Low-Temperature Dissociation of CO₂ on a Ni/CeO₂(111)/Ru(0001) Model Catalyst

The adsorption of CO₂ on CeO_2‑<i>x</i>(111) and Ni/CeO_2‑<i>x</i>(111)/Ru­(0001) surfaces has been studied with reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). On the maximal-oxidized CeO₂(111) surface physisorbed linear CO₂ and a CO₂^– species are identified at 97 K. The reduced CeO_2‑<i>x</i>(111) surface exhibits higher reactivity toward adsorbed CO₂, which leads to higher coverages of CO₂^– and promotes CO₂ dissociating into CO and an active oxygen species at higher temperature, reoxidizing the reduced CeO_2‑<i>x</i>(111) films. Deposition of Ni on the maximal-oxidized CeO₂ thin films leads to slight reduction of ceria films. Adsorption of CO₂ on Ni/CeO_2‑<i>x</i>(111) films causes dissociation at 97 K and leads to Ni-CO adsorbates plus partial oxidation of Ni nanoparticles. This process is inhibited when Ni nanoparticles on CeO₂ are fully oxidized. In contrast to the results reported for CO₂ adsorption on Ni single-crystals, where the dissociation temperature was found to be higher than 240 K, the much lower dissociation temperature (∼97 K) for CO₂ on Ni nanoparticles supported on CeO₂(111) suggests that the Ni/CeO₂ catalyst exhibits high activity toward CO₂ activation

Interaction of Au with Thin ZrO₂ Films: Influence of ZrO₂ Morphology on the Adsorption and Thermal Stability of Au Nanoparticles

The model catalysts of ZrO₂-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO₂ films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO₂ films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO₂ films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO₂ film through the procedure of first spin-coating a zirconium ethoxide (Zr­(OC₂H₅)₄) precursor onto a SiO₂/Si­(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous “sponge-like nanostructures” were obtained in this case. The second method was epitaxial growth of a ZrO₂(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10^–6 Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO₂ film as compared to those on the ordered ZrO₂(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO₂ surface than on the ZrO₂(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO₂(111) but not from porous ZrO₂. The enhanced thermal stability for Au on porous ZrO₂ can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures