Surface Acid Functionalization of Activated Carbons and Its Influence on the Copper-Support Interactions

Abstract

Two sets of commercial activated carbon (AC) were surface functionalized by an acid treatment using a) strong acid HNO3 (AC80 and AC90) and b) a weak acid C6H8O7 (ACC). The effect of the functionalization on the surface chemistry and structure of the AC samples was studied by means of TGA-DSC, XRD, FT-IR, N2 adsorption, and TPD and tested in the decomposition of isopropanol. The catalysts were synthesized by the deposition of copper using a recirculation impregnation method. Metal dispersion, average size of metallic domains and metal-support interactions were correlated with the surface functionalization by using XRD, SBET, SEM-EDS, AAS and N2O dissociative adsorption. The acid treatments showed no effect on the crystal structure; however, increments in the acid/basic site concentration and the surface oxygen-containing functional group (OFG) content were observed, mainly in the AC80 and AC90 samples. BET surface measurements show a small decrease due to OFGs and/or copper incorporation. Copper distribution was uniform and its content shows a direct correlation with the OFG concentration. XRD reveals the presence of Cu° and Cu2O in the Cu/AC80 and Cu/AC90 samples, possibly associated with an auto-reduction process assisted by the OFGs near the metallic centers. Cu/AC and Cu/ACC XRD patterns show no evidence of copper phases, due to low content and high dispersion. The HNO3-treated supports showed the higher dispersions and the smallest metallic domain sizes (30%—3.3 nm Cu/AC80 and 22%—4.5 nm Cu/AC90), while Cu/ACC showed a 17% dispersion and an average metallic domain size of 5.8 nm. The presence of surface OFGs promotes selective adsorption of Cu, an increase in metal dispersion, a decrease of domain size and the presence of alternative copper-derived phases, showing increased metal-support interactions.