Enhanced adsorption and degradation of methylene blue over mixed niobium-cerium oxide - Unraveling the synergy between Nb and Ce in advanced oxidation processes.

Formation of reactive oxygen species (ROS) via H2O2 activation is of vital importance in catalytic environmental chemistry, especially in degradation of organic pollutants. A new mixed niobium-cerium oxide (NbCeOx) was tailored for this purpose. A thorough structural and chemical characterization of NbCeOx along with CeO2 and Nb2O5 reference materials was carried out using TEM/STEM/EDS, SEM, XRD, XPS, EPR, UV-vis and N2 physisorption. The ability of the catalysts to activate H2O2 towards ROS formation was assessed on the basis of EPR and Raman measurements. Catalytic activity of the oxides was evaluated in degradation of methylene blue (MB) as a model pollutant. Very high activity of NbCeOx was attributed to the mixed redox-acidic nature of its surface, which originated from the synergy between Nb and Ce species. These two properties (redox activity and acidity) ensured convenient conditions for efficient activation of H2O2 and degradation of MB. The activity of NbCeOx in MB degradation was found 3 times higher than that of the commercial Nb2O5 CBMM catalyst and 240 times higher than that of CeO2. The mechanism of the degradation reaction was found to be an adsorption-triggered process initiated by hydroxyl radicals, generated on the surface via the transformation of O2-•/O22-.

The impact on the ring related vibrational frequencies of pyridine of hydrogen bonds with haloforms - a topology perspective.

Hydrogen bonds between pyridine (Py) and haloforms (CHX3, X = F, Cl, Br, I) and their impact on the ring related vibrational frequencies of pyridine were studied using a combination of solution phase FTIR and quantum mechanical DFT and ab initio calculations. With various possibilities for dimers that could potentially be formed between pyridine and haloforms, the calculations identified an intermolecular ring structure, which was established based on both the [Py-]N-involved hydrogen bond and the hydrogen bond between the alpha H on pyridine ([Py-]H) and the halogen atom on the haloform ([CHX2-]X), as the most energetically stable form. The formation of a ring between the two molecules makes the entire ring structure more rigid on one hand, and weakens the [Py-]N-involved hydrogen bond on the other hand. As a result, no significant shift was observed for ν12, and ν10 only experiences a moderate blue shift upon hydrogen bonding. The magnitude of the shift in ν10 is in the order: CHI3 > CHBr3 > CHCl3 > CHF3, according to calculations. FTIR experiments with pyridine and CHCl3/CHBr3/CHI3 in cyclohexane solution showed a consistent sequence. Strong correlation was observed between the values of ν10 and the various interatomic distances among [Py-]N, [Py-]H, [CHX2-]X and [CX3-]H, as well as other topological parameters involving the two bond critical points (BCP1 and BCP2) and the ring critical point (RCP). The percentage contributions from the internal coordinates were also estimated and were closely related to the magnitude of ν10. Moreover, the occupied frontier molecular orbitals of the hydrogen bonding complexes (from HOMO-4 to HOMO) were analyzed to explain their roles in the pyridine ring vibrations and their sensitivity to hydrogen bonding.