Metal redispersion

Heating palladium in UHV at 500°C leads to metal redispersion over y-alumina [XPS (119) see also Section III,A]. Purging the Pd/Al203 catalyst with ultrapure helium or argon at 500-600°C produces Pd+ species seen by ESR (134). IR spectra of the adsorbed CO on the catalyst pretreated in Ar at 600°C for 17 h show the presence of Pd+-CO and PdCT+-CO bands... 

Combination of 1 and 2 under carefully controlled conditions can lead to metals redispersion. 

The chemical stability tests were intended to quantify structure loss under conditions encountered in either catalyst manufacturing or post-regeneration metal redispersion processes ("rejuvenations") Zeolites were first calcined in dry air for two hours at 4 2dC, after which slurries of 20 g zeolite in 200 mL of treatment solution were prepared and stirred for two hours. The treated zeolites were collected on Buchner funnels, washed twice by reslurrying in 200 mL portions of water, dried at 110DC, and (optionally) calcined in dry air for one hour at 482°C. 

Redispersion of the Metal Function and Chloride Adjustment.-After coke elimination by burning, the next step in the catalyst regeneration technique is the restoration of the catalytic functions by metal redispersion and chloride adjustment. This is performed in... 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

Stevenson S, Dumesic JA, Baker RTK, Ruckenstein E (1987) (eds) Metal-support interactions in catalysis, sintering and redispersion. Van Nostrand Reinhold, New York Ferrari AM, Neyman KM, Mayer M, Staufer M, Gates BC, ROsch N (1999) J Pbys Chem B 103 5311... 

This is the basis of a generally applicable multigram synthesis routes to isolable 1-10 nm organosols of the transition metals that are easily redispersible in high concentration in organic phases (Figure 7) [3,183]. 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Although this method gives hydrosols in isolable form and redispersible in water in high concentration of metal, a scale-up of this is not possible. 

A synthetic alternative to this is the chemical reduction of metal salts in the presence of extremely hydrophilic surfactants have yielded isolable nanometal colloids having at least 100 mg of metal per litre of water [105], The wide range of surfactants conveniently used to prepare hydrosols with very good redispersibility properties include amphiphilic betaines A1-A4, cationic, anionic, nonionic and even environmentally benign sugar soaps. Table 3.1 presents the list of hydrophilic stabilizers used for the preparation of nanostructured colloidal metal particles, and Table 3.2 shows the wide variety of transition metal mono- and bi-metallic hydrosols formed by this method [105,120],... 

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

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