Mechanism Of Elementary Steps Research Articles

Kinetic modeling of dynamically operated heterogeneously catalyzed reactions: Microkinetic model reduction and semi-mechanistic approach on the example of the CO[formula omitted] methanation

As the production of green hydrogen from fluctuating resources and the use of Power-to-X technologies become more and more important in the energy sector and the chemical process industries, the interest in dynamically operated reactors is increasing. In order to apply model-based optimization methods for process intensification the dynamic reactor behavior needs to be predicted sufficiently accurate by the underlying model equations, in which the reaction kinetics play a key role. Microkinetic elementary step models are able to describe the dynamic behavior on the catalyst surface. However, these models are laborious to derive, parameterize and require comparably high computational effort to solve. For these reasons, simpler semi-mechanistic kinetic models based on a rate determining step at steady state conditions are much more frequently applied in numerical studies. However, the underlying assumptions of these models are questionable for dynamic operation since sorption processes and possible changes in the reaction mechanisms are not considered. We present a semi-mechanistic kinetic model approach based on so called rate-affecting steps and the dynamic change in surface coverages, which is able to describe the dynamic behavior of the catalyst surface. The applicability of our approach is demonstrated for model reduction of an elementary step mechanism developed for the COx methanation and for kinetic model development with synthetic experimental data. Both methods result in a kinetic expression that can accurately reproduce the dynamic kinetic behavior predicted by the elementary step model, with a significant reduction in computational effort for our new semi-mechanistic model, making this new class of model approach attractive for model-based optimization applications.

A Molecular Level Interpretation of the Underlying Chemical Phenomenon of Semi‐coke and Coke Formation: A Treatise from Ab Initio Calculations

AbstractChemistry of coke formation has been subject to a wide array of experimental investigations since the advent of industrial coal carbonization process. Roles of different additives in inducing chemical modifications have been examined in this regard. However, similar computational quantum chemistry calculations attempting to obtain fundamental insights into the plausible molecular mechanism operating during the resolidification steps of semi‐coke and coke formation are elusive. Thus, a rational molecular level understanding pertaining to the evolution of chemical structure during coke formation has not developed likewise. In this context, density functional theory calculations have been employed in the present work to comprehend the plausible molecular mechanism of elementary steps driving the semi‐coke and coke formation. Influence of an additional organic H‐donor species on the mechanism has also been explored to ascertain its participation in coke making. Mechanistic steps involved in semi‐coke formation are found to be crucial in determining the final coke structure. Experimental observations relating to the liberation of H• and also molecular hydrogen (H2) during the generation of semi‐coke structure are well corroborated with our investigated mechanistic features. Overall, the work is believed to offer a molecular level interpretation of a much‐exploited chemical phenomenon.

In-operando surface-sensitive probing of electrochemical reactions on nanoparticle electrocatalysts: Spectroscopic characterization of reaction intermediates and elementary steps of oxygen reduction reaction on Pt

In this paper we present an in-operando spectroscopic study of the reaction intermediates involved in ORR on electrified Pt nanoparticle electrocatalysts. To accomplish this, we have synthesized Ag-Pt core–shell nanoparticles, that contain a thin shell of Pt (~1 nm) uniformly deposited on a larger Ag core (~60 nm). We show that these nanostructures are ideal platforms for in-operando Plasmon Enhanced Raman Spectroscopy (PERS) of ORR on Pt. We combined these PERS studies with DFT calculations to assign the measured vibrational spectra to relevant surface intermediates as a function of electrocatalyst potential. These vibrational assignments were further verified and validated by performing PERS measurements using de-oxygenated and heavy oxygen (O2-18) saturated electrolyte. These studies allowed us to identity the presence of critical reaction intermediates (OH, O2, OOH, and H2O) as the function of operating voltage on the Pt surface and derive voltage-dependent elementary step mechanisms of the reaction.

Liquid-phase oxidation of cyclohexane. Elementary steps in the developed process, reactivity, catalysis, and problems of conversion and selectivity

