Early stages in the formation and burning of graphene on a Pt/Mg(Al)Ox dehydrogenation catalyst: A temperature- and time-resolved study

Abstract

Deposition of graphene-like coke during non-oxidative propane dehydrogenation was investigated on a 0.5%Pt/Mg(Al)Ox catalyst. The initial blocking of Pt sites by graphene plays an important role in establishing the excellent steady-state selectivity of Pt-based catalysts toward propylene. Temporal Analysis of Products (TAP) pulse-response experiments was used to demonstrate that during the initial nucleation of graphene-like coke, the blocked active sites are spontaneously recovered on the timescale of minutes after the dosing of the feed is discontinued. These observations suggest that an additional transport process is involved between the generation of coke precursors on Pt dehydrogenation sites and their subsequent assimilation into the growing graphene sheet. After continued exposure to the propane feed under atmospheric pressure flow conditions, extensive deposits of deformed graphene are formed on small Pt nanoparticles and shifted onto the support. Multiple layers of graphite are also formed on large nanoparticles. During subsequent oxidative catalyst regeneration (burning), some of these carbonaceous deposits are readily oxidized in air already at 650K, leading to significant recovery of Pt sites. However, those carbonaceous deposits that are less accessible to activated oxygen resist oxidation up to 800K. Ex situ TEM characterization of incompletely burned samples and isothermal pulsed oxidation provided evidence that transport phenomena on the surface determine the accessibility of graphene-like coke for oxidation at a given temperature.