Search Results (75)

Ammonia synthesis remains a cornerstone of global chemical manufacturing, essential for fertilizer production, energy storage, and emerging carbon capture technologies. This overview examines recent developments in the understanding of elementary reaction mechanisms in heterogeneous catalysis, with emphasis on transition metal thermocatalysts operating under the Haber–Bosch process. Traditionally, the dissociative adsorption of nitrogen (N₂) has been considered the rate-determining step. However, recent studies challenge this view, revealing possible shifts in rate-determining steps and suggesting that alternative mechanistic pathways may be operative. The discussion critiques studies that adhere strictly to the classic dissociative mechanism—often inferred from the reaction order of N₂—while overlooking alternative pathways that could offer more efficient catalytic routes and deeper mechanistic insight into ammonia synthesis. These insights offer a pathway toward more rational catalyst design and improved process efficiency in ammonia synthesis. Full article

Hydrogen (H₂) has become a more important alternative source in the current energy transition process. Beyond its role in clean energy production, it also serves as a key reactant in a wide range of industrial chemical transformations, such as hydrogenation and hydroprocessing. A fundamental step in many of these processes is the dissociation of hydrogen on catalyst surfaces. This short review provides an overview of the fundamental mechanisms involved in hydrogen dissociation over catalysts, with a specific emphasis on heterolytic pathways. Meanwhile, the influence of surface coordination environments on hydrogen activation is discussed, focusing on key factors—Lewis acid–base pairs, lattice oxygen and oxygen vacancies, and metal–support interfaces. With recognizing the significance of understanding the reaction mechanisms, we provide a critical review of experimental techniques, including spectroscopy, temperature-programmed methods, and kinetic analysis, that have been successfully applied or appear promising for probing active sites, reaction dynamics, chemisorbed intermediates, and elementary steps. Our goal is to highlight how these techniques contribute to a mechanistic understanding and to outline future directions, making this review a valuable resource for both new and experienced researchers. Full article

Density functional theory calculations have been performed to explore the detailed mechanism of a ruthenium-catalyzed dehydrogenative annulation between α-carbonyl phosphonium ylide (A) and sulfoxonium ylide (B). The proposed catalytic cycles consist of several elementary steps in succession, namely the C–H activation of ylide A, the insertion of ylide B, reductive elimination, protodemetallation, and an intramolecular Wittig reaction, in which C–H activation is rate-limiting, with a free energy barrier of 31.7 kcal/mol. As A and B are both capable of being a C–H activation substrate and a carbene precursor, there are potentially four competing pathways including homo-coupling reactions. Further calculations demonstrate that A is more reactive in the C–H activation step than B, while the opposite conclusion is true for the ylide insertion step, which can successfully explain the fact that the solely observed product originated from the use of A as the C–H activation substrate and B as the carbene precursor. Molecular electrostatic potential, charge decomposition, and electron density difference analyses were performed to understand the distinct behaviors of the two ylides and the nature of the key ruthenium–carbene intermediate. Full article

We address the scientific “time” concept in the context of more general relaxation processes toward the Wärmetod of thermodynamic equilibrium. More specifically, we sketch a construction of a conceptual ladder of chemical reaction steps that can rigorously bridge a description from the microscopic domain of molecular quantum chemistry to the macroscopic materials domain of Gibbsian thermodynamics. This conceptual reformulation follows the pioneering work of Kenichi Fukui (Nobel 1981) in rigorously formulating the intrinsic reaction coordinate (IRC) pathway for controlled description of non-equilibrium passages between reactant and product equilibrium states of an overall material transformation. Elementary chemical reaction steps are thereby identified as the logical building-blocks of an integrated mathematical framework that seamlessly spans the gulf between classical (pre-1925) and quantal (post-1925) scientific conceptions and encompasses both static and dynamic aspects of material change. All modern chemical reaction rate studies build on the apparent infallibility of quantum-chemical solutions of Schrödinger’s wave equation and its Dirac-type relativistic corrections. This infallibility may now be properly accepted as an added“inductive law” of Gibbsian chemical thermodynamics, the only component of 19th-century physics that passed intact through the revolutionary quantum upheavals of 1925. Full article

