No abstract

We report here the first synthesis of pure boron single-wall nanotubes by reaction of BCl 3 with H 2 over an Mg-MCM-41 catalyst with parallel, uniform diameter (36 ( 1 Å) cylindrical pores. The composition of the tubular structures observed in TEM was confirmed by electron energy loss spectroscopy, and the tubular geometry was confirmed by the presence of the characteristic spectral features in the Raman breathing mode region.

Palladium-based catalysts supported on metal oxides are attractive for methane combustion at low temperature. However, at temperatures below 450 °C, their tendency to deactivate hinders their usefulness. Catalytic deactivation in this temperature regime has been attributed to a water/hydroxyl inhibition effect. We investigated this effect to better understand the mechanism for catalytic deactivation. Comparative in situ FTIR transmission spectroscopy experiments at 325 °C revealed that hydroxyl accumulation occurs on the oxide supports during catalytic methane combustion and deactivation. The water/hydroxyl accumulation on the support is slow to desorb at this temperature. In light of our recent finding that oxygen from the support is utilized in the methane combustion process, we propose that hydroxyl/water accumulation on the support impedes the catalytic combustion reaction by hindering oxygen mobility on the support. We support this hypothesis by demonstrating that the presence of water on the catalyst inhibits oxygen exchange with the oxide support.

Highly ordered cobalt substituted MCM-41 samples were synthesized and characterized for application as catalytic templates for producing aligned single walled carbon nanotubes (SWNT). Highly reproducible Co-MCM-41 samples were successfully synthesized using alkyl templates with 10, 12, 14, 16, and 18 carbon chain lengths by direct incorporation of cobalt into the siliceous MCM-41 framework using a hydrothermal method; the pore size and the pore volume can be controlled precisely. The local environment of cobalt as determined by UV-vis spectroscopy is a mixture of tetrahedral and distorted tetrahedral structures similar to those observed in Co 3 O 4 . Cobalt atoms are uniformly distributed in the pores (about 30-40/pore) at nearly atomic dispersion probed by XAFS. Incorporation of cobalt into siliceous MCM-41 improves the structure, most likely by dehydroxylation and/or knitting the defective structure of the amorphous silica polymer. The optimum crystallization temperature and time were 100 °C and 4 days for siliceous MCM-41 and 6 days for Co-MCM-41, respectively. Co-MCM-41 is very stable against reducing and oxidation conditions at temperatures under 750 °C. The catalytic templates showed over 90% selectivity to SWNT with up to 4 wt % carbon yield. The growth of SWNT in the pores of Co-MCM-41 was confirmed by Raman spectroscopy and TEM. The catalytic template maintained its structure after successive reaction cycles, which suggests that Co-MCM-41 is a very stable template for producing SWNT under harsh reaction conditions.