Supported metals reduction step

Figure 3.10. Schematic representation of the elementary steps used in microkinetic simulations of the reduction of NO on supported metal particles [23]. The mechanism represented here incorporates adsorption and desorption steps, surface reactions such as NO dimerization and dissociation and N2, N20 and C02 formation, surface oxidation, and mobility of adsorbates. (Figure provided by Professor Libuda and reproduced with permission from Elsevier, Copyright 2005).

The SEA approach can be applied to a novel system in three steps (1) measure the PZC of the oxide (or carbon) and choose a metal cation for low-PZC materials and an anion for high-PZC materials, (2) perform an uptake-pH survey to determine the pH of the strongest interaction in the appropriate pH regime (high pH for low PZC and vice versa), and (3) tune the calcination/reduction steps to maintain high dispersion. Highly dispersed Pt materials have been prepared in this way over silica, alumina, and carbon. Other oxides can be employed similarly. For bimetallics, the idea is to first adsorb a well-dispersed metal that forms an oxide intermediate with a PZC very different to the support. In this way the second metal can be directed onto the first metal oxide by SEA. Reduction may then result in relatively homogeneous bimetallic particles. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts. (4)... 

The various stages of the preparation and thermal treatments of supported metal catalysts arc very schematically illustrated in Fig. 3. A very similar presentation was earlier given by van Delft et al. [16]. Typically, the support is impregnated with a metal salt (sec Section A.2.2.1.1) which serves as the metal precursor and should be well dispersed. Small metal particles may be formed by either direct reduction under mild conditions or by reduction after an intermediate oxidation step. Mild oxidation will lead to thin oxide films spread out on the support or to small oxide particles, where particles and film may also coexist. More severe treatments in oxidative atmospheres can lead to the... 

In summary, the way is paved to look at oxide-supported metal nanoparticles, prepared in solution, and to understand the formation of MNPs through calcination and reduction. However, there is still a way to go to identify the elementary steps in the interaction of the species from solution at the solid-liquid interface. Of course, this is what we really want. 

In the case of the phen complex, reduction of the monovalent state leads to copper metal, the initial electron transfer occurring either in the Cu 4s orbital or via a phen n orbital. Fast dissociation follows this monoelectronic reduction step. The redox orbitals involved during the reduction process of the copper catenate are likely to be ligand-localized. This is also supported by the small difference between the redox potentials of the Cu+ / and Cu°/ couples (A i/2 200 mV). Electrolysis of Cu.5+ in the cavity of an EPR spectrometer confirms the radical anion nature of the formally copper(O) complex obtained by one-electron reduction of the catenate g = 2.000 + 0.002, Ai/ = 39 G. 

After drying, the supported salt is generally calcined to convert the salt into the oxide which is then reduced to give the supported metal catalyst. At times the dried supported salt is reduced without the intermediate calcination step. In general, a more dispersed catalyst is obtained by the reduction of a calcined material than is found when the supported salt is reduced without prior calcination. ... 

In conclusion, bimetallic Pt-Sn/alumina catalysts prepared by successive impregnations with an intermediary reduction step and introduction of the tin salt (SnCU) under hydrogen are less sensitive to coke deactivation than catalysts prepared by coimpregnation. This behavior probably results from a more effective interaction between the two metals, leading to smaller platinum ensembles, as evidenced by the low hydrogenolysis activity. However, the amount of coke deposited on the whole catalyst depends on the nature of the feed and therefore on the nature of the dehydrogenated species which are more or less active precursors for coke deposition on the support. 

During the highly exothermic reduction step, LTWGS catalysts can lose activity [5] by sintering of the metallic phase, formation of a Cu-Zn alloy or changes in the textural characteristics of the support induced by temperature, the presence of steam and the reducing mixture composition... 

Catalytic liquid phase semihydrogenation of acetylenes is an important industrial and laboratory reaction, especially in fine chemical synthesis [1]. The use of supported metal catalysts for this selective hydrogenation readily facilitates the separation of organic products from the catalyst. However, liquid phase reactions with supported catalysts tend towards mass transport limitation [2] and, therefore, the support particles should be between 1 and 10 pm in size this avoids transport limitations and separation problems. With support particles of this size high temperature reduction in a flow of H2 gas is very difficult and to avoid this step it is possible to prepare supported metal particles by decomposing organometallic compounds under mild conditions [3-5]. 

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