Preparation of supported catalyst by adsorption of polyoxometalate on graphene oxide/reduced graphene oxide

Abstract

Adsorption of tungstophosphoric acid (H3PW12O40) as a polyoxometalate (POM) on graphene oxide (GO) and two reduced graphene oxide supports (RGO-a/RGO-b) for producing supported catalysts was studied using an equilibrium adsorption technique by ultraviolet–visible spectroscopy analysis. The surface properties of supports were analyzed by XPS to interpret their different adsorption properties. The samples were characterized by XRD, RAMAN, FTIR, SEM-EDX, and TEM analyses. The three most determinant parameters in adsorption phenomena are presence of oxygen functional groups on the support, the polarization of surface functional groups in accordance to the suspension pH, and the solvent type. The supported polyoxometalate on GO (GO/POM) prepared by adsorption in a suspension of 1:1 water/methanol mixture with pH = 3.5 yielded the highest adsorption capacity of 427 mg/g while the adsorption capacities of supported polyoxometalate on reduced samples were 74 mg/g and 54 mg/g, respectively. The pH-dependent behavior of ionizable surface oxygen functional groups was investigated and results revealed that also played a key role in the adsorption capacity. The highest POM adsorption was obtained in the pHs below isoelectric point of GO where POM anions can establish chemical bonds with the positive net surface charge. The highest adsorption capacity was obtained in the water-methanol mixture. The kinetics of adsorption mechanism was best described by a pseudo-second-order model.