How the addition of atomic hydrogen to a multiple bond can be catalyzed by water molecules.

Observational data show complex organic molecules in the interstellar medium (ISM). Hydrogenation of small unsaturated carbon double bond could be one way for molecular complexification. It is important to understand how such reactivity occurs in the very cold and low-pressure ISM. Yet, there is water ice in the ISM, either as grain or as mantle around grains. Therefore, the addition of atomic hydrogen on double-bonded carbon in a series of seven molecules have been studied and it was found that water catalyzes this reaction. The origin of the catalysis is a weak charge transfer between the π MO of the unsaturated molecule and H atom, allowing a stabilizing interaction with H2O. This mechanism is rationalized using the non-covalent interaction and the quantum theory of atoms in molecules approaches.

The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H2CCO ice versus hydrogenation of mixed H2CCO/H2O ice at 10 K.

Water ice plays an important role in reactions taking place on the surface of interstellar ice grains, ranging from catalytic effects that reduce reaction barrier heights to effects that stabilize the reaction products and intermediates formed, or that favor one reaction pathway over another, passing through water-involvement in the reaction to produce more complex molecules that cannot be formed without water or water-derived fragments H, O and OH. In this context, we have combined experimental and theoretical studies to investigate ketene (CH2CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH2CO and CH2CO/H2O ices. We show in the present study that with or without water, the ketene molecule reacts with H atoms to form four reaction products, namely CO, H2CO, CH4 and CH3CHO. Based on the amounts of CH2CO consumed during the hydrogenation processes, the CH2CO + 2H reaction appears to be more efficient in the presence of water. This underlines the catalytic role of water ice in reactions occurring on the surface of interstellar ice grains. However, if we refer to the yields of reaction products formed during the hydrogenation of CH2CO and CH2CO/H2O ices, we find that water molecules favor the reaction pathway to form CH3CHO and deactivate that leading to CH4 and H2CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H2O on the CH2CO + H reaction, whose potential energy barrier drops from 4.6 kcal mol-1 (without water) to 3.8 and 3.6 kcal mol-1 with one and two water molecules respectively.

Formation of CO, CH4, H2CO and CH3CHO through the H2CCO + H surface reaction under interstellar conditions.

The reaction of ketene (H2CCO) with hydrogen atoms has been studied under interstellar conditions through two different experimental methods, occurring on the surface and in the bulk of H2CCO ice. We show that ketene interaction with H-atoms at 10 K leads mainly to four reaction products, carbon monoxide (CO), methane (CH4), formaldehyde (H2CO) and acetaldehyde (CH3CHO). A part of these results shows a chemical link between a simple organic molecule such as H2CCO and a complex one such as CH3CHO, through H-addition reactions taking place in dense molecular clouds. The H-addition processes are very often proposed by astrophysical models as mechanisms for the formation of complex organic molecules based on the abundance of species already detected in the interstellar medium. However, the present study shows that the hydrogenation of ketene under non-energetic conditions may also lead efficiently to fragmentation processes and the formation of small species such as CO, CH4 and H2CO, without supplying external energy such as UV photons or high energy particles. Such fragmentation pathways should be included in the astrophysical modeling of H2CCO + H in the molecular clouds of the interstellar medium. To support these results, theoretical calculations have explicitly showed that, under our experimental conditions, H-atom interactions with the CC bond of ketene lead mainly to CH3CHO, CH4 and CO. By investigating the formation and reactivity of the reaction intermediate H3C-CO radical, our calculations demonstrate that the H3C-CO + H reaction evolves through two barrierless pathways to form either CH3CHO or CH4 and CO fragments.

Composition dependent reactivity of titanium oxide clusters.

Based on first-principles calculations of titanium oxide clusters, TinOm (n = 1-4), we reveal the composition dependent reactivity of titanium oxide clusters. Our interesting results include: (1) the reactivity depends on the ratio of O atoms in the clusters, with smaller O ratios associated with higher reactivity; (2) among the different titanium oxide species investigated, the most stable structures are TinO2n, but their reactivities are relatively lower than the clusters with a smaller O atom ratio; moreover, (3) when the O atom ratio is small, the reactivity required to form the Ti-Ti bond is larger than either the Ti-O or O-O bond between two interacting titanium oxide clusters. These results will be useful for designing efficient titanium oxide catalysts, or photocatalysts, in particular, for energy and environmental applications.

CO dissociation on magnetic Fe(n) clusters.

This work theoretically investigates the CO dissociation on Fen nanoparticles, for n in the range of 1-65, focusing on size dependence in the context of the initial step of the Fischer-Tropsch reaction. CO adsorbs molecularly through its C-end on a triangular facet of the nanoparticle. Dissociation becomes easier when the cluster size increases. Then, the C atom is bonded to a square facet that is generated as a result of the adsorption if it does not yet exist in the bare cluster, while the O atom is adsorbed on a triangular facet. In the most stable situation, the two adsorbed atoms remain close together, both having in common one shared first-neighbor iron atom. There is a partial spin quenching of the neighboring Fe atoms, which become more positively charged than the other Fe atoms. The shared surface iron atom resembles a metal-cation from a complex. Despite the small size of the iron cluster considered, fluctuations due to specific configurations do not influence properties for n > 25 and global trends seem significant.

Modeling localized photoinduced electrons in rutile-TiO2 using periodic DFT+U methodology.

We propose a theoretical model for photocatalytic processes on titanium dioxide, described by its most stable phase and surface, rutile-TiO(2)(110). The excitation induced by light promotes electrons from the valence band to the conduction band. In this context, one important requirement is having a correct value of the magnitude of the electronic gap. The use of GGA+U or LDA+U functional with an appropriate U value allows this. The U correction has little consequence on the adsorption strength itself on the TiO(2)(110) surface. For the ground state, it only yields a slight increase of the interaction strength of some test molecules; the surface basicity is somewhat enhanced. This is interpreted by the shift of TiO(2) vacant levels. Photoexcitation is taken into account by imposing two unpaired electrons per cell of the same spin. The size of the cell therefore determines the number of excitations per surface area; the larger the cell, the smaller the electron-hole surface concentration and the smaller the energy for electronic excitation. For the excited state, careful attention must be focused on the localization of the excited electron and of the hole which are crucial for the determination of the lowest electronic states and for the surface reactivity. We found that the excited electron is localized on a pentacoordinated surface titanium atom while the hole is shared by two surface oxygen atoms not too far from it. The electronic levels associated to the reduced titanium atoms are low in energy; the projected density of states is superposed onto the valence band.