Nitrogen Coverage Research Articles

A Molecular Beam Study of the NO + CO Reaction on Pd(111) Surfaces

Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system.

Selectivity-directing factors of ammonia oxidation over PGM gauzes in the Temporal Analysis of Products reactor: Secondary interactions of NH 3 and NO

Factors that direct the selectivity of ammonia oxidation for NO were determined previously (J. Catal. 227 (2004) 90) by the investigation of primary NH 3–O 2 interactions over pure Pt and Pt–Rh (95–5) alloy gauzes at 973–1173 K in the temporal analysis of products (TAP) reactor. A solid mechanistic understanding of the processes leading to by-products (N 2O and N 2) requires an analysis of secondary NH 3–NO interactions, which we investigated in the TAP reactor with isotopically labeled molecules. Our experiments under transient vacuum conditions indicate that these secondary processes determine the reaction selectivity for N 2O and N 2 in high-temperature ammonia oxidation over noble metal catalysts. Adsorbed oxygen species initiate the reaction of ammonia with nitric oxide. N 2O originates from the coupling of ammonia intermediates (NH x ) and nitric oxide. Different reaction pathways leading to N 2 have been identified, including primary (NH 3 oxidation) and secondary (NH x and H-assisted NO reduction) processes. The relative contributions of these routes depend on the surface coverage of nitrogen and hydrogen-containing species. A reaction scheme accounting for our experimental observations has been proposed, giving rise to an improved mechanistic description of the complex processes in ammonia burners.

The possible role of intermediate NH species in oscillations of the NO+H 2 reaction on noble metal surfaces

A comparative study of the thermodynamic properties of adsorbed NH n species ( n = 0, 1, 2, 3) on transition metal surfaces is performed by using the semi-empirical method of interacting bonds. The principal difference between single crystal surfaces exhibiting oscillatory behavior in the NO+H 2 reaction, and those surfaces which do not show such a behavior is that the combination reaction of NH species can easily proceed in the former case, whereas it is substantially endothermic on the latter surfaces. A trigger-like route for the oscillatory behavior is considered where the combination reaction of NH species operates as a temporary reaction pathway. This pathway practically does not contribute to the N 2 formation until the nitrogen coverage reaches some critical value, which ensures a sufficiently close distance between adjacent NH particles. The trigger pathway starts upon reaching that stage initiating the surface wave propagation, and stops immediately when the wave propagation is completed. The surface becomes then nearly clean and ready for the next oscillatory cycle. In this way, the feedback mechanism and the critical point of the regular wave initiation can be understood without any further assumptions. An alternative key reaction is also considered.

Arrays of magnetic nanodots on nitrogen-modified Cu(001) surfaces

A self-organized nanometre-scale pattern on a nitrogen-adsorbed Cu(001)-c(2 × 2)surface is used as a template for the fabrication of a magnetic nanodot array. Byadjusting the coverage of nitrogen on this surface, 5 nm × 5 nm square patches ofthe c(2 × 2)–N surface can be arranged squarely and separated by 2 nm wide stripsof clean Cu surface. Magnetic transition metals grow selectively on the clean Cusurface, and two-monolayers thick dots are dominantly formed at the intersectionsof the 2 nm wide Cu strips. Consequently, magnetic dots of a few nanometres indiameter are squarely organized with 7 nm intervals. Both the formation of thenitrogen-adsorbed nanopattern and the growth of the magnetic nanodotshave been studied in detail by using scanning tunnelling microscopy andelectron diffraction. Strain relief of the partly nitrogen-covered surface is thedriving force producing the square arrangement of the nitrogen-adsorbedpatches. The organization of the magnetic dots is ascribed both to selectivenucleation of the magnetic metal dots on the clean Cu surface and to thelarge growth rate at the intersections. The magneto-optical Kerr effectis employed to examine the magnetic properties of the Co dot array. Along-range order among double-layer dots is established before their percolationoccurs with increasing size. The origin of the novel magnetism is discussed.

