Surface reactions catalysts

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

As with the other surface reactions discussed above, the steps m a catalytic reaction (neglecting diffiision) are as follows the adsorption of reactant molecules or atoms to fomi bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-detemiming or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate detemiining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

Say the concentration of dissolved gas A is A . The series rates involved are from the gas to the interface where the concentration is Aj and from the interface to the surface of catalyst where the concentration is A, and where the reaction rate is T[wk,nA f At steady state,... 

The synthesis of methane from C02 and hydrogen was studied by Binder and White (11) over a reduced nickel catalyst (Harshaw Ni-88). The surface reaction between the C02 and hydrogen appeared to be rate controlling. The rate of reaction can be correlated by either of the following rather awkward equations ... 

For the first assumption, the value of Kw for the shift appears to be too high. It must be this high because it is necessary to make C02 appear while both C02 and CO are being consumed rapidly by methanation. The data may be tested to see if the indicated rate appears unreasonable from the standpoint of mass transfer to the gross catalyst surface. Regardless of the rate of diffusion in catalyst pores or the surface reaction rate, it is unlikely that the reaction can proceed more rapidly than material can reach the gross pill surface unless the reaction is a homogeneous one that is catalyzed by free radicals strewn from the catalyst into the gas stream. 

As our first approach to the model, we considered the controlling step to be the mass transfer from gas to liquid, the mass transfer from liquid to catalyst, or the catalytic surface reaction step. The other steps were eliminated since convective transport with small catalyst particles and high local mixing should offer virtually no resistance to the overall reaction scheme. Mathematical models were constructed for each of these three steps. 

Moreover, a specially active hydrogen species present in a reaction mixture (e.g. atomic hydrogen, protons) (83) or forming during the surface reaction (37) can penetrate into a metal catalyst lattice and become... 

The catalytic reaction can be subdivided into pore diffusion and chemisorption of reactants, chemical surface reaction, and desorption and pore diffusion of products, the number of steps depending upon the nature of the catalyst and the catalytic reaction. 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

The promotional kinetics described by equation (11.6) or by its equivalent equation (11.12) imply uniform distribution of the backspillover promoting species on the catalyst surface. This requires fast ion backspillover relative to its desorption or surface reaction. 

Forward rate constant for reversible surface reaction Exam. 10.2 Reverse rate constant for reversible surface reaction Exam. 10.2 Mass transfer coefficient for a catalyst particle 10.2... 

Thus it is important to obtain reliable models for catalyst deactivation and to investigate, whether it is possible to decouple the deactivation model from the kinetic model or if it is necessary to treat the catalyst deactivation as one of the surface reactions on the catalyst [45]. 

Note that the rates of product formation and reactant conversion indeed have the dimensions of mol per unit of time, and that these rates are proportional to the number of sites, or, in fact, the amount of catalyst present in the reactor. Also, in the case of a second order reaction, e.g. betv een adsorbed species A and B, we write the rate in the form r = Nk0j 0 by applying the mean-field approximation. Here the rate is proportional to both the total number of sites on the surface and the probability of finding a species A adjacent to a species B on the surface, the latter being proportional to the coverages of A and B. In the mean-field approximation A and B are distributed randomly over the N available sites this only tends to be valid when the adsorbents repel each other. Thus the rate is not r= k(N0/ )(N02,) since the reactants need to be on adjacent sites. Another important consideration is that we want the rate to be linearly proportional to the amount of catalyst in the reactor, in accordance with r = Nk0A0B for a second order surface reaction. 

It is important to realize that the assumption of a rate-determining step limits the scope of our description. As with the steady state approximation, it is not possible to describe transients in the quasi-equilibrium model. In addition, the rate-determining step in the mechanism might shift to a different step if the reaction conditions change, e.g. if the partial pressure of a gas changes markedly. For a surface science study of the reaction A -i- B in an ultrahigh vacuum chamber with a single crystal as the catalyst, the partial pressures of A and B may be so small that the rates of adsorption become smaller than the rate of the surface reaction. 

How relevant are these phenomena First, many oscillating reactions exist and play an important role in living matter. Biochemical oscillations and also the inorganic oscillatory Belousov-Zhabotinsky system are very complex reaction networks. Oscillating surface reactions though are much simpler and so offer convenient model systems to investigate the realm of non-equilibrium reactions on a fundamental level. Secondly, as mentioned above, the conditions under which nonlinear effects such as those caused by autocatalytic steps lead to uncontrollable situations, which should be avoided in practice. Hence, some knowledge about the subject is desired. Finally, the application of forced oscillations in some reactions may lead to better performance in favorable situations for example, when a catalytic system alternates between conditions where the catalyst deactivates due to carbon deposition and conditions where this deposit is reacted away. 

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