The literature data concerning features of the kinetics and mechanisms of elementary steps of liquid-phase oxidation of cyclohexane and its oxygen derivatives are considered and analyzed. A comparison of rates of intermolecular and intramolecular reactions of cyclohexylperoxyl radicals under the industrial conditions indicated a necessity to take into account intramolecular interactions. The occurrence of cross recombination of hydroperoxyl and α-hydroxyperoxyl radicals without chain termination in the course of cyclohexanol and 2-hydroxycyclohexanol oxidation was proved. A significance of degenerate branching reactions involving cyclohexyl hydroperoxide in the industrial process of cyclohexane oxidation at 423 K was evaluated. The influence of the electron-withdrawing functional groups on the reactivity of carbon–hydrogen bonds of organic compounds in the reactions with electrophilic peroxyl radicals was studied. The low conversion of a substrate in the industrial process are mainly caused by the radicalchain oxidation of cyclohexanone leading only to by-products. The catalysts of cyclohexane oxidation, viz., compounds of variable valence metals, affect the reaction rate and ratio of the yields of the target products (cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone) but exert no effect on their relative reactivity. The use of the catalytic additives increasing the yield of cyclohexanone in the step of cyclohexane oxidation in the production of caprolactam is revealed to be inexpedient.

Analysis of light-off during oxidation of reactant mixtures on Pt/Al2O3 using micro-kinetic models

An elementary step mechanism, which accounts for adsorption, desorption and surface reaction steps is proposed for co-oxidation of CO, propylene, hydrogen and ethane on Pt/Al2O3. Parameter estimation is used to determine the kinetic parameters for these steps, which depend on platinum loading and dispersion. A micro-kinetic model is proposed for water-gas shift reaction that captures the experimental trend observed during co-oxidation of CO/H2. This mechanism is used to predict hysteresis features during oxidation and co-oxidation reactions. The oxidation of all the individual reactants exhibits regular hysteresis and the model predicts the experimentally observed inverse hysteresis during co-oxidation of CO and C3H6. The inverse hysteresis is caused by surface intermediates formed during C3H6 oxidation. These surface intermediates block the active sites, decreasing the light-off activity of CO during ramp down. The developed micro-kinetic model can also explain steam reforming of propylene and ethane on Pt/Al2O3. Finally, we compare the predicted hysteresis during CO oxidation by global and micro-kinetic models. The predicted ignition temperature by both global and micro-kinetic models is almost same. However, the predicted extinction temperature by the global and micro-kinetic models is rather different. This behavior is due to different rate determining step during ramp down period.

Development of a multi-layer microreactor: Application to the selective hydrogenation of 1-butyne

A multi-layer microreactor (MR) was especially developed to assess the effects of the intensification of liquid-phase selective hydrogenation of 1-butyne (BY). The behavior of the system is described and the results of the regression of experimental data are reported. Various MR configurations, differentiated by the number of catalyst layers were constructed. A commercial Pd/Al2O3 catalyst, crushed and sieved to 37–44μm particle size, has been tested. The reaction was carried out in liquid phase, at three levels of temperature (35, 44 and 60°C), and covering a range of hydrogen partial pressure of 1.2–5bar. The kinetic expression used to analyze the experimental data was based on an elementary step mechanism. Fitting of the kinetic parameters allowed to reproduce the experimental results within an average deviation of 3.7%. The results presented in this study are compared to experimental data obtained previously on the same commercial catalyst, but using the original 2.3mm spherical pellets packed in a conventional fixed bed reactor (FBR). It is concluded from such a comparison that the measurements carried out in the MR are free from diffusion limitations and that the evaluation of such effects in the FBR was satisfactorily performed.

Theoretical insight into the mechanism of Pt(ІІ)-catalyzed [3+2] cycloaddition reactions of propadienyl silyl ethers with alkenyl ethers

By carrying out DFT calculations, we have performed a detailed mechanism study on the Pt(II)-catalyzed [3 + 2] cycloaddition reactions of propadienyl silyl ethers with alkenyl ethers. We calculate the mechanism proposed by Iwasawa et al. about the cycloaddition reactions of triisopropylsilyl (TIPS) 1,2-propadienyl ether (1a) with 2-methoxypropene (2a), tert-butyldiphenylsilyl (TBDPS) 1,2-propadienyl ether (1b) with 2a, and 1b with benzyl substituted alkenyl ether (2b), respectively. The calculated results show that the two reactions proceed according to similar elementary step mechanism. For the cycloaddition reaction of 1a with 2a, the products are cyclopentene derivative 3a and methylenecyclobutane derivative 4a. In contrast, for the cycloaddition reaction of 1b with 2a and 1b with 2b, due to the less electron-donating TBDPS group and the more steric repulsion between TBDPS group and the metal fragment, the predominant product is 3a.

