Oxidation of ethylene by Cu/TiO2: reducibility of Cu2+ in TiO2 as a possible descriptor of catalytic efficiency

Abstract

Catalytic oxidation using non-noble metal-based catalysts is a promising approach to mitigate pollution due to VOCs in the air. In this work, mesoporous Cu/TiO₂ catalysts containing different concentrations of Cu²⁺ (0.2, 1, 3, and 4 wt% Cu w.r.t. Ti) were synthesized using the sol–gel technique. The catalysts were characterized using inductively coupled plasma-optical emission spectrometry, XRD, Raman spectroscopy, N₂ physisorption, cyclic voltammetry, H₂-TPR and electron microscopy to understand the structure and composition. The thermal catalytic gas phase oxidation of ethylene was studied by heating a mixture of ethylene (1.5 vol%) and air (5.9 vol%) in the presence of the Cu/TiO₂ samples in the temperature range of 298 to 773 K. Cu/TiO₂ showed a higher catalytic activity compared to TiO₂ for the thermal oxidation of ethylene, indicating a strong promotion by doped copper ions. A volcanic behaviour in the catalytic activity was observed with different concentrations of Cu doping, with 1% Cu/TiO₂ showing a 99.5% ethylene conversion at 673 K and 100% selectivity to CO₂. The activity of 1% Cu/TiO₂ remained consistent without deactivation for 24 h. At low dopant concentrations of Cu (0.2 and 1% Cu/TiO₂), the reduction of Cu²⁺ to Cu⁺ was observed. An interplay of oxygen vacancies (O_V), Cu⁺, Cu²⁺ and Ti⁴⁺ may be involved in controlling the activity. DRIFT studies indicated the formation of surface bidendate carbonate as a possible intermediate.