Adsorption on palladium

When a negative potential (-0.4V) was applied to the PDl-coated Pd electrode, the 1987 cm band moved to 1983 cm , while the application of a positive potential (-1-0.4V) shifted the band position to 1991cm . The v(N=C) of the uncoordinated NC group remained at 2122 cm as the applied potential was changed. 

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A variety of catalysts including copper, nickel, cobalt, and the platinum metals group have been used successfully in carbonyl reduction. Palladium, an excellent catalyst for hydrogenation of aromatic carbonyls is relatively ineffective for aliphatic carbonyls this latter group has a low strength of adsorption on palladium relative to other metals (72,91). Nonetheless, palladium can be used very well with aliphatic carbonyls with sufficient patience, as illustrated by the difficult-to-reduce vinylogous amide I to 2 (9). 

Fio. 33. CD adsorption on alumina-supported platinum (40) and palladium (42) characterized by ATR-IR spectroscopy during catalysis. Top spectra taken at different CD concentrations in hydrogen-saturated CH2CI2. The CD concentration increases from bottom to top (0-3 x 10 M for platinum and 0 4.3 X 10 M for palladium). Vibrational bands indicative of various adsorption modes of CD (bottom) are marked. Bands indicative of species 2 are absent from spectra observed for CD adsorption on palladium (40,42). 

C.2.6. PM-IRAS of CO on Nanoparticle Model Catalysts. For completeness, we mention that investigations of CO adsorption on palladium nanoparticle catalysts were also carried out by PM-IRAS. The observed adsorbate species essentially agree with those observed by SFG, and References 175,306,307,453) provide more information. 

Fig. 56. CO adsorption on palladium nanoparticles grown at 90 K on Nb205/Cu3Au(l 00). (a) SFG spectra acquired in 10 mbar of CO at 110 K, after annealing of the model catalysts to the temperatures indicated. Values obtained for the peak position, resonant amplitude, peak width (FWHM), and phase <[) of the spectra are displayed in (b), both for on-top and bridge-bonded CO. Metal-support interactions resulting from annealing of Pd/Nb205 led to an irreversible loss of the CO adsorption capacity and formation of a mixed Pd-NbO, . phase reprinted from (523) with permission from Elsevier.

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). 

Eichler, B., Kim, S.C. Thermogravimetric determination of the enthalpy of astatine and radon adsorption on palladium surfaces. Isotopenpraxis 21, 180-183 (1985)... 

On the other side, H adsorption on palladium shows that the nanostructures are more reactive than the planar surface. Further, we have studied the H adsorption reaction on the Pdg cluster, increasing the amount of hydrogen. 

Lopez-Corral, I., German, E., Juan, A., Volpe, M.A., and Brizuela, G.P. (2011) DFX study of hydrogen adsorption on palladium decorated graphene. /. Phys. Chem. C, 115, 4315-4323. 

Toth, G. Studies of the Iodine Adsorption on Palladium Surfaces. Isotopenpraxis 4, 60 (1968), (In German). 22 25431... 

Dong W, Ledentu V, Sautet Ph, Eichler A and Hafner J (1998), Hydrogen adsorption on palladium a comparative theoretical study of different surfaces . Surf. Sci., 411,123-136. 

Conrad FI, ErtI G and Latta E E 1974 Adsorption of hydrogen on palladium single crystal surfaces Surf. Sc/41 435-46... 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

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-... 

Fig. 10. Spectra of CO adsorbed on palladium samples Pd-105, Pd-45, and Pd-15. The height of the arrow gives the intensity of the band at 2260 cm-1 after adsorption of N2 on the samples.

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

Figure 8.7 RAIRS spectra show that lateral interactions force CO to leave the twofold adsorption sites on palladium (IR frequency of about 1920 cm 1) when NO is coadsorbed, and push it to the on top site (adsorption frequencies above 2000 cm-1). Adsorbed NO gives rise to the absorption peaks below 1800 cm 1 (from Raval et al. [22]).

Low pressure studies Adsorption of CO. The experiments were performed in an ultra-high vacuum system described previously (1). The data obtained on palladium particles with a size smaller than 2 nm or larger than 3 nm will be discussed in turn. 

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