Evans Polanyi Relationship Research Articles

Activity and stability descriptors of Ni based alloy catalysts for dry reforming of methane: A density functional theory study

AbstractExperimentally determined catalytic activity and stability of alumina supported Ni and Ni3M (M = Fe, Co, Cu) alloy catalysts for the dry reforming of methane (DRM) were rationalized by density functional theory (DFT) studies. Ni3M slab models were prepared based on the experimental characterization data and DFT calculated segregation energies. First dehydrogenation step of CH4 which is the rate determining step in DRM was modeled on the Ni(111) and Ni3M(111) surfaces. Calculated reaction energetics show that Brønsted–Evans–Polanyi relationship holds true for this catalytic reaction system. A linear correlation was found between turnover frequency values of CH4 and the calculated dissociation energy of CH4. Interestingly, a linear correlation was found between percentage deactivation of catalysts and the calculated carbon adsorption energy. Based on these correlations, we propose that the dissociation energy of CH4 is a suitable activity descriptor and the adsorption energy of carbon is a suitable stability descriptor for these Ni and Ni3M alloy catalysts.

Surface electrocatalysis on high-entropy alloys

Electrocatalysis on high-entropy alloys (HEAs) is a very young field. There are only a few articles on the theoretical understanding of catalysis on these surfaces. Activity descriptors and a ‘theory of catalysis’ exist for uniform surfaces. In this article, we address scaling relations and Brønsted–Evans–Polanyi relations on HEAs which are fundamental for finding activity descriptors. We find that some relations hold, whereas others disappear on HEAs. This reveals some of the future challenges in modeling electrocatalysis on HEAs.

Adsorption and Reaction of Trimethyl and Triethyl Phosphite on Fe3O4 by Density Functional Theory

First-principle density functional theory (DFT) calculations are used to calculate the most stable structures and heats of adsorption of trimethyl and triethyl phosphites on an Fe3O4 surface in order to understand their behavior as lubricant additives. Previous surface science studies of phosphate and phosphite esters on iron oxide surface have shown that they react by sequentially desorbing the corresponding alcohol and aldehyde in equimolar amounts. This implies that the reactions are limited by the rate of alkoxide removal, followed by a rapid reaction to form the alcohol and aldehyde. It is found that the trialkyl phosphites, dialkyl phosphite, and monoalkyl phosphite all adsorb with the phosphorus atoms bound to surface Fe3+ ions, with the alkoxy groups close to parallel to the surface. The heat of adsorption increases as the alkoxide groups are removed. The postulate that the rate-limiting step in the decomposition of the phosphate esters is the sequential removal of the alkoxide group is tested by plotting the experimental activation energy obtained from temperature-programed desorption experiments versus the corresponding heats of adsorption calculated by DFT. A linear dependence shows that the reaction obeys the so-called Evans–Polanyi relation, thereby confirming the above postulate. The slope of the plot is close to unity and thus implies that the structure of the transition state of the reaction resembles the product.

What Does the Machine Learn? Knowledge Representations of Chemical Reactivity.

In a departure from conventional chemical approaches, data-driven models of chemical reactions have recently been shown to be statistically successful using machine learning. These models, however, are largely black box in character and have not provided the kind of chemical insights that historically advanced the field of chemistry. To examine the knowledgebase of machine-learning models-what does the machine learn-this article deconstructs black-box machine-learning models of a diverse chemical reaction data set. Through experimentation with chemical representations and modeling techniques, the analysis provides insights into the nature of how statistical accuracy can arise, even when the model lacks informative physical principles. By peeling back the layers of these complicated models we arrive at a minimal, chemically intuitive model (and no machine learning involved). This model is based on systematic reaction-type classification and Evans-Polanyi relationships within reaction types which are easily visualized and interpreted. Through exploring this simple model, we gain deeper understanding of the data set and uncover a means for expert interactions to improve the model's reliability.

