Surface reactions in catalysts

Results obtained in this study reveals that catalyst design based on (i) the primary knowledge of the reaction network and the mechanism of rsactions involved in it and (ii) the application of Controlled Surface Reactions in catalyst preparation and modification can be used to obtain highly active and selective catalysts for different organic reactions taking place in the presence of hydrogen. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

This was explained by having only the contribution of surface reaction in the case of batch processing, whereas micro reactors profit, in addition, from processing inside the pores of the catalyst beads. The penetration of the reaction solution into the pores is achieved here by applying pressure [2]. By this means, the number of available catalyst sites is increased. 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

Margitfalvi and coworkers (e.g. 73-76) have utilized as a means of catalyst preparation a controlled surface reaction in which a volatile Sn (or Pt) compound is allowed to react with Pt (or Sn) already present on a support. They employed conventional and transient kinetic approaches to study the mechanism of hydrocarbon reactions on these catalysts conversions were effected at atmospheric or lower pressures. These authors found a perplexing variety of activity patterns, depending upon the manner and sequence in which Pt and Sn was added. Depending upon preparation conditions, the added tin may either enhance or decrease Pt activity and increase or decrease the selectivity for hydrogenolysis (73). 

The whole of the internal surface area of a porous catalyst will be available for the catalytic reaction if the rates of diffusion of reactant into the pores, and of product out of them, are fast compared with the rate of the surface reaction. In contrast, if the reactant diffuses slowly but reacts rapidly, conversion to product will occur near the pore entrances and the interior of the pores will play no role in the catalysis. Ion exchange resins are typical examples of catalysts for which such considerations are important (cf. Sect. 2.3). The detailed mathematics of this problem have been treated in several texts [49-51] and we shall now quote some of the main theoretical results derived for isothermal conditions. The parameters involved tend to be those employed by chemical engineers and differ somewhat from those used elsewhere in this chapter. In particular, the catalyst material (active + support) is present in the form of pellets of volume Vp and the catalytic rates vv are given per unit volume of pellet (mols m 3). The decrease in vv brought about by pore diffusion is then expressed by an effectiveness factor, rj, defined by... 

In our discussion of surface reactions in Chapter 11 we assumed that each point in the interior of the entire catalyst surface was accessible to the same reactant concentration. However, where the reactants diffuse into the pores within the catalyst pellet, the concentration at the pore mouth will be higher than that inside the pore, and we see that the entire catalytic surface is not accessible to the same concentration. To account for variations in concentration throughout the pellet, we introduce a parameter known as the effectiveness factor. In this chapter we will develop models for diffusion and reaction in two-phase systems, which include catalyst pellets and CVD reactors. The types of reactors discussed in this chapter will include packed beds, bubbling fluidized beds, slurry reactors, and trickle beds. After studying this chapter you will be able to describe diffusion and reaction in two- and three-phase systems, determine when internal pore diffusion limits the overall rate of reaction, describe how to go about eliminating this limitation, and develop models for systems in which both diffusion and reaction play a role (e.g., CVD). 

Fig. 5.3. Arrhenius plot for temperature programmed surface reaction In(rate) as a function of 1/T, for the reaction of a pre-adsorbed CO in the flow of H2. Promoted and unpromoted Rh/SiOj catalysts [26].

This chapter deals primarily with the reactions of bulk tungsten and tungsten powder. Avast literature exists about their reactions. However, it is not so much the reactions of the bulk metal which attracted the interest of scientists and technicians as its reactions at the surface. Scientific as well as practical reasons boosted that research owing to the widespread application of tungsten as a catalyst and as an electron or ion emitting source. It would be far beyond the scope of this book to deal with surface reactions in detail. In this regard, reference is made to the very complete compilation elsewhere [1.99-1.101]. 

As in homogeneous catalysis, the existence of a most abundant catalyst-surface species (macs) or one or several low-abundance catalyst-surface species (lacs) can greatly simplify the mathematics of multistep surface reactions. In some cases, mathematics reduces to that of a single reaction step, so that the Hougen-Watson formula becomes applicable. 

Microkinetic modeling assembles molecular-level information obtained from quantum chemical calculations, atomistic simulations and experiments to quantify the kinetic behavior at given reaction conditions on a particular catalyst surface. In a postulated reaction mechanism the rate parameters are specified for each elementary reaction. For instance adsorption preexponential terms, which are in units of cm3 mol"1 s"1, have been typically assigned the values of the standard collision number (1013 cm3 mol"1 s 1). The pre-exponential term (cm 2 mol s 1) of the bimolecular surface reaction in case of immobile or moble transition state is 1021. The same number holds for the bimolecular surface reaction between one mobile and one immobile adsorbate producing an immobile transition state. However, often parameters must still be fitted to experimental data, and this limits the predictive capability that microkinetic modeling inherently offers. A detailed account of microkinetic modelling is provided by P. Stoltze, Progress in Surface Science, 65 (2000) 65-150. 

The dependence of the initial rate of oxidative decarboxylation of oxalic acid on the total acid concentration and the content of undissociated species of mono- and dianion in aqueous solutions in the presence of polymeric cobalt phthalocyanine has been examined [160]. Whereas the monoanion, [HC2O.C], of oxalic acid is shown to enter the reaction, the undissociated species does not. Furthermore, [C204 ] dianion retards the process. This is attributed to the fact that in catalyst A, active particles of CoPc(NaS03)4 are located mainly on the species surface while in catalyst B they are distributed within the species and are hardly accessible. 

Surface physics is an old and well-established science but when ultrahigh vacuum equipment became available it was apparent that the previous results were inconsistent with the new data and everything had to be measured over again. The analytical techniques developed in surface physics were a great help for surface chemistry as the surface reactions in catalysis were analyzed with them. One of those techniques is the BET (Brunauer-Emmett-Teller) technique to characterize surfaces Figure 6.2 shows the adsorption isotherms that are used to measure specific surface areas of heterogeneous catalysts and characterize their pores. 

In this section, the catalyst particle or the catalytic surface is assumed to be nonporous, and the 7-step sequence is reduced to a 5-step sequence with steps 1, 3-4-5, and 7. In this case, the only mass transfer resistance involved is between the fluid and the outer surfece of the particle. The rates of external mass transfer depend on (i) the temperature, pressure, and physical properties of the fluid phase under these conditions, (ii) the gas velocity relative to the solid surface, and (iii) the intrinsic rate of the surface reaction. In other words, the rate at which mass is transported from the fluid to the surface is determined by the relative magnitudes of ... 

Improved instrumentation to carry out surface analysis and to monitor chemical surface reactions in situ on small area catalysts over a wide pressure range (10- -10 Torr)... 

Chemical Surface Reactions in situ on Small Area Catalysts over a Wide Range of Pressures (10 - 10 torr)... 

The importance of including surface phenomena in the transient model of porous catalyst particles has been pointed out, e.g., by Sheintuch and Schmitz Q2). Experimental work by Lehr et al. (13) and by Denis and Kabel (14, 15) seems to support this argument. Elnashaie and Cresswell TT6, 17) also employed a mathematical model which accounts for adsorption, desorption, and surface reaction in studying porous catalyst pellets. 

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