Paired sites

Indicate that active sites are pair sites which contain reduced Mo and/or Co atoms next to Co and/or Mo Ions. 

At least one member of an active pair should probably exist to a greater extent on sulfided than on unsulflded catalysts. Exposed reduced metal sites, either slightly electropositive or uncharged, shown by Infrared to exist appreciably only on reduced sulfided catalysts, thus appear to be likely members of active pair sites. 

An exposed Mo Ion seems likely as the other member of an active pair site. Such Ions have been postulated as active sites on... 

Figure 11 shows schematically how such sites may be formed on Mo/alumlna or CoMo/alumlna catalysts. The pair site contains a reduced metal next to an acidic metal cation, either Co or Mo. Prestimably this dual site can remove S, leaving an olefin or aromatic molecule attached to the cation. This Is then hydrogenated by hydrogen from the reduced metal component of the site. 

Figure 11. Possible formation of active pair sites on CoMo/Al Oj catalysts.

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

The active site for type I hydrogen adsorption appears to consist of isolated, noninteracting Zn—O pair sites which are not affected by the oxida-... 

The spectrum of chemisorbed propylene in the CH and O—H stretching region is shown in Fig. 15. The band at 3593 cm-1 is clearly due to an OH frequency hence, dissociation accompanies propylene adsorption. Careful scrutiny of the region from 1500 to 2000 cm-1 reveals no band assignable to a ZnH band. Since the presence of adsorbed propylene has been found to block out the infrared active hydrogen chemisorption on the ZnO pair sites, we may assume that propylene adsorption occurs as follows ... 

Not all mutagenesis in IS. coli is dependent on SOS-processing. Mutations may arise quite simply during DNA replication if a base is substituted by or converted to another, incorrect, base. Consider the consequence of oxidative deamination of the base 5-methylcytosine to thymine. Replication followed by daughter strand segregation will result in a G C base pair having been mutated to an A T base pair. Sites containing 5-methylcytosine are hotspots for G C to A T transitions in 12. coli (24). 

The mechanism of phenol methylation on Cul-xCoxFe204 and the participation of Lewis acid-base pair site in the methylation step were recently demonstrated in detail [79]. 

The spectra obtained from the chemisorption of methanol onto catalyst above 100°C indicated the progressive oxidation of methoxy species to formate via dioxymethylene/HCHO and finally to CO, CO2, and H2. Phenol adsorbed on the surface Lewis acid-base pair site and dissociated to phenolate anion and proton. The formation of phenolate anion and proton were discerned from the strong intense C-0 stretching vibration and the disappearence of phenolic 0-H stretching vibration, respectively. Importantly, there were series of definite low intensity bands between 2050 and 1780 cm" that were identified as the out-of-plane aromatic C-H bending vibrations [79, 84-85]. These bending vibrations are possible only if the phenyl ring of phenol is perpendicular to the catalyst surface. 

Adsorption of aniline on Cui xZnxFc204 at <100°C indicates a simple molecular adsorption through N-atom on an acid-base pair site. However, above 100°C, N-H bond dissociates and aniline chemisorbed strongly on the catalyst surface. Chemisorbed aniline... 

It appears to us that the way forward to further industrial application of solid base catalysts may emerge from the discovery of unusual selectivity effects or new reactions that require well-balanced base and acid-base pair sites with relatively inexpensive solid catalysts that last long enough to process large quantities of reactants per unit mass of catalyst. 

Electrodes based on 9 but no nucleobase derivative [3.0 wt% 9 bis(2-ethylhexyl) phthalate ( dioctyl phthalate , DOP) as the membrane solvent] showed similar potentiometric responses to 5 -GMP and 5 -AMP (Figure 8a), which is not surprising because cation 9 cannot interact specifically with the base pairing site of nucleotides. The EMF slope (-29 mV decade" 0.1 M HEPES-NaOH buffer solution, pH 6.8) was much greater than in case of the electrode based on the macrocyclic pentaamine 1 (-15 mV decade" ) and corresponds to the slope as expected for a dianion according to the Nemstian equation. Extraction experiments confirmed that at this pH it is indeed the dianion that enters the organic phase. 

The terms of 1.2.5 have been discussed with reference to metallic surfaces but they can be applied to other adsorbents and catalysts and, in particular, to the pair-sites involved in heterolytic dissociative adsorption. 

The interaction of acetylene with Mg(001) was investigated by LEED (198-200). At 88 K, C2H2 molecules lie almost parallel to the surface, and neither molecule nor substrate distortions have been observed, indicating that only a weak physisorption occurs. Calculations with semiempirical potentials confirm the structure determined by LEED (199). The isomerization of d.v-2-butene on MgO has been reported (201). The dissociation of cyclopen-tadiene on a few active acid-base pair sites of totally dehydrated MgO was followed by IR spectroscopy, and the formation of surface hydroxyl groups and C5H5 species was proposed (202). Methanol decomposition (203) and ethanol decomposition (204, 205) have been reported. 

The site bound ions accounts for its hydration state and are grouped either as outer-sphere or as inner-sphere [9, 11, 14]. In the later case, it is assumed that the water molecules in the hydration shell do not participate. The ions directly interact with the phosphate charges and anionic ligands [9, 11, 14], Since both outer- and inner-sphere interactions lead to formation of ion pairs, site bound ions are describable in terms of an association constant satisfying the law of mass action [9, 11, 14—16]. 

III. Metal Pair Sites and Triplet Sites on Supports 63... 

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