The phases present and their crystal structure and microstructure in the nanocrystalline SnO 2 -TiO 2 system were studied in the compositional range Sn 1 -x Ti x O 2 (0.0 ≤ x ≤ 0.9). There is an apparent increase in the solubility limits in the solid solution compared to bulk crystalline SnO 2 -TiO 2 . No two phase region was observed with increasing TiO 2 content. Electron energy loss spectroscopy, infrared spectroscopy (FTIR), and X-ray diffraction (XRD) of the nanopowders showed that the apparent increase in solubility is related to the systematic Ti 4+ segregation on the particle surface (surface excess) at the SnO 2rich side, avoiding the nucleation of a second phase even at high Ti 4+ contents. Is this finding in accord with Raman spectra, which suggest localized Ti-rich sites in the absence of a second crystalline phase. Ti 4+ surface excess is also lead to a modification of the surface hydroxyls and a decrease in the crystallite size of the nanoparticles (with a concomitant increase in surface area), with expected implications to catalytic and sensorial properties of these nanoparticles.G. Rohrer-contributing editor Manuscript No. 36322.

MgAl ₂ O ₄ is a stable catalyst support with potential for replacing gamma-alumina in several applications. However, synthesis of magnesium spinel requires elevated temperatures to avoid phase separation (in MgO and Al ₂ O ₃ ) at low temperatures, leading to coarsening and reduction of active surface area. In this work, nano CaO-doped and undoped magnesium aluminate were successfully prepared by Ultrasonic Spray Pyrolysis (USP), using a simple adapted experimental set-up operating at 1100°C. During the process, the particles stay at high temperatures for a short period of time, allowing phase stability and limited coarsening. The influence of calcium oxide on the particles morphology and structure was investigated via X-ray diffraction, N 2 adsorption, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. The spinel nanopowders were obtained as spherical porous agglomerates of ~1 µm. The resulting powder showed low © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ crystallite sizes in the 5-10 nm range and high specific surface area from 110.0 to 76.6 m ² .g -1 .

Resumo O TiO2 tem sido objeto de diversos estudos devido ao seu excelente desempenho como fotocatalisador. Aditivos como o MgO têm sido introduzidos para melhorar o desempenho fotocatalítico do TiO2. No entanto, a físico-química destes sistemas de óxidos e suas relações com suas demais propriedades são pouco compreendidas. Neste trabalho, nanopartículas de xMgO-(1-x)TiO2 (0≤ x≤ 0,05) foram sintetizadas pelo método dos precursores poliméricos a 500 °C por 15 h. Os resultados de difração de raios X mostraram que somente a fase anatásio é formada e que o tamanho de cristalito diminui com o aumento da concentração de MgO. A lavagem dos pós em solução aquosa de HNO3 solubilizou grande parte do MgO evidenciando que ocorre excesso de superfície do aditivo. Análises de FTIR confirmaram que a lavagem em ácido altera a superfície das partículas e, portanto, é proposto que a redução do tamanho de cristalito ocorre devido à segregação de MgO na superfície.

The thermodynamic stability of nanocrystalline SnO 2 -TiO 2 solid solutions was studied experimentally. Microcalorimetry of water adsorption revealed a systematic decrease in the surface energy with increasing Ti 4+ content in the SnO 2 -rich compositions, consistent with previous reports of Ti 4+ segregation on the surface. The surface energy change was accompanied by an increase in the magnitude of the heat of water adsorption, also indicating a modification of the SnO 2 surface by Ti 4+ . Supporting the water adsorption data, calculations using high-temperature oxide melt solution calorimetry data also suggest a decrease in the interface energies. A thermodynamic analysis showed that the observed surface energy decrease is responsible for an increase in the stability of solid solutions in the nanophase regime. Although a miscibility gap is expected in this system from bulk phase diagrams, the surface energy contribution modifies the bulk trend and promotes extensive solid solutions when the surface area is above a critical value dependent on the surface energy and the bulk enthalpy of mixing.