Adsorbed Nitrogen Atoms Research Articles

溶鉄，溶融Ｆｅ‐ＣｒおよびＦｅ‐Ｃｒ‐Ｎｉ合金の気相との反応速度

The rates of nitrogen desorption of liquid iron and liquid Fe-Cr alloys are explained by the application of the mixed-control model including the mass transfer in the liquid phase and the chemical reaction at the interface where an adsorbed nitrogen atom collides with a nitrogen atom in the liquid phase and then a nitrogen molecule is formed. Although the rate of nitrogen desorption has been regarded as the chemical reaction control, the mass transfer in liquid iron may play an important role on the nitrogen desorption at the lower oxygen range.It is also found that the apparent mass transfer coefficient of nitrogen absorption kNI are illustrated by applying same mixed-control model.The oxygen absorptions of liquid iron, liquid Fe-Cr and Fe-Cr-Ni alloys from gas phase are represented by a model that the most of gaseous oxygen dissolves into liquid iron through oxide free interface, although some of them forms iron oxide at the interface and then dissolves into liquid iron. This model is valid for liquid iron in the range above 0.057atm PO2 and for liquid Fe-18Cr alloy above 0.003atm PO2 When the partial pressure of oxygen is lower than those values, however, the formation of oxide could be practically ignored, so that the rate of oxygen absorption is controlled simply by the mass transfer in gas phase. On the other hand, the amount of oxygen accumulated at the interface as oxide phase in liquid alloy should be larger than that in liquid iron under the same oxygen potential.

Reduction of NO by CO over a silica-supported platinum catalyst: Infrared and kinetic studies

The reduction of NO by CO was studied over a Pt/SiO 2 catalyst at 300°C. Reaction rate data were obtained together with in situ infrared spectra of species adsorbed on the catalyst surface. The performance of the catalyst depended strongly on the ratio of CO/NO in the reactor. For net reducing conditions (CO/NO > 1) the catalyst deactivated with time. Measurements of kinetics taken during a period of slow deactivation showed that the rate of NO reduction was first order in NO and inverse second order in CO. Reactivation of the catalyst was obtained with both oxidizing and reducing pretreatments. Under net oxidizing conditions (CO/NO In situ infrared spectra showed bands corresponding to CO chemisorbed on Pt and NCO and CN groups attached to the support as Si-NCO and Si-CN, respectively. Both the CO and NCO bands were more intense under reducing than oxidizing conditions. The kinetics and product selectivities observed under reducing conditions were found to be in good agreement with a mechanism based upon the dissociation of NO as the rate-limiting step. This mechanism further assumes that N 2 O is formed by reaction of adsorbed NO with an adsorbed nitrogen atom. It is proposed that the loss in catalyst activity is associated with a change in the electronic properties of the supported Pt crystallites, caused by the accumulation of large concentrations of NCO and CN groups on the support in areas adjacent to the crystallites.

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NO ad. The mean adsorption energy is about 25 kcal mole . Dissociation of adsorbed NO starts already at −120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N 2 occurs around 600 °C with an activation energy of about 52 kcal mole . A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of N ad + O ad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H 2. This yields an N ad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ −2, −8.8 and −14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. N ad as well as O ad causes a peak at −5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.

Sputtering of chemisorbed gas (nitrogen on tungsten) by low-energy ions

Flash filament techniques and mass spectrometry have been used to measure sputtering yields for nitrogen chemisorbed on tungsten and bombarded with noble-gas ions in the energy range up to 500 eV. The experimental results show that primarily nitrogen atoms rather than molecules are sputtered. Despite a high binding energy (∼6.7 eV/atom), we find high sputtering yields and low threshold energies. The results are found to be in reasonable agreement with simple theoretical estimates. It is sugguested that, in the investigated energy range, adsorbed nitrogen atoms are sputtered primarily as a consequence of direct knock-on collisions with impinging and/or reflected noble-gas ions. Estimates are also given for the yield of nitrogen atoms knocked off by sputtered tungsten atoms. This latter process is expected to dominate at much higher energies.

Mechanism and isotope effect in ammonia synthesis over molybdenum nitride

The mechanism of ammonia synthesis over molybdenum nitride has been studied by means of 15N tracer, deuterium isotope effect, and kinetics. The kinetic data as well as the deuterium isotope effect indicates that the rate-determining step is the chemisorption of nitrogen retarded by adsorbed nitrogen atoms, which is the same situation as concluded with the unpromoted iron. In agreement with this conclusion, it is found that no isotopic mixing takes place in the reactant nitrogen during the synthesis. The subsequent step of the synthesis, i.e., the hydrogenation of the adsorbed nitrogen, is evidently equilibrated. The deuterium isotope effect in the rate of hydrogenation of preadsorbed nitrogen is caused thermodynamically.

Field-Emission Microscopic Investigation of Catalysed Synthesis of Ammonia on Tungsten

FIELD-EMISSION microscopy was applied to the investigation of the mechanism of the catalysed synthesis of ammonia on tungsten. It was concluded from the result that (1) the dissociative adsorption of nitrogen is considerably accelerated by the presence of hydrogen, and that (2) the rate-determining step of the synthesis is either the formation of adsorbed imino-group NH(a) from adsorbed nitrogen atom N(a) and hydrogen atom H(a), or that of adsorbed amino-group NH2(a) from NH(a) and H(a) but not the dissociative adsorption of nitrogen, as is generally accepted.