Progress in Reaction Kinetics and Mechanism

The mechanism of the cyclic reaction N₂O(X¹Σ⁺) + CO(¹Σ⁺) → N₂(X¹Σ_g⁺) + CO₂(¹Σ_g⁺) catalysed by Y⁺ ions has been investigated on both singlet and triplet potential energy surfaces. The reactions were investigated by means of the relativistic effective core potential together with the Stuttgart basis sets on Y and the UB3LYP/6-311G** level of theory on non-metal atoms. The crossings involved between the singlet and triplet energy surfaces have been investigated by means of the intrinsic reaction coordinate approach used by Yoshizawa et al. Furthermore, both steps of the reaction are exothermic and the overall reaction is exothermic by 361.12 kJ mol⁻¹.

Rh catalysts supported on Fe₂O₃, Co₃O₄ and Fe₂O₃–Co₃O₄ mixed oxide were prepared by the co-precipitation method. The effect of the support on the performance of the Rh catalysts for the hydroformylation of dicyclopentadiene was investigated using X-ray photoelectron spectroscopy, H₂-temperature-programmed reduction, H₂-temperature-programmed desorption and Brunauer–Emmett–Teller analysis techniques. The results indicated that the Fe₂O₃–Co₃O₄ supported catalyst had a higher dispersion of Rh and thus more Rh⁺ sites. As a result, the Fe₂O₃–Co₃O₄ supported Rh catalyst exhibited higher activity compared with counterparts supported on Fe₂O₃ and Co₃O₄.

Pt-promoted S₂O₈^2–/ZrO₂–Al₂O₃ (SZA) catalysts were successfully prepared by the microemulsion (Pt-SZA-M) and impregnation (Pt-SZA-I) methods. The effects of the preparation parameters (surfactant-to-oil ratio, surfactant-to-cosurfactant ratio, types of cosurfactant and oil phase, etc.) on the catalytic activity of Pt-SZA-M catalysts in the isomerisation of n-pentane were investigated. The results showed that the optimal catalyst preparation conditions were: a cetyltrimethylammonium bromide (CTAB)/n-butanol mass ratio of 4:6, a (CTAB + n-butanol)/cyclohexane mass ratio of 3:7, a content of H₂PtCl₆ solution in microemulsion of 3.6 wt% and a N₂H₅OH/H₂PtCl₆ molar ratio of 25:1. A comparison of the isopentane yield for Pt-SZA-M with that for Pt-SZA-I was also performed. The results showed that the microemulsion method led to smaller and more uniform Pt particles (4.5 nm) on the SZA. In comparison with Pt-SZA-I, the isopentane yield for Pt-SZA-M was increased by 13.5% at 180 °C, showing the catalytic performance of Pt-SZA-M at lower temperature was improved significantly. At a reaction temperature of 230 °C, a pressure of 2.0 MPa, a hydrogen/n-pentane molar ratio of 4:1 and a weight hourly space velocity of 1.0 h⁻¹, the yield of isopentane for Pt-SZA-M reached 60.8%.

A number of Zn–S₂O₈^2–/ZrO₂–Al₂O₃ (Zn(x)–SZA) catalysts with different Zn mass fractions were synthesised and characterised by using X-ray diffraction, the Brunauer–Emmett–Teller method, and H₂ temperature-programmed reduction. The structure and isomerisation performance of Zn(x)–SZA catalysts were studied using n-pentane as a probe reaction. The results showed that a pure tetragonal ZrO₂ phase was formed on Zn(x)–SZA, and the ZrO₂ crystallite sizes of the tetragonal phase increased in the order: Zn(0.5)–SZA < Zn(1.0)–SZA < Zn(1.5)–SZA < Zn(2.0)–SZA < SZA. Zn can strengthen the interaction between persulfate ions and the support, promote the formation of stronger acidity, lead to a better dispersion of sulfate ions on the surface, and improve the redox performance of the catalysts. The Zn(1.0)–SZA catalyst exhibited the best catalytic activity for n-pentane isomerisation. At a temperature of 170 °C, a reaction pressure of 2.0 MPa, a molar H₂/n-pentane ratio of 4:1, and a weight hourly space velocity of 1.0 h⁻¹, the isopentane yield reached 58.0%.

It is important to understand the nature of formic acid (HCOOH) adsorption on transition metal surfaces so as to elucidate the mechanism of its decomposition in direct HCOOH fuel cells. Despite the overwhelming interest, not much is known about the adsorption behaviour of HCOOH on Pd(111) surfaces. This work presents a theoretical study of monomeric HCOOH adsorption on Pd(111) surfaces in the gas phase. The stable configurations of the adsorbed molecules on the surface are displayed, with some omitted in previous studies. Our calculations show that the most stable configuration is that with the hydroxyl O and H on C in HCOOH binding to the surface. The modes of adsorption of HCOOH dimer are presented on Pd(111) for the first time. It is found that both the monomer and dimer of HCOOH weakly interact with the surface. Our results are intended to provide valuable insights to understand the reactivity of HCOOH on Pd(111).

The kinetics and mechanism of the thermal decomposition reaction of 3-oxetanone in the gas phase were studied using quantum chemical calculations. The major products of this reaction are formaldehyde, ketene, carbon monoxide, ethylene oxide, ethylene and methyl radical. Formaldehyde, ketene, carbon monoxide and ethylene oxide are the initial decomposition products and other species are the products of ethylene oxide decomposition. The results of B3LYP and QCISD(T) calculations reveal that thermal decomposition of 3-oxetanone to ethylene oxide and carbon monoxide is more probable than to formaldehyde and ketene from an energy viewpoint. Moreover, quantum theory of atoms in molecules and natural bond orbital analysis indicate that 3-oxetanone decomposition to formaldehyde, ketene, carbon monoxide and ethylene occurs via a concerted mechanism and bonds that are involved in the transition states have a covalent character. Moreover, the calculated changes in bond lengths in the transition states reveal that bond breaking and new bond formation occur asynchronously in a concerted mechanism.

