Four NiO-based oxygen carrier materials (OCMs) were prepared by wet impregnation on different supports: alumina, silica, titania, and zirconia. Physicochemical properties of calcined OCMs were determined by X-ray diffraction, Brunauer-Emmett-Teller, and H2-temperature-programmed reduction (TPR) techniques. Alumina (Ni-Al) and titania (Ni-Ti) supported OCMs exhibited strong metal oxide-support interaction, forming mixed compounds such as NiAl2O4 and NiTiO3, respectively, along with NiO. In contrast, only NiO species were present in silica (Ni-Si) and zirconia (Ni-Zr) supported OCMs. The performance of these materials was evaluated in consecutive methane reduction-air oxidation (redox) cycles in a thermogravimetric analysis (TGA) unit. All OCMs, except Ni-Al, exhibited stable redox activity during 20 cycles in TGA. A high degree of oxygen utilization, i.e., nearly complete NiO reduction, was obtained with all OCMs except Ni-Al. The reducibility of Ni-Al OCM increased progressively during the first 12 of the 20 cycles, due to gradual conversion of surface NiAl2O4 species to easily reducible NiO with successive reduction-reoxidation, as confirmed by TPR characterization of the used Ni-Al OCM. The trends in carbon and hydrogen formation during the reduction cycles demonstrated that H2 is mainly formed by methane decomposition on all OCMs. The activity toward methane decomposition, i.e., toward carbon deposition and hydrogen production, was in the order Ni-Al > Ni-Si > Ni-Zr > Ni-Ti, which correlates qualitatively with their respective specific surface areas. Post characterization with transmission electron microscropy showed the presence of graphitic carbon forms on all OCMs irrespective of support. Filamentous carbon was detected only on Ni-Al and Ni-Zr OCMs.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology