Sn Speciation and Spectroscopic Signatures of Sn(II) Sites in Alumina-Supported PtSn Propane Dehydrogenation Catalysts

Abstract

The increasing propylene demand and supply gap accelerates the development of on-purpose propane dehydrogenation (PDH) technologies. In industrial PDH processes, Pt–Sn-based catalysts have had a major impact due to their specific catalytic performances, despite few shortcomings and the need for further optimization. Thus, large research efforts have been centered on identifying the roles of the Sn promoter, including the use of surface organometallic chemistry (SOMC). While, so far, SOMC has been mostly used to identify the role of Pt–Sn alloying, the focus here is to explore the speciation of the surface and interfacial Sn sites across synthetic steps. A Pt–Sn catalyst supported on alumina is thus prepared via SOMC, and the detailed spectroscopic and microscopy characterization allows monitoring the evolution of Sn sites evolving from SnIV sites after grafting to SnII sites, best described as SnII single atoms, upon thermal treatment. Subsequent grafting of Pt and H2 treatment yield small and narrowly distributed Pt–Sn nanoparticles, which are active, selective, and stable for PDH, while residual well-defined SnII sites remain. Notably, while SnII single atoms co-exist with the PtSn alloy, this study pinpoints that these SnII single atoms dispersed on alumina display specific 119Sn NMR signatures, which makes it difficult to distinguish SnII from Sn0 sites solely based on their XAS signatures.