, and (3) oxidation of the alkyl Co(III) complex to alkyl Co(IV). The precise control of protons and electrons by the three catalysts allows the elimination of strong acids and external reductants/oxidants that are required in the conventional methods.

Solvent-Assisted Ketone Reduction by a Homogeneous Mn Catalyst.

The choice of a solvent and the reaction conditions often defines the overall behavior of a homogeneous catalytic system by affecting the preferred reaction mechanism and thus the activity and selectivity of the catalytic process. Here, we explore the role of solvation in the mechanism of ketone reduction using a model representative of a bifunctional Mn-diamine catalyst through density functional theory calculations in a microsolvated environment by considering explicit solvent and fully solvated ab initio molecular dynamics simulations for the key elementary steps. Our computational analysis reveals the possibility of a Meerwein–Ponndorf–Verley (MPV) type mechanism in this system, which does not involve the participation of the N–H moiety and the formation of a transition-metal hydride species in ketone conversion. This path was not previously considered for Mn-based metal–ligand cooperative transfer hydrogenation homogeneous catalysis. The MPV mechanism is strongly facilitated by the solvent molecules present in the reaction environment and can potentially contribute to the catalytic performance of other related catalyst systems. Calculations indicate that, despite proceeding effectively in the second coordination sphere of the transition-metal center, the MPV reaction path retains the enantioselectivity preference induced by the presence of the small chiral N,N′-dimethyl-1,2-cyclohexanediamine ligand within the catalytic Mn(I) complex.

Theoretical evidence for the formation of perfluorocarboxylic acids form atmospheric oxidation degradation of fluorotelomer acrylates.

The atmospheric oxidation degradation of fluorotelomer acrylates (FTAcs) has been proposed as a potential source of perfluorocarboxylic acids (PFCAs) in remote locations. In this paper, detailed reactions of the main oxidant OH radicals with 4:2 FTAc in the atmosphere have been investigated by using density functional theory (DFT) calculation. All possible pathways involved in the oxidation process were presented and discussed. Based on the mechanism, transition state theory (TST) was used to predict the rate constants of the key elementary steps including the initial reactions of OH radical with n:2 FTAcs and the subsequent reactions of the main intermediates. Studies show that the reaction processes of OH radical addition to C = C bond are dominant and the fluorotelomer glyoxylate and formaldehyde are the major products. At 296K, the calculated overall rate constant of 4:2 FTAc with OH radical is 1.19 × 10-11 cm3 molecule-1s-1 with an atmospheric lifetime of 23.3h. In the atmosphere, fluorotelomer glyoxylate will continue to be oxidized, which will lead to the formation of PFCAs ultimately. In addition, atmospheric reactions of more carbons FTAc (CnF2n+1CH2CH2OC(O)CH = CH2, n = 6, 8, 10) are also discussed in the presence of O2/NOx.

On the Volmer reaction mechanism

Electrochemistry Proton-coupled electron transfer reactions at an electrochemical interface, such as electrosorption of a solvated proton (the Volmer reaction), are the key elementary steps in many electrocatalytic processes. Their characterization is challenging mainly because of the presence of an electric double layer (EDL). Using operando vibrational spectroscopy accompanied by theoretical analysis, Sarkar et al. identified the main stages of the Volmer reaction corresponding to proton discharge from tertiary ammonium with a benzonitrile vibrational tag to a silver electrode surface, and revealed the role of the EDL in this process. They anticipate that their mechanistic insights are general and may be useful in the rational design of materials for electrochemical energy storage and conversion. J. Am. Chem. Soc. 143 , 8381 (2021).

Influence of a Cu-zirconia interface structure on CO2 adsorption and activation.

CO2 adsorption and activation on a catalyst are key elementary steps for CO2 conversion to various valuable products. In the present computational study, we screened different Cu-ZrO2 interface structures and analyzed the influence of the interface structure on CO2 binding strength using density functional theory calculations. Our results demonstrate that a Cu nanorod favors one position on both tetragonal and monoclinic ZrO2 surfaces, where the bottom Cu atoms are placed close to the lattice oxygens. In agreement with previous calculations, we find that CO2 prefers a bent bidentate configuration at the Cu-ZrO2 interface and the molecule is clearly activated being negatively charged. Straining of the Cu nanorod influences CO2 adsorption energy but does not change the preferred nanorod position on zirconia. Altogether, our results highlight that CO2 adsorption and activation depend sensitively on the chemical composition and atomic structure of the interface used in the calculations. This structure sensitivity may potentially impact further catalytic steps and the overall computed reactivity profile.

Designing Copper-Based Catalysts for Efficient Carbon Dioxide Electroreduction.

The electroreduction of carbon dioxide (CO2 ) has been emerging as a high- potential approach for CO2 utilization using renewables. When copper (Cu) based catalysts are used, this platform can produce multi-carbon (C2+ ) fuels and chemicals with almost net-zero emission, contributing to the closure of the anthropogenic carbon cycle. Nonetheless, the rational design and development of Cu-based catalysts are critical toward the realization of highly selective and efficient CO2 electroreduction. In this review, first the latest advances in Cu-catalyzed CO2 electroreduction in the product selectivity and electrocatalytic activity are briefly summarized. Then, recent theoretical and mechanistic studies of CO2 electroreduction on Cu-based catalysts are investigated, which serve as programs to design catalysts. Strategies for devising Cu catalysts that aim at promoting different key elementary steps for hydrocarbon and C2+ oxygenates production are further summarized. Moreover, challenges in understanding the mechanism, operando investigation of Cu catalysts and reactions, and systems' influences are also presented. Finally, the future prospects of CO2 electroreduction are discussed.

Mechanistic investigation of Rh(i)-catalysed asymmetric Suzuki–Miyaura coupling with racemic allyl halides

Understanding how catalytic asymmetric reactions with racemic starting materials can operate would enable new enantioselective cross-coupling reactions that give chiral products. Here we propose a catalytic cycle for the highly enantioselective Rh(i)-catalysed Suzuki–Miyaura coupling of boronic acids and racemic allyl halides. Natural abundance 13C kinetic isotope effects provide quantitative information about the transition-state structures of two key elementary steps in the catalytic cycle, transmetallation and oxidative addition. Experiments with configurationally stable, deuterium-labelled substrates revealed that oxidative addition can happen via syn- or anti-pathways, which control diastereoselectivity. Density functional theory calculations attribute the extremely high enantioselectivity to reductive elimination from a common Rh complex formed from both allyl halide enantiomers. Our conclusions are supported by analysis of the reaction kinetics. These insights into the sequence of bond-forming steps and their transition-state structures will contribute to our understanding of asymmetric Rh–allyl chemistry and enable the discovery and application of asymmetric reactions with racemic substrates. A major drive in current chemistry research is to develop asymmetric versions of widely used carbon–carbon bond-forming reactions, such as Suzuki-Miyaura cross-couplings. Now, the origins of diastereo- and enantioselectivity in a Rh-catalysed cross-coupling of boronic acid and racemic allyl halides have been established.