Design of Active Centers in Ammonia Synthesis on Mo-Based Catalysts: A Theoretical Study

The strong Mo–N bond restrains the catalytic activity of metallic Mo in ammonia synthesis. In this study, the semi-empirical calculations in conjunction with the density functional theory calculations, Bronsted–Evans–Polanyi relationship and microkinetic modeling were used to evaluate the rate of ammonia synthesis on model active sites of Mo-based alloys, nitrides, and clusters with a modified Mo–N bond. It was found that active sites of binary alloys MoδMe1−δ (0 ≤ δ ≤ 1; Me = Co, Pt, Ir, Rh) show the synergetic behavior. The sites of ternary Mo3Me3N (Me = Mo, Co, Pt, Ir) and Mo2N-type nitrides revealed higher activities than sites on Mo planes due to an extra Mo bond with the lattice N atom. The sites of octahedral clusters Mo3Me3N (Me = Mo, Co, Ir, Pt) exhibited higher catalytic activity than the sites of nitrides because their Me–N bonds are weaker than Mo–N. It was also found that tetragonal Mo2Me2 (Me = Co, Pt, Ir) and bi-tetragonal clusters Mo3Me2 (Me = Co, Ir, Pt) are the best cases because their sites provide the optimal combination of local structure and thermodynamics. Catalytic activities of the most active sites, relative to the Fe–C7 center, were found to change in the row 18.4 (threefold site Mo2Ir1 in cluster Mo3Ir2), 7.3 (Mo2 in cluster Mo2Ir2), 3.9 (Mo2Pt1 in cluster Mo3Ir3N), 3.8 (M3 on alloy Mo0.78Ir0.22), 2.0 [Mo3Pt1 on the plane (100) of Mo3Pt3N], 0.57 [Mo3 on the plane (111) of Mo2N], and 0.03 [Mo4 on the plane Mo(110)–(1 × 2)]. The design of tailor-made catalytic sites suggested in this paper can probably be applied to other catalytic systems.

Solvent fluctuations in the solvation shell determine the activation barrier for crystal growth rates

Solution crystallization is a common technique to grow advanced, functional crystalline materials. Supersaturation, temperature, and solvent composition are known to influence the growth rates and thereby properties of crystalline materials; however, a satisfactory explanation of how these factors affect the activation barrier for growth rates has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of solvent fluctuations in the solvation shell of solute molecules attaching to the crystal surface. With increasing supersaturation, the average hydration number of the glutamic acid molecule decreases and can reach an asymptotic limit corresponding to the number of adsorption sites on the molecule. The hydration number of the glutamic acid molecule also fluctuates due to the rapid exchange of solvent in the solvation shell and local variation in the supersaturation. These rapid fluctuations allow quasi-equilibrium between fully solvated and partially desolvated states of molecules, which can be used to construct a double-well potential and thereby to identify the transition state and the required activation barrier. The partially desolvated molecules are not stable and can attach spontaneously to the crystal surface. The activation barrier versus hydration number follows the Evans-Polanyi relation. The predicted absolute growth rates of the α-glutamic acid crystal at lower supersaturations are in reasonable agreement with the experimental observations.

Brønsted–Evans–Polanyi relation for CO oxidation on metal oxides following the Mars–van Krevelen mechanism

Scaling relations are widely used to model catalytic reactions on metal surfaces, but they are less commonly applied to metal oxides. Oxygen vacancy formation energies have been suggested as a descriptor for the activity toward oxidation reactions via the Mars-van Krevelen mechanism. However, there is currently no function that maps oxygen vacancy formation energies to CO oxidation barriers. We have compiled a data set for CO oxidation on doped and pristine metal oxide surfaces as well as metal/metal oxide interface sites using density functional theory. Based on this data, we can predict CO oxidation barriers from oxygen vacancy formation energies using a Brønsted–Evans–Polanyi relation with a mean absolute error of 14 kJ/mol. Contrary to what is known for reactions on metal surfaces, we find that the scaling parameter α that describes the lateness of the transition state for CO oxidation on metal oxides is not constant.

