Using tapered element oscillating microbalance for in situ monitoring of carbon deposition on nickel catalyst during CO₂ reforming of methane

The present work aims at clarifying the carbon formation behavior through an in situ monitoring of carbon deposition on Ni/Al₂O₃ catalyst during CO₂ reforming of methane using the tapered element oscillating microbalance (TEOM). The carbon deposits were also characterized by transmission electron micrograph (TEM) and temperature-programmed oxidation with IR detector (TPO-IR). TEOM results and a kinetic study on carbon formation established that both CO in the products and CH₄ in the reactants can be the source of carbon formation during the CO₂ reforming reaction. In an equimolar CO₂-CH₄ reforming condition, CO in the product stream is likely the major source of carbon formation. TEM coupled with TPO-IR and TEOM revealed that carbon formed in CO₂ reforming from a feed without containing CO shows more filamentous carbon, which is more difficult to be oxidized, while carbon formed from a feed containing CO mostly encapsulates metal particles in the catalyst and is relatively easy to be oxidized. Replacing some of CO₂ with H₂O greatly inhibits carbon formation encountered in CO₂ reforming. Kinetic studies with TEOM showed that H₂ addition to the feed gas can enhance carbon formation from CH₄, probably by hydrogenating surface carbon species and clean the surface metal sites active for methane decomposition; addition of CO to the feed gas CH₄ + CO₂ can further enhance carbon formation more than that with H₂ addition alone; whereas H₂O addition can reduce carbon formation from both CO and CH₄, most likely by gasifying carbon species.