Progress in Reaction Kinetics and Mechanism

Hetero-/homogeneous micro-combustion of fuel-lean hydrogen–air mixtures in the plane channel containing a Pt/γ-Al₂O₃ catalyst washcoat was investigated numerically with detailed chemical kinetic mechanisms. The main theme of the paper is assessing the relative significance of the H₂ gas-phase reaction as compared with catalytic reaction at different plane channel heights H, inlet mass fluxes J, and equivalence ratios ϕ in plane micro-channels. The numerical model of microcombustion was employed, which included detailed gas-phase and surface catalytic reaction mechanisms, heat transfer mechanisms, and diffusion of multicomponent species. In order to sustain micro-combustion of fuel-lean hydrogen–air mixtures over Pt/γ-Al₂O₃ in the range of operating conditions, the solid wall temperature of the micro-combustors was maintained at 1400 K. In plane micro-channels, as the channel dimension is decreased, the micro-combustion characteristics of hydrogen–air mixtures are significantly impacted due to radical and effective heat losses to the walls. However, the catalytic walls contribute to sustain the H₂ gas-phase reaction in the micro-channels by decreasing heat losses to the walls due to the exothermic surface catalytic reaction, which also restrains the H₂ gas-phase reaction by extracting radicals owing to typically high absorption rates of the above-mentioned species at the walls. Therefore, the detailed radical chain reaction mechanisms can be significantly changed by the presence of surface catalytic reaction (wall reaction), and the radical accumulation in the gas-phase can be restrained. In the present work, the effects of the above-mentioned three key parameters on the interaction between the surface catalytic and gas-phase reactions are discussed. For fuel-lean hydrogen–air mixtures, in each case, the limiting values of the plane channel height, inlet mass flux, and equivalence ratio beyond which the gas-phase reaction become negligible as compared with the surface catalytic reaction are explored. The computational results indicate that variation of the hydrogen–air inlet mass flux at constant inlet equivalence ratio and plane channel height alters the balance between the diffusive and convective mass fluxes. Finally, the equivalence ratio of the hydrogen–air mixtures significantly effects the contribution of the H₂ gas-phase reactions to the entire H₂ conversion.

The kinetics of the entire process of the SAPO-34 catalyst for the transformation of methanol into olefins in the 400–450 °C range has been established by using the experimental data obtained in an isothermal fixed bed reactor. The model considers the activity change of SAPO-34 zeolite with time on stream. The activity of the catalyst consists of two parts: the stabilisation activity (while methanol is fully transformed) and the deactivation activity (while methanol is not all transformed). According to the activity of the catalyst, this study measured the evolution of each olefin with time on stream respectively.

The solvent effects on the reaction of 1-fluoro-2,4-dinitrobenzene with aniline was studied in various compositions of molecular-molecular solvent mixtures and molecular solvent-2-hydroxyethylammonium formate mixtures as media containing an ionic liquid. A mounting trend with the mole fraction of water in aqueous solution of alcohols up to a maximum at a mole fraction of 0.9 with respect to water was found, but a mild decrease with the mole fraction of ethyl acetate in methanol–ethyl acetate mixtures. A similar increasing trend was observed with increasing methanol, ethanol, 2-propanol and dimethylsulfoxide content in the mixtures containing the ionic liquid. Solvent effect investigations based on linear free energy relationship confirm that polarity has a major effect in molecular-molecular solvent mixtures, whereas polarity and hydrogen–bond acceptor ability were the dominant factors on reaction rate in molecular-ionic solvent mixtures. A nonlinear free energy relationship based on the preferential solvation hypothesis showed differences between the microsphere solvation of the solute and the bulk composition of the solvents. In these mixtures, the intermediate of the reaction was preferentially solvated by the ionic liquid.

The adsorption kinetics of adenine and adenosine on a ligand-exchange matrix, carboxymethyl diaminoethane poly(vinyl chloride) in cobalt form (cobalt-CMEDA-PVC), have been studied in a fixed-bed column system. The initial ligand concentrations ranged between 1.25×10⁻⁵ and 2.0×10⁻⁴ mol L⁻¹. The kinetic studies were carried out spectrophotometrically, in order to determine the ratecontrolling step of the adsorption. The kinetic data of the single-component systems were acquired from the breakthrough curves of the ligands on the basis of the equivalent points method. In this study, the homogeneous diffusion model was adapted to a fixed-bed column system to describe the differentiation in the ligand concentration of the effluent, beginning from breakthrough point as a function of time. The results obtained for the adsorption kinetics indicated that the diffusion step plays a significant role in the adsorption kinetics of the ligands, due to the fact that the adsorption performance was strongly affected by the initial concentration of ligands.

