Ruthenium chemisorption

Methane is a stable molecule and therefore hard to activate. As a result the sticking probability for dissociative chemisorption is small, of the order of 10 only, and ruthenium is more reactive than nickel. However, a stretched overlayer of nickel is significantly more active than nickel in its common form, in agreement with expectation. 

Ruthenium-copper and osmium-copper clusters (21) are of particular interest because the components are immiscible in the bulk (32). Studies of the chemisorption and catalytic properties of the clusters suggested a structure in which the copper was present on the surface of the ruthenium or osmium (23,24). The clusters were dispersed on a silica carrier (21). They were prepared by wetting the silica with an aqueous solution of ruthenium and copper, or osmium and copper, salts. After a drying step, the metal salts on the silica were reduced to form the bimetallic clusters. The reduction was accomplished by heating the material in a stream of hydrogen. 

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

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. 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

Throughout these studies, no product other than propane was observed. However, subsequent studies by Sinfelt et al. [249—251] using silica-supported Group VIII metals (Co, Ni, Cu, Ru, Os, Rh, Ir, Pd and Pt) have shown that, in addition to hydrogenation, hydrocracking to ethane and methane occurs with cobalt, nickel, ruthenium and osmium, but not with the other metals studied. From the metal surface areas determined by hydrogen and carbon monoxide chemisorption, the specific activities of... 

Ru3+ is not the only ion capable of improving the efficiency of n-GaAs based solar cells. Lead chemisorbed from a basic plum-bite solution,36 as well as Ir43 and Rh33 chemisorbed from acids32 are also effective. These ions are, however, not as strongly chemisorbed as Ru3+. Consequently, they are more readily desorbed and produce a lesser improvement. Experiments with combined plumbite and ruthenium chemisorption show a further small improvement.38... 

Saturated hydrocarbons other than methane do react with metal surfaces at 20°C. Clean metal surfaces such as ruthenium absorb saturated hydrocarbons and effect dehydrogenation of the hydrocarbon, e.g., cyclohexane is converted to benzene. In these cases, perhaps the initial chemisorption process comprises the formation of several C-H-metal three-center two-electron bonds. 

Figure 15. Representation of the interaction of cyclohexane (chair form) with Ru(0001) as suggested by Madey and Yates (31) in their classic study of hydrocarbon chemisorption on this basal plane of ruthenium. Three Ru—H—C three-center bonds are formed with three of the axial hydrogen atoms on one side of the chair form of cyclohexane. Weaker three-center Ru—H—C bonds may also be extant with equatorial C—H hydrogen atoms.

There is an elegant study (31) of hydrocarbon chemisorption by Madey and Yates for the basal plane, (0001), of ruthenium. 

The same types of catalysts used for HDS are also used for hydrodenitrogena-tion, and ammonia is used as a probe molecule to characterize these catalysts. An IINS investigation of NH3 on partially desulfided RuS2 (25) showed that the chemisorption was dissociative, forming NH2 groups on the coordinatively unsaturated ruthenium sites. 

The surface areas of a number of commercial palladium blacks were measured using the BET procedure as well as hydrogen chemisorption, electron microscopy and X-ray diffraction analysis. These data showed that these blacks had particle sizes ranging from about 7 to 140 nm and surface areas between 70 and 4 m2/g.20 Ruthenium blacks prepared by the reduction of different samples of ruthenium oxide and ruthenium chloride were found to have surface areas ranging from 3-20 m2/g.21... 

The catalysts were prepared by consecutive impregnation with aqueous solutions of Ru(N0)(N03)3 and Mg(N03)2. The support was an activated carbon (commercial one provided by ICASA, Spain, Sbet = 960.7 m g ) purified by treatment with HCl solution, to remove inorganic compounds. For comparative purposes, a ruthenium catalyst supported on a Y-AI2O3 (Puralox condea, Sbet = 191.9 m -g ) was also prepared by similar procedure. The impregnants were dried at 383 K and subsequently reduced. Before reaction and chemisorption measurements, samples were in situ reduced at 673 K for 2 h. Activity, selectivity and stability under reaction conditions were measured at atmospheric pressure in a fixed-bed quartz reactor kept at 823 K by cofeeding CH, CO2 and He as diluent. An equimolecular mixture of CH4 and CO2 (10% CH4, 10% CO2 and balance He) was adjusted by mass flow controllers (Brooks) and passed through the catalyst at a flow rate of 100 cm -min (space velocity = 1.2-10 h ). The effluents of the reactor were analysed by an on-line gas chromatograph with a thermal conductivity detector. 

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. 

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