Mechanochemically synthesized cobalt oxide-based particles are employed for the catalytic reduction of 4-nitrophenol (4NP), a toxic water contaminant. This reduction produces 4‑aminophenol (4AP), a less toxic, pharmaceutical precursor for drugs such as paracetamol. The indicated reduction has been completed previously using noble metals and/or catalysts requiring extensive solvent use, and time as part of their preparation. The cost and synthesis of these noble metal catalysts hinders the sustainable broad scale application as an environmental remediation solution. The catalyst synthesis explored in this study utilizes the green chemistry technique of vibratory ball-milling and annealing cobalt oxide-based particles at different temperatures, producing unique agglomerates with differing surface structure and catalytic properties. Additional investigation into the mechanism through temperature, pH, and change in pressure over the reaction is completed. Further analysis shows that these catalysts are efficient for the reduction of 4-amino-3-nitrophenol and 2-amino-5-nitrophenol with unique catalytic rates. Finally, it is found that the application of this reduction in a flow process has potential for use on a broader scale.