Metal redispersion oxidative

Redispersion of Metals after Oxidative Regeneration - Influence of Wetting. -... 

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. 

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... 

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. 

The preparation of real supported catalysts will involve the deposition of a precursor salt followed by decomposition and/or reduction to the final metallic state. We shall consider the influence of the precursors and the effect of oxidative pretreatments later. First, we consider how the shapes of supported metal particles will vary with time under reducing conditions, since this represents the working condition for most metal catalysts. A comprehensive review of sintering and redispersion in supported metals has been presented by Ruckenstein and Dadyburjor.232... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

It is for the above reasons that rejuvenation of supported metal catalysts is typically performed in oxidizing atmospheres. However, redispersion mechanisms may be much more complex than indicated since fragmentation of particles may also occur during the thermal treatments in oxidizing atmospheres (see subsection C). 

Other types of information which can be obtained using EELS are oxidation state, coordination number, and cluster size of heavier atomic number elements (25). Applicability and feasibility is just starting to be demonstrated. There are many problems associated with trying to obtain useful quantitative information on small volumes of catalyst. The major concern is radiation damage. Others include, contamination, sample preparation, beam reactivity (especially crucial for redox information), cluster migration and redispersion, support reactivity with the catalyst metals, and desorption of some elements under high vacuum. 

Oxidative redispersion of Pd and Cu aggregates in zeolite NaHY has been reported (76,143,205). First the metal (M = Pd or Cu) is oxidized to oxide particles ... 

Molecules that are able to coordinate to the metal ions to form complex ions can facilitate the oxidative redispersion of large metal aggregates. Treatment of Pt or Pd catalysts with CI2 in the presence of H2O, CO, NO, and NH3 or with O2 followed by ammoniation was found to be more efficient than the use of CI2 or O2 alone (207-209). 

The CO molecule is a well-known coordinating ligand for many transition metals. Although CO has been reported to be a reducing agent for metal ions such as Cu " and Ag" in zeolites (214,215), it also induces oxidative redispersion of Rh particles in zeolites at room temperature (704). With FTIR, it has been established that the presence of H2O is crucial for the reduction of Ag" in zeolite according to the following reaction ... 

This contribution focuses primarily on those metal oxides that are viewed as immediately relevant to electrocatalysis, but for additional information on the subject the reader is directed to a recent review article [2j]. It has been shown that after exposure to air, nanosized (e.g., 3-5 nm) colloidal Fe Co and Nl particles cannot be redispersed because they are immediately oxidized both in solution and in powder form. When the addition of oxygen is precisely controlled, e.g., by using stoichiometric amounts of O2 diluted in large excess of argon, an... 

An unusual reaction reported by Inoue et al. [66] is the direct oxidation of Ce metal in 2-methoxyethanol at temperatures between 200 °C and 250 °C. Most of the product obtained was bulk Ce02 as a yellow solid, but in addition, they obtained a brown solution of 2 nm Ce02 nanoparticles. The Ce02 nanoparticles could be salted out by the addition of NaCl, and redispersed into solution at will. The solutions obeyed the Beer-Lambert law for the concentration dependence of the optical extinction, suggesting that the nanoparticulate dispersion was a genuine solution. 

RedispersicHi of noble metals other than Pt as a result of oxidationAeduction treatments have also been observed in studies of model catalysts. Wang and Schmidt report that silica and alumina-supported Rh and Pt-Rh and Ir/silica can be redispersed by heating in air at 873 to 1023 K followed by reducing at lower temperatures indeed, diey indicate that a surface area increase of about 100% may occur by these processes. Their results are consistent with either crystallite splitting or oxide film formation in O2 fdlowed by breakup in Hj (see Figure 4). On the other hand, Ruckenstein and Chen observed that repeated cycling of model Pd/alumina in... 

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