Oxygen surface groups, which are typically present on the surface of activated carbon, have a decisive effect on the achieved dispersion of active phases, which affects the electronic properties of the surface sites and stabilizes the materials against sintering phenomena. Therefore, the desorption of oxygen surface groups during the decomposition–reduction of the metal salt precursors of the fresh carbon-based catalysts can lead to surface reconstruction of the metal nanoparticles, which can significantly affect their catalytic activity. In this work, the oxidation states of Ru atoms supported on both an activated carbon thermally treated to remove all oxygen groups and the untreated material were studied during the reduction process. The precursor to prepare these catalysts is Ru(NO)(NO3)3. The main results were obtained by using in situ X-ray absorption near-edge structure analysis of the Ru K-edge under conditions comparable to the temperature-programmed reduction experiments. The different reduction mechanism observed in each of the catalysts, of a multistep nature if oxygen surface groups are present and single-step in the case of clean graphitic surfaces, can be regarded as a typical metal–support interaction. Moreover, differences in both the final Ru oxidation state and the subsequent interaction of the active phase with the support are the main cause of the five-times higher catalytic activity observed in the presence of the catalyst without surface oxygen groups for the NH3 decomposition reaction.