Palladium cleaning surface

Platinum, palladium and the normal alloys of platinum used in industry are easily workable by the normal techniques of spinning, drawing, rolling, etc. To present a chemically clean surface of platinum and its alloys after fabrication, they may be pickled in hot concentrated hydrochloric acid to remove traces of iron and other contaminants —this is important for certain catalytic and high-temperature applications. In rolling or drawing thin sections of platinum, care must be taken to ensure that no dirt or other particles are worked into the metal, as these may later be chemically or elec-trolytically removed, leaving defects in the platinum. 

A variety of model catalysts have been employed we start with the simplest. Single-crystal surfaces of noble metals (platinum, rhodium, palladium, etc.) or oxides are structurally the best defined and the most homogeneous substrates, and the structural definition is beneficial both to experimentalists and theorists. Low-energy electron diffraction (LEED) facilitated the discovery of the relaxation and reconstruction of clean surfaces and the formation of ordered overlayers of adsorbed molecules (3,28-32). The combined application of LEED, Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), field emission microscopy (FEM), X-ray and UV-photoelectron spectroscopy (XPS, UPS), IR reflection... 

The adsorption of CO on nickel has received extensive study since large area nickel films are comparatively simple to prepare. The nickel system therefore offered the possibility of producing a clean surface on a metal crystallizing with a face centered cubie structure. The results obtained on this surface could then be compared with those measured on tungsten, which crystallizes with a body centered,cubic structure. Much of the earlier work carried out for CO adsorption on the elements nickel, palladium, and platinum was reviewed by Gundry and Tompkins in 1960 135). 

B. Marchon. HREELS Study of the Cyclomerization of Acetylene on Clean and Phosphorus-Covered Palladium (111) Surfaces. Surf. Sd. 162 382 (1985). 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

P-C-T data were obtained with a palladium sample ( 20 g) in the form of 1-mm rods (99.9% purity). Thus, the surface-to-volume ratio was quite small. The sample was not coated with palladium black but merely was cleaned mechanically after being annealed at 1100° K for several days. The sample had never been subjected to the hydride-phase transformation. 

C-Tracer studies of acetylene adsorption on alumina- and silica-sup-ported palladium [53,65], platinum [66] and rhodium [53] show the coexistence of at least two adsorbed states, one of which is retained on the surface, the other being reactive undergoing molecular exchange and reaction with hydrogen. Acetylene adsorption exhibits the same general characteristics as those observed with ethylene (see Sect. 3.2). However, there are important differences. The extent of adsorption and retention is substantially greater with acetylene than with ethylene. Furthermore, the amounts of acetylene retained by clean and ethylene-precovered sur-... 

Cleaning of the CO gas to remove metal carbonyls by cooling the CO container with liquid nitrogen removes all the surface contaminants indicated by Cls spectra, even after treatments at pressures of approximately 1 mbar for 5-6 h. Care has to be taken not to misinterpret such observations as being evidence of CO dissociation on palladium. 

The amount of CO adsorbed on the palladium particles can be deduced from CO-TDS (Fig. 3 Id). The CO-TDS spectrum was identical to that observed after dosing of the same amount of CO on the clean particles, demonstrating that the particles were fully covered with CO and that Ft was replaced from the palladium surface. FFowever, FF had not desorbed because the FF2-TDS experiment (Fig. 3Id) indicated that the overall amount of hydrogen was unchanged. Apparently, CO had displaced surface FF to the subsurface and bulk of the palladium nanoparticles (see schematics in Fig. 31 partial Ft spillover to the support is unlikely because no OFF groups or H2O were detected). 

These results indicate that the origin of the acceleration in the rate of acetylene cyclotrimerization due to the addition of hydrogen measured above (Fig. 1.4) arises from a combination of the formation of a more open ethylidyne-covered surface, and possibly also the removal of the ethylidyne once it has been formed to produce regions of relatively clean palladium. 

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