Studies of Muonium Reactivity with Uncapped Gold Nanoparticles and with Surface-Adsorbed Benzene on these NPs in Porous Silica Hosts

Gold nanoparticles (AuNPs) have been a subject of considerable interest in recent years due to both their magnetic and catalytic properties. This paper reports a two-fold study of the reactivity at 300 K of the isotopic hydrogen atom, muonium (Mu = mu(+)e(-)), (i) with bare uncapped AuNPs of different sizes encapsulated in mesoporous (SBA-15) silica hosts, forming a diamagnetic final state in the Mu + AuNP -> MuAuNP reaction, and (ii) with surface-adsorbed benzene on these NPs forming the muoniated cyclohexadienyl radical in the Mu + C6H6 -> MuC(6)H(6) addition reaction. The measured muon-spin relaxation rates, lambda(C), for the chemisorption reaction of Mu with the bare AuNPs show some variation with AuNP size. The Mu + C6H6 addition reaction has been studied over a range of benzene loadings both on bare silica and in the AuNP/silica samples. The measured muon-spin relaxation rates, lambda(Tot), exhibit a linear dependence on benzene concentration over the full range of loadings in both cases, in accord with an Eley-Rideal model of surface reactivity. Rate constants, k(Bz), were determined from this dependence which exhibit a 2-3-fold faster reaction rate on the AuNPs than on the bare silica, suggesting a catalytic effect due to benzene adsorbed on these NP surfaces.