This paper demonstrates that relative reactivity estimated under competition of several similar substrates can be applied to visualize the dynamics of changes in a catalyst under the conditions of a complex catalytic process. The fundamental advantage of the proposed approach is that the state of an active catalyst can be monitored throughout the catalytic reaction without differentiation of the kinetic data on the concentrations of reacted substances.
Iron oxide-modified 1Pd0.5Fe and 1Pd10Fe catalysts with a target content of 1 wt % Pd and 0.5 or 10 wt % Fe have been synthesized by the wet impregnation of Al2O3 with iron and palladium nitrates. The catalysts have been compared with each other and with a monometallic 1Pd catalyst in diclofenac (DCF) hydrodechlorination (HDC) in dilute aqueous solutions at 30°C in batch and flow reactors after high-temperature (320°C) and mild reduction (30°C), the latter being run in a batch or flow reactor. X-ray photoelectron spectroscopy (XPS) has revealed that, after reduction at 320°C, the catalysts contain mostly Pd0, Fe2+, and Fe3+. The Fe2+/Fe3+ ratio on the surface increases with a decrease in the iron content. The reduction of Pd2+ to Pd0 can occur even at 30°C; however, on the 1Pd0.5Fe surface, it is significantly less effective than that on 1Pd10Fe. According to XPS, temperature-programmed reduction, and diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO, modification with iron oxides leads to an increase in the palladium content on the surface compared with that on 1Pd, contributes to the formation of new Pd–O–Fe sites, and affects the reducibility of palladium. These effects enhance with an increase in the iron content. The iron-modified catalysts reduced at 320°C exhibit similar activity and stability in DCF conversion in flow and batch systems. The 1Pd10Fe catalyst, unlike 1Pd0.5Fe, is highly efficient and stable even after mild reduction at 30°C. Under flow conditions, it provides a DCF conversion comparable to that provided by 1Pd and a selectivity in the DCF HDC reaction that is higher than that provided by 1Pd, which is also active in hydrogenation.
Changes in the composition of immobilized [M(COD)Cl]2–NH2–C3H6–SiO2 and [M(COD)Cl]2–P(Ph)2–C2H4–SiO2 (where M = Ir, Rh) catalysts in the gas-phase selective hydrogenation of propylene, propyne, and 1,3-butadiene with parahydrogen (p-H2) have been studied by the XPS method. It has been proposed that the M/Cl atomic ratio should be used as an indicator of the structural stability of the anchored complex both at the sample synthesis stage and in the reaction. Based on comparison of XPS data and results of catalytic tests using parahydrogen-induced nuclear polarization, it has been shown that the stability of the anchored {[M(COD)Cl]2–Linker–SiO2} complex during hydrogen activation is a key factor in the catalytic behavior of the systems. The stability of the complex is affected not only by the chosen metal and linker, but also by the nature of the substrate subjected to hydrogenation.
The number of observations that demonstrate a significant effect of the size of droplets on the kinetics of chemical processes has increased with the expansion of the scope of applications of spray technology. The equations linking the concentrations of reagents, the volume of droplets, the initial composition of a solution, the composition of the gas medium, and the speed of processes are formulated within the framework of formal chemical kinetics. Using second order reactions (coupling, exchange, condensation, polymerization, and polycondensation reactions) as examples, it is shown that size kinetic effects occur when chemical processes are accompanied by changes in the droplet sizes in equilibrium with the gas medium. The results of computer simulation of condensation and polycondensation reactions are given, which reproduce size effects. Kinetic curves obtained by simulation of the polycondensation process are compared with experimental data.
The article is devoted to the analysis of possible spatiotemporal kinetic structures that can arise during catalytic oxidation reactions on metal surfaces at atmospheric pressure. The catalytic oscillatory reaction in a flow reactor is modeled using a 1D system of equations of the reaction–diffusion–convection type. The STM type oscillatory reaction model of catalytic oxidation is used as a kinetic model. The obtained results of mathematical modeling show the decisive influence of an axial mixing in the reactor on the development of spatiotemporal structures. It is also shown that, depending on the ratio of adsorption rates of reacting species, three different isothermal spatiotemporal structures can arise, namely a spatially inhomogeneous stationary state, regular and aperiodic “breathing structures”.
The influence of platinum additives on the properties of rhodium catalysts in the processes of steam reforming and autothermal reforming of diesel fuel was investigated. It was found that Rh/CZF was more active compared to the bimetallic sample Rh–Pt/CZF: the degree of fuel conversion in its presence was higher, and the concentration of reaction by-products was lower. The proposed two-zone Pt/CZF + Rh/CZF structured honeycomb catalyst demonstrated stable performance and high activity in the autothermal reforming of commercial diesel fuel. However, the presence of platinum in the frontal zone of the catalyst reduced its resistance to coking compared to the rhodium-containing sample. The results obtained are of practical significance in the development of efficient systems for the conversion of heavy hydrocarbons into synthesis gas.
