High-temperature Hydrothermal Treatment Research Articles

Preparation, active phase composition and Pd content of perovskite-type oxides

Perovskite-type oxides, containing Pd, were prepared via a combined sol–gel and combustion synthesis method and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and BET specific surface area (SSA). Their activity as three-way catalysts for the abatement of carbon monoxide, methane and nitrogen oxides, emitted from stoichiometrically operating natural-gas-fuelled vehicles, was investigated under simulated exhaust conditions. The preparation conditions concerning the complexing agent, the use of additives and the thermal treatment of the precursor solutions, were investigated. The La0.91Mn0.85Ce0.24Pd0.05Oz and La1.034Mn0.966Pd0.05Oz phases were the most active. Their activation by high-temperature hydrothermal treatment was ascribed to the migration of Pd out of the perovskite lattice and the formation of segregated PdO. The role of Pd was crucial for the catalytic activity of the active phase. A low Pd content favored the dispersion of oxidized Pdx+, 2≤x≤4, and thus, enhanced catalytic activity. Oxidized Pdx+ with x>2 appeared to be less active than Pd2+. The catalytic activity of La1.034Mn0.966Pd0.05Oz increased significantly when 8ppm SO2 were introduced in the reaction mixture.

Alumination pathways to mesoporous aluminosilicates with high-temperature hydrothermal stability.

Does the alumination pathway affect the high-temperature (steam) stability of mesoporous aluminosilicates? To what extent can the steam stability be optimised by choice of alumination pathway? This communication identifies alumination pathways that can be used to prepare mesoporous MCM-41 aluminosilicates with exceptional and unprecedented steam stability. The picture shows the XRD patterns of wet-grafted and dry-grafted Al-MCM41 before and after high-temperature hydrothermal (steaming) treatment.

Effects of fictive temperature and water content on electrical conductivity of silica glasses

Abstract Electrical conductivities of type I and IV silica glasses were measured as a function of fictive temperature and water content. Silica glasses with different fictive temperatures were produced by a heat treatment in ambient air and subsequent quenching. The water content in silica glasses was varied by a high temperature hydrothermal treatment. For type IV silica glass, the electrical conductivity decreased and the activation energy increased with increasing fictive temperature. These changes appear to be related to the decrease in specific volume of the silica glass with increasing fictive temperature. For type I glass, the changes were much smaller. Changes in water content produced different effects on electrical conductivities of different types of silica glasses; the conductivity of type I silica glass increased slightly while that of type IV silica glass decreased with increasing water content. These opposite trend in these two types of glasses can be attributed to the different content of aluminum, which interacts with protons introduced by the hydrothermal treatment.