The purpose of this paper is to review the basic steps in the development of the atomic–molecular theory with the support of the original documentation. The most important contributions are examined, from the first intimation of the theory by Dalton to the Avogadro hypothesis, intended to reconcile empirical relations observed by Gay-Lussac in gaseous reactions with the atomistic ideas. The genesis and the structure of Dalton’s theory are described, and attention is focused on the rules of the greatest simplicity by means of which the relative weights and sizes of elementary and compounds atoms were determined. The first striking achievements are outlined. A peculiar trait of the Daltonian principles is represented by the rejection of a simple relation between specific gravities of elastic fluids and weights of the ultimate particles. On the contrary, the fixed (and small) ratios of reacting volumes in gaseous reactions, a result discovered by Gay-Lussac, suggest that particles combine according to simple numbers. The equal volume/equal number of particles hypothesis was a substitute better than Dalton’s rules for the determination of the atomic weights and led to the prediction of elementary polyatomic molecules. Full article

Fuel cells/zinc–air cells represent a transformative technology for clean energy conversion, offering substantial environmental benefits and exceptional theoretical efficiency. However, the high cost and limited durability of platinum-based catalysts for the sluggish oxygen reduction reaction (ORR) at the cathode severely restrict their scalability and practical application. To address these critical challenges, this study explores a groundbreaking approach to developing ORR catalysts with enhanced performance and reduced costs. We present a novel Pd₃Cu alloy, innovatively modified with N-doped carbon aerogels, synthesized via a simple self-assembly and freeze-drying method. The three-dimensional carbon aerogel-based porous structures provide diffusion channels for oxygen molecules, excellent electrical conductivity, and abundant ORR reaction sites. The Pd₃Cu@2NC-20% aerogel exhibits a remarkable enhancement in ORR activity, achieving a half-wave potential of 0.925 V, a limiting current density of 6.12 mA/cm², and excellent long-term stability. Density functional theory (DFT) calculations reveal that electrons tend to transfer from the Pd atoms to the neighboring *O, leading to an increase in the negative charge around the *O. This, in turn, weakens the interaction between the catalyst surface and the *O and optimizes the elementary steps of the ORR process. Full article

The co-production of CO₂ continues to remain the bane of several hydrogen production technologies, including the steam reforming of methane and the dry reforming of methane processes. Efficient utilization of abundant greenhouse gas in the form of methane provides opportunities for the design of an innovative system that will maximize the use of such a raw material in the most environmentally friendly manner. The study of the mechanism of the pyrolysis of methane reactions over molten metals provides promise for improved hydrogen yield and methane conversion with a greater turnover frequency. Catalyst electronic properties computed via Density Functional Theory using the Quantum Espresso code provided data that were built into a database. Using Bismuth as the base metal, active transition metals including Ni, Cu, Pd, Pt, Ag, and Au of different concentrations of 5, 10, 15, and 25% were placed on 96 atoms of the base metal and relaxed to obtain the optimized geometric structures for the catalytic reaction studies. The kinetics of the individual elementary steps of the pyrolysis reaction at preset temperatures over the bi-metals were calculated using the Car-Parinello (CP) method and Nudge Elastic Band (NEB) computations. The collated data of the various pyrolysis of methane reactions over the different bi-metals was used to train machine learning models for the prediction of reaction outcome, catalytic performance, and efficient operating conditions for the pyrolysis of methane over molten metals. The turnover frequency, which is determined using the transition state energies of the fundamental reaction cycles, will be used to simulate the stability of the catalyst. Full article

The use of a single-atom FeN₄ catalyst on defective graphene (Fe-NC) has recently emerged as an effective method for the synthesis of amino acids. Herein, we investigated the mechanism of alanine formation on FeN₄-doped graphene using comprehensive density functional theory (DFT) computations. The alanine formation reaction begins with the activation of NO molecules on the surface, followed by their reaction with hydrogen atoms provided in the system. The computational results show that NO molecules can be effectively activated on Fe-NC, facilitating the subsequent alanine formation at a relatively lower potential. The potential-limiting step in alanine production involves either the formation of HNO* or HNOH* intermediates on Fe-NG, as the free energy changes (ΔG) in these two elementary steps are nearly equivalent. Notably, the formation of HNO* exhibits a higher activation energy (E_a) compared to HNOH* formation. This study provides valuable insights into the C–N coupling reaction and the mechanism of amino acid synthesis on single-atom catalysts. Full article

