Reduction and re-oxidation of molybdena and vanadia: DFT cluster model studies

Behavior of Molybdenum⁻Vanadium Mixed Oxides in Selective Oxidation and Disproportionation of Toluene

This study deals with the behavior of molybdenum–vanadium (Mo/V) mixed oxides catalysts in both disproportionation and selective oxidation of toluene. Samples containing different Mo/V ratios were prepared by a modified method using tetradecyltrimethylammonium bromide and acetic acid. The catalysts were characterized using several techniques: nitrogen adsorption–desorption isotherms, X-Ray diffraction (XRD), ammonia temperature-programmed desorption (TPD-NH₃), temperature-programmed reduction by hydrogen (H₂-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier-transform infrared-spectroscopy (FTIR) and ultraviolet-visible spectroscopies (UV–VIS). The XRD results evidenced the presence of orthorhombic α-MoO₃ and V₂O₅ phases, as well as monoclinic β-MoO₃ and V₂MoO₈ phases, their abundance depending on the Mo to V ratio, while the TPD-NH₃ emphasized that, the total amount of the acid sites diminished with the increase of the Mo loading. The TPR investigations indicated that the samples with higher Mo/V ratio possess a higher reducibility. The main findings of this study led to the conclusion that the presence of strong acid sites afforded a high conversion in toluene disproportionation (Mo/V = 1), while a higher reducibility is a prerequisite to accomplishing high conversion in toluene oxidation (Mo/V = 2). The catalyst with Mo/V = 1 acquires the best yield to xylenes from the toluene disproportionation reaction, while the catalyst with Mo/V = 0.33 presents the highest yield to benzaldehyde.

Enhanced Phosphorus Locking by Novel Lanthanum/Aluminum-Hydroxide Composite: Implications for Eutrophication Control

Lanthanum (La) bearing materials have been widely used to remove phosphorus (P) in water treatment. However, it remains a challenge to enhance phosphate (PO₄) adsorption capacity and La usage efficiency. In this study, La was coprecipitated with aluminum (Al) to obtain a La/Al-hydroxide composite (LAH) for P adsorption. The maximum PO₄ adsorption capacities of LAH (5.3% La) were 76.3 and 45.3 mg P g^-1 at pH 4.0 and 8.5, which were 8.5 and 5.3 times higher than those of commercially available La-modified bentonite (Phoslock, 5.6% La), respectively. P K-edge X-ray absorption near edge structure analysis showed that PO₄ was preferentially bonded with Al under weakly acid conditions (pH 4.0), while tended to associate with La under alkaline conditions (pH 8.5). La L_III-edge extended X-ray absorption fine structure analysis indicated that PO₄ was bonded on La sites by forming inner sphere bidentate-binuclear complexes and oxygen defects exhibited on LAH surfaces, which could be active adsorption sites for PO₄. The electrostatic interaction, ligand exchange, and oxygen defects on LAH surfaces jointly facilitated PO₄ adsorption but with varied contribution under different pH conditions. The combined contribution of two-component of La and Al may be an important direction for the next generation of commercial products for eutrophication mitigation.

Density-functional studies of hydrogen peroxide adsorption and dissociation on MoO3(100) and H0.33MoO3(100) surfaces

Hydrogen peroxide (H₂O₂) adsorption and dissociation mechanisms on MoO₃(100) and H_0.33MoO₃(100) surfaces were studied by means of density-functional computations.

Relative stability of low-index V(2)O(5) surfaces: a density functional investigation

Ab initio density functional calculations of the structural and electronic properties of V(2)O(5) bulk and its low-index surfaces are presented. For the bulk oxide and the (010) surface (the natural cleavage plane) a good agreement with experiment and with earlier ab initio calculations is found. For the first time, the investigations are extended to other low-index surfaces: (001) and (100). On both surfaces, termination conserving a bulk-like stoichiometry is preferred, but-in contrast to the (010) surface-a strong structural relaxation takes place. Relaxation reduces the surface energy from 1.16 to 0.48 J m(-2) for the (001) and from 0.61 to 0.55 J m(-2) for the (100) surface. Although the relaxed surface energies are still one order of magnitude higher than calculated for the (010) surface (0.047 J m(-2)), the Wulff construction demonstrates that (001) and (100) surfaces contribute about 15% of the total surface area of a V(2)O(5) crystallite, indicating a non-negligible role in the catalytic activity of V(2)O(5).

Quantum Chemical Study of Mechanisms for Oxidative Dehydrogenation of Propane on Vanadium Oxide

We have carried out a hybrid density functional study of mechanisms for oxidative dehydrogenation of propane on the (010) surface of V2O5. The surface was modeled using both vanadium oxide clusters and a periodic slab. We have investigated a Mars-van Krevelen mechanism that involves stepwise adsorption of the propane at an oxygen site followed by desorption of a water molecule and propene, and subsequent adsorption of an oxygen molecule to complete the catalytic cycle. The potential energy surface is found to have large barriers, which are lowered somewhat when the possibility of a triplet state is considered. The barriers for propane adsorption and propene elimination are 45-60 kcal/mol. The highest energy on the potential energy surface at the B3LYP/6-31G* level of theory is about 80 kcal/mol above the energy of the reactants and corresponds to formation of an oxygen vacancy after water elimination. Subsequent addition of an oxygen molecule to fill the vacancy is predicted to be energetically downhill. The reactions of propane at a bridging oxygen site and at a vanadyl site have similar energetics. The key results of the cluster calculations are confirmed by periodic calculations. Factors that may lower the barriers on the potential energy surface, including the interaction of vanadium oxide clusters with a support material and a concerted reaction with O2, are discussed.