CO₂ Reforming of Methane to Syngas: Deactivation Behavior of Nickel Aluminate Spinel Catalysts

CO₂ reforming of natural gas is one of the best ways to make low H₂:CO ratio syngas. Commercially, supported nickel catalysts are used. One of the drawbacks of this process is excessive coke formation believed to be caused by the sintering of metallic nickel particles. In order to slow down the nickel particle growth a series of nickel aluminate spinel catalysts were prepared. Since Ni is a part of a cubic close pack spinel structure, it was believed that Ni aggregation would be much slower. These spinel catalysts showed excellent initial reforming activity. However, an XRD analyses of the spent catalyst clearly showed the presence of metallic Ni, as expected, and alpha-alumina. The presence of alpha-alumina in the spent catalyst is very surprising since 1) no crystalline alumina was present in the fresh catalyst, 2) no alumina, with the exception of very dense and crystalline “diaspore”, yields alpha alumina at a temperature less than 1150°C, and 3) a NiO/alumina catalyst, prepared in the laboratory, did not yield alpha-alumina under reforming conditions (800–900°C). The results of the NiAl spinel catalyzed reforming, analyses of the spent catalyst, and a probable explanation for the alpha-alumina formation will be discussed in this paper.