Evans Polanyi Relationship Research Articles

Optimal design of PdAu/In2O3 catalysts for CO2 hydrogenation

Efficient catalyst design has garnered significant interest in recent decades due to its potential to address both the challenges of the greenhouse effect and energy shortages by facilitating the conversion of CO2 into valuable chemicals through catalytic reactions. To investigate maximizing the synergistic effects of supported PdAu catalysts, we conducted first-principles calculations on the activation and decomposition of CO2 and H2 on the PdAu/In2O3(110) system. The results demonstrate that the incorporation of a secondary metal (Au) into the supported Pd catalyst, in conjunction with precise control over Au concentration, exerts influence on both reactant binding energy and activation. The adsorption and activation of CO2 at the interface sites of Au4/In2O3(110) and PdAu3/In2O3(110) are not observed. The transition state for the dissociation of CO2 into *CO and *O is determined based on adsorbed CO2, providing insights into the properties of activated CO2. The Bronsted–Evans–Polanyi relation, which correlates activation barriers (Ea) with reaction energies (Er), was established for the CO2 dissociation mechanism on PdAu/In2O3(110) catalysts using equation E = 0.4Ea + 0.63. It was carried out to investigate the H2-dissociated adsorption processes and mobility energy on various PdAu/In2O3(110) catalysts. Finally, a highly efficient Pd2Au2/In2O3 catalyst for the hydrogenation of CO2 into methanol has been proposed. This research provides valuable insights into the hydrogenation of CO2 to methanol using bimetal-oxide catalysts and contributes to the optimization of the design of PdAu/In2O3 catalysts for CO2 reactions.

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.

Role of Steric Effects on Rates of Hydrogen Atom Transfer Reactions.

The influence of steric effects on the rates of hydrogen atom transfer (HAT) reactions between oxyradicals and alkanes is explored computationally. Quantum chemical density functional theory computations of transition states show that activation barriers and reaction enthalpies are both influenced by bulky substituents on the radical but very little by substituents on the alkane. The activation barriers remain roughly correlated with reaction enthalpies via the Evans-Polanyi relationship even when steric repulsion effects become important, although dispersion effects sometimes stabilize transition states. By making comparisons to previously developed Evans-Polanyi and modified Roberts-Steel relationships, we find that HAT reactions between bulky molecules remain well-described by these relationships.

Temperature-dependent ethylene dissociative adsorption on ruthenium

The dissociative adsorption of ethylene onto Ru(111) at different temperatures was computed systematically at the first time. At 105 K, ethylene dissociative adsorption has the co-adsorbed CH2C*+2H* and CH3C*+H* as the first and second stable surface intermediates. At over 330 K, CH3C*+H* is converted back into CH2C* accompanied by H2 desorption and the subsequent dissociation of CH2C* into HCC*, HC*+C* and 2C*. The computed Arrhenius activation barriers of the dissociation of CH2CH2 (0.18 vs. 0.22 ± 0.04 eV) and CH3C (0.54 vs. 0.52 ± 0.04 eV) agree perfectly with the available experimental values, and CH3C* represents the most stable surface species. Under CO co-adsorption, the most stable surface species are the co-adsorbed CH3C*+H*+3CO*. It is found that CO co-adsorption promotes H2 desorption and stabilizes CH3C* by blocking the surface sites for dissociation and raises the dissociation barrier compared to the clean surface (0.78 vs 0.54 eV). Brønsted–Evans–Polanyi relationship between the activation Gibbs free energy barrier and reaction Gibbs free energy is found for CH2C*+2H*+nCO* = CH3C*+H*+nCO* and CH2C*+2H*+nCO* = HCC*+3H*+nCO* (n = 0–3). Ethylene adsorption has di-σ and π adsorption configurations in very close energy, and H2 has adsorption energy of about 0.90 eV.

Non-noble metal clusters supported on titanium phthalocyanines for ammonia decomposition: A first-principles study

This study examined NH3 decomposition over the metal clusters on titanium phthalocyanines (M6@TiPc; M6 = Fe6, Co6, Ni6, Co5Ni, and Ni5Co). The M6 metal clusters could stably adsorb on TiPc. The Brønsted–Evans–Polanyi relations were found in the dehydrogenation step and the step for associative desorption of nitrogen. The latter was the rate-determining step for decomposing NH3. Moreover, the positive electric field promoted the dehydrogenation step by weakening the adsorption strength of NH3. In contrast, the negative electric field improved the step for associative desorption of nitrogen by reducing the adsorption strength of nitrogen. Overall, this study offers insight into the design of the high-performance M6@TiPc-based catalyst for decomposing NH3.

