Kinetics and Catalysis最新文献

In this work, lanthanum orthochromite (LaCrO₃; LCO) and lanthanum orthochromite doped with zinc in the B site ((LaCr_1–xZn_xO₃: x = 0.1, 0.2, 0.3; LCZO) were synthesized by the sol–gel citrate route and investigated with respect to their crystal structure, surface morphology and optical band gap energy by means of X-ray diffraction, scanning electron microscopy and UV–Vis spectrophotometry. Moreover, the band structure, density of states (DOS) and electronic density were calculated for LCO using the DFT–FP–LAPW method. The XRD analysis showed that LCO sintered at 1000°C and LCZO sintered at 1100°C crystallize in a perovskite-type orthorhombic structure. The computed band gap energy was about 2.93 eV for pure LCO oxide. On the other hand, the experimental results showed a remarkable decrease in the LCO optical band gap from 3.0 to 2.7 eV with increased Zn content. The prepared systems were also tested with respect to their catalytic activity in the oxidative dehydrogenation of 1-butene, where doping with Zn was found to improve the catalytic performance of LCZO catalysts, leading to a maximum selectivity to 1,3-butadiene of 35% at a zinc doping level of x = 0.2.

The work investigated the influence of the nature and amount of the applied noble metal, as well as the physicochemical properties of the initial TiO₂ on the process of photoelectrocatalytic reduction of CO₂. Catalysts of the compositions x% Me/TiO₂ (where Me = Ag, Au, Pt; and x = 0.1–2 wt %) were synthesized on the basis of titanium dioxide of the brands Evonik P25 (anatase : rutile ratio = 75 : 25) and EuroSupport (anatase >99%) by chemical reduction from metal salts using NaBH₄. The samples were characterized by X‑ray powder diffraction analysis, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy. The catalytic properties of the obtained samples were studied in the photoelectrocatalytic reduction of carbon dioxide in 0.1 M KHCO₃ electrolyte at a potential of –700 mV versus Ag/AgCl/KCl(sat) under irradiation at a wavelength of 371 nm. Data on the accumulation of total organic carbon in the solution showed that, for each supported metal, the activity of the samples based on EuroSupport anatase-rich TiO₂ is 20% higher than the activity of the samples based on Evonik P25 rutile-containing TiO₂ at identical values of current efficiency. A conclusion was made about the observed decrease in electron energy and the reducing capacity of the catalyst by the rutile phase in Evonik P25 TiO₂. Based on the relationship between the activity and utilization efficiency of metals in the CO₂ reduction process, Au was selected as the preferred metal for photoelectrocatalytic CO₂ reduction.

In this work, a series of composite photocatalysts based on graphitic carbon nitride and platinized titanium dioxide was synthesized and the reaction kinetics of photocatalytic hydrogen evolution from aqueous solutions of glucose under the visible light irradiation (440 nm) was studied. The graphitic carbon nitride was prepared by the thermal polycondensation of melamine at 600°C with the subsequent thermal exfoliation. The g-C₃N₄/Pt/TiO₂ composite photocatalysts were produced by the self-assembly of g-C₃N₄ and Pt/TiO₂. Varying the g-C₃N₄ content showed that the sample containing 10 wt % g-C₃N₄ had the highest activity. The dependence of the rate of hydrogen evolution on the initial glucose concentration obeyed the Langmuir–Hinshelwood equation; the adsorption constant was K_ads = 76 ± 14 M^–1, and the apparent rate constant was k_r = 0.69 ± 0.02 μmol/min. The photocatalyst 10% g-C₃N₄/1% Pt/TiO₂ was studied using X-ray photoelectron spectroscopy both before and after the reaction of hydrogen evolution. It was found that a portion of platinum in the initial photocatalyst was present in the oxidized state (+2), and platinum was completely reduced to a metallic state during the photocatalytic reaction. The activity of the photocatalyst 10% g-C₃N₄/1% Pt/TiO₂ under the solar simulator light irradiation (AM1.5G) was 560 μmol h^–1 g^–1.

The Ir–P@SiO₂ catalyst was prepared by immobilizing a dimeric Ir complex via interaction with –PPh₂ groups of 2-diphenylphosphinoethyl-functionalized silica gel. The catalyst was tested in hydrogenations of butynes (1-butyne and 2-butyne) with parahydrogen. The experimentally observed ¹H NMR spectra of hyperpolarized products are well-fitted with theoretical PASADENA spectra simulated with the use of spin density matrix formalism. The analysis of ¹H NMR spectral lines of the products reveals that the catalyst is stereoselective in such reactions and the syn-addition of p-H₂ takes place. Moreover, we demonstrate that the lineshape of hyperpolarized NMR signals of cis-2-butene is indicative of the mechanism of its formation. It was found that 2-butene produced during 1-butyne hydrogenation is formed from hyperpolarized 1-butene via isomerization. The maximum observed ¹H polarization levels for 1-butene were 5.2 ± 0.5%.

