Modeling micro-activity

As described above, the activity of a catalyst can be measured by mounting it in a plug flow reactor and measuring its intrinsic reactivity outside equilibrium, with well-defined gas mixtures and temperatures. This makes it possible to obtain data that can be compared with micro-kinetic modeling. A common problem with such experiments materializes when the rate becomes high. Operating dose to the limit of zero conversion can be achieved by diluting the catalyst with support material. 

If one can understand what the basic parameters of the reactants and the surface are that determine the reaction dynamics (activation barriers etc.) then given a micro-kinetic model one has a knowledge of the factors determining the catalytic activity of the catalyst. 

If the aim is to explore the mechanism of the reaction and understand which are the important parameters of the catalyst determining the activity, then a micro-kinetic model is needed. A micro-kinetic model is based on a detailed mechanism and independent information about the rates of the elementary steps involved and the stability of the intermediates. The micro-kinetic model is the synthesis of all the basic knowledge about a reaction over a given catalyst. 

In spite of the shortcomings of the modelling, the real strength is that they can be used to understand ( em variations in the catalytic activity from one system to another. The stability of the intermediates and the activation barriers are among the input parameters for the micro-kinetic model, and it is straight forward to calculate the effects of changes in stability for some or all the intermediates. 

The activation enthalpy for the catalytic reaction may be calculated from the micro-kinetic model as... 

In this work Micro Activity Tests with an aluminosilicate MCM-41 and a silica-containing VPI-5 were carried out using n-hexadecane as the model feed. Special emphasis was given to the catalytic activity, thermal stability and selectivity of the different materials studied in comparison with a commercial equilibrium FCC catalyst. Additionally, the possible structural and textural changes during the above-mentioned catalytic process were characterized. 

M..Forissier and J.R.Bemard, Modeling the Micro-activity Test of FCC Catalysts to Compute... 

Surface segregation of Pd in Pd—Rh catalysts suppresses NOx reduction [61]. De Sarkar and Khanra studied the segregation difference between Pd—Rh and Pt—Rh nanoparticles, and the influence of sulfur in fuel on CO oxidation and NO. They used Monte-Carlo (MC) simulation to predict the surface composition of PtsoRhso and PdsoRhso particles (2406 atoms for 4nm particles). TTiey used a micro-kinetic model to compare the activities of both soHds for reactions of CO -i- O2, CO -I- NO and CO -1- NO -1- O2, and found that Pt and Pd segregate to the particles surface, especially in the Pd catalyst, which is clearly better for CO oxidation, while Pt—Rh is a better catalyst for NO reduction. For both reactions, sulfur poisons the Pd—Rh catalyst more than the Pt—Rh catalyst [62]. 

Ertl et al. also reported that the apparent activation energy of the dissociated nitrogen adsorption obtained on Fe (111) promoted by K at near 400K is -12kJ mol. The values predicted by these micro-kinetic models are 13kJ mol -13kJ mol and -O.SkJ mol, respectively for the rate constants of steps 1 and 2 in the mechanism. Thus, the prediction by microkinetic model III is not successful. 

The synthesis conditions are 400°C, 50 bar, gas composition (H2 N2) 3 1 containing 5% NH3. The numbers are obtained by combining a micro-kinetic model describing ammonia synthesis rates with the linear relation existing between the potential energy and the activation energy for N2 dissociation. The known entropy barrier for N2 dissociation and the effect of adding electropositive promoters such as K and Cs have been taken into account in the model. 

Students in this group always worked simultaneously with the macro, sub-micro and symbolic levels. They did so trying to make the representation in one level be a support for the development of both a representation in another level and a plausible model that could explain the systems they observed. In Activity 3, when building... 

Figure 3.40 Experimental results for isoprene conversion in metallic and ceramic micro reactors. The metallic micro reactors were operated without catalyst to determine blank activity of the various construction materials. In addition, conversion data were calculated. (0) Calculated values for micro-channel reactor model (full symbols) experimental values for different reactor materials [27].

Hence the investigations made so far were aimed more at showing the capability of an automated micro-reactor system, using the Knorr synthesis as a model reaction however, to a certain extent, information on this synthesis itself was also gained (see Section 4.9.6.5) [20]. The Knorr synthesis of pyrazoles chosen is of industrial interest since by this route compounds with a wide range of biological activity can be produced. 

Pyrrolopyrazine 22 has an IC50 of 1.7nM against Syk [88] and significantly decreased ankle swelling in prophylactic and therapeutic rat CIA models, and micro-CT scans of ankle joints showed significant inhibition of bone erosion. While 22 is clearly a potent Syk inhibitor, no selectivity or in vitro functional data has been reported. The pyrrolopyrazine scaffold appears in a series of compounds, exemplified by 23, that are substituted in the pyrrole and pyrazine rings [89-92]. These compounds are claimed to inhibit Syk and JAK, but the limited data disclosed suggests that JAK is the dominant activity. 

Hinrichsen, Muhler, and co workers—micro kinetic analysis parameterized by redox model. Hinrichsen et al.317 investigated the elementary steps by micro kinetic analysis by applying temperature and concentration-programmed experiments over Cu/Zn0/Al203, and modeling the data with the simplified redox mechanism in the spirit of Ovesen, Topsoe, and coworkers.303 This included 3 steps (1) dissociative adsorption of H2 on Cu metallic surface (2) dissociative adsorption of H20 leading to an adsorbed H2 molecule and an O adatom and a reduction step by CO to form gas phase C02 and a free active site (see Scheme 71). 

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