Gold nanoparticles were generated inside silicate MCM-41 mesoporous materials by two methods, co-precipitation in the presence of a cationic surfactant (COP) and incipient wetness impregnation (IMP). The samples were characterized by XRD, nitrogen sorption, UV-vis, TEM, TOF-SIMS, and DTG/DTA methods and were tested in acetonylacetone (AcAc) cyclization/dehydration in the gas phase and NO reduction with propene in the presence of oxygen (NO-SCR) monitored by FTIR. Metallic gold was determined in both samples; as expected, its dispersion was lower on the impregnated material. In addition, An species surrounded by chloride ions was generated in silicate MCM-41 matrix prepared by the one-step synthesis with Si (sodium silicate) and An (HAuCl4) sources. This sample revealed high activity in AcAc cyclization to MCP (methylcyclopentenone), demonstrating the basic character of the surface. The basicity of this catalyst results from the presence of chloride. Moreover, this catalyst exhibits excellent performance in the electron transfer to oxygen in the NO-SCR reaction. The role of chloride anions as promoters in this reaction is demonstrated. (c) 2006 Elsevier Inc. All rights reserved.