DFT investigation of NH ₃ physisorption on CuSO ₄ impregnated SiO ₂

In this quantum chemical investigation, NH ₃ 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 NH ₃ molecule to the adsorption site of the dry adsorbents and as displacement of a H ₂O molecule by NH ₃ in the hydrated complexes. The surface of silica is represented by a hydroxyl group attached to a silsesquioxane cage, H ₇Si ₈O ₁₂(OH) and silica impregnated with CuSO ₄ by the most stable configuration of the cluster containing a CuSO ₄ ion pair placed adjacent to the silica cage. H ₂O is systematically added to the dehydrated adsorbents to investigate the role of water in NH ₃ adsorption. Modeling hydrated environments of each type of adsorbent is focused on H ₂O molecules that directly coordinate with the active sites. The results indicate that the binding energy of adsorbing NH ₃ onto the mixed adsorbent is greater than in pure silica. This enhanced binding in the mixed adsorbent is consistent with improved Bronsted acidity of the silanol in the presence of CuSO ₄.