Surface Hydroxyl-Determined Migration and Anchoring of Silver on Alumina in Oxidative Redispersion

Hydroxyl (OH) groups are widely present on an oxide surface, which have been recognized as a key factor affecting surface properties of the oxide and interaction of the metal overlayer with the oxide. Here, gamma-alumina (gamma-Al2O3) supports with different OH contents are prepared by calcinating pseudo-boehmite (AlOOH) at different temperatures. The surface OH effect on oxidative redispersion of supported Ag nanoparticles including surface migration and anchoring of Ag species has been explored using in situ X-ray diffraction and UV-visible spectroscopy, as well as ex situ X-ray photoelectron spectroscopy and transmission electron microscopy. We reveal that the dispersion capacity, i.e., the amount of anchored Ag species associated with the steady dispersion state is thermodynamically determined by the surface OH contents, while the dispersion rate related to the surface migration process is kinetically limited by surface OH densities at low Ag loading. The higher dispersion ability is observed on the support with higher OH contents, and the quicker dispersion occurs on the support with higher OH densities. The results reveal that both OH contents and OH densities are critical in the redispersion of metal particles on oxide surfaces, which can be used to manipulate the Ag-catalyzed CO oxidation reaction.