Kazuo Urabe

Activation of nitrogen by alkali metal-promoted transition metal: II. Isotopic exchange in molecular nitrogen over potassium-promoted ruthenium-carbon catalyst

The isotopic exchange in nitrogen has been investigated over the potassium-promoted ruthenium-carbon catalysts between 220 and 300 °C. The reaction readily takes place in this temperature range with rate R given by R = A exp(−22 000 ~ −23 000/RT)P12. From the 15N tracer experiment, all the adsorbed nitrogen was proved to be displaceable by the gas phase nitrogen. A slight retardation of the exchange rate is observed in the presence of hydrogen. The exchange rate and the ammonia synthesis rate are of the same magnitude.

Activation of nitrogen by alkali metal-promoted transition metal: VI. Hydrogen effect on isotopic equilibration of nitrogen and rate-determining step of ammonia synthesis on potassium-promoted ruthenium catalysts

Abstract The effect of hydrogen on isotopic equilibration of nitrogen was studied on ruthenium catalysts with and without potassium at PN2 = 40–150 mm Hg, PH2 = 7–180 mm Hg and 300–450 °C. The equilibration rate is increased on pure Ru but is decreased on RuK by the addition of hydrogen. The rate of ammonia synthesis is correspondingly faster than equilibration on pure Ru but is slower on RuK. The equilibration rate on RuK is decreased by condensing the ammonia produced in the presence of hydrogen and by increasing the H 2 N 2 ratio and approaches negligible value at H 2 N 2 = 3 , demonstrating that the rate-determining step of ammonia synthesis is the dissociation of nitrogen molecule. The equilibration rate on RuK in the presence of ammonia also decreases as hydrogen pressure increases, whereas the reaction order with respect to nitrogen pressure is higher than that in the absence of hydrogen, suggesting that the adsorbed hydrogen exhibits a retarding effect on the adsorption of nitrogen. The surface area of Ru-K was found to increase on hydrogen treatment of nitrogen covered surface, suggesting a corrosive adsorption of nitrogen to form a nitride on RuK.

Activation of nitrogen by alkali metal-promoted transition metal: IV. Effect of potassium on the kinetics of isotopic equilibration of nitrogen on ruthenium catalysts

The kinetics of isotopic equilibration of nitrogen was studied on ruthenium catalysts with and without added potassium at 280–480 °C and 40–200 mm Hg. The first order kinetics observed on Ru-Al2O3 is changed by the potassium addition into fractional order kinetics which approaches first order as the temperature increases. The change in kinetics is accompanied by an increase in apparent activation energy, whereas the equilibration activity is increased by two or three orders of magnitude. The kinetics are consistent with a Langmuir type rate equation for the dissociative chemisorption of nitrogen, from which the effective value of adsorption constant and thus the effective value of heat of chemisorption may be estimated. The change in kinetics as well as the estimated value of the heat of chemisorption (40 ± 6 kcal/mol on Ru-K) demonstrates that the addition of potassium intensifies the adsorption strength of ruthenium towards nitrogen.

Infrared active adsorbed nitrogen on alkali metal-promoted transition metal-alumina catalyst

Abstract An infrared band of chemisorbed nitrogen is observed at 2020 cm −1 on RuAl 2 O 3 -K catalyst which is active for the ammonia synthesis. Similar bands are observed on analogous catalysts with Rh and Re, and also with Na. This species is formed by the adsorption at above 200 °C and disappears gradually on hydrogenation at above 260 °C, resulting in the formation of ammonia, while the infrared band is ascribed to an undissociated form of adsorbed nitrogen. The large extinction coefficient observed suggests that the mode of adsorption is of end-on type. The heterogeneity of adsorption site is suggested on the basis of peak shift observed during adsorption and subsequent hydrogenation.

