Irreversible sintering

It is noteable that the spectra of samples calcined at 923 K, when the zeolite framework could well be irreversibly sintered, do not differ too much from those for the same zeolites after mild pretreatment. As a rule, the same sites were detected, and only the concentration of the most coordinatively unsaturated atoms increases after calcination, while the number of hydroxyl groups diminishes. Apparently, the method is sensitive to the local arrangement, that does not change greatly even if the lattice symmetry is lost. Adsorption on the defect sites arising after high temperature treatment can supply supplementary information about the composition of material or about the state of incorporated metal atoms before the calcination. 

Few comparative studies between nanowire and polycrystalline chemical sensors have been reported. Sysoev reported that, even if the nanoparticles had a higher response to 2-propanol vapors at first, after some days of operation the response of the nanoparticles decreases to the stable response of nanowires. This was ascribed to the irreversible sintering process in the nanoparticles that occurs due to high temperature operation. 

Improve Catalyst Life and Steadiness. Regeneration or replacement of a catalyst is expensive both in direct cost and in lost production represented by the down time. Lowering the rate of deactivation of the catalyst whether by fouling, by sintering, or any other irreversible process will improve the economics of a process. 

Panesh et al. [157] were the first to make an attempt to detect rare gas metastable atoms (RGMAs) with the aid of semiconductor sensors. The sensing element (a sensor) was represented by a sintered polycrystalline film of ZnO metastable atoms were obtained in a neon ambient by electron impact. It was shown that electrical conductivity of ZnO film irreversibly increases under the action of RGMAs. However, the signals obtained were too small and that did not allow one to utilize the sensing technique to survey the processes with participation of metastable atoms. 

Surface reconstruction has been earlier observed and reported in the literature [116]. Sequential reductive and oxidative thermal treatment usually leads to bulk transition from CoOx + La203 to LaCo03, respectively. On the other hand, the restoration of the perovskite structure is not observed under severe conditions at higher temperature. In those temperature conditions, the sintering of Co crystallites leads to irreversible redox cycle with the preferential formation of Co304 under lean conditions. 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

However, in most cases of poisoning, fouling, sintering, solid-state transformation, volatilization and attrition, the loss of activity is irreversible and definite, and in the best scenario only a fraction of the initial activity can be restored. 

Such a temperature-time-conductance curve for a vacuum-sintered sample is shown in Fig. 5. A sample, pretreated at 100°C as described above, was heated to 125°C at time zero. The conductance changes as a function of time were followed for 160 minutes (point B), and then the temperature was suddenly raised to 150°C. The time-dependent conductance changes proved to be quite reversible, contrary to the irreversible decrease of conductance with increasing temperature for samples kept in an oxygen atmosphere. 

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