Sintering during reduction

With Ni/AI2O3 catalysts, total nickel area increases with loading. Even up to about 40 wi % nickel the crystallites are sufficiently separated that extensive sintering during reduction does not occur. Above 50%, however. 

There are indications that these poorly sintered materials are unstable upon reduction of the copper oxides. Two separate studies, one with Cu—YZT and the other with Cu—GDC, have found that the Cu migrates out of the porous electrolyte matrix during reduction. 

Hydrogen chemisorption, HRTEM and QEXAFS all show that the final particle size is smallest after autoreduction. During autoreduction, the reduction proceeds in several steps. Two steps (at 210 and 240 °C) show a high reduction rate accompanied by sintering. During this sintering, metal particles in the range of 10-12 A are formed. The reduction is caused by... 

Species such as, Cl, or F that increase the mobility of metal atoms may cause either redispersion or increased sintering rates. The role of Cl in redispersion has been discussed elsewhere [16]. There is evidence that S and F poisons increase rates of sintering. For example, Erekson and Bartholomew [57] found that an unsupported Ni powder with particles having diameters of 2-6 pm was relatively stable during reduction in H2 at 725-775 K over a period of 18 h. However, after exposure for just 6 h to 0.2 ppm H2S/H2 at either 725 or 775 K, (but not below 725 K) most of the small particles had agglomerated to large (100-250 pm)... 

Water vapor produced during reduction is particularly problematic as it can inhibit the reaction or lead to hydrothermal sintering. 

According to this concept, the stabilizer function of alumina reduces to paracrystal-line lattice defects an analogous effect is to be expected with Cr203, Sc203, etc. [262], [296]. Another theory is based on the observation that during reduction part of the alumina precipitates with other promoters into the surface of the iron crystallite in a molecularly dispersed distribution [258], [297] or in small islands [254]. This patchy monolayer of alumina acts like a spacer between iron atoms of neighboring crystallites and prevents sintering by means of a skin effect [263]. (See also [101], [110], [253].)... 

RI1/AI2O3 1% O2/N2,1% H2/N2 at 600-800°C Rapid loss of dispersion within 2 hours at 600, 700, and 800°C aging in O2 at same temperatures causes rapid and complete oxidation to 2 3 sintering during high frequency reduction/oxidation is slower than in pure gases. 26... 

The results presented above allow us to conclude that the presence of NiO inside the support matrix leads to its sintering during the reduction stage, due to the shortening of Ni-Al or... 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

The results that have been obtained with the catalysts after reduction and passivation are the same as those after calcination, i.e. the textural and structural properties of the support material have completely been retained after the treatments (as determined with nitrogen physisorption. X-ray diflfiaction and transmission electron microscopy). Information concerning the metallic nickel particles has been obtained with X-ray diffraction and transmission electron microscopy. Diflractograms of the catalysts after passivation are shown in Fig. 8. The observed features are exactly the same as for the oxidic systems (Fig. 4) only very broad and low diffractions are visible for the catalyst ex citrate, whereas sharp, intense peaks with a broad onset are observed for the catalyst ex nitrate. Consequently the nickel particles of the catalyst ex citrate have resisted sintering during the reduction treatment, thereby conserving the high dispersion of the catalyst. These results were confirmed by transmission electron microscopy measurements (not shown) only very small nickel nanoparticles situated inside the mesopores were found for the catalyst ex citrate. 

Fig. 5 shows the SEM images of Fe-oxide(urea), Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) before the first reduction and after the third oxidation. Al and Cr cations are effective additives for redox performance of iron oxides, as described earlier. The SEM images of the samples before the first reduction showed that all the samples consisted of small particles with uniform size and the particles sizes of AI-Fe-oxide(urea) (Fig. 5 (b)) and Cr-Fe-oxide(urea) (Fig. 5 (c)) were smaller than that of Fe-oxide(urea) (Fig. 5 (a)), suggesting that the addition of Al and Cr cations into iron oxides divides the oxides into small particles. In the SEM image of the Fe-oxide(urea) after the third oxidation (Fig. 5 (a)), larger particles with different size were observed. Therefore, the iron species in Fe-oxide(urea) were sintered during redox cycles. On the other hand, the SEM images of... 

Honrglassing - Refers to the change in the shape of a cylindrical object made of polytetrafluoroethylene during sintering. A reduction in the diameter of the cylinder near the middle and an increase in the bottom diameter creates a resemblance in the shape of the sintered part to an hourglass. 

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