Estudo da adsorção de hidrogênio e sulfeto na superfície de paládio: aspectos experimentais (eletroquímica) e teóricos (ab initio e Teoria do Funcional da Densidade)

The adsorption of H and S2- species on Pd (100) has been studied with ab initio, density-functional calculations and electrochemical methods. A cluster of five Pd atoms with a frozen geometry described the surface. The computational calculations were performed through the GAUSSIAN94 program, and the basis functions adapted to a pseudo-potential obtained by using the Generator Coordinate Method adapted to the this program. Using the cyclic voltammetry technique through a Model 283 Potentiostat/Galvanostat E.G.&G-PAR obtained the electrochemical results. The calculated chemisorption geometry has a Pd-H distance of 1.55Å, and the potential energy surface was calculated using the Becke3P86//(GCM/DFT/SBK) methodology. The adsorption of S2- ions on Pd surface obtained both through comparison between the experimental and theoretical results, at MP2 level, suggest a S2- absorption into the metallic cluster. The produced Pd-(S2-) system was show to be very stable under the employed experimental conditions. The paper has shows the powerful aid of computational methods to interpret adsorption experimental data.

Modificação de zeólitas para uso em catálise

The use of zeolites and other molecular sieves as catalysts is discussed at an introductory level. The text includes a brief historic background on the use of zeolites in catalysis, and a discussion of some chemical and physical properties of silicalite, aluminosilicate, and aluminophosphate molecular sieves. The strategies currently used to chemically modify zeolites and related materials to produce catalysts with increased activity and selectivity are discussed, including the use of redox molecular sieves for hydrocarbon oxidation and the leaching of the active metals from the support.

Filme fino de ZrO2 enxertado sobre a superfície de sílica gel: preparação e propriedade de adsorção de Cr(VI)

The present experiment describes an easy procedure for obtaining SiO2/ZrO2 by reacting ZrOCl2 with SiO2 with the following characteristics: S BET = 500 m² g-1 and an average pore diameter of 6 nm. The material obtained presented 1.3 wt% ZrO2 content corresponding to 140 mumol g-1. The average density of ZrO2 onto SiO2/ZrO2 matrix is 2.8x10-11 mol cm-2. The adsorption isotherm for Cr(VI) showed a maximum of adsorption value (200 mumol g-1) at pH 2. The adsorption can be described by the reaction: =Zr(OH)2 + 2HCrO4- + 2H+ [(=Zr(OH2+)2) (HCrO4-)2]. Above the zero point of charge, i.e. pH > 5.5 due to the surface charge inversion, desorption of Cr(VI) occurs according to the reaction: [(=Zr(OH2+)2) (HCrO4-)2] + 6OH- (=ZrO2)2- + 6H2O + 2CrO4(2-).

Geoquímica e índice de geoacumulação de mercúrio em sedimentos de superfície do estuário de Santos - Cubatão (SP)

The concentrations of mercury in surface sediments (<63mum fraction) from Santos-Cubatão Estuary, located in State of São Paulo, southeast Brazil, were determined to obtain the level of contamination in this highly polluted area. Fifteen stations distributed though seven rivers were sampled (156 samples collected) from September 1997 to August 1998 (12 stations sampled monthly) and during March, June, September and December 1998 (three stations sampled quarterly). The levels of mercury varied from 0.10 to 6.77 ppm, with the index of geoaccumulation of the rivers varying between grades 1 and 3 (annual average). The Cubatão is the most polluted river and the main agent of Hg dispersion in the study area. Geochemical data revealed that Hg levels in surface from Santos-Cubatão Estuary at present are as elevated as in past (for instance, by comparison 1980's), when Cubatão city was known as one of the most polluted cities in the world.

Química organometálica de superfície aplicada à preparação de catalisadores heterogêneos bem definidos

The study of the reactions of organometallic complexes with the surfaces of inorganic oxides, zeolites and metals constitutes the basis of Surface Organometallic Chemistry (SOMC). The basic rules of organometallic chemistry are often valid when applied to surfaces and well-defined surface organometallic complexes can be obtained. These complexes can be used as heterogeneous catalysts or, by controlled reactions, can be transformed in other species useful for a given catalytic reaction. In some cases, these catalysts exhibit higher activity and/or selectivity than their analogous molecular complexes.

Tendências em modificação de eletrodos amperométricos para aplicações eletroanalíticas

The most relevant advances on the analytical applications of chemically modified electrodes (CME) are presented. CME have received great attention due to the possibility of electrode surface modification including chemisorption, composite generation and polymer coating. In recent years, the interest in CME has increased overall to improve the sensitivity and selectivity of the electroanalytical probes, considering the electron mediator incorporation and the new conducting polymers development. The general procedures employed for the electrode modification and the operational characteristics of some electrochemical sensors are discussed.

Modificação de óleos e gorduras por biotransformação

The oleochemical industry has a permanent interested in controlling the physical, functional and organoleptical properties of their products and in producing useful derivatives from their raw materials. The potential of biotechnology for developing novel or well-known products at more competitive costs meets the need of this industrial segment in expanding their goals. In this work some technical aspects, problems and perspectives related to the production of oil and fat derivatives using biotransformation techniques are discussed. Particular emphasis is given to the description of biotransformation processes using lipase as catalyst, in view of the great versatility of this enzyme class to mediate typical reactions in this technological sector.