110th Anniversary: Microkinetic Modeling of the Vapor Phase Upgrading of Biomass-Derived Oxygenates

Bio-oil produced from fast pyrolysis of biomass is a complex mixture of more than 200 compounds, including oxygenates and acids. As these species are highly undesirable in fuels, catalytic upgrading of biomass pyrolysis product vapors, also known as catalytic fast pyrolysis, is performed to upgrade the vapors to valuable fuels and chemicals. This work presents a detailed microkinetic model, composed of elementary steps, of the catalytic upgrading of acetic acid and acetone, two common oxygenates present in bio-oil. An automated network generator was utilized to construct a reaction network composed of 580 unique species and 2160 unique reactions. The kinetic parameters for each reaction in the network were estimated using transition state theory, the Evans–Polanyi relationship, and thermodynamic data. The resulting mechanistic model is able to describe experimental data presented in the literature for the transformation of acetic acid and acetone on HZSM-5 in a fixed-bed reactor, which is modeled as a plug-flow reactor. Additionally, the model solutions reveal vital information regarding the mechanism by which acetic acid and acetone are upgraded to valuable fuels and chemicals. In the first phase of the mechanism, acetic acid is converted to acetone via acylium ion addition to acetic acid; this is followed by decarboxylation of acetoacetic acid. The second phase is dominated by the self-aldol condensation of acetone, which is shown to occur predominantly through the keto form of acetone rather than the enol form, and subsequent deoxygenation reactions leading to olefins and aromatics. Finally, net rate analysis shows that aromatics are primarily formed via a pathway including aldol condensation of mesityl oxide, whereas olefins are produced from the addition of isobutene and subsequent cracking.

Control of surface reactivity towards unsaturated C[sbnd]C bonds and H over Ni-based intermetallic compounds in semi-hydrogenation of acetylene

Intermetallic compounds composed of transition metals and semimetals or post-transition metals have been shown to exhibit elevated selectivity in olefins and aromatics production and other reactions that require control of CC activation and functionalization. The vast number of element combinations available for intermetallic compound production necessitates down-selecting to isolate catalytically useful compositional spaces. In this study, we have investigated the semi-hydrogenation of acetylene over isoelectronic intermetallic compounds composed of nickel and the boron group elements at a bulk stoichiometry of 1:1. We have isolated that orbital overlap between IMC constituent elements tracks well with the nature of their bulk bonding, surface reactivity, and catalytic preference for the semi-hydrogenation of acetylene to ethylene. The trend isolated appears to be universal and marks a subspace within intermetallic compounds that is promising to focus further studies upon. Results also suggest the possibility of accessing New Brønsted–Evans–Polanyi relationships over a subspace of intermetallic compounds.

Implicit solvent effects in the determination of Brønsted-Evans-Polanyi relationships for heterogeneously catalyzed reactions.

Heterogeneously catalyzed reactions take place at the catalyst surface where, depending on the conditions and process, the reacting molecules are either in the gas or liquid phase. In the latter case, computational heterogeneous catalysis studies usually neglect solvent effects. In this work, we systematically analyze how the electrostatic contribution to solvent effects influences the atomic structure of the reactants and products as well as the adsorption, activation, and reaction energy for the dissociation of water on several planar and stepped transition metal surfaces. The solvent effects were accounted for through an implicit model that describes the effect of electrostatics, cavitation, and dispersion on the interaction between the solute and solvent. The present study shows that the activation energy barriers are only slightly influenced by the inclusion of the electrostatic solvent effects accounted for in a continuum solvent approach, whereas the adsorption energies of the reactants or products are significantly affected. Encouragingly, the linear equations corresponding to the Brønsted-Evans-Polanyi relationships (BEPs), relating the activation energies for the dissociation reaction with a suitable descriptor, e.g. the adsorption energies of the products of the reaction on the difference surfaces, are similar in the presence or in the absence of the solvent. Despite the associated uncertainties, this suggests that BEP relationships derived without the implicit consideration of the solvent are still valid for predicting the activation energy barriers of catalytic reactions from a reaction descriptor.