Efficient heterogeneous catalysts for chemoselective hydrogenation of terminal and disubstituted alkynes and alkynols to monoenes based on Pd–P particles have been proposed. The influence of a zeolite support (Na-ZSM-5, MSM-41) and a phosphorus modifier on the properties of palladium catalysts in the semihydrogenation of acetylenic compounds is considered. Promotion with phosphorus increases the activity of palladium catalysts in hydrogenation of various acetylenic compounds 2.5- to 30-fold without reducing the selectivity for monoenes. The high selectivity for monoenes is determined by both thermodynamic and kinetic factors. The possibility of changing the ratio of the rates of hydrogenation of triple and double bonds by varying the nature of the solvent and the structural order of the catalyst particles was demonstrated using hydrogenation of acetylenic alcohols as an example.
Results of studying the catalytic properties of systems based on complexes with [Pd(Cp)(L)n]m[BF4]m (where Cp = η5-C5H5; n = 2, m = 1: L = tris(ortho-methoxyphenyl)phosphine, triphenylphosphine, tris(2-furyl)phosphine (TFP); n = 1, m = 1: L = 1,1'-bis(diphenylphosphino)ferrocene, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane; n = 1, m = 2 or 3: L = 1,6-bis(diphenylphosphino)hexane) in the addition homo- and copolymerization of norbornene (NB) and NB derivatives have been described. It has been found that these complexes can be activated with Lewis acids (BF3⋅OEt2 or AlCl3). The productivity of the [Pd(Cp)(PPh3)2][BF4]/BF3⋅OEt2 catalyst system in NB polymerization can achieve 188 800 molNB ({text{mol}}_{{{text{Pd}}}}^{{ - 1}}). The homopolymerization of 5-methoxycarbonylnorbornene and the copolymerization of NB with 5-methoxycarbonylnorbornene or 5-phenylnorbornene in the presence of BF3⋅OEt2 and [Pd(Cp)(L)2][BF4] (L = PPh3 or TFP) has been studied. A hypothesis for the formation of the catalyst via the intramolecular rearrangement of the η5-Cp ligand into the η1-Cp form upon interaction with a Lewis acid has been proposed. The structure of the [Pd(Cp)(TFP)2]BF4 complex (I) has been determined by X-ray diffraction (XRD) analysis. In the crystal structure of complex I, the coordination sphere of palladium is characterized by a slight distortion of the square planar geometry of the central atom, while the cyclopentadiyl moiety is in an eclipsed conformation. Based on XRD data, the steric hindrance of the TFP ligand has been determined (cone angle is 149°).
It has been shown in tests that N-heteroaryl-substituted α-diphenylphosphinoglycines, namely, N-(pyrazin-2-yl) α-diphenylphosphinoglycine, N-(pyridin-2-yl) α-diphenylphosphinoglycine, and N‑(pyrimidin-2-yl) α-diphenylphosphinoglycine, which are synthesized by the three-component condensation of diphenylphosphine, the respective primary amine, and glyoxylic acid monohydrate, being combined with Ni(COD)2, where COD is cyclooctadiene-1,5, are capable of generating active forms of catalysts for the selective homogeneous dimerization and trimerization of ethylene to form butene-1 and hexene-1 as the main products. The studied organonichel catalyst systems provide a yield of short-chain (C4–C6) olefins at a level of 90% with a linear α-olefin selectivity of 97%. Studies of the effect of temperature on the homogeneous oligomerization of ethylene using the synthesized compounds have revealed that the process run at an optimum temperature of 80–105°C and an optimum ethylene pressure of 20–35 atm provides the highest butene-1 and hexene-1 selectivity. Under these conditions, the butene selectivity is recorded at a level of 71.4–72.6% (butene-1 selectivity of 69.3–71.1%), while the hexene selectivity is 20.6–21.2% (hexene-1 selectivity of 19.2–19.5%). The optimum duration of the oligomerization process at a temperature of 105°C is 1.5 h. The butene-1 and hexene-1 formation rate is 168.1 and 47.3 golig ({text{g}}_{{{text{Ni}}}}^{{ - 1}}) h–1, respectively.
In this work, we used X-ray photoelectron spectroscopy (XPS) to perform a comparative study of the interaction of NO2 with two samples of highly oriented pyrolytic graphite (HOPG), on the surfaces of which rhodium was preliminarily deposited by evaporation in a vacuum, at room temperature and a pressure of 10–5 mbar. Before metal deposition, one of the HOPG samples was annealed in a vacuum at 600°C, and the other was bombarded with argon ions followed by exposure to air at room temperature for 1 h in order to introduce strongly bound oxygen atoms into the surface composition. After the deposition of rhodium onto the two HOPG samples, two model catalysts designated as Rh/C and Rh/C(A)–O were prepared. It was found that the interaction of NO2 with Rh/C led to the oxidation of graphite with the destruction of the surface layer. The Rh particles remained in a metallic state, but they were introduced into the near-surface layer of the carbon support. On the contrary, when the Rh/C(A)–O sample was treated with NO2, the deposited rhodium was partially converted into Rh2O3, while the graphite was oxidized to an insignificant degree and retained its original structure. The role of surface oxygen in the stabilization of graphite with respect to oxidation in NO2 was discussed.
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