Molecular nitrogen sticking coefficients

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).

The sticking coefficient is about 0.9 for N2 adsorption on Re (0001) and Re (1120) faces at low temperatures ( 80K). The bond between adsorbed nitrogen and the catalyst surface was considered to be vertical to the molecular axis. The probability of dissociation process is between 10 -10 at high temperatures ( 218K) and depends on the concentration of surface defects. Two molecular forms (/ and 7) were considered as precursors for dissociation. ... 

The promoter action of adsorbed potassium has been attributed both to a stabilization of the molecular a-species as well as to an enhancement of the sticking coefficient for the formation of this a-species. On Fe(lll) the adsorption energy for a-N2 is increased locally in the vicinity of an adsorbed potassium atom from 31 to 44 kJ moP. Parallel to this increase in the adsorption energy for the precursor state its activation energy for dissociation is lowered, as becomes apparent from inspection of Fig. 3.10. Once the nitrogen molecule has become dissociated, the adsorbed N atoms formed near the K atoms diffuse away to the bare iron patches (when the temperature is above 400 K), so that finally the whole available surface is covered by Nad-... 

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