The reactivity of nitrogen oxide, NO, as a ligand in complexes with [Fe2-S2] and [Co2-S2] non-planar rhombic cores is examined by density functional theory (DFT). The cobalt-containing nitrosyl complexes are less stable than the iron complexes because the Co-S bonds in the [Co2-S2] core are weakened upon NO coordination. Various positions of NO were examined, including its binding to sulfur centers. The release of NO molecules can be monitored photochemically. The ability of NO to form a (NO)₂ dimer provides a favorable route of electrochemical reduction, as protonation significantly stabilizes the dimeric species over the monomers. The quasilinear dimer ONNO, with trans-orientation of oxygen atoms, gains higher stability under protonation and reduction via proton–electron transfer. The first two reduction steps lead to an N₂O intermediate, whose reduction is more energy demanding: in the two latter reaction steps the highest energy barrier for Co₂S₂(CO)₆ is 109 kJ mol⁻¹, and for Fe₂S₂(CO)₆, it is 133 kJ mol⁻¹. Again, the presence of favorable light absorption bands allows for a photochemical route to overcome these energy barriers. All elementary steps are exothermic, and the final products are molecular nitrogen and water. Full article

The increase in the resistance of mutant strains of Neisseria gonorrhoeae to the antibiotic ceftriaxone is pronounced in the decrease in the second-order acylation rate constant, k₂/K_S, by penicillin-binding protein 2 (PBP2). These changes can be caused by both the decrease in the acylation rate constant, k₂, and the weakening of the binding affinity, i.e., an increase in the substrate constant, K_S. A501X mutations in PBP2 affect second-order acylation rate constants. The PBP2^A501V variant exhibits a higher k₂/K_S value, whereas for PBP2^A501R and PBP2^A501P variants, these values are lower. We performed molecular dynamic simulations with both classical and QM/MM potentials to model both acylation energy profiles and conformational dynamics of four PBP2 variants to explain the origin of k₂/K_S changes. The acylation reaction occurs in two elementary steps, specifically, a nucleophilic attack by the oxygen atom of the Ser310 residue and C–N bond cleavage in the β-lactam ring accompanied by the elimination of the leaving group of ceftriaxone. The energy barrier of the first step increases for PBP2 variants with a decrease in the observed k₂/K_S value. Submicrosecond classic molecular dynamic trajectories with subsequent cluster analysis reveal that the conformation of the β₃–β₄ loop switches from open to closed and its flexibility decreases for PBP2 variants with a lower k₂/K_S value. Thus, the experimentally observed decrease in the k₂/K_S in A501X variants of PBP2 occurs due to both the decrease in the acylation rate constant, k₂, and the increase in K_S. Full article

Silica (SiO₂), accounting for the main component of fly ash, plays a vital role in the heterogeneous formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) in high-temperature industrial processes. Silica clusters, as the basic units of silica, provide reasonable models to understand the general trends of complex surface reactions. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. By employing density functional theory, this study examined the formation of 2-chlorothiophenolate from 2-CTP adsorbed on the dehydrated silica cluster ((SiO₂)₃) and the hydroxylated silica cluster ((SiO₂)₃O₂H₄). Additionally, this study investigated the formation of pre-PCTA/DTs, the crucial intermediates involved in PCTA/DT formation, from the coupling of two adsorbed 2-chlorothiophenolates via the Langmuir–Hinshelwood (L–H) mechanism and the coupling of adsorbed 2-chlorothiophenolate with gas-phase 2-CTP via the Eley–Rideal (E–R) mechanism on silica clusters. Moreover, the rate constants for the main elementary steps were calculated over the temperature range of 600–1200 K. Our study demonstrates that the 2-CTP is more likely to adsorb on the termination of the dehydrated silica cluster, which exhibits more effective catalysis in the formation of 2-chlorothiophenolate compared with the hydroxylated silica cluster. Moreover, the E–R mechanism mainly contributes to the formation of pre-PCTAs, whereas the L–H mechanism is prone to the formation of pre-PCDTs on dehydrated and hydroxylated silica clusters. Silica can act as a relatively mild catalyst in facilitating the heterogeneous formation of pre-PCTA/DTs from 2-CTP. This research provides new insights into the surface-mediated generation of PCTA/DTs, further providing theoretical foundations to reduce dioxin emission and establish dioxin control strategies. Full article

