Activated carbon surface chemistry: Changes upon impregnation with Al(III), Fe(III) and Zn(II)-metal oxide catalyst precursors from NO3− aqueous solutions

Abstract

Because of the relevance in heterogeneous catalysis, the changes produced in the surface chemistry of activated carbon (AC) upon impregnation with the Al3+, Fe3+ and Zn2+ ions from aqueous solution of NO3− salts at pH 2.91 for Al3+, 1.54 for Fe3+ and 5.16 for Zn2+ in two successive soaking and oven-drying steps are studied. The samples (A120, F120 and Z120) were analyzed in terms of elemental composition and by energy dispersive X-ray analysis (EDX), FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and measurement of pH of the point of zero charge (pHpzc). The process yield was 102wt% for A120, 114wt% for F120, and 103wt% for Z120. Chromene, pyrone and ether type structures are by far predominant in AC. The hydrolysis of metal ions markedly influences the pH of the impregnation solution and thereby the oxidation of surface functional groups of AC by O2 and NO3− dissolved in such a solution. The degree of AC oxidation is larger with the Fe3+ solution than with the Al3+ and Zn2+ solutions. Carboxylic acid groups for F120 and phenolic hydroxyl groups for Z120 and especially for A120 are formed from reducing structures of AC. The nitrogen content is higher by Z120>A120>F120>AC. pHpzc is 10.50 for AC, 5.20 for A120, 4.00 for F120 and 6.30 for Z120.