Ruthenium catalysts hydrogen chemisorption

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. 

The data in Table 1 show that ruthenium dispersion is higher for RU/AI2O3 catalyst than for the Ru/AC one. The addition of small amounts of MgO to carbon supported catalysts improves the dispersion of the metal, very probably because the MgO is avoiding sinterization processes. However, addition of higher amounts of MgO causes a diminution of the hydrogen uptakes. This fact indicates that a part of the metallic surface could be covered by MgO hindering the hydrogen chemisorption. 

Dispersion is defined as the ratio of surface atoms to total atoms in the metal crystallites, and it is determined from chemisorption measurements (26,29). A typical hydrogen chemisorption isotherm is shown in Figure 2.4 for a silica-supported ruthenium catalyst containing 5 wt% ruthenium. The quantity H/Ru in the right-hand ordinate of the figure is the ratio of the number of hydrogen atoms adsorbed to the number of ruthenium atoms in the catalyst. The catalyst was treated with a stream of hydrogen in an adsorption cell at 500°C, after which the cell was evacuated and cooled to room temperature for the determination of the isotherm. The adsorption is... 

Figure 2.4 Typical hydrogen chemisorption isotherm at room temperature for a silica-supported ruthenium catalyst containing 5 wt% ruthenium (28). (Reprinted with permission from Academic Press, Inc.)...

Figure 4.2 Influence of the state of dispersion of ruthenium-copper catalysts on the relationship between hydrogen chemisorption capacity and catalyst composition (10, 11). (Square points for large ruthenium-copper aggregates represent total hydrogen chemisorption triangular points represent strongly chemisorbed hydrogen, i.e., hydrogen which is not removed from the surface by evacuation of the adsorption cell at room temperature to a pressure of approximately 10 6 torr). (Reprinted with permission from Academic Press, Inc.)...

In the case of product inhibition, it is expected that the ruthenium surface area of the spent catalysts, as measured by hydrogen chemisorption, declined due to blocking of the active sites. However, no evidence for this assumption was found (Table 11.4), since the spent catalysts possessed ruthenium surface areas... 

OUphant JL, Fowler RW, PanneU RB, Bartholomew CH (1978) Chemisorption of hydrogen sulfide on nickel and ruthenium catalysts I. Desorption isotherms. J Catal 51 229-242... 

Aika et aV prepared a series of RU/AI2O3 catalysts with the same content of Ruthenium [2% (mass fraction)] supported on AI2O3 without the addition of the promoters, with RuCls, K2RUO4, Ru(acac)3, Ru(N0)(N03)3 and Ru3(CO)i2 as the precursors of ruthenium, respectively (Table 6.1). The activity and amount of hydrogen chemisorption were measured under the same reaction conditions as shown in Table 6.1. 

The dispersion of ruthenium, measured with chemisorption of H2 and Transmission Electron Microscopy (TEM), are given in Table 6.38. It is seen from Table 6.38 that the dispersions obtained by these two methods are not in agreement. The chemisorption of H2 considered only the strongly bound hydrogen and by assuming a surface stoichiometry of H/Ru = l. The calculation of TEM was assuming spherical ruthenium crystals without contact to the support. Kowalczyk et al ° considered that the dispersions obtained from chemisorption of H2 on Ru/carbon catalysts are not reliable. Therefore, a careful study of the TEM was conducted to obtain accurate estimates of the particle sizes and these numbers will also be used in the following discussion. 

Me]x-MCM-41 containing nanosized particles of platinum, palladium, rhodium, ruthenium and iridium were directly synthesised from surfactant stabilised spherical metal nanoparticles in the synthesis gel, and characterised with XRD, ICP-AES, TG/DSC, TEM, nitrogen physisorption and carbonmonoxide chemisorption, and Si MAS NMR. During the synthesis some agglomeration of the particles took place, but the metal particles were present inside the pore system of MCM-41. The matericils were active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbomene. 

Influence of the Reduction Temperatube. RuNaY zeolite, containing 5.6 % Ru by weight, is taken as a representative catalyst to illustrate the influence of the reduction temperature on the methanation activity. The dispersion of the ruthenium metal phase measured by desorption of chemisorbed hydrogen and by CO chemisorption is given in Table I. 

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