Coverage by Intermediates

An interesting aspect of the study of the surface chemistry of the catalyst is the determination of the coverages by intermediates on the surface of the working catalyst. Attempts have been made to observe the intermediates on the surface of the working catalyst by various techniques. Laser Raman spectroscopy shows the following intermediates H [647], N2 [473, 647], N [647], NH [647], NH2 [647], and NH3 [647]. 

N2 has been observed using infra-red spectroscopy [512] and secondary-ion mass-spectroscopy [379] on Fe surfaces exposed to N2 + H2. Arguments in opposition to N2 being the most abundant reaction intermediate the weakness of adsorption for this species [650] and the rapid decomposition of N2H4 to NH3 over Fe [651] 

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 

At conditions where the concentrations of NH3 in the gas phase is significant, the coverages by are high. At these conditions the sequence of 

Evidence for the existence of significant amounts of N has been found from interpretation of reaction orders [607,614,617,652-654], from a comparison of work function, electrical resistance and catalyst activity [655], from laser fluorescence [656], from the observation of N by electron spectroscopy on the catalyst [52] or a Fe single crystal [414, 603, 604] after exposure to NH3 synthesis conditions, and from thermodynamic estimates based on data measured for the intermediates on single crystal surfaces [550]. 

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The coverages by intermediates may be expressed by 0 and the partial pressures of the reactants and products. 

The coverage by intermediates are calculated from the the coverage by free sites, the equilibrium constants and the partial pressures of the gases. 

The reaction rate is calculated from the rate constant, the equilibrium constants and the coverage by intermediates. 

Expression (4.44) Is easy to analyze by considering, as before, different cases of the active center coverage by Intermediates. Some specific cases are considered following. 

As r decreases, increases in value, and the stationary coverage by intermediate Kj increases. For simplicity, let Zi = Z2 = Z- this situation,... 

However, as soon as the stationary coverage by intermediate Kj becomes significant—say, at (6ki/6k) 1... 

The internal variable K can be expressed here, like in Section 4.2, in terms of thermodynamic rushes of active centers and the active center coverage by intermediates. While... 

Find the apparent activation energy of the stationary catalytic process when step 1 is rate limiting and the active center coverage by intermediate K2 is high. What are the rate determining process parameters in this case 3. The stepwise catalytic process... 

Find the dependence of the apparent activation energy of the cata lyzed stepwise reaction on thermodynamic parameters of the processes under consideration when the active center is predominantly covered by the A sorbate. Consider the case when reaction 1 is rate hmiting and, additionally, the active center coverage by intermediate Kj is negligible in the absence of A. 

How will the stationary rate of the catalytic process change when there is a decrease in the size of the active component nanoparticles Is the surface coverage by intermediate Kj minor The rate hmiting step is reaction 1. What if the rate hmiting step is reaction 2 ... 

Thus, it is seen that in practical evaluation of electrocatalysis at various materials, the relative Tafel slope b values, and associated conditions of coverage by intermediates, are as important as the material dependence of logi o values, as discussed in Ref. 131. 

The adsorption behavior of intermediates is usually related to the difference ofTafel (dV/d logi) and potential relaxation ( — dV/d log t) slopes. In the simple case of potential relaxation of a process that does not involve appreciable coverage by intermediates, namely, 9 < 0.02, say, as for the HER at Hg, the kinetics of potential relaxation are derived from the following differential equation ... 

IX. Tafel Slopes and Potential Dependence of Coverage by Intermediates... 

At potential corresponding to the Tafel region and in the transition region, an inductive impedance also occurs as shown in Fig. 5.32c,d. This may be indicative of the relaxation of coverage by intermediate species to the dissolution of silicon. 

Evidence for the existence of significant amounts of has been deduced from interpretations of the reaction orders [512, 614, 658, 659], laser fluorescence [656, 660], and from the study of the dissociation of NH3 /Fe(110) by electron spectroscopy [661]. It is a complication in the deduction of the surface coverages by intermediates that the coverages depend on the operating conditions of the catalyst [396]. These variations may be sufficient to change the nature of the most abundant intermediate [396]. Experimental evidence has been found for changes in the nature of the most abundant reaction intermediates with temperature or promoter concentration [614]. 

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