Titanium Nanotubes Serve as Hydrogen Sensors
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of Amsterdam researchers G. Rothenberg, E.A.B. de Graaf, and A. Bliek have developed a cerium tungsten oxide catalyst that is able to cleanly and selectively oxidize hydrogen while maintaining excellent thermal stability. Moreover, the catalyst can be readily regenerated with oxygen and serve as an oxygen “reservoir.” As reported in the July 28 issue of Angewandte Chemie, Rothenberg and coworkers doped ceria with Bi, La, In, Mo, Pb, Sn, V, W, Y, and Zr to make 10 ceriabased bimetallic oxides by using a parallel synthesis technique based on boiling molten mixtures of nitrate precursors followed by calcination. The extent of substitution of Ce ions was limited to 10% so as to retain the cubic fluorite crystal structure. As the redox chemistry of substituted ceria is sensitive to induced structural defects and stresses in the crystal lattice, the researchers expected to see varied oxygen mobility, which would produce different levels of catalytic activity and thermal stability in these systems. Rothenberg said, “We had an idea that similar cations will create ‘small holes’ in
the fluorite lattice, without causing too much disturbance. This way, you keep the oxygen-exchange properties of the ceria, but also, hopefully, tune the selectivity so that the new material will oxidize hydrogen selectively.” The catalysts were screened in cyclic redox experiments with a gas mixture that simulates the effluents from an ethane dehydrogenation process. While the lanthanum-, indium-, and zirconiumdoped systems showed the highest activity, the tungsten-doped system showed >97% selectivity toward hydrogen. Moreover, this system showed excellent stability under sintering as well as negligible coke formation, properties that are of significance in commercial oxidative dehydrogenation reactions. The mechanism of the hydrogen selectivity remains to be determined. Rothenberg thinks that the small hydrogen molecules may diffuse into the lattice, reacting with labile oxygen atoms that are unavailable to the bulky hydrocarbon molecules. SARBAJIT BANERJEE
Titanium Nanotubes Serve as Hydrogen Sensors Titanium nanotubes used as hydrogen sensors is an example of materials properties changing dramatically when crossing the border between real-world sizes and nanoscopic dimensions. Craig A. Grimes, associate professor of electrical engineering and materials science and engineering at The Pennsylvania State University, and colleagues have found that hydrogen entering an array of titanium nanotubes flows around all the surfaces, but also splits into individually charged atoms and permeates the surface of the nanotubes. These hydrogen ions provide electrons for conductivity. The change in conductance signals that hydrogen is present at a concentration above the background level. The researchers report in the August 1 issue of Sensors and Actuators B: Chemical and an earlier issue of Advanced Materials (April 2003) that because the nanotubes are in close contact with each other (see figure), the contact points become highly conductive relative to th
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