An EXAFS study of the coordination chemistry of hydrogen hexachloroplatinate (IV): 2. Speciation of complexes adsorbed onto alumina

Abstract

The changes in coordination chemistry that occur as chloroaquohydroxo platinate complexes adsorb over alumina have been studied using extended X-ray absorption fine structure (EXAFS) analysis at the advanced photon source (APS) at Argonne National Laboratory. Our earlier study of Pt complexes in the liquid phase [Appl. Catal. A: Gen. 232 (2002) 219] is used as a starting point. Samples were prepared both in excess solution with 200 ppm chloroplatinic acid (CPA), and using pore volume impregnation with higher CPA concentrations to give loadings of 0.25 to 4.25 wt.% Pt. The changes in coordination chemistry of chloroaquohydroxo platinate complexes adsorbing over alumina need not invoke a “triple layer” theory, which incorporates surface grafting reactions, but are more readily explained by refinements to the “double layer” theory which is the kernel of the Revised Physical Adsorption model [Chem. Eng. Sci. 56 (2000) 2365]. The changes in speciation of adsorbing CPA complexes appear to be influenced first by the change in the bulk pH brought on by the oxide buffering effect, second by the additional pH change at the (single) layer of adsorption at the alumina surface, and third by the chloride concentration at this local level. At low surface loadings, pH shifts are minimized and the amount of chloride near the adsorption plane appears to be low. Platinum complexes adsorb with low Pt–Cl/Pt–O bond ratios, even when present in the bulk solution as hexachlorides. The adsorbed species appear to behave as in the liquid phase at the pH of the adsorption plane with no excess chloride. At high surface loadings such as with pore volume impregnation, the pH shifts toward the alumina PZC are very large but all chloride is kept in the pore volume. At high Pt loadings, high chloride concentration appears to dominate the Pt speciation and Pt–Cl coordination in the adsorbed complexes remains high, as occurs in the liquid phase in excess chloride. The adsorption of Pt from solutions in which zero-valent species are thought to initially exist can be explained by mechanisms in which either rapid OH–H 2O exchange occurs at the higher pH of the adsorption plane, creating a dianionic adsorbing complex, or a surface protonation reaction, which creates a dianionic adsorbed complex in concert with surface charging. Both mechanisms are consistent with an electrostatic adsorption mechanism.