DFT investigation of NH3 physisorption on CuSO4 impregnated SiO2

Abstract

In this quantum chemical investigation, NH₃ physisorption onto a model of copper sulfate impregnated silica is compared with pure silica and copper sulfate adsorbents. The physisorption process is modeled as direct binding of the NH₃ molecule to the adsorption site of the dry adsorbents and as displacement of a H₂O molecule by NH₃ in the hydrated complexes. The surface of silica is represented by a hydroxyl group attached to a silsesquioxane cage, H₇Si₈O₁₂(OH) and silica impregnated with CuSO₄ by the most stable configuration of the cluster containing a CuSO₄ ion pair placed adjacent to the silica cage. H₂O is systematically added to the dehydrated adsorbents to investigate the role of water in NH₃ adsorption. Modeling hydrated environments of each type of adsorbent is focused on H₂O molecules that directly coordinate with the active sites. The results indicate that the binding energy of adsorbing NH₃ onto the mixed adsorbent is greater than in pure silica. This enhanced binding in the mixed adsorbent is consistent with improved Brønsted acidity of the silanol in the presence of CuSO₄.