Activation energy reactor surface material

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

In a general CVD process, the substrate is heated to a substantially high temperature, e.g., higher than 300°C, but the reactor itself is generally not heated. Therefore, the activation of the starting material in vapor phase is done by the thermal energy provided by the substrate surface. Here, the important factor is (1) thermal activation and (2) the creation of the activated chemically reactive species both occur at the surface of the substrate. 

The previous sections described techniques employed for parameter estimation. These thermodynamic and kinetic parameters are input to a microkinetic model that is solved numerically to describe material balances in a chemical reactor (e.g., a PFR). This section describes tools for the subsequent model analysis, which can be used in multiple ways. Initially during mechanism development, they can be used to assess which reactions and reactive intermediates are important in the model, which helps the modeler to focus on important features of the surface reaction mechanism. During this process, simulated macroscopic observables, for example, global reaction orders and apparent activation energies can be compared directly to experimental data. Then, once the model describes experimental data reasonably well, analytical tools can be used to develop further insights into the reaction mechanism, with apphcations that include catalyst design [50]. 

In yet another method [42], the reaction for pyrolysis of l,2-dichloro-2,2-difluoroethane in the presence of hydrogen was carried out in the absence of a catalyst in an essentially empty reactor at a temperature >400°C. In the absence of a catalyst refers to the absence of a conventional catalyst. A typical catalyst has a specific surface area and is in the form of particles or extrudates, which may optionally be supported to facilitate the dehydrochlorination reaction by reducing its activation energy. The reactors that are suitable are quartz, ceramic (SiC), or metallic reactors. In this case, the material constituting the reactor was chosen from metals such as nickel, iron, titanium, chromium, molybdenum, cobalt or gold, or alloys thereof. The metal, chosen more particularly to limit corrosion or other catalytic phenomena, may be bulk metal or metal plated onto another metal. 

Two other crucial factors are mass transfer and heat transfer. In Chapter 3 we assumed that the reactions were homogeneous and well stirred, so that every substrate molecule had an equal chance of getting to the catalytic intermediates. Here the situation is different. When a molecule reaches the macroscopic catalyst particle, there is no guarantee that it will react further. In porous materials, the reactant must first diffuse into the pores. Once adsorbed, the molecule may need to travel on the surface, in order to reach the active site. The same holds for the exit of the product molecule, as well as for the transfer of heat to and from the reaction site. In many gas/solid systems, the product is hot as it leaves the catalyst, and carries the excess energy out with it. This energy must dissipate through the catalyst particles and the reactor wall. Uneven heat transfer can lead to hotspots, sintering, and runaway reactions. 

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