Total Oxidation of Formaldehyde over MnOx-CeO2 Catalysts: The Effect of Acid Treatment
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Abstract
The effect of acid treatment in mixed MnOx–CeO2 samples has been investigated in the catalytic total oxidation of formaldehyde. The acid treatment has no effect on the textural and redox properties of the materials when Mn is stabilized in a MnOx–CeO2 solid solution (Mn content below 50%). However, these properties were found to be highly altered by acid treatment when the solubility limit of Mn in the ceria was exceeded (Mn content above 50%). This enabled access to the primary porosity and oxidized the manganese species to a higher oxidation state via a Mn dismutation reaction. As a result, the catalytic activity of pure manganese oxide, after chemical activation, in the oxidation of formaldehyde is greatly improved—at 100 °C, the conversion of formaldehyde is increased by a factor of 5 and the corresponding intrinsic reaction rate by 1.4. Combined in situ surface analysis unambiguously identified formate species as a result of formaldehyde oxidation at room temperature on the chemically activated pure MnOx. The evolution of various surface species was monitored by increasing the temperature and in situ FTIR, and XPS results provided direct evidence of the desorption of monodentate formate species into formaldehyde and the oxidation of bidentate-bridging formate species. Changes in the average oxidation state of surface manganese confirmed the participation of oxygen from MnOx in the formation of formate species at room temperature and their transformation into CO2 and H2O when increasing the temperature.
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