Turnover frequency ammonia synthesis

Figure 8.25. Predicted volcano plots for ammonia synthesis, showing the turnover frequency versus the relative bonding strength of N atoms to the surface for ammonia concentrations of 5%, 20%, and 90%. The left-hand panel corresponds to conditions of...

Another test of validity is to check the performance of the model against experimental rate data obtained far from equilibrium. The microkinetic model presented in Table 7.3.1 predicts within a factor of 5 the turnover frequency of ammonia synthesis on magnesia-supported iron particles at 678 K and an ammonia concentration equal to 20 percent of the equilibrium value. This level of agreement is reasonable considering that the catalyst did not contain promoters and that the site density may have been overestimated. The model in Table 7.3.1 also predicts within a factor of 5 the rate of ammonia synthesis over an Fe(lll) single crystal at 20 bar and 748 K at ammonia concentrations less than 1.5 percent of the equilibrium value. 

Fig. 7. Structure sensitivity in the ammonia synthesis over rhenium single-crystal faces. The turnover frequencies (TF) are given as NH3 molecules/cm2/s (PIotai = 20 atm, H2 N2 = 3 1, Tcryslai = 870 K). Schematics of the atomic structure of each surface are given above each bar. From Ref. 85.

Ammonia synthesis reaction rate about three kinds of catalysts with different loading amount of ruthenium and promoters were studied. Based on O2 chemisorption data, the relationship between the turnover of frequency (TOF) and the ruthenium particle size is shown in Fig. 6.51. A monotonic increase in TOF02 vs do2 is characteristic for each system. Extrapolation of the results to small crystallite diameters suggests (Fig. 6.51) that extra fine particles smaller than 0.7nm 0.8 nm (critical size) might be totally inactive. Analogous trends in the surface activities were found (not shown) when the amount of adsorbed CO were used instead of O2 uptake for the particle diameter and TOF calculation. 

But this proposal is not so strict scientifically due to the lack of clear definition of activity. The catalytic activity of ammonia synthesis catalyst can be expressed by outlet ammonia concentration of converter, conversion ratio of ammonia, reaction rate and rate constant of kinetics and turnover frequency of ammonia (TOF). 

Fig. 10.1 Calculated turnover frequencies for ammonia synthesis as a function of...

The calculations show that there are few free sites and that the surface may be viewed as a surface nitride with a few vacancies. As the rate of ammonia synthesis may be expressed as a turnover frequency multiplied by the coverage... 

From the numerical models on the synthesis of ammonia the turnover frequency is readily available. The turnover frequency depends on the operating conditions [396]. The temperature is a particularly important parameter as the turnover frequency increases by 5 orders of magnitude from 500 to 1000 K at 10.1 MPa and a 28% approach to equilibrium [396]. 

FIGURE 7.8 Turnover frequency (TOP) of ammonia synthesis as a function of the dissociative chemisorption energy of nitrogen. Top panel Experimental data from Aika et al. (1973). Middle panel Result of the microkinetic model for stepped metal surfaces (blue Une). Reaction conditions are 673 K, 100 bar, Hj N2 ratio of 3 1, and y = 0.1. The effect of potassium promotion has been included (red Une). Effects of promotion will be discussed in Chapter 12. Lower panel Microkinetic model using a two-site model for the adsorption of intermediates. Adapted from Vojvodic et al. (2014). 

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