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This grant is on the measurement of partial oxidation of methanol (POM) catalysis of Cu and Cu oxide nanostructures, via specialized in situ ultra-high vacuum (UHV) transmission electron microscope (TEM) and environmental scanning tunneling microscope (STM) with Professor Guangwen Zhou at Binghamton University, where the gas reaction is:

(POM): CH3OH + 1/2 O2 = 2H2 +CO2

in order to investigate the possible the catalytic reaction between methonal and oxygen with the catalytic aids of Cu and Cu oxide. Two post-docs, Dr. Zhenyu Liu (University of Pittsburgh), and Dr. Congkang Xu (Aug. 2009) (Binghamton University) have been working on this project. The following is a summary of the results from the on-going experiments and the plan for the next phase of the project.

By using in situ methods, we can create nano-oxides and systematically study the effects of oxide structure, strain and defects on the POM reaction. Due to their large lattice mismatch between Cu2O and the Cu substrate, Cu2O islands formed on Cu surfaces are elastically strained when the island size is small. We expect that the strain state in the oxide islands can affect their catalytic activity. We are carrying out the methanol oxidation over Cu surfaces with oxide islands having different island size distribution, defect structures, and surfaces. The information of catalysts under reaction conditions is critical for a fundamental understanding of the reactivity and durability of a catalyst.

To prepare the Cu films, a UHV dual e-gun evaporator system was purchased from Pascal via University of Pittsburgh and accepted in 2009. Single crystal Cu(001) and Cu(011) were produced by e-beam evaporation onto single crystal NaCl. In situ oxidation of the Cu films within the UHV-TEM or STM led to the formation of copper oxide islands.

The specialized capabilities of the in situ UHV-TEM include gas reactions from 10-6 to 10-4 torr in situ or up to atmospheric pressures, with the electron beam off, and temperature range from room temperature to 1000°C. A residual gas analyzer (RGA) wasl purchased (~7K), by leveraging cost-sharing from the University of Pittsburgh, and installed to the in situ UHV TEM for monitoring gas species during the catalytic partial oxidation of methanol. The EXTORR RGA allows partial pressure measurements from 10-5 Torr to 10-11 Torr, an additional electron multiplier can increase the sensitivity down to 10-14 Torr. This experimental tool has provided unique and critical data of metal oxidation in a wide pressure and temperature range needed for a fundamental understanding of the atomistic kinetics. A Gatan CCD camera was added in 2008 to capture dynamic structural changes in real time, and chemical reaction experiments. The CCD camera captured the structural changes during the chemical reactions. Initial RGA studies suggests that the probe needs to be moved closer to the sample region for measurements of the gas changes and will be pursued next.

To complement the in situ TEM study at the University of Pittsburgh that focuses on the correlation between surface chemistry and surface reactivity of Cu catalysts, we are employing in-situ STM to elucidate the effect of surface structure on the surface reactivity. We are carrying out methanol oxidation over these surfaces and use in situ STM/AFM to visualize the generation, motion, and annihilation of these surface defects during the catalytic reaction. The Cu surface will then be oxidized in situ inside the STM chamber to form oxide islands. The formation of neighboring oxide islands will modify the atomic and electronic structure of these surface defects, and therefore affect their reactivity. These effects are being investigated by comparing the behavior of the generation, motion, and annihilation of surface defects with and without neighboring Cu2O islands during the methanol oxidation.

The surface activity is expected to be altered by surface roughness because the surface roughness affects many important physical and chemical processes such as gas adsorption, desorption, ionization, dissociation and transport of reactants on the catalyst surface. However, the microscopic and mechanistic understanding of these correlations is still lacking. Our in situ UHV TEM results reveal that the roughness of Cu(110) surfaces can be altered through in situ heating. When the Cu(110) surface is heated up to ~800ºC, surface faceting can be observed. Our AFM and TEM results indicate that these changes in the surface morphology at the different temperatures are irreversible, i.e., the surface still remains roughened or faceted after being cooled to room temperature. This provides us unique opportunities to carry out in situ studies of the correlation between surface roughness and surface reactivity of Cu catalysts during the methanol oxidation at the lower temperature (the typical temperature for the catalytic methanol oxidation is ~200ºC or lower).

In situ STM and TEM are being employed to monitor the roughness evolution of Cu surfaces under the methanol oxidation. In conclusion, we plan to determine quantitatively the dependence of surface reactivity on oxide nanostructures, strain, defects and surface roughness by correlating RGA data with the reaction kinetics.