Bimetallic catalytic reduction

In bimetallic catalysts prepared by catalytic reduction of copper by hydrogen, copper is deposited as three-dimensional agglomerates which are located, at low copper loadings, on the edges, corners, and rims of the parent metallic particles. The mechanism of deposition can be transferred from that proposed in corrosion and involving a local electrochemical cell ... 

The additive loading at saturation of the parent catalyst is dependent on the experimental conditions. So, different Pt-Re/Al203 bimetallic catalysts were prepared by catalytic reduction of Re04 by hydrogen. 

In summary, the technique of catalytic reduction for the preparation of bimetallic catalysts can be extensively used with a variety of parent metals and re-ductants. However, some structure sensitivities of the reduction reactions become apparent and the modifying metal can be selectively deposited on specific sites of the parent-supported metal. Furthermore, such structure sensitivity depends on the nature of the re-ductant, and a given modifier can be deposited, according to the reductant used, selectively onto different parts of the metallic surface. In fact, a bimetallic catalyst can be tailored to provide the optimum activity, selectivity and lifetime for a given reaction. 

The catalytic reduction of O2 by ferrocene (Fc) derivatives (one-electron reductants like cytochrome c) with a monomer cobalt porphyrin and cofacial dicobalt porphyrins was examined in the presence of perchloric acid (HCIO4) in PhCN (168). This finding allowed for the difference between the single and bimetallic systems to be clarified. 

The dioxygen reduction site of the key respiratory enzyme, cytochrome c oxidase [E.C. 1.9.3.1], is a bimetallic catalytic center comprised of a heme iron adjacent to a Type 2 mononuclear copper center (see Cytochrome Oxidase). The recent solution of the X-ray crystal structure of this enzyme revealed an entirely unanticipated covalent modification of the protein structure, a cross-link between a histidine and tyrosine side chain (23) within the active site (Figure 2)." This extraordinary posttranslational modification has been confirmed by peptide mapping and mass spectrometry, and has been detected as a conserved element in cytochrome c oxidases isolated from organisms ranging from bacteria to cows. The role of the cross-linked structure in the function of cytochrome c oxidase is still controversial." " ... 

The catalytic activity toward hydrogenation reactions has been indeed observed in such bimetallic colloidal systems. The SERS spectrum of p-nitrobenzoate (PNBA) adsorbed on colloidal silver and the SERS spectra observed on Ag/Pd nanoparticles, immediately after the addition of PNBA and after 1 week, are shown in Fig. 20.7 A, B, and C, respectively. In the bimetallic colloid, instead of the SERS spectrum of PNBA (spectrum A), a different spectrum is obtained (spectrum B) that slowly evolves toward a different spectral feature, which becomes predominant after a week (spectrum C). This modification may be related to the initial formation of p-aminobenzoate as a result of the catalytic reduction of the nitro group, followed by slow oxidation to azodibenzoate by atmospheric oxygen (see Fig. 20.8). 

Bimetallic platinum-rhenium catalysts were prepared either by coimpregnation or by catalytic reduction (redox reaction). 

Table 2 shows conyersion values obtained during cyclopentane hydrogenolysis on the two series of Pt-Re bimetallic catalysts (Cl Coimpregnation, CR Catalytic reduction). The different catalysts were activated by direct reduction (R) or by calcination followed by reduction (C). 

Catalyst presulfidation induces a strong decrease of the amount of coke deposited on bimetallic Pt-Re catalysts, the effect being mote pronounced on catalysts prepared by catalytic reduction (Table 4). However, coke deposits are less toxic for cyclohexane dehydrogenation on catalysts activated by direct reduction, i.e. on catalysts where Pt-Re interaction is high. 

Bimetallic Pt-Re catalysts prepared by catalytic reduction and activated by reduction (close interaction) are less sensitive to sulfur adsorption than catalysts prepared by the classical coimpregnation technique. 

In the course of the catalytic reduction, deposition of the second metal (or the third) can occur on both the parent metal and the support, depending on the nature of the support and the operating conditions. Effectively, in the case of bimetallic Rh-Ge [41] and Pt-Sn [38] catalysts, it was observed that the amount of Ge or Sn deposited is higher on alumina than on silica supported catalysts (Fig. 

After preparation by catalytic reduction and after drying overnight at 70 °C under nitrogen flow, Pt-Cu/Al203 catalysts were tested for the nitrate reduchon as is or after air exposure at ambient temperature [34]. The highest activity was obtained in the latter case. This allows one to increase Pt accessibility in bimetallic catalysts, whereas the coverage of Pt particles by Cu in the just-prepared catalysts is probably... 

