Silicon nanocrystals were synthesized and deposited on different substrates. In order to clarify the photoluminescence (PL) emission of these agglomerates, we studied the luminescence spectra of several samples as a function of the exposure to air, oxygen, nitrogen, and rare gases. Measurements at different pressures reveal a strong enhancement of the PL at atmospheric pressure, when the sample is in air. In contrast no significant PL is observed for clean Si quantum dot in rare-gas atmosphere and in air at low pressure. Different behavior is detected in oxygen and in nitrogen as a function of the pressure. These data point out a catalytic role of the surface adsorption adding significant information for clarifying the PL mechanism. A comparison of our results, including the decay-time spectra with data and models of literature, demonstrates the important role of the phonon interaction in the relaxation and decay …