Adsorption Of Molecular Nitrogen Research Articles

Reactions of small cobalt clusters with N2: implications for cluster structure

The geometrical structures of small cobalt clusters are probed via the adsorption of molecular nitrogen on their surfaces. In contrast to nickel clusters, the adsorption of nitrogen causes most cobalt clusters to change structure; in some cases multiple structural changes occur as the nitrogen coverage is increased. Although this complicates the determination of structure for the bare clusters, we are able to propose structures for most of the clusters in the Co3Co16 size range. Again in contrast to nickel, most cobalt clusters appear to adopt close-packed structures that resemble bulk packing. For example, while Ni13 is most likely an icosahedron, Co13 appears to have an fcc or hcp structure. The structural differences between the two metals probably reflects the increased importance of d-orbital binding in cobalt compared to nickel.

Nickel cluster structure determined from the adsorption of molecular nitrogen: Ni49-Ni71

The geometrical structures of nickel clusters in the size range from 49 to 71 atoms are studied by the chemical probe method. Saturation coverages of molecular nitrogen are determined for each cluster and from this data specific structures are proposed (except for Ni66 and Ni67). The results indicate that icosahedral packing is the dominant structural configuration throughout this size range, in agreement with earlier results based on water and ammonia adsorption. In addition, it seems that for clusters larger than Ni54 the excessive strain in the surface of the 55-atom regular icosahedron often leads to rear-rangements of the surface atoms to relieve that strain. Ni55, in particular, is found to have two isomers, the regular icosahedron and a structure in which a single apex atom is displaced to the center of an opposite face. Ni71 occurs as a 55-atom regular icosahedron with a 16-atom cap. The results suggest that the atoms in the cap adopt an ABA configuration relative to the underlying icosahedron rather than an icosahedral arrangement. For some clusters the saturation with nitrogen causes a small degree of surface reconstruction that leads to the adsorption of additional nitrogen molecules.

Fractionation of Nitrogen Isotopes on Mars: The Role of the Regolith as a Buffer

We have measured the adsorption of molecular nitrogen (N 2) on palagonite, and modeled the adsorbed nitrogen inventory on the martian regolith. We were motivated by the fact that models of isotopic evolution predict stronger N 2 fractionation than reported by Viking. Possible scenarios for reconciling models with the observation include a heavy CO 2 atmosphere early in the planet's history, continued outgassing of N 2 throughout the history of Mars, or a substantial adsorbed inventory. In this paper we investigate the plausibility of the last explanation. We find that the regolith reservoir of adsorbed N 2 is inadequate by itself to buffer the atmospheric isotopic composition, but may play a role depending on the total regolith surface area available.

Adsorption of molecular nitrogen on rhodium supported on dealuminated zeolite Y; the formation of well-defined surface dinitrogen complexes

The surface complexes RhI(CO)(N2)+ with ν(CO) 2062 cm–1 and ν(N2) 2252 cm–1 and RhI(N2)2+, ν(N2) 2244 + 2218 cm–1, which are stable in flowing nitrogen up to 250 °C, have been synthesised from RhI(CO)2+ on highly dealuminated zeolite Y (DAY, US-Ex).

On the structure sensitivity of ammonia synthesis on promoted and unpromoted iron

In recent years the techniques of surface science have been applied to the elucidation of the details of the mechanism of ammonia synthesis on Fe. In particular Ertl and co-workers have demonstrated the strongly anisotropic nature of nitrogen adsorption on well-defined Fe single-crystal surfaces. Thus the (111) plane is by far the most active for such dissociation, a property thought to be due to the presence of exposed seven-coordinated Fe atoms in this surface. The trend in the efficiency of nitrogen dissociation on these planes has been shown by Spencer et al. to parallel their activity for ammonia synthesis under kinetically limited conditions; thus the (111) plane shows 400 times the activity of the relatively smooth (110) surface. Ertl et al. have also investigated the effects of the presence of alkali metal on the nitrogen adsorption kinetics and have found that the main effect on the energetics is an increase in the heat of adsorption of molecular nitrogen, and a consequent reduction in the energy barrier to dissociation at the surfaces. They then postulate that the effect of promotion in industrial ammonia synthesis is to bring all planes to a similar activity (1b), with the (111) plane being least affectedmore » and the (110) being the most affected. The purpose of the present paper is to present data from the kinetic modeling of ammonia synthesis using the data derived by Ertl et al. and to examine whether the thesis described above regarding promoter action is correct.« less