Mechanochemistry breaks with expectations

Tensile strain of a solid surface can result in either strengthening or weakening of bonds with adsorbates. Adsorption energies of different adsorbate/site combinations may be shifted in different directions — a striking violation of the Bronsted–Evans–Polanyi relation.

A DFT approach for methanol synthesis via hydrogenation of CO on gallia, ceria and ZnO surfaces

A systematic theoretical study of the consecutive hydrogenation reactions of the CO molecule for the methanol synthesis catalyzed by different oxides of Zn, Ce and Ga is reported in this work. First, the CO hydrogenation with the formation of formyl species (HCO) was analyzed, followed by the successive hydrogenations that lead to formaldehyde (H2CO), methoxy (H3CO) and, finally, methanol (H3COH). The co-adsorption with H, in almost all the intermediate species, allows the corresponding hydrogenation reaction. Oxygen vacancies promote the reactivity in the generation of both formaldehyde and methoxy species. The formation of these species involves an important geometric difference between the initial and the final states, leading to high activation barriers. Comparing the surfaces studied in this work, we found that ZnO (0001)vacO has shown to be of a greater interest for methanol synthesis. However, the foregoing is not the most relevant of our results, but, instead, that the Brönsted Evans Polanyi (BEP) relationships between the initial or the final states and the transition states (TS) allowed to find a very good correlation between surface structure and reactivity.

Bimodal Evans–Polanyi Relationships in Dioxirane Oxidations of sp3 C–H: Non-perfect Synchronization in Generation of Delocalized Radical Intermediates

The selectivities in C-H oxidations of a variety of compounds by DMDO have been explored with density functional theory. There is a linear Evans-Polanyi-type correlation for saturated substrates. Activation energies correlate with reaction energies or, equivalently, BDEs (ΔH‡sat = 0.91*BDE - 67.8). Unsaturated compounds, such as alkenes, aromatics, and carbonyls, exhibit a different correlation for allylic and benzylic C-H bonds (ΔH‡unsat = 0.35*BDE - 13.1). Bernasconi's Principle of Non-Perfect Synchronization (NPS) is found to operate here. The origins of this phenomenon were analyzed by a Distortion/Interaction model. Computations indicate early transition states for H-abstractions from allylic and benzylic C-H bonds, but later transition states for the saturated. The reactivities are mainly modulated by the distortion energy and the degree of dissociation of the C-H bond. While the increase in barrier with higher BDE is not unexpected from the Evans-Polanyi relationship, two separate correlations, one for saturated compounds, and one for unsaturated leading to delocalized radicals, were unexpected.

Kinetic Requirements of Aldehyde Transfer Hydrogenation Catalyzed by Microporous Solid Brønsted Acid Catalysts

Brønsted acids in a confined environment catalyze transfer hydrogenation of aldehydes (CnH2nO, n = 3–6) by kinetically relevant hydride transfer between hydride donors (e.g., cyclohexadiene, tetralin, cyclohexene, or 3-methyl-1-pentene) and protonated aldehydes. The hydride ion affinity difference between the donor and acceptor pair determines the reaction enthalpy, which reflects the thermodynamic tendency, of hydride transfer. For this class of reactions, the hydride ion affinity difference appears to be the empirical kinetic descriptor for predicting the rates of hydride transfer and, in turn, of transfer hydrogenation through the Brønsted–Evans–Polanyi relation.

Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions

Bio-oil produced from biomass fast pyrolysis often requires catalytic upgrading to remove oxygen and acidic species over zeolite catalysts. The elementary reactions in the mechanism for this process involve carbenium and oxonium ions. In order to develop a detailed kinetic model for the catalytic upgrading of biomass, rate constants are required for these elementary reactions. The parameters in the Arrhenius equation can be related to thermodynamic properties through structure–reactivity relationships, such as the Evans–Polanyi relationship. For this relationship, enthalpies of formation of each species are required, which can be reasonably estimated using group additivity. However, the literature previously lacked group additivity values for oxygenates, oxonium ions, and oxygen-containing carbenium ions. In this work, 71 group additivity values for these types of groups were regressed, 65 of which had not been reported previously and six of which were newly estimated based on regression in the context of...