Interaction of nitric oxide with Pt(1 0 0). A fast X-ray photoelectron spectroscopy study

The interaction of nitric oxide (NO) with the clean, reconstructed (1 0 0) surface of platinum is studied using synchrotron X-ray photoelectron spectroscopy. It is found that molecular adsorption at 200 K yields two different molecular NO species. Above 250 K, NO is found to adsorb both dissociatively and molecularly, giving rise to an oxygen/NO coadsorption structure with a molecular coverage that is half of that at 200 K. At 375 K, adsorption is found to be exclusively dissociative. Furthermore, it is found that NO dissociation yields two different atomic oxygen species. No significant coverage of atomic nitrogen is observed indicating fast combination of nitrogen atoms upon NO dissociation.

Stepwise dehydrogenation of NH 3 at the Ru( [formula omitted]) surface

The thermal dehydrogenation of NH 3 was studied on Ru(1 1 2 ̄ 0) using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Following NH 3 exposure at 85 K, the expected three dehydrogenation steps could be separated, allowing to isolate successively NH 2, NH and N on the surface. The Ru(1 1 2 ̄ 0) surface is highly reactive as indicated by the complete dehydrogenation of NH 3 for exposures up to 0.2 L. During annealing to 300 K both the channels for desorption and dehydrogenation are opened. Following adsorption at 85 K, the obtained nitrogen coverage of 0.11 ML is low, while for dissociative adsorption of NH 3 at 300 K, a maximum coverage of 0.38 ML is achieved, and the stability of NH 2 is increased.

Effect of coverage and temperature on the kinetics of nitrogen desorption from Rh(111) surfaces

The kinetics of molecular nitrogen desorption from Rh(111) single-crystal surfaces covered with atomic nitrogen were characterized by both isothermal and temperature-programmed experiments. A complex kinetic behavior was observed for this system not describable by typical Arrhenius expressions even if coverage-dependent activation energies are considered. In fact, the possibility of developing an analytical expression for those desorption rates is voided by the fact that they depend not only on surface temperature and nitrogen coverage but also on the mode of preparation of the system. Isotope labeling experiments support the idea of the formation of surface nitrogen islands and of preferential recombination and desorption at their edges. Coadsorbed oxygen, a typical by-product in NO reduction catalysis, behaves kinetically in a way similar to surface nitrogen as far as N2 desorption is concerned. The recombination of two surface nitrogen atoms is typically slower than the reaction between one nitrogen and one adsorbed NO molecule, and therefore the catalytic reduction of NO most likely occurs via the latter pathway.

Self-ordering in two dimensions: nitrogen adsorption on copper (1 0 0) followed by STM at elevated temperature

We have followed the adsorption and the desorption of nitrogen on the copper (1 0 0) surface in real time using scanning tunneling microscopy at elevated temperatures. This gives an unprecedented view, especially of the adsorption behavior at very low coverage. Growth proceeds by splitting of square-shaped islands of critical size without loss of C 4 symmetry. The equilibrium structures are analyzed in terms of their dimensionality and shape showing that the phase diagram comprises two periodic domain morphologies as a function of nitrogen coverage: two-dimensional (2D) array of compact nitrogen islands at low and high coverage and a quasi 1D arrangement of striped domains (rows of squares) at intermediate coverages. Furthermore, we investigated the sensitivity of the system to small changes in the elastic properties of the substrate by pre-deposition of gold on the bare copper surface. Finally, the comparison all our observations in the light of existing model predictions for long-range interactions in 2D systems reveals crucial differences and, therefore, these models have to be extended to include short range relaxation and interactions in order to reproduce the fundamental aspects of this system.

