Subject catalysts

For commercial appHcation, catalyst activity is only one of the factors to be considered. Equally important is catalyst life, but Htde has been pubHshed on this aspect. Partly because of entrainment losses and partly through loss of acid as volatile triethyl phosphate, the catalyst loses activity unless compensating steps are taken. This decline in activity can be counteracted by the periodic or continuous addition of phosphoric acid to the catalyst during use, a fact that seems to have been disclosed as early as 1940 (94). A catalyst subjected periodically to acid addition could remain in service indefinitely, according to a report by Shell (91). A later Shell patent (85) states that complete reimpregnation with acid is required every 200 mn-days. 

Except for those catalysts subjected to the previously mentioned conditions, which lead to irreversible transformation of the active phase and/or the support material, the HDT catalysts are regenerable [37], Through a systematic and careful procedure, the spent catalyst is unloaded from the reactor and regenerated by specialized companies. The possibility of in situ regeneration is also commercially offered and the decision, on which method would be used, is typically based on economical considerations [38],... 

Because the reaction is driven by protonation of the carbonyl functionality, reacting species were expected to be localized on the bed of the acid catalyst subjected to microwave irradiation. Hexane was used as a nonpolar solvent to minimize solvent absorption and superheating. Elimination of catalyst superheating in a continuous-flow reactor was most probably the reason why no significant differences were observed between the reaction rates under the action of microwave and conventional heating. 

The rate of reaction with a catalyst subject to degradation with time on stream is,... 

The role of various elements in the supported catalyst requires additional work to fully understand their function. There is also a dearth of information dealing with the nature of the surface of the newer catalysts subjected to various pretreatment and exposed to model compounds such as alkyl-substituted dibenzothiophene or other feedstocks. There is a need to correlate that data from such characterization with kinetic and mechanistic studies. 

Catalysts subjected to heating in air may exhibit accelerated hydroxide or carbonate formation, before a loss of water or carbon dioxide could take place. Phase changes may also occur, for example, in alumina by interaction of a partially hydrated phase with water... 

As the purpose of this review is to focus on characterization of catalysts in the working state, we have selected investigations in which it was demonstrated that the catalyst was indeed in an active state during the measurement. This restriction implies that the catalytic activity was actually measured simultaneously with the XAFS data, substantially limiting the types of catalysts and reactions that can be investigated by the technique, as is to be discussed later. There is a plethora of papers in the literature that include reports of catalysts subjected to some type of treatment, followed by measurement of XAFS data these papers are not reviewed here. 

If, therefore, for a given monofunctional catalyst subject to scheme X with certain ki, a F-component is added, draining of C into the new product D can occur in the manner of a consecutive reaction (scheme XII) for which kinetic behavior has been variously analyzed (e.g., in ref. 10). For example, the product compositions of C and D for ki = 0.2, t = 10, and variable (Kika) are plotted in Fig. 6. 

The results from catalyst characterization measurements are given in Table 1. It can be seen that the atomic surface ratio of Or and Al, which can be used as an indicator of the Chromium dispersion, after a few regenerations is considerably diferent from that of the fresh catalyst. This is especially true of the series B catalyst, subjected to more severe regeneration conditions. In the series B samples the Cr/AI ratios measured are different for catalyst samples from the entrance and from the exit regions of the reactor. The Na/AI surface ratio also changes, as can be seen from the results of series A, in which there is a considerable difference between the Na/AI ratios at different positions in the bed, as well as with respect to those of the fresh catalyst. 

In Fig. 1, as an example, the results of evolution of combustion products (CO, CO2 and H2O) at 500°C, for Samples 1 and 4, are shown. Sample 1 corresponds to the catalyst deactivated at 2 h time on stream, and Sample 4 corresponds to the same catalysts subjected to aging during 1 h. In Fig. la, it can be observed that for the most hydrogenated coke. Sample 1, with H/C = 2.0 5, the beginning of the combustion and the maximum peak of production of CO and CO2 correspond to lower values of temperature than those corresponding to the combustion of coke subjected to aging (Fig. lb). Sample 4, with H/C = 1.15. 

