Importance Of Oxidation State Research Articles

Effect of variable oxidation states of Mn+n ion impregnated TiO2 nanocomposites for superior adsorption and photoactivity under visible light

This paper deals with the importance of oxidation state and ionic size of Mn(II) and Mn(VII) co-catalyst impregnated TiO2 for the observed electrokinetic, adsorption and photocatalytic properties of Mn+n-TiO2 nanocomposites. Due to difference in net electronic charge and ionic size of Mn(II) and Mn(VII) ions, the zeta potential, streaming potential and surface charge demand of Mn(II)–TiO2 and Mn(VII)–TiO2 nanocomposites are greatly varied. The absorption edge of Mn+n-TiO2 nanocatalysts revealed the red shift (515–550 nm) with increased oxidation state (from II to VII) of Mn+n due to 6A1g → 4T1g transition. The photoluminescence (404–530 nm) of Mn(II)–TiO2 is highly quenched relative to Mn(VII)–TiO2 during 340 nm excitation. Further, the HRTEM analysis confirmed the presence of Mn(VII) (∼20–45 nm) and Mn(II) (∼40–60 nm) nanodeposits over TiO2 surface. Binding energies at 639.1 and 640.3 eV showed the presence of Mn in (0) and (II) oxidation state for Mn(II)–TiO2 and 646 eV alongwith a satellite peak for Mn(VII)–TiO2 co-catalyst. As a result, the adsorption and photocatalytic degradation of cationic (Fuchsin blue and methylene blue) and anionic (Salicylic acid and Salicylaldehyde) pollutants by Mn+n-TiO2 nanocomposites are notably improved relative to bare TiO2 depending on their respective oxidation state and surface morphological features.

High catalytic activity of PdOx/MnO2 for CO oxidation and importance of oxidation state of Mn

PdOx/MnO2 has been examined as a catalyst for CO oxidation using a conventional flow reactor at reaction temperatures between 50 and 150°C. In the reaction conditions of GHSV (gas‐hourly‐space‐velocity) of 1.22 × 105/h and CO concentration of 2000 ppm, PdOx/MnO2 showed higher catalytic activity compared with PdOx/Mn2O3, which had been previously reported as an effective catalyst due to the cooperative action of Pd and Mn2O3 for this reaction. The reason for higher activity of PdOx/MnO2 than PdOx/Mn2O3 has been investigated using TPR (temperature‐programmed reduction) and XPS studies. TPR showed that PdOx/MnO2 could be reduced by CO at much lower temperature than PdOx/Mn2O3. During the experiment of reduction and oxidation, XPS showed that the valence of Mn in the PdOx/MnO2 was between 4+ and 3+, which is higher than that of Mn in the PdOx/Mn2O3 catalyst of which the valence has been reported to be between 3+ and 2+. It is known that in this catalyst system the support supplies oxygen onto Pd, where the oxidation occurs with adsorbed CO, and the ability of the support to provide oxygen improves the performance of the catalyst. Therefore, it was concluded that the readiness of MnO2 to be reduced with maintaining a higher oxidation state showed higher CO oxidation activity than Mn2O3 as support for PdOx.