Humberto V. Fajardo

Selective synthesis of vinyl ketone over SnO2 nanoparticle catalysts doped with rare earths

Ultrafine particles of tin oxide, doped with the rare earths Ce, Y and La respectively, were prepared using the polymeric precursor method. The novel application of nanostructured tin oxide, undoped and doped, to the CC bond formation of an α,β-unsaturated compound (methyl vinyl ketone; MVK) is presented. The promotion of SnO2 by rare earths results in a large increase in the catalytic activity and basicity (total basic sites). The catalytic behavior observed in SnO2 samples suggests that the control of the catalytic activity and the selectivity of the products takes place by the segregation of a layer of a rare earth compound, with the increase of the heat-treatment temperature. The structural variations of the ultrafine particles were characterized by means of BET, XRD, CO2 chemisorption and Raman.

Influence of Rare Earth Doping on the Structural and Catalytic Properties of Nanostructured Tin Oxide

Nanoparticles of tin oxide, doped with Ce and Y, were prepared using the polymeric precursor method. The structural variations of the tin oxide nanoparticles were characterized by means of nitrogen physisorption, carbon dioxide chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. The synthesized samples, undoped and doped with the rare earths, were used to promote the ethanol steam reforming reaction. The SnO2-based nanoparticles were shown to be active catalysts for the ethanol steam reforming. The surface properties, such as surface area, basicity/base strength distribution, and catalytic activity/selectivity, were influenced by the rare earth doping of SnO2and also by the annealing temperatures. Doping led to chemical and micro-structural variations at the surface of the SnO2particles. Changes in the catalytic properties of the samples, such as selectivity toward ethylene, may be ascribed to different dopings and annealing temperatures.

Gadolinium-doped cerium oxide nanorods: novel active catalysts for ethanol reforming

The gadolinium-doped ceria nanorods (Gd0.2Ce0.8O2−x) were synthesized by hydrothermal treatment. It was shown that the use of microwave heating during hydrothermal treatment decreases the treatment time required to obtain gadolinium-doped ceria nanorods and that oriented attachment is the dominant mechanism responsible for anisotropic growth. It was clear that Gd0.2Ce0.8O2−x nanorods were more catalytically active than commercial CeO2 in the ethanol reforming reaction.

Ce1−xSmxO1.9−δ nanoparticles obtained by microwave-assisted hydrothermal processing: an efficient application for catalytic oxidation of α-bisabolol

Heterogeneous catalysts based on Sm-doped ceria were employed for the first time in the liquid-phase oxidation of α-bisabolol. Nanometer-sized catalysts were obtained by microwave-hydrothermal synthesis and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), Raman spectroscopy and N2-physisorption. The influence of Sm doping, temperature and the solvent used on the catalytic behavior was investigated. Conversions up to 84% and a combined selectivity for the products up to 77% were obtained for Ce0.9Sm0.15O1.85−δ catalysts. The reactions were highly selective for the epoxidation products (only bisabolol oxides A and B were obtained) and shown to be strongly dependent on the temperature and solvent employed. Best results were achieved for higher Sm concentrations, which indicate that changes in the textural properties due to doping produced a significantly more active catalyst.

Obtenção e caracterização de carbono ativado a partir de resíduos provenientes de bandas de rodagem

In this work, the preparation of activated carbons through the pyrolysis of elastomers, arising from car threads of tire waste, was investigated. The material was processed at 500, 620 and 700 °C, under N2 atmosphere, by using potassium hydroxide as activating agent. The resulting products were characterized by physisorption of N2 at 77 K, through Brunauer, Emmet and Teller isotherms, and scanning electron microscopy. The carbons obtained display a characteristic structure of mesoporous materials and the pyrolysis temperature has strong influence on the specific area and porous volume distribution. The activated carbon obtained at 700 °C has high specific area and compact structure. It exhibited high performance for adsorption of methylene blue solution, removing 1.1 x 10-1 g of the dye per gram of carbon in less than 300 s.

Catalytic Properties of AgPt Nanoshells as a Function of Size: Larger Outer Diameters Lead to Improved Performances

We report herein a systematic investigation on the effect of the size of silver (Ag) nanoparticles employed as starting materials over the morphological features and catalytic performances of AgPt nanoshells produced by a combination of galvanic replacement between Ag and PtCl6(2-) and PtCl6(2-) reduction by hydroquinone. More specifically, we focused on Ag nanoparticles of four different sizes as starting materials, and found that the outer diameter, shell thickness, and the number of Pt surface atoms of the produced nanoshells increased with the size of the starting Ag nanoparticles. The produced AgPt nanoshells were supported into SiO2, and the catalytic performances of the AgPt/SiO2 nanocatalysts toward the gas-phase oxidation of benzene, toluene, and o-xylene (BTX oxidation) followed the order: AgPt 163 nm/SiO2 > AgPt 133 nm/SiO2 > AgPt 105 nm/SiO2 > AgPt 95 nm/SiO2. Interestingly, bigger AgPt nanoshell sizes lead to better catalytic performances in contrast to the intuitive prediction that particles having larger outer diameters tend to present poorer catalytic activities due to their lower surface to volume ratios as compared to smaller particles. This is in agreement with the H2 chemisorption results, and can be assigned to the increase in the Pt surface area with size due to the presence of smaller NPs islands at the surface of the nanoshells having larger outer diameters. This result indicates that, in addition to the overall diameters, the optimization of the surface morphology may play an important role over the optimization of catalytic activities in metal-based nanocatalysts, which can be even more pronounced that the size effect. Our data demonstrate that the control over surface morphology play a very important role relative to the effect of size to the optimization of catalytic performances in catalysts based on noble-metal nanostructures.