Theoretical investigation on the Pt(ІІ)-catalyzed [3+2] cycloaddition reactions of propargyl ether derivatives with n-butyl vinyl ether

By carrying out density functional theory (DFT) calculations, we have performed a detailed mechanism study on the Pt(II)-catalyzed [3+2] cycloaddition reactions of two propargyl ether derivatives, 2-(3-methoxyprop-1-ynyl) aniline derivative (1A) and phenol derivative (2A), with n-butyl vinyl ether. The calculated results show that the two reactions proceed according to similar elementary step mechanism. For both the two reactions, the elimination of the methoxy group is the rate-determining step. The present results provide a theoretical validation for the mechanism proposed by Iwasawa et al.

Double Duty for a Conserved Glutamate in Pyruvate Decarboxylase: Evidence of the Participation in Stereoelectronically Controlled Decarboxylation and in Protonation of the Nascent Carbanion/Enamine Intermediate,

Pyruvate decarboxylase (PDC) catalyzes the nonoxidative decarboxylation of pyruvate into acetaldehyde and carbon dioxide and requires thiamin diphosphate (ThDP) and a divalent cation as cofactors. Recent studies have permitted the assignment of functional roles of active site residues; however, the underlying reaction mechanisms of elementary steps have remained hypothetical. Here, a kinetic and thermodynamic single-step analysis in conjunction with X-ray crystallographic studies of PDC from Zymomonas mobilis implicates active site residue Glu473 (located on the re-face of the ThDP thiazolium nucleus) in facilitating both decarboxylation of 2-lactyl-ThDP and protonation of the 2-hydroxyethyl-ThDP carbanion/enamine intermediate. Variants carrying either an isofunctional (Glu473Asp) or isosteric (Glu473Gln) substitution exhibit a residual catalytic activity of less than 0.1% but accumulate different intermediates at the steady state. Whereas the predecarboxylation intermediate 2-lactyl-ThDP is accumulated in Glu473Asp because of a 3000-fold slower decarboxylation compared to that of the wild-type enzyme, Glu473Gln is not impaired in decarboxylation but generates a long-lived 2-hydroxyethyl-ThDP carbanion/enamine postdecarboxylation intermediate. CD spectroscopic analysis of the protonic and tautomeric equilibria of the cocatalytic aminopyrimidine part of ThDP indicates that an acidic residue is required at position 473 for proper substrate binding. Wild-type PDC and the Glu473Asp variant bind the substrate analogue acetylphosphinate with the same affinity, implying a similar stabilization of the predecarboxylation intermediate analogue on the enzyme, whereas Glu473Gln fails to bind the analogue. The X-ray crystallographic structure of 2-lactyl-ThDP trapped in the decarboxylation-deficient variant Glu473Asp reveals a common stereochemistry of the intermediate C2α stereocenter; however, the scissile C2α-C(carboxylate) bond deviates by ∼25-30° from the perpendicular "maximum overlap" orientation relative to the thiazolium ring plane as commonly observed in ThDP enzymes. Because a reactant-state stabilization of the predecarboxylation intermediate can be excluded to account for the slower decarboxylation, the data suggest a strong stereoelectronic effect for the transition state of decarboxylation as supported by additional DFT studies on models. To the best of our knowledge, this is a very rare example in which the magnitude of a stereoelectronic effect could be experimentally estimated for an enzymatic system. Given that variant Glu473Gln is not decarboxylation-deficient, electrostatic stress can be excluded as a driving force for decarboxylation. The apparent dual function of Glu473 further suggests that decarboxylation and protonation of the incipient carbanion are committed and presumably proceed in the same transition state.

Kinetics of the N2O+CO reaction over steam-activated FeZSM-5

The kinetics of the N2O reduction by CO as well as the direct N2O decomposition have been investigated over steam-activated FeZSM-5. To this end, steady-state experiments at different temperatures, partial reactant pressures, molar feed N2O/CO ratios, and space times were carried out in an integral fixed-bed micro-reactor. Different empirical and microkinetic models were evaluated in order to describe the experimental data and derive quantitative kinetic information. Depending on the molar feed CO/N2O ratio and temperature, the reduction occurs alone in the catalyst bed or simultaneously with direct N2O decomposition. The orders for N2O and CO in the N2O + CO reaction are 0.51 and 0.72, respectively, in contrast with the characteristic first-order behaviour of the direct N2O decomposition. The latter can be properly described by a two-step oxygen transfer mechanism. The scavenging mechanism (Eley-Rideal type), where gas-phase CO effectively eliminates adsorbed atomic oxygen properly describes the N2O + CO reaction. The removal of O* by CO is two orders of magnitude faster than by N2O, but the elimination of O* remains as the rate-determining step. The N2O activation generating adsorbed oxygen is 2–8 times faster than the O* elimination by CO. Elimination of O* by CO is more sensitive to temperature than N2O activation. The elementary step mechanism has been used to predict the fraction of free/oxidised sites. The inlet concentration of CO and temperature determines the degree of reduction of the catalyst surface. The obtained activation energies in direct N2O decomposition (141 kJ mol−1) and N2O reduction by CO (62 kJ mol−1) were in excellent agreement with values reported in the literature.

