Kinds Of Possible Modes Research Articles

Investigation on adsorption and decomposition of H2S on Pd (1 0 0) surface: A DFT study

Density functional theory (DFT) together with periodic slab models was employed to investigate the adsorption and decomposition of H2S on the Pd (1 0 0) surface. Different kinds of possible modes of H2S and HS species adsorbed on the surface were identified. It was found that H2S prefers to adsorb on the top sites, while HS, S and H adsorb preferentially on the hollow sites. Additionally, the optimum co-adsorption configurations for HS/H and H/S were determined. It was revealed that the co-adsorbed species repel each other slightly on the Pd (1 0 0) surface. Finally, the potential energy profile of H2S dissociation on the Pd (1 0 0) surface was constructed. The energy barriers of the first and second H-S bond scission are similarly low, implying that the dissociation of H2S on the Pd (1 0 0) surface is facile both kinetically and thermodynamically. The distinct differences over Pd (1 1 1) and Pd (1 0 0) surface suggests that the decomposition of H2S over Pd-based catalyst is a structure-sensitive reaction.

Insight into the adsorption and decomposition mechanism of H2S on clean and S-covered Au (1 0 0) surface: A theoretical study

Abstract Density functional theory (DFT) was employed to investigate the adsorption and decomposition of H2S on clean and S-covered Au (1 0 0) surface. Different kinds of possible modes of H2S, as well as HS, S and H adsorbed on both surfaces were considered. It was proposed that H2S prefers to adsorb on the top sites, HS and H adsorb preferentially on the bridge sites, while S atom is opt to occupy the hollow site. In addition, the optimum co-adsorption configurations for HS/H, H/S, H2S/S, HS/S, HS/HS and H/S/S were determined. It was revealed that the co-adsorbed species repel each other slightly on Au (1 0 0) surface and that co-adsorption tends to weaken the adsorbate–substrate interaction due to the presence of atomic sulfur. Finally, the pathways and transition states of H2S dissociation were also investigated. Through comparing the activation barrier on different surfaces, it was found that the presence of S atom can facilitate the first H S bond scission and go against the second dissociation of HS species. The distinct differences between Au (1 0 0) and Au (1 1 1) surface suggest that the decomposition of H2S over Au-based catalyst is a structure-sensitive reaction.

Fundamental studies about the interaction of water with perfect, oxygen-vacancy and pre-covered oxygen Cu2O(1 1 1) surfaces: Thermochemistry, barrier, product

The adsorption and dissociation of H2O on three types of Cu2O(111) surfaces, including perfect, oxygen-vacancy and pre-covered oxygen surfaces have been systematically investigated using periodic density functional slab model calculations. Different kinds of possible modes of H2O adsorbed on those surfaces are identified. Our results first show that the bonding of dissociated species (HO, H and O) for H2O is substantially stronger than that of H2O on Cu2O(111) surface. Then, on perfect surface, H2O mainly exist in the form of molecular adsorption, in which CuCUS site is chemisorption, OSUF site is physisorption with a weak hydrogen bond. On oxygen-vacancy surface, dissociative adsorption of H2O occurs predominantly, suggesting that oxygen-vacancy exhibits a strong catalytic activity toward H2O dissociation; meanwhile, a small quantity of chemisorption H2O adsorbed at CuCUS site also exists. On pre-covered oxygen surface, H2O is typical of physisorption due to a hydrogen bond interaction between H of adsorbed H2O and pre-covered oxygen. Further, the dissociation mechanisms of molecular adsorption H2O, OH from dissociative adsorption H2O, as well as single OH group on different surfaces, leading to the final products H and O atoms, give out the thermochemistry, barrier of each elementary reaction, which indicates that OH species is energetically the most stable product on those surfaces. Pre-covered oxygen on Cu2O(111) surface can act as a promoter to facilitate the first dehydrogenation of molecular adsorption H2O leading to OH species, which is more favorable both thermodynamically and kinetically than that of molecular adsorption H2O on other two surfaces. Finally, the vibrational frequencies for the adsorbed H2O and OH species on Cu2O(111) surfaces can be applied to guide surface vibrational spectroscopy in experiment. Our calculation may be a worthwhile theoretical example for the interaction of H2O with other metal oxide surface.

