Anion-modified Oxides Research Articles

CATALYSTS OF THE ISOMERIZATION PROCESS OF n-HEXANE

Information on the isomerization of n-hexane is systematized. It has been shown that the process of isomerization of n-alkanes is a priority issue in solving the problems of improving the quality of modern automobile gasolines and their compliance with environmental requirements. The use of low-octane fractions as raw materials determines the high efficiency of the process in addition to the production of high-value isomerizate. The obtained isomerizate meets all the requirements for motor fuels: high impact resistance, easy volatility, absence of benzene, olefins and sulfur. The article presents thermodynamic data, provides general information about modern catalytic systems used in the isomerization of n-alkanes, and shows their mechanisms of action. Comparative data on Friedel-Crafts catalyst, chlorinated alumina, zeolites, sulfated and tungsten-containing zirconium, and heteropolyacids are summarized. In the process of isomerization of the pentane-hexane fraction, platinum-containing catalysts based on chlorinated aluminum oxide, zeolites and sulfated zirconium dioxide are used in industry. SO4/ZrO2-based catalysts can be considered the most promising due to their high performance indicators. The current work is mainly focused on the systematic study of the physico-chemical regularities of the formation and isomerization activity of catalysts based on anion-modified oxides, depending on their composition and preparation conditions. Keywords: n-hexane, isomerization, thermodynamics, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, catalytic systems.

Formation of Effective Copper-Based Catalysts of Methanol Synthesis

The regularities of formation of the composition and structure of a highly efficient catalyst for methanol synthesis have been revealed. The catalyst was obtained by thermal treatment of the combined CuZnAl(Cr) hydroxocarbonate followed by the reductive activation of the anion-modified zinc oxide precursor. The composition and structure of hydroxocarbonate are determined by the nature of components and their ratio. The thermal decomposition of hydroxocarbonates at 550–650 K formed the oxide catalyst; at higher temperatures, the product was an inactive mixture of oxides. The oxide catalyst is mixed zinc oxide containing the Cu2+ and Al or Cr3+ cations and residual (СО3)2– and ОН– anions, i.e., the anion-modified oxide.

Controlling the Catalytic Properties of Copper-Containing Oxide Catalysts

The results of a systematic study of the formation of Cu–Zn, Cu–Zn–Al, Cu–Zn–Cr, Cu–Zn–Si, Cu–Cr, and Cu–Si oxide catalysts with a widely varied ratio between their components are generalized within the chemical approach developed by G.K. Boreskov to establish the quantitative relation between their chemical composition and catalytic activity. Simultaneously, their catalytic properties, such as selectivity and activity, are measured under the same conditions in the methanol synthesis and dehydrogenation and water gas shift reactions, whose common feature is a reductive reaction medium. The activity of Cu–Zn–Al–Cr— Si-oxide catalysts in all the studied reactions is governed by the Cu0 nanoparticles formed on their surface in the process of reductive activation. Nanoparticles of different catalysts have similar sizes (3–8 nm). However, the ratios between the catalytic activities per unit of the copper surface area for catalysts with various structures of their oxide support (spinel, wurtzite, zincsilite, or silica type) are appreciably different in each reaction. The relation between the chemical composition of a catalyst and its catalytic activity in a certain reaction is established by the chemical composition of its precursor representing a hydroxo compound, i.e., the nature of the selected cations and the quantitative ratio between them. The decomposition of hydroxo compounds to oxides (and the further activation of oxides) should be performed at medium temperatures, providing the incomplete elimination of ОН– and CO32- anions, i.e., the formation of anion-modified oxides. The structure of the latter and the type of interaction between Cu0 nanoparticles and an oxide support are governed by the structure of the hydroxo precursor compound.

Electrochemical investigation of the roles of oxyanions in chemical–mechanical planarization of tantalum and tantalum nitride

Nitrate, sulfate, and phosphate oxyanions are shown to serve as effective surface-modifying agents for low-pressure chemical–mechanical planarization (CMP) of Ta and TaN barrier layers of interconnect structures. The surface reactions that form the basis of this CMP strategy are investigated using cyclic voltammetry, open circuit potential and polarization resistance measurements, and impedance spectroscopy. The results suggest that forming structurally weak layers of surface oxides is crucial to chemically controlling the CMP of Ta/TaN at low polish-pressures. It is shown that in oxyanion-based slurries, this can be accomplished by modifying the sample surfaces with anion-incorporated oxide films of Ta or TaN, which, in turn, can readily be removed with moderate abrasion. Electrochemical results elaborate the reaction mechanisms that lead to anion-modified oxides, such as Ta2O5(1−x)(NO3)10x , Ta2O5(1−x)(SO4)5x , and Ta2O5(1−x)(PO4)10x/3 on both Ta and TaN surfaces in pH-controlled solutions of KNO3, K2SO4, and KH2PO4 solutions, respectively.

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.

Physico-chemical studies of the temperature range for the formation of anion-modified oxides

The effect of metal ions on the temperature range for the exstence of anion-modified oxides formed during the formation of Mg and Zn oxides and solid solutions of Cu, Al, Cr and Ni ions in these oxides through thermal decomposition of the respective hydroxo compounds, has been established.

Low-temperature preparation of oxide catalysts

Anion-modified oxide compounds are shown to be formed in the low-temperature synthesis of oxide catalysts, which causes differences in the composition and state of transition metal ions and, hence in catalytic properties of oxide catalysts prepared at low and high temperatures.