Kinetics and Catalysis最新文献

The physicochemical and catalytic (CO₂ hydrogenation) characteristics of Mo-containing catalysts were studied. The catalysts containing 8 and 15 wt % Mo oxide were prepared by impregnation of γ‑Al₂O₃ with ammonium paramolybdate, followed by drying and calcination at 500°C. The introduction of Mo oxide reduced the pore volume of the support and increased the average pore size, indicating that molybdenum oxide was distributed in the support pores. According to the X-ray diffraction analysis, the calcinated catalyst did not contain the crystalline MoO₃ phase. According to the Raman spectra, oxygen-containing formations were present on the catalyst surface, with Mo atoms tetrahedrally and octahedrally coordinated to the oxygen atoms. The impregnated MoO₃ was partially reduced with hydrogen during linear heating, starting from 320°C. The hydrogenation of CO₂ (gas composition, vol %: 30.7 CO₂, 68 H₂, the rest was N₂; 0.5 g sample) was studied under conditions of linear heating to 400°C. The main reaction was the reverse reaction of CO steam reforming. The contribution of methanation to CO₂ hydrogenation was small. An increase in the temperature and pressure had a positive effect on CO₂ conversion. When the pressure increased from 1 to 5 MPa, the CO content was approximately doubled. In the CO₂ hydrogenation, appreciable activity (although significantly lower compared to that of Mo-containing catalysts) was also exhibited by γ-Al₂O₃, preliminarily heated to 400°C in an H₂ flow. The activity of alumina also increased with pressure.

Based on analysis of the catalytic properties of 4%Ru–13.6%Cs/Sibunit and 4%Ru–5.4%Ba–7.9%Cs/Sibunit in the ammonia decomposition (10⁵ Pa, 350–470°C) and ammonia synthesis processes (6 × 10⁵–5 × 10⁶ Pa, 400–430°C), an analytical expression for nitrogen formation/consumption rate in the reversible reaction N₂ + 3H₂ ( rightleftharpoons ) 2NH₃ has been derived to correctly describe the dependence of the chemical reaction rate on the partial pressures of the reaction mixture components for both the forward and reverse reactions. The approach used to derive the kinetic equation is based on the assumption that the adsorption sites of the ruthenium surface are filled with hydrogen, which is subsequently displaced by nitrogen during competitive interaction. Using the proposed kinetic equation, the equilibrium constants and apparent activation energies for ammonia synthesis and decomposition in the presence of 4%Ru–13.6%Cs/Sibunit and 4%Ru–5.4%Ba–7.9%Cs/Sibunit catalysts have been determined; the values are in good agreement with the published data.

Cu–Ni/SiO₂ catalysts were prepared by coprecipitation method and used in the dehydrogenation reaction of secondary butyl alcohol to methyl ethyl ketone (MEK). The crystal structure, reduction characteristics, element valence state and dispersibility of the catalysts were investigated using X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H₂-TPR), inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), X-ray Auger electron spectroscopy (XAES) and high resolution transmission electron microscopy (HRTEM). The role of Ni component in the dehydrogenation reaction of secondary butyl alcohol was analyzed. The results showed that the conversion of secondary butyl alcohol increased to over 99% when using the Cu–Ni/SiO₂ catalyst. The addition of nickel component to Cu/SiO₂ inhibited the agglomeration of copper nanoparticles. The interaction between copper and nickel was strengthened due to the formation of the Cu–Ni compound. This resulted in change to the valence state and improved the dispersion of copper species on the catalyst surface. The Cu⁺/(Cu⁺ + Cu⁰) ratio increased with the addition of nickel component to Cu/SiO₂, which may be responsible for the enhancement of the secondary butyl alcohol conversion. However, the addition of the nickel component increased the reduction temperature of the catalysts and deteriorated their reduction characteristics, which leads to insufficient reduction, resulting in a high content of Cu⁺ species remaining in the catalyst. Therefore, side reactions can occur, which are detrimental to the selectivity and yield of MEK. The selectivity to MEK can reach 98% with the Cu/SiO₂ catalyst, whereas that for the Cu–Ni/SiO₂ catalyst was 97%.

The main reasons for the promoting effect of phosphorus on the properties of Pd–P/ZSM-5 catalysts during direct synthesis of H₂O₂ from H₂ and O₂ under mild conditions are considered based on the data obtained by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and inductively coupled plasma mass spectrometry (ICP MS). The introduction of phosphorus in the catalyst affects the particle size and the electronic state of palladium in the surface layer, as well as the surface concentration of the phosphate and phosphite ions. The yield of H₂O₂ increases when the particle size of the Pd–P catalysts decreases, when the side process of H₂O₂ decomposition is inhibited by the phosphate and phosphite surface ions, and when the hydrogen solubility in the solid solutions of phosphorus in palladium decreases.

The dependence of the catalytic activity of Co foil in ethylene oxidation on the degree of oxidation of the Co surface during stepwise oxidation of foil has been studied. The experiments were performed at 500–800°C by the pulse method using alternating pulses of a 0.2%C₂H₄–0.25%O₂–1%Ar–He test mixture and a 1%O₂–1%Ar–He oxidative mixture. The degree of oxidation of the Co foil surface varied from total reduction to oxidation of ~100 cobalt oxide “monolayers.” The XRD, SEM, and EDS studies showed that CoO formed at the first stage of stepwise oxidation (from 0 to ~60 oxide “monolayers”) at all temperatures under study, and changes in the surface morphology could be observed. In this state the samples had a relatively high activity in both partial and total oxidation of ethylene at 500–600°C. At 700–800°C, however, there was no total oxidation, and the rate of partial oxidation was much lower than at 500–600°C. At the second stage of Co surface oxidation (from ~60 to ~120 oxide “monolayers”) at 500–600°C, Co₃O₄ formed, and gradual ordering of oxide crystals was observed. In this state, the samples exhibited constant (at 500°C) or extreme (at 600°C) activity in the total oxidation of ethylene. In contrast, an increase in the temperature to 800°C led to a drastic decrease in the catalytic activity of Co foil in the given range of the degrees of oxidation.

