Monodisperse Pt and Rh monometallic and (Rh,Pt) bimetallic nanoparticles (as small as ~ 1 nm) were synthesized using dendrimers as the scaffold and the capping agent. Dendrimer encapsulated nanoparticles were supported onto mesoporous silica (SBA-15) by sonication. Catalysts were active for ethylene hydrogenation without any pretreatment, which could be due to the mesoporous silica preventing the dendrimer from collapsing onto the nanoparticle surface. After a mild reduction (423 K), activity was maximized due to reduction of the oxidized Rh and Pt in the as-synthesized nanoparticles and was comparable to data obtained from single crystals and other nanoparticles, which is expected for a structure insensitive reaction such as ethylene hydrogenation. Moreover, the particle size was preserved after low temperature treatments. Reduction at higher temperatures (473, 573, and 673 K) led to decreased activity and was likely caused by sintering. Catalytic activity was further demonstrated under oxidizing conditions with CO oxidation.