Monometallic Cu

Ion exchange type reactions have been utilized to produce catalysts generating hydrogen. Zeolite confined copper(O) nanoclusters were synthesized 

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Collman et al., 2007b]. However, when the electron delivery was slow, only the bimetallic forms manifested ORR catalysis (it was presumed that the monometallic Cu-free (Fe-only) catalysts degraded rapidly under these conditions) [Collman and Boulatov, 2002 Collman et al., 2007b]. 

Comparison of the Cu K-edge EXAFS signals for the monometallic Cu/Si02 and the bimetallic Ru-Cu/Si02 catalyst, on the other hand, provides clear evidence for the proximity of ruthenium to copper atoms in the latter. This is seen in the different shape of the measured EXAFS signal and the distorted inverse transform of the first coordination shell. Note that the intensity of the latter is weaker for the bimetallic catalyst, while the region between k=8 and k=15 A-1 has become more important, which points to the presence of a scattering atom heavier than copper in the first coordination shell. The reduced intensity in the Cu Fourier transform of the bimetallic catalyst is indicative of a lower coordination of the copper, which is characteristic of surface atoms. 

A-Aminophthalimide (118) can also be added to olefins in an asymmetric fashion. Thus, reaction of A -enoyl oxazolidinone 122 with 118 and lead tetraacetate in the presence of the camphor-derived chiral ligand 120 provides aziridine 123 in 83% yield and with 95% ee <020L1107>. Other useful chiral ligands include imine 121, derived from the condensation of 2,2 -diamino-6,6 -dimethylbiphenyl with 2,6-dichlorobenzaldehyde. The corresponding monometallic Cu(I) complex was found to be very efficient in chiral nitrogen transfer onto chromene derivative 124 using (Ar-(p-toluenesulfonyl)imino)phenyliodinane (PhI=NTs) to provide aziridine 125 in 87% yield and 99% ee <02JOC3450>. 

Cycloaddition with Azides, Alkynes, Alkenes and Allenes. The copper-catalysed azide-alkyne cycloadditiOTi reaction is typically catalysed by simple, monometallic Cu(I) salts. However, the mechanism of catalysis was recently determined to involve a bimetalhc process. Similar bimetallic mechanisms have also been discovered in the cycloaddition of alkynes with alkenes, allenes and other alkynes using Au catalysts. This reaction is discussed for its broad application to many areas of chemistry and for the potential of bimetalhc catalyst design to enhance the reaction. 

We investigated on structure of CuPd (2 1) bimetallic nanoparticles by XRD [71]. Since the XRD peaks of the PVP-protected CuPd nanoparticles appeared between the corresponding diffraction lines of Cu and Pd nanoparticles, as shown in Figru e 11, the bimetallic alloy phase was clearly formd to be formed in CuPd (2 1) bimetallic nanoparticles. We also characterized Ag-core/Rh-shell bimetallic nanoparticles, which formed during simple physical mixing of the corresponding monometallic ones, by XRD coupled with TEM [148]. 

The reduction of metal hydroxides or oxides powder by polyol was first reported by Figlarz and co-workers, which gave rise to fine powders of Cu, Ni, Co and some noble metals with micrometer sizes (polyol process) [32,33]. The polyol process was first modified for the preparation of PVP-protected bimetallic and monometallic nanoclusters such as Pt/Cu, Pd/Pd, Pt/Co, Pt, Pd, etc. [34-38]. The previous results definitely revealed that Pt, Pd, Cu and Co in these PVP-protected metal or alloy nanoclusters were in a zero-valent metallic state. 

Figure 6.16 Fourier transforms of Ru and Cu K-edge EXAFS spectra of Ru/Si02, Cu/Si02 and Ru-Cu/Si02 catalysts before and after exposure to oxygen at room temperature. The data show that almost all Cu in the bimetallic Ru-Cu catalyst is oxidized, while Ru is hardly affected. The monometallic Ru and Cu catalysts are oxidized to a limited extent only (from Sinfelt etal. [39]).

The results presented here refer to monometallic catalysts employed in a reduced state. After the reduction step, the monometallic catalysts M/support (M = Pt, Ni, Cu, Rh support = Si02, AI2O3) are cooled to between 298 and 423 K, and at this point the reaction between the monometallic catalyst and the organotin compound SnR4 (R = butyl, menthyl) takes place. 

Figure 6.3 Variation of SnBu4 concentration in the impregnation solution (mmol L ) as a function of time (min). Monometallic catalyst Cu/Si02. Reaction temperature 363 K ( CuSn-OM). (Reproduced from Reference [47].)...

Figure 6.7 shows the TPR diagrams for the CuSn-BM and Cu/Si02 catalysts. The bimetallic system CuSn-BM (Sn/Cu = 0.15), like the monometallic one, shows only one significant area of hydrogen consumption at about 493 K, displaced... 

Another a, i-unsaturated aldehyde analyzed is cinnamaldehyde. Its liquid-phase hydrogenation has been studied in our research group [20, 51, 94], using Pt, Ni and Cu-based tin-modified hi- and organobimetaUic catalysts (in all cases with Si02 as support). The catalytic results obtained showed that in aU cases there was a marked promoting effect of Sn on the selectivity to cinnamic alcohol (UOL). The specific modification of the monometallic systems due to Sn addition from the application of SOMC/M markedly increases the selectivity to UOL, especially in the case of Ni, where it goes from zero selectivity for the monometallic to 25% for the NiSn catalyst. Pt-based systems modified by Sn yield the best Suol values. 

Monometallic nanocatalyst systems copper nanocatalysts supported on silica (Cu/Si02)... 

In bimetallic catalysts, the influence of Cu as a second metal in catalytic activity and selectivity is also closely related to the nature of the supports. However, as a general rule Ni-Cu alloying promotes an improvement in selectivity values. In this respect, the most interesting results were obtained in Ni-CU/AIPO4 catalyst containing 20 wtX of both Ni and Cu because the selectivity increased to 98% and the catalytic activity maintained the same level as in Ni/AlPO4 monometallic catalyst. 

High selectivity was also observed on a silica-supported Fe-Cu catalyst prepared by coprecipitation (333 K, 10 atm H2, ethanol)286 and over polymer-protected colloidal Pd-Pt cluster catalysts (303 K, 1 atm H2, ethanol)287,288. In contrast with the above observation, the activity of the bimetallic alloy was 1.4-3 times higher than that of the monometallic Pd cluster reaching the maximum activity at a composition of Pd/Pt = 4 1. 

The pH of the solution can also play an important role. For nitrate reduction, reaction was faster in lower pH solutions (Horold et al. 1993 Liidtke et al. 1998) however, acidic solutions can dissolve the catalyst metal and negate the effects of the faster rates. The Cu in the bimetallic nitrate-reducing catalyst dissolved at pH values < 5 and deactivation was observed (Horold et al. 1993) Pd dissolution in a monometallic catalyst became significant below a pH of 4. (Munakata et al. 1998)... 

These results were interpreted in terms of a substantial surface enrichment in Cu, driven by Cu s lower heat of sublimation [23]. The reactivity of these catalysts for CO oxidation, and the clear spectroscopic evidence for surface Pt - CO species indicate that, at least for the heterogeneous systems, particle surface stoichiometries are very sensitive to metal-adsorbate interactions. Similar arguments were presented for the PtAu/silica system, in which monometallic Au particles severely sinter under dendrimer removal conditions. In this case, the retention of small bimetallic particles after activation was attributed to the strength of Pt-silica interactions, which effectively anchored the bimetallic nanoparticles to the support [24],... 

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