Vol 60, No 4 (2019)

Literature data on palladium clusters obtained by the interaction of Pd(II) compounds in solutions with hydrogen are summarized and analyzed. The conclusion is drawn on the influence of ligands and reduction conditions on the polynuclearity of clusters and on the autocatalytic nature of their formation. Palladium clusters become active in the selective hydrogenation of alkynes and dienes only after their treatment with hydrogen containing traces of oxygen or hydrogen peroxide. Clusters containing phosphide ligands are active in the homogeneous hydrogenation of unsaturated hydrocarbons with a high selectivity being of a thermodynamic nature. Most of the distinctive features of nanocluster synthesis in solution are also observed in the synthesis of metal-containing nanoparticles on the support surface from precursors. Catalysts for the liquid-phase selective hydrogenation of phenylacetylene on the Pd supported on coal with a nanoparticle size of 2.8 ± 0.2 nm are obtained. This size is comparable with the size of clusters in solution. The mechanism for the reduction of palladium acetate on carbon is also autocatalytic and is similar to the same process in solutions. A kinetic study of the selective hydrogenation of phenylacetylene to styrene in the presence of Pd supported on coal shows that it is impossible to adequately describe the experimental data in the framework of the Langmuir, Hinshelwood, or Eley–Rideal mechanisms. It is proposed to use this mechanism in the framework of cluster (polynuclear) active sites on the surface of Pd particles capable of adsorbing more than one substrate molecule or its fragments on the same site. This approach makes it possible to describe the mechanisms of other reactions occurring in the presence of nanoscale heterogeneous supported catalysts: deoxygenation of fatty acids obtained from renewable raw materials to paraffins on supported palladium catalysts and olefins on supported nickel sulfide catalysts.

This article considers pioneering works by M.I. Temkin in the field of adsorption, the theory of elementary stages of surface processes and general questions describing condensed phases using as examples the processes on surfaces in dense monolayers and in the volume phase. A brief review of the subsequent development of his ideas in these areas is given. The questions of the correct way to take into account the cooperative behavior of adsorbed species in equilibrium and surface reactions and the principle of self-consistent description of the rates of stages and the equilibrium state of the reactants for heterogeneous surfaces and nonideal reaction systems are discussed.

The kinetic regularities of the catalytic oxidation processes that proceed via a stepwise redox mechanism are considered. The main assumptions of the Mars–van-Krevelen model widely used to describe such processes are analyzed, and their correspondence to actual catalytic systems—typical catalysts for the partial oxidation of light alkanes (oxidative coupling of methane, oxidative dehydrogenation of C₂₊ alkanes, oxidative cracking of C₃₊ alkanes)—is examined. Certain features of the composition and operating conditions of these catalysts require some revision of existing notions about the activation of molecular oxygen, including factors that determine its interaction with active sites and participation in the catalytic cycle. Particular attention is paid to: the thermodynamic and kinetic conditions for the realization of the redox mechanism (thermochemistry of oxide systems, the lifetime of oxygen in a bound state); suitability of the redox-type mechanisms in the case of catalysts that do not contain cations with a variable oxidation state; chemical and phase transformations, which are part of the catalytic redox cycle; interpretation of the results of kinetic experiments in the study of heterogeneous-homogeneous processes and the influence of free-radical reactions on the values of experimentally determined kinetic parameters.

The conversion of hexadecane on a 4% Ni/Al₂O₃ catalyst in a temperature range of 20–300°C was studied using IR spectroscopy and catalytic methods. It was found that the dehydrogenation of hexadecane occurred at 20–100°C with the subsequent formation of aromatic products, but the rates of these processes were very low. As the reaction temperature was increased to 200°C, the 4% Ni/Al₂O₃ catalyst exhibited a maximum activity and high selectivity for the formation of 1-hexadecene, and aromatic compounds and cracking products were present in the reaction products. As the reaction temperature was further increased, the catalytic activity significantly decreased. This was due to the fact that polyaromatic deposits gradually accumulated on the catalyst surface in a temperature range of 200–300°C.