Cu–Au nanoparticles produced by the aggregation of gas‐phase metal atoms for CO oxidation

AbstractAlloy nanoparticles (nanoalloys) are widely applied in heterogeneous catalysts, advanced electrodes, biomaterials, and other areas. The properties of nanoalloys can be tuned to a significant extent by their structures and compositions, which are governed by the employed synthetic procedure. Often such synthesis occurs in non‐equilibrium conditions and yields nanoalloys with structures and properties that are different from those obtained in thermodynamic equilibrium. In this work, we characterize how the non‐equilibrium conditions during the synthesis of Cu–Au alloys via physical vapor deposition (PVD) affect their morphology, composition, electronic structure, and reactivity in CO oxidation. We used molecular dynamics to simulate the PVD synthesis of Cu–Au nanoalloys through the non‐isothermal aggregation of Cu and Au atoms at a 3:1 ratio in the Ar atmosphere to obtain realistic structures of Cu–Au nanoparticles. Due to the different aggregation kinetics of Au and Cu atoms, the average Au concentration in the obtained Cu–Au particles varied between 14% and 50% depending on the nanoparticle size and the aggregation time. Density functional simulations revealed that the reactivity of the obtained Cu–Au clusters toward CO and oxygen as well as Brønsted–Evans–Polanyi relations for CO oxidation significantly depend on whether the clusters had fcc, icosahedral, or amorphous structures and do not strongly correlate with the d‐band centers of the adsorption sites. Our study highlights the importance of the non‐equilibrium character of nanoalloy structure and composition for their electronic structure and catalytic properties. The performed analysis of the reactivity of Cu–Au clusters with realistic structures in CO oxidation will help the optimization of Cu–Au catalysts for this societally important reaction.

Quantifying the error propagation in microkinetic modeling of catalytic reactions with model-predicted binding energies

Energy prediction models, such as scaling relations, Brønsted–Evans–Polanyi relations and machine-learning prediction models, are widely employed to accelerate the calculations of energies of reaction intermediates and the rational design of catalysts. However, error propagation from predicted binding energies of reaction intermediates and transition states to the calculated reaction rates would result in the misidentification of optimal catalysts. In order to assess and quantify the error propagation in determining kinetic information for catalyst design, here we make energy error simulation based on DFT-calculated binding energies of reaction intermediates within the network of methane dry reforming reaction, then employ these simulated energies to microkinetic modeling. The results suggest that the microkinetic results would have different tolerance to the error indicators of predicted binding energy. Further detailed analyses show that binding energies with low variance are more likely to result in less significant error propagations during the calculation of reaction energy and activation energy, leading to more reliable kinetic information. Finally, several directions are discussed regarding the minimization of error propagation in microkinetic modeling based on predicted binding energies, and suggestions are provided for future studies.

Thiol‐Ene Polymerization of Natural Monomers: A DFT Study

AbstractThis study investigates the thiol‐ene reaction mechanism of natural monomers using quantum chemical tools. The structure‐reactivity relationship for these monomers is elaborated by the aid of radical stabilization energies (RSEs), intermolecular interactions, the curve‐crossing model, Hammond Postulate, and Evans–Polanyi relationships. Geometry optimizations, frequency, and energy calculations are performed at the M06‐2X/6‐31++G(d,p) level of theory. A crucial parameter for thiol‐ene polymerization, which is the ratio of the propagation reaction (kp) and the chain transfer reaction (kct) rate constants (kp/kct), is calculated in order to elucidate the thiol‐ene reaction mechanism. In light of these energetic and kinetic calculations, it is suggested that linalool, d‐limonene, terpinolene, and γ‐terpinene are considered natural monomer candidates for thiol‐ene polymerization due to their almost equal magnitude of the rate constants (kp/kct≈1). The thiol‐ene polymerization potential of some specific unsaturated fatty acids is another prominent finding in this study.

Examination of the Brønsted-Evans-Polanyi relationship for the hydrogen evolution reaction on transition metals based on constant electrode potential density functional theory.

In the search for efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), the hydrogen binding energy is often used as a descriptor to represent the catalytic activity. The success of this approach relies on the Brønsted-Evans-Polanyi (BEP) relationship. In this study, we used constant electrode potential density functional theory calculations to examine this relationship. Eight fcc metals with a low hydrogen adsorption concentration of 1/9 were used as the model systems. We found that the HER kinetic barriers are indeed correlated to the . Both the s of the hollow site and less favourable top site correlate to the kinetic barriers; however, the correlation is better for the latter. This behaviour leads to a set of equations for estimating the HER kinetic barriers with improved accuracy that can be used to predict the HER performance of the materials with a low hydrogen adsorption concentration. This work demonstrates the importance of calculating the of a suitable adsorption site to establish good BEP relationships.

