In the present study, Mg0.25Fe2.75O4nanocatalyst for reduction of emissions in diesel engine was fabri-cated via urea-nitrate combustion method. The effect of urea concentrations as fuel on its structuralcharacteristics and Oxygen Storage Capacity (OSC) was thoroughly investigated. The results of variouscharacterization analyses showed that fuel ratio significantly affects crystallinity, textural, size andmorphology of particles, and thermal stability of nanocatalyst. These are mainly due to increase incombustion temperature and duration. The Mg0.25Fe2.75O4fabricated at fuel ratio of 2.5 times of stoi-chiometric ratio exhibited the lowest weight loss/high crystallinity (less than 1 wt %) and lowest latticeparameter (8.346 Å) which are related to well diffusion of Mg cations into ferric oxide as host. The Ramanand FTIR analyses showed the existence of strong peaks correspond to formation of tetrahedral andoctahedral sublattices and also the formation of spinel structure of MgFe2O4. The reduction in crystallinesize, obtained with higher amount of fuel, resulted in a higher surface area (23.6 m2/g), higher porevolume (0.0950 cc/g) and proper particle size distribution (25e35 nm). These cause a better catalyticactivity. Moreover, the enhanced OSC (8661mmol H2/g) along with shifting the reduction peaks to lowertemperatures, as major attributes in relation to catalytic technologies, confirmed the strong effect of fuelratio of 2.5 on fabrication process of Mg0.25Fe2.75O4. Finally, the highest reduction in HC (33%), CO (14-17%), PM1.0(16%), and CO2(12%) achieved, using the optimum sample-mixed diesel fuel at 90 ppm.

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