Nanofabrication of Pt particles on SiO2, with particle sizes and separations down to similar to 20 nm, and mon complex structures consisting of Pt particles deposited on top of CeO2 islands, have been made by electron beam lithography and lift-off techniques. Under catalytic reaction conditions (nonflammable mixture of H-2 + O-2 in Ar at 1000 K, 1 atm) the originally deposited, disk-shaped, and polycrystalline particles, transform to three-dimensional crystalline particles. This restructuring is attributed to the surfactant role of oxygen (promoting Pt mobility), and the nonwetting of late transition metals on oxides. The exothermic H-2 + O-2 reaction is a possible additional driving force. The catalytic kinetics on 3D crystalline particles with well-defined facet planes were explored by Monte Carlo simulations. The latter demonstrate that the catalytic kinetics on nm sized, crystalline particles may be absolutely unique and nonpredictable from single-crystal studies, primarily due to the diffusive mass transport between differently oriented facets on the particles.