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Our original goal of the proposed work is to directly image nanoparticle catalysts while catalyzing reactions. We planned to produce these nanoparticles via laser vaporization, and imaging would be achieved with hard x-ray spectroscopic imaging.

We have designed and performed a few important experiments through which we have learned much about catalysis of both pre-made nanoparticle catalysts and those made in situ. Three major achievements are (1) designed and constructed a laser vaporization nanoparticle production device that can be interfaced to either a reaction gas manifold and a mass spectrometer or an ultrafast x-ray source; (2) measured the turnover frequency (TOF) of the nanoparticles produced in situ in a background gas of methane and carbon dioxide and the TOF is two orders of magnitude greater than that measured using a conventional reactor with flowing reaction gases. ; (3) constructed an x-ray source to image the plume of the nanoparticles. The flux of the x-rays was not high enough to allow us to perform spectroscopic imaging. We are still attempting to image the plume spectroscopically, but it may not be possible with this new source. Improvement to both the x-ray source and the laser vaporization device are needed to permit such imaging. We are preparing a manuscript to report our findings.

In addition, we performed measurements of TOF using pre-made Pt nanoparticles to confirm that smaller nanoparticles may be more active with respect to a similar reaction of steam reforming of methane (SRM). After systematically investigating the results, we devised a simple analytical formula to model the reactivity (TOF) as a function of size, which is that the TOF is inversely proportional to the average coordination number of the surface atoms divided by the total atoms of the nanoparticles.

We have recently advanced this idea to the next level after we carefully examined the carbon dioxide reforming of methane (CRM) using the same Pt nanoparticles. We further concluded that the TOF may be inversely proportional to the radius of the nanoparticles, and the activation energy is linearly proportional to the radius of nanoparticles. These simpler relationships will need a correction when the particle size is below 2 nm, at which substrates and other effect may arise. The results will be reported in another manuscript that is being prepared.

These pre-made nanoparticles greater than 2 nm, which may be much bigger than those created in situ in the plume via laser vaporization. The TOF values were maximized in the CRM reaction by Pt nanoparticles, and they were of the order of 500 per site per sec, which is much greater than we had measured before, as well as those measured by others. However, this value is still two orders of magnitude less than that of the nanoparticles created in the plume. Since no substrate is used, the enhanced TOF may solely be due to the size of nanoparticles, or the temperature of the nanoparticles.

Our results shown here suggest that it is even more desirable now to directly image the nanoparticles and understand the physical and chemical state of these nanoparticles. We are working toward that goal in the lab now, and we also plan to write a proposal to the fourth generation synchrotron facility at Stanford to further explore this process.