Direct H abstraction by molecular oxygen from unsaturated C3–C5 hydrocarbons: A theoretical study

AbstractPotential energy surfaces for the H abstraction reactions by molecular oxygen from unsaturated closed‐shell C3–C5 hydrocarbons, including allene and propyne (C3H4), propene (C3H6), diacetylene (C4H2), vinylacetylene (C4H4), 1,3‐butadiene (C4H6), butene isomers (C4H8), cyclopentadiene (C5H6), and cyclopentene (C5H8), which were earlier identified among the main pyrolysis products of jet fuel components, have been explored at the CCSD(T)‐F12/cc‐pVTZ‐f12//ωB97XD/6‐311G** + ZPE(ωB97XD/6‐311G**) level of electronic structure theory. The results of the ab initio calculations were then utilized in transition state theory calculations of the reaction rate constants. The H abstraction reactions by O2 appeared to be most favorable from the allylic sites and from allene and propyne forming the propargyl radical, with the reactions involving the cyclic allylic sites in cyclopentadiene and cyclopentene exhibiting the lowest reaction endothermicities and barriers. They are followed by the reaction of O2 with cyclopentene producing cyclopent‐1‐en‐4‐yl and the reactions with vinylacetylene and 1,3‐butadiene forming the resonantly stabilized i‐C4H3 and i‐C4H5 radical products. The H abstractions from primary and secondary vinylic sites as well as from acetylenic sites are significantly less favorable and unlikely to compete. The calculated rate constants have been validated against the previous experimental and theoretical results for propene and butene isomers. All considered reactions of H abstraction by O2 are predicted to be rather slow, with their rate constants at 1500 K being on the order of 10–17‐10–16 cm3 molecule–1 s–1 and exhibiting a well‐defined Arrhenius behavior. Modified Arrhenius expressions for the reaction rate constants both in forward and reverse directions and Evans–Polanyi relationship between the activation and reaction energies have been generated and proposed for kinetic combustion models. The H abstractions by O2 forming the resonantly stabilized radicals were concluded to likely contribute to the initiation of oxidation of the unsaturated C3–C5 species produced in the pyrolysis of the important components of jet fuels.

Kinetic Study of the Free Radical Copolymerization of Methyl Methacrylate with 2-Perfluorooctyl Ethyl Methacrylate by Quantum Computational Approach

Fluorinated copolymers with perfluoroalkyl side chains have widespread use in applications requiring superior technology due to their unique surface properties. Kinetic analysis of copolymerization of fluorinated acrylates with conventional acrylates is necessary to synthesize such copolymers efficiently. However, kinetic investigation of such reactions are limited in the literature due to the experimental difficulties. In this study, the kinetic of copolymerization of methyl methacrylate with 2-perfluorooctyl ethyl methacrylate in toluene medium using AIBN initiator was investigated using quantum chemistry postulates as an alternative to experimental methods. Reaction rate constants (kp) for propagation were determined using transition state theory. A terminal effect models were used to examine four different addition reactions involving monomeric and dimeric radicals and monomers for both self- and cross-propagation. Reactant and product conformations were optimized with a DFT method using PBE0 function. The Evans-Polanyi relationship was used to calculate the rate of self- and cross-propagation of monomers. The results showed that the reactivity ratio of 2-perfluorooctyl ethyl methacrylate was found to be higher than that of methyl methacrylate. In addition, it was observed that the reaction conditions caused the random polymer structure due to the different rate constants in self and cross propagation.

Reveal Brønsted–Evans–Polanyi relation and attack mechanisms of reactive oxygen species for photocatalytic H2O2 production

The generation of H+ by active species attacking proton donor (PD) is a key step in photocatalytic H2O2 production (PHP). Deeply analyzing the involved mechanism may be a crucial factor to break through the bottleneck of H2O2 yield. Here, nitrogen-deficient and boron-doped g-C3N4 are synthesized for PHP and the interaction mechanisms between active species and PD are revealed. The prepared photocatalyst exhibits a high H2O2 production rate (455 μM h−1 g−1). The 1O2, •O2- and hole promoted by modification site dominate twelve different reaction mechanisms, respectively. For the oxidation pathways dominated by hole, H2O2 generation is limited by the dissociation of PD or desorption of H+ on catalyst surface. Interestingly, •O2- and 1O2 could directly attack PD, or diffuse into solution to react with PD to produce H2O2 with lower barriers. Moreover, there is a synergistic effect between doped-boron, 1O2 and water bridge, which further reduces the reaction barrier.

Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study.

The applicability of the Evans–Polanyi (EP) relationship to HAT reactions from C(sp3)–H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, for HAT from the C–H bonds of 56 substrates to CumO•, spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C–H bond dissociation enthalpies (BDEs) spanning 27 kcal mol–1 has been calculated using the (RO)CBS-QB3 method. The log kH′ vs C–H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C–H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG⧧HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol–1, indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi’s principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)–H donors.

Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State.

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH3CN and H2O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.

