Kinetics and Catalysis最新文献

In heterogeneous catalytic reactions, the structure of the catalyst and the nature of its surface-active sites are essential for a comprehensive understanding of the reaction mechanisms. Both physical and chemical adsorption technologies play pivotal roles in addressing these issues. Physical adsorption technology has emerged as a preferred method for determining the specific surface area, pore structure, and diffusion properties of porous catalytic materials due to its wide applicability and reliability of test results. Meanwhile, chemical adsorption is valuable for characterizing the type and quantity of active sites on catalysts, including acidic and basic sites, as well as metal dispersion and the adsorption of reactant gases. A thorough understanding of how physical and chemical adsorption technologies are applied in catalytic research can enable researchers to accurately investigate catalytic reaction mechanisms. This review begins by outlining the principles behind physical and chemical adsorption technologies. It then presents various studies showcasing the use of different probe molecules in examining the structure and active sites of catalysts. Finally, we discuss potential future directions for the development of physical and chemical adsorption methods, offering new insights for more advanced adsorption techniques and their applications in catalytic research.

The interaction of hydrazine monohydrate with nickel on various carriers has been investigated using a set of physicochemical methods. Hydrazine monohydrate was adsorbed on catalysts in forms both active and inactive in the IR region of the spectrum. The adsorption sites of hydrazine monohydrate particles were localized on the carrier. No correlation was found between the spectral manifestations of a number of catalysts under study and their catalytic activity in hydrogen formation. The surface hydrazine complexes activated by adsorption on the carrier diffused onto metal clusters, where the main reactions of hydrogen formation occurred. It was found that the reaction energy decreased the size and, apparently, rearranged the structure of the clusters with the appearance of centers suitable for efficient intramolecular dehydrogenation of hydrazine. This process was most effective on smaller clusters, possibly because a stronger Me−H bond was formed on them. Adsorption of hydrazine monohydrate through hydrogen atoms was possible on these clusters. These conditions ensured the predominant formation of hydrogen at low temperatures. An increase in temperature promoted a competing reaction of ammonia formation associated with the N–N bond rupture in the adsorption complex, which led to the formation of NH₂ complexes and then ammonia.

n-Heptane is an important component of liquid motor fuels, which is included in the composition of reference fuel mixtures for determining the octane number of gasoline, and it is also the most important component of surrogate fuels. Despite a large number of experimental and numerical studies on the kinetics of its oxidation and combustion, there are very few data on the composition of intermediate products of its combustion at elevated pressures. In this work, we used flame-sampling molecular-beam mass spectrometry (MBMS) to study the chemical structure of flames of premixed stoichiometric (ϕ = 1.0) and rich (ϕ = 1.5) n-heptane/O₂/Ar mixtures stabilized on a flat-flame burner at pressures of 2, 4 and 6 atm. The mole fraction profiles of key intermediate products of n-heptane combustion, including atoms and radicals, and acetylene and propargyl as precursors of polycyclic aromatic hydrocarbons (PAHs) were measured. Numerical simulation of the flame structure were performed using various detailed chemical-kinetic mechanisms of heavy hydrocarbon combustion proposed in the literature. It was found that the models adequately described the experimentally observed changes in the composition of combustion products both with increasing pressure and with increasing fuel equivalence ratio. Although experiments in rich flames showed a complete absence of acetylene in the final combustion products, all models predicted a high acetylene content, which indicated the need for further study of the kinetics of C₂H₂ conversion in combustion of rich mixtures.

Highly porous SiO₂ supports with the MCM-41 structure were synthesized by a template method. The Pt–Ga catalysts for propane dehydrogenation were prepared using an impregnation method. The structure of the synthesized samples was studied by low-temperature nitrogen adsorption and X-ray diffraction (XRD) analysis, and the characteristics of catalyst reduction were studied by temperature-programmed reduction in hydrogen (TPR-H₂). The catalytic properties were tested in the reaction of propane dehydrogenation, and the influence of a hydrogen additive to the reaction mixture on the activity of the catalysts was examined. The developed approach demonstrated the possibility of obtaining MCM-41-based materials characterized by an ordered mesoporous structure (mesopores of 3–4 nm in size) and a high specific surface area, which makes them promising for use as catalyst supports. It was found that the introduction of hydrogen into the reaction mixture led to an increase in the stability and activity of platinum catalysts.

