Efficient Thermal Reactions of Sulfur Dioxide on Ice Surfaces at Low Temperature: A Combined Experimental and Theoretical Study

Abstract

The interaction of sulfur dioxide (SO 2 ) gas with a crystalline ice surface at low temperature was studied by analyzing the surface species with low energy sputtering (LES) and reactive ion scattering methods and the desorbing gases with temperatureprogrammed desorption mass spectrometry. The study gives direct evidence for the occurrence of efficient hydrolysis of SO 2 with low energy barriers on the ice surface. Adsorbed SO 2 molecules react with the ice surface at temperatures above ∼90 K to form anionic molecular species, which can be detected by OH − , SO 2 − , and HSO 3 − emission signals in the LES experiments. Heating the sample above ∼120 K causes the desorption of SO 2 gas from the surface-bound hydrolysis products. As a result, the hydrolysis of SO 2 on an ice surface is most efficient at 100− 120 K. The surface products formed at these temperatures correspond to metastable states, which are kinetically isolated on the cold surface. Quantum mechanical calculations of a model ice system suggest plausible mechanistic pathways for how physisorbed SO 2 is transformed into chemisorbed HSO 3 − species. HSO 3 − is formed either by direct conversion of physisorbed SO 2 or through the formation of a stable H 2 SO 3 surface complex, both involving proton transfer on the ice surface with low energy barriers. These findings suggest the possibility that thermal reactions of SO 2 occur efficiently on the ice surface of Jovian satellites even without bombardment by high-energy radiation.

Proton Transport and Related Chemical Processes of Ice

“…The surface propensity of excess protons is well supported by theoretical calculations. ,,,,− Density functional theory (DFT) calculations indicate that the hydronium structure is energetically stabilized at the ice surface compared with that in the tetrahedral ice lattice. ,,− The calculated surface segregation energy of hydronium varies sensitively with respect to the local structure of the ice surface, particularly the spatial arrangement of dangling surface hydrogens, in a wide range of 20–80 kJ mol –1 . Hydroxide is also stabilized at the ice surface with a comparable magnitude of surface segregation energy .…”

Entropy-Driven Spontaneous Reaction in Cryogenic Ice: Dissociation of Fluoroacetic Acids

“…Therefore, the configurational entropy of protons may operate for a range of ice-related phenomena involving proton transfer. For example, hydrolyses of HF (p K aq = 3.2) and SO 2 (p K aq of H 2 SO 3 = 1.9) − in ice at low temperature have been observed. Ayotte and coworkers explained this reactivity of HF in ice by suggesting that HF is intrinsically a strong acid; its weak acidity in aqueous solution results from the large negative entropy of F – hydration.…”

Sulfur Ice Astrochemistry: A Review of Laboratory Studies

“…Programmed heating of the physisorbed gas to 120 K causes desorption from the ice surface and thus separates it from the chemisorbed hydrolysis products. Quantum chemical calculations suggest that the mechanism of formation of these products is similar to the equations outlined above (Bang et al 2017).…”

“…Recent work by Bang et al (2017) has also shown that thermally-driven reactions between H2O ice and gas phase SO2 are possible. Their study showed that SO2 molecules adsorbed at the surface of a H2O ice can react with the ice at temperatures of > 90 K. The primary reaction products are SO2 -, HSO3 -, and OH -.…”