A. A. Dulov

Hydrogenation activity and surface composition of perovskite-type mixed rhenium oxides

Abstract Perovskite-type mixed rhenium oxides having the composition A 4 B Re 2 □ O 12 (where A = Ba, Sr; B = Co, Ni; □ = a vacancy in the cation lattice) have been synthesized by solid-state reaction of stoichiometric amounts of the starting oxides and metallic rhenium at temperatures up to 1100°C. It has been shown that the synthesized oxides have high catalytic activity for the hydrogenation of benzene. The sample where A = Ba and B = Ca is inactive for this reaction. In the continuous series of Ba 4- x Sr x CoRe 2 □O 12 solid solutions, isomorphous substitution of Ba with Sr increases catalytic activity. On Sr 4 CoRe 2 □O 12 and Sr 4 NiRe 2 □O 12 , benzene is hydrogenated completely (200–300°C, P H 2 = 100–120 atm, t = 2–3 h, 1% of Re). Catalyst activity is increased with increasing ionization potential of A (Ba∠Sr) and B (Ca∠Ni∠Co) cations. On the basis of a phase analysis of the surfaces of Sr 4 CoRe 2 □O 12 and Sr 4 NiRe 2 □O 12 made by thermovacuum electric conductivity curves (TEC), the surface phases formed in the course of the catalysis retain the semiconductor nature of conductivity. XPS of Sr 4 CoRe 2 □O 12 and Sr 4 NiRe 2 □O 12 after catalysis reveals rhenium in reduced Re 4+ and Re 6+ states, which is possibly involved in the catalytic sites.

Computer simulation analysis of conductivity of supported polydisperse systems

Effects of the support texture and the supported phase distribution on the conductivity of an individual grain or a granulated system have been analyzed theoretically. Experimental criteria are discussed, which allow one to judge the porous structure and sites of localization of a conducting component on the basis of the concentration dependence of the conductivity.

Surface phase analysis of catalysts and other disperse materials using electroconductivity: The TVE-curves method

The method of thermovacuum electroconductivity (TVE) curves for the analysis of surface phases in disperse systems is described. The theoretical basis of the application of electric measurements for the investigation of the surface of disperse objects is presented. Examples of the use of the TVE-curves method for characterization of diverse catalytic systems and the solving of some problems of heterogeneous catalysis are discussed. The TVE-curves method is an efficient tool for the control of the phase-structural state of the surfaces of objects of a semiconductor type.

Theoretical description of pores in the simulation of composite materials

The influence of porosity on the electric conductivity and the dielectric constants of composite materials was studied using computer simulation (the Monte Carlo method). A new approach to the simulation of porous systems that treats pores as a component equivalent to the solid phase components was proposed. Within the framework of this approach, an analysis of the influence of the micro- and macroporosity on the electric conductivity was carried out. It was established that the concept of a barrier-disordered system is also valid for pores. It was shown for the first time that pores can serve as one of the factors that forms the contact barrier distribution function. The proposed theoretical models were in good agreement with the experimental data.

Surface solid-state interactions in copper-nickel-cement catalysts for hydrogenation of oxygen

The surface phase composition of alumocalcium cement-supported CuO and CuO-NiO catalysts prepared by chemical mixing has been studied using the method of thermo-vacuum curves of electric conductivity. The deactivation of these catalysts due to overheating to 800 °C under conditions of hydrogenation of oxygen is rationalized by the partial extraction of CuO (and NiO) from the stabilizing structure of the support solid solutions and by sintering of the extracted oxides and the reduced metallic phase. Complete regeneration of the CuO-NiO-talum catalysts can be achieved if a considerable amount (20%) of copper hydroxocarbonate is added.

Computer models for electrical conductivity of multicomponent solid-state systems

Models for the computer simulation of electrical conductivity of polycrystalline materials and mixtures are proposed. The effect of model parameters on the concentration behavior of conductivity is considered. The correspondence between the model parameters and the characteristics of physical objects is established, thus making it possible to analyze the spatial relations in solid-state systems.

Analysis of electric conductivity of binary solid-state catalysts by computer simulation

Experimental and model studies of concentration dependencies of electric conductivity have been performed for a number of binary mechanical mixtures. The physical origins of various peculiarities are discussed with Fe3O4+MgO, Fe3O4+NiO, and Ni+ZrO2 systems considered as examples. The degree of friability of the systems and the degree of homogeneity of the component distribution are estimated. A considerable mutual influence of the phases is observed, which is, however, not a chemical interaction.

Phase analysis of the surface of supported copper-cobalt oxide catalysts by means of electrical conductivity thermal-vacuum curves

Conclusions1.Using the method of thermovacuum curves of electrical conductivity it has been shown that in copper-cobalt oxide catalysts supported on talyum obtained from the hydroxycarbonates of the metals in the presence of ammonia water and calcined at 400°C, a solid solution of CuO and Co3O4 forms, i.e., a copper cobaltite of variable composition with the spinel structure.2.The support facilitates the interaction between the oxides and stabilizes the surface copper cobaltite observed to form at CuO contents as low as ∼5% at ∼200°C.3.The copper oxide has a tendency to concentrate at the surface of the particles, which should lead to the enrichment of the surface in copper cobaltite compared to the interior of the catalyst.

Preparation and properties of coprecipitated catalysts of NiO-TiO2 system

1. A method was developed for the preparation of coprecipitated catalysts of the NiO-TiO2 system, which made it possible to prepare samples with a large value of the specific surface and practically devoid of Fe and Cl impurities. 2. Based on an analysis of the results of a complex study of the system by many physical methods it was concluded that the structure characteristics and properties of these catalysts are caused by reaction between the components of the system, which takes place even during the precipitation step. 3. A possible, mechanism for the deactivation of coprecipitated catalysts was discovered, which consists in the formation of surface layers of the inactive reaction product of the components (NiTiO3), which shield the active phase (NiO).