Catalyst reconstruction

Fig. 8. Typical CO concentration and reaction rate profiles in the porous Pt/y-Al203 catalyst reconstructed by particle-packing method. Mean hydraulic diameter of macropores = 300 nm, macroporosity =18.1%. Free space corresponds to macropores, solid gray corresponds to mesoporous y-Al203 with dispersed Pt. Length of the section edge 10 pm. Boundary /.. yco 1%, y0j = 0.5%. (a) T 513 K, (b) T = 533 (Koci et al., 2007a) (see Plate 2 in Color Plate Section at the end of this book).

It is important from the point of view of thermodynamics of nonequi librium processes that catalysts are operating usually at the condition of large affinities for the catalyzed stepwise process (AfS > RT)—in other words, the process occurs far from thermodynamic equilibrium. Following we discuss the manifestation of only the phenomena of the nonequifibrium catalyst reconstruction under conjugation involving the catalyst transfor mations and the conjugating catalyzed reaction. 

The other gronp of methods is based on sudden jumplike disturbances of the concentration of reactants after establishing a defined steady state of the reactor system. From the different shapes of responses at the reactor outlet, conclusions can be drawn concerning reaction mechanisms such as rate-determining steps and catalyst reconstruction phenomena. 

The different carbon sources also readily facilitate catalyst reconstruction, again suggesting reactant-metal complex formation. If different faces on the catalyst are generated from the different carbon sources, then different SCMs can be formed. 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

FIG. 13 Snapshot configuration of the catalyst surface obtained for the ZGB model with local reconstructions using lattices and patches of side L = 129 and Lp = 3, respectively, and taking 7 = 0.331 and = 0. B species , A species. Empty sites are left white. Notice the formation of clusters of both species surrounded by empty sites. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Oxide surfaces, and in particular oxide films, are versatile substrates for the preparation of model catalysts. Quite a few of these systems show nanoscale reconstructions, which can be employed as templates for the growth of ordered model catalysts of reduced complexity. In order to efficiently control the growth of nanostructured metal particle arrays, two conditions have to be met. First, the template must provide sites of high interaction energy that trap the deposited metals. Second, the kinetics of the growth process must be carefully controlled by choosing... 

Another example of the flexibility of the Pt catalyst is the reconstruction of a stepped Pt(l 11) crystal with adsorbed sulfur upon exposure to CO [25]. Single-crystal Pt(l 1 1) cut at an angle of approximately 5° from the (1 1 1) direction consists of numerous terraces with a width of 20-60 A separated by steps with single-atom height. The adsorption of sulfur atoms restructures the clean stepped Pt(l 1 1) surface with single-atom steps into a sulfur-adsorbed surface with double-atom... 

The BET surface area of the catalysts is summarised in Table 3. The enhancement could be explained in case of MO-s with the reconstruction of the lamella structure. The reason of enhancement in the presence of 212 is still not known. All the other cases significant decrease can be observed. The surface area of metallic part of the used RNi-s shows increase from A to C, with the increasing temperature of the catalyst production, indicating growing Ni distribution. 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

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