Liquid-phase hydrogenation of methyl oleate on a Ni/α-Al 2O 3 catalyst: A study based on kinetic models describing extreme and intermediate adsorption regimes

The kinetics of the hydrogenation of methyl oleate on a Ni/α-Al 2O 3 catalyst was studied in the absence of mass-transport limitation, at 398 ≤ T ≤ 443 K and 3.7 ≤ P H 2 ≤ 6.5 bar. The kinetic modeling was performed on the basis of elementary step mechanisms involving different regimes of competition between hydrogen and methyl oleate. Admitting a distinction between occupied-sites and covered-sites by the large molecule of methyl oleate, a rigorous proposal was made to link the seemingly separate kinetic models corresponding to the extreme modes of competitive and non-competitive adsorption, without having to draw the common distinction between two types of surface sites. General rate equations were formulated without expressing opinion a priori on whether the adsorption regime is competitive or non-competitive. Then, typical LHHW rate equations for both extreme adsorption regimes were straightforwardly derived as special cases. Statistical results demonstrated the inadequacy of the models approaching non-competitive adsorption to describe the experimental data but results did not allow a definite discrimination between rival models with competitive and semi-competitive adsorption. A mechanistic model featuring dissociative adsorption of hydrogen, molecule of methyl oleate interacting with a single atom of Ni, and second insertion of hydrogen as RDS, proved to be the best candidate to describe the experimental data satisfactorily with physically reasonable parameters. As a distinctive feature, the model considering semi-competitive adsorption gave additional indication that the adsorbed molecule of methyl oleate could cover up to seven surface sites. From this finding, the semi-competitive model seems to be more realistic than the competitive one.

Interactions of Au cluster anions with oxygen.

Experimental and theoretical evidence is presented for the nondissociative chemisorption of O2 on free Au cluster anions (Aun-, n=number of atoms) with n=2, 4, 6 at room temperature, indicating that the stabilization of the activated di-oxygen species is the key for the unusual catalytic activities of Au-based catalysts. In contrast to Aun- with n=2, 4, 6, O2 adsorbs atomically on Au monomer anions. For the Au monomer neutral, calculations based on density functional theory reveal that oxygen should be molecularly bound. On Au dimer and tetramer neutrals, oxygen is molecularly bound with the O-O bond being less activated with respect to their anionic counterparts, suggesting that the excess electron in the anionic state plays a crucial role for the O-O activation. We demonstrate that interplay between experiments on gas phase clusters and theoretical approach can be a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi 2S 3 and Bi 2O 3 semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies ( D≈ 10 −16 cm 2 s −1). Solid-state transformation of sulfide to the oxide film is a consequence of OH − ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.

Origin of unusual catalytic activities of Au-based catalysts

Experimental evidences for the non-dissociative chemisorption of O 2 are presented on even-numbered free Au anion clusters (Au n −, n=number of atoms) up to Au 20 − at room temperature. Our result indicates that the formation of the activated di-oxygen species is the key of the unusual catalytic activities of Au-based catalysts. No correlation between geometrical structures of Au n − and the activities towards O 2 adsorption was found, showing that site-specific chemistry disappears for Au-nanocatalysis. We demonstrate that interplay between cluster physics and surface chemistry is a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

Elementary steps and mechanism of electrodeposition of Al from complex hydride ions in tetrahydrofuran baths

Electrocrystallization of Al cannot be achieved from aqueous media, but is possible from low melting-point organic salts, and from aromatic and etheric solutions. In the present paper studies are reported on the kinetics and mechanisms of electrodeposition of Al from plating baths of varying ratios of AlCl3 and LiAlH4 dissolved in tetrahydrofuran (THF). A complementary paper, which follows, deals with the topic of nucleation and growth processes involved in Al phase electrocrystallization and provides experimental results on that process. Steady-state cathodic and anodic Tafel polarization relations are determined, complemented by ac impedance studies. Back-reaction corrected Tafel plots enable anodic and cathodic transfer coefficients (α) to be evaluated together with corresponding stoichiometric numbers (ν). Critical evaluation both of the present α and ν results, and previous α values in the literature, enables a mechanism of elementary steps in Al deposition from the hydrido-chloride baths to be proposed.

