Impacts of vanadium and coke deposits on the CO2 gasification of nickel catalysts supported on activated carbon from petroleum coke

Abstract

One of the ways to dispose of spent carbon-based catalysts is gasification. In this study, the kinetics of CO2 gasification of Ni/activated carbon (AC) catalysts, produced from petroleum coke, were investigated at 700–900°C using a thermogravimetric analyzer (TGA). The catalysts were coked by exposure to ethylene at 400–600°C and/or impregnated with 3 or 8wt% vanadium, which is typically deposited during hydrodemetalization (HDM) of heavy oil. The gasification rates of all samples were 30–400 times greater than the rate of gasification of uncatalyzed (i.e., no metal) AC. Rate profiles as a function of conversion were different for all samples, which suggested that nickel was promoted/inhibited as a gasification catalyst in different ways depending on the composition of the sample. Transmission electron microscopy was used to analyze samples before and after gasification. The particle morphologies and chemical analysis (energy-dispersive X-ray) help to explain the different gasification behaviors. In particular, the nickel particles on the coked Ni/AC sample became encapsulated by graphite and sintered over time from 20nm initially to 150nm after 80% conversion (∼10min at 900°C). For the Ni/AC catalysts impregnated with vanadium, the AC support was decorated with ∼10nm nickel particles and wetted with a layer of vanadium after preparation at 500°C. After heating to 900°C, however, sintering of both metals was evident as well as the formation of new vanadium species including vanadium carbide in some areas. The interaction between nickel and vanadium lowered the initial gasification rate but also slowed the deactivation of the nickel during gasification.