DeNOx Reactions on MgO(100), ZnxMg1-xO(100), CrxMg1-xO(100), and Cr2O3(0001): Correlation between Electronic and Chemical Properties of Mixed-Metal Oxides

A highly selective and sensitive H2S sensor at low temperatures based on Cr-doped α-Fe2O3 nanoparticles

Cr-doped α-Fe₂O₃ nanoparticles were synthesized by one-step hydrothermal reaction and showed high sensitivity and selectivity to H₂S at low temperature.

Template-free synthesis of nanoparticle-built MgO and Zn-doped MgO hollow microspheres with superior performance for Congo red adsorption from water

A template-free route was developed to synthesize MgO and Zn-doped MgO hollow microspheres with ultrahigh adsorption performances and excellent reusability.

Mesoporous (ZnO)(x)(MgO)(1-x) nanoplates: template-free solvothermal synthesis, optical properties, and their applications in water treatment

Mesoporous (ZnO)x(MgO)1-x nanoplates were synthesized from a solution containing zinc acetate and magnesium acetate by a template-free solvothermal synthetic method followed by subsequent calcination. After thermal treatment, the plate-like morphology was retained. The formation of pores was due to thermal decomposition of Mg(OH)2 and the release of H2O. The optical properties of the mesoporous (ZnO)x(MgO)1-x nanoplates had been investigated by UV-vis absorption and cathodoluminescence (CL) emission spectroscopy. The UV-vis absorption spectra showed the band gap variation of the as-prepared samples due to the presence of ZnO in the MgO nanostructures. The CL spectra showed strong broad peaks in the visible range from 450 to 700 nm, indicating significant oxygen vacancy defects on the surface of the (ZnO)x(MgO)1-x nanoplates. Moreover, the samples were evaluated as photocatalysts for the UV-induced degradation of methyl orange (MO) in aqueous solution. The (ZnO)x(MgO)1-x nanoplates showed high photocatalytic performance and thus would be promising candidates for polluted water treatment.

In situ ambient pressure studies of the chemistry of NO2 and water on rutile TiO2(110)

The adsorption of NO(2) on the rutile TiO(2)(110) surface has been studied at room temperature in the pressure range from approximately 10(-8) torr to 200 mtorr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Atomic nitrogen, chemisorbed NO(2), and NO(3) were formed, each of which saturates at pressures below approximately 10(-6) torr NO(2). Atomic nitrogen originates from decomposition of the NO(x) species. For pressures of up to 10(-3) torr, no significant change in the NO(x) surface species occurred, suggesting that environmentally relevant conditions with typical NO(2) partial pressures in the 1-100 ppb range can be modeled by ultrahigh vacuum (UHV) studies. The chemisorbed surface species can be removed by in situ annealing in UHV: all of the NO(x) species disappear around 400 K, whereas the N 1s signal associated with atomic nitrogen diminishes around 580 K. At higher pressures of NO(2) (p(NO(2)) > or = 10(-6) torr), physisorbed NO(2) and adsorbed water, which was likely due to displacement from the chamber walls, appeared. The water coverage grew significantly above approximately 10(-3) torr. Concurrently with co-condensation of water and NO(2), the population of NO(3) species grew strongly. From this, we conclude that the presence of NO(2) and water leads to the formation of multilayers of nitric acid. In contrast, pure water exposure after saturation of the surface with 200 mtorr NO(2) did not lead to a growth of the NO(3) signals, implying that HNO(3) formation requires weakly adsorbed NO(2) species. These findings have important implications for environmental processes, since they confirm that oxides may facilitate nitric acid formation under ambient humidity conditions encountered in the atmosphere.

Interaction of oxygen with TiN(001):N↔O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.

Activation energy for hydrogen abstraction from methane over Li-doped MgO: A density functional theory study

Hydrogen abstraction from methane over Li-doped MgO is studied by means of density functional theory. The generalized synchronous transit method is applied to determine the transition state of the reaction. This method allows a transition state search that is more comprehensive compared with previous studies. The convergence of the calculated activation barrier with respect to cutoff energy, k-point mesh, vacuum layer thickness, and number of ionic layers in the crystal slab is examined. The activation barrier is calculated to be 0.745+/-0.01 eV (71.9+/-1.0 kJ/mol).

NO2 Adsorption on Ultrathin θ-Al2O3 Films: Formation of Nitrite and Nitrate Species

Interaction of NO2 with an ordered theta-Al2O3/NiAl(100) model catalyst surface was investigated using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The origin of the NO(x) uptake of the catalytic support (i.e., Al2O3) in a NO(x) storage catalyst is identified. Adsorbed NO2 is converted to strongly bound nitrites and nitrates that are stable on the model catalyst surface at temperatures as high as 300 and 650 K, respectively. The results show that alumina is not completely inert and may stabilize some form of NO(x) under certain catalytic conditions. The stability of the NO(x) formed by exposing the theta-Al2O3 model catalyst to NO2 adsorption increases in the order NO2 (physisorbed or N2O4) < NO2 (chemisorbed) < NO2- < NO3-.

Low-Temperature Reduction of NO2 on Oxidized Mo(110)

The reactions of nitrogen dioxide (NO(2)) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO(2) reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N(2), and trace amounts of N(2)O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO(2) to yield N(a) and O(a). N(2) forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO(2) reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO(2). The selectivity for N(2) formation over N(2)O is greater for NO(2) as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO(2) reaction than by O(2) reaction, indicating that NO(2) is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N(2)O(4), NO(2), and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N(2)O(4) desorbs by 200 K, leaving only NO(2) and NO on the surface. Infrared spectroscopy of NO(2) on (18)O-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.

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.

Chemistry of NO2 on oxide surfaces: formation of NO3 on TiO2(110) and NO2<-->O vacancy interactions

Synchrotron-based high-resolution photoemission, X-ray absorption near-edge spectroscopy, and first-principles density functional (DF) slab calculations were used to study the interaction of NO(2) with a TiO(2)(110) single crystal and powders of titania. The main product of the adsorption of NO(2) on TiO(2)(110) is surface nitrate with a small amount of chemisorbed NO(2). A similar result is obtained after the reaction of NO(2) with polycrystalline powders of TiO(2) or other oxide powders. This trend, however, does not imply that the metal centers of the oxides are unreactive toward NO(2). An unexpected mechanism is seen for the formation of NO(3). Photoemission data and DF calculations indicate that the surface nitrate forms through the disproportionation of NO(2) on Ti sites (2NO(2,ads) --> NO(3,ads) + NO(gas)) rather than direct adsorption of NO(2) on O centers of titania. Complex interactions take place between NO(2) and O vacancies of TiO(2)(110). Electronic states associated with O vacancies play a predominant role in the bonding and surface chemistry of NO(2). The adsorbed NO(2), on its part, affects the thermochemical stability of O vacancies, facilitating their migration from the bulk to the surface of titania. The behavior of the NO(2)/titania system illustrates the importance of surface and subsurface defects when using an oxide for trapping or destroying NO(x)() species in the prevention of environmental pollution (DeNOx operations).