Characterizing the Reduction of NixMg1-xAl2O4
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
The reduction of NixMg1-xAl2O4 in H2 to form nickel metal and a remnant oxide was characterized by XRD, HTXRD, TGA, pycnometry, TEM, and SEM. The aim of the work was to investigate the dynamics of the system to better understand its capabilities and limitations for catalysis applications. ZrO2 was added to the majority of samples to discourage the transformation of metastable spinel phases to Ɵ or α-Al2O3. As the reduction progresses, one O2- is lost for each Ni2+ which reduces to Ni metal. The temperature of the onset of reduction was shown to vary by composition in flowing 4% H2/Ar via TGA, with NiAl2O4 beginning to reduce at ~ 780 °C. The onset temperature of lower nickel compositions were quite close to each other, starting at ~ 900 °C for Ni0.75Mg0.25Al2O4. Ni0.25Mg0.75Al2O4 and Ni0.5Mg0.5Al2O4 were shown to form Ni metal and a nonstoichiometric spinel of the same Mg-Al ratio as the starting composition. NiAl2O4 and Ni0.75Mg0.25Al2O4 were found to become unstable as full reduction was approached, and metastable spinel, Ɵ-Al2O3, and α-Al2O3 formed sequentially given sufficient time at temperature. A phase diagram was constructed in a previously uninvestigated region of the NiAl2O4 – MgAl2O4-Al2O3 ternary phase diagram using the phase stability of the remnant spinel as indication of the edge of the spinel stability phase field. Rietveld refinements were performed on all compositions reduced at temperatures from 650 to 1100 °C to quantify structural changes in the spinel and phase fraction, crystallite size and microstrain in all phases. The formation of non-stoichiometric spinel upon reduction was confirmed by density measurements of the reduced specimens using helium pycnometry. Significant progress was made towards understanding the dynamics of an important catalyst system. The majority of nickel metal was present as faceted crystallites on the surface, explaining previously observed high catalytic activities. Subsequent studies can use the phase stability and kinetic results of this work to identify additives to stabilize the metastable spinel structures. Good candidates were identified in ZrO2 and Nb2O5, and TiO2 was found to promote the formation of corundum.
Advisory committee members: Steven Pilgrim, Matthew Hall, Nathan Mellott. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University