Koshi Sekizawa

Low temperature oxidation of methane over Pd catalyst supported on metal oxides

Supported Pd catalysts were investigated for low temperature oxidation of methane for catalytic combustor. Both, Pd/ZrO2 and Pd/SnO2 demonstrated excellent activity in spite of its low surface area. The activity of Pd/ZrO2 was strongly dependent on the crystal phase of the support materials. ZrO2 with a monoclinic phase enhanced the activity than that with a tetragonal phase. The catalytic activity of Pd/SnO2 was affected by the preparation procedure. Impregnation of Pd on SnO2 using Pd(C5H7OO)2 aqueous solution was most effective in enhancing the catalytic activity. It is considered that catalytic activity is strongly influenced by the existence of interaction between palladium and support materials.

Oxidation of methane over Pd/mixed oxides for catalytic combustion

Abstract Palladium catalysts supported on mixed oxides (Pd/Al 2 O 3 –MO x ; M=Co, Cr, Cu, Fe, Mn, and Ni) were investigated for the low-temperature catalytic combustion of methane. Although the surface area decreased with increasing NiO in Pd/ m Al 2 O 3 – n NiO, Pd/Al 2 O 3 –36NiO demonstrated an excellent activity due to the small particle size of palladium. Also, the catalytic activity strongly depended on the composition of the support. Temperature-programmed desorption of oxygen revealed that the catalytic activity in the low-temperature region depends on the adsorption state of oxygen on palladium. The activity was enhanced when the amount of adsorbed oxygen increased. In-situ XRD analysis indicated that the PdO phase was thermally stabilized on Pd/Al 2 O 3 –36NiO.

Low temperature oxidation of methane over Pd/SnO2 catalyst

Abstract Catalytic activities of supported Pd were investigated for low temperature oxidation of methane. Pd/SnO2 catalysts demonstrated excellent activity for methane oxidation in spite of their low surface area. The catalytic activity of Pd/SnO2 was strongly affected by the preparation procedure. Impregnation of Pd on SnO2 using aqueous solution of Pd(CH3COO)2 was most effective in enhancing the catalytic activity. The catalytic activity was also improved when well-crystallized SnO2 was employed as a support material. TEM observations revealed that catalytic activity is strongly influenced by the dispersion state of Pd. For the active catalysts, strong interaction between Pd and SnO2 support was observed in the adsorption of oxygen.

Oxidation of methane over Pd-supported catalysts

Abstract Pd catalysts supported on A1 2 O 3 -based mixed oxides were investigated for catalytic combustion. Pd/ m Al 2 O 3 -n NiO catalysts demonstrated excellent catalytic activity. Although the surface area decreased with increasing NiO content in the support, the catalytic activity increased inversely. Supported Pd was characterized by X-ray line broadening method, XPS, and by temperature programmed desorption (TPD) of oxygen. The core level binding energy (BE) of palladium increased with decreasing crystal size. This particle-size-induced BE shift depended on the crystalline phase of the support. The crystalline phase of the support affected the catalytic activity and the interaction between palladium and the support. Pd/Al 2 O 3 -36NiO demonstrated excellent activity for catalytic combustion at low temperatures. Pd particle size and support material play important roles in determining catalytic activity.

Property of Pd-supported catalysts for catalytic combustion

Abstract The catalytic properties of Pd/Sr 0.8 La 0.2 XAl 11 O 19 (X = Al and Mn) and Pd/Al 2 O 3 MO χ (M = Co, Cr, Cu, Fe, Mn and Ni) catalysts were investigated for use in catalytic combustion. The activity of Pd/Sr 0.8 La 0.2 Al 12 O 19 initially increased with rise in temperature, but decreased at high temperatures (ca. 700°C). The drop in catalytic activity was steep when the supported Pd particles were sintered after calcination above 1000°C. The activity drop accompanied dissociation of PdO into metallic Pd. Such significant drop in catalytic activity can be avoided by the use of Mn-substituted hexaaluminate (X = Mn) as a catalyst support, due to its activity for combustion. The drop could be also avoided by use of Pd/A1 2 O 3 -NiO catalysts calcined at low temperatures. Although the surface area decreased with increasing NiO content, the catalytic activity increased. Pd particle size is an important factor in determining catalytic activity.

Thick-film coating of hexaaluminate catalyst on ceramic substrates for high-temperature combustion

Abstract Structural and chemical stabilities of substituted hexaaluminate catalyst films coated on some ceramic substrates were investigated for high-temperature combustion applications. The thermal stability of the hexaaluminate catalyst films on α-SiC substrate was greatly enhanced by the insertion of both, a neat hexaaluminate and mullite intermediate layer. Pure alumina substrate was preferable in depositing the substituted hexaaluminate catalyst film to pure mullite, mullite-zirconia composite, or partially stabilized zirconia substrates. The thermal stability of the hexaaluminate catalyst films, coated on these oxide ceramic substrates, greatly depended on the extent of diffusion of components between the film and substrate at high temperatures. The substituted hexaaluminate microparticles in the film rearranged during the sintering process above 1400°C, and their (00l) plane oriented parallel to the surface of the substrate. Manganese introduced in the substituted hexaaluminate films or disks gradually decreased with an increase in the heat-treatment temperature because of its volatilization.

Thick-film coating of hexaaluminate catalyst on ceramic substrates and its catalytic activity for high-temperature methane combustion

Thick-film coating of hexaaluminate catalyst on thermally stable ceramic substrates and the catalytic activity of the film for methane oxidation were investigated for high-temperature combustion applications. The hexaaluminate film coated on pure alumina substrate retained the initial composition after calcination at 1200°C and displayed slightly lower catalytic activity than bulk hexaaluminate catalyst calcined at the same temperature. At the calcination temperatures of 1400 and 1600°C, migration of the components of the film and substrate occurred and the catalytic activity decreased considerably. The hexaaluminate catalyst film heat-treated at 1600°C was almost inactive for methane oxidation, which is attributable to volatilization of Mn during the heat treatment. By direct coating of the hexaaluminate on aluminum titanate honeycomb followed by calcination at 1200°C, the film exfoliated and cracks appeared in the aluminum titanate honeycomb. With an alumina intermediate layer inserted between the film and honeycomb, the thermal stability of the hexaaluminate film on the substrate calcined at 1200°C was significantly improved and the catalytic activity of the film was comparable to that of bulk hexaaluminate catalyst.

Role of A- and B-cations in catalytic property of substituted hexaaluminate (ABAl11O19-α) for high temperature combustion

Cation-substituted hexaaluminate compounds, ABAl 11 O 19−α , were investigated for application to high temperature catalytic combustion. Two series of modifications of the compounds was made by cation substitution; substitution of large cations in the mirror plane with lanthanides ions, and substitution of transition metals for Al site in the spinel block. In a series of AMnAl 11 O 19−α (A=La, Pr, Sm and Nd), surface area and catalytic activity increased with an increase in ionic radius of lanthanides. La 3+ is superior as the large cation in the mirror plane of the hexaaluminate to other tri-valent cations with small ionic radii. The catalytic activities of LaBAl 11 O 19−α (B=Cr, Mn, Fe, Co, Ni, and Cu) were enhanced when Mn and Cu were employed as the B-site substituents. Althoug Mn and Cu were also effective substituents for enhancing catalytic activity in Ba-based hexaaluminate compounds, their activity was low as compared with the La-based catalysts. These results indicate that the redox cycle of transition metal in hexaaluminate lattice and catalytic activity appears to be affected sensitively with the electronic or structural effect of large cation in the mirror plane.

Preparation of hexa-aluminate catalyst thick films on α-SiC substrate for high temperature application

The coating of Mn-substituted hexa-aluminate catalysts (BaMnAl11O19-α and Sr0.8La0.2MnAl11O19-α) on α-SiC substrates was investigated for high temperature application above 1000 °C. The thermal stability of the hexa-aluminate catalyst films on the α-SiC substrates was significantly affected by reactivity between the oxidized layer (SiO2) of the surfaces of the substrates and the coated layers. The Mn-substituted hexa-aluminate films were thermally less stable due to their high reactivity than unsubstituted hexa-aluminates. The Sr0.8La0.2MnAl11O19-α film directly coated on the substrate exfoliated from the substrate even after heating at 1200 °C due to its high reactivity to the SiO2 layer, whereas the BaMnAl11O19-α film was stable after the same treatment. The thermal stability of the Mn-substituted hexa-aluminate film could be improved by insertion of an Al6Si2O13 intermediate layer between the film and the SiC substrate. Additional coating of a highly crystallized Ba0.75Al11.0O17.25 intermediate layer underneath the BaMnAl11O19-α catalyst layer was also effective. The Al6Si2O13 and Ba0.75Al11.0O17.25 intermediate layers suppressed the diffusion of SiO2 from the substrate and subsequent reaction between SiO2 and the hexa-aluminate film.

Catalytic Combustion of Methane Over Metal Oxide Catalysts

Cation-substituted hexaaluminate compounds, ABAl 11 O 19-μ (A = La, Pr, Sm, and Nd; B = Cr, Mn, Fe, Co, Ni, and Cu) were investigated for application to high temperature catalytic combustion. Two series of modifications of the compounds was made by cation substitution; substitution of large cations in the mirror plane with lanthanides ions, and of transition metals for Al site in the spinel block. In a series of AMnAl ll O 19-μ , surface area and catalytic activity increased with an increase in ionic radius of lanthanides. La 3+ is superior as the large cation in the mirror plane of the hexaaluminate to other tri-valent cations with small ionic radii. The catalytic activi- ties of LaBAl 11 O 19-μ , were enhanced when Mn and Cu were employed as the B-site substituents. Although Mn and Cu were also effective substituents for enhancing catalytic activity in Ba-based hexaaluminate compounds, their activity was low as compared with the La-based catalysts. These results indicate that the redox cycle of transition metal in hexaaluminate lattice and cata- lytic activity appears to be affected sensitively with the electronic or structural effect of large cation in the mirror plane.