Direct dehydrogenation pathway

The direct dehydrogenation pathway favors high turnover efficiencies at low overpotentials through a direct conversion of formic acid to the carbon dioxide (CO2) product [Eq. (3.1)] ... 

Abstract Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects. 

A common method for improving formic acid electrooxidation activity is through the incorporation of foreign adatoms in sub- or monolayer coverages onto metal electrocatalyst surfaces (substrates). Adatoms are usually deposited onto the metal surface either by under potential deposition (UPD) or by irreversible adsorption [17]. The two dominant reaction enhancement mechanisms for the direct dehydrogenation pathway, as described in Sect. 3.3 of the previous chapter for formic acid electrooxidation, are the third-body and electronic effects. The type of enhancement mechanism due to adatom addition is dependent on the substrate/adatom... 

The DiSalvo group at Cornell Uifiversity has intensely studied intermetallics for formic acid electrooxidation and observed significant enhancements in turnover efficiencies [16, 46, 72-79]. Table 4.3 compares the activity of several extended intermetallic surfaces in comparison to a Pt baseline [16]. The onset potential relevant to enhanced reactivity through the direct dehydrogenation pathway was most impacted by the addition of Sb. The introduction of both Sn and Sb into the Pt unit cell negatively impacted the anodic peak current. While Bi increased the peak current, it had an adverse impact on the onset potential. It increased the onset potential by 0.06 V and nearly doubled the peak current. The key challenges related to intermetallics for DFAFCs are surfactant-free synthesis methods and reduced nanoparticle sizes (>10 nm) to improve mass activity of the catalyst [74, 75, 80]. Mastumoto et al. compared the mass activity of PtPb 10-20 nm intermetallic particles to a commercial nanocatalyst [79], During a 9 h hold at 0.197 V vs. RHE, the PtPb intermetallic catalyst demonstrated over a twofold sustained mass activity over that of Pd. 

Cyclohexane dehydrogenation represents another classical example for isotopic studies. Balandin s sextet mechanism predicted direct dehydrogenation of cyclohexane over several metals, assuming a planar reactive chemisorption of the reactant. Cyclohexene is also readily dehydrogenated to benzene. The use of hydrocarbons labelled with established the true reaction pathway. T6tenyi and co-workers[ °di] reacted a mixture of [ 0]-cyclohexane and inactive cyclohexene on different metals and measured the specific radioactivity of the fractions (cyclohexane, cH, cyclohexene, cH= and benzene, Bz) in the product at low conversion values (Table The... 

Demirci investigated the degree of segregation and shifting of d-band centers by metal alloy combinations to improve the direct liquid fuel cell catalyst activity through electronic promotion of the dehydrogenation pathway [57]. He focused on Pt- and Pd-based catalyst for formic acid electrooxidation and looked at the potential impact of surface adatom adsorption of other 3d, 4d, and 5d transition metals. The criteria he imposed for improved catalytic activity on Pt and Pd... 

Kinetic analysis of propane ammoxidation with V-Sb-0 catalysts has shown that the reaction proceeds through propylene as the key intermediate (130). The first step in the propane ammoxidation reaction is the oxidative dehydrogenation of propane to propylene. Essentially, all the products of the reaction derive from conversion of the propylene intermediate. The kinetic results also suggest that there is a lesser direct reaction pathway from propane to acrylonitrile. 

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

Studies with unpromoted and Pt-promoted solid acids, specifically, H morde-nite,308 Pt Beta-zeolite,313 and acidic polyoxometalate cesium salts314,315 applied in the presence of hydrogen, showed that the isomerization of n-butane follows a monomolecular pathway. This consists of dehydrogenation on the metal site and isomerization on acidic sites. A new study using isotope labeling provided direct... 

Magnus and coworkers have published full details55 on the direct a- or /J-azido functionalization of triisopropylsilyl (TIPS) enol ethers using an iodosylbenzene-TMS-azide combination (equation 13) the w-pathway, favoured at —78 °C, is an azide radical addition process, whereas the -pathway, favoured at —15 to — 20 °C, involves ionic dehydrogenation. Attempts to extend the /3-functionalization to other TMSX derivatives failed. 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

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