Adsorption states of nitrogen

The study on the pattern of nitrogen chemisorption is very important for the understanding of reaction mechanism of ammonia synthesis. But the chemisorption of nitrogen on the surface of transient metals is very complex. Taking chemisorption on iron catalyst for ammonia synthesis as an example, it was found that the formation rate of ammonia depends on pre-adsorption temperature if the adsorption of nitrogen was preceded first on the surface of activated catalyst, and then hydrogen was passed on surface of pre-adsorbed nitrogen. It is considered that there exist two adsorption patterns The first one is of L-type which appeared at 200°C of pre-adsorption temperature, in which the formation rate of ammonia was proportional to second one is of H-type which appeared at 400°C 40°C of 

There is very different behavior for the two-type adsorption states. For instance, H-type nitrogen has no effect on CO adsorption, while L-type nitrogen inhibits CO adsorption. L-type is more active for ammonia synthesis than H-type. If iron nitride was hydrogenated to ammonia, the formation rate of ammonia was also proportional to P, indicating that the H-tjrpe adsorption state of nitrogen is similar to that on iron nitride, i.e., one nitrogen atom is coordinated with several iron atoms, 

However, there still is no common sense about that the nitrogen adsorption on iron catalyst for ammonia S3mthesis is dissociation or not. 

Based on molecule adsorption of N2, the reaction mechanism is as follows, 

The relation between [N ] determined by Aiiger energy spectroscopy and ph2 illustrates that the adsorption of N2 on the surface of ammonia synthesis catalyst was dissociated. If its adsorption was in molecular state, N should be independent of PH2 aud was constant, as it plays no role in kinetics. 

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The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

On Rh surfaces, only desorption of N2 and NO is observed in the TDS. Figure 6 shows TDS for NO on two stepped Rh surfaces, Rh(533) [structure 4(111) X (100)] and Rh(410) [structure 4(100) X (100)]. The adsorption of NO and the effect of preadsorbed O and N on the adsorption of NO have been studied on these surfaces and on many other Rh surfaces by Janssen et al. [28). The N atoms are markedly more strongly bound on (100) terraces than on (111) terraces. The presence of steps does not affect the thermal stability of Nads on Rh. At higher NO exposures, repulsive N-N and N-0 interactions lower the thermal stability of Nads- Following saturation NO exposure, N2 desorbs in a single state from (100) terraces at 750 K. From (111) terraces several desorption states of nitrogen appear at temperaturs between 450 and 700 K. An important observation is that recombination of Nads and Oads to give NO is more favorable than the 2 Nads N2 reaction when Rh with precovered Oads is exposed to NO. 

The Horvath and Kawazoe (HK) method [39] was developed to determine the PSD of active carbons from nitrogen adsorption isotherm. All pores are assumed to have slit shape. This method rests on the assumption that the adsorption state of a pore is either empty or completely fiUed. The demarcation pressure between these two states is called the pore-filling pressure, and it is a function of pore width. The equilibrium of a pore exposed to a bulk phase of constant chemical potential is obtained from the minimization of the following grand thermodynamic potential ... 

The nitrogen adsorption isotherm is determined for a finely divided, nonporous solid. It is found that at = 0.5, P/P is 0.05 at 77 K, gnd P/F is 0.2 at 90 K. Calculate the isosteric heat of adsorption, and AS and AC for adsorption at 77 K. Write the statement of the process to which your calculated quantities correspond. Explain whether the state of the adsorbed N2 appears to be more nearly gaslike or liquidlike. The normal boiling point of N2 is 77 K, and its heat of vaporization is 1.35 kcal/mol. 

Since nitrous oxide was cut off from the feed stream, the sum of evolved nitrous oxide and nitrogen is equal to the adsorbed amount of nitrous oxide on the catalyst in stationary state of the reaction. This amount is extremely small compared to that on CuO and this fact also implys that the adsorption of nitrous oxide could be the slowest step in the overall reaction of nitrous oxide decomposition on MgO. 

Although no high resolution studies have been reported for ammonia interaction with iron surfaces, two main states of adsorption were recognized. At 80 K adsorption is entirely molecular with a characteristic N(ls) binding energy of 400 eV, but on warming the adlayer to 290 K the N(ls) intensity is mainly at 397 eV, typical of chemisorbed nitrogen adatoms with only a small contribution at 400 eV. 

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