Reaction-based kinetic model for the reduction of supported NiO oxygen transfer materials by CH4

Abstract

Abstract The focus of this study is to assess the reaction rate-based kinetics of three NiO-based oxygen transfer materials (OTMs) supported on ZrO2, Al2O3 and SiO2 under CH4 reduction. Both NiO weight change data and gaseous phase composition from TGA-MS tests are utilized for the elucidation of the reaction scheme. We show that mainly CH4, as well as CO and H2 (primary NiO reduction products by CH4), are involved in the reduction process under a varying extent that depends on the support used. The developed kinetic model is first order dependent on the concentration of the main reductant at each reaction and a function of the NiO/Ni concentration. On ZrO2, NiO is exclusively reduced by CH4 via both total and partial oxidation. On the other hand, in addition to CH4, CO and H2 also contribute to NiO reduction on Al2O3 and SiO2. Still, total and partial CH4 oxidation are the dominant routes. In the case of SiO2 in particular, the reduction rate of NiO by CO and H2 is the highest among all OTMs. The CH4 decomposition reaction, catalyzed by the metallic nickel sites, also takes place via the Langmuir-Hinshelwood mechanism after the completion of the NiO reduction. The rate of this reaction correlates with the surface area of the materials due to the higher population of active metallic nickel sites on the surface, leading to carbon deposition in the order NiO/Al2O3 > NiO/SiO2 > NiO/ZrO2.