The use of atomic force microscopy (AFM) to study the surface topography of commercial fluid cracking catalysts (FCCs) and pillared interlayered clay (PILC) catalysts

Abstract

An atomic force microscope operating in contact or Tapping Mode TM has been used to study the surface morphology, nanostructure, clay plates packing and conformation while providing nanometer-scale features of FCCs surfaces not readily accessible by other microscopic techniques. Contact mode micrometer-scale (15μm x 15μm) AFM images have revealed that the topography and molecular organization of the surface of several commercial FCCs are fairly heterogenous in nature, frequently containing discontinuities represented by deep trenches, valleys and crater-like openings with micrometer dimensions. Surfaces are in general, composed of short stacks of plates with voids or pores between these stacks resulting from materials occlusion between plates, from missing plates, missing stacks of plates and from misaligned stacks of plates. Gross structural differences between fresh and equilibrium FCCs, were not observed. However surfaces of equilibrium FCCs may contain debris possibly representing NiO and V 2 O 5 deposits, in agreement with chemical analysis. Not all equilibrium microspheres contain surface debris. Thus AFM images allow the distinction of old and young FCC fractions in equilibrium FCC samples. Coke deposits during gas oil cracking at MAT conditions, are imaged as raised surface features representing molecules or cluster of molecules. Contact-mode AFM images of pillared interlayered clays (PILCs) cracking catalysts having alumina clusters as the structure supporting pillars, represent the catalyst surface as a collection of white spots in an hexagonal arrangements having nearest neighbor and lateral distances in agreement with the repeat distances of the clay siloxane layer; evidenced of surface alumina debris was not observed an all the extraframework alumina introduced by the pillaring reaction is located in the clay interlamellar space. After exposure for 5h to 100% steam at 760° C and 1 atm, the structural parameters of the surface disappear when the PILC was prepared using montmorillonite and were retained when the PILC was prepared from rectorite. Thus PILCs collapse is the result of the clay (single) silicate layer hydrothermal instability and it occurs irrespective of the hydrothermal stability of the pillars used. In contrast to FCCs, coke deposition from gas oil cracking at MAT conditions, form on the surface of pillared rectorites a layer geometrically similar to graphite that can be easily removed by heating in air at 600°C without affecting the PILC's structure or cracking activity.