Sticking coefficient of nitrogen

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,...

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). 

The statistical thermodymamic Fowler-Guggenheim treatment of adsorption was applied, and the equilibrium constant of each reaction step in terms of partition functions was calculated, after the introduction of various additional assumptions. The experimental data of Ertl for a potassium-covered iron surface (at optimum potassium coverage) were used, but a much lower value for the nitrogen atom chemisorption energy than quoted by Ertl was adopted. This value and the sticking coefficient of nitrogen were recognized as the parameters on which the results depend most sensitively. 

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).

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. 

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. 

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. 

Ertl reported that the sticking coefficients of dissociated nitrogen adsorption obtained on Fe (111) at 430K were in the range of 5 x 10 for clean surface to 4 X 10 for optimum surface promoted by K. The values estimated by the analysis of micro-kinetics are 1.4 X 10 1.2 X 10 and 1.2 x 10 for model I, II and III,... 

The microkinetics analysis, as a useful and powerful tool to interpret, harmonize and consolidate the study of catal3dic phenomena, can describe various results obtained at wide experimental conditions. For ammonia synthesis reaction discussed in this section, the microkinetic models are evaluated from the experimental data such as the sticking coefficient of dissociated nitrogen adsorption, the spectrum of programmed-temperature desorption of adsorbed nitrogen as well as the kinetics of ammonia synthesis at industrial conditions and at laboratory conditions are far from equilibrium. 

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. 

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. 

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 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. 

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