High Performances of Pt/ZnO Catalysts in Selective Hydrogenation of Crotonaldehyde

Hydrogenation of crotonaldehyde in the gas phase, at atmospheric pressure and 353 K over Pt/ZnO catalysts, was studied. Two types of precursor, (Pt(NH3)4(NO3)2 and H2PtCl6, referred to as A and B catalysts, respectively, were used for catalyst preparation. Before the catalytic experiments the catalysts were reduced at different temperatures. The reducibility of the support and the catalysts was followed by TPR. Catalysts were also analysed by XPS and XRD. Rapid deactivation during time on stream was observed. The A and B catalysts showed different dependence on the reduction temperature. Thus, the A catalyst had the highest activity when reduced at 473 K; a further increase in the reduction temperature led to a decrease in the activity, but at 673 K both catalysts A and B showed nearly the same activity. On the B catalyst, the crotyl alcohol selectivity reached a value as high as 75–80%, whatever the reduction temperature. The B catalyst was better dispersed than the A catalyst and formed a PtZn alloy at low reduction temperature (473 K). It contained about 5 wt% chloride, whatever the reduction temperature. In contrast, Pt metal particles were only formed on the A catalyst, reduced at 473 K, and then showed low selectivity in crotyl alcohol. However, when the reduction temperature was increased, activity decreased and crotyl alcohol selectivity increased parallel to Pt–Zn alloy formation. One can speculate that Pt sites, when alloyed to Zn, formed Ptδ−–Znδ+ entities, on which the crotonaldehyde adsorbed by the carbonyl group rather than by the C=C double bond. On the B catalyst, the high selectivity observed, whatever the reduction temperature, led us to assume that besides the alloying effect, chlorine has an important promotor effect by increasing the polarity of Znδ+ in the PtZn catalytic sites and facilitating the carbonyl adsorption. A reaction network and mechanism were put forward. Kinetic models, developed from the proposed elementary step mechanisms, were used to discuss the influence of support and promoters on reaction selectivity.

Kinetics of α-Pinene Isomerization

The kinetics of pinene isomerization over clinoptilolite with formation of several products has been investigated in a batch reactor at atmospheric pressure. Initially mainly camphene and limonene were produced, and only at relatively high conversions were other secondary products formed. Selectivity values at particular conversions were seen to be independent of catalyst pretreatment, although activity was different. The observed kinetic regularities were modeled on the basis of an elementary step mechanism.

Kinetics of Thymol Hydrogenation over a Ni-Cr2O3 Catalyst

The kinetics of thymol hydrogenation over commercial nickel-chromia catalyst with formation of four menthol diastereoisomers have been investigated in a batch reactor at constant hydrogen pressure (0.4-4 MPa) at temperatures between 373 and 433 K in n-hexane and cyclohexane solutions. The rate of thymol consumption was independent of conversion. Initially mainly more thermodynamically unstable cis isomers (neomenthol and neoisomenthol) were produced and only at relatively high conversions trans isomers (menthol and isomenthol) were formed. However the stereoselectivity values at a particular conversion were seen to be independent of hydrogen pressure and reaction temperature. The observed kinetic regularities were modeled based on an elementary step mechanism

Kinetics of Crotonaldehyde Hydrogenation over a Titania-Supported Platinum Catalyst

Selective hydrogenation of [alpha],[beta]-unsaturated aldehydes into allylic alcohols remains a difficult task to achieve, especially in the gas phase, when working with heterogeneous catalysts. Crotonaldehyde hydrogenation is a complex reaction that has been modeled using results from a standard gas flow system. The obtained results give rise to the formation of a reaction network and an elementary step mechanism derived from steady-state kinetics. The selectivity of the system to give crotyl alcohol is explained in terms of the surface electronic gas model.

On the separability of catalyst activity and kinetic behavior

The separability of catalyst activity and kinetic behavior is examined for several reaction systems described by elementary-step mechanisms. It is shown that separability is directly related to the form of the elementary steps in a mechanism. Three general types of mechanisms are described. Activity and kinetics are always separable for type 1 mechanisms. The converse is true for type 2 mechanisms provided that the reactant surface coverage is niether extremely high nor low. For type 3 mechanisms, the most commonly encountered type, separability of kinetics and activity results when one of the steps in the mechanism is rate-controlling (surface reaction, adsorption etc.), and, conversely, nonseparable behavior is the direct result of the lack of a single rate-controlling step. It is shown that previous reports of nonseparable behavior can be adequatley described by using type 3 mechanisms. The use of nonseparable activity and kinetics for model discrimination based on structural differences between mechanisms is also discussed.