Precursor mediated and direct adsorption of molecular nitrogen on Fe{111}

In this article, we will summarize the kinetic models for nitrogen dissociation on Fe {111} and will present new data on the adsorption mechanism for the π-bonded α-N2 state, which was identified as the immediate precursor for dissociation. Our results reveal that adsorption into the α state can occur via the more weakly bonded γ state, but also by direct impact from the gas phase. The sticking coefficient via the γ state precursor depends directly on its concentration, and therefore depends on the temperature of the substrate and the gas pressure. Contrary to our previous estimates, the kinetic parameters for precursor mediated adsorption into the α state in the limit of zero coveragecannot explain the high temperature dissociation probability. The direct channel from the gas phase into the α state needs to be included to explain the experimental results. We also discuss the stabilization of γ-N2 by preadsorbed α-N2, i.e. an attractive interaction between alike molecules but in different bonding configurations.

Thermodynamic and kinetic parameters of molecular nitrogen adsorption on Fe{111}

We describe thermal desorption experiments and their computer simulations together with thermodynamic equilibrium measurements for molecular nitrogen adsorption on an Fe{111} surface. Three distinct nitrogen adsorption states, labeled δ, γ, and α with increasing desorption temperature, are detected and it is found that adsorption into the π-bonded α state occurs via the δ and γ states at low substrate temperatures. The δ state can act as a precursor for the γ state at T <100 K, but at higher temperatures the direct channel from the gas phase into the γ state predominates the adsorption/desorption kinetics. The activation energy of desorption for γ-N2 is found to be ∼10% lower than the isosteric heat of adsorption indicating the possibility of dynamical effects in the desorption process into vacuum. It was also observed that preadsorbed α-N2 increases the heat of adsorption for γ-N2 which is evidence for a mutually attractive interaction between those two states.

The role of σ-donation in the bonding of molecular nitrogen on clean and modified Ru(001) surfaces

We demonstrate that the adsorption of molecular nitrogen on modified Ru(001) is, in contrast to CO adsorption, largely governed by its σ-charge donation to the metal, which can be promoted by coadsorbed oxygen. Preadsorbed potassium inhibits nitrogen adsorption for σ K > 0.08, thus enabling us to determine a minimum distance of 4.25 Å for the repulsive K-N 2 interaction.

Adsorption of nitrogen on potassium promoted Fe(111) and (100) surfaces

Dissociative nitrogen adsorption through N 2 ⇌ N 2, ad → 2 N s is the rate limiting step in ammonia synthesis over iron catalysts. This model study deals with the “promoter” action of potassium by studying the influence of preadsorbed K on the energetics and kinetics of molecular and atomic nitrogen adsorption on Fe(111) and Fe(100). The heat of adsorption of molecular nitrogen is locally increased to about 11 kcal/mole in the vicinity of an adsorbed K atom, which phenomenon simultaneously lowers the activation barrier for dissociation. As a consequence the effective sticking coefficient for dissociative nitrogen adsorption is markedly increased. This effect is less pronounced for Fe(111) than for Fe(100) so that the difference in activity observed with the K-free surfaces is eliminated. The increase of the N 2 adsorption energy is attributed to the enhanced electron density of the substrate (due to charge transfer from K) which enables a more pronounced “backbonding” contribution to the adsorption bond.

5261. Adsorption of molecular nitrogen on copper clusters and nickel films at 7 K: D Schmeiβer et alVacuum,31 (10–12), 1981, 439–442

5261. Adsorption of molecular nitrogen on copper clusters and nickel films at 7 K: D Schmeiβer et alVacuum,31 (10–12), 1981, 439–442

Adsorption of molecular nitrogen on nickel. I. Cluster-model theoretical studies

Adsorption of molecular nitrogen on nickel. I. Cluster-model theoretical studies

Adsorption of molecular nitrogen on copper clusters and nickel films at 7 K

UV photoemission spectra of nitrogen matrices doped with Cu atoms and small clusters were measured at 7 K. On copper clusters as well as on nickel films we have found a surface N2 species which has experienced a rigid shift of about 1 eV to smaller binding energies and which we interpret in essence as physisorbed N2. From the relative intensities of the 4σ, 1π and 5σ levels and from a weakly indicated split of the 1π level into two components separated by ⪅ 0.5 eV, tentative conclusion of a non-random, probably side-on alignment of the N2 molecules has been drawn.