Adsorption Behavior of N2, Water, C6 Hydrocarbons, and Bulkier Benzene Derivative (TMB) on Na–X Zeolite and Its K+-, Rb+-, and Cs+-Exchanged Analogues

The hydrothermal crystallization of X-type zeolite with a Si/Al ratio of 1.15 was achieved from the Na2O–Al2O3–SiO2–H2O system at 368 K under static conditions. The post-synthesis modification was carried out by a conventional ion-exchange technique to obtain K+-, Rb+-, and Cs+-exchanged samples with different degrees of exchange. All the samples were characterized using chemical analysis, IR, SEM, powder XRD, low-temperature nitrogen adsorption, and equilibrium sorption uptakes of different probe molecules. The relative intensities of the XRD peaks of cation-exchanged zeolite were found to be affected to different extents, depending on the nature and the concentration of nonframework cationic size, without any shift in the positions of reflection. The sorptive properties of the K-, Rb-, and Cs-exchanged samples were studied using nitrogen, water, and different C6 hydrocarbons including bulkier benzene derivative 1,3,5-trimethylbenzene (TMB) as probe molecules. The trend observed in chemical potential estimated as a function of nitrogen coverage indicates different sorption selectivity because of differences in the cationic size and population. Sorption uptake kinetics for probe molecules such as water, n-hexane, cyclohexane, benzene, and TMB were also studied. The samples with higher degrees of exchange and/or cationic size have shown a decrease in hydrophilic character due to the formation of irregular networks of water molecules connected with preadsorbed water molecules, framework oxygen ions, and nonframework cations. Among C6 hydrocarbons including TMB, the benzene molecule is found to be the most promising probe for the estimation of openness of structure and surface heterogeneity as well.

Self-organisation of adsorbed nitrogen on (100) and (410) copper faces: a SPA-LEED study

The self-organisation of nitrogen nanostructures at different coverages on a (100) copper surface is studied by spot profile analysing low energy electron diffraction (SPA-LEED). The existence of two surface states with a domain of coverage leading to coexistence of the two states as already observed by scanning tunneling microscopy (STM) [Leibsle and Robinson, Phys. Rev. B 47 (1993) 15 865; Leibsle et al., Surf. Sci. 317 (1994) 309; Leibsle, Surf. Sci. 440 (1999) L835] and low energy electron diffraction (LEED) [Sotto et al., Surf. Sci. 371 (1997) 36] is confirmed. In the first state, the surface is organised in square shape islands separated by bare copper 〈100〉 rows. This work shows that the surface periodicity depends on the preparation of the nitrogen overlayer. When nitrogen coverage is obtained by adsorption with a sample temperature near 320°C, the periodicity does not vary with coverage and is found to be equal to 55±2 Å. At coverages below 0.75±0.05 and if the nitrogen is deposited at room temperature followed by an anneal at 320°C, during long periods of time, the periodicity evolves to large values (∼97±3 Å). During thermal desorption, the long range order with decreasing coverage is maintained but the surface periodicity also evolves continuously to large values (∼100 Å). However, a surface periodicity of 55±2 Å seems to be a characteristic length of this system. The second surface state corresponds to large c(2×2)N domains separated by 〈110〉 trenches [Leibsle and Robinson, Phys. Rev. B 47 (1993) 15 865; Leibsle et al., Surf. Sci. 317 (1994) 309; Leibsle, Surf. Sci. 440 (1999) L835]. Nitrogen adsorption on a (410) stepped face induces a reconstruction into a (810) face with double step height. The complex behaviour of this film growth is discussed in the light of existing theories about the driving force leading to nanostructuration.

Fabricating nanometer-scale Co dot and line arrays on Cu(100) surfaces

We have templated Cu(100) surfaces with self-assembled arrays of atomic nitrogen islands and then used these islands as masks for Co growth. This method of nanolithography enables the creation of Co dot arrays with dot densities 4×1012 cm−2 (27 teradots/in.2). Adjusting the nitrogen coverage also enabled the creation of arrays of Co lines spaced 10 nm (0.01 μm) apart.