Molten gallium trichloride emerges as a promising solvent for oxidative metal recycling. The use of supercritical fluid enhances the performance and kinetics of metal dissolution due to significantly lower viscosity in the reaction media. Additionally, the dual molecular nature of gallium trichloride, existing as edge-sharing ES-Ga₂Cl₆ dimers at low temperatures and high pressure, or flat trigonal GaCl₃ monomers in the vicinity of the critical point and low pressures, creates the possibility to tailor the chemical geometry to a particular metallic species. Nevertheless, the mechanism of dimer dissociation, local structure, and atomic dynamics in supercritical gallium trichloride fluids are not known. Using first-principles molecular dynamics, validated by comparison with our high-energy X-ray diffraction results, we illustrate the elementary steps in dimer dissociation. These include the formation of intermediate corner-sharing CS-Ga₂Cl₆ dimers, the partial disproportionation of GaCl₃ monomers at high temperatures and low pressures, changes in the local environment of molecular entities, and unusual atomic dynamics in supercritical fluids. Full article

This paper deals with the proposition of a kinetic model for the direct synthesis of DME via CO₂ hydrogenation in view of the necessary optimization of the catalytic system, reactor design, and process strategy. Despite the fact that DME synthesis is typically treated as a mere combination of two separated catalytic steps (i.e., methanol synthesis and methanol dehydration), the model analysis is now proposed by taking into account the improvements related to the process running over a hybrid catalyst in a rational integration of the two catalytic steps, with boundary conditions properly assumed from the thermodynamics of direct DME synthesis. Specifically, the CO₂ activation step at the metal–oxide interface in the presence of ZrO₂ has been described for the first time through the introduction of an ad hoc mechanism based on solid assumptions from inherent studies in the literature. The kinetic modeling was investigated in a tubular fixed-bed reactor operating from 200 to 260 °C between 1 and 50 bar as a function of a gas hourly space velocity ranging from 2500 to 60,000 NL/kg_cat/h, in a stoichiometric CO₂/H₂ feed mixture of 1:3 v/v. A well-detailed elementary mechanism was used to predict the CO₂ conversion rate and identify the key reaction pathways, starting with the analysis of the implicated reactions and corresponding kinetic mechanisms and expressions, and finally estimating the main parameters based on an appropriate modeling of test conditions. Full article

Surface geometry at the atomic level is an important factor related to the activity of a catalytic site. It is important to identify sites with high activity to comprehend the performance of a given catalyst. In this work, it is proposed that the optimal surface for a given reaction step should satisfy the condition $\frac{\partial E}{\partial x_i}{|}_{TS}=0$, where E is the transition state energy and x_i is any variable characterizing the surface. Taking three elementary steps as examples, it is shown that the optimal site found by this method has significantly reduced TS (transition state) energy compared with facets commonly applied in previous studies, and, thus, it can be several orders more active. The method provides an insight into the geometric impact of catalysis, gives a blueprint for an ideal catalyst surface structure, and, thus, provides guidance for catalyst development. Full article

The stereoselective polymerization of conjugated dienes promoted by using transition metal complexes has attracted much interest in both industrial and academic environments for the relevance of polydienes as synthetic rubbers and for the challenging reaction mechanisms. Among the different transition metal complexes, those based on group IV have been demonstrated to be versatile and efficient catalysts. Titanium complexes are generally more active than zirconium complexes. A rare exception to this trend is represented by a series of Zr(IV) complexes supported by (anilidomethyl)pyridine ligands that, after activation by using Al(iBu₂H)/MAO, were found to be highly active affording exclusively cis-1,4-polybutadiene. To rationalize this unexpected trend and to obtain more insights into the parameters that control the reactivity of group IV complexes, a theoretical investigation of the entire polymerization mechanism, employing density functional methods, was undertaken. In the framework of the widely accepted polymerization scheme, the different intermediates featuring h⁴ (both cis and trans) coordination of the monomer and h¹ or h³ (syn or anti)allyl coordination of the growing chain were scrutinized. Subsequently, the effects of the metal center on the free-energy profiles of the elementary steps involved in the reaction were examined. The results presented herein aim to achieve a better knowledge of the influence of the metal on the polymerization rates and on the stereoselectivity of the reaction. Full article