Studies on the development of a homogeneous chloride-free two-stage process (target reaction (first stage) and catalyst regeneration (second stage)) for propylene oxidation to acetone with oxygen in the presence of a ({text{Pd}}_{{{text{aq}}}}^{{{text{2 + }}}}) + Mo–V–P heteropoly acid (HPA-x, where x is the number of V atoms) catalyst have been described. A kinetic equation of the target reaction has been derived; a mechanism of the reaction has been proposed. It has been shown that the most efficient catalysts are those based on high-vanadium modified (non-Keggin) HPA-x_M compositions. It has been found that the kinetics of C₃H₆ oxidation in the presence of Keggin HPA-x and HPA-x_M solutions is identical; however, only ({text{Pd}}_{{{text{aq}}}}^{{{text{2 + }}}}) + HPA-x_M catalysts are technologically feasible. They exhibit high thermal stability (up to 180°C) and, therefore, can be rapidly regenerated with oxygen. Owing to this feature, catalysts based on HPA-x_M compare favorably with those based on Keggin HPAs-x, the thermal stability of which is limited to 140°C. The possibility of rapidly regenerating the catalyst has made it possible to close the two-stage catalytic cycle of C₃H₆ oxidation to acetone with oxygen and opened up prospects for the practical implementation of the process in the presence of ({text{Pd}}_{{{text{aq}}}}^{{{text{2 + }}}}) + HPA-x_M. The catalyst has effectively passed stability tests.

The advantages of using SAXS along with the masking liquid technique for determining the sizes of supported metal particles over the standard TEM and XRD methods, usually used for these purposes, were shown in the case of Au/C catalysts. Particle size distributions of gold in a wide range (1–50 nm) were obtained from the SAXS data, including for all size fractions present in the samples. The mass fractions of X-ray amorphous gold particles smaller than 4 nm (W_SAXS) were determined. The oxidative treatment of the carbon support before the deposition of the metallic gold precursor complexes has a significant effect on the size distribution of gold particles in the final catalyst. A comparison of the experimental rates of CO oxidation with an excess of moist air at 40°C on Au/C catalysts with the W_SAXS values found for these catalysts showed that the catalytic activity increased exponentially as W_SAXS increased. The Au/C catalysts with W_SAXS ≥ 80% showed high activity in the oxidation of CO.

Developing a facile and accurate method for assessing catalytic activities of supported metals can significantly promote the preparation and application of supported metal catalysts. Herein, we synthesized five MCM-41 materials functionalized with Schiff-base groups, which were employed as the supports for copper cations. The chemical structures and coordination of copper cations on the supports were characterized by Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The grafting densities were calculated by analyzing the results of thermal gravimetric analysis (TGA) and elemental analysis (EA). The microstructures of these supported copper catalysts were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The oxidation of thioanisole to methylphenyl sulfoxide was employed to determine the catalytic activities of these supported copper catalysts while their o-Ps annihilation properties were evaluated by the positron annihilation lifetime spectroscopy. It is found that the o-Ps annihilation properties were linearly correlated with their catalytic activities. Therefore, the catalytic activities of supported copper species surrounded by different functional groups can be easily determined through o-Ps annihilation.

The work presents an approach for studying the kinetics, mechanism, and reactivity of intermediates of a wide class of redox reactions for which the rate-limiting step is the redox decomposition of the intermediate complex. The approach was applied to the investigation of the oxidation of oxalic acid (H₂Ox) by cerium(IV) in a sulfuric acid medium as part of the Belousov–Zhabotinsky oscillating reaction (BZ reaction) catalyzed by cerium ions. Experimental, mathematical, and computational methods that are typically used to study metal complexes in a stable oxidation state were kinetically generalized to variable-valence metal complexes and were used to determine the characteristics of intermediate complexes of the cerium(IV)–oxalate reaction and derive a general equation for its rate based on a set of equations describing the rapid achievement of pre-equilibrium in the system and the subsequent nonequilibrium process. A quantitative model of the process was proposed; it included two parallel reaction pathways, for which two different cerium(IV)–oxalate intermediate complexes were identified and characterized. The complexes have similar reactivity, which may be due to the similarity of the structure of their inner coordination spheres and the inner-sphere mechanism of electron transfer in the complexes. Using the developed model, a diagram of the yields of all main species of cerium(IV) under the conditions of the BZ reaction was constructed, which indicates the need to take into account the formation of intermediate complexes of the composition CeOHOx(_{n}^{{3 - 2n}}) (n = 1, 2) in the oxidation of oxalic acid under these conditions. The main difference between the presented model of the cerium(IV)–oxalate reaction as part of the BZ reaction and the previous models is the explicit consideration of the participation of intermediate complexes of cerium(IV) with oxalic acid anions and sulfate background anions in the reaction.

A comparative study was made of the differential selectivity of the Sonogashira reaction with a pair of competing arylacetylenes under “ligand- and copper-free” conditions by varying nature and concentration of aryl halides and bases common to competing substrates. The change in the differential selectivity with varying nature of aryl halides is consistent with the proposal that aryl halide and arylacetylene are activated sequentially according to a mechanism that is linear in terms of chemical kinetics. The observed lack of influence of the nature and concentration of the base on the selectivity of the reaction under competition between arylacetylenes indicates that the step of their activation is virtually irreversible.