Platinum nanoparticle model catalysts: Fabrication, characterization, and reaction studies

Platinum nanoparticles supported on silica have been fabricated using electron beam lithography (EBL), which produced ordered two dimensional arrays with particle diameters of 20 +/- 2 nm, uniform interparticle distances (230 +/- 2 nm), and uniform height (15 +/- 1 nm). Due to the narrow size distribution of the particles and the long range (mm2) order, the arrays produced using EBL were applied as models for supported metal catalysts.; The thermal, chemical, and mechanical stability of Pt nanoparticles supported on silica has been measured with transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM studies provided information about the array periodicity, particle dimensions, and the crystallinity of individual particles. Before heat treatments, individual Pt nanoparticles were found to be polycrystalline with crystalline domain sizes of 4 to 8 nm. After heating to 1,000 K in high vacuum (10-7 Torr) or 1 atm H 2, the crystalline domain sizes within individual particles grew larger, without noticeable deformation of the array. A similar enlargement of crystalline domains was seen in 1 atm 02 at a lower temperature of 700 K. Using contact mode AFM, the height, periodicity and adhesion of the particles were determined. On a newly prepared sample, Pt particles were displaced from the silica support by the AFM tip with approximately 10 nN lateral force. The interfacial adhesion energy between the Pt and SiO2 after was on the order of 1 mJ/m 2, which is relatively weak bonding. After heating, the Pt particles could not be displaced by the AFM tip, suggesting that heat treatments had increased the bonding between the Pt and SiO2. The stability and uniformity of the nanoparticle arrays make them ideal model catalysts for reactions in either oxidizing or reducing conditions.; The reactivity of the Pt/SiO2 arrays was compared to a Pt foil for cyclohexene dehydrogenation and hydrogenation in the presence of hydrogen at 400 K. The overall reactivity of the Pt particle arrays was higher by a factor of two, the selectivity towards dehydrogenation was three times higher, and the rate of deactivation was about the same as the Pt foil. Since the primary difference between the nanoparticle array and the Pt foil was the interface between the Pt and the SiO2, the interfacial region was most likely responsible for the changes in reactivity on the arrays. Using AFM, scanning electron microscopy (SEM), and temperature programmed desorption (TPD), the arrays were characterized before and after being exposure to reaction conditions. AFM images of a sample cleaned by ion sputtering showed that the pattern of the Pt nanoparticle array was replicated in the silica during the sputtering process.