Reduction and oxidation kinetic modeling of NiO-based oxygen transfer materials

Abstract

Abstract This study investigates the redox kinetics of four NiO-based oxygen transfer materials (OTMs) supported on Al 2 O 3 , TiO 2 , SiO 2 and ZrO 2 . The OTMs were tested and evaluated under twenty consecutive methane reduction/air oxidation cycles. Several solid-state kinetic models were consecutively and optimally screened for each OTM at different redox cycles. The use of different supports resulted in different forms of the redox kinetics. NiO/Al 2 O 3 and NiO/TiO 2 reduction kinetics were rate-determined by chemical reaction and fitted via the Unreacted Shrinking Core Model. On the other hand, NiO/ZrO 2 and NiO/SiO 2 reduction was found to proceed via nucleation and subsequent nuclei growth and was suitably described with Avrami-Erofeev models. The use of different types of kinetic models is attributed to differences in strength of metal-support interactions. Specifically, the strong interaction between NiO and Al 2 O 3 and TiO 2 creates NiO-support interfaces where Ni nuclei are formed very fast, rendering the chemical reaction the governing step of the reduction process. When NiO is supported on SiO 2 and ZrO 2 , the interaction is weak and NiO basically behaves like free NiO, reducing via the slow formation of homogeneous nuclei on the surface and the subsequent faster growth of Ni domains. Regarding Ni oxidation kinetics, all OTMs were rate-determined by nucleation and nuclei growth. NiO/Al 2 O 3 and NiO/TiO 2 OTMs followed an Avrami-Erofeev model approach for all considered cycles, while NiO/ZrO 2 oxidation kinetics were described via a Prout-Tompkins model that considered a short but still, significant nucleation period.