The catalytic behavior of Ag-loaded fumed SiO2 has been investigated for diesel soot oxidation. The diesel soot generated by burning pure Mexican diesel in laboratory was oxidized under air flow in presence of the catalyst inside a tubular quartz reactor in between 25 and 600 degrees C. UV-vis optical spectroscopy was utilized to study the electronic state of silver in Ag/SiO2 catalysts. The soot oxidation was seen to be strongly enhanced by the presence of metallic silver on the catalyst surface, probably due to the formation of superoxide ion O-2(-). During the catalytic process, the Ag2O species on the catalyst acts as a strong oxidizing agent which gets reduced readily by the soot carbon to metallic Ag. The Ag/SiO2 catalysts exhibited strong performance for diesel soot oxidation below 300 degrees C. Mechanical mixing of extra SiO2 with 3% Ag loaded SiO2 resulted a further increase of soot oxidation rate, probably due to the increased interactions of O-2(-) and soot particles through the highly porous surface of SiO2. (C) 2012 Elsevier B.V. All rights reserved.