Ammonia synthesis activity of a Raney ruthenium catalyst

Abstract A Raney ruthenium catalyst prepared from a Ru Al ( 1 3 ) alloy exhibits a high activity for ammonia synthesis. The apparent activation energy of ammonia synthesis is lower on the Raney catalyst than on pure ruthenium by 5 kcal/mol. The specific activity with respect to the number of surface ruthenium atoms determined by chemisorption of hydrogen as well as carbon monoxide is three times higher on the Raney catalyst at 300 °C, and the addition of metallic potassium brings about a further 12-fold increase in the activity, retaining the lower activation energy. The higher specific activity of the Raney catalyst discloses a promoter effect of aluminum.

Potassium-dinitrogen-ruthenium complex as an active catalyst for nitrogen fixation

Abstract Potassium uptake by metallic ruthenium is enhanced and stabilized by the presence of nitrogen, accompanying a significant nitrogen uptake. The stabilized part of potassium uptake which can not be removed by evacuation at 350 °C is in a stoichiometric relation to the nitrogen uptake, suggesting that the uptake is caused by formation of a ternary compound (KN 2 Ru) n . Electric resistance of ruthenium film seems to be decreased slightly by formation of the ternary compound. The potassium adsorbed on this compound gives rise to an enhanced activity for the isotopic equilibration of dinitrogen. The dinitrogen nature of the ternary compound is supported by formation of hydrazine in addition to ammonia and a dinitrogen complex of ruthenium upon hydrolysis as well as ethanolysis.

Activation of nitrogen by alkali metal-promoted transition metal: VIII. Reactivity of sorbed nitrogen on Ru-KAl2O3 catalyst

A considerable amount of nitrogen is absorbed by Ru-KAl2O3 while nitrogen is hardly adsorbed by RuAl2O3 or KAl2O3. Although the absorbed nitrogen can be converted to ammonia by hydrogen treatment above about 270 °C, the rate of hydrogenation is much slower than the rate of ammonia synthesis on the catalyst. The isotopic mixing between the sorbed and gaseous nitrogen also becomes detectable above about 270 °C, while the isotopic equilibration of nitrogen takes place readily at 250 °C. There can be at least three different species of sorbed nitrogen, the absorbed one being predominant in amount and much less reactive.

Isotopic equilibration of nitrogen on potassium-promoted transition metal catalysts

The equilibrium showed a trend similar to that previously reported for ammonia synthesis, i.e., the ruthenium/potassium catalyst was the most active one. The catalysts were prepared by impregnation of activated carbon with a transition metal chloride, followed by hydrogen reduction and addition of 17-20Vertical Bar3< potassium metal. The apparent activation energies of ruthenium, rhodium, osmium, and iridium were 22-26 kcal/mole, which probably reflected their activation energy of adsorption. The less noble metals rhenium, molybdenum, iron, cobalt, and nickel had apparent activation energies of 31-40 kcal/mole, which probably corresponded to their activation energy of desorption because of their greater affinity to nitrogen.

Infrared absorption spectra of adsorbed dinitrogen on alkali-promoted ruthenium in the 500 cm−1 region

An infrared band of chemisorbed nitrogen as well as of carbon monoxide was observed in the 500 cm −1 region on Ru dispersed in a KBr matrix and promoted with K or Na. The band was unaffected by evacuation at room temperature, while it disappeared upon hydrogen treatment at 350 °C, simultaneously forming ammonia. The band shift caused by the change from K to Na as well as the above features is in accord with the behavior of the 2020 cm −1 band observed on Ru/Al 2 O 3 /K. Thus the band is assigned to the Ru N stretching vibration of chemisorbed dinitrogen.

Infrared spectra of adsorbed dinitrogen on alkali metal-promoted transition metal–alumina catalyst

I.r. absorption peaks of dinitrogen on Ru–, Rh–, and Re–Al2O3 catalysts promoted by Na or K, in which form N2 can readily react with H2 to form NH3, are observed at 2020–2040 cm–1.