Article pubs.acs.org/IECR
Effect of Mg, Ca, and Sr on CeO2 Based Catalysts for the Oxidative Coupling of Methane: Investigation on the Oxygen Species Responsible for Catalytic Performance Víctor J. Ferreira,† Pedro Tavares,‡ José L. Figueiredo,† and Joaquim L. Faria*,† †
LCM − Laboratory of Catalysis and Materials − Associate Laboratory LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal ‡ Centro de Química − Vila Real, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal S Supporting Information *
ABSTRACT: CeO2 catalysts modified with earth alkaline metals (M = Mg, Ca, and Sr) were prepared by the citrate method in a M:Ce molar ratio of 1:1. Different analytical techniques including diffuse reflectance Ultraviolet−Visible (DR UV−vis) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature programmed desorption of CO2 (TPD-CO2) were used for the characterization of the catalysts. The materials were tested in the oxidative coupling of methane (OCM). The results of calcined samples revealed that the charge transfer from cerium to oxygen ions is shifted when CeO2 is doped, indicating defects on the catalyst surface. A linear correlation was found between the amount of surface basic sites and the ratio of the oxygen species O2− and O22− to lattice oxygen. This behavior is crucial for the CH4 conversion and selectivity to C2H6 and C2H4 at 700 °C. Ca-doped CeO2 catalyst revealed the best performance in the OCM, which can be attributed to the similar ionic radii of Ca2+ and Ce4+. systems.3 The O22− peroxide ion was also proposed to be an active species, especially at temperatures higher than 750 °C on catalytic systems such as Na2O2,4 Ba/MgO, Na/La2O3, and La2O3.5 On the other hand, superoxide species O2− were observed on catalysts such as LaOF,6 Ba/Nd2O3,7 and Y2O3− CaO.8 The influence of cerium on the oxide systems investigated for the oxidative coupling of methane is of particular interest because of its excellent redox properties. Cerium added to Li/MgO and Na/MgO was studied by Radha et al.,9 who observed improved selectivity and activity. This was attributed to the presence of different active sites. Shahri et al. showed that 5 wt % Ce promotes the activity and stability of the Mn−Na2WO4/SiO2 catalyst. However, from the catalytic point of view, other studies revealed that CeO2 exhibited low selectivity to coupled hydrocarbon products,10 and Choudhary et al. studied the influence of the rare earth oxide(s) precoated on catalyst supports on the conversion, selectivity, yield, and product C2H4/C2H6 and CO/CO2 ratios. They reported a poor performance when CeO2 was precoated on the commercial support (CH4 conversion