Um modelo para a superfície líquida no estudo da dinâmica do espalhamento de Xe e Ne pelo esqualano

In this work we present a theoretical model to investigate the scattering of Xe and Ne by a liquid squalane surface. The liquid surface is modeled as a grid of harmonic oscillators with frequencies adjusted to experimental vibration as frequencies of the liquid squalane and the atom-surface interaction potential is modeled by a Lennard-Jones function. The three dimensional description of the dynamics of the process which occurs at the gas-liquid interface is obtained by the classical trajectory method. The general characteristics of the dynamics of the scattering process are in good agreement with experimental data.

Aplicação e modificação química da sílica gel obtida de areia

The silica gel was obtained from sand and its surface was modified with POCl3 to produce Si-Cl bonds on the silica surface. Ethylenediamine was covalently bonded onto the chlorinated silica surface. The adsorption of the chlorides of divalent cobalt, nickel and copper was qualitatively studied to show that the bonding of ethylenediamine onto the silica gel surface produces a solid base capable of chelating metal ions from solution. The experiments illustrate the extraction of silica gel, its reactivity, the development of modified surfaces and its application in removing metal ions from water and are deigned for undergraduate inorganic chemistry laboratories.

Biomateriais: deposição de hidroxiapatita sobre superfície de Ti-cp modificada por aspersão térmica

The osseointegrated titanium implants are reliable and safe alternatives to treatments for long periods of time. For surface modification, thermal aspersion of TiO2 was used. The samples with and without TiO2 were treated with NaOH and SBF in order to obtain a layer of HA. Characterization was done by SEM and FTIR. The images of HA obtained by SEM show a uniform morphology and a porous structure with spherical particles. The IR spectra show that the surface of Ticp/ TiO2 is more favorable for the HA deposit, as can be seen by the increase of the crystalline structure and the very intense and defined bands of the OH group of HA that is verified about 3571 and 630 cm-1. Thus the Ticp/ TiO2 surface presents a satisfactory nucleation of HA when compared to Ticp.

Modificação química e física do amido de quirera de arroz para aproveitamento na indústria de alimentos

The study evaluated the efficiency of chemical (phosphorylation) and physical (extrusion) modifications of the starch of broken rice. Results demonstrated a reduction in the moisture content of extruded and phosphorylated broken rice and an increase in the ash content of phosphorylated broken rice. Both phosphorylation and extrusion increased cold water binding capacity, swelling power, and solubility. Extruded and phosphorylated pastes were stable under refrigeration, but only extruded paste was stable when submitted to freezing. Phosphorylated paste had the lowest viscosity and the highest stability during heating, while the extruded one gelatinized without heating, but had higher losses during heating.

Efeito do triacilglicerídeo de cadeia média, da fibra e do cálcio na disponibilidade de magnésio e de zinco pelo método in vitro e metodologia de superfície de resposta

Nutritional therapy with enteral diets became highly specialized in the last years. This work aims to study the effect of the components of a formulation, namely fiber, calcium and medium-chain triglycerides, for dialysability of minerals. Analysis of multiple variables was done using response surface methodology. The level curve showed that the tertiary interaction MCT-fiber-calcium was the one that presented the highest synergism in the formulation. The proportion of 33% MCT, 25% fiber and 42% calcium, gave the best formulation for availability of magnesium.

Efeito antagônico do ferro e do zinco em uma formulação de alimentação enteral utilizando planejamento de misturas da metodologia de superfície de resposta

Nutritional therapy with enteral diets became substantially specialized over the last years. This work's aim was to study the effect of the components of a formulation: fiber, calcium and medium-chain triglycerides for dialysability of minerals. Analysis of multiple variables was carried out using response surface methodology. The level curve showed an antagonic effect of interaction between iron and zinc. TCM was the variable responsible for characterizing competition between the two minerals in the formulation. More studies on foods are needed for knowing the effecs of minerals in formulations.

Otimização de método para determinação de flavonóis e flavonas em frutas por cromatografia líquida de alta eficiência utilizando delineamento estatístico e análise de superfície de resposta

This work optimized the HPLC conditions for the simultaneous determination of luteolin, apigenin, myricetin, quercetin and kaempferol in aglycone form, as well defined the best conditions for hydrolysis/extraction of these flavonoids in fruits, using the statistical central composite design and response surface analysis. A reverse phase method was developed using a gradient of methanol/water acidified with 0.3% formic acid as mobile phase and a photodiode array detector. The samples were extracted with methanol/water (50:50 v/v) at 90 ºC. The optimum time and HCl concentration varied for the different fruits investigated, demonstrating the necessity of optimizing these conditions for each fruit analyzed. Good recovery (87.1 to 96.3%), repeatability and linearity were obtained.

Aplicação da metodologia de superfície de resposta no estudo da produção e extração da poligalacturonase

The aim of this work was to verify the effects of initial medium moisture content (U), addition of ammonium sulphate (N) and of potassium phosphate (P) in the production of the polygalacturonase through the solid-state fermentation, using cashew apple husk as substrate and Aspergillus niger CCT0916 as transformation agent. We also studied the best extraction conditions of the produced enzyme. The best condition of production was with U of 40%, 1% of N and 0% of P being reached an activity of the poligalacturonase of 10.1 U/g. The best extraction condition is an agitation system with a time of 100 min and a solvent-fermented medium volume ratio of 5 mL/g.