Effect of Crystal Phase of MnO₂ with Similar Nanorod-Shaped Morphology on the Catalytic Performance of Benzene Combustion

Four manganese oxides with different crystal structures in the similar nanorod-shaped morphology were synthesized and investigated for the catalytic combustion of benzene. The physicochemical properties of the materials were characterized by N₂ physisorption-desorption, transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectra, O₂ temperature programmed desorption (O₂-TPD) and temperature programmed surface reaction (TPSR). The catalytic activities on benzene combustion over various manganese oxides decreased in the order: γ-MnO₂ > β-MnO₂ > α-MnO₂ > δ-MnO₂. The activation energy of benzene combustion over γ-MnO₂ (67.4 kJ/mol) was lower than that over α-MnO₂ (69.2 kJ/mol), β-MnO₂ (79.1 kJ/mol) and δ-MnO₂ (85.2 kJ/mol). The TPSR results with or without gaseous oxygen revealed that the reaction pathway was occurred via MVK mechanism. The surface adsorbed oxygen species concentration and low temperature O₂ desorption of these manganese oxides, determined by XPS and O₂-TPD, decreased in the order of γ-MnO₂ > β-MnO₂ > α-MnO₂ > δ-MnO₂, are in good agreement with the sequence of their catalytic performance on benzene combustion. It is concluded that higher surface adsorbed oxygen species ratio and better low temperature O₂ desorption were crucial to the superior catalytic performance of the α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂ materials.