Takashi Aida

Textural properties of layered double hydroxides: effect of magnesium substitution by copper or iron

Abstract Layered double hydroxides (LDHs), in which magnesium was partially substituted by iron or copper, were synthesized by a coprecipitation method. The materials were characterized by X-ray diffraction, X-ray fluorescence, FT-IR spectroscopy, N2 adsorption, scanning electron microscopy and transmission electron microscopy. The treatment of the adsorption–desorption isotherms with different computation models permitted the determination of several parameters, useful for thorough characterization of the microstructures of the samples. The substituted samples preserved the hydrotalcite-type layered structure though their textural properties underwent important modifications. For copper substituted LDH, a relatively uniform porous structure with emphasized mesoporous characteristics emerged, the mesopores were enlarged and the specific surface area was decreased. On the contrary, for iron substituted LDH, the microporosity features developed, nonuniformity and constrictions in the porous structure was accentuated, the pore size was decreased and the specific surface area was increased. Microscopic morphology characteristics contributed to establish the textural properties of the samples.

Mg-V-Al mixed oxides with mesoporous properties using layered double hydroxides as precursors: catalytic behavior for the process of ethylbenzene dehydrogenation to styrene under a carbon dioxide flow

A new catalyst for the process of ethylbenzene dehydrogenation to styrene under a CO2 atmosphere was obtained using vanadium-substituted layered double hydroxides as precursors. After calcination, vanadium-substituted hydrotalcite-like samples gave mixed oxides with a high surface area and strong mesoporous characteristics; the XRD analysis indicates the formation of Mg3V2O8 and Mg2Al2O4 in bulk while the XPS results point to the presence of a mixture of V5+ and V3+ ions on the surface. Catalytic tests show that the styrene yield and selectivity increase when the vanadium content of the sample increases. The higher catalytic activity is attributed to the formation of V5+ on the surface that is promoted by the presence of CO2 as a cofeed gas. The FTIR adsorption of pyridine shows that aluminum contributes to Lewis-type acidity of the catalyst and is important for establishing features of the catalyst deactivation process.

Effect of diffusion and adsorption–desorption on periodic operation performance of NO–CO reaction over supported noble metal catalysts

Abstract Effects of intrapellet diffusion and of adsorption along a plug-flow reactor on periodic operation performance of NO–CO reaction over Pt/Al 2 O 3 and Rh/Al 2 O 3 were investigated by reaction experiments and by model calculations. The model calculation for the effect of intrapellet diffusion on the time-averaged conversion of NO showed that the effect was significant under periodic operation at cycling periods longer than several tens of seconds in the case of strong adsorption even when it had no significant effects on steady-state performance. A two-stage catalyst bed which consisted of Pt/Al 2 O 3 at the inlet- and Rh/Al 2 O 3 at the outlet-side attained a higher time-averaged conversion of NO than a similar one but with the reverse placement. Model calculations for the effect of adsorption–desorption along the catalyst bed showed that the result described was caused by deformation of concentration cycling pattern due to strong adsorption of NO and CO. Calculated breakthrough curves could simulate the experimental ones quite satisfactorily.

In-Situ Active Site Formation in Co Oxidation On Alumina

For the development of automotive catalysts which may fit the condition of developing countries, catalytic activity of alumina for CO oxidation was studied. It was proposed that the carbon formed in-situ acted as an active site for CO oxidation. the carbon active site was also checked by methanol oxidation on alumina which showed temperature hysteresis during consecutive heating and cooling operations. Alkali-treated Alumina did not show any indication of the temperature hysteresis. The optimal temperature for maximum carbon depostion was confirmed by thermogravimetric analysis to be 450–500 C, which well explains the hysteresis. CeO2−Al2O3 showed remarkably higher activities for complete oxidation. It seems that alumina has reasonably satisfactory activity in total clean-up of exhaust gas.

Modeling of a SMB-type Reactor for Hydrogen Storage by Iron Oxide with a Reversible Reaction Mechanism

Steam oxidation of iron metal which is the reverse reaction of the reduction of iron oxides as a hydrogen storage material was conducted under the condition of 673K with varied partial pressures of water vapor. The kinetic model of the steam oxidation of iron metal was expressed as the ordinary shrinking-core model with film diffusion control by experimental data obtained from thermogravimetric measurement. The apparent rate constant of steam oxidation was expressed as the Langmuir type equation against the partial pressure of water vapor. A model of a simulated moving bed-type reactor to solve the problem of over-reduction of materials was made. In our previous work (forthcoming), the kinetic model of the reduction of iron oxide with hydrogen was expressed as a shrinking-core model with reaction control, and the modeling of a simulated moving bed-type reactor was made with an irreversible reaction model. In this work, the modeling of a simulated moving bed-type reactor was achieved with the model of a reversible reaction consisting of reduction rate and oxidation rate.

Ammonia oxidation over Cu-based metal oxides under microwave irradiation

Ammonia oxidation over copper based oxides (CuO, La 2 CuO 4 , CuTa 2 O 6 and Cu-MOR) under microwave irradiation was investigated. Effects of the catalysts, amount and particle size of CuO, diameter of the reactor, microwave power and cycle number were examined. CuO was found to be most effective among the catalysts tested due to its high efficiency for microwave absorption. The highest temperature reached for the catalyst was 792 K. And the absorption efficiency became higher when the amount and particle size of the catalyst, the diameter of the reactor and the microwave power were greater, and no significant change was observed when the microwave irradiation was repeated. The results of the NH 3 oxidation under microwave irradiation were almost the same as those obtained by conventional heating by an electric furnace except the negligible NO formation at high temperatures.

Oxidative Coupling of Propane with a Two-Layered Catalyst Bed Reactor

To realize process intensification, active species like radicals and their mass transfer need to be considered. A two-layered catalyst bed reactor was used to investigate the participation of radicals in two different reaction processes. Two-layered catalyst bed (2LCB), mixed catalyst beds (MCB) and separated catalyst beds (SCB) were compared in oxidative coupling of propane. The dependency on the contact time for the concentration of propylene in the effluent was almost constant at experimental conditions. However, that of benzene with 2LCB and MCB was higher than SCB, especially at a lower flow rate. The quartz sand layer for SCB between the catalyst beds prevented the highly active intermediate generated in the first layer from being transferred in the gas phase to the second layer. This result supports the idea of the participation of the active intermediate in the direct coupling to benzene.