Cu-Ru system

In bimetallic catalysts, Cu-Ru is an important system. Combinations of the Group Ib metal (Cu) and Group VIII metal (Ru)-based catalysts are, for example, used for the dehydrogenation of cyclohexane to aromatic compounds and in ethane hydrogenolysis involving the rupture of C-C bonds and the formation of C-H bonds (Sinfelt 1985). Here we elucidate the structural characteristics of supported model Cu-Ru systems by EM methods, including in situ ETEM. 

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

in situ ETEM, ED and EDX are powerful methods to provide 

The behaviour of a model Cu-Ru catalyst on carbon, in CO and H2 in ETEM show sheet-like Cu particles and smaller Ru particles in CO, whereas primarily larger spherical Cu particles and smaller Ru ones are observed in H2 (figure 5.25), 

The studies reported in the literature have suggested that the surface tension of Cu depends on its surrounding environment it is higher in vacuum and varies as vacuum H2 CO. Well-rounded particles are likely to form when the surface tension is low. In CO, the surface tension is lowered to the extent that the Cu prefers to spread out as sheets rather than as three-dimensional spherical particles. Experiments carried out on real (practical) powder catalysts are consistent with the data from the model systems. As in the model systems, sintering by Cu particles is dominant, the particles growing to several tens of nanometres. The type and extent of sintering depend on the exact composition of the bimetallic catalyst. For Cu Ru, ETEM studies show the sintering of Cu to be primarily by particle coalescence. 

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In the Cu/Ru system, ruthenium may function as a reservoir for atomic hydrogen, which is accessible via spillover to neighboring copper. Kinetically controlled spillover of hydrogen from ruthenium to copper (5) is consistent with the observed optimum reaction rate at an intermediate copper coverage. 

The rate enhancement observed for submonolayer Cu deposits may relate to an enhanced activity of the strained Cu film for this reaction due to its altered geometric and electronic properties. Alternatively, amechansim whereby the two metals cooperatively catalyze different steps of the reaction may account for the activity promotion. For example, dissociative Hj adsorption on bulk Cu is unfavorable due to an activation barrier of approximately 5 kcal/mol . In the combined Cu/Ru system, Ru may function as an atomic hydrogen source/sink via spillover to/from neighboring Cu. A kinetically controlled spillover of Hj from Ru to Cu, discuss above, is consistent with an observed optimum reaction rate at an intermediate Cu coverage. 

ARUPS results have identified unique electronic interface states for the Cu/Ru(0001) system. These states are not present in either metal separately but exist because of the abrupt change in properties at the interface. 

That CO chemisorption is perturbed on strained-layer Ni is not surprising in view of CO chemisorption behavior on other metal overlayer systems. For example, on Cu/Ru it has been proposed that charge transfer from Cu to Ru results in decreased occupancy of the Cu 4s level. This electronic modification makes Cu more nickel-like , and results in an increase in the binding energy... 

A model system to study the effects of tensile strain is Cu on Ru(0001). Cu has a 5.5% smaller lattice parameter than Ru. Each Cu layer grown on Ru(0001) presents a specific pattern of surface reconstruction due to the layer-dependent relaxation of the strain [69]. The first Cu layer is pseu-domorphic with Ru(0001) [70], e.g. it is laterally expanded by 5.5% from a nearest neighbor distance (nnd) of 2.55 A in Cu(lll) to 2.70 A. The Cu atoms occupy hep sites (i.e. the continuation of the Ru lattice) with a Cu-Ru distance at the interface of 2.10 A as determined by LEED [71]. 

Alternatively TEMPO can be reoxidized by metal salts or enzyme. In one approach a heteropolyacid, which is a known redox catalyst, was able to generate oxoammonium ions in situ with 2 atm of molecular oxygen at 100 °C [223]. In the other approach, a combination of manganese and cobalt (5 mol%) was able to generate oxoammonium ions under acidic conditions at 40 °C [224]. Results for both methods are compared in Table 4.9. Although these conditions are still open to improvement both processes use molecular oxygen as the ultimate oxidant, are chlorine free and therefore valuable examples of progress in this area. Alternative Ru and Cu/TEMPO systems, where the mechanism is me-... 

