Vol 59, No 6 (2018)

An active cobalt catalyst supported on functionalized carbon nanotubes was prepared and tested for CO hydrogenation to produce liquid hydrocarbons in the Fischer–Tropsch synthesis. The catalyst characterization was carried out using different methods including X-ray diffraction, transmission electron microscopy and BET surface area equation. The kinetic experiments were performed in a fixed-bed reactor under following conditions: T =200–240°C, P = 15–30 bars, GHSV = 0.5–1.5 nL \({\text{g}}_{{{\text{cat}}}}^{{ - 1}}\) h^–1 and H₂/CO feed ratio (mol/mol) = 1–2.5. Based on various mechanisms and Langmuir–Hinshelwood–Hougen–Watson type rate equations, eleven kinetic expressions for CO consumption were tested and the best-fitted model is achieved. In kinetic rate development, various rate-determining steps (RDS) are considered for evaluation of RDS effects. The kinetic parameters were estimated with nonlinear regression method using Levenberg–Marquardt method to make refined optimization. The obtained energy of activation was 78 kJ/mol for optimal kinetic model. The obtained results show that the best models for proposed elementary reactions involve the formation of surface species as RDS rather than syngas adsorption.

This work is a theoretical and practical study on the synthesis and characterization of a ZnO photocatalyst doped with \({\text{SO}}_{4}^{{2 - }}.\) X-ray diffraction analysis, band gap energy ranges and BET surface area measurements demonstrated that \({\text{SO}}_{4}^{{2 - }}\) had an impact on the photocatalyst properties of ZnO, when this material was used for the production of oxidizing agents by the action of ultraviolet radiation. Significant changes in the band gap energy were observed for ZnO doped with 3 mol of \({\text{SO}}_{4}^{{2 - }}.\) The photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated, a toxic pesticide found in agricultural effluents in Sinaloa (Mexico). Degradation process was monitored in photoreactors of 500 mL, in which the maximum degradation efficiency was achieved after 6 h. A removal efficiency of 38.4% (1.55 mmol/L) was found with an undoped ZnO, while an efficiency of 82.3% (3.4 mmol/L) was observed with \({\text{SO}}_{4}^{{2 - }}\) doped ZnO. Based on the results obtained, the photocatalysts used in this study can be considered as promising materials to mitigate pollution problems coming from agricultural activities in these regions.

N-Hydroxyphthalimide (NHPI) is an efficient organic catalyst in the oxidation reactions of organic compounds occurring via a radical mechanism, often used together with redox-active ions or transition metal complexes. In this work the catalytic action of NHPI is studied together with Cu(II), Fe(III), and Mo(VI) compounds in the reaction of aerobic oxidation of cyclohexene in an acetonitrile solution at 60°C. It was found that iron(III) benzoate accelerates the reaction by rapidly generating the active form of the phthalimide-N-oxyl radical (PINO) catalyst, but does not cause decomposition of the hydroperoxide. The oxidation product is 2-cyclohexenyl hydroperoxide formed with a selectivity of 85% at a cyclohexene conversion of 50%. Copper(II) acetate initiates oxidation and is capable of catalyzing the radical decomposition of the hydroperoxide and secondary oxidation of allyl oxygenates. When reaching a cyclohexene conversion close to 80%, the overall selectivity to the main products, 2-cyclohexenyl hydroperoxide and 2-cyclohexen-1-on, was 70%. The addition of iron(III) and molybdenum(VI) compounds led to the intensive generation of hydroperoxide and its activation as an electrophilic reactant capable of cyclohexene epoxidation. As a result of the use of the multifunctional three-component NHPI–Mo(VI)–Fe(III) catalyst, cyclohexene oxidation by molecular oxygen occurred with the formation of epoxycyclohexane. The selectivity to the products of cyclohexene epoxidation was close to 50%, which is a value expected from theory.

