Hydrogen adsorbed palladium

Figure 3. Maximum hydrogen adsorbed at 30-mm. hydrogen pressure as function of palladium concentration on silica gel surface...

Methods for monitoring the amount of adsorbed hydrogen on palladium mesowire are described by Monty [6] and Penner [7]. 

In the hydrogenation of methylenecyclohexane, 16, over a palladium catalyst, 1-methylcyclohexene, 17, is also produced by double bond isomerization (Eqn. 5.4). >5 In an ethanol solution both of these isomers are hydrogenated over palladium at essentially the same rate but, as depicted in Fig. 5.1, when a 1 1 ethanohbenzene solvent is used the rate of hydrogenation of 17 is decreased but the hydrogenation of 16 is not affected initially, but drops off significantly after only a short reaction time. At this point analysis of the reaction mixture showed that the only olefin present was 17 which, as shown by the bottom curve in Fig. 5.1, was hydrogenated very slowly under these conditions. These results indicate that benzene evidently competes with 17 for adsorption sites on the catalyst but that 16 is more readily adsorbed than is benzene. > 5... 

Interestingly, the same catalyst is used for the conversion of furfiiral to furan, so that the question may be asked why THF is not made directly from furfural. The explanation lies in the fact that the conversion of furfural to furan liberates carbon monoxide which has such a high heat of adsorption on palladium that hydrogen cannot be adsorbed simultaneously. The heat of adsorption of carbon monoxide on palladium exceeds the heat of adsorption of hydrogen on palladium by 14 kcal/mole (58.576 kJ/mole). 

Finally, dynamic Monte Carlo simulations are very useful in assessing the overall reactivity of a catalytic surface, which must include the effects of lateral interactions between adsorbates and the mobility of adsorbates on the surface in reaching the active sites. The importance of treating lateral interactions was demonstrated in detailed ab initio-based dynamic Monte Carlo simulations of ethylene hydrogenation on palladium and PdAu alloys. Surface diffusion of CO on PtRu alloy surfaces was shown to be essential to explain the qualititative features of the experimental CO stripping voltammetry. Without adsorbate mobility, these bifunctional surfaces do not show any catalytic enhancement with respect to the pure metals. 

It was a heterogeneous process—a reduotion of the red quinone formed in the anodic oxidation of its leuco-form by hydrogen adsorbed on colloidal palladium, manifested by the increase of the anodic wave of the leuco-form in the presence of colloidal palladium and gaseous hydrogen. 

The INS spectrum of hydrogen adsorbed by Raney palladium at 400 mbar corresponded to a mixture of p-PdH (480 cm ) and a-PdH (565 cm ) [27]. After dihydrogen adsorption at a lower pressure, 2 mbar, the INS showed peaks due both to the palladium hydrides and to adsorbed hydrogen. A broad peak at 975 cm" was assigned to the two unresolved vibrations of three-coordinated (Csv) hydrogen [27]. 

H. Jobic, J.P. Candy, V. Perrichon A. Renouprez (1985). J. Chem. Soc. Farad. Trans. I, 81, 1955-1961. Neutron-scattering and volumetric study of hydrogen adsorbed and absorbed on Raney palladium. 

I. J. Braid, J. Howard J. Tomkinson (1983). J. Chem. Soc. Faraday Trans. II, 79, 253-262. Inelastic neutron-scattering study of hydrogen adsorbed on impure palladium black. 

J. Howard, T.C. Waddington C.J. Wright (1978). Chem. Phys. Lett., 56, 258-62. The vibrational spectrum of hydrogen adsorbed on palladium black measured using inelastic neutron scattering spectroscopy. 

The promoting action is not the same for inactive as for activated carbon, and dissimilar effects are found with different types of activated carbons. Sabalitschka and Moses40 studied the influence of the carrier on a palladium catalyst employed for the hydrogenation of maleic and fumaric acids. They found that the activity of the catalyst depends on the extent to which the palladium compound is adsorbed prior to reduction. With carriers of low adsorbing power, a more efficient catalyst can be prepared by using the readily adsorbable palladium hydroxide rather than the less adsorbable chloride. With good adsorbents, equally efficient catalysts can be prepared with cither of the palladium compounds. 

Coke formation during acetylene hydrogenation over palladium catalyst could promote hydrogen transfer to acetylene and ethylene adsorbed molecules leading to a decrease in ethane selectivity and an increase in ethylene loss. 

Model catalysts (Section 2.3) permit the use of the same techniques of examination as single crystals. Palladium particles formed on alumina-coated NiAl( 100) adsorbed ethene in both the tt- and a-forms, but the latter was favoured with increase in particle size this could indeed be the basis for the weak size dependence noted previously (Section 7.2.2) in alkenehydrogenation. Hydrogen adsorbed more strongly on small particles, and in a now familiar way shifted the adsorbed states of ethene towards the r-form this reacted with weakly-held hydrogen atoms to form ethane. ... 

Wagner and Hauffe [47] first applied a perturbation method to the study of a reaction between oxygen and adsorbed hydrogen on palladium by measuring electrical conductivity response to the change of oxygen flow. A very early application was the pulse method which... 

In one example, colloidal Pd particles are obtained by the electrolysis of aqueous solutions of palladium chloride at pH 1 in a two-layer bath in the presence of a hydrocarbon solvent and epoxy dianic resin or PVA. Electrolysis results in flie formation of colloidal palladium organosols stabilized by the chemisorption of tiie polymer. Metal-lopolymers containing up to 90-95% of Pd remain after the removal of solvent and residual electrolyte. They are formed under high cathode polarization where concomitant elimination of hydrogen adsorbed on the nanoparticles (5.5-7.S nm in diameter) occurs. 

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