Methanol microkinetic model

Figure 4.32. Methanol decomposition over Pt as determined from a microkinetic model. Adapted...

In terms of catalytic kinetics, the implications of the dynamic changes in catalyst morphology during methanol synthesis are dramatic. Figure 16a shows the agreement between the predictions of a static microkinetic model and the measured rates of methanol synthesis catalyzed by Cu/ZnO/A1203... 

Fig. 16. (a) Comparison of the calculated rate with the measured rate of methanol synthesis catalyzed by Cu/ZnO/A1203. The calculated rate was obtained from a static microkinetic model, (b) The corresponding comparison estimated by use of a dynamic microkinetic model [adapted from Ovesen et al. (59)]. 

In the present paper, we will discuss in some detail the results of methanol synthesis catalyst since in this case, the dynamic changes occurring in the catalyst structure have been described in some detail and it has been possible to use this insight to formulate a dynamic microkinetic model. 

In the following section, we will discuss the kinetic implications of the dynamical changes in catalyst morphology during methanol synthesis. First, we will present an analysis of steady state kinetic experiments using a static, microkinetic model where it is assumed that the number of sites are constant. Then, we will introduce the dynamic aspect into the microkinetic modeling and also discuss some recent transient experiments. 

Comparison of the calculated rate with the measured rate of methanol synthesis over a Cu/ZnO/AhOs catalyst. The calculated rate is obtained from the static microkinetic model. Inlet gas compositions 12% CO, 2.1% CO2, 85.9% H2 (solid circle), 17 9% CO, 6.7% CO2, 75.4% H2 (empty triangle) [21]. 

The Langmuir Hinshelwood kinetic model based on this reaction scheme is formulated assuming that all reactions are in equilibrium except for reaction steps (2), (4), (7) and (11). Reaction steps (2), (4) and (7) are all steps which may be slow during the shift reaction [5,6], whereas reaction step (11) represents the slow step for methanol synthesis [8], The kinetic and thermodynamic parameters are taken from available Cu single crystal experiments. We call this type of model a static microkinetic model since the number of active sites are assumed constant (i.e., independent of reaction conditions) [21]. 

The above findings also show that it is necessary to include the dynamic aspect in a microkinetic model to be able to account quantitatively for the rate of methanol synthesis. A dynamic microkinetic model of the methanol synthesis was recently presented [21]. To introduce the dynamic aspect into the microkinetic model, it is necessary to have a description of the changes in particle shape, i.e. surface area, with change in the gaseous environment. As the change in particle shape is induced by change in contact surface free energy between the Cu particle and the support, we will first present results for such calculations. 

In the dynamic, microkinetic model, it is furthermore taken into account that the rate of methanol synthesis is different over the three low index facets [21]. The observed rate of the methanol synthesis for a given catalyst is therefore an average of the rates over the exposed facets and can be expressed as... 

In the kinetic modelling of catalytic reactions, one typically takes into account the presence of many different surface species and many reaction steps. Their relative importance will depend on reaction conditions (conversion, temperature, pressure, etc.) and as a result, it is generally desirable to introduce complete kinetic fundamental descriptions using, for example, the microkinetic treatment [1]. In many cases, such models can be based on detailed molecular information about the elementary steps obtained from, for example, surface science or in situ studies. Such kinetic models may be used as an important tool in catalyst and process development. In recent years, this field has attracted much attention and, for example, we have in our laboratories found the microkinetic treatment very useful for modelling such reactions as ammonia synthesis [2-4], water gas shift and methanol synthesis [5,6,7,8], methane decomposition [9], CO methanation [10,11], and SCR deNO [12,13]. 

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