Simulation, of ethylene hydrogenation

Snapshot of a Pd(lOO) surface during a simulation of ethylene hydrogenation at 298 K, 25 torr of ethylene and 100 torr of dihydrogen. [Figure from First-Principles-Based Monte Carlo Simulation of Ethylene Hydrogenation Kinetics on Pd, by E. W. Hansen and M. Neurock, m Journal of Catalysis, Volume 196 241, copyright 2000 by Academic Press, reproduced by permission of the publisher and authors.]... 

Figure 16 A single snapshot from the simulation of ethylene hydrogenation over the well-dispersed Pdg3 5%Aug 25% alloyed surface. Ethylene adsorbs on both Pd and An sites. Atomic hydrogen however prefers only the threefold fee sites of Pd. 

Finally, dynamic Monte Carlo simulations are very useful in assessing the overall reactivity of a catalytic surface, which must include the effects of lateral interactions between adsorbates and the mobility of adsorbates on the surface in reaching the active sites. The importance of treating lateral interactions was demonstrated in detailed ab initio-based dynamic Monte Carlo simulations of ethylene hydrogenation on palladium and PdAu alloys. Surface diffusion of CO on PtRu alloy surfaces was shown to be essential to explain the qualititative features of the experimental CO stripping voltammetry. Without adsorbate mobility, these bifunctional surfaces do not show any catalytic enhancement with respect to the pure metals. 

Hansen, E., Neurock, M. First-principles-based Monte Carlo simulation of ethylene hydrogenation kinetics on Pd. J. Catal. 2000,1%, 241-52. 

E. W. Hansen and M. Neurock,/. Catal., 196, 241 (2000). First-Principles-Based Monte Carlo Simulation of Ethylene Hydrogenation Kinetics on Pd. 

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

The following heat flux profile was generated from independent simulations of the heat transfer in the firebox. First tube 23 kcal/m s (96 kJ/m s) second tube 20 (84) third tube 19 (80) fourth tube 17 (71) fifth tube 15 (63) sixth, seventh, eighth, ninth, and tenth tubes, 14 (59). With this heat flux profile, the conversion, temperature and total pressure profile of Fig. 2 was obtained. The agreement with the industrial data is really excellent. Also, the product distribution is in complete agreement as can be seen from Fig. 3 the simulated yields for ethylene, hydrogen, and methane, for example, are, respectively, 47.92, 3.79, and 3.49 the... 

Figure 2.34 simulates the copolymerization of ethylene and propylene in the presence of hydrogen. Ethylene, being the faster comonomer, has a much steeper radial concentration profile than propylene. In the same way, hydrogen reacts much more slowly and also diffuses rather fast and therefore has a flat radial concentration profile. The effect of these profiles on the CLD and CCD is clear polymer made near the surface of the particle will have higher molecular weight and ethylene fraction than the polymer made near the center of the particle. This modeling approach was first proposed by Soares and Hamielec for a version of the PFM [70]. 

In addition to temperature effects, we can also simulate changes in reaction kinetics as we increase the partial pressure for each of the reactants. A series of simulation runs were carried out at different partial pressures of ethylene and hydrogen to examine their effect on the rate. The measured turnover frequencies were fit to the following power law expression in order to establish the reaction orders of hydrogen (x) and ethylene (y). 

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