The decomposition processes and reactivity of C₄F₉0CH₂O• radical formed from C₄F₉0CH₃ (HFE-7100) have been studied by density function theory computational methods. All calculations were performed at B3LYP and mPW1PW91 levels of theory with the 6-311G(d,p) basis set. The calculated barrier heights were further improved by QCISD(T)/6-31G(d)//MP2/6-31G(d) methodology to obtain better rate constants. Five possible pathways were investigated: reaction with O₂, reaction with OH radical, C-O bond dissociation, release of H radical and finally rearrangement of the radical and then C-O bond cleavage with energy barriers of 6.35 (6.09) [12.12], 12.85 (16.87) [7.51], 17.05 (21.77) [28.34], 20.3 (20.75) [18.13], 32.60 (31.50) [32.63] and 16.07 (18.73) [20.04] kcal mol⁻¹, respectively (the values in the parentheses for mPW1PW91 and in the brackets for the QCISD(T) method). Rate constants were calculated by utilising canonical transition state theory in the temperature range of 200–400 K and 1 atm pressure, and Arrhenius diagrams were plotted. The results showed that H elimination and H abstraction pathways are dominant for degradation of C₄F₉0CH₂O• radical in the atmosphere. A smooth transition from the reactants to products on the corresponding potential energy surface was confirmed by intrinsic reaction coordinate calculations.

The kinetics and mechanism of nitrogen monoxide (NO) adsorption and oxidation on a tungsten oxide nanocrystalline surface have been studied. First principle calculations were performed within the framework of density functional theory with the ONIOM method at the (B3LYP/LANL2DZ:UFF) level. The activation energy for the rate-determining step of the reaction was determined as 0.45 eV. NBO analysis confirmed that charge transfer has occurred from the adsorbate to the surface. Density of state spectral analysis showed that the Fermi level was shifted to higher values and confirmed the effective interaction between the nanocrystal and adsorbate. HOMO–LUMO analysis showed that crystal band gap was reduced after adsorption. Quantum reactivity indices showed that the chemical hardness was reduced after adsorption which corresponded to the band gap reduction. The electronic chemical potential was shifted to more positive values while the electrophilicity of the surface was reduced after adsorption. These parameters predict that charge transfer has occurred to the surface.

Electro-oxidation of catechol derivatives has been studied in the presence of N-methylbenzylamine as nucleophile in phosphate buffer (c=0.15 M, pH=7)/ethanol solution using cyclic voltammetry and controlled-potential coulometry (CPC). Voltammetric data indicate that a 1,4-Michael addition of N-methylbenzylamine from its NH group to the electrochemically derived o-quinone via an ECE mechanism has occurred. Moreover, the progress of coulometry confirmed our electrochemical mechanism. The electrochemical synthesis of quinone derivatives has been successfully accomplished using CPC in a two-compartment cell and the products were characterised by FTIR and NMR spectroscopy and elemental analysis.

The thermal stability and decomposition kinetics of riboflavin were investigated by nonisothermal thermogravimetric analysis (TGA) experiments in an inert atmosphere. For kinetic analysis, riboflavin was heated from room temperature to 800 °C with five different heating rates (5,10,15,20 and 30°C min⁻¹). From the thermal decomposition process, it was found that there are two main stages of pyrolysis. In the DTG thermograms, the temperature peaks at maximum weight loss rate changed with varying heating rate. The kinetic parameters of decomposition including apparent activation energy (E_a) and lnA (pre-exponential factor) under an inert atmosphere have been evaluated from the model-free isoconversion methods of Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO) and Friedman over the whole temperature domain. It was found that values of E_a and lnA decrease with increasing conversion (α). The reaction order does not have a significant influence on the process because of the high value of the pre-exponential factor. The apparent activation energy distributions with conversion calculated by these methods ranged between 15.18 and 236.55 kJ mol⁻¹, and varied over a broad range in a complex manner depending on the heating rate.

Electrochemical oxidation of catechol has been studied in the presence of thioacetamide (TAM) in a phosphate buffer solution (0.2 mol L⁻¹, pH=6.0) at the surface of glassy carbon electrode by means of cyclic voltammetry. After the cyclic voltammetric investigation of the reaction mechanism of TAM with catechol, TAM has been determinated by means of the differential pulse voltammetry technique. The results showed that the cathodic peak current of catechol reduced in the presence of various concentrations of TAM and that catechol can participate in a 1,4-Michael addition reaction with TAM under an EC mechanism. The decreasing of the cathodic current of catechol showed a linear relationship with the TAM concentration that can be used for the determination of TAM with a detection limit less than 3 μmol L⁻¹.