Interactions of nitrogen and hydrogen on iron surfaces

Interactions between nitrogen and hydrogen on single and polycrystalline Fe surfaces were studied by thermal desorption and Auger spectroscopy under UHV conditions and by applying total pressures up to about 1 atm. Molecularly adsorbed nitrogen, N 2,ad, is displaced from the surface by admission of hydrogen. Adsorbed hydrogen in turn inhibits adsorption of molecular nitrogen and as a consequence lowers the rate of dissociative nitrogen adsorption (which proceeds through N 2,ad). Ammonia synthesis is therefore most favourably run under conditions where the stationary H ad-concentration is not too high, which will usually be fulfilled at the rather high reaction temperatures (≳ 700 K), even if the H 2 pressure is high. Adsorbed atomic nitrogen, N s, on the other hand, blocks the sites for hydrogen adsorption, so that also the stationary N s-concentration should not be too high. That this latter condition will be fulfilled with a stoichiometric N 2+H 2 mixture at T≳580K (as long as NH 3 decomposition plays no important role) was demonstrated by means of high pressure studies in which the stationary N s-concentration was determined after evacuation. These measurements showed unequivocally that under the stated conditions dissociative nitrogen adsorption is the rate-limiting step of ammonia synthesis. Complications arise from bulk dissolution and surface segregation of atomic nitrogen which processes are affected by the presence of hydrogen.

Adsorption of molecular nitrogen by the W(110) plane

Characteristics of the adsorption of nitrogen on the (110) plane of tungsten were determined by thermal desorption and work function measurements. The low temperature γ-N 2 state desorbs with first order kinetics and an activation energy of 6 kcal mole −1. The absence of isotope mixing between 14N 2 and 15N 2 demonstrates γ-N 2 is adsorbed molecularly. Monolayer coverage shows a decrease of 0.19 eV in work function. A Topping model plot indicates the layer is immobile at 123 K.

Hydrogen activation on zinc oxide

Three types of hydrogen adsorption have previously been characterized on zinc oxide at low temperatures, designated Types I, II and III. One of these, Type III, is a molecular form of adsorption which can be specifically titrated by nitrogen adsorption on 10–20% of the surface with formation of a molecular nitrogen species. This selective titration by a molecular nitrogen adsorption was employed to study the participation of molecularly adsorbed hydrogen in the allotropie (ortho-para) hydrogen conversion and isotopic (H 2D 2) hydrogen exchange. The promotional effect of oxygen on the conversion reaction was also investigated. These results at low temperatures clearly indicate that the conversion reaction proceeds predominantly by a molecular paramagnetic mechanism involving this type III hydrogen below 140 °K, while the exchange reaction proceeds via a Bonhoeffer-Farkas mechanism involving type I hydrogen at both low temperature and also at room temperature. At low temperatures, the exchange and conversion reactions are independent, but this is not necessarily true at room temperature. A mathematical model is presented for the paramagnetic conversion based on Wigner's theory involving longer times of interaction. The result is an enhancement of the conversion rate by several orders of magnitude with the deuterium conversion being enhanced more than the molecular hydrogen conversion.

Another rate equation for the catalytic synthesis of ammonia

A rate equation for the ammonia synthesis is given based on the assumption that the adsorption of molecular nitrogen on one site is the rate-determining step. The equation represents satisfactorily the experimental data.

The influence of crystallite size on the adsorption of molecular nitrogen on nickel, palladium and platinum: An infrared and electron-microscopic study

A specified study was made of the phenomenon of infrared-active nitrogen adsorption on nickel, the existence of which was first shown by Eischens and Jacknow. The study combines an infrared spectroscopic investigation of the adsorbed nitrogen with an electron-microscopic determination of the metal particle size distribution for the various nickel-on-silica samples. It is shown that this form of nitrogen adsorption is not limited to nickel but also exists on platinum and palladium (and, doubtless, also on other metals). However, this type of adsorption is measurable only if the diameter, d, of the metal crystallites is within the range ~ 15 Å < d < ~ 70 A. This is because crystals within this range possess a large number of special surface sites which are absent, or much scarcer, on larger or smaller crystals. This has appeared from a study of their surface structure in which use was made of marble models and of mathematical computations. The infrared activity of the nitrogen adsorbed on these special sites is due to the induction of a transition-moment by the strong polarizing field of the special sites. The observed absorption band is, in fact, the nitrogen Raman band (2331 cm −1) which has shifted under the influence of the Stark effect; in nickel, palladium and platinum it is observed at 2202, 2260 and 2230 cm −1 resp. Measurement of the heat and entropy of adsorption revealed that the initial heat of adsorption is about 12 kcal/mole and that the adsorption of the molecules is immobile at low coverage. Although the high heat of adsorption might suggest chemisorption, it is made plausible, i.a. in view of the non-specificity of the phenomenon to a particular metal, that this type of nitrogen adsorption is, in fact, physisorption.