Cermet CoZr/AlO_x(OH)_y/Al catalysts containing different amounts of the active component, the CoZr nanocomposite, were studied in Fischer–Tropsch synthesis (FTS). The CoZr nanocomposite was prepared by mechanochemical treatment of metallic Co and Zr powders. The cermet catalysts were obtained by mixing the CoZr nanocomposite powder with aluminum powder, followed by hydrothermal treatment. A disproportionate increase in the efficiency of the active component was observed with an increase in its content in the cermet. It was shown that in the catalyst with a higher content of the active component, the CoZr nanocomposite, the amount of aluminum hydroxo compounds (products of aluminum oxidation by water that block the cobalt surface on CoZr particles) is lower. The selectivity to the main syngas conversion products remained unchanged regardless of the fraction of the CoZr nanocomposite in the cermet. A comparison was made between the performance of the studied catalysts and oxide and cermet FTS catalysts known from the literature in terms of C₅₊ hydrocarbon production.

The presence of sulfur compounds resistant to hydrogenolysis complicates the production of commercial diesel and marine fuels with low sulfur content. This study examines the effect of catalyst composition and temperature (110–150°C) on the low-temperature oxidative removal of benzothiophene (BT) and dibenzothiophene (DBT) from hexadecane (model feedstock) using carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs), as well as Pd-based catalysts supported on them, with oxygen as the oxidant. The catalytic oxidation of BT to sulfone (BT-SO₂) showed a positive effect of both nitrogen doping of CNTs and the introduction of small amounts of Pd (0.2 wt %), which stabilized atomic/clustered palladium species on pyridine-like nitrogen centers (Pd²⁺–N_Py). Most of the BT-SO₂ formed during the experiment (6 h) underwent thermal decomposition or oxidation with SO₂ release, reducing the need for an additional sulfone extraction step in diesel fuel processing. In the catalytic oxidation of DBT to sulfone, nitrogen doping of CNTs had no effect. The highest DBT conversion to sulfone and its further oxidation were observed in the presence of the 0.2 wt % Pd/CNT catalyst, in which palladium was present as nanoparticles with a size of 1.1 nm.

High-temperature oxidation of NH₃ with air to NO oxide on platinum alloy gauzes is used in the industrial production of nitric acid, which is employed to manufacture agricultural fertilizers. To enhance the efficiency of the catalytic gauzes, particular attention is paid to the etching processes initiated by the oxidation of NH₃ on platinum group metals (PGMs) and their alloys. In this study, high-resolution scanning electron microscopy (HR SEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS) were used to investigate and characterize microrelief, morphology, structure, and chemical composition of the etching structures on Pt(poly) after NH₃ oxidation with air at Т = 1133 K and a pressure of 3.6 bar. A microgranular structure with 50−150 µm grains having different etching structures was detected on the Pt(poly) surface. Crystal growth pyramids about 80 nm in height were observed on the surface of grains with decreased etching and surface structure close to {111} faces. On the surface of grains with increased etching and surface structure close to {100} and {110} faces, we detected etch pits represented by regular, similarly oriented etch grooves with the dimensions (length × width × depth) of 3.0 × 0.7 × 0.35 µm. The etching structures contained Pt, Fe, Si, Al, Mg, O, N, and C. The elements Pt, O, and C had high concentrations (12.2–53.7 at %), while the other elements had low ones (0.1−4.8 at %). The analysis of the chemical composition of surface and subsurface layers of the catalyst showed that the detected О and N atoms were absorbed on defects and gradually accumulated in subsurface Pt layers during NH₃ oxidation. Oxide particles with a size of ~100 nm containing Fe₂O₃, MgO, and SiO₂, which were observed at the vertices of pyramids and inside the etch grooves, can be formed in the oxidizing medium from Fe, Si, and Mg impurities that moved to the catalyst from the flow of reagents and from the reactor during NH₃ oxidation. These oxide particles can be involved in the etching processes on platinum alloy catalysts under the conditions of NH₃ oxidation.

