ZSM-5 and BEA zeolites modified with Pd nanoparticles by laser electrodispersion. The structure and catalytic activity in CO and CH4 oxidation

Small amounts of nearly 2-nm Pd nanoparticles (0.01-0.03 wt.%) were deposited onto the surface of the ZSM-5 and BEA zeolites by laser electrodispersion. Materials thus prepared were studied by low-temperature nitrogen adsorption, thermo-programmed desorption of ammonia, transmission electron microscopy, and X-ray photoelectron spectroscopy. They were also tested in total catalytic oxidation of CO and CH4. It was demonstrated that the oxidation state of Pd and the character of distribution of isolated metal particles or their aggregates over the external surface or in the bulk of the zeolite is affected by the palladium content and by the zeolite nature. The activity of the Pd/ZSM-5 catalysts decreases with increasing Si/Al ratio of the zeolite and depends on the oxidation state of the metal. In the most active catalysts 65-75% of Pd occur as metal, whereas 35-25% make up oxidized palladium. The oxidation of carbon monoxide on the ZSM-5 catalyst (Si/Al = 15, 0.01 wt.% Pd) whose surface is uniformly covered with isolated Pd nanoparticles begins at 90 degrees C, which is 70-80 degrees C lower compared to CO oxidation on other Pd-modified ZSM-5 and BEA zeolites containing aggregates of palladium nanoparticles.