Thermal deactivation

Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts, where both the active metal and ceria were dispersed onto high-surface-area y-Al203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavy-duty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. 

As mentioned in Section 10.2 above, both ceria and ceria-zirconia contain relatively weakly-bound oxygen when freshly prepared, e.g., in high-surface-area form. The thermal stability of this oxygen may differ in the two materials, however, as shown in steady-state CO-oxidation measurements performed by Bunluesin et al. [11] on model planar catalysts. In these experiments, films of ceria and ceria-zirconia were subjected to calcination treatments over a wide range of temperature before noble 

Irrespective of the underlying reason, ceria-zirconia certainly retains a much higher oxygen storage capacity than ceria in model Pd catalysts after high-temperature redox aging, intended to simulate automotive exhaust, as shown in Table 10.1. 

As already emphasized above, the ability of a three-way catalyst to utilize its OSC depends critically on the noble metal. Not only does the metal need to be in contact with the oxygen-storage material, but it must also be sufficiently well dispersed in 

In addition to the dilution effect described above, the surface area of the oxygen-storage material also figures into a potentially more serious deactivation mode, loss of noble metal by deep encapsulation [19,20], As ceria-zirconia sinters, some of the noble metal particles supported on it may become trapped, either within single grains or at grain boundaries of the dense ceramic, as shown by the TEM micrograph in Fig. 10.9 [20]. 

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An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

Pore volume is an indication of the quantity of voids in the catalyst particles and can be a clue in detecting the type of catalyst deactivation that takes place in a commercial unit. Hydrothermal deactivation has very little effect on pore volume, whereas thermal deactivation decreases pore volume. 

The decreases in microactivity and surface area are strong functions of thermal deactivation in the regenerator and the presence of metal in the feed. 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

The remainder of the work on Ni(II) complexes involves the use of chelating ligands in which the carbene is functionalised with pendant heteroatom donor(s). The picolyl-functionalised NHC dicationic complex 29 (Fig. 4.11) was tested for ethylene polymerisation after treatment with MAO [34]. This complex was found to be highly active in a preliminary test (330 kg moF bar h" ), giving predominantly linear polyethylene. Unfortunately this work does not seem to have been followed up. The same system was active for norbomene polymerisation (TOF = 24 400 h" over 1 h). Maximum activity was achieved at 80°C whereafter thermal deactivation became significant, although the nature of this deactivation was not studied. The phenoxide-functionalised carbene complex 30 (Fig. 4.11) was also... 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

G. Kiss, C. E. Kliewer, G. J. DeMartin, C. C. Culross and J. E. Baumgartner, Hydro-thermal deactivation of silica-supported cobalt catalysts in Fischer-Tropsch synthesis, J. Catal., 2003, 217, 127-140. 

The temperature dependence of luminescent metal complexes can be controlled by molecular design that affects the energy gap between the emitting state and the deactivating d-d or by altering the preexponential factor for thermal deactivation. The sometimes large temperature dependencies of lifetime and quantum yields for metal complexes also suggest their use as temperature sensors. 

Since thermal deactivation from high-lying electronically excited states occurs on the picosecond timescale [70, 71] for conjugated molecules, excitation energy... 

Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type monomers can be overcome by changing to heterofunctional AB-type monomers. Indeed, IIMU has been subjected to bulk polycondensation using lipases as catalyst in the presence of 4 A molecular sieves. At 70 °C, CALB showed 84% monomer conversion and a low molecular weight polymer (Mn 1.1 kDa, PDI 1.9). No significant polymerization was observed with other lipases (except R cepacia lipase, 47% conversion, oligomers only) and in reference reactions with thermally deactivated CALB or in the absence of enzyme. Further optimization of the reaction conditions (60wt% CALB, II0°C, 3 days, 4 A molecular sieves) gave a polymer with Mn of 14.8 kDa (PDI 2.3) in 86% yield after precipitation [42]. 

Kwon, K.S., and M.H. Yn, Effect of glycosy-lation on the stability of alphal -antitrypsin toward urea denaturation and thermal deactivation. Biochim Biophys Acta, 1997. 1335(3) 265-72. 

The quantum yield of the primary act of spectral sensitization is limited by competitive processes fluorescence (fl), thermal deactivation of the excited dye molecule by internal conversion (ic), and intersystem crossing to the triplet manifold (isc). The sum of the quantum yields of sensitization and all competitive processes is one ... 

It is simple to demonstrate that a catalyst operated in the absence of poisons (54, 55) still can show significant activity loss, albeit to a much smaller degree than in their presence. This deactivation process is induced by thermal effects. A separation of chemical and thermal deactivation requires considerable efforts. 

Enhancement of Stability of Proteases against Oxidation and Thermal Deactivation 293... 

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