Identification of the structure of the Bi promoted Pt non-oxidative coupling of methane catalyst: a nanoscale Pt3Bi intermetallic alloy

Abstract

Recently, stable non-oxidative conversion of methane (NOCM) for up to 8 h with a C₂ selectivity greater than 90% has been reported over Pt–Bi/ZSM-5 at moderate temperatures (600–700 °C). In this study, we show that the structure of the bimetallic nanoparticles on Pt–Bi/ZSM-5 catalyst is similar to Pt–Bi/SiO₂. EXAFS indicates the formation of Pt-rich bimetallic Pt–Bi nanoparticles with Pt–Bi bond distance of 2.80 Å. The XRD spectra (on SiO₂) are consistent with cubic, intermetallic surface Pt₃Bi phase on a Pt core. The Pt₃Bi structure is not known in the thermodynamic phase diagram. In all catalysts, only a small fraction of Bi alloys with Pt. At high Bi loadings, excess Bi reduces at high temperature, covering the catalytic surface leading to a loss in activity. At lower Bi loadings with little excess Bi, the Pt₃Bi surface is effective for non-oxidative coupling of CH₄ (on ZSM-5) and propane dehydrogenation (on SiO₂).