A mechanistic study of H2S adsorption and dissociation on Cu2O(1 1 1) surfaces: Thermochemistry, reaction barrier

The interaction mechanism of H2S with different Cu2O(111) surfaces, including perfect, oxygen-vacancy and sulfur-containing surfaces, have been systematically studied using periodic density functional calculations. Different kinds of possible modes of H2S, as well as the resultant SH and S species adsorbed on these surfaces are identified. Two types of pathways via molecular and dissociative adsorption processes are mapped out. Our results show that sulfur species (H2S, SH and S) interact with surface Cu centers; H2S exists in the form of molecular adsorption on perfect and sulfur-containing surfaces; the dissociative adsorption of H2S occurs predominantly on oxygen-vacancy surface, suggesting that oxygen-vacancy exhibits a strong catalytic activity toward the dissociation of H2S. On the other hand, the dissociation processes of the molecular and dissociative adsorption H2S, leading to final product S species on these Cu2O(111) surfaces, show that the overall dissociation process is exothermic. Meanwhile, with respect to molecular adsorption H2S, the activation barrier and reaction energy of the overall dissociation process on perfect and oxygen-vacancy surfaces indicate that H2S can easily dissociate into S species. Importantly, in the case of dissociative adsorption of H2S, the dissociation of H2S into S species is a spontaneous process with respect to molecular adsorption H2S. However, on sulfur-containing surface, the presence of surface S atom goes against the HS bond-breaking process both thermodynamically and kinetically. Finally, the vibrational frequencies for the adsorbed H2S, SH and S species on these surfaces have been obtained, which can be applied to guide surface vibrational spectroscopy in experiment.

Adsorption and dissociation of O 2 on CuCl(1 1 1) surface: A density functional theory study

The adsorption and dissociation of O 2 on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O 2 adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O 2 lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O–O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O 2 adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O 2 −), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O 2 dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.

Adsorption and dissociation of O 2 on the Cu 2O(1 1 1) surface: Thermochemistry, reaction barrier

The adsorption and dissociation of O 2 on the perfect and oxygen-deficient Cu 2O(1 1 1) surface have been systematically studied using periodic density functional calculations. Different kinds of possible modes of atomic O and molecular O 2 adsorbed on the Cu 2O(1 1 1) surface are identified: atomic O is found to prefer threefold 3Cu site on the perfect surface and O vacancy site on the deficient surface, respectively. Cu CUS is the most advantageous site with molecularly adsorbed O 2 lying flatly over singly coordinate Cu CUS–Cu CSA bridge on the perfect surface. O 2 adsorbed dissociatively on the deficient surface, which is the main dissociation pathway of O 2, and a small quantity of molecularly adsorbed O 2 has been obtained. Further, possible dissociation pathways of molecularly adsorbed O 2 on the Cu 2O(1 1 1) surface are explored, the reaction energies and relevant barriers show that a small quantity of molecularly adsorbed O 2 dissociation into two O atoms on the deficient surface is favorable both thermodynamically and kinetically in comparison with the dissociation of O 2 on the perfect surface. The calculated results suggest that the presence of oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of O 2 and can obviously improve the catalytic activity of Cu 2O, which is in agreement with the experimental observation.

Adsorption and dissociation of H 2 on the Cu 2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu 2O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H 2 adsorbed on the Cu 2O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O SUF, Cu CUS and O vacancy sites are the adsorption active centers for H adsorbed on the Cu 2O(1 1 1) surface, and for H 2 adsorption over perfect surface, Cu CUS site is the most advantageous position with the side-on type of H 2. For H 2 adsorption over deficient surface, two adsorption models of H 2, H 2 adsorbing perpendicularly over O vacancy site and H 2 lying flatly over singly-coordinate Cu–Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu CUS atom, suggesting that the dissociative adsorption of H 2 is the main dissociation pathway of H 2 on the Cu 2O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu 2O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H 2.