N. A. Gaidai

The mechanism of carbon dioxide hydrogenation on copper and nickel catalysts

The mechanism of the reaction of CO2 with H2 on copper and nickel catalysts was studied by means of isotope, non-steady-state, and steady-state methods. Staged schemes of the process were proposed. The slow step of CO formation on the test catalysts is the reaction of the adsorbed carboxylate complex with a hydrogen atom. It was shown that hydrogen adsorption on the copper catalyst is dissociative in character. A formyl complex and hydrogen are involved in the slow step of methane formation on the nickel catalyst. It was found that the methane formation proceeds via a consecutive scheme through CO.

Kinetics and mechanism of the oxidative dehydrogenation of isobutane on cobalt, nickel, and manganese molybdates

The kinetics of oxidative dehydrogenation of isobutane in the presence of atmospheric oxygen on manganese molybdate has been studied. The experiments have been carried out in a circulation flow reactor at 470–530°C. The form of kinetic equations and the mechanism of the formation of isobutene, carbon oxides, and cracking products on manganese molybdate are similar to those found previously for the same reaction on cobalt and nickel molybdates. The highest yields of isobutene and propene (isobutane cracking products) are achieved on Co0.95MoO4. The mechanism of the process has been investigated by the unsteady-state response method. Manganese molybdate contains the largest amount of reactive oxygen, whereas nickel molybdate contains the smallest amount of reactive oxygen. The earlier conclusion that molybdate lattice oxygen and chemisorbed oxygen play the main role in the formation of iso-C4H8 and in deep oxidation processes, respectively, is confirmed.

Kinetics of benzene and toluene hydrogenation on a Pt/TiO2 catalyst

The kinetics and mechanism of benzene and toluene hydrogenation on a Pt/TiO2 catalyst were studied in steady-state and non-steady-state regimes in the presence or absence of strong metal–support interactions (SMSI). It was found that the kinetics and mechanism of the test reactions were independent of SMSI. The observed effect of a decrease in the catalytic activity in the SMSI state was due to structural changes in the active centers of the catalyst and to the presence of strongly bound hydrogen species on the surface; this was also supported by thermal-desorption data.

Kinetics of carbon monoxide methanation on nickel catalysts

The kinetics of CO methanation in excess H2 on CaO- and CeO2-doped nickel catalysts supported on Al2O3 and TiO2 was studied at atmospheric pressure in a temperature range of 180–240°C. It was found that the same rational fractional rate equation corresponding to the reaction taking place at high surface coverages, is valid for all of the catalysts. The activity of nickel catalysts in the methanation reaction and their adsorption capacity with respect to reaction mixture components depend on the nature of the support and dopants.

Reaction mechanism of CO methanation on nickel catalysts, as studied by isotopic and nonstationary methods

Kinetic isotope effects were measured upon the replacement of hydrogen by deuterium in the reaction of carbon monoxide methanation on nickel catalysts supported on TiO2 and γ-Al2O3. Data on the mechanism of the process were obtained with the use of a nonstationary method. A step-scheme was proposed, in which the interaction of oxygen-containing compounds with hydrogen is a slow step of the process.

Kinetic and Mechanistic Study of the Oxidative Dehydrogenation of Isobutane over Cobalt and Nickel Molybdates

The kinetics and mechanism of the oxidative dehydrogenation of isobutane on nickel and cobalt molybdates are studied. Cobalt molybdate is found to be more active than nickel molybdate. The rate laws and mechanisms for the formation of isobutene, carbon oxides, and cracking products are the same for both catalysts. Isobutene is formed via the redox mechanism with the participation of lattice oxygen. The formation of carbon oxide occurs with the participation of chemisorbed oxygen. The steps of the mechanism are proposed.

Towards efficient catalysts for the oxidative dehydrogenation of propane in the presence of CO2: Cr/SiO2 systems prepared by direct hydrothermal synthesis

Cr/SiO2 catalysts (Cr loading in the 0.25–2.0 wt% range) have been prepared by direct hydrothermal synthesis in the presence of templating agents, in order to attain porous systems with high specific surface area (in the 600–1000 m2 g−1 range), and then characterized and tested in the oxidative dehydrogenation of propane in the presence of CO2 or CO2 + O2 as an oxidant. The extent and regularity of mesopores decreased significantly by increasing the Cr content (X-ray diffraction, N2 adsorption, transmission electron microscopy), but this did not limit the catalytic performances of the catalysts with higher Cr loadings. In all cases, the only chromium species found were surface chromates (diffuse reflectance electronic spectroscopy and X-ray absorption near edge spectroscopy), accompanied by Bronsted acid centres (infrared spectra of adsorbed NH3). All catalysts appeared stable towards irreversible deactivation, even after ca. 900 min of testing, and propene yields as high as 40% were attained. The combination of spectroscopic and catalytic results allowed us to rationalize, at least in part, the role of different oxidants in defining the chromium oxidation state, and a tentative correlation of the oxidation state of Cr species during the reaction (Cr2+/Cr3+) with selectivity to propene is suggested.

Kinetic Models of Catalyst Deactivation in Paraffin Dehydrogenation

The results of studying the deactivation of both unpromoted platinum–alumina catalysts and those promoted with K, Li, In, Sn, and W in the dehydrogenation of lower and higher paraffins are discussed. The main reason for catalyst deactivation is found to be coke formation. The rate laws of coke formation and paraffin dehydrogenation in the non-steady-state regime of the reaction are derived. The catalyst sulfuring is found to enhance its stability. The effects of oxygen and water impurities on the reactions and coke formation are studied.

Transient response studies of isobutane oxidative dehydrogenation over molybdenum catalysts

Kinetics and mechanism of isobutane oxidative dehydrogenation were studied over cobalt and nickel molybdate catalysts. The data obtained in nonstationary and stationary regimes showed that kinetics and mechanism are the same over both catalysts. Isobutene and carbon oxides are primary reaction products. Lattice oxygen takes part in dehydrogenation reactions. Carbon oxides formation proceed by the interaction with adsorbed oxygen. Cobalt molybdate is more active and selective catalyst for isobutane oxidative dehydrogenation. It was shown that nickel molybdate catalyst is stable only at high oxygen concentration while cobalt molybdate catalyst can work at lower oxygen concentrations.

Influence of the preparation conditions for catalysts CrOx/SiO2 on their efficiency in propane dehydrogenation in the presence CO2

Several sets of chromium oxide catalysts supported on silica gel were prepared by precipitation. The optimization of the preparation conditions can considerably enhance the activity and stability of the catalysts in propane dehydrogenation in the presence of CO2. The most stable systems with the chromium content <3 wt.% retain their initial activity after long-duration tests and regenerations. Introducing the active phase on the support in several steps makes it possible to enhance the stability of the samples with a high content of chromium (7 wt.%). It was found that with an increase in the acidity of a solution of chromium nitrate for catalyst preparation (pH from 3 to 1) the rate of their deactivation increases during the work. When the acidity of this solution decreases (pH from 3 to 5), the yield of propylene in propane dehydrogenation increases. As found by UV diffuse reflectance spectroscopy and X-ray diffraction analysis, the CrOx/SiO2 catalysts are characterized by the agglomeration of the chromium oxide phase during the work accompanied by a decrease in dispersion and specific activity. The degree of agglomeration increases with an increase in the chromium content.