Phase transitions and crossovers in reaction-diffusion models with catalyst deactivation

The activity of catalytic materials is reduced during operation by several mechanisms, one of them being poisoning of catalytic sites by chemisorbed impurities or products. Here we study the effects of poisoning in two reaction-diffusion models in one-dimensional lattices with randomly distributed catalytic sites. Unimolecular and bimolecular single-species reactions are considered, without reactant input during the operation. The models show transitions between a phase with continuous decay of reactant concentration and a phase with asymptotic non-zero reactant concentration and complete poisoning of the catalyst. The transition boundary depends on the initial reactant and catalyst concentrations and on the poisoning probability. The critical system behaves as in the two-species annihilation reaction, with reactant concentration decaying as t^{-1/4} and the catalytic sites playing the role of the second species. In the unimolecular reaction, a significant crossover to the asymptotic scaling is observed even when one of those parameters is 10% far from criticality. Consequently, an effective power-law decay of concentration may persist up to long times and lead to an apparent change in the reaction kinetics. In the bimolecular single-species reaction, the critical scaling is followed by a two-dimensional rapid decay, thus two crossovers are found.Comment: 8 pages, 7 figure

Controle da qualidade microbiológica das águas de consumo na microbacia hidrográfica Arroio Passo do Pilão.

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Avaliação de genótipos de trigo e de outros cereais de inverno ao crestamento, em solo com e sem aplicação de calcário.

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Utilização de casca de pinhão para fabricação de compósitos.

Resumo

Características da madeira de seis espécies de eucalipto plantadas em Colombo - PR.

Foram analisadas as propriedades da madeira de seis espécies de Euca/yptus plantadas em Colombo, PR. As espécies estudadas foram Euca/yptus badjensis, E. panicu/ata, E. robusta, E. smithii, E. vimina/is e o híbrido cambiju. As variáveis foram densidade básica, poder calorífico, composição química, retratibilidade e o processo de secagem. Adicionalmente, os resíduos do tronco e os ramos foram quantificados e analisados como fonte de energia. Foram observadas diferenças significativas entre espécies para algumas propriedades da madeira. E. panicu/ata produziu a madeira mais densa e mais homogênea com relação à variação da densidade no sentido longitudinal. A madeira do híbrido cambiju mostrou-se dimensionalmente mais estável e, juntamente com a madeira de E. panicu/ata, foram aquelas de melhor comportamento no processo de secagem. E. panicu/ata e E. smithii produziram lenha de melhor qualidade.bitstream/item/119090/1/digitalizar0022.pd

Caracterização da composição nutricional do fruto e da farinha da casca da jabuticaba.

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Participação da Embrapa Solos em congressos: ano 2009.

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Thermal tools to evaluation of decayed and weathered wood polymer composites prepared by in situ polymerization.

This study aims to apply thermal tools in the evaluation of decayed and weathered wood polymer composites prepared by in situ polymerization of methyl methacrylate (MMA) using glycidyl methacrylate (GMA) and methacrylic acid (MAA) as cross-linkers. The pine wood samples were impregnated in a vacuum/pressure system and polymerized in an oven at 90°C for 10h, using benzoyl peroxide at 1.5 wt% as catalyst. The untreated wood and composites were exposed to in vitro decay tests with Trametes versicolor and Gloeophyllum trabeum fungi, and to artificial weathering. The weight loss after tests was measured, and the characterization was performed by thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis. The mass loss caused by exposure to fungi was evidently higher in untreated wood in relation to the composites, ~2.5 to 10 times - the composites with GMA and MAA showed the highest resistance to both fungi. The composites without cross-linkers showed the higher mass loss in the artificial weathering tests (>11%), due to the leaching of part of poly(MMA) formed inside wood. By TGA and DSC analysis, we observed shifting in the temperature of thermal events related to polysaccharides and lignin after exposed to decays tests ? more significant changes were for Trametes versicolor tests. The thermograms related to weathered samples showed different results for each composite. The untreated wood and the composite without cross-linker presented loss in lignin, meanwhile the composites with cross-linkers presented degradation in the copolymer formed onto surface of wood. Keywords: TGA, DSC, pinewood, methacrylate, additivesCBRATEC