Catalyst ageing and poisoning

As we have pointed out in relation to stability, it is only in theory that the catalyst is found intact at the end of the reaction. All catalysts age and when their activities or their selectivities have become insufficient, they must be regenerated through a treatment that will return part or all of their catalytic properties. The most common treatment is burning off of carbon, but scrubbing with suitable gases is also frequently done to desorb certain reversible poisons hydrogcnolysis of hydrocarbon compounds may be done when the catalyst permits it, as well as an injection of chemical compounds. When the treatment docs not include burning off carbon deposits, it is often called rejuvenation. 

Another opportunity is in understanding and improving how SCR catalyst systems age. In HD applications the catalyst may need to operate for a million kilometers. Thermal aging and poisoning need to be better understood to allow tighter control and high deNOx efficiency throughout the useful life. 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

Mowery, D.L., Graboski, M.S., Ohno, T.R. et al. (1999) Deactivation of Pd0-Al203 oxidation catalyst in lean-burn natural gas engine exhaust aged catalyst characterization and studies of poisoning by H20 and S02, Appl. Catal. B 21, 157. 

Figure 1.19 AES data from a Ru/Al203 catalyst aged in a reaction (CO+H2) mixture containing trace amounts of H2S [148], Spectra are shown for the sample before (a) and after (b) sputtering with an Ar+ beam for 2 min. The difference between the two spectra indicates the presence of S on the surface but not the subsurface of the poisoned catalyst. (Reproduced with permission from Elsevier.)...

In Section V, deactivation of catalyst pellets and reactor beds during residuum hydroprocessing is considered. The chemical nature of the metal deposits is described, including a discussion of the physical distribution of these poisons in aged catalysts and reactor beds. Models to predict... 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

BR with narrow MMDs (Mw/Mn > 3.5) and a low solution viscosity can also be obtained by the use of a multi-component catalyst system which comprises the following six components (1) Nd-salt, (2) additive for the improvement of Nd-solubility, (3) aluminum-based halide donor, (4) alumoxane, (5) aluminum (hydrido) alkyl, and (6) diene. The solubility of the Nd-salt is improved by acetylacetone, tetrahydrofuran, pyridine, N,N-dimethylformamide, thiophene, diphenylether, triethylamine, organo-phosphoric compounds and mono- or bivalent alcohols (component 2). The catalyst components are prereacted for at least 30 seconds at 20 - 80 °C. Catalyst aging is preferably performed in the presence of a small amount of diene [397,398 ]. As the additives employed for the increase of the solubility of Nd salts exhibit electron-donating properties it can be equally well speculated that poisoning of selective catalyst sites favors the formation of polymers with a low PDI. 

Assuming that the metals and other poisons on catalyst are low, we can expect that traditional catalyst steaming will be sufficient to simulate catalyst deactivation. Keyworth et a) [16] recommend to make a composite of several steamings in order to address the age distribution of equilibrium catalyst in a commercial unit. Beyerlein et al [17, 21] critically question the possibility of improving catalyst ageing procedures, which rely onfy on steam treatment at constant temperature for varying times. 

Characterization of catalysts with respect to their propensity to be inhibited by IS must take into account the fact that the phenomena depend upon the whole catalytic reaction system (catalysts, degree of ageing and/or poisoning, inhibiting effects, operating conditions) and... 

Figure 4 shows the dibenzothiophen conversion activities of the aged and regenerated catalysts relative to the fresh catalyst, versus amounts of metals. The activities are considered to be proportional to the remained active sites on the catalyst surface because effectiveness factors are assumed to be unity in this test. An activity loss of the regenerated catalysts is considered to be caused by metal poisoning. 

---