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Days15n16 - CE 561 Lecture Notes Fall 2009 Days 15 and 16...

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CE 561 Lecture Notes Fall 2009 p. 1 of 12 Days 15 and 16: Reactions at Surfaces For the next couple of lectures, we will consider reactions at surfaces. We will primarily look at heterogeneous catalysis at gas-solid interfaces, where the solid surface serves to accelerate an overall reaction with gas phase reactants and products. However, much of what we do will also be applicable to situations where material is deposited or removed at a gas-solid interface (chemical vapor deposition or etching, respectively) and in situations where the gas-solid interface is replaced by a liquid-solid interface, or even a fluid-fluid interface. This material is covered in chapter 2 of the textbook by Froment and Bischoff, of which we will make much more use during the remainder of the semester. A much more detailed treatment of surface reactions can be found in “Principles of Adsorption and Reaction on Solid Surfaces”, by Richard Masel (a chemical engineer at the University of Illinois) (John Wiley and Sons, New York, 1996). A heterogeneous catalyst accelerates a reaction that has fluid phase reactants and products (so the solid is nominally unchanged by the reaction). In general, it does this by stabilizing reactive intermediates whose formation would be energetically unfavorable in the fluid phase. The great advantage of heterogeneous catalysis over homogenous catalysis is that, because the catalyst is in a separate phase from the reacting mixture, it can stay in the reactor while the reactants and products flow through. We will not discuss the chemistry of particular catalytic systems, but will try to develop some basic approaches that can be applied to a range of systems. For a catalytic reaction occurring in a porous catalyst pellet (the traditional case in chemical engineering applications), we can envision the overall reaction as a series of sequential steps: (1) Transport of reactants to the exterior surface of the catalyst pellet. (2) Transport of reactants to active sites inside the pores. (3) Adsorption of reactants onto the surface at active sites. (4) Reaction on the surface. (5) Desorption of products from the surface. (6) Transport of products out of the pores to the pellet surface. (7) Transport of products from the pellet surface to the bulk fluid. For the generic catalytic reaction A B, this can be illustrated schematically as A B A B A B A B 1 5 4 3 2 6 7
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CE 561 Lecture Notes Fall 2009 p. 2 of 12 We will first focus on steps 3, 4, and 5 – which we might call the reactive steps, as opposed to steps 1, 2, 6, and 7, which involve only transport and not chemical transformations. In subsequent lectures, we will consider the effects of transport limitations that can be both external (steps 1 and 7) and internal (steps 2 and 6) to the catalyst pellet. For situations other than heterogeneous catalysis with a porous pellet catalyst, some of these steps may be missing. In a CVD reactor, for example, the surface is generally flat, smooth, and nonporous. Then, the transport steps can be simplified or eliminated, but the reactive steps remain essentially the same.
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