Synthesis and catalytic activity of copper and iron oxide based catalysts in carbon dioxide methanation

Abstract

The high content of carbon dioxide (CO2) in sour crude natural gas can cause damage to the pipeline system and reduce natural gas quality. Green technology via catalytic methanation reaction was found to be the best method for sour gas sweetening, whereby methane (CH4) is produced thus increasing the gas quality. In this study, the manganeseruthenium (Mn/Ru) oxide catalysts were modified through the addition of copper (Cu), chromium (Cr), iron (Fe), vanadium (V) and zinc (Zn) to produce excellent methanation catalysts. The catalysts were prepared via the wet impregnation method, followed by ageing process for one day and calcination at various temperatures for 5 hours, and tested on simulated natural gas (CO2/H2) using a Pyrex glass reactor with an internal diameter of 10 mm at atmospheric pressure. The catalysts have undergone several optimizations such as calcination temperatures, various loading amount of catalysts, weight hourly space velocity (WHSV) as well as reproducibility, regenerability and stability testing. The results showed that Ru/Mn/Cu(10:30:60)-Al2O3 catalyst calcined at 1000oC was the most active, with 98.5% CO2 conversion and 19.7% CH4 yield achieved at 220oC. The second most active catalyst was Ru/Mn/Fe(5:35:60)-Al2O3 with 93.2% CO2 conversion and 19.2% CH4 yield achieved at 270oC. The Cu based catalyst was verified by response surface methodology-central composite design (RSM-CCD) and the optimum conditions were with the loadings of 60% of Cu, 29.5% of Mn and 10.5% of Ru at calcination temperature of 1010oC with 1200 mL/g-1h-1 WHSV to achieve the 96.6% of CO2 conversion, while the experimental result gave 98.5% CO2 conversion, which was 1.9% higher than the suggested value. For Fe based catalyst, the optimum conditions were with the loadings of 60% of Fe, 34.5% of Mn and 5.5% of Ru at calcination temperature of 1010oC with 1200 mL/g-1h-1 WHSV to achieve the 96.6% of CO2 conversion, while the experimental result gave 95.5% which was 1.1% less than the suggested value. Analysis of the results of the characterization by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) for Cu based catalyst showed the active species were RuO2, Mn3O4 and CuO, while field emission scanning electron microscopy (FESEM) assigned the presence of small particles that were homogeneously distributed. For Fe based catalyst, the active species were RuO2, Mn3O4 and Fe3O4 with small particles that distributed homogeneously on the catalyst surface as shown in the FESEM micrograph. Energy dispersion X-ray (EDX) analysis for both catalysts also confirmed the presence of all elements in the prepared catalysts. From the nitrogen adsorption (NA) analysis, a higher surface area and macroporous property of the materials may have contributed to the higher catalytic activity. Temperature programmed desorption (TPD) results also confirmed that both catalysts showed superior performance for sorption of CO2, while the temperature programmed reduction (TPR) gave reduction sites at lower temperatures. The Ru/Mn/Cu(10:30:60)-Al2O3 catalyst was more efficient towards CO2 conversion, exhibited good reliability and reproducibility as well as regenerability compared to the Ru/Mn/Fe(5:35:60)-Al2O3 catalyst. Furthermore, the mechanistic study by Fourier transform infrared (FTIR) spectroscopy suggested that Cu based catalyst has more tendency to form bridged bidentate carbonate and bidentate carbonate species, whereas the Fe based catalyst has more tendency to form monodentate species in the initial state, then forming the formate when it was hydrogenated, and to finally release methane.