Reaction-based kinetic model for the reduction of supported NiO oxygen transfer materials by CH4
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2019 Elsevier B.V. The focus of this study is to assess the reaction rate-based kinetics of three NiO-based oxygen transfer materials (OTMs) supported on ZrO 2 , Al 2 O 3 and SiO 2 under CH 4 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 CH 4 , as well as CO and H 2 (primary NiO reduction products by CH 4 ), 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 ZrO 2 , NiO is exclusively reduced by CH 4 via both total and partial oxidation. On the other hand, in addition to CH 4 , CO and H 2 also contribute to NiO reduction on Al 2 O 3 and SiO 2 . Still, total and partial CH 4 oxidation are the dominant routes. In the case of SiO 2 in particular, the reduction rate of NiO by CO and H 2 is the highest among all OTMs. The CH 4 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/Al 2 O 3 > NiO/SiO 2 > NiO/ZrO 2 .
