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The goal of this project is to create a reactive environment in which the activity of nanoparticles levitated in space and without the influence from any substrates can be studied. In this project, such an environment is created by laser vaporizing a metal target in gaseous reactant background. The significance of this work is to afford the ability to explore the intrinsic reactivity of nanoparticles, free of any influence of the substrate.

It is known that smaller nanoparticles may be much more reactive than larger ones, especially for those reactions such as steam reforming of methane that is catalyzed by single platinum surface atoms in a nanoparticle through the activation of C-H bond in methane. We have recently observed such enhanced activity using premade Pt nanoparticles of narrow size distributions. The article is submitted for publication. In that article, we discovered, for the first time, a mathematical function to quantitatively describe the relationship between the activity and the size of nanoparticles. In our submitted work, the smallest nanoparticles are 5 nm in diameter.

As the nanoparticles becomes smaller, it is increasingly more difficult to study the intrinsic activity exhibited by nanoparticles alone because they have to be supported on a substrate, which can influence the activity as discussed in the literature in recent years. Using the levitated nanoparticles, we can avoid the influence from the substrate.

This ACS-PRF grant allows us to carry out the exploration using premade Pt nanoparticles to further prove our case. In the meantime, we have redesigned our ultrafast x-ray source, as shown elsewhere in this report. This new source uses a metal nanoparticle-coated plastic tape as the x-ray target onto which a terawatt laser beam is focused to generate picosecond x-ray pulses. We used this source to acquire x-ray absorption spectra of various thin films. The main advantage of this new source is that samples can be placed very close to the source, only a few millimeters away. Because our source emits x-rays in all directions, this greatly increases the flux at the sample. The x-ray absorption data is also shown in this report. We have just designed the laser vaporized nanoparticle module, which is also shown in this report. Next we will connect this module to our reforming reactor to prove that nanoparticles generated in situ are chemically active. This is the first time when one can study the chemical activity of nanoparticles created in situ in a reactive environment at ambient pressure. The module is designed to interface with the ultrafast x-ray source so that we can image the chemical state of nanoparticles during reactions in the next step. The total x-ray pathway length through the sample is long enough to result in fast data acquisition times using the ultrafast x-ray source. In addition to these planned experiments, we will also take the module to Stanford Synchrotron Radiation Lab (SSRL) toward the end of this year to perform various measurements and study the in situ response of nanoparticles in reactive environment at high temperatures. These newly planned experiments will help expand the understanding of how nanoparticles behave at high temperatures, which is closely related to the aforementioned suspended nanoparticles.This grant gives us the opportunity to establish my research group as the pioneer in this new field that is closed associated with several topics such as catalysis, physical chemistry, atomic and molecular optics, and materials chemistry. It help anchor our scientific methodology in this new field and create more new concepts and experiments as we venture into this new territory of knowledge.This naturally leads to the teaching of students. On the premade nanoparticles front, a graduate student helped with the project and he is now a postdoctoral in the department of chemistry at UC Los Angeles. A senior chemistry undergraduate student worked on this project as well and he is a graduate student at Caltech. Currently another senior chemistry undergraduate student is working on the project on the premade nanoparticles side. Another female senior chemistry undergraduate student just joined her.The ultrafast x-ray front was more difficult to attack, and in early 2007 we hired an undergraduate student of physics major, who is Hispanic. He is terrific in the lab. However, after graduation in 2007, he could not get into graduate school due to his low GPA. His talents were there, so he was hired for this project and he is doing fantastically well. We hope that he will learn and accomplish enough so that our project can be completed soon and he can enter a graduate school after training in this group. Along the way, we also hired another talented person, who was in UC Davis Bio-engineering PhD program but changed his mind and wanted to learn and do ultrafast x-ray work. Since we could not keep the only graduate student who showed some remote interest in doing x-ray work in the summer 2008, we are keeping the two people hired so far to run the project. We are almost ready to test the whole thing - levitated nanoparticles created by laser vaporization for high temperature catalysis while imaging the catalysts with ultrafast x-ray pulses. The second student may enter our chemistry program in the near future.