Graphitization of alumina as a way to stabilize its textural characteristics under hydrothermal conditions

Abstract

Gamma alumina is a widely used support for catalysts of various processes. However, in the case of aqueous-phase processes, hydrothermal conditions result in transformation of γ-Al 2 O 3 to boehmite, which is accompanied by degradation of the porous structure and leads to rapid catalyst deactivation. Here, we demonstrate a simple approach to stabilize the textural characteristics of γ-Al 2 O 3 under hydrothermal conditions, based on preliminary graphitization of its surface by chemical vapor deposition of ethylene. Different duration (5–240 min) revealed the optimal degree of alumina surface coverage with carbon, ensuring the preservation of its textural characteristics from collapse during hydrothermal treatment (HTT) at 200 °C. Materials characterization was performed by X-ray diffraction, transmission electron microscopy, nitrogen physisorption, and Raman spectroscopy. It is shown that initial γ-Al 2 O 3 during HTT transforms into boehmite with a dramatic decrease in the surface area from 217 to 42 m 2 /g. Whereas the preliminary graphitization of its surface maintains high surface area of the material after HTT, which is achieved by forming a C@boehmite composite (294 m 2 /g). The presented approach makes it possible to modify the surface of alumina supports with a layer of graphite-like carbon and can be used to improve or maintain the catalysts activity. • C@Al 2 O 3 samples were obtained by ethylene CVD for 5–240 min. • Graphitization degree of Al 2 O 3 surface varied from 6 to 100%. • Nanocrystalline graphite with graphene fragment size less than 5 nm is formed in C@Al 2 O 3 . • Monolayer carbon coating prevents Al 2 O 3 texture collapse in hydrothermal conditions. • Mesoporous C@boehmite composite with high surface is formed by hydrothermal treatment of C@Al 2 O 3 .