Theoretical Study of Epoxidation Reactions Relevant to Hydrocarbon Oxidation

Advances in computational modeling tools have allowed for the construction of detailed chemical models of oxidation processes to enhance the understanding of mechanisms and answer questions of interest in diverse fields such as DNA damage, lubricant oxidation, food chemistry, and art conservation. The construction of detailed kinetic models is facilitated by the use of kinetic correlations, such as Evans–Polanyi relationships, where the activation energy, EA, is related linearly to the heat of reaction, ΔHR: EA = E0 + α ΔHR. In this work, we present an Evans–Polanyi relationship based on properties determined from hybrid G4 quantum chemical calculations for an epoxidation reaction of peroxy species. We explore a broader chemical space at a higher level of theory than previous reports, and as a result, the importance of several key structural features, such as alkylhydroperoxy functional groups, that can strongly influence the trends observed was revealed. We suggest a subdivision of the reaction family in...

Pressure-Dependent Rate Rules for Intramolecular H-Migration Reactions of Hydroperoxyalkylperoxy Radicals in Low Temperature

Intramolecular H-migration reaction of hydroperoxyalkylperoxy radicals (•O2QOOH) is one of the most important reaction families in the low-temperature oxidation of hydrocarbon fuels. This reaction family is first divided into classes depending upon H atom transfer from -OOH bonded carbon or non-OOH bonded carbon, and then the two classes are further divided depending upon the ring size of the transition states and the types of the carbons from which the H atom is transferred. High pressure limit rate rules and pressure-dependent rate rules for each class are derived from the rate constants of a representative set of reactions within each class using electronic structure calculations performed at the CBS-QB3 level of theory. For the intramolecular H-migration reactions of •O2QOOH radicals for abstraction from an -OOH substituted carbon atom (-OOH bonded case), the result shows that it is acceptable to derive the rate rules by taking the average of the rate constants from a representative set of reactions with different sizes of the substitutes. For the abstraction from a non-OOH substituted carbon atom (non-OOH bonded case), rate rules for each class are also derived and it is shown that the difference between the rate constants calculated by CBS-QB3 method and rate constants estimated from the rate rules may be large; therefore, to get more reliable results for the low-temperature combustion modeling of alkanes, it is better to assign each reaction its CBS-QB3 calculated rate constants, instead of assigning the same values for the same reaction class according to rate rules. The intramolecular H-migration reactions of •O2QOOH radicals (a thermally equilibrated system) are pressure-dependent, and the pressure-dependent rate constants of these reactions are calculated by using the Rice-Ramsberger-Kassel-Marcus/master-equation theory at pressures varying from 0.01 to 100 atm. The impact of molecular size on the pressure-dependent rate constants of the intramolecular H-migration reactions of •O2QOOH radicals has been studied, and it is shown that the pressure dependence of the rate constants of intramolecular H-migration reactions of •O2QOOH radicals decreases with the molecular size at low temperatures and the impact of molecular size on the pressure-dependent rate constants decreases as temperature increases. It is shown that it is acceptable to derive the pressure-dependent rate rules by taking the average of the rate constants from a representative set of reactions with different sizes of the substitutes. The barrier heights follow the Evans-Polanyi relationship for each type of intramolecular hydrogen-migration reaction studied. All calculated rate constants are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression at pressures varying from 0.01 to 100 atm and at the high-pressure limit. Furthermore, thermodynamic parameters for all species involved in these reactions are calculated by the composite CBS-QB3 method and are given in NASA format.

