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Heterogeneous-homogeneous radical reactions

Mechanistic studies of heterogeneous-homogeneous oxidation reactions over solid catalysts


It was found that at high temperatures reactions of catalytic combustion may proceed via so-called heterogeneous-homogeneous mechanism. This means that the surface oxidation reactions and the radical reactions in gas phase are interlinked and proceed together. Free radicals are formed on the catalyst surface from adsorbed molecules and desorb into the gas phase. These radicals participate in gas phase reactions proceeding simultaneouslly with the oxidation reactions on the catalyst surface and interlinked.

Oiginal studies of free radicals generation upon interaction of a series of alcohols and amines with oxygen over Pt and oxide catalysts were carried out. The compounds studied were: normal alcohols C1-C6, normal amines C1, C3, C4, tert-C4H9OH, tert-C4H9NH2, (n-C4H9)2NH, cyclohexanol, cyclohexylamine. The catalyst used were: transition metal oxides of the fourth period: TiO2, V2O5, Cr2O3, MnO2, Fe2O3, Co3O4, NiO, CuO, ZnO, supported oxide catalyst 34% CuCr2O4/ γ-Al2O3, γ-Al2O3, and platinum catalyst 0.64% Pt/γ-Al2O3.

Experiments on radicals desorption from catalyst surface were carried out in a quartz flow reactor at temperatures 658-893 K and total pressure 13.3 kPa using the kinetic method of freezing out radicals from the gas phase with their ESR identification.

Alkyl peroxy radicals (RO2) are formed from all amines studied over γ-Al2O3, and 0.64% Pt/γ-Al2O3. The radicals formation from amines was shown to proceed via breaking of C-N bond in the presence of oxygen. Modelling of ESR spectra of hydrocarbon radicals obtained by photolysis of peroxy radicals showed that they are represented by a mixture of alkyl and allyl radicals. When amine-oxygen mixtures were passed over oxide catalysts no formation of gas-phase radicals were detected. It was established that over oxide catalysts radicals recombination and decay take place.

Heterogeneous-homogeneous process of radicals generation on the catalyst surface in case os amines was oserved over all types of catalysts studied. For a series of normal alcohols studied over combustion catalyst 34% CuCr2O4/γ-Al2O3 efficient activation energies and radicals accumulation rates were found to be close. The rate of radicals formation over the catalysts studied at 600-1000 K is in the range of 1010-1012 particles/m2*s*torr, which is sufficient for initiation of gas phase reactions over catalysts under conditions of catalytic fuel combustion.

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Z.R. Ismagilov, S.N. Pak, V.K. Yermolaev. Heterogeneous-Homogeneous Reactions Involving Free Radicals in Procasses of Total Catalytic Oxidation. Journal of Catalysis 139 (1992) 197-201.

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