Dimensional effects in controlled structure supported catalysts derived from layered synthetic microstructures. Final progress report for period March 1, 1997 - February 28, 2000
Several heterogeneous catalytic reactions show size dependence, whereby the specific rate changes with the average diameter of supported metal particles in the nanometer range. Geometric arguments relating the size dependence to the relative concentration of active sites on idealized crystal particles cannot account for all the observed results. In an effort to overcome the geometric limitations of supported particles, the authors had previously created novel supported metal catalysts called Layered Synthetic Microstructures (LSMs) by the physical vapor deposition of alternating thin films of Ni and silica onto 3-in. Si wafer substrates. Subsequent lithography followed by wet etching left an array of micron-sized towers. Relative catalytic rate measured for ethane hydrogenolysis showed that LSMs produced similar size effects as previously found with supported particles. In the current work, experiments were accomplished using LSMs with a wider range of metals (Ni, Pt, Ir, Rh, Ru, etc.) and supports (SiO{sub 2} and Al{sub 2}O{sub 3}). Dry etching with Ar ions was used. It was found that a distinction can be made between several types of size effects due to the well-defined geometry of LSM catalysts. Rates in some systems are truly size dependent, while in other systems rates are clearly dependent on the metal-support interface. In addition, a lift-off process was developed for fabrication of all kinds of LSMs without resorting to either wet or dry etch techniques.