Dynamic response of physisorbed hydrogen molecules on lanthanide-modified zirconia nanoparticles

摘要

Ultrafine lanthanide (Ln = Ce and Nd)-modified zirconia powders synthesized by a coprecipitation method exhibit high surface areas and adsorption sites that are essential for catalytic applications. We report a study of the surface chemistry of Ce(sub 0.1)Zr(sub 0.9)O(sub 2) and Nd(sub 0.1)Zr(sub 0.9)O(sub 1.95) powders. First, the specific surface area and porosity are characterized by nitrogen isotherm-adsorption measurements. Second, the motion of hydrogen molecules physisorbed on Ce- and Nd-doped zirconias is studied by inelastic neutron scattering. Nitrogen adsorption-desorption isotherm measurements yield a BET surface area (26.1 m(sup 2)/g) and mesopore size ((approximately)5 nm radius) in Ce(sub 0.1)Zr(sub 0.9)O(sub 2) as compared to those (72.3 m(sup 2)/g and (approximately)3 nm) in Nd(sub 0.1)Zr(sub 0.9) O(sub 1.95). The vibrational densities of states of H(sub 2) on Ce(sub 0.1)Zr(sub 0.9)O(sub 2) and Nd(sub 0.1)Zr(sub 0.9)O(sub 1.95) were measured at 20 K over the 0-200 meV energy range for three hydrogen coverage. The spectra for both samples consist of two parts: a sharp peak at (approximately)14.5 meV and a broad component extending beyond 200 meV. The sharp peak corresponds to transitions from the J=0 to J=1 rotational states of bulk hydrogen molecules, and its intensity decreases with decreasing H(sub 2) coverage. The broad component corresponds to overdamped motion of surface adsorbed hydrogen molecules. The major difference in the latter component between the Ce- and Nd-doped samples is an excess of intensities in the 5-14 meV region in Nd(sub 0.1)Zr(sub 0.9)O(sub 1.95). The confined motion of adsorbed H(sub 2) on the different micropore and mesopore surfaces of Ce(sub 0.1)Zr(sub 0.9)O(sub 2) and Nd(sub 0.1)Zr(sub 0.9)O(sub 1.95) is discussed.