A surface chemistry investigation of the low pressure nitridation of Si(100) using 1,1-dimethylhydrazine

The adsorption and decomposition of 1,1-dimethylhydrazine (DMHy) on Si(100) and the low pressure nitridation of Si(100) using DMHy were investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). DMHy dissociatively adsorbs on Si(100) at 310 K with the cleavage of both N–N and N–H bonds. TPD and XPS show that dimethylamine desorption results in the complete removal of all the carbon on the surface by ∼600 K. Nitridation of the Si(100) surface between approximately 600 and 720 K using DMHy results in the adsorption of only NH x species with carbon contamination below XPS detection. In this temperature window, the nitrogen species and coverage are very similar to that found using ammonia (NH 3) as the nitridant.

Role of adsorbed nitrogen in the catalytic reduction of NO on rhodium surfaces

The role of surface nitrogen in the kinetics of the NO+CO conversion reaction on Rh(111) under steady-state catalytic conditions was explored by using collimated molecular beams and mass spectrometry detection. Two types of kinetically different nitrogen atoms were identified on the surface. The buildup of a critical nitrogen coverage was determined to be required for the start of the nitrogen recombination step to N2. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly with increasing reaction temperature, and becomes almost insignificant above 600 K. The actual value of this coverage is quite insensitive to the ratio of NO to CO in the reaction mixture, but displays an inverse correlation with the steady-state reaction rate under most conditions. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms. Isotope switching experiments indicated that the two types of kinetically different nitrogens are not likely to represent different adsorption sites, but rather similar adsorption states with adsorption energetics modified by their immediate surrounding environment on the surface. The data are explained here by a model in which the nitrogen atoms form surface islands and where the atoms at the perimeter of those islands react preferentially via N+N recombination to N2.

A Molecular Beam Study of the Kinetics of the Catalytic Reduction of NO by CO on Rh(111) Single-Crystal Surfaces

Steady-state rates for the catalytic reaction of NO with CO on Rh(111) surfaces have been measured under isothermal conditions by using a molecular beam approach with mass spectrometry detection. Systematic studies were carried out as a function of surface temperature, NO+CO beam composition, and total beam flux. A maximum in reaction rate was observed between 450 and 900 K, the exact temperature depending on the NO:CO beam ratio. Indeed, a synergistic behavior was seen where the loss in reactivity induced by increasing the CO concentration in the beam is partly compensated by a higher surface temperature. The data presented here are consistent with the rate-limiting step of the overall NO reduction process being the surface recombination of atomic nitrogen atoms resulting from fast dissociation of NO adsorbed molecules. Temperature-programmed desorption and CO titration experiments were also performed after the isothermal kinetic runs in order to estimate the surface coverages of the reactants during the steady-state reactions. The NO+CO conversion rate was found to be directly proportional to the coverage of atomic oxygen on the surface. The relation between reaction rates and nitrogen coverages, however, proved to be much more complex; an inverse correlation was in fact seen in most cases between those two parameters. The build-up of a critical coverage of atomic nitrogen was found to be necessary to trigger the nitrogen recombination step to N2. This critical coverage of strongly held nitrogen was determined to not depend in any significant way on the composition of the beam, but to decrease with reaction temperature in all cases.

Nitridation of the GaAs(001) surface: Thermal behavior of the (3×3) reconstruction and its evolution

The nitridation of the GaAs(001) surface using a radio frequency atomic nitrogen plasma source in a molecular beam epitaxy growth chamber has been studied. The resulting nitrogen induced GaAs(001) (3×3) reconstruction was investigated by in situ reflection high energy electron diffraction and x-ray photoemission spectroscopy (XPS). It was found that this reconstruction is only obtained in the temperature range 400–580 °C with a very low dose of atomic nitrogen. The nitrogen coverage corresponding to the (3×3) reconstruction was determined by quantitative XPS to be 0.30±0.09 ML. Below 400 °C an As–N species of disordered structure was found on the GaAs(001) surface. Subsequent annealing at about 500 °C produced the (3×3) reconstruction. Above 580 °C, nitridation lead to direct formation of β-GaN islands. In addition, the (3×3) reconstruction was found to be unstable to both exposure to atomic hydrogen and annealing. The N desorption activation energy of the (3×3) was estimated to be 2.75±0.55 eV. A surface phase diagram of the (3×3) has thus been deduced.

SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID‐ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid‐Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid‐Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom‐based stream classification did not correspond to Omernik’s ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid‐Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse‐scale factors, such as land cover/use in watersheds, and by site‐specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid‐Atlantic streams.

The (3 × 3) reconstruction and its evolution during the nitridation of GaAs(001)

Nitridation of GaAs(001) was performed using a radio frequency nitrogen plasma source in a molecular beam epitaxy chamber. The nitrogen-induced GaAs(001) (3 × 3) reconstruction was investigated by reflection high energy electron diffraction (RHEED) and X-ray photoemission spectroscopy (XPS). The temperature dependence of the (3 × 3) reconstruction as well as its evolution during further nitridation was observed by RHEED; we found that the (3 × 3) reconstruction could be obtained in the temperature range 400–580 °C by a very low dose of atomic nitrogen deposition. The nitrogen coverage in the (3 × 3) reconstruction is determined to be in the range 0.2–0.4 ML using XPS. We also discuss the atomic model of the (3 × 3) reconstruction and the energetics of its formation and stability.

The structure of Ni39

The geometrical structure of Ni39 is probed via molecular adsorption of nitrogen on its surface. Nitrogen uptake patterns are determined at various reaction temperatures, and the results are interpreted in terms of the number and nature of nitrogen binding sites on possible structures. It is found that an adsorbate-induced isomerization occurs at partial nitrogen coverage, but that at saturation a reverse isomerization returns the cluster to its initial structure. The two lowest energy structures calculated by Wetzel and DePristo [T. L. Wetzel and A. E. DePristo, J. Chem. Phys. 105, 572 (1996)] are completely consistent with the observed saturation and isomerization behavior. The structures consist of atom caps packed around a central pentagonal bipyramid, and reflect a marked change from the octahedral structure determined earlier for Ni38.

Selective MOVPE of GaN and AlxGa1−xN with smooth vertical facets

Selective metalorganic vapor-phase epitaxy (MOVPE) of GaN and AlxGa1−xN was investigated as a function of growth conditions such as substrate temperature and partial pressure of NH3. GaN hexagonal microprisms (HMPs) with vertical {11̄00} facets were grown when the surface coverage of atomic nitrogen was rather low, while GaN hexagonal micro-pyramids with {11̄01} facets were grown when the surface coverage was higher. This is because the growth rate of a GaN {11̄00} surface is more sensitive to the nitrogen surface coverage than a {11̄01} surface. The GaN HMPs have atomically smooth top surfaces because the growth mode is a balance between adsorption and desorption of film forming precursors and also because of the finite-area effect. HMPs and stripes of Al0.05Ga0.95N with smooth vertical facets were also fabricated. Stimulated emission was observed at room temperature from photopumped GaN HMPs 50μm in diameter. The longitudinal modes having a 0.33nm separation indicate an inscribed hexagonal optical path in the GaN HMP.

Isothermal Kinetic Study of the Decomposition of Nitric Oxide over Rh(111) Surfaces

The kinetics of the thermal decomposition of NO on Rh(111) surfaces was probed both by temperature-programmed desorption (TPD) experiments and by isothermal measurements using an extension of the so-called King and Wells collimated beam method. The TPD studies corroborated previously reported results, including the existence of two distinct N2desorption peaks, the first of which displays apparent first-order kinetics. The isothermal work proved that the adsorption of NO is precursor mediated at low temperatures, and that it is not affected significantly by the presence of coadsorbed nitrogen and/or oxygen atoms at any temperature below 900 K. The rate of molecular nitrogen production was found to be significant above 450 K and to be controlled by the recombination of atomic nitrogen below 600 K, but the experimental data could not be reproduced in a satisfactory manner by any empirical rate law unless the order in nitrogen coverage was set to be less than unity. Such an observation is interpreted here as being the result of the slow diffusion of nitrogen atoms across the surface prior to their recombination. A strong additional effect due to lateral repulsion between nitrogen and/or oxygen atoms was also inferred from the data.