This can be classed as a decomposition reaction, because a compound is decomposing into simpler substances.
It can also be classed as a redox reaction. The oxidation number of Ni decreases (reduction) from +3 in Ni₂O₃ to 0 in Ni, while the oxidation number of O increases (oxidation) from -2 in Ni₂O₃ to 0 in O₂.
