Coke deposition mechanisms of propane dehydrogenation on different sites of Al2O3 supported PtSn catalysts

Propane dehydrogenation (PDH) Pt-based catalysts are facing the serious challenge of coke deactivation. The locations would greatly influence the coke formation, while the detailed mechanism is not fully explored. Herein, the coke mechanisms on different locations including Al2O3, Sn, Pt, and Pt-Sn sites were deeply investigated via in situ Fourier transform infrared spectroscopy (FTIR) technology, and the key factors triggering catalyst coke deactivation were proposed. Excessive dehydrogenation of propyl species is a crucial initial step in the formation of coke, whether at metal sites or supports. These propyl species on Al2O3 supports then cyclize to form monocyclic aromatic and bicyclic aromatic species, while those on SnOx sites cyclize to form monocyclic aromatic species. As for the Al2O3 supported PtSn catalysts, the strong dehydrogenation function and the interaction between Pt and Al2O3 supports trigger the complex coke formation mechanism. The surface Pt sites with saturated coordination are prone to coke deposition, leading to rapid deactivation at the initial stage of the reaction. However, the low-coordination Pt sites with ultra-small size are found to be highly resistant to coke formation in the PDH reaction, which selectively catalytic the PDH. Owing to the metal-support interaction, the extensive active hydrogen species generated from Pt can regulate the formation of coke precursors on the Al2O3 supports. Furthermore, the effect of hydrogen co-feed on coke deposition is also explored. The hydrogen co-feed inhibits the coke formation and results in a higher H/C ratio (3.96) for aromatic coke precursors. This study can enhance the understanding of the coke formation in PDH, which is important to designing efficient PDH Pt-based catalysts.