Sacrificial hydrogen reduction

Recently, our group has invented a modified successive alcohol reduction process to prepare inverted core/shell structured bimetallic nanoclusters. In practice, PVP-capped Pd-core/Pt-shell bimetallic nanoclusters 2 nm in size were obtained. This method is based on the fact that precious metals like Pd have the ability to adsorb hydrogen and split it to form metal hydride on the surface of a precious metal. Hydride on precious metal has a very strong reducing ability, implying a quite low redox potential. As illustrated in Fig. 3.23, the Pt ions added into the dispersion of Pd nanoclusters are reduced by hydride on the surface of preformed Pd nanoclusters without oxidizing Pd atoms to Pd ions, and are deposited on the Pd nanoclusters to form Pd-core/Pt-shell bimetallic nanoclusters. Since the coreduction of Pd and Pt ions produces only Pt-core/Pd-shell bimetallic nanoclusters and this structure is controlled by nature, Pt-core/Pd-shell can be called a normal core/shell structure. The structure of Pd-core/Pt-shell is opposite to the normal structure. Thus, we call this an inverted core/shell structure. 

In the modified successive reduction process to produce the inverted core/shell structure, hydrogen is additionally used for control of the structure. We call this the sacrificial hydrogen reduction method. This method can be very useful in controlling layered bimetallic and trimetallic structures in general. 

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Sacrificial Hydrogen Reduction. In the previous session, successive reduction of metal ions is sometimes successful to synthesize core/shell structured bimetallic fine particles. However, sometimes it is not successful. For example, Schmid and coworkers could synthesize Au/Pd bimetallic fine particles, while we could not. Compared to these, the size of the Pd core particles is much larger in Schmid s case than in our case. During the reduction of AuCH-, the oxidation of Pd° atoms may also occur in the system of Schmid and coworkers. However, because of their large size, before the decomposition of Pd core particles, generated Au atoms may precipitate on them, thus also protecting Pd from further oxidation. 

Fig. 9.1.5 Preparation process of Pd-core/Pt-shell (inverted core/shell) structured bimetallic clusters by a sacrificial hydrogen reduction. (From Ref. 69.)...

Infrared Spectroscopy. Infrared (1R) spectroscopy is also used for understanding the structure of the bimetallic nanoparticles. Carbon monoxide can be adsorbed on the surface of metals, and the 1R spectra of the adsorbed CO depend on the kind of metal. These properties are used for analyzing the surface structure of metal nanoparticles. The inverted core/shell structure, constructed by sacrificial hydrogen reduction, was probed by this technique (44). 

Fig. 3.23 Sacrificial hydrogen reduction method to synthesize inverted Core/shell-structured bimetallic nanoclusters by successive reduction.

Reduction of two different precious metal ions by refluxing in ethanol/water in the presence of PVP gave a colloidal dispersion of core/shell structured bimetallic nanoparticles. In the case of Pd and Au ions, e.g., the colloidal dispersions of bimetallic nanoparticles with a Au core/Pd shell structure are produced. In contrast, it is difficult to prepare bimetallic nanoparticles with the inverted core/shell (in this case, Pd-core/Au-shell) structure. The sacrificial hydrogen strategy was used to construct the inverted core/shell structure, where the colloidal dispersions of Pd-cores are treated with hydrogen and then the solution of the second element, Au ions, is slowly added to the dispersions. This novel method, developed by us, gave the inverted core/shell structured bimetallic nanoparticles. The Pd-core/Au-shell structure was confirmed by FT-IR spectra of adsorbed CO [144]. 

Given the structural similarity of intermediates A and E, and also of B and D, it seems obvious that each reaction step in the cycle is potentially reversible. The equilibrium is shifted, however, into the desired direction by the steric shielding of the sacrificial hydrogen acceptor, which makes the reductive elimination of TBA an essentially irreversible process. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

It is possible to use isolated, partially purified enzymes (dehydrogenases) for the reduction of ketones to optically active secondary alcohols. However, a different set of complications arises. The new C H bond is formed by delivery of the hydrogen atom from an enzyme cofactor, nicotinamide adenine dinucleotide (phosphate) NAD(P) in its reduced form. The cofactor is too expensive to be used in a stoichiometric quantity and must be recycled in situ. Recycling methods are relatively simple, using a sacrificial alcohol, or a second enzyme (formate dehydrogenase is popular) but the real and apparent complexity of the ensuing process (Scheme 8)[331 provides too much of a disincentive to investigation by non-experts. 

The synthesis of the module is provided in Scheme 10.5 (Kushner et al. 2007). Double alkylation of ethyl acetoacetate followed by guanidine condensation afforded alkenyl-pyrimidone intermediate 24 (Kushner et al. 2007). Isocyanate 25 was coupled to pyrimidone 24 to yield 26. Upon dimerization in DCM, RCM effectively cyclized the two UPy units (Mohr et al. 1997 Week et al. 1999). A one-pot reduction and deprotection through hydrogenation using Pearlman s catalyst gave diol module 27. Finally, capping 27 with 2-isocyanatoethyl methacrylate at both ends provided the UPy sacrificial cross-linker 28, which was thoroughly characterized by H- and C-NMR, Fourier transform IR (FTIR), and mass spectrometry. 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

An organic molecule can be used as the sacrificial donor in a reduction half reaction. Generally there is no net energy storage, but (depending on the reaction) hydrogen may be evolved at the same time as a surplus reaction product. For example, the reaction... 

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