Nitrogen Molecule Adsorption on Cationic Tantalum Clusters and Rhodium Clusters and Desorption from Their Nitride Clusters Studied by Thermal Desorption Spectrometry.
Adsorption and desorption of N2 molecules onto cationic Ta and Rh clusters in the gas phase were investigated in the temperature range of 300-1000 K by using thermal desorption spectrometry in combination with density functional theory (DFT) calculations. For Ta6(+), the first N2 molecule was found to adsorb dissociatively, and it remained adsorbed when Ta6(+)N2 was heated to 1000 K. In contrast, the second and the subsequent N2 molecules adsorbed weakly as a molecular form and were released into the gas phase when heated to 600 K. The difference can be explained in terms of the activation barrier between the molecular and dissociative forms. On the other hand, when Ta clusters were generated in the presence of N2 gas by the laser ablation of a Ta rod, isomeric clusters, TanNm(+), having heat resistivity were formed. For Rh6(+), N2 adsorbed molecularly at 300 K and desorbed totally at 450 K. These results were consistent with the DFT calculations, indicating that the dissociative adsorption of N2 is endothermic.
Comparative cluster reaction studies of the V, Nb, and Ta series
Reactions of the group 5 transition-metal clusters (V{sub n}, Nb{sub n}, and Ta{sub n}) have been investigated in the gas phase by using a fast-flow chemical reactor. Dissociative chemisorption of D{sub 2} and N{sub 2} is found to be size selective for these bare metal clusters, as well as for the cluster monoxides V{sub n}O and Ta{sub n}O and monocarbides V{sub n}C. The effect of a single atomic impurity (C or O) wanes for clusters larger than 10 atoms and becomes negligible for clusters larger than 15-20 atoms. This loss of sensitivity to impurities is consistent with the loss of cluster size specificity, which occurs in the same size range. Evidence of structural isomerism is found for Nb{sub 9}, Nb{sub 11}, Nb{sub 12}, and Ta{sub 12}, which react with N{sub 2} with biexponential reaction kinetics. The rate of chemisorption of ethane (C{sub 2}H{sub 6}) displays a steady increase with cluster size for V{sub n} and Nb{sub n}, but Ta{sub n} clusters are only slightly reactive. Finally, the rate of addition of a second ligand to a metal clusters is considered, and examples are analyzed for Nb{sub n}N{sub 2} + N{sub 2}.
Correlations for mass transfer between fluidized spheres and a liquid
Abstract Data on mass transfer between the solid particles and the liquid in a fluidized bed have been analyzed. After a convenient selection, various correlations have been derived. They show that for a given fluidized system, the Sherwood number is independent on the Reynolds number.