Small-sized metal nanoparticles (NPs) are of long standing interest to both magnetism and heterogeneous catalysis. Recent muSR studies of Au NPs encapsulated in mesoporous silica at TRIUMF showed clear evidence of a reaction of the hydrogen-like muon-electron bound state muonium (Mu) with the Au NP surfaces. Preliminary results suggest faster chemical addition for smaller NPs, consistent with an anticipated enhanced catalytic effect. We propose to measure GNPs of different sizes in order to investigate whether and how the reactivity depends on the particle size. Radio Frequency muSR is a sensitive method both to confirm the final state species and to directly measure its formation rate. The study should also reveal the magnetic nature of the surface from the RF line widths.