HD formation

Alumiojuffl resists corrosion not because of its position in the electrochemical series but because of the ra Hd formation of a coherent, inert, oxide layer. Contact with grafihite, Fe. Ni. Cu, Ag or Pb is disastrous for corrosion resistance, the effect of contact with steel, Zn and Cd depends on pH and exposure conditions. Protection is enhanced by anodizing the metal this involves immersing it in 15-20% H2SO4 and connecting it to the positive terminal so that it becomes coated with alumina ... 

Another interesting response in hydrogen metabolism was observed by Hoch et al. [27] They found that N2-fixing organisms exposed to D2 in the presence of N2 formed HD. They referred to this as an exchange reaction. Later Bulen [28] indicated that it was improper to refer to this as an exchange reaction as it involved electron transfer. He also pointed to the factors in common between H2 inhibition and HD formation. 

There has been some controversy about the need for N2 in the formation of HD. Burgess et al. [29] reported that N2 was not required. They used argon as their diluent gas and took the word of the supplier that it was free of N2. Not only is commercial argon seldom free of N2, but it is difficult to remove the last traces of N2, and very little N2 is required to support HD formation. To settle this difference in experimental observations, Li and Burris [30] made it a point to rid their diluent gas of contaminating N2. One can absorb N2 on molecular sieve at liquid N2 temperature the problem is that argon liquefies and freezes before you get down to die temperature of liquid N2. So Li used neon as his inert gas and captured any contaminating N2 on molecular sieve in a liquid N2 bath. When the atmosphere above the nitrogen-ase system was carefully freed of N2 there was no formation of HD. 

N as a tracer, ammonia 106 exchange reaction 105 glutamic add 106 GOGAT system 106 radioactive tracer 106 N2 and HD formation 109 N2 fixation in non-leguminous plants, actinomycetes 110 actinorhizal systems 110 NAD 74 nanocrystal 263 nanocrystalline materials 171... 

A difficnlty in measnring D/H isotope ratios is that, along with the H2+ and HD+ formation in the ion source, H3+ is produced as a by-prodnct of ion-molecule collisions. Therefore, a H3+ correction has to be made. The amonnt of H3+ formed is directly proportional to the number of H2 molecules and H+ ions. Generally the H3+ current measured for hydrogen from ocean water is on the order of 16% of the total mass 3. The relevant procedures for correction have been evaluated by Brand (2002). 

Sellmann, D. Sutter, J. Biological N2 fixation Molecular mechanism of the nitrogen-ase catalyzed N2 dependent HD-formation, the N2 fixation inhibition and the open-side FeMoco model, Perspectives in Coordination Chemistry , Vol. 5 Eds. Trzeciak, A. M. Sobota, D. Ziolkowski, J. University of Wroclaw Poland, 2000. 

Sellmann s model is claimed to be consistent with the severe constraints imposed on this N2-dependent HD formation from D2 and protons. Other modeling studies have shown that protons can be transferred from acidic H2 ligands in cationic Ru-H2 complexes to N2 ligands in W(N2)2(P)4 complexes (P = phosphine donor), in some cases even forming ammonia [Eq. (22)] (88,89). 

Collision System Lattice Type Coverage (ML) HD Formation H2 or D2 Formation Sticking Bulk H or D Reflection... 

Figure 11 Experimental HD formation rates for H atoms incident on D-covered Ni(100) surfaces. The different initial D atom coverages are listed in the figure. The metal temperature is 120K. The beam flux, starting at t = 0, is 0.07 ML/s. Taken from Ref. [24].

Kiippers and co-workers have successfully used the random walk form to reproduce the behavior observed on Cu(l 11), Pt(lll) and Ni(l 0 0) [26]. They demonstrate that the different behaviors with regard to the short time (pre-saturation) HD formation rates can be explained in terms of the relative rates of hot atom reaction and sticking. We have used our kinetic equations to derive approximate analytical expressions for initial reaction rates and product yields as a function of the initial surface coverage, and these have compared well with the experimental findings of the Kiippers group [37]. 

