V. A. Sadykov

Monolith perovskite catalysts of honeycomb structure for fuel combustion

New method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated. The influence of defect structure of these compounds as well as surface segregation on their catalytic properties is discussed. Basic stages of the monolith perovskite catalysts preparation are optimized. The experimental samples of monolith catalysts of various shapes are obtained, possessing high activity, thermal stability and resistance to catalytic poisons.

Oxide catalysts for ammonia oxidation in nitric acid production: properties and perspectives

Abstract This paper generalizes the results of long-term efforts aimed at research and development of industrial oxide catalysts for ammonia oxidation in the nitric acid production within two-bed (Pt gauzes+monolithic oxide layers) technology of the high pressure process. Main factors determining performance of precious metals and oxides in the high-temperature ammonia oxidation are considered. The surface oxygen bonding strength determined by the surface atomic structure appears to be the most important. From this point of view, existing approaches to synthesis of mixed oxide systems including perovskites with controlled nitric oxide selectivity and good stability in the high-temperature process of ammonia oxidation are analyzed. Main features of the bulk oxide monolithic catalysts production technology and principles of a two-bed system design based upon the process mathematical modeling are briefly outlined. Proven economic benefits of this technology recently commercialized in Russia at nitric acid plants are debated.

A role of surface nitrite and nitrate complexes in NOx selective catalytic reduction by hydrocarbons under oxygen excess

NO adsorption over three types of catalytic systems, such as cation-exchanged zeolites, transition metal oxides supported on γ-Al2O3, and partially stabilised tetragonal ZrO2 (PSZ) was studied by using the TPD method. NO forms several surface complexes having different desorption temperatures. TPD results compared with catalytic properties of these systems in the selective reduction of NOx by propane under oxygen excess showed that strongly bound nitrites and nitrates appeared to be true intermediates in this reaction.

Honeycomb catalysts for clean-up of diesel exhausts based upon the anodic-spark oxidized aluminum foil

Abstract The technology of the aluminum foil anodic spark oxidation in the water-based electrolytes has been applied to form strongly adhering protective alumina layer at the surface. Basic features of the oxidation were studied by SEM and XRD. Secondary supports, promoters and modifiers including rare-earth elements were used to increase the thermal stability of these composites up to 900°C. These materials were assembled as thin wall honeycomb supports loaded with a number of active components. The catalysts were tested in the reactions of CO and CHx oxidation, NOx selective reduction by hydrocarbons and demonstrated a high performance and a low pressure drop at high space velocities. City diesel buses field tests of converters equipped with those catalysts demonstrated their high and stable performance in clean-up of exhausts from SOF, CO, gaseous hydrocarbons and NOx.

Microstructural and spectroscopic investigations of the supported copperalumina oxide system: Nature of aging in oxidizing reaction media

Abstract By means of electron microscopy, infrared spectroscopy of adsorbed CO, XPS, and ESR the state of the copper oxide component of the Cu Al O system (CuO/γ-Al 2 O 3 and bulk CuAl 2 O 4 catalysts) has been investigated. Several types of copper Al oxygen entities were detected on the catalyst surfaces: isolated ions, weak magnetic associates, two- and three-dimensional clusters, defect CuO phase. The dependence of their distribution on composition, preparation, and pretreatment conditions and their reducibility by carbon monoxide have been analyzed. The enthalpy of oxygen adsorption on copper atoms in various structures has been calculated by a semiempirical, interacting-bond method. The most reactive species are three-dimensional clusters reducing to Cu 0 by CO even at 163 K. Prolonged treatment in reaction media (CH 3 OH, CH 2 O, O 2 at 673–873 K) annealed extended defects in CuO particles and destroyed the clusters, forming perfect CuO particles and bulk aluminate.

Specific features of the defect structure of metastable nanodisperse ceria, zirconia, and related materials

Models of the defect structure and microstructure of the CeO2, ZrO2, Ce-Me1-O (Me1 = La, Sm, Zr), and Zr-Me2-O (Me2 = Ca, Sr, Ba) nanomaterials are discussed. For ceria-based fluorite, the appearance of weakly bound oxygen and the mobility of bulk oxygen are due to distortions in the Ce-O coordination sphere and the appearance of interstitial oxygen atoms. For pure and doped zirconia, the phases forming in the intermediate temperature range are characterized by metastable structural networks differing from those observed in the equilibrium phases. The change in the local environment of the Zr cations (eight-atom coordination sphere) from a square antiprism in the initial salts to a distorted fluorite-like polyhedron in zirconia and the principle of structural conformity between hydrolyzed cations and the terminal hydroxyls of the Zr complexes in solution are the factors determining the genesis and structural features of the metastable phases. The defect structure and microstructure of the complex fluorite-like oxides have an effect on the state of the supported active component, favoring the formation of clustered species in the vicinity of extended defects in the support. Some examples of this effect in different types of reactions are provided.

