Tunneling metal particles could be used to control catalyst pore size and shape
Chemists in japan who have been exploring the structural changes that catalysts undergo during operation have made a surprising observation: Platinum particles deposited on a porous zeolite support can dig out new, well-defined pores in the zeolite surface. This phenomenon could turn out to be useful as a technique to control the structure of catalysts, the researchers believe.
Hitoshi Kato of the Japan Fine Ceramics Center in Nagoya and coworkers made the discovery while observing platinum catalysts under conditions similar to those inside an automobile catalytic converter [Angew. Chem. Int. Ed., 43, 1251 (2004)]. The researchers used a [Pt(NH3)4](OH)2 solution to deposit platinum particles on zeolite ZSM-5, a SiO2-Al2O3 ceramic. The catalyst was then heated to 800 °C and exposed to an atmosphere containing trace amounts of CO, CO2, NO, H2O, and other compounds--similar to the exhaust gas of a car engine.
After 100 hours, the team examined the zeolite crystals under an electron microscope and saw that the platinum particles had vanished. The researchers subsequently found that the particles had sintered into larger particles and burrowed into the zeolite surface. The resulting pores have the same diameter as the particles, they note, and have a hexagonal cross section matching the zeolite lattice.
Kato thinks the platinum coupled with components of the exhaust gas catalyzes formation of gaseous SiO or Si(OH)4, allowing the platinum to tunnel away.
"The number of pores, as well as their shape and size, could be controlled by the diameter of the platinum particles, the duration of heating, the type of zeolite selected, and the orientation of the crystals," Kato says.
ADAPTED FROM ANGEW. CHEM. INT. ED.
TIME PASSAGES Platinum particles are visible on a zeolite surface before sintering (top left), but after treatment the particles disappear and pores form (bottom left). A cross section (right) reveals that the platinum particles have burrowed into the zeolite.