MnOx nanoparticles were synthesized by spray pyrolysis, with and without citric acid assistance. The prepared MnOx particles were calcined between 600 and 1000 degrees C and characterized by Brunauer-Emmett-Teller, X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction. The use of citric acid made it possible to reduce the particle size and increase the surface area of the MnOx particles. The highest surface area and reduction activity were achieved with citric acid. As a result, the MnOx nanoparticles prepared with citric acid and calcined at 600 degrees C showed the highest catalytic activity for the oxidation of benzene using ozone.