Catalytic Methane Combustion on the Hydrothermally Synthesized MnO<sub>2</sub> Nanowire Catalysts
Ehsan Akbari, Seyed Mehdi Alavi, Mehran Rezaei, Afsanehsadat Larimi
Abstract
The various morphologies (wire, rod, tube, and flower) of the MnOx nanocrystalline catalysts were prepared using the hydrothermal and solution methods, and the prepared samples were evaluated for catalytic combustion of methane. The results revealed that the wire-like MnO2 catalyst showed an excellent performance in the methane combustion reaction. The light-off temperature related to 90% of the CH4 conversion was about 400 °C for the wire-like sample, which was higher than those obtained in other catalysts with different morphologies. The influence of the different synthesis factors including the hydrothermal temperature, pH value, aging time, and calcination temperature on the textural properties and activity of the catalysts was investigated. The obtained results confirmed that varying the preparation factors had a considerable effect on the structure and activity of the catalysts. The sample synthesized under the specific conditions (Thydro. = 240 °C, pH = 3, aging time = 24 h, and Tcal. = 400 °C) displayed the highest efficiency in the methane catalytic combustion process. Furthermore, the effect of various operating parameters including the gas hourly space velocity, feed (O2/CH4) ratio, pretreatment condition, type of solvents, and catalytic stability in the existence of CO2 and steam was studied on the catalytic performance. The results revealed that the catalyst stability of the optimal MnO2 sample reduced to some extent with the concurrent concentration of CO2 and water vapor in the reactant feed stream. In the presence of CO2 or/and H2O, the initial CH4 activity at 350 °C was below 20%. However, the long-term stability showed high stability under the dry feed at 450 °C for 50 h.