Catalysts metal crystallite formation

Grubbs and coworkers (35) while examining Rh and Co catalysts derived from 14 reported the loss of infrared CO stretches and visual darkening of the catalysts after use for hydrogenation of olefins, aldehydes or ketones, cyclohexene disproportionation to benzene and cyclohexane or the cyclotrimerization of a wide variety of acetylenes. Stille (36) using a rhodium catalyst prepared from 14 observed activity for the hydrogenation of benzene that increased with reuse, a phenomenon usually associated with metal crystallite formation. Rhodium catalysts of 15 and 16 used to hydroformylate octene-1 revealed a loss of carbonyl adsorptions and a loss in catalytic activity upon reuse (37). 

There are two views on the origin of enantiodifferentiation (ED) using Pt-cinchona catalyst system. In the classical approach it has been proposed that the ED takes place on the metal crystallite of sufficient size required for the adsorption of the chiral modifier, the reactant and hydrogen [8], Contrary to that the shielding effect model suggest the formation of substrate-modifier complex in the liquid phase and its hydrogenation over Pt sites [9],... 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

With Pd- or Pt-containing catalysts the problem arises how to discriminate between reduced Ni and the reduced metal. Temperature-programmed reduction experiments ( 5) have shown that Pd is reduced arond 80 C. Reduction of Ni starts at 200 to 300 C. Reoxidation and rereduction point to a possible Pd-Ni alloy formation. We have studied Pd-NiSMM and Pt-NiSMM samples after reduction at 350 and 450 C by TEM combined with electron microprobe analysis. Metal crystallites with a maximum diameter of 20 nm are formed. Part of them contain Pd and Pt, respectively. Because of the background of lattice Ni2+, reduced Ni is difficult to distinguish by this technique. Since, moreover, large metal crystallites are observed from which no Pd or Pt signal is obtained at all, it seems reasonable to assume that these crystallites are reduced Ni. The presence of alloys cannot be ruled out. 

Most methods deal with the formation of metal particles on a support that is preformed since this leads to simpler preparation processes. There is an important route, however, typically used for metal-SiCh and metal-AI2O3 catalysts, which involves (Table I) the coprecipitation in a precursor form (hydroxides, nitrates, carbonates, silicates, etc.) of both the support and the active phase from a solution 37a,b, 38, 41). The advantage is to produce an intimate mixing of metal precursor and support. The precipitate leads on calcination to a support with the active component dispersed throughout the bulk as well as at the surface. After reduction to the final catalyst, it is difficult to obtain metal crystallites of uniform size 42, 43) because of the presence of both the oxides (of the support and of the active metal) and other intermediate compounds [e.g., nickel alumi-nate or silicate for the Ni/Al203 42) and Ni/SiCh 43) systems, respectively] that have different reducibilities. 

Results were interpreted in terms of the formation of islands of CO molecules where the island size was dictated by the number of Pt atoms in a terrace-type ensemble on the metal crystallite. Consistent with this interpretation, re-reduction of the sample, which is known to produce smoother particles with fewer steps and edges and more extended terraces, led to a shift in the frequency at which this invariance was observed (2076 cm" ) and also a change in the % CO conversion which was achieved at the corresponding temperature (Fig. 4.10). This methodology was then applied to a supported Pt-Rh catalyst, where it was shown that the ensembles... 

Li et reported a novel method of obtaining nickel oxide particles with controlled crystalline size and fibrous shape, highly dispersed on in situ produced carbon, inhibiting further growth of Ni particles. On the other hand, Ni/CFC (filamentous carbon) catalysts were shown to have sufficient efficiency in low-temperature methane decomposition. Thus, the use of CFG, whose textural properties can be modified by their activation with Hg or COg, opens up the possibility of its application as a support in heterogeneous catalysis. Methane decomposition over Ni-loaded activated carbon (AC) was also investigated. XRD results showed absence of NiO with only Ni metal crystallites formed in the catalyst even if calcined in Ar, which eliminates the inevitable reduction step with other supports. However, the formation of NisC during the process leads to deactivation of the catalysts. Filamentous carbon formation is... 

On the catalysts with large metal crystallites size, the strong adsorption of the dehydrogenated species leads to the formation of coke deposits. Other mechanism involves the formation of highly dehydrogenated fragments that may combine to form more toxic coke deposits. 

The high-temperature shift process is typically carried out in adiabatic reactors at an inlet temperature above 300°C and with a temperature increase up to 500°C. The catalyst is a robust Fe-Cu-Cr catalyst [68], Chromium, which prevents sintering, is present as an iron chromium internal spinel [175] [361], The activity is improved by promotion with a low percent copper, which will be present as small metallic crystallites on the iron chromium spinel [232] [266], This will also inhibit the formation of hydrocarbons at low H2/CO ratios [96], It was shown [235] [391] that the activity for this reaction could be related to the phase transition into iron-carbide, which is a Fischer-Tropsch catalyst ... 

The introduction of magnesia to what seems an already complicated mixture is interesting mainly because it was also included in other nickel catalysts such as the raschig-ring catalysts for steam reforming. It is now realized that the molecular dimensions of magnesia are similar to those of cobalt and nickel oxides, and that magnesiirm can replace cobalt and nickel in solid solution within a crystalhne lattice. This can make catalyst reduction easier and result in the formation of smaller, more stable metal crystallites. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

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