| In an effort to understand the molecular ingredients of catalytic activity and selectivity toward the end of tuning a catalyst for 100% selectivity, advanced nanolithography techniques were developed and utilized to fabricate well-ordered two dimensional model catalyst arrays of metal nanostructures on an oxide support for the investigation of reaction selectivity. In-situ and ex-situ surface science techniques were coupled with catalytic reaction data to characterize the molecular structure of the catalyst systems and gain insight into hydrocarbon conversion in heterogeneous catalysis.;Electron beam lithography (EBL) was employed to create platinum nanoparticles on an alumina (Al2O3) support. The Pt nanoparticle spacing (100--150-nm interparticle distance) was varied in these samples, and they were characterized using x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), both before and after reactions. The TEM studies showed the 28-nm Pt nanoparticles with 100 and 150-nm interparticle spacing on alumina to be polycrystalline in nature, with crystalline sizes of 3--5 nm. The nanoparticle crystallites increased significantly after heat treatment. The nanoparticles were still mostly polycrystalline in nature, with 2--3 domains. The 28-nm Pt nanoparticles deposited on alumina were removed by the AFM tip in contact mode with a normal force of approximately 30 nN. After heat treatment at 500°C in vacuum for 3 hours, the AFM tip, even at 4000 nN, could not remove the platinum nanoparticles. The increase of adhesion upon heat treatment indicates stronger bonding between the Pt and the support at the metal-oxide interface.;Due to the high cost and low throughput (mm2) of electron beam lithography, as it is a serial process, another fabrication technique was developed to produce a 2nd generation model catalyst system. Size reduction lithography (SRL) is a new photolithographic process whereby through sacrificial layer depositions and selective etching, wafer-scale silicon nanowire molds with 7-nm features can be produced. When coupled with nanoimprint lithography (NIL), a polymer stamping technology, the pattern is transferred into an oxide-coated (silica or alumina) Si(100) wafer that will ultimately become Pt nanowire or nanodot model catalyst arrays. This low-cost, high-throughput parallel process yields the necessary metal surface area (cm2) to study those industrially significant reactions that have low turnover (≤10 -2 s-1). The SRL and NIL processes, along with the final catalyst system was characterized via SEM, XPS, and AFM. (Abstract shortened by UMI.). |
