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

The study investigates the relationship between the surface basicity and catalytic activity for C 2 + formation over equimolecular mixtures of alkaline-earth oxides with Nd 2O 3. The experimental results led to the idea that methane is converted to products on two distinct types of active sites by independent pathways. The synergetic effect between alkaline-earth oxide and a series of rare-oxide on the OCM catalytic activity have been also studied. The work investigates the relationship between the surface basicity and catalytic activity for C 2 + formation over equimolecular mixtures of alkaline-earth oxides (BeO, MgO, CaO and SrO) with Nd 2O 3. 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 C 2 + was determined by measuring the amount of evolved CO 2 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 C 2 +. The turnover frequency (TOF) values were calculated by quantitatively determining the total number basic sites retaining CO 2. 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 C 2 + 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.