Cobalt nanoparticles for CO hydrogenation
We are currently studying mono-dispersed Co nanoparticles prepared by colloidal chemistry. In order to prepare 2D model samples, we supported them on gold foil using the Langmuir-Blodgett method. High-resolution scanning electron microscopy (SEM) images showed that the nanoparticles were packed on the gold surface as a single layer (Figure 1)

In-situ XAS was carried out at Advanced Light Source (ALS) using a windowed, temperature-controlled gas-flow sample cell (Figure 2) able to measure Co L-edge and O K-edge of nanoparticles and adsorbates/ligands. Figure 3 is an example of in-situ XAS measurement showing the reduction of oxidized Co nanoparticles in the presence of flowing H2 at elevated temperatures.

Figure 3. In-situ XAS of oxidized 10 nm Co nanoparticles in the presence of flowing H2 at increasing temperatures

Au nanoparticles for CO oxidation
Gold nanoparticles with narrow size distribution are being studied for their activity in the CO oxidation of carbon monoxide with NO and oxygen. The changes in the nanoparticle electronic structure under reaction conditions are being evaluated using ambient pressure photoelectron spectroscopy (APPS). The detail of the instrument can be found here (link to APPES at ALS Experiment). Figure 4 shows some selected TEM pictures of the Au nanoparticles supported over silicon oxide.

Figure 4. High resolution TEM micrograph of the Au nanoparticles dispersed over silicon oxide

Surface chemistry of copper in the presence of H2O and CO2
Weinvestigate the chemical nature of copper and copper oxide (Cu2O) surfaces in the presence of carbon dioxide and water at room temperature using Ambient Pressure Photoelectron Spectroscopy.  This is important in applications such as catalysis, solar cells, water pipe corrosion, electrical motor rotor and brushes. While copper oxide is inactive, polycrystalline cobalt foil is reactive when exposed to CO2 (0.1 torr), forming several species which remain adsorbed onto the surface. These include carbonate (CO32-), CO2δ- and C0. The addition of 0.1 ML Zn to the Cu results in complete conversion of CO2δ- to carbonate, as it is shown in Figure 5. In a mixture of 0.1 torr H2O and 0.1 torr CO2, new species are formed, including hydroxyl, formate and methoxy, with H2O providing the hydrogen needed for the formation of hydrogenated species.

Adsorption of water on oxide thin films
One of the main topics of research in our group is the molecular scale origin of wetting.  Within this effort we study the initial stages of water condensation (6 molecular layers aprox.), on oxide surfaces as Cu2O and Al2O3, using Ambient Pressure Photoelectron Spectroscopy at relative humidity values (RH) from 0 to >90%. We found that water adsorbs first dissociatively on oxygen vacancies producing adsorbed hydroxyl groups in a stoichiometric reaction: O(lattice) + Vacancies + H2O = 2OH.  The reaction is completed at ~ 1% RH and is followed by adsorption of molecular water. The thickness of the water film grows with increasing RH. The first monolayer is completed at ~ 15% RH on both oxides and is followed by a second layer at 35-40% RH. At 90% RH, about 6 layers of H2O film have been formed on Al2O3 (Figure 6).

Using photoemission spectroscopy under ambient conditions of pressure and temperature we have determined the steps and the dynamics of the oxidation of palladium (111). As the chemical potential of O2 increases a chemisorbed phase forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by first-order transformation into bulk PdO. Different activation energies for these two oxidation stages may cause the well-known Pd low temperature oxidation/reduction hysteresis.

G. Ketteler, D.F. Ogletree, H. Bluhm, H. Liu, E.L.D. Hebenstreit, M. Salmeron, J. Am. Chem. Soc. (accepted)