Catalyst deterioration

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia sHp can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

This reaction is highly exothermic the excessive temperature increase reduces ethylene oxide yield and causes catalyst deterioration. Overoxidation can he minimized hy using modifiers such as organic chlorides. 

Promoters are usually added to a catalyst during catalyst preparation (classical or chemical promotion). Thus if they get somehow lost (evaporation) or deactivated during prolonged catalyst operation, this leads to significant catalyst deterioration. Their concentration cannot be controlled in situ, i.e. during catalyst operation. As we will see in this book one of the most important advantages of electrochemical promotion is that it permits direct in situ control of the amount of the promoter on the catalyst surface. 

The performance of most catalysts deteriorates with time3-5. The rate at which the deterioration takes place is not only an important factor in the choice of catalyst and reactor conditions but also the reactor configuration. 

To examine the catalyst deterioration under the present superheated liquid-film conditions, a long-term tetralin dehydrogenation over the carbon-supported platinum catalyst (Pt/C) was carried out under the superheated liquid-film conditions (heating temperature 240°C) at the amount ratio of 1.1 g/0.5 mL/min [13]. Figure 13.21 shows the time courses of reaction rate and conversion in the tetralin dehydrogenation. High conversion (around 50%) was maintained for longer than 5 h. 

Successful catalytic alkylation of isobutane with ethylene has been accomplished in one commercial installation using aluminum chloride catalyst (I). The chief product of the reaction is 2,3-dime thy lbutane, a hydrocarbon having very high aviation octane ratings. Ethylene has also been alkylated with isobutane in a thermal process to give 2,2-dimethylbutane as the chief product component (6). When sulfuric or hydrofluoric acid alkylation with ethylene is attempted, the ethylene forms a strong bond with the acid, and fails to react with isobutane. The net result is the formation of little or no product, accompanied by excessive catalyst deterioration. 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

Although catalyst deterioration for methane oxidation was clearly demonstrated in the laboratory test under steady-state conditions at 500°C, as shown in Fig. 15, it seems to be less discernible in car tests based on various driving cycles. Thus, data obtained on Ford vehicles, subjected to the 1977 Federal Certification Test over 50,000 miles, indicated that catalysts can be essentially inactive in removing methane even at zero miles, under these conditions. 

The low sulfur content of E85 should be a benefit in reducing catalyst deterioration compared to vehicles using gasoline. Insufficient data have been gathered to date to determine whether this effect is significant. 

Furthermore, the organometallic compounds (of which nickel and vanadium are the principal constituents) that are present to varying degrees in all residua and in the majority of heavy oils cause catalyst deterioration. Deposition of these metals in any form on to the catalyst leads to catalyst deactivation but the exact mechanism of deactivation is still subject to speculation. Nickel tends to be deposited throughout the catalyst whereas vanadium is usually more concentrated in the outer layers of the catalyst. In either case, catalyst deactivation is certain whether it be by physical blockage of the pores or destruction of reactive sites. 

Catalysts. Chromia prepared by the method of Burwell and others (3, 4). Weight 0.86 gram, Treated with continuous flow of hydrogen and thiophene for 2 hours at 400° C. before use. This particular catalyst deteriorated rather rapidly. 

Thus, the accelerated catalyst aging test can be considered to accurately reproduce the tendency of catalyst deterioration in the Z-Former demonstration test. In the case of a sequence consisting of 24 hours per cycle, it is possible to predict catalyst life in 1/4 to 1/8 the time required by the demonstration test assuming reaction/regeneration in... 

The activity of the catalysts decreases with time, depending on the flue gas conditions to which they are exposed. The main causes of catalyst deterioration are ... 

The causes of catalyst deterioration can be grouped into two general classifications those causing normal aging, and those causing abnormal aging (221). 

The previous errors addressed heterogeneity on a small scale. Now we examine heterogeneity on a large scale the scale of the lot over time or space. The long-range nonperiodic heterogeneity fluctuation error is nonrandom and results in trends or shifts in the measured characteristic of interest as we track it over time or over the extent of the lot in space. For example, measured characteristics of a chemical product may decrea.se due to catalyst deterioration. Particle size distribution may be altered due to machine wear. Samples from different parts of the lot may show trends due to lack of mixing. 

Figure 7. XPS of the catalysts deteriorated by SOji (a) after reaction without SOj (b) after reaction with SO,.

Figure 9. Cu K-edge EXAFS spectra for CuHM (A) and CuNZA (B) catalysts deteriorated by...

Reaction (79) results in catalyst deterioration and Step (81) forms colloidal... 

Benzoic acid is formed in oxidation at a roughly similar level to primary-EBHP (up to 1% of EB consumed), via the facile oxidation of benzaldehyde. To mitigate possible corrosion effects or catalyst deterioration downstream, the acid is partially removed from the EBHP before the epoxidation step, using an alkaline washing step. The remaining benzoic acid is removed via the heavy end stream, partly in ester form. 

For the MTG reaction [3] carbenes have been suggested as initial species, leading to ethylene as the first olefin formed. This seems also to be a plausible mechanism in our case. Thus in the initial step methylchloride may form a surface carbene and HC1 catalyzed by acid and basic sites in the zeolite. Surface carbenes can subsequently react with each other to give ethylene. Another possibility may be that a carbene molecule reacts with methylchloride to ethyl-chloride, which leads to ethylene after HC1 elimination. Since no ethylchloride has been observed under our reaction conditions, the former reaction sequence may rather apply. Higher olefines may be formed by reaction of ethylene with methylchloride, followed by HC1 elimination, or higher condensation of surface carbenes. The product HC1 may cause catalyst deterioration during long reaction times. 

The exchange of halogen atoms between simple aromatic compounds (R(])X and (jiX R = -F, -Cl, -CH3 X = -Cl, -Br, -1) in the presence of Cu-HZSM-5 occurred selectively through ipso substitution. 4-chlorotrifluoromethylbenzene in the presence of bromobenzene gave also a bromo product with a good selectivity. Unfortunately the catalyst deteriorated so that more appropriate experimental conditions have to be found for the reaction to be of a pratical interest. Other sources of bromine should be investigated. 

The alkaline earth metal addition to the Pd catalyst improved the hydrocarbon oxidation activity. Similar phenomena have been observed on Pd/Ba and Pd/La catalysts, and it is concluded that the suppression of hydrocarbon chemisorption on Pd by the addition of Ba or La allows the catalytic reaction to proceed smoothly under reducing conditions( 16,20). On the other hand, the alkali metal addition, especially K or Cs, to the Pd catalyst deteriorated the hydrocarbon oxidation activity. 

Reaction temperature. Lower temperatures enhance isobutene conversion, prolong catalyst life, decrease formation of byproducts like diisobutene and tert-butyl alcohol. Higher temperatures cause catalyst deterioration, favor decomposition of product, and reduce yield. 

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