The use of iron oxide catalysts in advanced oxidation processes (AOP), such as the conversion of hydrogen peroxide to reactive oxygen species (ROS), is relatively inefficient at circumneutral pH values. Silica-containing iron oxide nanoparticles derived from a one-pot process show higher conversion rates and more efficient utilization of ROS compared with those made of plain iron oxide. The as-formed particles (50–200 nm) were characterized by TEM, SEM, BET, and DLS and used in a reaction to degrade phenol. In a period of 8 h, all particles investigated showed higher phenol degradation while degrading significantly less hydrogen peroxide. The higher conversion rate can be attributed to the interaction of iron with the silica shell, which alters the redox processes and favors the formation of stronger oxidants during H2O2 decomposition. Production of such nanomaterials by a simple, environmentally friendly, and upscalable route is particularly appealing due to its simplicity in a one-pot process.