Basal plane catalysts

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Sulfided Mo and Co-Mo catalysts, used in hydrotreating reactions, contain Mo as M0S2. This compound has a layer structure consisting of sandwiches, each of a Mo layer between two S layers. The chemical activity of M0S2 is associated with the edges of the sandwich where the Mo is exposed to the gas phase the basal plane of the anions is... 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

From the Co EXAFS results alone one cannot conclude whether the Co atoms are located at edges or basal planes but a comparison of the Co EXAFS data with the above Mo EXAFS results indicates that the edge position is the most likely one. This Co location is illustrated in Figure T For the unsupported catalysts, many of these "surface" positions may be present at internal edges (i.e., at the "domain" boundaries). Recently, direct evidence confirming the edge position has been obtained by combining MES results (to ensure that Co is present as Co-Mo-S in the samples studied) with ir spectroscopy (lU) or with analytical electron microscopy (l ) ... 

Graphene is also used as catalyst support in PEFCs as it offers high conductivity, facile electron transfer and large surface area [151,152]. The planar structure of graphene allows its edge and basal planes to interact with the nanoparticles of the electrocatalyst [100],... 

The inner cavity of carbon nanotubes stimulated some research on utilization of the so-called confinement effect [33]. It was observed that catalyst particles selectively deposited inside or outside of the CNT host (Fig. 15.7) in some cases provide different catalytic properties. Explanations range from an electronic origin due to the partial sp3 character of basal plane carbon atoms, which results in a higher n-electron density on the outer than on the inner CNT surface (Fig. 15.4(b)) [34], to an increased pressure of the reactants in nanosized pores [35]. Exemplarily for inside CNT deposited catalyst particles, Bao et al. observed a superior performance of Rh/Mn/Li/Fe nanoparticles in the ethanol production from syngas [36], whereas the opposite trend was found for an Ru catalyst in ammonia decomposition [37]. Considering the substantial volume shrinkage and expansion, respectively, in these two reactions, such results may indeed indicate an increased pressure as the key factor for catalytic performance. However, the activity of a Ru catalyst deposited on the outside wall of CNTs is also more active in the synthesis of ammonia, which in this case is explained by electronic properties [34]. 

As stoich. [Ru(0)((N 0)p7CH3CN it oxidised primary alcohols to aldehydes, secondary alcohols to ketones, alkenes to aldehydes, tetrahydrofuran to y-butyrolactone. Styrene, cis- and tran -stilbenes gave benzaldehyde and adamantane gave 1-adamantol exclusively, while cyclohexanol gave cyclohexanone, suggesting that the complex is an effective oxidant for unactivated C-H bonds [636]. Immobilisation of the catalyst within Nation films on a basal plane pyrohtic graphite electrode was achieved, but the... 

In catalysis, one does not expect the activity of a catalyst to be proportional to its surface area, since there is good evidence that in many instances catalytic action is limited to certain active regions which may constitute only a small fraction of the total surface area (129). As would be expected, the same reasoning holds true for gas-carbon reactions. Carbon is a multicrystalline material, which can present varying degrees of surface heterogeneity depending upon the size and orientation of the crystallites. In the broadest sense, two main orientations of crystallites in the carbon surface need be considered—(1) crystallites with their basal planes parallel to the surface and (2) crystallites with their basal planes perpendicular to the surface. 

Notwithstanding the impressive number of investigations of HDS catalysts, many fundamental questions remain unanswered, and most of them are obviously related to the fact that traditional spectroscopic techniques are not capable of unequivocally mapping the real-space atomic structure. For instance, what is the preferential shape of the M0S2 crystallites The basal planes of the M0S2 slabs are chemically inert, and it is therefore clear that the HDS activity is associated with the... 

Its molecular structure (Figure 37) consists of a centrosymmetric dimer with a bridging H2Al(OR)( U-OR)2Al(OR)H2 entity. The Ta atoms are approximately square pyramidal, with the four phosphorus atoms forming the basal plane (Ta lies 0.64 A out of it). The relatively short Ta—A1 distances are comparable to those found in other transition metal aluminum complexes (Ta—Al 2.79-3.13 A). The hydrogen atoms have not been located, but were evidenced by chemical and spectroscopic techniques (IR 1605, 1540 cm 1 HNMR 16.30p.p.m.). The Ta—(ju-H2)A1 unit is relatively stable, and (54) is inert to carbon monoxide or trimethylamine. It is a poor catalyst in the isomerization of 1-pentene. Formation of complexes analogous to (54) may explain the low yields often obtained from alkoxoaluminohy-drides and metal halides. 

The hydration water in the interlamellar space is acidic and, therefore, able to work as an acid catalyst. The nature of the acid sites is discussed with respect to the origin of the lattice charge (tetrahedral or octahedral). The mechanism of cross-linking smectites with oligomeric inorganic cations shed light on the reactivity of the basal planes as well. 

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