This work is dedicated to a numerical study of the chemistry and mechanism of combustion for binary fuels consisting of hydrogen and C₂ hydrocarbons. These fuels are considered very promising because they allow effective control over combustion characteristics by adjusting the ratio of the fuel components. It was found that replacing part of the hydrogen with acetylene in a stoichiometric premixed H₂/air flame at a constant equivalence ratio results in a noticeable increase in the flame temperature, with a slight decrease in the flame speed. In the case of ethylene, the decrease in flame velocity is more significant. The flame temperature initially drops by 3 K and then slightly increases as the ethylene concentration rises. When part of the hydrogen is replaced by ethane, both the temperature and flame velocity decrease. According to the modeling results, partial replacement of hydrogen with acetylene, ethylene, or ethane in flames reduces the concentration of chain carriers (H, O, and OH), with the most significant effect on the concentration of the H atom. This indicates the determining influence of H on the speed of these flames, which is confirmed by the correlation between the normalized H concentrations and flame velocities. It was found that the higher concentration of H in the H₂/C₂H₂/air flame is due to the significant contribution of the C₂H₂ + O ( rightleftarrows ) HCCO + H step to H atom formation. The rate of this step in flames with added ethylene and ethane is significantly lower due to the much lower concentration of C₂H₂. On the one hand, the consumption of chain carriers during interactions with carbon-containing compounds negatively affects the H₂/C₂H₂/air flame speed, while on the other hand, the increase in flame temperature has a positive effect due to the high endothermicity of acetylene.

The increasing demands of the hydroxyacetone (also known as acetol) market require more efficient acetol production technologies. The development of new effective catalysts for glycerol dehydration to acetol at mild condition is an important task. In this study, dehydration of glycerol to acetol on microporous sodium mordenite (MOR) modified by conventional ion exchange with Cu²⁺, Ag⁺ and Ag⁺Fe²⁺ was investigated within a temperature range from 100 to 250°C. The transition metal content after ion exchange procedure was determined by EDX: 4.6 wt % of Cu for CuMOR, 2.8 wt % of Ag for AgMOR, 2.3 and 0.2 wt % of Ag and Fe, respectively, for AgFeMOR. H₂-TPR indicates the presence of different types of active sites (Cu²⁺, Ag⁺, ({text{Ag}}_{m}^{{n + }}), Fe⁺²). The glycerol conversion and the acetol yield over the zeolite samples increase in the CuMOR < AgFeMOR < AgMOR series. For AgMOR, glycerol conversion achieves ~90% at 175°C (with an acetol yield of about 76%), the presence of Fe²⁺ reduces the catalytic performance of the catalyst. The copper-exchanged catalyst exhibits the poorest catalytic performance. The proposed reaction mechanism of glycerol dehydration over mordenite involves a preparative step (glycerol as reducing media, forms a catalyst by interacting with cations introduced into the zeolite), and then on these formed neutral metallic sites the conversion of glycerol to the final product occurs; the selectivity of the reaction is mainly determined by steric constraints.

Ag-mordenite with atomic Si/Al ratio 6.5 was characterized by FTIR spectra of adsorbed CO. Besides H-bonding with surface OH groups and side-on interaction with oxygen atoms of siloxane bridges, adsorption on Ag⁺ ions occurs, which accounts for the band at 2193–2178 cm^–1. The exact position of this band depends on the coverage, it shifts to higher wavenumbers upon desorption, until disappearance after heating in vacuum at 300°C. Frequency growth is accompanied by increase in the integrated absorption coefficient up to 8.0 cm/μmol, which is much higher than that of a free CO molecule. We assume that the changes in band position and absorbance are due to the transition from di- to monocarbonyl, whose bands are not resolved in the fundamental vibration region, but the splitting of overtone band confirms such supposition. The position of the high-frequency component of overtone band is even higher than the double frequency of the fundamental vibration, indicating that the anharmonicity of adsorbed CO is vanished or becomes negative. High intensity of overtone bands infers on nonlinear dependence of molecular dipole on the vibrational coordinate. Spectra registered at elevated temperature do not show bands that could be assigned to O-bonded CO species. Quantum chemical calculation confirms the absence of linkage isomerism for CO bound to Ag⁺ cations.