Organic capping agents are necessary for metallic nanoparticle preparation via colloidal method; however, complete removal of the capping agent and cleaning the metal surface is a well-known challenge in application. In this Article, we show that polyvinyl alcohol (PVA)-stabilized palladium nanoparticles (Pd NPs) were prepared and immobilized on activated carbon (AC). Different acids (HCl and H2SO4) were used to adjust the pH, thus enhancing the adsorption of the colloid on the support. The catalysts were characterized by TEM, CO-chemisorption, XRF, N2 physisorption, XPS, TGA, and temperature-programmed reduction MS. Activity of the catalyst was tested using nitrite hydrogenation in aqueous phase and formic acid decomposition in gas phase as probe reactions. The results showed that chlorine, introduced via HCl, efficiently suppressed the interaction of the Pd NPs with PVA. Clean Pd NPs were obtained without any significant sintering after reduction in H2/N2 at mild temperature (200 °C). The influence of acid on PVA thermal stability was also investigated. Differences in catalytic activity in gas phase versus liquid aqueous phase indicated that the extent of PVA covering the Pd NPs is phase-dependent.