Desorption, catalytic cycle

In drafting a catalytic cycle as in Eqs. (132)-(135) we naturally have to ensure that the reaction steps are thermodynamically and stoichiometrically consistent. For instance, the number of sites consumed in the adsorption and dissociation steps must be equal to the number of sites liberated in the formation and desorption steps, to fulfill the criterion that a catalyst is unaltered by the catalytic cycle. 

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

Desorption is important both because it represents the last step in a catalytic cycle and because it is also the basis of temperature-programmed desorption (TPD), a powerful tool used to investigate the adsorption, decomposition and reaction of species on surfaces. This method is also called thermal desorption spectroscopy (TDS), or sometimes temperature programmed reaction spectroscopy, TPRS (although strictly speaking the method has nothing to do with spectroscopy). 

Since this scheme regenerates the original coordinatively unsaturated Ti+2 centers upon desorption of the aromatic, it could, in principle, represent a catalytic cycle for heterogeneous alkyne cyclization. The present study reports a test of that h3T>othesis—the feasibility of catal5hic cyclotrimerization—on a reduced Ti02 surface in UHV. 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

Endo and coworkers98 were able to catalyze the Diels-Alder reaction between acrolein and 1,3-cyclohexadiene by using a novel organic network material built up of anthracene-bisresorcinol derivatives which were held together by intermolecular hydrogen bonds. The suggested catalytic cycle was composed of sorption of the reactants in the cavities of the material, a pre-organized intracavity reaction, and desorption of the adduct. 

The rate of product desorption can also influence the kinetics of a surface-catalyzed reaction. Consider the following simple catalytic cycle ... 

Reactor control models for monoliths require a more detailed study of the time scales of all occurring subprocesses, because of their dynamic character. Under dynamic circumstances, the rates of the individual elementary steps of a catalytic cycle, such as adsorption, surface reaction, and desorption, are not equal to each other anymore, since the time scales of the corresponding processes may differ by many orders of magnitude. Therefore, accumulation effects on the catalyst surface have to be taken into account as well, which demands that continuity equations for surface species be included in the model. Such aspects may even play a role in the steady state if the kinetics depend on rate-determining steps that change according to the concentration level of the reactants... 

A comparison of the different types of motion of the three different variants allows the correlation of the enzyme diffusion behavior with specific stages of the catalytic cycle. TLL, an enzyme which cannot interact strongly with phospholipid bilayers, was found to diffuse quickly on the POPC multilayers with no specific preference for the edge or the top of the layer. The motion detected is most likely associated with weak adsorption and desorption of the enzyme on the layer since the diffusion constant is 100 times slower than that expected for free diffusion in solution [42]. These motions correspond to parts A and eventually B of the catalytic cycle shown schematically in Fig. 25.8. 

Fig. 25.8. Proposed catalytic cycle. While in solution, the enzyme remains in the closed form, with the lid covering the active site (a). Binding of the enzyme to the surface (b) promotes lid displacement and exposure of hydrophobic residues that interact with the phospholipid interface (c), thereby stabilizing the open form. Partitioned substrate accesses the active site (d), resulting in the formation of the enzyme-substrate and enzyme product complex (e). Hydrolysis is followed by product desorption and the enzyme diffuses along the substrate or into solution...

A subsequent step in the sequence is the formation of Ti-hydroperoxo species from the reaction of H2O2 and tetrahedral Ti sites, more likely Ti tripodal sites. This Ti-hydroperoxo species has been proposed as an intermediate in the gas-phase epoxidation of propylene by analogy with the well-known chemistry for oxidations in the liquid phase with H2O2 and TS-1 [105,106,401,446]. In gas-phase reactions, hydroperoxide species have been inferred by D2 isotopic experiments [431] and detected by ex situ INS [432] and in situ UV-vis measurements [433]. Other species in this simplified sequence include adsorbed propylene on a Ti-hydroperoxo site and adsorbed PO on a Ti tripodal site. Desorption of PO and water results in the original Ti species, which closes the catalytic cycle. 

