Raul Miranda

Bulk synthesis of silicon nanowires using a low-temperature vapor–liquid–solid method

Silicon nanowires will find applications in nanoscale electronics and optoelectronics both as active and passive components. Here, we demonstrate a low-temperature vapor–liquid–solid synthesis method that uses liquid-metal solvents with low solubility for silicon and other elemental semiconductor materials. This method eliminates the usual requirement of quantum-sized droplets in order to obtain quantum-scale one-dimensional structures. Specifically, we synthesized silicon nanowires with uniform diameters distributed around 6 nm using gallium as the molten solvent, at temperatures less than 400 °C in hydrogen plasma. The potential exists for bulk synthesis of silicon nanowires at temperatures significantly lower than 400 °C. Gallium forms a eutectic with silicon near room temperature and offers a wide temperature range for bulk synthesis of nanowires. These properties are important for creating monodispersed one-dimensional structures capable of yielding sharp hetero- or homointerfaces.

Denitrogenation of piperidine on alumina, silica, and silica-aluminas: The effect of surface acidity

Abstract Alumina, silica, and three silica-aluminas having 10, 50 and 90 wt% SiO 2 were prepared and characterized for the total acidity, acid strength distribution, and Bronsted/Lewis acidity. The characterization involved the techniques of ammonia chemisorption, temperature-programmed desorption (TPD), and FT-IR spectroscopy. The effect of surface acidity on denitrogenation selectivity was studied by means of the reactions of piperidine in a continuous-flow fixed-bed reactor operated at atmospheric pressure and 320–340°C. The overall piperidine conversion to various products increases with the total acidity of the catalysts. However, the denitrogenation selectivity depends only on the concentration of Bronsted acid sites, demonstrating the participation of those sites in the deamination mechanism and hence showing evidence for the Hofmann elimination pathway. The other important reactions of piperidine were alkylation, condensation, disproportionation, and dehydrogenation, to yield alkylpiperidines, 2,3,4,5-tetrahydropyridine, pyridine, 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydro-3-methylquinoline, 5,6,7,8-tetrahydroquinoline, and decahydroquinoline.

Platinum-sulfated-zirconia. Infrared study of adsorbed pyridine

Pyridine adsorption on sulfated zirconia (SO2−4-ZrO2) provides evidence for infrared bands characteristic of both Brønsted and Lewis acid sites. Samples treated at 100°C retain water and have a higher fraction of Brønsted acidity than when the sample is treated at 400°C. The fraction of Brønsted acid sites observed for SO2−4-ZrO2 is the same in the presence or absence of supported Pt. Based on pyridine adsorption, exposure to gaseous hydrogen at 100 or 150°C did not significantly alter the fraction of Brønsted acid sites following the exposure to hydrogen.

Catalytic conversion of alcohols: role of sodium in altering the alkene products obtained with alumina catalysts

In order to study the catalytic conversion of alcohols to alkenes, catalysts were prepared by impregnating Degussa aluminum oxide C with aqueous NaNO/sub 3/ and then drying and calcining the catalysts. The alcohols studied were 2-butanol and 2-octanol; but since a serious secondary isomerization effect present when 2-butanol is converted is eliminated when a mixture of the two alcohols is converted, the mixture was used for this study. The results indicated that two types of dehydration sites must occur on the alumina catalyst. One type designated A is more active for dehydration, and their selectivity from 1-butene and cis-2-butene in about equal amounts with only a small fraction of trans-2-butene. The other type of sites, B sites, form isomers in nearly equal amounts.

NEURAL-NETWORK-AIDED DESIGN OF AUTOMOBILE EXHAUST CATALYSTS

A priori design of catalysts is not yet possible. Such task would demand unavailable scientific knowledge of the correlations among synthesis parameters and resulting solid state and surface structures, on the one hand, and among those atomic-level structural details and their catalytic functions, on the other hand. To avoid testing every possible combination, therefore, the applied chemist or chemical engineer must identify empirical correlations underlying the existing experimental data base. The ability of artificial neural networks to identify complex correlations and to predict the result of experiments has recently generated considerable interest in various areas of science and engineering. In this paper, neural networks are used to identify composition-performance relationships in automobile exhaust catalysts. This work employs an artificial neural network technique to do a sensitivity analysis of the conversions of pollutant gases as a function of the catalyst composition and the operating conditi...

Side reactions in quinoline hydrodenitrogenation

Abstract The mechanism for hydrodenitrogenation (HDN) of quinoline to propylbenzene, propylcyclohexene and propylcyclohexane is well established. In this paper the formation of substantial amounts of species formed from HDN intermediates is reported for a cobalt-molybdenum/alumina catalyst. As many as eighteen species were found in reaction mixtures at concentration levels greater than 1%, with indoline and hydrindane being the most significant. The results suggest that Bronsted acidity may be responsible for the formation of indoline and hydrindane.

Sol-gel preparation and characterization of niobia thin films for catalytic sensor applications

Thin films of niobium pentoxide were deposited on quartz plates by spin-coating a solution of niobium ethoxide in ethanol. The films were tested as potential selective gas sensors by exposing them to various reducing environments. It was found that they were selective and sensitive to ammonia. This sensitivity was attributed to the formation of donor-acceptor complexes between chemisorbed ammonia and Lewis acid sites on the solid.

Supported and sulfided Pb-Mo oxides: Active and stable hydrotreatment catalysts

A newly developed PbMo/Al2O3 HDS catalyst shows activity and stability that are comparable to or better than the traditional CoMo/Al2O3. Activity is optimum when the atomic ratio Pb∶Mo is 1∶6. At that ratio, the generated surface phase(s) display maximum degree of Mo coordinative unsaturation, as measured by low temperature oxygen chemisorption.

NEURAL NETWORK MODELING OF TRANSITION METAL - ZEOLITE EXHAUST CATALYSTS

Abstract Recent research in automobile exhaust catalysts has addressed the substitution of platinum-group metals Pt, Pd and Rh by metals such as Cu, Co, Ag, Zn, Mn and Sr exchanged or impregnated on zeolites, TiO2 or Zro2 carriers. These catalysts have the potential of becoming good alternatives to the commercial three-way catalysts to convert pollutant hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). This paper describes a technique based on neural networks, to correlate the catalyst synthesis variables and resulting exhaust conversion. The optimum catalyst composition and operating conditions for a specified exhaust conversion are determined A back-propagation algorithm was used to train the feed-forward neural network consisting of two hidden layers with 45 and 60 neurons in the first and second hidden layers respectively, with optimum values of the learning factor and momentum gain coefficient. The effects of the operating and compositional parameters on NOx conversion by Cu-ZSM-5 we...