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 = μ+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 → MuĊ6H6 addition reaction. The measured muon-spin relaxation rates, λC, 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, λTot, 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, kBz, 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.