A convolutional neural network was used to solve the problem of segmentation of XPS spectra. The developed combination of recognition using a machine learning model and a post-processing algorithm provided fast automatic analysis of XPS data. The results of determining the positions and areas of peaks were in good agreement with both the results of manual analysis and handbook values. The proposed approach was applied to analyze the XPS spectra of heterogeneous catalysts (Pd/Al₂O₃ and Sr₂TiO₄) and chemical compounds used in the preparation of catalysts (AgCl and TiO₂).

A comparative study of the interaction of palladium supported on the surface of highly oriented pyrolytic graphite (Pd/HOPG) with oxygen and nitrogen dioxide was carried out using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The samples were prepared by vacuum deposition of metallic Pd onto HOPG; the size of the resulting particles was ≤5 nm. The samples were treated in oxygen at room temperature or 150°C and a pressure of 20 mbar. The interaction with NO₂ was carried out at room temperature and a pressure of 10^–6 mbar. For the two oxidants used, O₂ and NO₂, fundamental differences in their effects on Pd/HOPG were found. At room temperature, the treatment in O₂ was limited by the formation of some amount of oxygen-containing compounds C_xO_y on the surface of the carbon support. After increasing the temperature to 150°C, a portion of palladium was converted into a PdO oxide. When NO₂ was used as an oxidizing reagent, intense oxidative destruction of graphite occurred to a depth of 10–15 graphene layers. Palladium remained in a metallic state, but its particles penetrated deep into the carbon support, which led to significant screening of its lines in the X-ray photoelectron spectrum and the disappearance of particles in SEM images. A possible mechanism for the interaction of Pd/HOPG with O₂ and NO₂ has been proposed, which allowed us to explain the different behaviors of these oxidizing molecules.

The influence of the nature and concentration of a medium modifier (Brønsted acid, halide) on the yield of H₂O₂ in direct synthesis under mild conditions was studied using two samples of Pd–P catalysts supported on ZSM-5 zeolite in the H and Na forms. The acid modifier and the halide have opposite effects on the yield of H₂O₂. Unlike HCl, potassium chloride has an inhibitory effect on the catalyst activity, partially suppressing the formation of H₂O₂. Based on the experimental data, a hypothesis was formulated about the diverse nature of the active sites responsible for H₂O₂ formation in direct synthesis and H₂O₂ hydrogenation. The main functions of the acid modifier in direct synthesis under the action of the Pd–P catalysts are considered.

The review discusses the most significant catalytic processes involving solutions of Mo–V–P heteropoly acids (HPA-x, where x is the number of V atoms), which have been developed at Boreskov Institute of Catalysis of the Siberian Branch of the Russian Academy of Sciences. The prospects for using modified HPA-x solutions as oxidation and bifunctional catalysts are shown. The unique thermal stability and oxidation capacity of these systems make it possible to approach the implementation of optimum routes to synthesizing substances exhibiting activity in delignification, the development of highly selective key stages of the synthesis of vitamins K and E, and the neutralization of highly toxic gases (CO) without their predrying.

The review is devoted to an analysis of the metathesis of lower olefins on molybdenum-containing heterogeneous catalysts. Individual aspects of the cross-metathesis reaction are analyzed: the main industrial processes based on metathesis are described, the kinetic and thermodynamic issues of the reaction are considered, and the mechanisms of individual stages are discussed. The relationship between the structure of catalysts and their catalytic activity is demonstrated. Particular attention in this review is given to methods for improving catalytic systems based on molybdenum oxide.

The influence of structural defects in TiO₂-based photocatalysts, prepared by pulsed laser ablation (Nd:YAG laser: λ 1064 nm, 180 mJ, 7 ns, 20 Hz) followed by thermal treatment in He and air, on the photocatalytic hydrogen evolution was studied. The phase composition and optical properties of the samples were determined by X-ray diffraction, Raman spectroscopy, and diffuse reflectance spectroscopy at all stages of preparation and processing. Photostimulation of dark TiO_x samples revealed the appearance of absorption associated with the presence of oxygen vacancies (VO)/Ti³⁺ ions. A temperature-programmed oxidation (TPO) study of the sample surface in a flow of 10% O₂ in helium showed that the defects in the structure of titania were resistant to reoxidation with oxygen. According to the photocatalytic activity data in the hydrogen evolution reaction under LED irradiation at 375 nm, the high concentration of volume and surface defects in dark titanium dioxide prepared by pulsed laser ablation plays a more significant role in the process of photocatalytic hydrogen evolution than the ordered structure formed in TiO₂. The maximum efficiency was achieved due to the partial ordering of the dark TiO₂-400 structure as a result of heat treatment in air at 400°C with preservation of volume defects. Modification of the defective TiO₂-400 support with platinum by chemical and photoreduction methods showed that the defective structure of the support led to high dispersion of Pt and a ~77-fold increase in the apparent quantum yield during H₂ photoevolution regardless of the modification method. An additional increase in activity is associated with the strong metal–support interaction (SMSI) effect, which facilitates electron transfer from defects in the TiO₂-400 structure to Pt subnanoclusters.