Density functional theory calculations with different functionals have been performed to investigate the kinetics and mechanism of the thermal decomposition of oxalic acid in the gas phase and in solution in aniline, N-methylaniline, quinoline and DMSO. Five major possible mechanisms for this reaction have been proposed. Comparison between the theoretical data and experimental results shows that in the most probable path of this reaction CO, CO₂ and H₂O are produced. Moreover, this reaction is a little faster in the gas phase than in solvents. Quantum theory of atoms in molecules analysis indicates that C–C and O–H bond formations in the transition states have a covalent nature, because the ratios of kinetic energy density to potential energy density at the corresponding bond critical points are smaller than 0.5. Based on natural bond orbital analysis, during transition state formation, the interaction energy between the lone pair of electrons of the oxygen atom and the π* molecular orbital of the critical C–O bond is reduced and solvents affect the charge transfer process.

The mechanism of alcoholysis of acylpalladium(II) complexes relevant to the alternating copolymerisation of ethene and carbon monoxide has been investigated theoretically in detail. The solvolysis of acylpalladium(II) complexes is an important step in palladium-catalysed reactions. Based on experimental studies, two mechanisms have been proposed for this process, which consist of a concerted reductive elimination and an insertion mechanism (reductive elimination via a Meisenheimer intermediate). Both mechanisms include deprotonating of an acylpalladium(II) complex and according to our calculations, any mechanism involving this step, has an energy barrier higher than that of the rate-determining step. We propose a new mechanism for the insertion in which proton transfer to Pd is simultaneous with an inner-sphere attack of the alkoxide ligand (OCH₃) at the carbon atom of the palladium-bound carbonyl group (new Meisenheimer intermediate). Considering solvent effects, the activation energies of the two mechanisms and other contingent mechanisms were calculated and compared with each other and the experimental results.

We report the role of hydrogen spillover in n-hexane isomerisation over carbon nanotubes or SiC mixed with hierarchical zeolite beta. Here we infer that: (1) hydrogen spillover is a necessary step; (2) spilled-over H hydrogenates isomerised olefin at Brønsted acid sites only; and (3) closeness between acid and metal sites at the nanoscale enhances the rate of reaction, though this is not a requirement for this reaction to proceed. The economic advantage of the spillover phenomenon (necessitating less Pt on the catalyst) has been highlighted.

Quantum chemistry calculations have been performed to investigate the kinetics and mechanism of the cycloaddition reaction between cyclopentadiene and N-alkylmaleimides in the gas phase and different solvents. To investigate the effects of side-chain length on the cycloaddition reaction, the rate constants and kinetic parameters of the reaction between cyclopentadiene with N-methyl-, N-ethyl-, N-propyl- and N-butyl-maleimide were calculated. The results obtained indicate that the reaction in solvents is faster than in the gas phase. Moreover, the dipole moments of the transition states are larger than those of the reactants. Therefore, the reactions in the most polar solvent (water) are faster than in ethanol, n-hexane, 2,2,2-trifluoroethanol (TFE) and acetonitrile. Quantum mechanics-molecular mechanics (QM/MM) calculations on the reactions using the explicit solvent model for water and TFE indicate that hydrogen bond interactions of the solvents have a key role in the rate of the reaction and these are more important than the polarity of the solvent. Natural bond orbital analysis reveals that the charge transfer between the reactants in solvents is more than in the gas phase. Finally, HOMO–LUMO analysis indicates that solvents increase the reactivity of the reactants in comparison to the gas phase.

The intrinsic kinetics of the Fischer–Tropsch synthesis reaction in the conversion of synthesis gas into liquid hydrocarbons in a spinning basket reactor over a cobalt catalyst supported on carbon nanotubes were studied. The catalyst was synthesised by the impregnation wetness technique and the reactor tests were done at 215–245 °C, 20 bar, a H₂/CO molar ratio of 2 and a gas hourly space velocity of 2.4–12 Nl g_cat⁻¹ h⁻¹. To develop an appropriate kinetic model for syngas consumption, five different mechanisms were considered as possibilities for the Fischer–Tropsch reaction, each with a rate-determining step. The rate equations obtained were based on the Langmuir–Hinshelwood–Hougen–Watson model and statistical analyses were used for comparing the resulting values. The activation energies were found to be limited within the range 89–145 kJ mol⁻¹ and the adsorption constants of hydrogen and carbon monoxide were in the range −32 to −76 kJ mol⁻¹ and −13 to −64 kJ mol⁻¹ respectively.

The theoretical molecular adsorption energies, vibrational frequencies and total density of states of carbon monoxide (CO) on the (100), (110) and (111) surfaces of the face-centred cubic (FCC) crystalline phase of metallic cobalt were investigated using density functional theory calculations. The on-top adsorption state and three surface coverages were used for comparison of the results. The geometries of cobalt FCC surfaces, as well as those with adsorbed CO molecules and the CO binding energies were calculated with the generalised gradient approximation (GGA-D) using the revised revPBE-D₃(BJ) functional. The theoretical results for adsorption energies of carbon monoxide were proportional to the electron density of the cobalt surfaces, according to the following order: FCC (100) > FCC (110) > FCC (111). For CO adsorbed on the surface of cobalt metal the C–O distance increases, producing a weakening of the bond and the calculated stretching frequency decreases when compared with the isolated molecule.