Kinetika i kataliz

The number of observations demonstrating a significant effect of droplet sizes on the kinetics of chemical processes has increased with the expansion of the scope of application of spray technology. The equations linking the concentrations of reagents, the volume of droplets, the initial composition of the solution, the composition of the gas medium and the speed of processes are formulated within the framework of formal chemical kinetics. Using the example of second-order reactions (coupling, exchange, condensation, polymerization, polycondensation), it is shown that size kinetic effects occur when chemical processes are accompanied by changes in the droplet sizes in equilibrium with the gas medium. The results of computer simulation of condensation reaction and polycondensation process reproducing size effects are presented. Kinetic curves obtained by modeling the polycondensation process are compared with experimental data.

Processes of photocatalytic hydrogen generation from the formic acid water solution under vis-light irradiation with tantalum contained metal-ceramic silicon nitride-based composites were investigated depending on pH of the solution and hydrogen peroxide adding. These compounds were obtained by self-propagated high temperature (SHS) synthesis in the way of the ferrosilicoaluminum (FSA) and silicon-aluminum powders ignition in a nitrogen atmosphere with the tantalum addition. During the investigation it was found out that the reaction rate of the hydrogen production without hydrogen peroxide can be described within the Langmuir–Hinshelwood mechanism. There is the reaction mechanism changing simultaneously with a formic acid concentration increasing in the presence of H₂O₂. The most significant reaction rate of hydrogen production from HCOOH is observed with the Fe-contained composite synthesized from FSA in the solution system without H₂O₂ addition, the reaction turns of frequency (TOF) is 4.55 µmol/min.

Using the method of wet impregnation of alumina with iron and palladium nitrates, 1Pd0.5Fe and 1Pd10Fe catalysts modified with iron oxides were prepared with a target content of 1 wt % Pd, 0.5 or 10 wt % iron. The catalysts were compared with each other and with the monometallic catalyst 1Pd in the hydrodechlorination (HDC) of diclofenac (DCF) in dilute aqueous solutions at 30°C in batch and flow reactors after high-temperature (320°C) and mild (30°C) reduction; the latter was carried out in a batch or flow reactor. Using X-ray photoelectron spectroscopy (XPS), it was shown that after reduction at 320°C the surface of catalysts contains mainly Pd⁰, Fe²⁺ and Fe³⁺. The surface Fe²⁺/Fe³⁺ ratio increases as the iron content decreases. The reduction of Pd²⁺ to Pd⁰ is possible already at 30°C, but it proceeds much worse on the surface of 1Pd0.5Fe compared to 1Pd10Fe. According to XPS data, temperature-programmed reduction and infrared spectroscopy of diffuse reflection of adsorbed CO, modification with iron oxides increases the palladium content on the surface compared to 1Pd, promotes the emergence of new Pd–O–Fe centers, and affects the ability of palladium to be reduced. These effects increase with increasing iron content. Iron-modified catalysts reduced at 320°C showed similar activity and stability in the conversion of DCP in flow-through and batch systems. Unlike 1Pd0.5Fe, the 1Pd10Fe catalyst is highly efficient and stable even after mild reduction at 30°C. Under flow conditions with comparable DCF conversion, it provides increased selectivity in the HDC reaction of diclofenac compared to 1Pd, which is also active in hydrogenation.

Present paper demonstrates that relative reactivity estimated under competition of several similar substrates can be applied for the demonstrative visualization of the dynamics of active catalyst in a complex catalytic process. The fundamental advantage of the proposed approach is that the state of an active catalyst can be monitored throughout the catalytic reaction without differentiation of the kinetic data on the concentrations of the substances reacted.

Changes in the composition of anchored [M(COD)Cl]₂–NH₂–C₃H₆–SiO₂ and [M(COD)Cl]₂–P(Ph)₂–C₂H₄–SiO₂ (where M = Ir, Rh) catalysts in reactions of gas-phase selective hydrogenation of propene, propyne and 1,3-butadiene with parahydrogen (p-H₂) were studied using XPS. The atomic ratio M/Cl has been proposed as an indicator of the stability of the structure of the anchored complex, both at the stage of sample preparation and in the reaction. Based on a comparison of XPS data and the results of catalytic testing using parahydrogen-induced polarization, it is shown that the stability of the anchored {[M(COD)Cl]₂–Linker–SiO₂} complex during hydrogen activation is a key factor in the catalytic behavior of systems. Such stability is influenced not only by the chosen metal and linker, but also by the nature of the hydrogenated substrate.