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This project was on the development of the experimental tools, specifically, in situ ultra-high vacuum (UHV) transmission electron microscope (TEM) and environmental scanning tunneling microscope (STM), with the goal of investigating the partial oxidation of methanol (POM) reaction catalyzed by Cu and Cu oxide nanostructures in situ:

(POM):     CH3OH + 1/2 O2 = 2H2 +CO2

Two post-docs, Dr. Zhenyu Liu (University of Pittsburgh), and Dr. Congkang Xu (Binghamton University) were supported on this project.   Matt France, a research technician in Yang’s group, modified the in situ UHV TEM for POM studies and the development of the dual e-gun UHV e-beam evaporator. Currently, a MS student in Zhou’s group is continuing on this project to generate further data for publication.

The following is a summary of the results from the modifications and experiments as well as suggestions for future improvements. 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 of different shapes. Our in situ UHV TEM results also revealed 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.

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.   Modifications to this instrument were completed for the study of the POM reaction.  A residual gas analyzer (RGA) was purchased, by leveraging cost-sharing from the University of Pittsburgh, and installed onto 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. A Gatan CCD camera was added in 2008 to capture dynamic structural changes and chemical reaction experiments in real time.

By using in situ methods, we created various shapes of Cu₂O nano-oxides (domes, hollow pyramids, and nanorods) where the goal was to study the effects of oxide structure, strain and defects on the POM reaction.  We carried out the methanol oxidation over Cu surfaces with Cu₂O islands but only limited information on the POM gas reaction due to limited RGA sensitivity. In order to try to achieve the sensitivity required to have a reasonable chance of seeing any production of H₂ and CO₂ from the reaction of CH₃OH and O₂ in the presence of the Cu and/or Cu₂O, on such a small sample would require the highest sensitivity possible. The RGA probe needs to be moved closer to the sample region for measurements of the gas changes and will be pursued next.

It should be noted that the probability of seeing the hydrogen would be very low, since it does not ionize easily, but the CO₂ would be the most easily observed by RGA. This maximum sensitivity could be achieved by using a capillary tube installed near the sample and then leading out through the column wall to the RGA housing.  The RGA would then need to be pumped with a dedicated turbo and rough pump system.  This would be required to reach the level of 1x10^-6 Torr for the optimum pressure to use the electron multiplier.  At higher oxidation process pressures (up to atmosphere) the installation of an orifice would be required to maintain the vacuum at the RGA and still be able to sample the gases around the sample.   Hence, the RGA pumping system and the TEM would need further modification.  

To complement the in situ TEM study at the University of Pittsburgh, we also employed in situ STM to elucidate the effect of surface structure on the surface reactivity. 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. 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).

A gas handling system was built for carrying out the catalytic reactions. We carried out preliminary methanol oxidation over these surfaces and used in situ STM/AFM to visualize the generation, motion, and annihilation of these surface defects during the catalytic reaction. The Cu surface was oxidized in situ inside the STM chamber to form oxide islands. The formation of neighboring oxide islands  is expected to 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.

In situ STM and TEM were extensively modified and employed to study the POM reaction as catalyzed by Cu and Cu oxides.  We complemented these efforts with significant university cost-sharing on new equipment (e.g. dual e-gun UHV e-beam evaporator, CCD camera and RGA for the in situ UHV TEM, and an environmental STM/AFM instrument).   Successful installation of the new equipment was accomplished, and we created various Cu orientations and Cu₂O oxide island shapes for initial POM studies. However, extensive further modification of the in situ UHV-TEM is needed to achieve very high sensitivity of the POM gasses. A journal publication is under preparation from the research results.