Several bifunctional catalysts for dimethyl ether (DME) synthesis from syngas are prepared on the basis of commercial methanol-synthesis Megamax 507 catalyst. Commercial HZSM-5 zeolites with a SiО₂/Al₂О₃ ratios of 23, 80, and 307 and γ-alumina were used as dehydration components. Physicochemical characteristics of zeolites and alumina are studied: temperature-programmed desorption of ammonia, the porosity, and the specific surface area. The activity of catalyst in DME synthesis is studied in a microcatalytic flow-type setup at a pressure of 3 MPa in a temperature range of 200–260°С with a productivity based on syngas of up to 30000 L \({\text{kg}}_{{{\text{cat}}}}^{{ - 1}}\) h^–1. The composition of syngas was (vol %): CO, 21; CO₂, 6; Н₂, 67; N₂, 6 . It is shown that zeolites, especially with silica/alumina ratios of 23 and 80, are more active than alumina in methanol dehydration to DME, but in the presence of these zeolites, traces of hydrocarbons were detected at 260°С. The zeolite with a silica/alumina ratio of 307 is the most interesting of the studied zeolites. Hydrocarbons are almost not formed on it, and its activity in methanol dehydration is somewhat higher than that of alumina. The behavior of the methanol-synthesis component of the bifunctional catalyst is studied: the apparent activation energy of methanol synthesis and the degree of approaching to equilibrium are estimated depending on the catalyst load.

A series of model bimetallic Pd–Ag catalysts with different ratios of metals and size distributions of supported particles was prepared by successive metal deposition onto a modified surface of highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structural and electronic properties, as well as the morphology of nanoparticles at all stages of catalyst preparation. The use of the method made it possible to obtain bimetallic particles with various structures: alloyed, core–shell, and those consisting of individual monometallic particles. The XPS method with synchrotron radiation was used to determine the patterns in the formation of alloyed bimetallic particles, their chemical composition, structure, and the ranges of thermal stability under conditions of ultrahigh vacuum.

Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to study microstructure, morphology and chemical composition of the surface and near-surface layers of polycrystalline wire of commercial platinoid gauzes containing Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) after ammonia oxidation (10 vol % NH₃) by air at 1133 K for 50 h in the presence of these gauzes. Upon the completion of the catalytic reaction of ammonia oxidation, reconstruction (catalytic etching) of the surface layer on the backside of gauze wire (in the direction of the gas flow) was observed, in which the regions with different degrees of etching were identified. The analysis of these regions showed that the catalytic etching of the platinoid wire is initiated by etching the surface layer in the region of grain boundaries and dislocations in the course of highly exothermic catalytic reaction of ammonia oxidation by oxygen penetrated in the regions of defects. The regions with minimal etching contain smooth grains with crystalline terraces, 50 nm high, and with etching pits with size of ~72 nm in a concentration of 4.2 × 10⁸ cm^–2. The region with medium etching includes rough grains with etching pits with size of ~85 nm in a concentration of 2.5 × 10⁸ cm^–2. The regions with maximal etching consist of recrystallized grains with large pores with sizes of 350–400 nm in concentration of 8.9 × 10⁶ cm^–2. These grains are separated by voids with a width of 1–5 μm and a depth of 10 μm, which increases the specific surface area in the surface layer of wire. The growth of the specific surface area of the platinoid wire is accompanied by an increase in the volume rate of ammonia oxidation and, as a result, local overheating due to the high exothermicity of the reaction. With increasing temperature, the rate of diffusion of metal atoms increases, which, in turn, accelerates etching in this region. These processes lead to increasing the region of etching along the wire, which points to the autocatalytic regime of etching of platinoid gauzes in ammonia oxidation by oxygen.

The parameters of deuterium adsorption on the surface of gold nanoparticles supported onto highly oriented pyrolytic graphite were determined. It was established that surface coverage with the adsorbate at a level of single nanoparticles began at a graphite–gold interface; thereafter, the entire surface was filled.