Interface And Thermal Stability Of Au/ZrO₂ Model Catalyst Studied By Photoemission Spectroscopy

Highly dispersed supported-Au nanocatalysts have been extensively studied for their unique catalytic activity and high selectivity. Au/ZrO₂ is of significant interest as active catalysts in many catalytic reactions such as CO oxidation,water gas shift (WGS) reaction and selective hydrogenation of unsaturated compounds. To date, most of studies have been focused on the nanosize effect, the chemical state of gold, the preparation method, and zirconia crystal phase because of there extremely importance in determining the catalytic performance. However, only a few reports are addressed on the interfacial properties and thermal stability investigations of Au/zirconia. In order to understand interface interaction and catalytic mechanism of Au/ZrO₂ at the atomic-molecular level, this thesis is devoted to a comparative study of the growth, interfacial electronic structures and thermal stabilities of nanosized Au particles on ZrO₂(111) and porous ZrO₂ film surfaces using synchrotron radiation photoemission spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The main results of this thesis can be summarized as follows:1. Thin ZrO₂ films have been prepared by two different methods. The first method is using surface reaction to prepare ZrO₂ thin films. The procedure is spin-coating a zirconium ethoxide (Zr(OC2H5)4) precursor onto the SiO₂/Si(100) substrate at room temperature, followed by calcination at 500℃for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous"sponge-like"nanostructural ZrO₂ films were obtained in all cases. The second method is to epitaxially grow ZrO₂(111) films on Pt(111) through vacuum evaporation of Zr metal onto Pt(111) surface in 1×10^-6 Torr of oxygen at 550 K followed by annealing at 1000 K. In this way, an ordered ZrO₂(111) film is obtained where LEED pattern shows p(1×1) coexisting with (2×2).2. The growth of Au on the ZrO₂ thin films and their interfacial properties were investigated by photoemission spectroscopy. At room temperature, Au initially grows as two-dimensional (2D) islands on the ZrO₂(111) thin films up to 0.1 ML, followed by three-dimensional (3D) growth with a number density of ¹.4x10(12)particles/cm2. Au atoms most likely form Au^δ- initially. Gold forms smaller particles on porous ZrO₂ films as compared to those on ZrO₂(111) films at the same coverage. In addition, on both surfaces there is no significant interfacial interaction between Au and ZrO₂ thin films3. Thermal annealing experiments demonstrate that on both surfaces Au particles experience significant sintering before desorption. However, Au particles are more thermally stable on the porous ZrO₂ surface than on the ZrO₂(111) surface. By annealing to surface to 1100 K, Au particles desorb completely from ZrO₂(111) , but not from porous ZrO₂. This can be attributed to the stronger bonding of Au atoms on the surface defect sites. In addition, pore structures are efficient at restricting Au migration or diffusion, also enhanced the thermal stability of Au particles on porous surfaces.