The reaction mechanism of selective catalytic reduction of nitrogen oxides by hydrocarbons in excess oxygen: Intermediates, their reactivity, and routes of transformation

The main features of the mechanism of selective reduction of nitrogen oxides by hydrocarbons (methane, propane, and propylene) in excess oxygen catalyzed by systems containing transition metal cations are considered. A combination of steady-state and non-steady-state kinetic studies, in situ Fourier-transform infrared (FTIR) spectroscopy, temperature-programmed desorption, and theoretical analysis of bond strengths and spectral data for adsorption complexes made it possible to determine reliably that surface nitrate complexes are key intermediates at real temperatures of catalysis. The rate-limiting step in these reactions includes the interaction of these complexes with hydrocarbons or their activated forms. Factors are considered that determine the structure, bond strength, and routes of nitrate complexes transformations under the action of hydrocarbons. Mechanistic schemes are proposed for the reaction of various types of hydrocarbons in which the determining role belongs to the formation of organic nitro compounds in a rate-limiting step. Their further fast transformation with the participation of surface acid sites resulting in the formation of ammonia, which is a highly efficient reducing agent, though not limiting the whole process, but determines nevertheless both the selectivity to the target product, molecular nitrogen, and the selectivity of hydrocarbon consumption for nitrogen oxide reduction.

Honeycomb-supported perovskite catalysts for high-temperature processes

Pechini route [US Patent No. 3,330,697 (1967)] was used for supporting perovskite-like systems on thin-wall corundum honeycomb support to prepare catalysts for high-temperature processes of methane combustion and selective oxidation into syngas. In this preparation, the surface of corundum monoliths walls was shown to be covered by strongly adhering porous perovskite layer formed by rounded crystals. At high temperatures when pore diffusion is expected to affect catalysts performance in fast reactions, this spatial distribution of the active component could be attractive. In the kinetically controlled region of methane oxidation, samples prepared via Pechini route possess activity comparable with that of samples made via support wet impregnation with mixed nitrate solutions, when an active component is uniformly distributed across the wall thickness. Corundum-supported lanthanum manganite and ferrite are the most active in the reaction of methane combustion, while its selective oxidation into syngas effectively proceeds on supported lanthanum cobaltite and nickelates. Corundum-supported perovskites are more thermally stable as compared with those on γ-alumina support.

Pt-Supported Nanocrystalline Ceria-Zirconia Doped with La, Pr or Gd: Factors Controlling Syngas Generation in Partial Oxidation/Autothermal Reforming of Methane or Oxygenates

Nanocrystalline CeO2-ZrO2 (Ce:Zr 1:1) samples doped with La, Pr or Gd cations (containing up to 30 at.%) were prepared via the Pechini route. Pt (1.4 wt.%) was supported via impregnation with H2PtCl6 solution followed by drying and calcination. The samples’ surface features were studied by SIMS and FTIRS of adsorbed CO. The oxygen mobility was characterized by the dynamic oxygen isotope exchange and H2 TPR. Catalytic activity was studied in the flow installation using diluted feeds (0.7% CH4 +0.5% O2 or 1% C3H6O + 0.5% O2 +0.5% H2O in He). In the selective oxidation of methane (POM), the catalytic activity correlates with Pt dispersion controlled by the oxidized sample’s ability to stabilize Pt2+ cations as precursors of small reactive Pt clusters formed under reaction conditions. This is favoured by a larger doping cation (La) and a developed network of nanodomain boundaries. At comparable Pt dispersion, the highest performance was demonstrated by a La-doped system, which correlates with the highest surface/near-surface oxygen mobility controlled by the strength of Ce-O bonds in the surface layer. In the autothermal reforming of acetone, the activity trends differ from those in POM because of the more prominent role of the oxygen mobility required to prevent surface coking.

Mechanochemical Synthesis and Catalytic Properties of the Calcium Ferrite Ca2Fe2O5

The formation of the real structure of calcium ferrite prepared by the calcination of a mechanochemically activated hydroxide mixture at 600–1100°C was studied by X-ray diffraction analysis, electron microscopy, thermal analysis, Moessbauer spectroscopy, IR spectroscopy, small-angle X-ray scattering, and secondary-ion mass spectrometry. It was found that low-temperature calcium ferrite is an anion-modified oxide, in which the ordering of oxygen vacancies was incomplete. Regions with a disordered structure were detected on the surface of crystallites. As the calcination temperature was increased, the brownmillerite crystal structure was improved and the intercrystalline boundaries were formed and then annealed. At the surface, these processes were accompanied by a change in the predominant form of adsorbed NO from nitrosyl to dinitrosyl species. An increase in the specific catalytic activity of samples with calcination temperature can be associated with the perfection of the brownmillerite structure and with a change in the state of adsorption centers.