Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S–O bonds

Synthesis of Magnesium Oxide Nanoplates and Their Application in Nitrogen Dioxide and Sulfur Dioxide Adsorption

In this research, nanostructured magnesium oxide was synthesized through the sol-gel calcination or hydrothermal calcination method using various surfactants. The X-ray diffraction pattern of the materials confirmed that all the prepared magnesium oxide samples were single phase without any impurity. The scanning electron microscopy images and specific surface area values showed that sodium dodecyl sulfate was the most suitable surfactant for the synthesis of magnesium oxide nanoplates with the diameter of 40–60 nm, the average thickness of 5 nm, and a specific surface area of 126 m2/g. This material was utilized for nitrogen dioxide and sulfur dioxide adsorption under ambient condition. The saturated adsorption capacities of magnesium oxide were 174 mg/g for nitrogen dioxide and 160 mg/g for sulfur dioxide, making the magnesium oxide nanoplates a promising candidate for toxic gas treatment.

The Adsorption Properties of Cu and Ni on the Ceria(111) Surface

First-principles electronic structures calculations of the adsorption properties of Cu and Ni on the ceria(111) surface are presented. The adatoms (Cu, Ni) are adsorbed strongly at the hollow site on the CeO2(111) support. Metal induced gap states (MIGS) appear in the O2p-Ce4f gaps and the Cu and Ni adatoms are oxidized to Cu+ and Ni+ mainly by their next nearest neighbor Ce ion, which experiences a conversion of Ce4+→Ce3+. The bonding mechanisms for the Cu-ceria(111) and Ni-ceria(111) systems are proposed.

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.

Adsorption and Reaction of SO2 with a Polycrystalline UO2 Film: Promotion of S−O Bond Cleavage by Creation of O-Defects and Na or Ca Coadsorption

To characterize UO(2) for its possible use in desulfurization applications, the interactions of molecular sulfur dioxide (SO(2)) with a polycrystalline uranium dioxide film have been studied by means of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy ion scattering (LEIS). The stoichiometric, oxygen-deficient, calcium-precovered and sodium-precovered UO(2) surfaces have been characterized. The changes in oxide reactivity upon creation of oxygen vacancies and coadsorption of sodium and calcium have been studied. After creation of a reduced UO(2-x) surface (x approximately 0.44) via Ar(+) sputtering, the U 4f XPS spectrum shows conspicuous differences that are good indicators of the surface stoichiometry. Molecular SO(x) formation (x = 2-4) is observed after SO(2) deposition onto stoichiometric UO(2) and onto UO(2) precovered with small amounts (<1 ML) of Na or Ca; complete dissociation of SO(2) is not observed. Heating leads to desorption of the SO(x) species and to transformation of SO(2) to SO(3) and SO(3) to SO(4). On oxygen-deficient UO(2) and on UO(2) precovered with large Na or Ca coverages (> or =4 ML), both the formation of SO(x)= species and complete dissociation of SO(2) are observed. A higher thermal stability of the sulfur components is observed on these surfaces. In all cases for which dissociation occurs, the XPS peak of atomic sulfur shows similar structure: three different binding states are observed. The reactivity of oxygen-deficient UO(2) and sodium- and calcium-precovered UO(2) (coverages > or = 4 ML) is attributed to charge transfer into the antibonding LUMO of the adsorbed molecule.

The adsorption and dissociation of Cl2 on the MgO (001) surface with vacancies: Embedded cluster model study

The adsorption of Cl(2) at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F(+), or F(2+) center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption of Cl(2) at the edge and corner oxygen sites and the results show that Cl(2), energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl(2) on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl(2) approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl(2) adsorption at the oxygen vacancy site (F, F(+), or F(2+) center), both large adsorption energies and rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital of Cl(2). It suggests, at the oxygen vacancy site, that Cl(2) prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl(2) to atomic Cl is virtually barrierless.

Effects of carbon on the stability and chemical performance of transition metal carbides: A density functional study

Density functional theory was employed to study the stabilities and chemical activities of transition metal carbides. Here we take the well-known Mo carbides and Ti carbides as an example. Different kinds of structures including the bulk surfaces [Mo(2)C(001), MoC(001), and TiC(001)] and metcars [Mo(8)C(12) and Ti(8)C(12)] are taken into consideration. Systematic studies show that by raising the C coordination number of the metal atoms in the carbides, in general the stability of the carbides increases (metcars are an exception since they include both high-coordinated and low-coordinated metal atoms.); at the same time, the chemical activities of the carbides decrease due to a downshift of the metal d-band center (ligand effect). Considering the better catalysts those that combine high stability and moderate chemical activity, our results suggest that the catalytic potential of Mo carbide systems should decrease in the following sequence: Mo(8)C(12)>Mo(2)C(001) or MoC(001)>pure Mo(110). In spite of having the largest C/Mo ratio, the metcar appears as the most attractive system. Our studies also indicate that the "magic" behavior of metcars is not unique for Mo carbides. Similar behavior is also observed for Ti carbides. This implies that nanoparticles like metcar species could exhibit better performances than the corresponding bulk metal carbides as catalysts.

Activation of gold on titania: adsorption and reaction of SO(2) on Au/TiO(2)(110)

Synchrotron-based high-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of gold with titania and the chemistry of SO(2) on Au/TiO(2)(110) surfaces. The deposition of Au nanoparticles on TiO(2)(110) produces a system with an extraordinary ability to adsorb and dissociate SO(2). In this respect, Au/TiO(2) is much more chemically active than metallic gold or stoichiometric titania. On Au(111) and rough polycrystalline surfaces of gold, SO(2) bonds weakly and desorbs intact at temperatures below 200 K. For the adsorption of SO(2) on TiO(2)(110) at 300 K, SO(4) is the only product (SO(2) + O(oxide) --> SO(4,ads)). In contrast, Au/TiO(2)(110) surfaces (theta;(Au) < or = 0.5 ML) fully dissociate the SO(2) molecule under identical reaction conditions. Interactions with titania electronically perturb gold, making it more chemically active. Furthermore, our experimental and theoretical results show quite clearly that not only gold is perturbed when gold and titania interact. The adsorbed gold, on its part, enhances the reactivity of titania by facilitating the migration of O vacancies from the bulk to the surface of the oxide. In general, the complex coupling of these phenomena must be taken into consideration when trying to explain the unusual chemical and catalytic activity of Au/TiO(2). In many situations, the oxide support can be much more than a simple spectator.