Synthesis, characterization and catalytic properties of nanocrystaline Y2O3-coated TiO2 in the ethanol dehydration reaction

In the present study, TiO 2 nanopowder was partially coated with Y 2 O 3 precursors generated by a sol-gel modified route. The system of nanocoated particles formed an ultra thin structure on the TiO 2 surfaces. The modified nanoparticles were characterized by high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) analysis, Zeta potential and surface area through N 2 fisisorption measurements. Bioethanol dehydration was used as a probe reaction to investigate the modifications on the nanoparticles surface. The process led to the obtainment of nanoparticles with important surface characteristics and catalytic behavior in the bioethanol dehydration reaction, with improved activity and particular selectivity in comparison to their non-coated analogs. The ethylene production was disfavored and selectivity toward acetaldehyde, hydrogen and ethane increased over modified nanoparticles.

Gas-phase selective conjugate addition of methanol to acetone for methyl vinyl ketone over SnO2 nanoparticle catalysts

Neste trabalho e apresentada a reacao de condensacao aldolica entre metanol e acetona promovida por nanoparticulas de dioxido de estanho, nao dopadas e dopadas com as terras raras Ce e Y. Diversas condicoes de relacao molar metanol/acetona foram avaliadas, objetivando a obtencao com elevada seletividade do composto a, b-insaturado, metil vinil cetona. As nanoparticulas foram preparadas empregando-se o metodo dos precursores polimericos. As amostras foram caracterizadas por meio de adsorcao de N2 para determinacao de area superficial especifica (BET), difracao de raios X (DRX), adsorcao quimica de CO2, espectroscopia de fotoeletrons (XPS) e microscopia eletronica de transmissao (MET). O comportamento catalitico observado para as amostras de SnO2 sugere que a relacao molar metanol/acetona e a dopagem com as terras raras desempenham um papel importante na atividade catalitica do material e na seletividade aos produtos reacionais. Os resultados de DRX e XPS apontam para a formacao de uma camada de segregacao, para as amostras dopadas, devido ao tratamento termico imposto. Essa camada de segregacao afeta diretamente o comportamento catalitico do material frente a reacao de condensacao.

Processing effects of nanometric rare earth-doped tin oxides on the synthesis of methyl vinyl ketone

Nanoparticle catalysts based on SnO2 were processed by two different methods, víz.the polymeric precursor and high-energy mechanical milling. The sample morphology and catalytic performance in the aldol condensation between methanol and acetone were modified by the processing and rare earth doping.

Sub-15 nm CeO2 nanowires as an efficient non-noble metal catalyst in the room-temperature oxidation of aniline

We described herein the facile synthesis of sub-15 nm CeO2 nanowires based on a hydrothermal method without the use of any capping/stabilizing agent, in which an oriented attachment mechanism took place during the CeO2 nanowire formation. The synthesis of sub-15 nm CeO2 nanowires could be achieved on relatively large scales (∼2.6 grams of nanowires per batch), in high yields (>94%), and at low cost. To date, there are only a limited number of successful attempts towards the synthesis of CeO2 nanowires with such small diameters, and the reported protocols are typically limited to low amounts. The nanowires displayed uniform shapes and sizes, high surface areas, an increased number of oxygen defects sites, and a high proportion of Ce3+/Ce4+ surface species. These features make them promising candidates for oxidation reactions. To this end, we employed the selective oxidation of aniline as a model transformation. The sub-15 nm CeO2 nanowires catalyzed the selective synthesis of nitrosobenzene (up to 98% selectivity) from aniline at room temperature using H2O2 as the oxidant. The effect of solvent and temperature during the catalytic reaction was investigated. We found that such parameters played an important role in the control of the selectivity. The improved catalytic activities observed for the sub-15 nm nanowires could be explained by: i) the uniform morphology with a typical dimension of 11 ± 2 nm in width, which provides higher specific surface areas relative to those of conventional catalysts; ii) the significant concentration of oxygen vacancies and high proportion of Ce3+/Ce4+ species at the surface that represent highly active sites towards oxidation reactions; iii) the crystal growth along the (110) highly catalytically active crystallographic directions, and iv) the mesoporous surface which is easily accessible by liquid substrates. The results reported herein demonstrated high activities under ambient conditions, provided novel insights into selectivities, and may inspire novel metal oxide-based catalysts with desired performances.