Boosting the Rhodamine B adsorption of bimetallic Ce/Zr UiO-66: Effect of valence states of cerium precursor

Two kinds of Ce-doped UiO-66 were synthesized by using CeCl3·7H2O and (NH4)2Ce(NO3)6 as cerium precursors, namely Ce(III)- and Ce(IV)-UiO-66. The effect of valence states of cerium precursors on adsorption performance towards Rhodamine B (RhB) was investigated for the first time. Ce atoms are in two valence states of +3 and + 4, where Ce3+ ions turn their surfaces negatively charged. The higher cerium content (24.8%) and ratio of Ce3+/Ce4+ ions (1.4:1) of Ce(IV)-UiO-66 made its surface more negatively charged, which is contributed to form stronger electrostatic interaction with cationic dyes. The adsorption amounts of RhB were 239.5 mg/g and 431.7 mg/g for Ce(III)- and Ce(IV)-UiO-66, which are 2.38 and 5.11 times higher than pristine UiO-66, respectively. The well-fit Langmuir model and pseudo-second-order model suggest a monolayer chemisorption process. The adsorption mechanism suggests that the electrostatic interaction and the complexation interaction between Ce3+ and RhB are dominant factors affecting RhB adsorption. Since Ce3+ severed as adsorption sites, the higher Ce content and stronger electrostatic interaction of Ce(IV)-UiO-66 together promoted its RhB adsorption than Ce(III)-UiO-66. The obtained results help to understand how the mixed-valences of incorporated metals affect the adsorption behaviors of bimetallic MOFs deeply.