H2 as reducing agent

Hydrogenases (Hases) catalyze the reversible electron transfer from H2 to NAD) ) [15]. Reports on the use of Hases for the in situ regeneration of 

NADH [16] and NADPH [17] date back to the 1980s. Table 8.3 summarizes the performance of some Hase-promoted reduction reactions. 

Product Scale Catalyst (loading) Yield (%) TN (NAD(P)) Reference 

TbADH ADH from Jhermoanaerobacter brodcu i /Hase I hydrt enase I from Pyrococcus Juriosus A. eutrophus Alcal enes eutrophus (whole c Us-containing Hase) C. oxydam Gluconobacter oxydans R. aOropha Ralstonia eutropha. 

Another issue is the poor water solubility of H2 often necessitating autoclave setups for Hase catalysis. However, we expect that further research on solvent-stable Hases will enable using these promising enzymes in media that offer higher H2 

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H2, CO or synthesis gas is usually used for the reduction of iron catalyst for the Fischer-Tropsch synthesis. However, a substantial amount of research results show that whether at low temperatures or at high temperatures, H2 as reducing agent is more suitable than the CO for the reduction of iron oxide. 

Sodium borohydride reductions of gold(I) complexes give Au clusters at RT if sodium borohydride in ethanol is dropped slowly into a suspension of the Au(I) complex in the same solvent. The immediate coloring of the reaction mixture (mostly red), even after only a few drops of the borohydride have been added, indicates fast formation of Au clusters. In view of the complicated composition of these compounds the fast formation is surprising. The use of H2 and CO with HjO as reducing agents in the synthesis of gold clusters has been described (see Table 1, Method A, 8.2.2.2). 

Compare the relative efficiencies of H2 and CO as reducing agents for metal oxides for example, if cobalt oxide, CoO, were to be reduced to the pure metal, which would do a better job and at what temperatures ... 

Birch reductions use those blue solutions, with their solvated electrons, as reducing agents. The reduction of NH3 to NH2 and H2 is quite slow, and a better electron acceptor will get reduced in preference. In the example above, the electrons go into benzene s lowest lying antibonding orbital (its LUMO). The species we get can be represented in several ways, all of them radical anions (molecules with one excess, unpaired electron). 

Citronellal, an aldehyde with a trisubstituted double bond, was hydrogenated to citronellol over a ruthenium catalyst poisoned with lead acetate in 90-100% yields (eq. 5.22)46 or over chromium-promoted Raney Ni in 94% yield in methanol at 75°C and about 0.31 MPa H2.47 Court et al. studied the selective hydrogenation of citral (1, eq. 5.24) to citronellol over unsupported Nij. o catalysts, prepared by reduction of mixtures of metal iodides with naphthalene-sodium as reducing agent, in cyclohexane and in 2-propanol at 80°C and 1.0 MPa H2.48 Higher yields of citronellol were obtained in 2-propanol than in cyclohexane, primarily via citronellal as the predominant intermediate. The yields of citronellol for the overall hydrogenation in 2-propanol over Mo-promoted catalysts were Mo0 03 96%, Mo0 06 98%, and Mo012 96%. 

Magnesium hydride is obtained by interaction of Mg and H2 at ca. 500°C. An active form is readily made by interaction of PhSiH3 and Bu"Mg in ether-heptane solvents in presence of tmen. The solvated pyrophoric powders are used as reducing agents and hydride sources.13 The hydrides of Ca, Sr, and Ba are also made by direct interaction (Section 2-13). For Ca, Sr, and Ba the structure is of the PbCl2 type, whereas MgH2 has the rutile structure.14... 

Reduction processes are frequently involved in doping of materials prepared for specific applications. Bai <5Sr(5MgF4 (< < 0.55) was doped with Sm2+ by addition of Sm metal to the charge for crystal growth [48], Eu2+ is the key ion in fluorescent lamp phosphors for emission of blue light. Respective reduction of Eu3+ is frequently achieved in H2 atmosphere, but in alkaline earth fluoride phosphates, Sn2+ may act as reducing agent [49]. 

When CO alone is used as reducing agent, e.g. for reduction of Cu(hfac)(l,5-cod) (10c), the copper films are contaminated mostly with significant amounts of carbon impurities, while the use of CO/H2 mixtures results in the formation of high-purity metallic copper films. Under the CVD conditions applied, it is proposed that the copper(I) species is transported as Cu(hfac)(CO) in the gas phase . 

In the presence of both H2 and CO as reducing agents the situation wall be even more complicated due to competition betw een CO and H2 and, possibly, formation of specific products and intermediates such as HCN. This topic was addressed briefly at the beginning of this section. 

The catalytic reduction of nitrates using H2 or NH4 as reducing agent was investigated [73-75] using Pd-Cu catalysts supported on y-alumina. The direct electrochemical reduction of nitrate or indirect reduction by electrochemically produced hydrogen has not received much attention and could be an interesting area of research. 

Many chemistries are available to deposit tungsten (see chapter VI). Here we will only discuss the most frequently used ones SiH4/WF6 and H2/WF6. In these chemistries silane and hydrogen act as reducing agents for the tungsten source, respectively. The overall equations can be written as ... 

Nafion 117 membranes were electroless coated with Pt from a solution ofH2PtCl6 and hydrazine (N2H4) as reducing agent.58 The coated membrane was used to make an MEA of proton exchange membrane fuel cell which was tested at 60°C in a saturated H2/O2 system. The maximum current density of about 80 mA/cm2 at 0.3 V was obtained in this system. 

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