Highly Active Noble‐Metal‐Free Copper Hydroxyapatite Catalysts for the Total Oxidation of Toluene

Hydroxyapatite (Hap) - supported copper materials prepared by the wet impregnation method were designed as noble metal-free catalysts for toluene total oxidation. Cu/Hap with different Cu loading (from 2.5 wt% to 20 wt%), calcined at 400 °C, were characterized by inductively coupled plasma optical emission spectroscopy, N2 physisorption, X-ray diffraction, Raman, infrared, X-ray photoelectron and time-of-flight secondary ion mass spectroscopies. Tenorite CuO phase was detected in all materials, while additional libethenite Cu2(PO4)OH phase was observed for sample with 20 wt% Cu. The presence of libethenite was accompanied by the formation of Ca2CO3H+ ions at the hydroxyapatite surface. Residual NO3- species in interaction with Cu and Ca were also found, their amount increasing with Cu content in the sample. Interestingly the specific activity in the toluene total oxidation increased with the decrease in Cu content in the catalyst. The rate per mole of copper was increased 10 times when the copper content was 4 times reduced. This noticeable result could be related to the presence of acid sites with moderate strength as well as fine dispersed CuO species on the hydroxyapatite, allowing the toluene molecules activation and their oxidation through a redox mechanism. Moreover Cu2.5wt%/Hap showed remarkable stable catalytic performance for 45 h time-on-stream, providing evidence that this material has a high potential for Volatile Organic Compound catalytic removal application.