Víctor A. de la Peña O’Shea

Direct evidence of the SMSI decoration effect: the case of Co/TiO2 catalyst

First direct images of cobalt nanoparticles covered by a few atomic layers thick TiO(x) moieties after reduction treatment of a Co/TiO(2) system with the simultaneous formation of Co-O-Ti bonds confirm the development of the SMSI decoration effect.

Electronic and magnetic structure of bulk cobalt: The alpha, beta, and epsilon-phases from density functional theory calculations

The geometric, electronic and magnetic properties of the three metallic cobalt phases: hcp(alpha), fcc(beta), and epsilon(epsilon) have been theoretically studied using periodic density functional calculations with generalized gradient approximation (GGA) and plane wave basis set. These results have been compared with those obtained with GGA+U approach which have shown a noticeable improvement with regard to experimental data. For instance, the cohesive energy values predicted by GGA are overestimated by approximately 25%, whereas GGA+U underestimate them by 14%-17%. On the other hand, magnetic moment values are underestimated in GGA while are overestimated for GGA+U approach by almost the same amount. Besides, the introduction of U parameter gives rise to an electronic redistribution in the d-band structure, which leads to variations in the magnetic properties. Moreover, a higher attention has been paid in the study of the electronic and magnetic properties of the epsilon-phase that has not described previously. These studies show that this phase possesses special properties that could lead to an unusual behavior in magnetic or catalytic applications.

Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures

Photocatalytic H2 generation is investigated over a series of Ga-modified ZnO photocatalysts that were prepared by hydrothermal methods. It is found that the structural, textural, and optoelectronic properties remarkably depend on the Ga content. The photocatalytic activity is higher in samples with Ga content equal to or lower than 5.4 wt %, which are constituted by Zn1-xGaxO phases. Structural, textural, and optoelectronic characterization, combined with theoretical calculations, reveals the effect of Ga in the doped ZnO structures. Higher Ga incorporation leads to the formation of an additional ZnGa2O4 phase with spinel structure. The presence of such a phase is detrimental for the textural and optoelectronic properties of the photocatalysts, leading to a decrease in H2 production. When Pt is used as the cocatalyst, there is an increase of 1 order of magnitude in the activity with respect to the bare photocatalysts. This is a result of Pt acting as an electron scavenger, decreasing the electron-hole recombination rate and boosting the H2 evolution reaction.

The Role of Co-catalysts: Interaction and Synergies with Semiconductors

It is well known that the selection of an appropriate semiconductor is the key to obtain the desired results in a photocatalytic reaction. In the previous chapters, it has been shown which are the requirements for several photocatalytic processes, including pollutant removal, water detoxification or solar fuel production (from water splitting or CO2 photoreduction), and how semiconductor engineering can help to improve the intrinsic physico-chemical and catalytic properties of a single-phase photocatalyst attending to solve the main drawbacks in photocatalytic reactions. However, in some cases, the modification of the photocatalyst with an appropriate co-catalyst leads to an improvement in the photoactivity. The use of co-catalysts leads to (1) a better charge separation and a decrease in the recombination rate, because these co-catalysts act as electron attractors; (2) an enhancement in the activity or a selectivity control in the redox reactions, because of their performance as specific catalytic active sites for reaction evolution; and, in some cases, (3) a broadening in the light operating conditions from UV to visible wavelengths.