Artur J.S. Mascarenhas

Adsorption of monovalent metal atoms on graphene: a theoretical approach

This work investigates, using first-principles calculations, electronic and structural properties of hydrogen, lithium, sodium, potassium and rubidium that are adsorbed, in a regular pattern, on a graphene surface. The results for H-graphene (graphane) and Li-graphene were compared with previous calculations. The present results do not support previous claims that the Li-C bond in such a layer would result in an sp(2) to an sp(3) transition of carbon orbitals, being more compatible with some ionic character for the covalent bond and with lithium acting as an electron acceptor in a bridging environment. Calculations were also performed for the Na, K, and Rb-graphene systems, resulting in a similar electronic behaviour but with a more pronounced ionic character than for Li-graphene. Energy calculations indicate the possible stability of such ad-graphene layers, with only the Li-graphene being possible to be spontaneously obtained.

One-step synthesis of alkyltrimethylammonium-intercalated magadiite

Cetyltrimethylammonium- and tetradecyltrimethylammonium-intercalated magadiites were prepared by direct syntheses, starting from sodium metasilicate (Na2O/SiO2 = 1.0) and nitric acid. Total substitution of sodium by cetyltrimethylammonium or tetradecyltrimethylammonium cations was not achieved in the range of surfactant: silicon molar ratios used in this study. When a phosphoniun-based surfactant replaces the ammonium surfactants in the same procedure, the result of the synthesis is a mixture of quartz and unmodified surfactant. If dodecylammonium bromide is used, an MCM-41 molecular sieve is obtained. The substitution of the silicon source by tetramethylammonium silicate or of nitric acid by hydrochloric, hydrofluoric, or acetic acids also yields MCM-41 molecular sieves, indicating that the formation of magadiite is greatly dependent on the presence of sodium cations and nitrate anions.

Synthesis and characterization of magnetic mesoporous particles.

Magnetic mesoporous particles were synthesized and their magnetic and structural properties are reported. The synthesis procedure consists of four steps: (i) preparation of magnetite colloidal nanoparticles; (ii) growth of a silica layer; (iii) development of the mesoporous structure and (iv) template removal. Two different methods for the template removal were studied and their effectiveness was discussed. Magnetization and Mössbauer spectroscopy measurements showed superparamagnetic behavior for the particles at room temperature. X-ray diffraction and nitrogen adsorption measurements showed a mesoporous MCM-41 structure with 2.48nm pore diameter and 1023m(2)/g total area.

Effects of additional gases on the catalytic decomposition of N2O over Cu-ZSM-5

N2O decomposition into N2 and O2 was investigated in the presence of O2, NO, CO2, CO, CH4, SO2 and water vapor. Activity inhibition was observed in the presence of water vapor, and oxidant gases, whilst the reductant gases, enhanced the catalytic activity, in the temperature range of 350–550°C.

A Combined High-Resolution X-ray Powder Diffraction, Computational, and XPS Study of the Local Structure of Extra-Framework Copper Ions in Over-Exchanged Cu-MCM22 Zeolite

Local structure and site distribution of extra-framework copper ions in over-exchanged Cu-MCM22 zeolite were determined by a combination of high resolution X-ray powder diffraction and computational analysis. X-ray diffraction data suggested the presence of three Cu sites in six-membered rings and one site in a five-membered ring close to the interlamellar region, inside the MCM-22 supercage, whereas no Cu ions were found within the sinusoidal channels. First principle molecular orbital DFT calculations were employed to obtain, for the first time, an accurate structural description of the Cu(I) sites in the supercage, adding a structural and energetic interpretation to previous IR and EPR studies. The combined experimental and computational study suggested that Cu(I) sites facing 6-MRs are particularly stable. In general 5- or 4-fold coordination sites are located in 6-MRs while 2- or 3-fold coordination sites are located in 5-MRs. Three preferentially occupied sites were found in copper-exchanged MCM-22. X-ray photoelectron spectroscopy suggested the formation of dispersed Cu close to the surface of MCM-22 crystallites, easily reduced to Cu(I) under ultrahigh vacuum conditions.

