A. A. Firsova

Mechanism of CO oxidation in excess H2 over CuO/CeO2 catalysts: ESR and TPD studies

The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO2 catalysts in excess H2 is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO2 at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu+ carbonyls on Cu2O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu0 of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O2 on Cu0 at T ∼ 150°C. The activation energies of CO, CO2, and H2O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu+ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model.

Selective oxidation of CO in the presence of hydrogen on CuO/CeO2 catalysts modified with Fe and Ni oxides

The selective oxidation of CO in the presence of hydrogen on CuO/CeO2 systems containing Fe and Ni oxides as promoters was studied. The catalysts containing 1–5 wt % CuO and 1–2.5 wt % Fe2O3 supported on CeO2 and the CuO/CeO2 systems containing 1–2.5 wt % NiO were synthesized, and their catalytic activity as a function of temperature was determined. It was found that the additives of Fe and Ni oxides increased the activity of the CuO/CeO2 catalysts with a low concentration of CuO. In this case, the conversion of CO at 150°C approached 100%. At the same time, these additives had no effect on the activity of the CuO/CeO2 systems at a CuO concentration of 5 wt % or higher, which exhibited an initially high activity in the above temperature region. The forms of CO adsorption and the amounts of active sites for CO adsorption and oxidation were studied using temperature-programmed desorption. It was found that the introduction of Fe and Ni additives in a certain preparation procedure facilitated the formation of an additional amount of active centers associated with CuO. Data on the temperature-programmed reduction of samples (the amount of absorbed hydrogen and the maximum temperature of hydrogen absorption) suggested the interaction of all catalyst components, and the magnitude of this interaction depended on the sample preparation procedure. With the use of Mössbauer spectroscopy, it was found that the procedure of iron oxide introduction into the CuO/CeO2 system was responsible for the electron-ion interactions of catalyst components and the reaction mixture.

Mechanochemical activation of methane oxidation catalysis

Abstract Mechanochemical activation of methane oxidation catalysts is used for two aims: 1) for identification of free-radical centres at the oxide surfaces and 2) for preparation of new active catalysts of partial methane oxidation. Kinetics of interaction of free-radical defects at the surface of mechanically activated SiO2 with methane was studied. It shows possible ways of formation of formaldehyde and C2 products. Mechanical treatment of the MoO3 + SiO2 or V2O5 + SiO2 mixtures give rise to the formation of encapsulated (MoO2 in SiO2) catalyst. Such catalyst gave 100% selectivity of oxidation of CH4 into CH2O at 500–600°C and low conversions.

Pt-containing pillared clay catalysts for CO oxidation

Platinum has been introduced into pillared clay as a complex with the organosilicon amine N’-[3-(trimethoxysilyl)propyl]diethyltriamine, as a complex with the organosilicon amine and zirconyl chloride, as an ammine complex, and by impregnation with a chloroplatinic acid solution followed by hydrogen reduction. The catalytic activity of the Pt-containing clays in CO oxidation in excess hydrogen was also studied. The last procedure yields the most active Pt-containing pillared clay. Calcium has been introduced into pillared clay by ion exchange, and it was found that the catalytic activity of the clay decreases with increasing Ca content.

Methane Reforming with Carbon Dioxide on Cobalt-Containing Catalysts

Cobalt- and iron-containing catalysts supported on MgO, ZrO2, γ-, θ-, and α-Al2O3 were synthesized and studied in the CO2 reforming of methane. The CoO/α-Al2O3 systems are the most active and stable. The dependence of the catalytic activity and the degree of reduction on the amount of supported CoO was studied. In the active catalysts, CoO is weakly bound to the support and can readily be reduced to metal cobalt. Coke formed in the course of the reaction does not affect the activity of the CoO/α-Al2O3 catalyst.

Study of the mobility of lattice oxygen in complex oxide catalysts for partial oxidation of propylene to acrolein

The amount of oxygen in the lattice of solids that participates in the elementary stages of partial propylene oxidation is determined for two types of Co-Mo-Bi-Fe-Sb-K-O catalysts (I, II) differing in the method of introduction of antimony and potassium. Two independent methods are used: (1) on the basis of the yield of the oxygen-containing products of propylene oxidation by oxygen of the catalyst in a pulse regime and (2) with the use of Möessbauer spectroscopy. Coincidence of the results obtained by both methods indicates that the active oxygen of the catalyst lattice is formed during redox transformations of iron(III) molybdate entering the composition of the catalysts. Data on the reduction of the catalysts in a pulse regime at various temperatures, which were processed in the framework of the diffusion model, allowed the estimation of the rate constants for diffusion of the lattice oxygen. An increase in the mobility of the lattice oxygen in catalyst I, which is modified with a small amount of antimony as compared to catalyst II, results in an increase in the overall productivity of the sample and in a decrease in the selectivity of propylene oxidation to acrolein. This correlates with the increase in the total amount of the lattice oxygen participating in the process.

Modification of catalysts

Abstract The effect of additives on metal oxide and silver catalysts to improve the activity and selectivity of various processes is considered. Additives influence the kinetic parameters of a process (rate and activation energy) and the reaction mechanism (structure of surface compounds, energy of their bond with the catalyst). Modification causes the formation of new compounds in the bulk and at the surface of the catalyst, thereby altering the composition of active sites and inducing formation of other sites. The effects of species added to multicomponent oxide systems, as well as various aspects of modification by acid-base additives are considered. Specific features of the influence of additives and the role of modified multifunctional catalytic systems are also discussed.

Mechanochemical Activation of Cu–CeO2 Mixture as a Promising Technique for the Solid-State Synthesis of Catalysts for the Selective Oxidation of CO in the Presence of H2

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO2 was developed. It was established that metallic copper was oxidized by oxygen from CeO2 in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu1+, Cu2+, and Ce3+ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H2 method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO2. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO2 catalyst.

Effect of bismuth on structural rearrangement in Co-Mo-Fe catalysts for the gentle oxidation of propylene

Conclusions1.The introduction of bismuth into Fe-Co-Mo catalysts affects the degree of local lattice distortion in the neighborhood of the Fe3+ ions.2.Formation of a metastable β-FeMoO4 phase results from thermal vacuum activation of Co-Mo-Bi-Fe catalysts, and from the interaction of such catalysts with propylene.3.The necessary and sufficient condition for the formation of β-FeMoO4 is the generation of a critical concentration of point defects (anion vacancies) in the neighborhood of the β-CoMoO4 interface.