A Kinetic Study of the Reactions of Sulfate Radicals at the Silica Nanoparticle−Water Interface

Abstract

Sulfate radicals, SO 4 •-, were generated using flash photolysis of aqueous S 2 O 8 2solutions and the reactions of the inorganic radicals with the surface of suspended silica nanoparticles (NP) investigated. In the presence of colloidal silica no absorption traces due to SO 4 •radicals are observed at 100 µs after the flash of light. However, two transient species with absorption maxima around 320 and 600 nm are formed. A kinetic analysis of the experimental results indicate that SO 4 •radicals are adsorbed on the NP surface, leading to the formation of an adduct, with λ max ≈ 320 nm ( ≈ 7000 cm -1 M -1 ), and showing similar reactivity to that observed for the sulfate radical in aqueous solutions. The NP-sulfate radical adducts react with adsorbed water, and with single and geminal SiOsites with reaction rate constants of 1.5 × 10 14 × e -(58(12)kJ/mol)/RT s -1 , < 10 3 × e -(2(17)kJ/mol/RT s -1 and < 10 11 × e -(46(13)kJ/mol/RT s -1 , respectively. Two different SiO • surface defects, showing similar spectra (λ max ≈ 600 nm) but different reactivities, are formed from the reaction of NP-sulfate radicals and deprotonated geminal and single silanols.

Reactions of Cl•/Cl2•− Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the Atmosphere

“…Transient formation is only observed for suspensions of pH > 6.5 in agreement with it being formed after reactions (12) and (13). The observed absorption spectrum resembles that of the SiO • surface defects formed on silica glasses (21,22) and on silica NS (19,20) (also shown in Fig. 4) and is thus assigned to the SiO • surface radicals.…”

“…The previous results clearly indicate that Cl • and Cl 2 •− radicals interact with the NP surface. Moreover, in aqueous suspensions of pH < 5, silica NS have almost all its surface silanols undissociated (pK ag ≈4.5 and pK as ≈8.5 for the deprotonation of geminal and single silanols, respectively [19,20]). The lack of reaction between the Cl • /Cl 2 •− radicals and the NP surface at pH < 5, and the higher rates observed with increasing pH, clearly indicate that the deprotonated silanols are the reactive surface species, as shown in .…”

“…The absorbance of SiO • surface radicals strongly depends on the pH of the suspensions and despite that a broader absorption band was observed for suspensions of higher pH, it was not possible to differentiate the absorption of SiO • radicals formed from geminal or single silanols (19,20). Therefore, average absorption coefficients for SiO • radicals were estimated for each pH of the suspensions relative to the reported average value at 600 nm and pH 8.3, < ε 600 > pH 8 = 2100 M −1 cm −1 (20).…”

“…3, whose slope and intercept yield k 12 = (9 ± 6) · 10 7 M )1 s )1 and k 13 = (7 ± 4) · 10 5 M )1 s )1 , respectively, at 293 K. The error bars in these rate constants were estimated from a propagation error analysis considering the uncertainty in the reported values K 2,3 , K ag and the fraction of geminal and single silanols. For the batch of NP used here, pK ag = 8.5 ± 0.5 and (80 ± 5)% of single silanols applies (19), while the uncertainty in the value of K as is of no significance in the pH range employed in the present paper.…”

“…Moreover, in aqueous suspensions of pH < 5, silica NS have almost all its surface silanols undissociated (pK ag %4. 5 and pK as %8.5 for the deprotonation of geminal and single silanols, respectively [19,20]). The lack of reaction between the Cl • ⁄ Cl 2…”