The used mixture of H2, D2 and N2 (16.0 cm3/min of H2 and D2 each and 79,2 cm3/min of N2 0dC, 1 atm) was analyzed with the MS. It was possible to let the gas flow either through the reactor to the MS or directly bypassing the reactor or through the "equilibrium reactor". In this way three different gas compositions could be analyzed 1) The gas going directly to the MS, i.e. the same as the reactor inlet, 2) The outlet of the reactor, 3) The equilibrium composition. By this method it was easy to compensate for HD in the D. feed and calculate the rate of the HD formation reaction. While bypassing the reactor with the H2-D2 mixture, the reactor was flushed with nitrogen instead. 

The reactor was considered being a ideal plug flow reactor when the rate constant for the HD formation was calculated. Because of experimental restrictions it was difficult to determine the HD formation rate when the H2-D2 reaction was close to equilibrium This will... 

Figure 2. Deactivation profiles for experiments of type 1 and 2. Rate constants for HD formation in mol/(s, g cat., Pa) 10. The first GC-analyses were done 1.5 minutes after a stop and are marked with gray squares and rombs. In the end of the type 1 runs, three stops without exposure to hydrogen were performed (marked with black squares and rombs).

The results from the H -D2 experiments are shown in Figures 2 and 3. In Figures 4 and 5 the propane dehydrogenation conversion just before an H2-D2 experiment have been related to the HD formation rate. Experiments from all the runs were used. The TOF, based on the number of hydrogen chemisorption sites on a fresh catalyst, were calculated from the rate... 

For the fresh catalysts the rate of the HD formation is much larger on the monometallic platinum catalyst than on Pt-Sn. The TOF for the fresh Pt- n and Pt catalyst is estimated to 150 and 540 s"1 respectively. 

After the Pt-Sn catalyst was exposed to the reaction mixture for two minutes, the HD formation rate fell to about a third compared to the activity for the fresh catalyst. For the platinum catalyst, the activity fell to l/30th after only 15 seconds dehydrogenation reaction,... 

The dehydrogenation reaction and the H2-D2 reaction are proportional in activity as seen in the linear parts at higher conversions in Figures 4 and 5, During this period the coke will build up on the metal giving rise to the fast deactivation. After 10 h on stream, the activity for the HD formation was about 15% for the Pt-Sn and 0.6% for the Pt catalyst, of the original activity. The rate remained at this level even if the deactivation went on for another HOh,... 

Figure 5. Propane conversion immediately before an experiment versus the rate constant for HD formation in (mol/s, g cat., Pa) 10 9, for the Pt catalyst...

The continuous decrease in the propane dehydrogenation activity, with constant HD formation rate can have different origins. Calculations indicate that diffusion limitations may play a role in the propane dehydrogenation, but not in the H2-D2 reaction, when the amount of coke in the pores of the catalyst is high. Different structure sensitivity for the two reactions might also contribute to this effect. Somorjai [8] showed that the H2-D2 reaction is structure sensitive. For the propane dehydrogenation, on the other hand, fiiloen et al. [9] found that only one Ft atom is necessary for the reaction to proceed. 

The H2-D2 equilibration reaction was shown to be useful as a probe for measuring the metal area not covered by coke, on Pt/Al203 and Pt-Sn/AL03 catalysts deactivated during propane dehydrogenation. Problems with the method are the effect that the repeated stops have on the dehydrogenation deactivation profile, and the difficulties in correlating the HD formation rate to free metal area. 

The fresh Pt catalyst was found to be much more active than the bimetallic one, for the HD formation. After the catalysts were exposed to the dehydrogenation reaction mixture, the activity fell to only a fraction of the original activity. The decrease was much higher for the Pt than the Pt-Sn catalyst. 

Intermediates in the Fixation of Dinitrogen. There are still no spectroscopically detected intermediates in the reduction of dinitrogen to ammonia. We believe, however, that the electron balance studies with nitrogenase under dinitrogen/dihydrogen and dinitrogen/dideuterium atmospheres point to the existence of such intermediates. The stoichiometry for the HD formation reaction as determined experimentally is shown in Reaction 6. At first glance, this resembles the substrate reactions... 

Modeling the Nitrogenase Catalyzed N2 Dependent HD Formation with Diazene Complexes / 664... 

Severe constraints were found to be imposed upon this HD formation (154). (1) Electron balance studies showed that one electron is needed per HD formed such that Eq. 60 holds. 

---