Fernando Cunha Peixoto

Optimal wavelet signal compression as an efficient alternative to investigate molecular dynamics simulations: application to thermal and solvent effects of MRI probes

Currently, cancer is one of the most serious problems facing humanity, and one of the most widely used and versatile diagnostic techniques is magnetic resonance imaging (MRI), which may require an injection of contrast agents (CAs). CAs are paramagnetic compounds capable of increasing the relaxation rate of water protons, thereby obtaining clearer MRI examination images. In this work, the solvent and thermal effects on spectroscopic parameters of [Fe(H2O)6]2+, [Mn(H2O)6]2+, [Gd(H2O)8]3+, and γ-Fe2O3, coordinated to explicit water molecules, were studied in light of the classical and quantum methods. For the classical part of the simulation, a new method of selection of configurations from the molecular dynamics simulations, denominated optimal wavelet signal compression algorithm (OWSCA), is presented. Our findings indicate that OWSCA leads to a good agreement with the experimental available data and the thermal effects greatly influence our system. Thus, the molecular dynamics calculations are important for this type of system. Our results show the γ-Fe2O3 as a promising alternative to conventional contrast agents.

A study of equivalent electrical circuit fitting to electrochemical impedance using a stochastic method

Display Omitted A mathematical development regarding the minimization of residual least squares is presented.A statistically valid objective function of the complex nonlinear regression problem is obtained.A global search stochastic optimization algorithm is used on an electrochemical impedance system modelling problem.The population based stochastic method was used directly to obtain confidence regions to the estimated parameters.A Sensitivity Analysis was performedto assess the influence of each parameter onreal and imaginary parts of the model. Modeling electrochemical impedance spectroscopy is usually done using equivalent electrical circuits. These circuits have parameters that need to be estimated properly in order to make possible the simulation of impedance data. Despite the fitting procedure is an optimization problem solved recurrently in the literature, rarely statistical significance of the estimated parameters is evaluated. In this work, the optimization process for the equivalent electrical circuit fitting to the impedance data is detailed. First, a mathematical development regarding the minimization of residual least squares is presented in order to obtain a statistically valid objective function of the complex nonlinear regression problem. Then, the optimization method used in this work is presented, the Differential Evolution, a global search stochastic method. Furthermore, it is shown how a population-based stochastic method like this can be used directly to obtain confidence regions to the estimated parameters. A sensitivity analysis was also conducted. Finally, the equivalent circuit fitting is done to model synthetic experimental data, in order to demonstrate the adopted procedure.

Kinetics of Nitrate Hydrogenation in Water on Alumina and Niobia Supported Palladium-Copper Catalysts

Abstract The kinetics of nitrate hydrogenation in water over niobia- and alumina-supported Pd-Cu catalysts were examined. A Langmuir-Hinshelwood kinetic model fitted concentration data, providing consistent kinetic and thermodynamic parameters. The kinetic model assumed a bimolecular surface reaction as the rate-determining step, with two types of sites. Pd-Cu/gamma-Al2O3 was active and selective under the reaction conditions. However, Pd-Cu/Nb2O5 deactivated during time on stream, and a deactivation function was added to the kinetic model to best fit the experimental data.

MODELAGEM MICROCINÉTICA DO SISTEMA DE HIDROGENAÇÃO CATALÍTICA DE NITRATO EMPREGANDO CATALISADOR Pd-Sn/Al2O3

O excesso de nitrato na agua favorece inumeros problemas em termos ambientais, de saude animal e dos seres humanos. Em criancas altas concentracoes de N-Nitrato podem produzir intoxicacao e, em casos extremos, ate leva-la a morte por metemoglobinemia. A reducao catalitica de nitrato pode ser aplicada na remediacao das aguas contaminadas por nitrato. Uma modelagem completa do sistema de hidrogenacao de nitrato e producao de N 2 e NH 4 + em catalisador Pd-Sn/ γ-Al 2 O 3 , efetuado com controle de pH, foi desenvolvido c onsiderando as reacoes sobre a superficie do catalisador, os efeitos de transferencia de massa gas-liquido e liquido-solido e as relacoes de equilibrio na fase liquida. O modelo fenomenologico desenvolvido e constituido por um sistema de equacoes diferenciais, que traduz o balanco de massa dos reagentes e produtos no reator. Codificado em Scilab 5.3.3, o modelo foi usado para simular a evolucao de variaveis de estado do problema e mostrou estar em concordância com resultados experimentais.