Structure and selectivity of metal catalysts: revisiting bimetallic zeolite systems
Abstract
The structures of Pt–Co/NaY and Ru–Co/NaY have been investigated by X-ray photoelectron spectroscopy (XPS), Extended X-ray absorption fine spectra (EXAFS) and temperature programmed spectroscopy. Formation of the bimetallic particles inside the zeolite framework, the effect of pre-treatments using O2 /H2 , He/H2 and He/H2 /O2 /H2 on the structure of the small bimetallic particles have also been studied. The structure of the samples was related to the activity and selectivity in CO hydrogenation. Under mild oxygen treatment the Pt–Co bimetallic particles are separated leaving the platinum particles in the supercage and transferring the Co2+ ions into the sodalite cage or into the hexagonal prisms. Direct oxygen treatment on Ru/NaY results in the migration of the ruthenium particles to the external surface of the NaY, while the presence of cobalt in Ru–Co/NaY stabilizes the ruthenium inside the supercage. Conversely, EXAFS and XPS measurements indicate the reduction of Co2+ ions facilitated by ruthenium and platinum, but the catalyst structure can be described by a ‘cherry-type' model containing cobalt as its kernel. As a result the major difference between mono- and bimetallic samples is mainly controlled by the location of the metal particles and the extent of reducibility of Co2+ ions. The olefin/paraffin ratio changes depending on the location of the metal particles, and the chain growth depends on the residence time of the species inside the zeolite. The major parameters governing the general behavior of zeolite-supported bimetallic particles, are discussed.