First-Principles Characterization of NOxAdsorption on MgO

Efficient Conversion of NO to NO2 on SO2-Aged MgO under Atmospheric Conditions

The NO-NO₂ cycle determines the formation of O₃ and hence plays a critical role in the oxidizing capacity of troposphere. Traditional view concluded that the heterogeneous oxidation of NO to NO₂ was negligible due to the weak reactivity of NO on aerosols, compared to the homogeneous oxidation process. However, the results here reported for the first time that SO₂ can greatly promote the heterogeneous transformation of NO into NO₂ and HONO on MgO particles under ambient conditions. The uptake coefficients of NO were increased by 2-3 orders of magnitudes on SO₂-aged MgO, compared to the fresh sample. Based on spectroscopic characterization and density functional theory (DFT) calculations, the active sites for the adsorption and oxidation of NO were determined to be sulfates, where an intermediate [SO₄-NO] complex was formed during the adsorption. The decomposition of this species led to the formation of NO₂ and the change of sulfate configuration. The formed NO₂ could further react with surface sulfite to form HONO and sulfate. The conversion of NO to NO₂ and HONO on the SO₂-aged MgO surface under ambient conditions contributes a new formation pathway of NO₂ and HONO and could be quite helpful for understanding the source of atmospheric oxidizing capacity as well as the formation of air pollution complexes in polluted regions such as the northern China.

Adsorption of Trinitrotoluene on a MgO(001) Surface Including Surface Relaxation Effects

A thorough investigation of 2,4,6-trinitrotoluene (TNT) adsorption on a MgO(001) surface was carried out using density functional theory (DFT) combined with periodic boundary conditions. Four different initial orientations of the TNT molecule, adsorbed on two different representations of the MgO(001) surface, were investigated. In the first surface representation, there were two fixed layers of atoms and in the second the surface had three layers, with the uppermost fully relaxed in geometry optimizations. Electron density difference maps for each case were computed and provided a detailed picture of the interactions. The results showed a physical adsorption process for both surface representations. In the most favorable situation—TNT adsorbed on the surface with three layers—the computed adsorption energy was −9.89 kcal/mol. The importance of allowing the uppermost layer of the surface to fully relax upon molecular desorption was shown.

Periodic DFT study of acidic trace atmospheric gas molecule adsorption on Ca- and Fe-doped MgO(001) surface basic sites

The electronic properties of undoped and Ca- or Fe-doped MgO(001) surfaces, as well as their propensity toward atmospheric acidic gas (CO2, SO2, and NO2) uptake was investigated with an emphasis on gas adsorption on the basic MgO oxygen surface sites, O(surf), using periodic density functional theory (DFT) calculations. Adsorption energy calculations show that MgO doping will provide stronger interactions of the adsorbate with the O(surf) sites than the undoped MgO for a given adsorbate molecule. Charge transfer from the iron atom in Fe-doped MgO(001) to NO2 was shown to increase the binding interaction between adsorbate by an order of magnitude, when compared to that of undoped and Ca-doped MgO(001) surfaces. Secondary binding interactions of adsorbate oxygen atoms were observed with surface magnesium sites at distances close to those of the Mg-O bond within the crystal. These interactions may serve as a preliminary step for adsorption and facilitate further adsorbate transformations into other binding configurations. Impacts on global atmospheric chemistry are discussed as these adsorption phenomena can affect atmospheric gas budgets via altered partitioning and retention on mineral aerosol surfaces.

Nitrite and nitrate formation on model NOx storage materials: on the influence of particle size and composition

A well-defined model-catalyst approach has been utilized to study the formation and decomposition of nitrite and nitrate species on a model NO(x) storage material. The model system comprises BaAl(2x)O(1+3x) particles of different size and stoichiometry, prepared under ultrahigh-vacuum (UHV) conditions on Al(2)O(3)/NiAl(110). Adsorption and reaction of NO(2) has been investigated by molecular beam (MB) methods and time-resolved IR reflection absorption spectroscopy (TR-IRAS) in combination with structural characterization by scanning tunneling microscopy (STM). The growth behavior and chemical composition of the BaAl(2x)O(1+3x) particles has been investigated previously. In this work we focus on the effect of particle size and stoichiometry on the reaction with NO(2). Particles of different size and of different Ba(2+) : Al(3+) surface ion ratio are prepared by varying the preparation conditions. It is shown that at 300 K the reaction mechanism is independent of particle size and composition, involving initial nitrite formation and subsequent transformation of nitrites into surface nitrates. The coordination geometry of the surface nitrates, however, changes characteristically with particle size. For small BaAl(2x)O(1+3x) particles high temperature (800 K) oxygen treatment gives rise to particle ripening, which has a minor effect on the NO(2) uptake behavior, however. STM shows that the morphology of the particle system is largely conserved during NO(2) exposure at 300 K. The reaction is limited to the formation of surface nitrites and nitrates, which are characterized by low thermal stability and completely decompose below 500 K. As no further sintering occurs before decomposition, NO(2) uptake and release is a fully reversible process. For large BaAl(2x)O(1+3x) particles, aggregates with different Ba(2+) : Al(3+) surface ion ratio were prepared. It was shown that the stoichiometry has a major effect on the kinetics of NO(2) uptake. For barium-aluminate-like particles with high Al(3+) concentration, the formation of nitrites and nitrates on the BaAl(2x)O(1+3x) particles at 300 K is slow, and kinetically restricted to the formation of surface species. Only at elevated temperature (500 K) are surface nitrates converted into well-defined bulk Ba(NO(3))(2). This bulk Ba(NO(3))(2) exhibits substantially higher thermal stability and undergoes restructuring and sintering before it decomposes at 700 K. For Ba(2+)-rich BaAl(2x)O(1+3x) particles, on the other hand, nitrate formation occurs at a much higher rate than for the barium-aluminate-like particles. Furthermore, nitrate formation is not limited to the surface, but NO(2) exposure gives rise to the formation of amorphous bulk Ba(NO(3))(2) particles even at 300 K.

