Key role of terminal hydroxyl groups and visible light in the reactive adsorption/catalytic conversion of mustard gas surrogate on zinc (hydr)oxides

Abstract

Rhombic Zn(OH)2 and flower-like ZnO nano-particles were synthesized with a controlled precipitation rate. The materials were characterized by XRD, FTIR, potentiometric titration, nitrogen adsorption, TA-MS, MS-MS, SEM and UV–vis–NIR. A commercial ZnO was used as a reference material. Hydroxyl groups were formed on the surface when a slow addition of NaOH was used. The surface area of Zn(OH)2 was higher (350%) in comparison to that of ZnO. The presence of hydroxyl groups increased the band gap from 3.05eV for ZnO to 3.22eV for Zn(OH)2. The materials were used as reactive adsorbents of mustard gas surrogate, 2-Chloroethyl ethyl sulfide (CEES). On the surface of Zn(OH)2, more CEES was absorbed than of that of ZnO. Moreover, the capacity on Zn(OH)2 significantly increased under a visible light exposure. The results indicated the paramount role of terminal OH groups in the reactive adsorption process. The porosity was also found important since it affects the distribution of these OH active centers. Ethyl vinyl sulfide (EVS) was detected on the surfaces of both samples. At light, hydroxyethyl ethyl sulfide (HEES) was detected only on Zn(OH)2. Thus the absorption of photons led to the formation of the ethyl ethyl sulfide (EES) cations, which were transformed to less toxic EVS by dehydrohalogenation. With water and hydroxyl groups on the surface, the light irradiation promoted the formation of hydroxyl radicals. They reacted with the EES radicals and formed HEES via a hydrolysis pathway. No conversion of CEES was found in the dark.