The Ir/SiO2 and K ion-promoted Ir/SiO2 catalysts were fully characterized and catalytically studied using the ring-opening reaction of 1,3dimethylcyclohexane (1,3-DMCH) as a probe reaction. This reaction could take place during the catalytic process underwent either by gasoline or by diesel fuel for enhancing of octane numbers or cetane numbers (CNs), respectively. The Ir catalysts were characterized by chemisorption of CO and H-2, temperature-programmed techniques, X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The addition of potassium ions to Ir/SiO2 catalyst produce severe superficial changes that are reflected in its ability for catalyzing the ring-opening reaction of 1,3-dimethylcyclohexane and the selectivity to primary product from substituted to unsubstituted C-C cleavages. Ir dispersions slightly increased with rising K surface density up to 3.1 atoms nm(-2), but strongly decreased at higher K loadings due to the non-uniform decorative effect of potassium over Ir particles. This contribution also reveals that the opening of C-C bonds at substituted or unsubstituted positions can be tuned varying the promoter loading. That is, the dicarbene reaction path typically occurs on Ir/SiO2 catalyst, facilitating the formation of branched products through the opening of C-C bonds at unsubstituted positions. On the other hand, the metallocyclobutane intermediates become operative over K ion-promoted Ir/SiO2 catalysts. This involves a metal atom and three C atoms to form cyclic intermediate specie that undergoes the opening of C-C bonds at substituted positions and facilitates the formation of unbranched products, which are more desirable in the production of diesel fuel. (C) 2007 Elsevier B.V. All rights reserved.