Carbon Nanotubes Functionalized by NO2: Coexistence of Charge Transfer and Radical Transfer

The adsorption of NO(2) molecules on a series of zigzag (n,0) single-walled carbon nanotubes (SWNTs) (n = 6-12) have been investigated by first-principles methods. The results indicate that the tube diameter and the concentration of NO(2) gas determine the interactions between NO(2) molecules and the tube. The chemisorption of a single NO(2) is only possible for the tube with a small diameter (n < 10), while the second NO(2) molecule can be chemisorbed for all tubes studied here. The additions of more NO(2) molecules in different patterns have also been considered for a (8,0) tube, and the NO(2) groups prefer the pair arrangement with stronger binding energy. According to the results of band structure calculations, overall, the SWNT exhibits a p-type response upon exposure to NO(2) gas and the electron charge transfer is an important reason for explaining the enhancement of conductivity of the tube. Moreover, it is interesting that, accompanied by charge transfer, the chemisorption of NO(2) also leads to the radical transfer from the NO(2) group to the carbon atoms at the ortho and para sites of the six-membered ring. As a result, the SWNT possesses the radical characteristic, which facilitates the further functionalization of the tube wall.

Ab Initio Molecular Dynamics Simulation of NO Reactivity on the CaO(001) Surface

Ab initio molecular dynamics (MD) is used to investigate NO reaction processes on the (001) surface of CaO. A novel path is proposed for the first steps of nitrogen oxides reactivity catalyzed by the CaO surface. The mechanism consists of the formation of anionic dimers, adsorbing on the surface cations, at the expense of oxidized NO species adsorbed on surface anions. The complete charge-transfer process takes place in two steps, producing first monovalent anionic dimers (NO)2- and, later on, divalent anionic dimers (NO)2(2-). These redox processes cause spin quenching and are observed in the short time scale of the ab initio MD simulation at 300 K. The results presented provide a rationalization of a recent electron spin resonance (ESR) investigation indicating that the spectroscopy is silent to most of the nitrogen oxide species adsorbed on CaO powders, despite deposition of paramagnetic NO molecules at room temperature.

Theoretical Studies of Manganese and Iron Superoxide Dismutases: Superoxide Binding and Superoxide Oxidation

Density-functional calculations indicate that the second sphere of coordination around the metal centers of manganese and iron superoxide dismutases (MnSODs and FeSODs) plays an important role in the binding of O2(-). In these systems, O2(-) prefers to bind to Mn or Fe in end-on configurations. For human and E. coli MnSODs, the bound O2(-) forms hydrogen bonds with the tyrosine and glutamine amino acid residues in the second sphere of coordination. In the cases of E. coli and T. elongates FeSODs, hydrogen bonding occurs between the bound O2(-) and the tyrosine amino acid only because the glutamine is too far away for an effective bonding interaction. The manner in which the O2(-) binds to the metal center in MnSODs and FeSODs can affect the rate of subsequent protonation and determine the mechanism for the formation of H2O2. Both Mn- and Fe-containing superoxide dismutases contain a metal-bound solvent molecule that has been suggested to be involved in the uptake of a H+ upon reduction of the metal center [Bull, C.; Fee, J. A. J. Am. Chem. Soc. 1985, 107, 3295; Miller, A.-F.; Padmakumar, K.; Sorkin, D. L.; Karapetian, A.; Vance, C. K. J. Inorg. Biochem. 2003, 93, 71]. Using density-functional theory, we confirm this suggestion and show the involvement of the second sphere of coordination in the process. We show that the oxidation of superoxide by Mn- or Fe-containing superoxide dismutases is facilitated by a cooperative effect between superoxide binding, protonation of the OH- bound to the metal, and electron transfer from the superoxide molecule to the oxidized metal. In particular, proton transfer through tyrosine-34 on the absence of a bound superoxide is uphill while, once superoxide is bound, the energetic barrier is lowered. It is this barrier that likely keeps the resting state (Mn(III)SOD) of the enzyme with a bound hydroxide, instead of a water. This work provides a model for the mechanism of reaction of superoxide with the oxidized form of the metal within Mn- and FeSODs.