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

Abstract

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 + C 6 H 6 → MuC ̇6H 6 addition reaction. The measured muon-spin relaxation rates, λ C , for the chemisorption reaction of Mu with the bare AuNPs show some variation with AuNP size. The Mu + C 6 H 6 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, λ 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.

Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuĊ6H6) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

“…Benzene consequently adopts a canted orientation on the AuNP surface, explaining the linear dependence of λ Mu on [Bz] seen in P1. This also facilitates Mu reactivity with bare AuNP sites forming diamagnetic final states 3,5 due to its chemisorption at these sites, as a possible probe of surface magnetism, in sync with a more recent paper elsewhere. 65 Figure 5 compares ALC spectra for MuC ̇6H 6 on the 8 nm AuNP at an expected monolayer loading of 7.6 Torr Bz (a) with the 30 Torr Bz loading from Figure 4 (b), while Figure 6 compares results for two higher Bz loadings in the 10 nm AuNP/silica sample.…”

“…Such a dependence is in marked contrast to an asymptotic trend that would have been the case were Bz, at multilayer loadings, lying flat on the NP surface, like "stacked dinner plates" due to π-stacking interactions. 5,71 This linear dependence seen for λ Mu in paper P1 resulted from the weak interaction of Bz with the AuNPs and its canted orientation on the NP surfaces, allowing for many more bare AuNP sites as well as more Bz surface-adsorbed sites for Mu to react with than would have been the case were Bz lying flat. Still, one cannot exclude the possibility that some Bz and hence some MuC ̇6H 6 radicals could also be located at silica sites, in the face of the uncertainties in sample morphologies discussed in P1.…”

“…For reference, the highest loading in the present study (Table 1) is comparable. Note the loss of the Δ1 resonance in the upper scan for the bare silica, but which, though highly broadened, is still clearly seen in the fit for the AuNP data in part b. for the AuNP sample at this 30 Torr Bz loading, likely due to the formation of diamagnetic final states on the AuNP surfaces, 3,5 for which there is no clear route were MuC ̇6H 6 located at SiO 2 /SiOH sites. Were Bz to be located at silica sites in the AuNP/silica samples, little or no difference in ALC amplitudes would be expected, contrary to the observation in Figure 4.…”

“…47,66−70 Though the positions of these ALC resonances are insensitive to MuC ̇6H 6 binding sites, their amplitudes are not. Even at the higher loading of 83 Torr in Figure 6a Figure 3b, the peak amplitudes in the silica are still almost 3 times higher, indicating a chemisorption reaction of Mu with the AuNPs forming a diamagnetic final state, 3,5 thus reducing the ALC amplitudes seen. There is a difference in the widths on the 8 nm AuNP at 30 Torr Bz in Figure 5b, which is narrower than for both the 10 nm AuNP and the bare silica at comparable loadings (Figure 3a, Table 1), indicating a stronger BE for MuC ̇6H 6 to the smaller AuNP.…”

“…1,2,4,19 However, these can change the nature and stability of metal NPs 20 and hence their surface reactivity. Recently, we explored hydrogen atom interactions with AuNPs utilizing the light hydrogenic H atom "muonium", composed of a positive muon and an electron (Mu = μ + e − , mass 0.114 amu 5,7,21−23 ), in studies of its surface magnetism 3,4 and reactivity on AuNPs, 3,5,23 encapsulated in mesoporous (SBA-15) silica hosts. 3,11,23−25 The porous silica not only prevents the aggregation of the AuNPs but importantly also provides for measurements on uncapped NP surfaces.…”