The concept of virtual pressure is however very useful whenever an adsorption- desorption step in a catalytic cycle at the steady state is not at equilibrium. 

Let us consider a desorption step (d) and ask how far from equilibrium it takes place in a catalytic cycle, irrespective of its detailed mechanism. All we want to know is how we can measure the affinity A that drives desorption of a molecule M adsorbed (a) on a site ... 

Thus, if values of virtual and real fugacity in the gas phase can be measured at the same value of surface coverage, the affinity of the desorption step can be obtained, and with it a measure of the one-way behavior of the adsorption step. In particular, with a large value of A, the desorbed product will not appear as an inhibitor of the turnover freqency of the catalytic cycle, since in that case v[Pg.97]

Step 6. Products desorb from active sites on the interior catalytic surface. This desorption process generates vacant sites that are available to participate in a catalytic cycle. 

Clearly, the catalytically active sites are not directly accessible to the reacting molecifles. The catalytic cycle for zeolite-catalyzed reactions generally encompasses several steps including reactant adsorption, diffusion to the active sites, product formation, diffusion to the surface and product desorption. Because the pores of zeolites have dimensions close to those of the adsorbing... 

While the first-mentioned describes the way in which a catalyst is formed, employed, separated or deposited, made up, and regenerated or recovered, the catalytic cycle is the visual interpretation of a complec reaction mechanism by subdividing the overall reaction into a series of ad- and desorption steps (with heterogeneous catalysts) or arranging the intermediates of a homogeneously catalyzed reaction in a logical sequence to form a closed cycle [18],... 

Catalysis is the study of materials that can accelerate reactirms and control reaction mechanisms. After a catalytic cycle, namely adsorption of reactants, reactions and desorption of products, the catalyst is restored to its initial state. An ideal catalyst... 

The complexity of the catalytic reaction is a common thread through most of the chapters that follow. We describe the issues associated with the different time and length scales that underpin the chemical events that constitute a catalytic system. For example, a typical time scale for the overall catalytic reaction is a second with characteristic length scales that are on the order of 0.1 micron. The time scales for the fundamental adsorption, desorption, diffusion and surface reaction steps that comprise the overall catalytic cycle, however, are often 10 sec or shorter. The time scales associated with the movement of atoms, such as that which must occur for surface reconstruction events, may be on the order of a nanosecond. The vibrational frequencies for adsorbed surface intermediates occur at time scales on the order of a few picoseconds. The different processes that occur at these time scales obey different physical laws and, hence, require different methods in order to calculate their influence on reactivity. In this book we will show how the... 

A proper kinetic description of a catalytic reaction must not only follow the formation and conversion of individual intermediates, but should also include the fimdamental steps that control the regeneration of the catalyst after each catalytic turnover. Both the catalyst sites and the surface intermediates are part of the catalytic cycle which must turn over in order for the reaction to remain catalytic. The competition between the kinetics for surface reaction and desorption steps leads to the Sabatier principle which indicates that the overall catalytic reaction rate is maximized for an optimal interaction between the substrate molecule and the catalyst surface. At an atomic level, this implies that bonds within the substrate molecule are broken whereas bonds between the substrate and the catalyst are made during the course of reaction. Similarly, as the bonds between the substrate and the surface are broken, bonds within the substrate are formed. The catalyst system regenerates itself through the desorption of products, and the self repair and reorganization of the active site and its environment after each catalytic cycle. 

Sabatier s principle provides a kinetic rmderstanding of the catalytic cycle and its corresponding elementary reaction steps which include adsorption, surface reaction, desorption and catalyst self repair. The nature of the catalytic cycle implies that bonds at the surface of the catalyst that are disrupted during the reaction must be restored. A good catalyst has the unique property that it reacts with the reagent, but readily becomes liberated when the product is formed. This will be further discussed in Section 2.2, where we describe the kinetics of elementary surface reactions and their free energy relationships. 

Catalysis consists of a reaction cycle made up of several elementary reaction steps. For a zeolite, the catalytic cycle contains at least the following four elementary steps adsorption, diffusion, substrate activation and desorption. 

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