Designing Active Sites for Structure-Sensitive Reactions via the Generalized Coordination Number: Application to Alcohol Dehydrogenation

Identifying the structure of the most active site is essential to improve the performance of supported metal catalysts. For structure-sensitive reactions, in silico design cannot be easily achieved combining the scaling relations and Bronsted–Evans–Polanyi relations, which are only built on energy-based descriptors. We used here the generalized coordination number as a structural descriptor and established that low-coordinated sites are desirable when using Co and Cu to perform the acceptor-less alcohol dehydrogenation reaction.

Insight into Polyethylene and Polypropylene Pyrolysis: Global and Mechanistic Models

Pyrolysis of polyolefins has been proposed as a potential resource recovery strategy by converting macromolecules into valuable fuels and chemicals. Due to variations in possible backbone structures, chain-length distributions, and arrangements of pendant groups, their decomposition behavior via pyrolysis can be complex. In the present work, a review of historical data and empirical models for two distinct polyolefins, polyethylene (PE) and polypropylene (PP), is provided followed by a comparison to recent mechanistic models. The characteristic sigmoidal behavior of linear polymer decomposition is captured with global, lumped-species, and mechanistic models of high-density polyethylene. The PE model was extended to simulate PP using the same reaction families and reaction family parameters, but with distinct rate coefficients that accounted for the difference in the structure of PP with its pendant methyl groups compared to PE as manifested through heats of reaction embedded in the Evans–Polanyi relationship, Eₐ= E₀ + γ×ΔHᵣₑₐcₙ. The change in structure and its associated kinetic parameters resulted in no sigmoidal conversion, consistent with experimental reports for atactic PP. This suggests that mechanistic modeling could be an important complement to global model studies to understand when other effects are at play in the pyrolytic decomposition of polymers such as PP.

Scaling of Transition State Vibrational Frequencies and Application of d-Band Theory to the Brønsted–Evans–Polanyi Relationship on Surfaces

Semiempirical energy relations provide a means of estimating thermodynamic properties. Specifically, linear scaling relationships (LSRs) and Brønsted–Evans–Polanyi (BEP) relationships correlate adsorption energies between adsorbates across surfaces and reaction energies with activation barriers, respectively. Although vibrational scaling relations (VSRs) exist between adsorbates at identical sites, scaling between vibrational frequencies of adsorbed local minima and transition states is lacking. Here, we present density functional theory calculations for AHX (A = C, N, O) diffusions on transition metal surfaces and reveal linear scaling between frequencies of local minima and the transition state between those minima. Using d-band theory and linear muffin tin orbital theory (LMTO), we derive the slopes of these transition state vibrational scaling relations (TSVSRs) and, in so doing, provide a rigorous theory extending the original BEP relations developed for solution chemistry to surface chemistry. With a single reference DFT calculation, we predict the slopes and quantify uncertainty in the predictions.

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO2 by means of first-principles calculations using realistic amorphous periodic SiO2 models. We evaluated the three reaction steps in PDH, namely, the C–H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H–H coupling to release H2, regenerating the initial Ga–O bond and closing the catalytic cycle. Our work shows how Brønsted–Evans–Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga–O pairs on such sites of realistic amorphous SiO2 models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C–H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO2 for the PDH reaction.

The Brønsted-Evans-Polanyi relationship in oxygen exchange of fuel cell cathode material SrCo0.9Ta0.1O3-δ with the gas phase.

Perovskite related oxides ABO3-δ exhibiting mixed ionic-electronic conductivity (MIEC) possess large deviations from the oxygen stoichiometry. When providing excellent application potential, this feature also makes it very difficult to study the reaction mechanism between such oxides and molecular oxygen, also known as the oxygen reduction reaction. The complexity of the theoretical interpretation of kinetic experiments originates from the significant dependence of the kinetic and equilibrium properties of MIEC oxides on δ. It is proposed to consider such grossly nonstoichiometric oxides having different oxygen nonstoichiometry as chemical homologues participating in the oxygen exchange reaction and forming a series continuous in δ. The continuous homologous series approach is considered using the example of SrCo0.9Ta0.1O3-δ, an SOFC cathode material. The equilibrium and kinetic properties of the oxide were studied by new methods of oxygen partial pressure relaxation and oxygen release. Linear free-energy relationships have been discovered in the homologous series: thermodynamic and kinetic enthalpy-entropy compensations, as well as the Brønsted-Evans-Polanyi relation. A relationship has been established between the change in the observed LFERs and the morphotropic phase transition in the oxide.

Mo single atoms in the Cu(111) surface as improved catalytic active centers for deoxygenation reactions

The surface Mo-doped Cu(111) catalyst feature improved performance towards deoxygenation reactions, acting as a single-atom alloy capable of breaking Brønsted–Evans–Polanyi relations for carbonyl bond scissions.