Desorption rate constraints

The vapor pressure of materials as well as the desorption rate of adsorbates increase with rising temperature. Therefore, once closed, UHV chambers are subjected to a bake-out procedure, that is, the whole vacuum system is heated to 150-250 °C for many hours while pumping. Likewise, filaments and the samples themselves have to be preheated to temperatures higher than in the following experiments in order to outgas unwanted contaminants (specific sample cleaning, see Section 3.1.4.1). Obviously, bake-out and outgassing temperatures put constraint on the materials that can be used for and in a UHV system. 

MN acetylation is most likely related to the high polarity and bulkiness of the products with limitations in the reaction rate by product desorption. Dealumination would have a positive effect on the acetylation rate because of the decrease in the zeolite hydrophilicity and of the increase in the rate of diffusion of the bulky products owing to elimination of extra-framework A1 species. Curiously, in anisole acetylation, the Si/Al ratio of the HBEA zeolite had practically no effect on the reaction rate. However it is worth noting that most of the tested samples had Si/Al ratios between 11 and 30. Like for 2-MN acetylation,[28,32] the performance of HBEA zeolites in anisole acetylation depends on their crystallite size.[17] This was shown by comparing the activities of samples with large size (0.1-0.4 pm) and of a nanosize sample (0.01-0.02 pm) prepared within the pores of a carbon black matrix. The superior performance of the nanosize sample was ascribed to a decrease in diffusional constraints limiting the desorption of the bulky and polar p-methoxyacetophenone product from the BEA micropores. 

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

The main reactions of product formation (chain desorption) according to (ref. 1) have been already visualized above the dissociative reaction of an alkyl species ( 15) to yield the a-olefin or its associative reaction with hydrogen 0) to yield the paraffin. These chemi-desorption reactions are the slow steps of the Fischer Tropsch mechanism. The paraffin chemi-desorption is e.g. about four times slower than the olefin chemi-desorption (refs. 1,2) and the value of the rate constant of product desorption (paraffin plus olefin) is e.g. only 1/5th to 1/1Oth of the rate constant of chain prolongation. These constraints are most essential for establishing a Fischer Tropsch system. 

This picture of a substantial phosphorus constraint is drastically altered when the presence of the inorganic labile (i.e., sorbed) phosphorus pool is taken into account (Scenario C). Desorption of phosphate occurs in response to increased rates of removal from the soil solution. Consequently, the reduction in soil solution phosphorus concentration over 1730 levels is only 9% for 1981-1990. This contrasts with the 25% reduction in Scenario B. Consequently, the enhancements of Gp and Np are more similar to the no-P-constraint case, though a full expression of the COi-in-duced growth response is still not possible. Accordingly, the rate of net carbon accumulation by the ecosystem is 6.5 mol m - year", substantially more than Scenario B, but about 20% less than what is modeled to be the case if no phosphorus limitations to plant production occurred. 

With an adequate amount of ammonia being adsorbed on the SCR substrate, high NOx reduction rate can be realized. It is thus believed that consistent SCR NOx reduction can be ensured by ammonia storage control. However, besides the NOx reduction, tailpipe ammonia slip constraint is another objective needs to be taken into account. From the SCR model in Eq. (14.29) and the ammonia adsorption/desorption reactions in Eq. (14.4), it can be seen that high ammonia... 

---