Change in Surface Acidity of Nano-Ball Allophane upon Zinc Adsorption and its Mechanisms
Effect of zinc adsorption on the change in surface acidity of two nano-ball allophane samples with varying Si/Al ratio was studied. Zinc adsorption caused the enhancement in the surface acidity of nano-ball allophane samples. The enhancement in the surface acidity after zinc species adsorption is attributed to the inductive effect on the SiOH groups near the adsorption sites. Molecular orbital calculations indicated that Zn2+ ion has the possibility to accelerate deprotonation of the SiOH functional groups near the adsorption site. The result showed that the surface acidity of the allophane sample with a higher Si/Al ratio (KnP) was stronger than the KyP sample with lower Si/Al ratio. This tendency was observed under RH between 0 and 76%; then, the acid strength for the two nano-ball samples was the same at RH of 100%. The stronger acidity of the KnP sample is attributed to its a higher number of accessory polymeric silanol groups attached to the main frame structure, which cause the enhancement of the Brønsted acidity. The higher increase in the acid strength of the KnP could be also attributed to its higher adsorptive capacity for zinc species.
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