Monometallic catalysts

Fig. 3. Monometallic mechanism of formation for polymer chain growth on transitionmetal catalyst, where (D) represents a coordination vacancy.

Low pressure operation became routine with the appHcation of new catalysts that are resistant to deactivation and withstand the low pressures. The catalysts are bimetallic most incorporate rhenium as well as platinum (95). The stmctures of these catalysts are stiU not well understood, but under some conditions the two metals form small alloylike stmctures, which resist deactivation better than the monometallic catalyst. 

The conclusion may be drawn that the data obtained of comparative studies of olefin polymerization by the one-component catalyst (TiCl2) and two-component systems (TiCl2 + AlEtxCl ) confirm the concept of monometallic active centers on the surface of titanium chlorides developed by Cossee and Arlman (170-173). 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

The catalyst remained stable for a long diile (over 50 h) under reaction conditions at the above ten erature. In order to try to tmderstand this interesting behavior, let us first focus our attention on the behavior of monometallic cobalt and platinum mordenite samples. 

From a general point of view, a monometallic catalyst can be considered as surface metal atoms linked together, forming an ensemble on the surface [160]. 

Herein we briefly mention historical aspects on preparation of monometallic or bimetallic nanoparticles as science. In 1857, Faraday prepared dispersion solution of Au colloids by chemical reduction of aqueous solution of Au(III) ions with phosphorous [6]. One hundred and thirty-one years later, in 1988, Thomas confirmed that the colloids were composed of Au nanoparticles with 3-30 nm in particle size by means of electron microscope [7]. In 1941, Rampino and Nord prepared colloidal dispersion of Pd by reduction with hydrogen, protected the colloids by addition of synthetic pol5mer like polyvinylalcohol, applied to the catalysts for the first time [8-10]. In 1951, Turkevich et al. [11] reported an important paper on preparation method of Au nanoparticles. They prepared aqueous dispersions of Au nanoparticles by reducing Au(III) with phosphorous or carbon monoxide (CO), and characterized the nanoparticles by electron microscopy. They also prepared Au nanoparticles with quite narrow... 

Bimetallic nanoparticles (including monometallic ones) have attracted a great interest in scientific research and industrial applications, owing to their unique large sur-face-to-volume ratios and quantum-size effects [1,2,5,182]. Since industrial catalysts usually work on the surface of metals, the metal nanoparticles, which possess much larger surface area per unit volume or weight of metal than the bulk metal, have been considered as promising materials for catalysis. 

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

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]. 

Simplifying the synthesis of the catalysts (e.g., the highly optimized convergent synthesis of monometallic series 2 metalloporphyrins required between 14 and 17 steps [Collman et al., 2002d]—FeAc (Fig. 18.21a) is available in 7 steps). 

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