Showing papers by "José Maria C. Bueno published in 2010"

Effect of the CeO2 content on the surface and structural properties of CeO2–Al2O3 mixed oxides prepared by sol–gel method

Abstract: Mixed xCeO2–Al2O3 oxides with different CeO2 content (x = 1–20 wt%) were prepared by sol–gel method. The effect of the CeO2 content, atmosphere and temperature treatment during synthesis on the structure and properties of the xCeO2–Al2O3 was investigated. xCeO2–Al2O3 xerogels prepared from aluminum tri-sec-butoxide and Ce(III) nitrates have a pseudo boehmite-like structure. The Ce(III) ions stabilized under a reductive atmosphere randomly occupy vacant sites in tetrahedral and octahedral positions of Al2O3. The crystallization of γ-Al2O3 is suppressed and the textural properties of the mixed xCeO2–Al2O3 oxides are stabilized at high temperature. Conversely, the treatment xCeO2–Al2O3 xerogels in an oxidative atmosphere shows a progressive oxidation of Ce(III) to Ce(IV) with crystallization of γ-Al2O3 and formation of CeO2 with fluorite-like structure as the temperature rises. The effect of CeO2 on the catalytic performance xCeO2–Al2O3-supported Pt catalysts in partial oxidation of methane reaction was tested. The Pt catalysts were obtained by two methods: (i) impregnation of the xCeO2–Al2O3 supports with H2PtCl6·6H2O and (ii) one-step preparation by addition of Pt precursor during the sol–gel synthesis. CeO2-containing catalysts showed higher stability than Pt/Al2O3 catalyst obtained by impregnation method. The effect of the support was less evident when the catalysts were obtained by the one-step method. Pt catalysts based on sol–gel oxides preparation show higher thermal stability and result in higher Pt sintering resistance than conventional ones.

Formation of Al-rich nanocrystalline ZSM-5 via chloride-mediated, abrupt, atypical amorphous-to-crystalline transformation

Abstract: The efficient hydrothermal synthesis of highly crystalline, nano ZSM-5 particles, rich in framework Al content (Si/Al = 25) and low in the degree of intergrowth, in a medium of unusually high NaCl concentration was followed by examining the solids formed with 29Si, 27Al, 13C MAS NMR, TEM, DRIFT, and XRD spectroscopies. ZSM-5, in the form of 60 nm primary crystallites present as 100–150 nm dispersed cubic clusters, were crystallized from an amorphous phase via an abrupt, solid state transformation within 15 min at 165 °C. A model of the crystallization process was proposed, different from the typical nucleation-crystal growth process, in which aluminosilicate oligomers of low connectivity was salted out as amorphous solid as a result of high NaCl concentration immediately upon mixing of the ingredients.