Hybrid platforms of graphane–graphene 2D structures: prototypes for atomically precise nanoelectronics

First-principles calculations demonstrate that line/ribbon defects, resulting from a controlled dehydrogenation in graphane, lead to the formation of low-dimensional electron-rich tracks in a monolayer. The present simulations point out that hybrid graphane-graphene nanostructures exhibit important elements, greatly required for the fabrication of efficient electronic circuits at the atomic level.

Evaluation of the Cu Doping Effects in CeO2 Catalytic Supports Obtained by Combustion Reaction

This work has for aim to synthesize CeO2 catalytic supports doped with Cu2+ by combustion reaction method. Thus were obtained catalytic supports with the composition Ce1-xCuxO2 and the effect caused by doped element in the structure of the CeO2 was evaluated. The concentration value (x) of the Cu doped took over the values of 0.0; 0.3 and 0.5 mol. The catalytic supports developed were submitted to the structural characterization by X-ray diffraction, morphologic analysis by SEM and textural analysis by means of adsorption-dessorption of N2 by BET method. The results showed that the doped element (Cu) was not completely incorporated in the CeO2 structure and was evidenced that how much larger the quantity of Cu larger maid the disorder of the atomic structure of the obtained material. The analyzed supports present mesoporous nature structure.

Spectroscopic and catalytic studies on Cu-MCM-22: Effect of copper loading

Copper exchanged MCM-22 zeolites with varying copper contents were prepared, characterized and tested in the nitrous oxide decomposition. Over-exchanged samples showed high activity, comparable to very active Cu-ZSM-5 catalysts. The nature of the copper species was investigated by TPR-H 2 , FTIR-CO and FTIR-NO, evidencing at least three different species: exchanged Cu 2+ ions, cationic oligomeric species and oxidic phases. Upon NO adsorption, mononitrosilic complexes Cu I -(NO) were formed, and completely oxidized to Cu II -(NO) with increasing NO pressure. Nitrate complexes were extensively formed by reaction with the oxidic phases in the sample with 183% cation exchange level.

30-P-13-Transition metal exchanged-MCM-22 catalysts for N2O decomposition

Publisher Summary This chapter presents transition metal-exchanged-MCM-22 catalysts for nitrous oxide (N 2 O) decomposition. Transition metal-exchanged-MCM-22 catalysts have shown high activity on the decomposition of N 2 O, comparable to the well-known ZSM-5 catalysts. Copper (Cu)-MCM-22 is highly active, but suffers inhibition by excess oxygen. Cobalt (Co)-MCM-22 is not as active as Co-ZSM-5 because of the formation of cobalt monoxide (CoO) but is nearly insensitive to oxygen (O 2 ). Fe-MCM-22 presents an intermediate behavior and shows low tolerance to oxygen. Activities and oxygen tolerance are explained based on the nature of the active species formed in each case.

Cu-MCM-22 zeolite: A combined X-ray powder diffraction and computational study of the local structure of extra-framework copper ions

Abstract Local structure and site distribution of extra-framework copper ions in Cu-exchanged MCM-22 zeolite have been determined by synchrotron radiation X-ray powder diffraction analysis combined with ab-initio molecular orbital (DFT) calculations. Three Cu sites (S I , S II and S III ) in 6-membered rings and one site (Svi) in a 5-membered ring close to the interlamellar region, were located inside the MCM-22 supercage, whereas no Cu ions were found within the sinusoidal channels. The existence of two families of Cu sites (in 5- and 6-membered rings respectively) with different steric hindrance and adsorptive capacity, as indicated by previous FTIR study of adsorbed NO [A. Frache, et al., Langmuir, 18 (2002) 6875], was fully confirmed in this study.