Carbon deposition on Ni/YSZ composites exposed to humidified methane

Carbon formation on Ni/YSZ cermets in high-temperature processes involving hydrocarbons can be a severe problem. Understanding the mechanisms of carbon formation is necessary for developing strategies to avoid or minimize the problem. In this study, Ni/YSZ pellets (70/30 NiO/YSZ) have been exposed to humidified methane at temperatures between 773 K and 1073 K. The carbon formed on these pellets and/or any structural changes in the pellets was studied with X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO). At temperatures below 873 K, carbon fibres dominated. At higher temperatures, the majority of the carbon dissolved into the Ni particles. After TPO and removal of the carbon, the Ni structure was damaged to various extents depending on the exposure temperature. The amount of carbon deposited was significantly reduced by placing zirconia-doped ceria pellets on each side of the Ni/YSZ pellet. The addition of a ceria layer did not completely eliminate the carbon formation but the carbon that was formed was weakly attached to the Ni/YSZ such that the structure was not irreversibly changed. The role of the ceria is beyond simply reforming the methane. (c) 2006 Elsevier B.V. All rights reserved.