Nitrogen sticking coefficient

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

Figure 7.19. (Left-hand side) Comparison between experimental sticking coefficients of N2 on Fe(l 11) and the prediction on the basis of Eq. (57) with an activation energy of 0.03 eV. (Right-hand side) Potential energy diagram for molecular nitrogen dissociating on Fe(l 11).

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

Figure 8.27. Enhancement ofthe sticking coefficients of nitrogen by promoting the three basal plane of iron with potassium. [Adapted from G.A. Somorjai and M. Materer, Top,...

Assuming a sticking coefficient equal to unity, a residual atmosphere (p = 10 Pa) composed of oxygen and nitrogen, and room temperature, about lO sec are needed to build a hundredth of a monolayer, which corresponds to the detection threshold of photoelectron spectroscopy. 

Table 10.1 Dependence of the typical mean free path A and time for monolayer coverage r (assuming a sticking coefficient of one) on pressure P for nitrogen at 20°C.

The effusate which condensed on liquid nitrogen cooled copper collection targets was assayed by X-ray fluorescence. The Eu La radiation was determined at the peak maximum (26 = 36.84°, graphite analyzing crystal) by an external standard technique. Previous data (15, 19) have indicated the sticking coefficient of gaseous europium halide on chilled copper is approximately unity. 

Figure 2. Temperature coefficient and surface coverage dependence of sticking coefficient of nitrogen on tungsten...

Figure 4.9. (a) The sticking coefficients of O2 on the flat Pt(l 11) and stepped Pt[14(l 11) x (111)1 crystal faces as a function of oxygen coverage [94]. (b) The sticking coefficients of nitrogen as a function of step density on various crystal faces of tungsten (95). 

Similar conclusions had already been reached many years ago by Emmett and Brunauer [45], who measured the uptake of nitrogen by commercial catalysts and concluded likewise that the sticking coefficient is only on the order of 10. ... 

FIGURE 5.3. Arrhenius diagram Oog Sq versus 1/7) for the sticking coefficient for dissociative nitrogen surface at a Ru(0001) surface, and the influence of a small concentration of Au atoms blocking the "active sites" at the steps [12]. 

The consequences for the kinetics of adsorption and desorption are quite obvious. The occupation of adsorption sites by a second species will usually reduce the sticking coefficient. This may lead to the effect that under steady-state conditions, the more strongly held adsorbate is not necessarily most abundant on the surface, as will become obvious in Section 6.3, with the oxidation of CO on Pt. A second species may, however, also increase the sticking coefficient and therefore acts as promoter. This will be outlined in Section 6.1 with the effect of K on the sticking coefficient for nitrogen in catalytic ammonia synthesis. 

FIGURE 6.5. The sticking coefficient for dissociative nitrogen adsorption on Fe (111), open circles [16], open square [19], and filled squares [20], experimental data. Full line and desk-dotted line theoretical data for the indirect and direct path, respectively [22]. 

The reaction proceeds along the same elementary steps as with the Fe catalyst, and again successful microkinetic modeling on the basis of experimentally derived parameters could be achieved [46]. Again, dissociative nitrogen adsorption is rate limiting, where the sticking coefficient is markedly affected by the presence of atomic steps [47] whose role as "active sites" had been discussed in Chapter 5. 

Catalysts play an important role in overcoming the activation barrier in ammonia synthesis. It is weU known that strong N=N triple bond and the low sticking coefficient of the molecule nitrogen limit the choice of catalyst. However, the mechanism of ammonia formation on an electrocatalyst seems to be different from that of the conventional catalyst. The information about the conventional catalyst in the Haber-Bosch process and the electrocatalyst in the electrocatalytic membrane reactor are described in this section. 

Fig. 9. Arrhenius plot of the initial sticking coefficient for dissociative nitrogen adsorption on Fe(lll). Experimental results (0-0, Ertl and co-workers , Grunze and 0, Al-strup and co-workers) compared with theoretical data along the indirect (full line) and direct (dash-dotted line) routes. From Ref (35).

The role of potassium as an electronic promoter is in increasing the sticking coefficient for dissociative adsorption of nitrogen by stabilizing both the a-state and the transition state. In the presence of adsorbed K alone, the sticking coefficient was found to be increased up to 4 x 10 , whereby the enhancement was stronger for Fe(llO) and Fe(lOO) than for Fe(lll) and so the differences between the clean surfaces became almost eliminated. 

The reaction proceeds along the same elementary steps as with the Fe catalyst outlined in the earlier text and again successful modeling of the kinetics in terms of the rate parameters for the elementary steps could be achieved (52). Similarly as with Fe, dissociative chemisorption of nitrogen is rate limiting, and the sticking coefficient on pure Ru at room temperature was found to be extremely low (only 10 (53). However, contrary to Fe, adsorbed hydrogen inhibits N2 ad-... 

Table 2.2 lists the TPD data obtained on the sm-face of single crystal W. From weaker interaction between adsorbed nitrogen and surface W atoms, it can be concluded that 7 state occurs for all three W faces (100), (110) and (111) and a only for (111), confirmed by FEM and pore-probe emission spectroscopy. There is also much difference in values of N2 sticking coefficients 0.3-0.6(Refs. 73-75) for (100) face, 0.01-0.005 for (110) and 0.08 for (111), confirming that / adsorption... 

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