UNDER-COORDINATED SURFACE Al3+ IONS IN γ-Al2O3: THEIR ROLE IN ANCHORING AND STABLIZATION OF CATALYTICALLY ACTIVE PHASES, AND IN THE γ-to-θ PHASE TRANSITION

摘要

The importance of under-coordinated sites (defects) on metalnand metal oxide surfaces has long been recognized in heterogeneousncatalysis. These defects sites can serve as active centers fornreactions, as well as anchoring sites for catalytic phases. In reduciblenoxides, for example, the interaction of metal phases with the reducednsites leads to strong metal support interaction (SMSI), that lends highnthermal stability to the active metal phase. γ-Al2O3, is the mostnwidely used industrial oxide support material due its high thermalnstability, high surface area and inertness. Precious metal catalyticnphases exhibit high dispersion and at the same time, remarkablenstabilities on γ-Al2O3. In spite of the extensive studies aimed atnunderstanding the anchoring and stabilities of γ-Al2O3-supportednmetal catalysts, the sites where the metal atoms/clusters anchor tonhave not been determined unambiguously. In our previous studiesnwe have clearly shown that metal oxide (e.g., BaO) binds tonpentacoordinate Al3+ sites, and they remain stable to highntemperatures, and also stabilize the γ-phase of alumina to highntemperatures. In this presentation we are going to summarize thenresults of our recent studies on the multiple roles of undercoordinatednAl3+ sites on the γ-Al2O3 surfaces in anchoring and stabilizing Ptnparticles, as well as in the γ-to-θ phase transformation. In thesenstudies we apply a series of spectroscopy (NMR, EXAFS, TPRX,nXRD) and microscopy (STEM) techniques to gain insight into thenprocesses taking place on the γ-Al2O3 surfaces as they interact withnmetal phases at high temperatures