Post-mortem characterization of coke-induced deactivated alumina-supported Co–Ni catalysts

The physicochemical properties of used catalysts obtained from propane steam reforming under steam-to-carbon (S:C) of 0.8 and 1.6 at operating temperatures of 773–873 K were investigated using BET, H2H2 chemisorption, total organic carbon (TOC) content analysis, XRD, TEM, as well as carbon reactivity analysis via gravimetric temperature-programmed (TPO–TPR and TPR–TPO–TPR) runs. These independent fingerprinting techniques provided good agreement on the nature and quantity of carbon deposited as a function of steam-reforming history. It is shown that the surface area and bulk phase characteristics were not irreversibly damaged by carbon deposition. However, TOC, XRD and TGA weight analyses showed a higher carbon content at lower S:C ratio and temperature. The weight derivative profiles for TPO and TPR suggest two types of carbon phases—atomic CαCα and naphthalenic CβCβ. The deactivation-causing CβCβ was observed in the TEM micrographs as thick layers of filamentous carbon. This was corroborated by the crystalline carbon peak recorded in the XRD pattern. The study further showed that regeneration via reductive–oxidative carbon removal was more efficient than an oxidative–reductive route since the former produced a regenerated Co–Ni catalyst without substantial loss in surface properties (e.g., BET area, percent metal dispersion and active particle size). Carbon removal kinetics was attended by a relatively low activation energy (54–58kJmol-1) symptomatic of primary carbon deposition on metal sites. Solid-state rate data for the oxidative regeneration were adequately fitted by an Avrami–Erofeev reaction-order model.