Surface palladium

As catalyst for the Rosenmund reaction palladium on a support, e.g. palladium on barium sulfate, is most often used. The palladium has to be made less active in order to avoid further reduction of the aldehyde to the corresponding alcohol. Such a poisoned catalyst is obtained for example by the addition of quinoline and sulfur. Recent reports state that the reactivity of the catalyst is determined by the morphology of the palladium surface." ... 

Y. Matsumoto, T. Onishi, and K. Tamam, Effects of Sulphur on a Palladium Surface on the Adsorption of Carbon Monoxide and the Adsorption and decomposition of nitric oxide, J.C.S. Faraday I 76, 1116-1121 (1980). 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

Figure 5. Model of the palladium surface during 1-pentyne hydrogenation. (Reprinted from Reference [65], 2006, with permission from Elsevier).

Size-selected palladium atoms were deposited on an in. sv /M-prepared MgO(lOO) thin film at 90 K the palladium surface concentration was about 1% of a monolayer. Comparison of ab initio calculations and FTIR studies of CO adsorption provided evidence for single Pd atoms bond to F centres of the MgO support with two CO molecules attached to each palladium atom.24... 

This review has highlighted the key contributions of modern surface science to the understanding of the kinetics and mechanism of nitrogen oxide reduction catalysis. As discussed above, the conversion of NO has been taken as the standard to represent other NOx, and CO has typically been used as the reducing agent in these studies. The bulk of the work has been carried out on rhodium and palladium surfaces, the most common transition metals used in three-way catalytic converters. 

Relatively few investigations involving palladium carbonyl clusters have been carried out, partly because palladium per se does not form stable, discrete homometallic carbonyl clusters at room temperature in either solid or solution states.114,917-922 Nevertheless, solution-phase palladium carbonyl complexes have been synthesized with other stabilizing ligands (e.g., phosphines),105,923 and carbon monoxide readily absorbs on palladium surfaces.924 Moreover, gas-phase [Pd3(CO)n]-anions (n = 1-6) have been generated and their binding energies determined via the collision-induced dissociation method.925... 

Comparing the spectra of Pd-45 and Pd-105 with those of Pd-115 and Pd-190, respectively, we see that in the samples prepared from palladium acetate the 1982 cm-1 band is absent, or at any rate strongly reduced in intensity. Hence, it is not impossible that the 1982 cm-1 band is due to CO adsorbed on a palladium surface that is partially covered with chlorine. 

Fig. 5 Plot of H2 STY at 200 °C versus palladium surface area for some catalysts reported in the literature.

Species D is most likely an ethylidyne complex which forms from self-hydrogenation on the palladium surface. Such species along with species E have been suggested to be part of the compounds formed from platinum and acetylene. 

Fig. 53. Steady-state rate of C02 production on various palladium surfaces as a function of the substrate temperature. The maximum rate has been normalized to the same value for all samples (/5<5).

Kenneth and Steve Shoulders report an experiment in which a previously deuteron-loaded palladium cathode was subjected to the impact of a charge cluster [22]. Where the charge cluster impacted the deuteron-loaded palladium a visually-evident, explosive-like reaction occurs (Fig. 8). The palladium cathode was then subjected to an X-ray analysis of the impact crater (see chart in Fig. 8). Typically, the X-ray analysis shows a considerable number of elements not seen when scanning the nearby palladium surface. Such elements as oxygen, calcium, silicon, and magnesium are detected in the exploded region where a charge cluster impacted the palladium. 

The number of bands which are observed is a function of the amount of CO which is on the palladium surface. This is shown in Fig. 7. In this experiment the CO was added in small doses and the spectrum observed after each addition. Spectrum A shows that a band at 5.45 p is first to appear. In B a second band appears at 5.3 p. New bands at 4.85 and 5.2 p are observed in the third spectrum C. These bands become more intense in the fourth D and fifth E spectra with the 4.85-p band showing a greater increase than the 5.2-p band. Although there is some overlapping, it is evident that the longer-wavelength bands are produced first and in some cases may be complete before the shorter-wavelength bands are detectable. Evacuation removes the bands in the reverse order of their appearance. 

M.L.I. Lundstrom, Monitoring of hydrogen consumption along a palladium surface by using a scanning light pulse technique, J. Appl. Phys., 86(2) (1999) 1106-1113. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

Second, we shall present our approach as to how to probe the palladium surfaces in more complex Pd/support catalysts, especially when the so-called metal-support interactions are expected. We shall develop our idea of how to use such chemical probes as a catalytic reaction (alkane catalytic conversion) or chemisorption in order to see important changes in the catalytic behavior. When possible, an adequate reference to available data from more sophisticated physical techniques is made. 

After HTR of the Pd/Si02 catalyst, oxygen is consumed (at room temperature) in a much greater amount than is needed to cover the palladium surface the additional 02 is required for silicon oxidation of silicon to SiOz. If the oxidation process is restricted only to surface Si species, then the oxygen consumption suggests the presence of Pd2Si (207). 

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