Crystal Phases, Defects, and Dynamics of Adsorbed Hydroxyl Groups and Water in Pure and Lanthanide-Modified Zirconia: A Neutron-Scattering Study

The role of lanthanide doping in zirconia as a means of stabilizing and promoting catalytic reactions was studied by neutron spectroscopy. The crystal structure of high-surface-area Ln0.1Zr0.9O1.95 (Ln = La and Nd) powders prepared by a coprecipitation method were found to be composed of mixed phases of tetragonal and cubic symmetry, which can be stabilized over a temperature range (up to ∼ 1000°C) pertinent to catalytic applications. A real-space correlation function, obtained from a Fourier transform of the filtered residual diffuse scattering, showed evidence of static, oxygen vacancy-induced atomic displacements along the pseudocubic 〈111〉 and other directions. The dynamics of hydrogen atoms associated with the surface hydroxyl groups and adsorbed water molecules on Ln0.1Zr0.9O1.95 and pure ZrO2 over a frequency range of 0-4400 cm−1 was investigated by neutron inelastic scattering. The stretch vibrations of surface hydroxyl groups on monoclinic ZrO2 were found to have slightly higher frequencies than those for Ln0.1Zr0.9O1.95. At a submonolayer coverage of water the O-H stretch bands broaden and shift to lower energies. At higher coverage three bands, corresponding to the O-H stretch, H-O-H bend, and librational motion of water molecules, were observed, indicating the influence of hydrogen bonding. The neutron results were compared with infrared data and ab initio calculations for similar zirconia systems found in the literature.