Catalyst resistance to deactivation

Low pressure operation became routine with the appHcation of new catalysts that are resistant to deactivation and withstand the low pressures. The catalysts are bimetallic most incorporate rhenium as well as platinum (95). The stmctures of these catalysts are stiU not well understood, but under some conditions the two metals form small alloylike stmctures, which resist deactivation better than the monometallic catalyst. 

Kinds of Catalysts To a certain extent it is known what lands of reactions are speeded up by certain classes of catalysts, but individual members of the same class may differ greatly in activity, selectivity, resistance to deactivation, and cost. Since solid catalysts are not particularly selective, there is considerable crossing of lines in the classification of catalysts and the kinds of reactions they favor. Although some trade secrets are undoubtedly employed to obtain marginal improvements, the principal catalytic effects are known in many cases. 

Nickel catalysts used in steam reforming are more resistant to deactivation by carbon deposition if the surface contains sulfur, or gold. Explain why these elements act as promoters. Would you prefer sulfur or gold as a promoter Explain your answer. 

Based on these considerations the development of perovskite-based catalysts could be an alternative due to their ability to stabilised well-dispersed active sites more resistant to deactivation and to their tolerance to poisoning effects. Consequently, those materials... 

The Pt-Re system has been studied extensively since the 1970s because adding Re to AhOs-supported platinum catalysts increases the resistance to deactivation of the catalysts used in naphtha reforming by preventing coke deposition. By using carbonyl precursors, well-defined bimetalhc species have been prepared. A proper characterization of these species allowed a relationship to be established between their structure and their catalytic behavior. Table 8.3 shows several Pt-Re bimetaUic catalytic systems prepared using different carbonyl species in which Pt-Re interactions determine the catalytic behavior. 

Noble Metal Catalysts. Noble metal-based catalysts have been widely used in reforming reactions, and are logical choices for ATR. Results of reaction studies generally suggest that these catalysts are comparable in activity to Ni-based catalysts, but they appear to be somewhat more resistant to deactivation. 

II (Ni-Mo) and catalyst I (Co-Mo). However, catalyst V (Ni-Mo) showed the highest activity after 96 hr, and its activity decreased very little with time on-stream. Catalyst VI (Ni-Co-Mo) also had a high resistance to deactivation but had a much lower initial activity. Catalyst... 

As the trend toward utilizing heavier petroleum feedstocks continues, the hydro-treating processes used to upgrade such stocks become increasingly important. Difficulties are encountered in the development of catalysts with high resistance to deactivation. Another important challenge is that of designing three-phase reac-... 

The catalyst support impacts the rate of a catalyzed reaction, the reaction pathway (quantities and species of intermediates and products) and the resistance of the catalyst to deactivation. In DBCP reactions, powders had higher rate constants than beads, presumably due to reduced mass transfer limitations alumina yielded a faster rate than C, which had a faster rate than PEI/silica. Sorptive capabilities of the supports may also play an important role Kovenklioglu found that supports which sorbed 1,1,2-TCA more strongly had higher reaction rates, and Farrell concluded that TCE sorption to Fe cause higher reaction rates on Pd/Fe electrodes than on pure Pd electrodes. It is also clear that supports influence reaction products, but the correlation between a given support and pathways/products it promotes is not yet understood. The choice of support can also affect its resistance to deactivation this implies that catalyst supports may be tailored to maximize activity over the long term. 

Under steady-state conditions the conversion does not change to any appreciable degree, while under oscillatory conditions both the maximum and minimum conversions of the cycle as well as the mean conversion remain constant within 1-2%. This observation, of course,does not agree with prior observations by Schmitz and coworkers for the same reaction [l9]. We cannot explain at this point the apparent resistance of our catalyst system to deactivation. Possible reasons, we could think of, are the much higher ratio of surface area to geometric area of our catalyst, the use of a Pyrex-glass reactor and our lower experimental temperatures. 

The assumption of global kinetic control is probably valid for only a handful of catalytic reaction processes. Nevertheless, some typical simulation results of the model of catalyst deactivation under kinetic control are presented here in order to emphasize some of the unique percolation-type aspects of the problem. The overall plugging time 0p, i.e., the time at which the catalyst becomes completely deactivated is shown is Figure 1, where it is plotted versus Z, the average coordination number of the network of pores, (in industrial applications, of course, the useful lifetime of the catalyst is significantly smaller than 0p). Note that as Z increases, (higher values of Z mean a more interconnected catalyst pore structure) 0p increases, i.e., the catalyst becomes more resistant to deactivation. The dependence of normalized catalytic activity (r/rQ) ([Pg.176]

The larger pore zeolites (H-Y) show rapid deactivation in contrast to the more restrictive HZSM-5 and the non-zeolitic catalysts (silica-alnmina, MCM-41), since coke deposits can accumulate inside the channel system of large pores. In contrast, coke deposits only on the outer surface of the zeolites having narrow pores. Fnrthermore the catalysts having weaker acid sites of lower density are better at tolerating coke deposition. For instance, the mild acidity of clays (and their pillared versions, e.g. pillared montmoriUonite) show good resistance to deactivation by coking. 

A good catalyst is then one that has an optimized assembly of the catalyst, the support, and any promoters. The resultant geometry and architecture of the porosity must give good accessibility, large surface area, and local surface properties that induce high reactivity, good chemical selectivity, and resistance to deactivation. 

Hydro- denitrogenation Al-M/ YAI2O3 impreg. withNi- Mo. The Al-M showed significant activity for nitrogen removal, but activity was low compared to other catalyst (eg. deahuninated Laponite, commercial hydrotreating catalyst and y alumina). Resistance to deactivation was comparable to y alumina, but not as effective as dealuminated Laponite. 69... 

The bimetallic Pt-Re/Al203-Cl catalyst is the most widely used in naptha reforming. The addition of Re strongly improves the stability of the traditional monometallic Pt catalyst. Such improvement is explained by a double effect of rhenium stabilization of the metallic phase on the support and higher resistance to deactivation by coke deposition [1-8]. Nevertheless the role and the nature of the interaction between Pt and Re are the subject of many controversies [9-14]. 

---