The performances of bimetallic Rh-Ge/Si02 and Rh-Ge/Al203 catalysts prepared by catalytic reduction were measured for another a,p-unsaturated aldehyde, namely citral (Fig. 9.7). The addition of Ge to Rh by the surface redox reaction promoted the hydrogenation of citral to the unsaturated alcohols (geraniol/nerol), while the saturated aldehyde (dtroneUal) was the main product on the monometallic Rh catalyst [41]. 

Moreover, these catalysts obtained by catalytic reduction were more active and selective than bimetallic Rh-Ge systems prepared by successive impregnations. 

Then, the preparation by catalytic reduction of multimetallic catalysts for application in water denitration was extended to other bimetallic nanoparticles such as Pt-Ag [35], Pd-Cu [33, 35-37], Pd-Sn [38], Cu-Pt (8), Cu-Pd [8] or to trimetallic nanoparticles such as Pd-Sn-Au [46], deposited on various supports. 

Numerous studies were devoted to the use of bimetallic catalysts, promoted either by an active metal such as rhenium or iridium, or an inactive one such as tin or germanium. These different catalysts were generally prepared by co-impregnation or successive impregnations. Moreover, most of the catalysts prepared by redox reactions were only evaluated in model reactions. For example, Corro et al. [87] prepared Pt-Sn/Al203-Cl catalysts by catalytic reduction or co-impregnation and compared their resistance to coking under cyclopentane feed. Thus, catalysts prepared by the surface redox reaction were less sensitive than the others to deactivation. This result was explained in terms of a more effective interaction between... 

Figure 9.11 (a) n-Heptane conversion and (b) yield in toluene obtained with trimetallic Pt-lr-Sn/Al203-Cl prepared by catalytic reduction, after 5 h ( ) and 65 h ( ) or presulfided bimetallic Pt-lr/Al203 catalyst after 5h (x) or65h ( ) of reaction (0.3vrt.% Pt, 0.3vrt.% Ir reaction conditions ... 

Quantitative rate data on the catalytic reduction of nitrates in drinkable water are relatively scarce. One of the first works concerning kinetics is that of Tacke and Vorlop who employed a Pd-Cu bimetallic catalyst containing 5wt.% of Pt and 1.25 wt.% of Cu in a slurry reactor. Measurements of the initial rates resulted in a power-law rate expression. They reported a power of 0.7 with respect to the nitrate concentration, and an independency on the hydrogen partial pressure providing this pressure exceeded 1 bar. Pintar efa/. reported a complete kinetic model of the Langmuir-Hinshelwood type written in the form... 

In analogy to the Schrock cycle, Nishibayashi et aL postulated a Chatt-like reaction mechanism, where the bimetallic complex breaks into two monometallic fragments in solution, followed by catalytic reduction at a single metal center. However, the proposed intermediates could not be observed, and the reaction mechanism remained unclear. In order to address this problem, DFT calculations on the mechanism of the ammonia synthesis catalyzed by the Nishibayashi system were performed. Importantly, Batista and coworkers found that the bimetallic complex is the effective catalyst instead of the monometallic species that was originally postulated to play this role. Moreover, the dinitrogen-bridged dinuclear structure remains intact throughout... 

Elucidating the function of distal Cub in O2 reduction by HCOs has recently been the major focus of biomimetic studies of HCOs ". Cub has long been assumed to facilitate 0-0 bond heterolysis, or stabilize peroxo-level intermediates against dissociation, by forming a bridged Feaj-ZO j-Cug unit upon O2 binding Such a situation would imply that a bimetallic catalytic site is required for efficient O2 reduction. This would be comparable to the dual site mechanism of O2 reduction at Pt , which is supported by some recent results . However, no peroxo-level intermediates of any kind have ever been observed during O2 reduction by HCOs and it remains uncertain whether an intermediate where... 

Catalytic mechanisms are proposed that invoke both aurophilic di-gold(I) intermediates and covalently bonded di-gold(II) entities. The proposed intermediates are semi-supported or unsupported, according to the definition of Figure 11.16. Geometries and energies of putative intermediates are calculated within density-functional theory the computations support a bimetallic pathway. Reductive elimination, induced by an arylboronic acid, proceeds with retention of stereochemistry at carbon. 

Bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic, optical and catalytic properties compared to pure metallic nanopartides [24]. Cu-Ag alloy nanoparticles were obtained through the simultaneous reduction of copper and silver ions again in aqueous starch matrix. The optical properties of these alloy nanopartides vary with their composition, which is seen from the digital photographs in Fig. 8. The formation of alloy was confirmed by single SP maxima which varied depending on the composition of the alloy. 

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