Catalyst ageing mechanism

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138). 

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.

In other instances, reaction kinetic data provide an insight into the rate-controlling steps but not the reaction mechanism see, for example, Hougen and Watson s analysis of the kinetics of the hydrogenation of mixed isooctenes (16). Analysis of kinetic data can, however, yield a convenient analytical insight into the relative catalyst activities, and the effects of such factors as catalyst age, temperature, and feed-gas impurities on the catalyst. 

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

Utilities using post-combustion SCR-supported ammonia injection for NOx control as well as those using ammonia conditioning to improve electrostatic precipitator performance will produce fly ash that contains ammonia compounds. The ammonia is primarily physically adsorbed onto the fly ash particles as sulphate and bisulphate species. In many cases, the residual ammonia levels are quite low (<50ppm) however, elevated concentrations can occur as the catalyst ages or due to mechanical problems with the ammonia injection system. While elevated ammonia concentrations in fly ash do not negatively impact pozzolanic properties, it can reduce ash marketability due to odour concerns. For this reason, several processes have been developed to remove or reduce the amount of ammonia in fly ash. 

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

It has been proposed that, as an increase in the conversion of vacuum residue in the commercial fixed-bed reactors, a coke-controlled catalyst deactivation regime appears in the last bed, where coke blocks the active sites as well as decreases the diffusivity. The activity and diffusivity tests were conducted for aged and regenerated catalysts, which were used in the commercial reactors, to investigate mechanisms of the deactivation by coke and metal deposition. The effects of residue conversion, reactor position, and time on-stream on the deactivation were investigated, comparing the catalysts aged at different conditions. 

Considering the importance of Ru island size, recent studies have been conducted by our group to determine the aging mechanism of some commercially available PtRu catalysts age. PtRu black electrocatalysts were obtained from Tanaka and Johnson-Matthey (referred to hereafter as Tk and JM respectively) and were observed ageing by potential cychng between 0-0.8 V vs. RHE. Both materials (1 1 Pt Ru) were found to have slightly... 

Experimental studies have demonstrated that conventional methanol-synthesis catalysts deactivate slowly in a slurry reactor, even with a concentrated, CO rich feedstream. The catalyst activity correlates with the BET surface area and the rate of deactivation increases rapidly with temperature. This limits the utility of temperature programming as a means for maintaining a constant methanol production rate as the catalyst ages. Continuous catalyst addition and withdrawal is the preferred means to maintain constant methanol production. The key mechanical and process features of this technique were demonstrated In the pilot plant. 

These deposits which act as a poison constitute a measurement of the accumulated work of the catalyst, their analysis and pilot plant tests allow to obtain information which contributes to a better understanding of the poisoning and aging mechanisms of these systems. 

Understanding catalyst deactivation, fouling and aging mechanisms is of paramount importance for developing better catalysts. This is particularly true for catalysts used in the fluid catalytic cracking (FCC) process. 

Figure 2 Ethanol oxidation. Temperature programmed experiments on alumina supported catalysts. MM mechanical mixture of Pt/AfiOr and Rh/Al203 Cl Pt-Rh/Al20j catalyst prepared by coimpregnation, a = fresh catalysts b = aged catalysts white column first oxidation cycle black column second oxidation cycle.

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