Together with exhaust gases from motor vehicles with diesel engines, soot and toxic nitrogen oxides are released into the environment. In this work, we consider the possibility of removing diesel soot on the surface of the Pt–Pd/MnO_x–Al₂O₃ oxidation catalyst with a low Pt–Pd content. The rate and activation energy of the catalytic oxidation of diesel soot with oxygen and NO_x were studied in the isothermal mode and compared with the kinetic characteristics of the oxidation of charcoal soot and two samples of synthetic soot (Printex U, Vulkan XC-72). The physicochemical properties of soot, such as nano- and microstructure, elemental composition, composition of surface functional groups, determine its oxidative reactivity, which is higher in mixtures with NO_x than in oxygen-containing mixtures.

An ordered mesoporous titanium–zirconium Ti_xZr_1–xO₂ matrix for introducing catalytic nanoparticles was synthesized by self-assembly using titanium isopropoxide and zirconium oxychloride as precursors and amphiphilic triblock copolymer F127 as a template. The process of self-assembly occurs without the addition of an acid to preserve the morphology and structure of the catalytic nanoparticles. When controlling the initial molar ratios of the copolymer to metal precursors, titanium–zirconium nanocomposites with controlled texture and composition were obtained in a wide range of titania content, from 15 to 80 mol % TiO₂. The structural and phase properties of the composites were studied by X-ray diffraction, low-temperature nitrogen adsorption, and transmission electron microscopy. Composites have an ordered mesoporous structure, a high specific surface area, a large pore volume, and a uniform pore size distribution. Catalytic coatings of 1 wt % Pd–Zn/Ti_xZr_1–xO₂ (x = 1.0, 0.8, 0.5) on the inner surface of a capillary reactor were prepared by the dip-coating method using a colloidal solution of Pd–Zn nanoparticles. The developed catalytic coatings based on titanium–zirconium composites exhibit high activity and selectivity (> 96%) in the hydrogenation of 2-methyl-3-butyn-2-ol.

As found using in situ small-angle X-ray scattering (SAXS), a porous structure consisting of compact particles with sizes from 2 to 8 nm was formed as a result of the sol–gel synthesis of silicate, silicate–phosphate, and silicate–phosphate–silver gels from tetraethoxysilane. It was established that the aggregation of silica particles in the presence of nitric acid proceeds according to a coalescence mechanism. The presence of phosphoric acid in the system changes the mechanism of aggregation for Ostwald ripening. The introduction of silver nitrate into the system does not significantly affect the size of silica particles and the mechanism of their aggregation.

Mole fractions and formation-depletion pathways of the first aromatic ring and its precursors, acetylene and propargyl radical as well as those of naphthalene are numerically investigated during the co-combustion of 1,3-butadiene/H₂ mixtures in low pressure premixed laminar flames conditions. The molar fraction of H₂ in the mixture is varied from 0 to 60% by adding 10% of hydrogen to the neat butadiene flame and by keeping the inert mole fraction (argon) and the equivalence ratio constants. The effect of hydrogen is investigated by using a modified Chemkin II version and by differentiating its chemical effect from its thermal and dilution effects. The obtained results indicate that the butadiene/H₂ oxidation process is controlled by the synergism between hydrogen dilution and chemical effects. Regardless of the hydrogen amount added to the fuel mixture, two routes are evolved in the benzene formation process, the direct C₄ + C₂ route and the indirect C₃ route, evolving fulvene as an intermediate species. Besides, the collected data reveal that hydrogen doping induces a noticeable decrease in the concentrations of all the studied species inferring that the hydrogen blended fuels are less prolific in soot formation than the neat 1,3-butadiene fuel. The observed decrease is dependent on the hydrogen initial concentration in the fuel mixture as well as on the nature of its effect.

In this study, we report a simple, efficient and green protocol for the synthesis of pyrano[2,3-c]pyrazole derivatives by the use of the natural phosphate K09 as a mild and efficient heterogeneous catalyst. The catalytic efficiency of K09 was compared with other heterogeneous catalysts to determine the best catalyst for said conversion. Easy recovery of the catalyst and its reusability, room temperature reaction conditions, short reaction time, excellent yields, are some of the important features of this protocol.