On the nature of active sites and catalytic activity for OCM reaction of alkaline-earth oxides-neodymia catalytic systems Original Research Article

The work investigates the relationship between the surface basicity and catalytic activity for C2+ formation over equimolecular mixtures of alkaline-earth oxides (BeO, MgO, CaO and SrO) with Nd2O3. The concentration as well as the evolution of surface carbonate species with temperature, reflecting the basic properties of mixed oxides, was analyzed by TPD, XPS and IR methods to obtain information on the reaction mechanism. The concentration of surface basic sites contributing to the formation of C2+ was determined by measuring the amount of evolved CO2 in the 300–820 °C temperature range. The experimental data showed that the higher was the catalyst basicity the better was the efficiency for selectively converting methane to C2+. The turnover frequency (TOF) values were calculated by quantitatively determining the total number basic sites retaining CO2. The experimental results led to the idea that methane is converted to products on two types of active sites by independent pathways. The alkaline active sites are responsible for C2+ formation whereas the acidic sites with low affinity for carbonate formation are responsible for methane combustion. The synergetic effect between a selected alkaline-earth oxide (MgO) and a series of rare-oxide on the OCM catalytic activity has been also investigated.