Masayoshi Kobayashi

Application of transient response method to the study of heterogeneous catalysis: I. Nature of catalytically active oxygen on manganese dioxide for the oxidation of carbon monoxide at low temperatures

Abstract The nature and the behavior of the surface oxygen species on manganese dioxide were studied in detail by means of the transient response method. The surface oxygen on manganese dioxide during the oxidation of carbon monoxide at −15 °C was classified into two parts, O s h and O s l , and O s h was found to be responsible for the catalytic activity of this oxide. The amount of O s h determined by the transient response method was in good agreement with that obtained by the analysis with the KI method which was used in our previous work. It was concluded that the O s h is in the forms of O 2 − or O − and is rapidly regenerated with the gaseous oxygen during the catalytic oxidation of carbon monoxide. This regeneration seems to take place instantaneously upon the release of electrons as the sequence of the reactions between the ionized oxygen species and the gaseous carbon monoxide followed by the desorption of carbon dioxide formed. The O s l , probably in the form of oxygen molecules, can transform into O s h , but the rate is very slow.

Distribution of oxidation power of surface oxygen species on manganese dioxide during the oxidation of carbon monoxide

Abstract The oxidation power of surface oxygen species on manganese dioxides and their catalytic activities were measured simultaneously. The results showed that the distribution of oxidation powers varies depending upon the ambient gas atmospheres and that the catalytic activity for the oxidation of carbon monoxide is closely related to the oxygen species of higher oxidation powers. Discontinuous changes were observed both in the oxidation power distribution and in the catalytic activity at a certain partial pressure of carbon monoxide and it was attributed to the change of crystallographic structure of the surface layer of the catalyst. The mode of the variation of the distribution of oxidation power suggests the existence of two different kinds of oxygen: One is dependent in its amount on the partial pressure of carbon monoxide and the other is not. Based upon these results, a new reaction mechanism was proposed.

Application of transient response method to the study of heterogeneous catalysis: II. Mechanism of catalytic oxidation of carbon monoxide on manganese dioxide

Abstract The mechanism of the oxidation of carbon monoxide over manganese dioxide was studied by a transient response method. To follow the transient behavior, measurements were made on the time dependencies of the reaction gas composition and of the electrical conductivity of the catalyst. The analyses of the response curves showed that the gaseous carbon monoxide reacts directly with the surface oxygen species, which is negatively charged, to form some intermediates, which successively decompose to form carbon dioxide. It was found that the amount of carbon dioxide existing on the surface during the reaction under steady states was larger than the equilibrium amount, which was calculated from the adsorption isotherm specified by the Langmuir equation, and it was suggested that the desorption step of carbon dioxide is one of the slower steps in the overall reaction.

Application of transient response method to the study of heterogeneous catalysis: III. Simulation of carbon monoxide oxidation under an unsteady state

Abstract A likely model for carbon monoxide oxidation over manganese dioxide has been proposed from previous response experiments and the reaction mechanisms were analyzed kinetically by computer simulation of unsteady state reaction data. The rate constants of all elementary steps were successfully determined by the non-linear parameter optimization technique and it was found that the kinetic results were quite consistent with the experimental findings discussed in the previous paper and this substantiates the potential use of the transient response method in the studies of heterogeneous catalysis.

Transient behavior of carbon monoxide: Oxidation on red lead (Pb3O4)

Abstract The transient behavior of carbon monoxide oxidation on red lead (Pb 3 O 4 ) caused by the stepwise change in the concentrations of reaction components was investigated in detail. The experimental results indicated that the reaction between gaseous carbon monoxide and adsorbed oxygen is the slowest step in the overall reaction and that the adsorption of oxygen during the reaction is not in partial equilibrium. Based on these results, a possible reaction mechanism is proposed and the kinetic parameters in each elementary step are determined by means of a computer simulation technique. A characteristic overshoot-type behavior of the response curve of effluent carbon dioxide due to the stepwise change of carbon monoxide in the feed stream can be attributed to the slow adsorption of oxygen on Pb 3 O 4 . The response curves under different conditions can be also well explained with this interpretation.

Short range structure of ferritin core.

The short range order structure of ferritin core has been studied by the radial distribution function (RDF) method with its “structure analogue” and a iron(III) hydroxide oxide gel. The observed reduced RDFs, G(r)obss, of the specimens depicted in Fig.1 have been derived by the method applied to their electron diffraction patterns. In comparison with Fig.1, Fig.2shows the calculated ones, G(r)cales, that have been estimated on the basis of the structural models of the ferritin core and of the gel structure model (GSM).It was found that the GSM gives G(r)cales which is the m ost similar to that of the ferritin core except intensity correlation. This suggests that all the structure models that have been proposed to date for the ferritin core may require some modification.

Oxygen species contributing to the formation and the decomposition of an intermediate in the complete oxidation of ethylene over a silver catalyst

The oxygen species formed on silver by the decomposition of N2O contribute to the formation of an intermediate (In) in the complete oxidation of C2H4; the oxygen species from gaseous O2 contribute to the decomposition of In.

Reactivity of oxygen species adsorbed on silver for CO oxidation

Oxygen species adsorbed on silver from gaseous oxygen show higher activity for the oxidation of CO compared with those adsorbed from nitrous oxide at 20 °C.

Desorption of an intermediate in the complete oxidation of C2H4 over silver catalyst

It was found by applying the transient response method that a stable intermediate was formed during the complete oxidation of C2H4 over a silver catalyst which could be desorbed as acetic acid in a stream of pure H2, whereas it was not desorbed in pure He and it was decomposed to CO2 and H2O in a mixture of O2–He.