Cecilia B. Mendive

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

Kinetic and mechanistic investigations of multielectron transfer reactions induced by stored electrons in TiO2 nanoparticles: a stopped flow study.

The kinetics and the mechanism of various multielectron transfer reactions initiated by stored electrons in TiO(2) nanoparticles have been investigated employing the stopped flow technique. Moreover, the optical properties of the stored electrons in the TiO(2) nanoparticles have been studied in detail following the UV (A) photolysis of deaerated aqueous suspensions of TiO(2) nanoparticles in the presence of methanol. The reduction of common electron acceptors that are often present in photocatalytic systems such as O(2), H(2)O(2), and NO(3)(-) has been investigated. The experimental results clearly show that the stored electrons reduce O(2) and H(2)O(2) to water by multielectron transfer processes. Moreover, NO(3)(-) is reduced via the transfer of eight electrons evincing the formation of ammonia. On the other hand, the reduction of toxic metal ions, such as Cu(II), has been studied mixing their respective anoxic aqueous solutions with those containing the electrons stored in the TiO(2) particles. A two-electron transfer is found to occur, indicating the reduction of the copper metal ion into its non toxic metallic form. Other metal ions, such as Zn(II) and Mn(II), could not be reduced by TiO(2) electrons, which is readily explained on the bases of their respective redox potentials. The underlying reaction mechanisms are discussed in detail.

Structural properties of mullite-type Pb(Al1–xMnx)BO4

Abstract We report on the structural characterization of the mullite-type PbAl1-xMnxBO4 series using neutron, synchrotron and in-house X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculations. The planar geometry of the BO3 group changes only slightly over the whole composition range. The rigid BO3 group plays the dominant roles in the thermal contraction in the a-direction followed by expansion in the b- and c-directions, leading to a correlation a · b/c ~ unity. The unit-cell volume at zero-pressure and 0 K was obtained, as well evaluated as the isothermal bulk-modulus from pressure dependent synchrotron X-ray diffraction using a diamond anvil cell as well as DFT calculations. Thermal expansion of the metric parameters was modeled using a first-order Grüneisen approximation for the zero-pressure equation of state. We used the double-Debye-double-Einstein- Anharmonicity model to calculate the temperature-dependent internal energy of the crystalline end members. The simulation helped to understand the anisotropic thermal expansion and together with the experimental and calculated bulk moduli to approximate the thermodynamic Grüneisen parameters.

The role of nanoparticulate agglomerates in TiO2 photocatalysis: degradation of oxalic acid

The simultaneous bimodal study of the photocatalytic oxalic acid degradation by aqueous TiO2 suspensions revealed that particular systems possess the capacity to protect a certain amount of oxalic acid from oxidation, thus hindering, to some extent, the photocatalytic reaction. While measurements of the oxalic acid concentration in the bulk liquid phase indicated full photocatalytic degradation; in situ pH-stat measurements allowed the quantification of the amount of oxalic acid remaining in the part of the nanoparticulate agglomerates where light could apparently not access. An explanation for this phenomenon takes into account the possibility of the formation of TiO2 agglomerates in which these molecules are hidden from the effect of the light, thus being protected from photocatalytic degradation. Studies of different TiO2 materials with different particle sizes allowed a deeper exploration of this phenomenon. In addition, because this property of encapsulating pollutant molecules by photocatalytic systems is found to be a reversible phenomenon, P25 appears to be more convenient and advantageous as compared to the use of large surface area photocatalysts.Graphical AbstractFig.: Deaggregation of TiO2 particle agglomerates upon UV illumination.

Liebau density vector: a new approach to characterize lone electron pairs in mullite-type materials

Abstract The bismuth 6s2 lone electron pair (LEP) in mullite-type Bi2M4O9/10 (where M = Al, Fe, or Mn) was characterized by means of several parameters derived from experimental and theoretical calculations. The Wang-Liebau eccentricity (WLE) parameter proved to be very useful to quantify the stereochemical activity of the LEPs. Calculations of electronic distributions (three-dimensional charge density difference isosurfaces) were used as independent measurements, which validated the relevance of the WLE parameter for the characterization and quantification of LEPs. The distribution of the Bi 6s2 electrons around the nucleus was evaluated and the maximum of electron density calculated. The spatial orientation of this electron density with respect to the nucleus is expressed as “Liebau density vector”. Therefore, this vector is ascribed to be a key result of this work as a proof that the purely geometrically defined Wang-Liebau vector indeed points towards the maximum electron density of the LEP. The LEP stereochemical activity was studied in terms of the type of structure (Bi2M4O9 or Bi2M4O10) and the nature of M. The effect of exchanging bismuth by lanthanum as well as the relative stabilities of Bi2M4O9 or Bi2M4O10 structures were calculated and discussed.

Hydrogen Production by Heterogeneous Photocatalysis

The necessity for achieving a sustainable society depending on clean energy systems has led to the development of technology as well as research in renewable energy. One target is the storing of natural energy, such as solar energy, for instance, as transportable fuels. Hydrogen has a huge potential for becoming one of such fuels as long as it can be produced from renewable or vast sources at low costs with the lowest harmful emissions. Therefore, the grand challenge relies on the design of a photocatalytic system that can effectively utilize sunlight as the energy source and water as a substrate to generate hydrogen by the splitting reaction, which leads also to the concomitant generation of oxygen.