How hydroxylation affects hydrogen adsorption and formation on nanosilicates

Silicate dust constitutes one of the primary solid components of the Universe and is thought to be an essential enabler for complex chemistry in a number of astronomical environments. Hydroxylated silicate nanoclusters (MgO)x(SiO2)y(H2O)z, where strongly absorbed water molecules are dissociated on the silicate surface, are likely to be persistent in diffuse clouds. Such precursor species are thus also primary candidates as seeds for the formation and growth of icy dust grains in dense molecular clouds. Using density functional calculations we investigate the reactivity of hydroxylated pyroxene nanoclusters (Mg4Si4O12)(H2O)N (N= 1−4) towards hydrogen physisorption, chemisorption and H2 formation. Our results show that increased hydroxylation leads to a significant reduction in the energy range for the physisorption and chemisorption of single H atoms, when compared to bare silicate grains and bare bulk silicate surfaces. Subsequent chemisorption of a second H atom is, however, little affected by hydroxylation. The H2 reaction barrier for the recombination of two chemisorbed H atoms tends to follow a linear correlation with respect to the 2Hchem binding energy, suggestive of a general Brønsted–Evans–Polanyi relation for H2 formation on silicate grains, independent of dust grain size, composition and degree of hydroxylation.

Effect of transition metal-doped Ni(211) for CO dissociation: Insights from DFT calculations

Density functional theory slab calculations were performed to investigate the adsorption and dissociation of CO over pure and M-doped Ni(211) (M=Fe, Co, Ru and Rh) with the aim to elucidate the effect of transition metal doping for CO activation. Doping the step edge of Ni(211) with Fe, Co and Ru is found to enhance the binding of CO in the initial state (IS) (in the sequence by the improvement degree: Fe>Ru>Co) as well as the co-adsorption of C and O in the final state (FS) (Ru>Fe>Co). In contrast, Rh doping is unfavorable both in the IS and in the FS. Analysis of the overall potential energy surfaces (PES) suggests CO dissociation is facilitated by Fe, Ru and Co doping both kinetically and thermodynamically, wherein Fe and Ru behave extraordinary. Interestingly, Fe substitute is slightly superior to Ru in kinetics whereas the contrary is the case in thermodynamics. Rh doping elevates the energy height from 0.97eV on Ni(211) to 1.32eV and releases 0.39eV less heat relative to Ni(211), again manifesting a negative effect. Besides the classical Brønsted–Evans–Polanyi relationship, we put forward another two neat linear relations, which can well describe the feature of CO dissociation. The differences of CO adsorption and activation in the IS over pure and doped Ni(211) surfaces are rationalized via electronic structure analysis. The findings presented herein are expected to provide theoretical guidance for catalyst design and optimization in relevant processes.

Methane formation from successive hydrogenation of C over stepped Ni and Ni3Fe surfaces: Effect of surface composition

As an integral part in methanation, CH4 formation from successive hydrogenation of atomic C was systematically studied over stepped Ni(211) and Ni3Fe(211) surfaces via periodic density functional theory calculations. The effect of surface composition was explicitly examined by taking into account of two termination structures of Ni3Fe(211) (the surfaces with NiNi- and NiFe-type step are denoted as Ni3Fe(211)-AA and Ni3Fe(211)-AB, respectively). Both alloyed surfaces are found to benefit the initial C hydrogenation through enhancing H adsorption as well as weakening the binding strength of C. Because CH2 generation and subsequent CH2 hydrogenation favor different sites over such stepped surfaces, the step of CH2 migration was proposed and incorporated into the mechanism. The hydrogenation reactions manifest significant structural sensitivity since Ni3Fe(211)-AB is superior to Ni3Fe(211)-AA in lowering the overall potential energy surfaces towards CH4 formation. In combination with our previous work focusing on CO activation and dissociation, the present results confirmed the improved activity of Ni3Fe alloy catalyst for CO methanation suggested experimentally and the Ni3Fe(211)-AB termination is further identified to dominate the promoting effect. A newly proposed Brønsted–Evans–Polanyi relationship is also found to hold for the successive hydrogenation steps herein.