Palladium—carbon monoxide adsorption

The following test for palladium is based on a combination of the activation of carbon monoxide by adsorption on finely divided palladium and the activation of Mo i in phosphomolybdic acid. Through these dual effects, palladium salts bring about a catalytic acceleration of the redox reaction (1). 

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). 

The effects of tin/palladium ratio, temperatnre, pressnre, and recycling were studied and correlated with catalyst characterization. The catalysts were characterized by chemisorption titrations, in situ X-Ray Diffraction (XRD), and Electron Spectroscopy for Chemical Analysis (ESCA). Chemisorption studies with hydrogen sulfide show lack of adsorption at higher Sn/Pd ratios. Carbon monoxide chemisorption indicates an increase in adsorption with increasing palladium concentration. One form of palladium is transformed to a new phase at 140°C by measurement of in situ variable temperature XRD. ESCA studies of the catalysts show that the presence of tin concentration increases the surface palladium concentration. ESCA data also indicates that recycled catalysts show no palladium sulfide formation at the surface but palladium cyanide is present. 

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. 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

A systematic attempt to correlate the catalytic effect of different surfaces with their adsorptive capacity was made by Taylor and his collaborators. Taylor and Burns, for example, investigated the adsorption of hydrogen, carbon dioxide, and ethylene by the six metals nickel, cobalt, palladium, platinum, iron, and copper. All these metals are able to catalyse the hydrogenation of ethylene to ethane, while nickel, cobalt, and palladium also catalyse the reduction of carbon monoxide and of carbon dioxide to methane. 

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. 

Water adsorbs into the walls of a glass container, but that adsorption is the extent of its adhesion. Some adsorbed molecules react chemically with some types of containers in a process called chemical adsorption (chemisorption. For example, carbon monoxide chemisorbs with palladium, but not with gold). The bonds resulting from chemisorption can hold molecules to the surface with far greater force than would exist with only physical attraction. It is also possible for a molecule (that normally would not chemisorb with the container wall) to break up when hitting the wall s surface. At that point the molecule s constituent parts chemisorb with the container walls. When an adsorbed gas reacts with the materials of a container, it is called reconstruction (for example, the reconstruction of iron with oxygen is rust). 

Carbon monoxide undergoes activated adsorption on the surface of palladium oxide. The maximum for this process, at about 350 mm. pressure, is at about 100°C. The gas taken up during activated adsorption can only be recovered as C02 for the most part (57). In a CO-air stream a slight initial reduction of PdO occurs at 23°C., but in the absence of oxygen, there is no reduction below 76°. This process of reduction decreases in rate with time and does not go to completion below 156°. Carbon dioxide, when present in the gas phase, inhibits the reduction of the palladium at 100°C. because it is adsorbed strongly by the PdO (56). Catalysts have been prepared by the deposition of palladium and platinum on asbestos, on silica gel, and on charcoal. 

Palladium nanoparticles (nm-Pd) were synthesized by ship-in-a-bottle technique in supercages of NaA zeolite. The behaviors of electrodes of thin film of nm-Pd accommodated in NaA zeolite were characterized by cyclic voltammetry. The results illustrated that the nm-Pd possess particular properties for hydrogen reaction, i.e. in contrast to hydrogen absorption on massive palladium electrode, the surface processes of hydrogen adsorption-desorption become the dominant reaction on electrodes of thin film of nm-Pd. The processes of adsorption and desorption of carbon monoxide on the electrodes were studied using in situ electrochemical FTIR reflection spectroscopy. It has been revealed that in comparison with CO adsorbed on a massive Pd electrode, the IR absorption of CO adsorbed on nm-Pd particles accommodated in NaA zeolite has been enhanced to about 36 times. 

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). 

Al-Ammar AS, Webb G (1978) Hydrogenation of acetylene over supported metal catalysts Part 1 - Adsorption of [ C] Acetylene and [ C] ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium. J Chem Soc Earaday Trans 74 195 Al-Ammar AS, Webb G (1979) Hydrogenation of acetylene over supported metal catalysts Part 3 - [ C] tracer studies of the effect of added ethylene and carbon monoxide on the reaction catalyzed by silica-supported palladium, rhodium and iridium. J Chem Soc Faraday Trans 75 1900... 

Palladium, etc. Carbon monoxide chemisorption has often been used to avoid the problem of hydrogen aZ>sorption into the metal but suitable conditions can be found to minimize absorption while still forming the monolayer. The subject is still being studied, e.g., the variation of solubility with metal dispersion. Small Pd particles of 7 nm mean size can also be completely oxidized to PdO at 538-580 K and therefore the oxygen uptake (on reduced and evacuated Pd at 580 K) measures total Pd atoms, Pdt. As the oxygen adsorption at room temperature measures surface atoms, Pdg, then the dispersion Pdg/Pdt is readily obtained from two gas uptake measurements. 

The degradation of the catalytic activity with reaction time or thermal sintering was analytically studied by the determination of the specific surface area of the exposed palladium particles, by the chemical adsorption of carbon monoxide the distribution of palladium particles, by an electron microscope and the crystal imperfection, by X-ray diifraction analysis, and so on. 

Leung, L.-W.H. and Weaver, M.J. (1987) Extending surface-enhanced Raman spectroscopy to transition-metal surfaces carbon monoxide adsorption and electrooxidation on platinum- and palladium-coated gold electrodes. Journal of the American Chemical Society, 109, 5113-5119. 

As with all catalysts, palladium and its alloys are susceptible to poisoning [69]. Catalysts must be designed with resistance to poisoning, and proper precautions must be taken to minimize exposure of the membranes to catalyst poisons [69]. Typical poisons for palladium include H2S and other compounds of sulfur such as carbon disulfide (CS2), carbonyl sulfide (COS), aromatic thiophenes and mercap-tans (thiols, R-SH). Palladium is poisoned by the Group VA elements, P, As, Sb and Bi, the halides (Cl, Br, I), and Si, Pb and Hg. Alkenes and unsaturated organic compounds can poison palladium as can elemental carbon deposited from decomposition of carbonaceous materials. Carbon monoxide in high concentrations and at low temperatures can form a monolayer which blocks adsorption and dissociation of molecular hydrogen. Carbon monoxide can also decompose to produce car-... 

Ladas, S., Poppa, H. Boudart, M. The adsorption and catalytic oxidation of carbon monoxide on evaporated palladium particles. Surface Science 102, 151-171 (1981). Kieken, L. Boudar, M. In New frontiers in catalysis Proceedings of the 10th international congress on catalysis, Budapest, July 19-24, 1992, Studies in surface science and catalysis, Guczi, L., Solymosi, F. Tetenyi, P. (Eds.), Elsevier, Amsterdam, (1993). 

In a liquid state high-resolution NMR study of adsorption on colloidal palladium, [33] it was found that under 3 atm. of 99% CO, the resonance which would correspond to carbon monoxide adsorbed on the 7.0 nm crystalline palladium colloid stabilized in methanol solution with PVP could not be directly... 

Further reports by Dobos et al (33,34) confirmed the initial results, but also showed that the response of the device was variable, depending on its previous history. The authors predicted a response characteristics of the device based on a Langmuir type of adsorption of the carbon monoxide on to the palladium surface ... 

Such a coverage dependence of the heat of adsorption is shown in figure 16, where the heat of adsorption of carbon monoxide on the palladium (1(X)) surface is shown as a function of surface coverage. At low coverages, the heat of adsorption remains by and... 

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