Jose Gracia

First principles study of C3N4 carbon nitride nanotubes

We investigate the structural and optical properties of carbon nitride (C3N4) nanotubes (CNNts) built from condensed heptazine cores (C6N7) with different chirality and connection patterns. In particular, heterocycles in a hexagonal ordering exhibit the lowest energy configuration for the CNNts studied. Overall, heptazine-based CNNts are energetically preferred over triazine motifs. Correspondingly, recent experimental data show the prevalence of heptazine units in synthesized CNNts. Nitrogen-nitrogen lone pair repulsions prevent CNNts from presenting smooth tubular surfaces. Corrugation in general stabilizes C3N4 nano-structures with respect to extended conformations in comparison with pure carbon compositions. In connection to experiment, hexagonal nanotubes show optical properties that are almost independent of the chirality and tube diameter. CNNts show potential for similar applications as carbon nanotubes, and may even improve on the performance in some specific areas, as they have stable semiconducting parameters, and have polarized C-N bonds together with surface holes, which suggest them as better hosts as storage devices.

Chemistry of O- and H-Containing Species on the (001) Surface of Anatase TiO2: A DFT Study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO(2) (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H(2)O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO(2) surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O(2) adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O(2). In the adsorbed OOH, the O-O bond is considerably lengthened and thus weakened.

The beneficial effect of hydrogen on CO oxidation over Au catalysts : a computational study

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O2 hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O–O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O2 and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H2O and CO2. CO is selectively oxidized in a H2 atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.

On the Role of Ferromagnetic Interactions in Highly Active Mo-Based Catalysts for Ammonia Synthesis

Reactions involving nitrogen fixation and transfer are of great industrial interest. In this regard, unveiling all the physical principles that determine their activity would be enormously beneficial for the rational design of novel catalysts with improved performance. Within this context, this work explores the activity of bulk molybdenum-based transition metal nitrides in ammonia synthesis. Our results highlight that the most active compositions show increasing ferromagnetism in the metal-nitrogen bonds, which constitute the active sites. We observe that the total spin accumulated in the bonds at the active sites is a physically meaningful descriptor to discriminate optimum catalysts. Higher activities are associated with ferromagnetic phases, and the underlying reason is an enhanced overlapping of the electronic wavefunctions; which also make the reaction steps spin-sensitive. These finding provides strong evidence of the general influence of electrons magnetic moment in catalysis, being part of the specific field of spintro-catalysis.