Catalyst deactivation thermal degradation

Continuous exposure of catalysts to high temperatures may cause an alteration in its components and gradually lead to its deactivation. Thermal degradation may have an undesirable impact on both the catalyst substrate and noble metal load in various ways. Thermal degradation covers two phenomena sintering and solid-state transformation. 

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

Deactivation in Process The active surface of a catalyst can be degraded by chemical, thermal, or mechanical factors. Poisons and... 

Sintering is an important mode of deactivation in supported metals. The high surface area support (carrier or substrate) in these catalysts serves several functions (l) to increase the dispersion and utilization of the catalytic metal phase, (2) to physically separate metal crystallites and to bind them to its surface, thereby enhancing their thermal stability towards agglomeration, and (3) in some cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

The thermal degradation of waste HDPE can be improved by using suitable catalysts in order to obtain valuable products. However, this method suffers from several drawbacks. The catalysts are deactivated by the deposition of carbonaceous residues and Cl, N compounds present in the raw waste stream. Furthermore, the inorganic material contained in the waste plastics tends to remain with the catalysts, which hinders their reuse. These reasons necessitate a relatively high purity of waste plastics, containing very low concentrations of a contaminant. Thus, various pretreatments are required to remove all the components that may negatively affect the catalyst. 

Waste-plastic-derived oil that was prepared by thermal degradation of municipal waste plastics at 410°C was dehydrochlorinated to remove chloroorganic compounds using various catalysts such as iron oxide, iron oxide-carbon composite, ZnO, MgO and red mud. The iron oxide catalysts were effective in removing the chloroorganic compounds. MgO and ZnO catalysts were deactivated during the reaction by HCl, which is produced by the dehydrochlorination of chloroorganic compounds. Iron oxide and its carbon composite were found to be stable in the dehydrochlorination of municipal waste plastic derived oil [19]. 

In a previous work [13], we reported on the preparation of carbon-supported bimetallic Bi-Pd catalysts by the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K and described their catalytic properties in glucose oxidation. The formation of various BixPdy alloys (BiPd, BiPds, Bi2Pds) or, at least, associations on the surface of these catalysts during the activation step was heavily suspected. Alloy formation in supported bimetallic Pd-based catalysts has been mentioned several times in the literature in die presence of other promoting elements, like Pb or Te [14-16] and is sometimes assumed as responsible for the deactivation of the catalysts. 

In the present review, the principle causes of SCR catalyst deactivation are considered under five categories, and discussed in the order of (1) sulfur compounds, (2) alkali metal and alkaline earth metal compounds, (3) arsenic and other heavy metal compounds, (4) fouling or masking by deposits, with pore blocking or surface coating, and (5) thermal degradation. 

Thermal degradation is a physical process leading to catalyst deactivation because of sintering, chemical transformations, evaporation, etc. 

Lead pickup on the catalyst increased with increasing catalyst temperature (I, 2). The initially fast deactivation of the catalyst for hydrocarbon conversion was associated with selective thermal degradation and poisoning of sites required for oxidation of the more difficult hydrocarbons such as methane (2, 3). Carbon monoxide conversion, on the other hand, remained relatively unaffected. 

To address the deactivation behavior in more detail, Uemichi et al. recently examined the change in activity of a silica-alumina catalyst with 13 wt% alumina as a function of time on stream. At a reaction temperature of 723 K, the SA catalyst accumulated over 12 wt% coke on the catalyst after 250 min time on stream. The liquid yield increased slightly from 60 wt"/o to approximately 70 wt% as the coke built up on the catalyst. The limited effect of the coke on the reaction was attributed to the inability of coke deposits to block completely the large pores (f/p,ave = 4.4 nm) of the amorphous catalyst. Although SA showed no activity toward cracking of -octane, the reactivity of polyethylene was substantially enhanced in the presence of the catalyst. This was attributed to the facile reaction on the catalyst of olefins which could be formed from thermal degradation of polyethylene at the temperatures used in this study. 

Zeolite X was also examined for the decomposition of polyethylene in a series of articles by Ayame and co-workers.Using both a batch reactor and a fixed bed tubular flow reactor, the rate of conversion of polyethylene was enhanced in the presence of CaX and NaX. Deactivation was observed in the flow studies, as the rate of formation of gaseous products decreased by a factor of three after 2.5 hours time on stream when the reaction was carried out at 750 K with 4.0 g of catalyst and a polyethylene flow rate of 7.23 x 10 g min . When a CaX catalyst was used, C4 species were observed in the highest yield. In the batch reaction, the yield of iso-C4 species was increased dramatically compared with thermal degradation, as thermal degradation afforded no iso-C4. 

One particular advantage of chemically anchored as opposed to physically adsorbed species such as Rh complexes on 7-AI2O3 is that no migration of Rh occurs over the surface of the support. At the same time the absence of solvent stabilization in gas-phase reactions gives rise to a greater potential for deactivation of catalyst due to thermal degradation or poisoning. 

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