Although several mixed oxide catalysts have been developed commercially for the selective oxidation of propylene, the investigation of their fundamental physical and chemical properties has resulted in only a slow and steady accumulation of information. It also appears that attempts to correlate data from different investigations have frequently resulted in unsatisfactory interpretations. It seems that some of this uncertainty arises from correlations between results obtained from different catalysts subjected to different pretreatments and assessed under different evaluation conditions. Hence, the comprehensive description of the bulk and surface properties of a single catalyst, their interdependence, and their influence on catalytic performance is in most cases quite unclear. 

Table 1 presents the properties of vanadium-magnesium oxide catalysts subjected to the heat treatment. The temperature of the heat treatment determines both the textural and the catalytic properties of the catalyst. Similar to the dehydrogenation of ethylbenzene into styrene [10,11], the most active catalysts occurred to be those... 

Activity data of catalysts subjected to high temperature treatments at 700°C and 800°C and that of hydrothermally treated samples are given in Table 2. The percentage loss of catalyst activity during deactivation tabulated in Table 2 is a helpful index to rank catalysts with respect to their resistance to deactivation. 

Fig. 31. Copper 2p core level electrons in LaMn04Cu06O3 catalyst subjected to various pretreatments (a) in a flow of nitrogen at 523 K (b) sample a after 6h on-stream (c) reduced in hydrogen at 573 K for 12 h (d) sample c after 6 h on-stream. With permission from Rodriguez-Ramos et al. (1991).

Spent Catalyst Analysis. Analysis of catalysts subjected to car and engine testing by x-ray fluorescence (XRF) revealed the expected con-... 

Values of total metal dispersion of flesh and regenerated catalysts are reported in Table 1. It can be seen that the rejuvenation treatment improved the metal dispersion of the catalysts. The dispersion values of the catalysts subjected to a single buming-off step (no rejuvenation) are about 30-35 % for all the samples studied (results not shown). To further confirm this behavior some additional runs were perfcnmed using cyclohexane dehydrogenation as a test reaction of the activity of the metallic sites (11). A firesh laboratory prepared Pt-Re/AbOs... 

Performance evaluation of catalysts subjected to forced deactivation. 

Recently, the group reported the synthesis of a-fluoroamides from a-fluoroenals using chiral triazolium salt F9 as the catalyst.Subjecting a... 

The activity of the catalysts measured by the degree of ammonia transformation to NO (Xj) decreased with increasing calcination temperature from 10(X)°C to 11(X) C. Fig. 3 presents the results of analogous measurements taken for the KA catalyst prepared in a larger lot (50 kg) and used in experiments in a large laboratory and semi-technical scale. The results of X-ray diffraction studies have confirmed that the catalyst before the heat treatment (PA-0) consists of CO3O4. Similarly, no other phases besides to C03O4 have been found in a catalyst calcined at 1C)00 C and then annealed at 8(X) C. This is true for a catalyst subjected... 

Figure 4.43. Effect of Mo content (atomic percentage) on the methanol oxidation activity of (W o)C. Legend catalyst subjected to alkaline treatment, o catalyst without alkaline treatment. Conditions 323 K, 0.5 Vj je, 1 M H2SO4, 4 M CH3OH [213]. (Reproduced by permission of ECS— The Electrochemical Society, from Kawamiua G, Okamoto H, Ishikawa A, Kudo T. Tungsten molybdenum carbide for electrocatalytic oxidation of methanol.)...

Martinez-Arias, A., Cataluna, R., Conesa, J.C., and Soria, J. Effect of copper-ceria interactions on copper reduction in a Cu/Ce02/Al203 catalyst subjected to thermal treatments in CO. J. Phys. Chem. B 1998,102, 809-817. 

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