The chemical behavior of monolayer coverages of one metal on the surface of another, i.e., Cu/Ru, Ni/Ru, Ni/W, Fe/W, Pd/W, has recently been shown to be dramatically different from that seen for either of the metallic components separately. These chemical alterations, which modify the chemisorption and catalytic properties of the overlayers, have been correlated with changes in the structural and electronic properties of the bimetallic system. The films are found to grow in a manner which causes them to be strained with respect to their bulk lattice configuration. In addition, unique electronic interface states have been identified with these overlayers. These studies, which include the adsorption of CO and H2 on these overlayers as well as the measurement of the elevated pressure kinetics of the methanation, ethane hydrogenolysis, cyclohexane dehydrogenation reactions, are reviewed. 

Data for Ru and Pd systems are consistent with such a route. Where data are available, most Ru and Rh systems (and earlier Cu , Cu , Ag systems) reveal larger 4// values and sometimes positive AS values, and Hj activation is probably genuinely heterolytic. This also seems realistic chemically in terms of the relatively inaccessible higher oxidation states that would be required for the two-step process. More extensive kinetic data, including volumes of activation , would be valuable. Kinetic isotope effects (D2 vs. Hj, or DjO vs. H2O for water-soluble species) are generally small and offer little insight into the different mechanisms of H2 activation . ... 

Figure 25.11 Selected hydrogen thermal desorption traces obtained from a bimetallic Cu—Ru surface (Cu coverage = 0.7 monolayers on a Ru(0001) surface) as a function of adsorption temperature The top curve (a) was obtained after the system had received a saturation exposure at 100 K curve (b) Hj desorption trace after a saturation exposure at 230 K. The dashed line indicates the direct superposition of (a) onto (b). The bottom curve (c) represents the difference (b) — (a) and, hence, is equal to the amount of hydrogen spilled over from Ru to Cu sites at 230 K. After Goodman and Peden [88].

Fig. 13. Relative Ru-speciftc activity as a function of Cu coverage (ML) on Ru(0001) (dashed lines) and Cu atomic ratio on silica-supported Cu/Ru catalysts (points and solid curves) for the ethane hydrogenolysis (a) and cyclohexane dehydrogenation (b) reactions. Note that the atomic ratio reported for the supported system probably underestimates the surface coverage of Cu since Cu resides predominantly on the Ru surface in these catalysts. From Ref. 159. Data for silica-supported catalysts taken from Ref. 184.

For Cu-based systems (e.g., CuCl/Na-ZSM-5 [96]), the Cu Auger parameter (vide supra) is a powerful tool to prove intra-zeolite guest location. Special effects were reported for Co phthalocyanine, where an inequivalence of the N atoms, which is not observed in the XPS of solid phthalocyanine, can be detected when the molecule is dispersed in a zeolite [202]. With Ru and Os phthalocyanine (but not with the Co-, Ni-, and Fe-based complexes), the oxidation state of the metal and, consequently, its BE, has been reported to be higher in intra-zeolite locations than in the bulk solids [200,202]. 

There seems to be comparatively little information available on the structures of bimetallic particles,and although certain systems of particular industrial relevance (e.g. Pt-Re and Pt-Sn), and others having great scientific interest (e.g. Ni-Cu, Ru-Cu), have been intensively studied, the emphasis has been on chemical... 

Apart from copper-based complexes several other metals have been used as well. Fe (56-59), Ni (60), Ru (15), etc have been used to some extent. Especially noteworthy here is the work by Sawamoto and co-workers. As indicated in the Introduction, Sawamoto and Matyjaszewski simultaneously pioneered ATRR Matyjaszewski started off with the use of copper, whereas Sawamoto spent most of his efforts on ruthenium-based catalysts. Recent work from Sawamoto and co-workers shows that the Ru-based complexes can compete with the Cu-based systems on many fronts. Although not yet perfect it seems that a specific Fe-based catalyst is the first to pol5unerize vinyl acetate via an ATRP mechanism (61). 

Jansson, K., and Nygren, M. (1996) Synthesis and catalytic properties of perovskite-related phases in the La-Sr-Co Cu Ru O system. /. Mater. Chem., 6, 97-102. 

Fig. 57. The QE linewidth of CeMjSij systems as function of temperature [(M = Au, Pd, Cu, Ru (from Severing et al. 1989a) and Cu (from Horn et al. 1981a)].

Comparisons with the lanthanide compounds are difficult due to the complexity found in the different systems. Most of the lanthanide compounds order at much lower temperature (< 20 K) than the U compounds. A good example is the recent work of Loidl et al. (1992) on the compounds CeM2Gc2, where M = Cu, Ru, Ag, and Au. All except M = Ru are antiferromagnets, and the M = Ag compound has a... 

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