H2 activation

Turning to the XANES results (Figure 8.4), upon reduction at 350°C, the extent of reduction is found to be higher for the H2-activated air calcined catalysts. This is evident in the shoulder at the edge (-7,709 eV), which is a measure of metallic content, as well as the lower white line intensity for the activated air calcined catalyst at -7,725 eV. The catalysts appear to contain a combination of mainly Co metal and CoO, in agreement with the interpretation of TPR profiles previously discussed. 

Figure 8.7 demonstrates that the H2-activated catalysts that were calcined using nitric oxide resulted in higher initial CO conversion rates on a per gram of catalyst basis in a CSTR reactor at 220°C and 280 psig, and using a Fl2/CO ratio of 2.5. 

Results of EXAFS Fitting3 of H2-activated Catalysts for Data Acquired Near the Co K Edge... 

FIGURE 8.7 CO conversion vs. time on stream in the CSTR (220°C, 280 psig, H2/CO = 2.5) at (squares) 10 NL/g.at/h and (circles) 20 NL/g.at/h for H2-activated (filled circles) air calcined and (unfilled circles) NO calcined catalysts, including (top) 15% Co/Si02 and (bottom) 25% Co/Si02. The NO calcined catalysts clearly exhibit higher CO conversion rates on a per gram of catalyst basis. 

The results confirm that the novel metal nitrate conversion method using nitric oxide in place of air advocated by Sietsma et al. in patent applications WO 2008029177 and WO 2007071899 leads to, after activation in H2, catalysts with smaller cobalt crystallites, as measured by EXAFS and hydrogen chemisorption/ pulse reoxidation. In spite of the lower extent of cobalt reduction for H2-activated nitric oxide calcined catalysts, which was recorded by TPR, XANES, EXAFS,... 

Results of Hydrogen Chemisorption/Pulse Reoxidation Measurements over H2-activated Alumina-Supported Cobalt Catalysts Calcined at 350°C Using either Flowing Air or 5% Nitric Oxide in Nitrogen... 

FIGURE 9.10 Thermodynamic view of cobalt surface segregation in the presence of CO (and H2). Activating catalyst restructuring under Fischer-Tropsch conditions. 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

The elimination of HC1 was proposed to occur also during the H2 activation with the [Pd(PNP)Cl]Cl complexes (PNP = bis-2-(diphenylphosphino)ethyl benzy-lamine, bis-2-(diphenylphosphino)ethyl amine or tris-2-(diphenylphosphino)ethyl amine) [24, 25]. Based on the findings of 31P 1H - and 1H-NMR investigations, the hydride [HPd(PNP)]Cl was detected under H2 atmosphere. The alternative mechanism which involves the oxidative addition of H2 with formation of a Pd(IV)-dihydride intermediate, appeared less likely on the basis of thermodynamic considerations. 

H2 activation by protonation of the coordinated allene in 8 with formation of the palladium hydride intermediate 9 and propene. 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

A kinetic analysis of the styrene hydrogenation catalyzed by [Pt2(P205H2)4]4 [66] was indicative of the fact that the dinuclear core of the catalyst was maintained during hydrogenation. However, three speculative mechanisms were in agreement with the kinetic data, which mainly differ in the H2 activation step. This in fact can occur through the formation of two Pt-monohydrides, still connected by a Pt-Pt bond, or through the formation of two independent Pt-monohydrides. The third mechanism involves the dissociation of a phosphine from one Pt center, with subsequent oxidative addition of H2 to produce a Pt-dihy-dride intermediate. 

An interesting application of this method is the preparation of diamond films which may be obtained from a precursor such as CH4, C2H2 and H2 activated by heating, microwaves, etc. typically at 600-1000°C at a reduced pressure. The direct deposition from the gas to the surface results in the formation of metastable diamond whereas, according to the phase diagram (see Fig. 5.37), the production of stable bulk diamond requires very high pressure and temperature. Kinetically, the... 

Modifications in the H2-activating site of the NAD-reducing hydrogenase from R. eutropha... 

Hembre, R. T., McQueen, J. S. and Day, V. W. (1996) Coupling H2 to electron transfer with a 17-electron heterobimetallic hydride A Redox Switch model for the H2-activating center of hydrogenase./. Am. Chem. Soc., 118, 798-803. 

Massanz, C., Fernandez, V. M. and Friedrich, B. (1997) C-terminal extension of the H2-activating subunit, HoxH, directs maturation of the NAD-reducing hydrogenase in Alcaligenes eutrophus. Eur. J. Biochem., 245, 441-8. 

Happe RP, Roseboom W, Egert G, et al. 2000. Unusual FTIR and EPR properties of the H2-activating site of the cytoplasmic NAD-reducing hydrogenase from Ralstonia eutropha. FEBS Lett 446 259-63. 

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