Pd adsorption

Figure 6.4 Illustrative Pd adsorption over alumina results of Contescu and Vass [2] (a) uptake of Pd chlorides at low pH and Pd ammines at high pH (b) summary of analyses.

On F centers the mechanism is similar. The two acetylene molecules bind easily to the Pd/Fsc complex, they form C4H4 with a barrier of about 1 eV, then add a third acetylene which reacts with C4H4 to form benzene with a barrier of 0.9 eV. Thus, the barriers are at most of 1 eV, consistent with a reaction temperature of 300 K. Also in this case, once formed, benzene desorbs easily. It should be noted that the barriers for the reaction occurring on Pd atoms adsorbed on low-coordinated F or F centers are always significantly higher, hence inconsistent with the TPR experiment. This suggests that the most likely Pd adsorption sites, at least from the point of view of the reaction barriers, are the F andp"" centers located at the terraces of the MgO surface. 

The combination of two catalytic elements was realized in a study of formic acid oxidation on Pt(lOO) and Pt(lll) modified by the adsorption of palladium [Pdaj + Pt(lOO) and Pdjd + Pt(lll) systems]. While the presence of adsorbed palladium on Pt(lOO) resulted in a considerable lowering of the oxidation potential and the absence of self-poisoning under open circuit conditions, the activity of Pt(lll) substrate did not change significantly by Pd adsorption. However, a deactivation of the Pdaj + Pt(lOO) system is observed when the oxidation of formic acid takes place. This deactivation is analyzed in terms of slow formation of an adsorbed species blocking the initial step of formic acid oxidation on palladium sites. 

Fig. XVII-20. Isosteric heat of adsorption of Xe on a stepped Pd surface [8(100) x (110)]. (From Ref. 111.)...

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

Rendulic K D, Anger G and Winkler A 1989 Wide-range nozzle beam adsorption data for the systems H2/Ni and H2/Pd (100) Surf.Sci. 208 404... 

Gi-op A, Wilke S and Scheffler M 1995 6-dimensional quantum dynamics of adsorption and desorption of H2 at Pd(IOO)-steering and steric effects Phys.Rev. Lett. 75 2718... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

Figure A3.10.24 UPS data for CO adsorption on Pd(l 10). (a) Clean surface, (b) CO-dosed surface, (c) Difference spectrum (b-a). This spectrum is representative of molecular CO adsorption on platinum metals [M].

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Clotet A and Paoohlonl G 1996 Aoetylene on Cu and Pd(111) surfaoes A oomparatlve theoretloal study of bonding meohanlsm, adsorption sites, and vibrational speotra Surf. Sol. 346 91... 

Olsen R A, Philipsen P H T, Baerends E J, Kroes G J and Louvik O M 1997 Direct subsurface adsorption of hydrogen on Pd(111) quantum mechanical calculations on a new two-dimensional potential energy surfaced. Chem. Phys. 106 9286... 

Parallel Pore and Solid Diffusion Control With a linear isotherm, assuming equilibrium between the pore fluid and the solid adsorbent, batch adsorption can be represented in terms of an equivalent solid diffusivity = ( pD i + ppD, )/( p + pp Q). Thus, Eqs. (16-96) and (16-99) can be used for this case with D, replaced by D. ... 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

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