Platinum, adsorption

The understanding of activation effects on the electrochemical behaviour of ordered platinum surfaces will allow the possibility of correlating the classical knowledge of platinum adsorption behaviour with the new observations. 

These examples show that adsorption of water molecules on platinum electrodes depends on the solution components. If the energy of the solute adsorption is higher than that of water molecules, water tends to adsorb on the top of the primary solute layer, which is directly bound to the platinum adsorption sites. If the interaction of organic molecules with platinum is weak, water adsorbs directly onto the electrode surface. In the... 

M. Schreier, and J. R. Regalbuto, A fundamental study of R tetra ammine impregnation of silica 1 The electrostatic nature of platinum adsorption, J. Catal. 225, 190-202 (2004). 

The Al-OH groups involved in this reaction are likely to be the basic ones, since the adsorption reaction is probably to be thought of as analogous to the hydrolysis of the PtClk2 ) species that occurs in aqueous solution at higher pH values. The fact that the platinum adsorption capacity of a typical y-alumina is of the order of 1.5 pmol/m, while the amount of basic OH groups is typically 3 to 3.5 pmol/m2, as determined by titration with Mo02(acetylacetonate)2, fits in nicely with this idea. 

Concentration profiles of platinum adsorption on impregnation of Y alumina pellets with aqueous chloroplatinic acid, a) Standard adsorption, no additives and b) NaNOj added equimolar to the chloroplatinic acid. (Redravm using data from Ref 80.)... 

Class 3 additives are materials such as phosphoric acid and citric acid that can compete with the metal for adsorption sites. While Class 1 and Class 2 additives can control the depth and amount of metal adsorbed leading either to uniform or egg shell catalysts. Class 3 species interfere with platinum adsorption and can give entirely different adsorption profiles. This approach is used, specifically, for the preparation of egg white and egg yolk type catalysts. Fig. 13.11 shows that the platinum distribution is displaced from the surface of the... 

Adsorption Method Iodine separation by the platinum adsorption technique was first reported by Toth (1961) and Lin et al. (1963). Oak Ridge National Laboratory has adopted an adsorption procedure (Case and Acree, 1966) for the purification of I produced from fission by a distillation process. In 1966, in comparison with other methods of L Baker (1966) has concluded that the adsorption technique is the most efficient and economical method. In this method, a metallic Pt plate or felt is used to adsorb carrier-free iodine from an acidic solution containing target materials. The Pt plate or felt with iodine activity is then removed from the solution and thoroughly washed with water. The activity is then desorbed in a slightly basic solution. Application of electrical current could enhance the desorption process. Nearly quantitatively the adsorbed iodine activity can be desorbed from the Pt surfaces. 

Berzins, A.R., et ah, Isothermal chemisorption upon oxide-supported platinum, Adsorpt. Sci. Technol., 1(1), 51-76(1984). 

Li, Y. et al. 2009. A first-principles study of nitrogen-and boron-assisted platinum adsorption on carbon nanotubes. Carbon 47 850-855. 

It is characteristic of these specimens that Iheir potential is not established during the experiment. The positive shift of the potential continues to increase throu out the whole experiment. On the basis of results obtained during hydrogenation of DMEC in water with single-shot addition of the compound and with proportioned delivery it can be concluded that strong adsorption of the reaction product takes place in water, since the potential of the catalyst does not return to the reversible hydrogen potential at the end of the experiment. This is not observed on platinum or on catalysts which are rich in platinum adsorption of the reaction product is evidently more clearly expressed on ruthenium than on platinxmi. 

Ruch and Bartell [84], studying the aqueous decylamine-platinum system, combined direct estimates of the adsorption at the platinum-solution interface with contact angle data and the Young equation to determine a solid-vapor interfacial energy change of up to 40 ergs/cm due to decylamine adsorption. Healy (85) discusses an adsorption model for the contact angle in surfactant solutions and these aspects are discussed further in Ref. 86. 

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].

The impurities usually found in raw hydrogen are CO2, CO, N2, H2O, CH, and higher hydrocarbons. Removal of these impurities by shift catalysis, H2S and CO2 removal, and the pressure-swing adsorption (PSA) process have been described (vide supra). Traces of oxygen in electrolytic hydrogen are usually removed on a palladium or platinum catalyst at room temperature. 

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

The discussion in the previous section suggests that adsorption of pyridine on the catalyst is a necessary prerequisite for the formation of 2,2 -bipyridine but as platinum catalysts, which are poisoned by... 

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). 

The experimental investigation was performed by depositing copper films on the (100) -surface of a platinum single crystal. It was found that the reconstruction of the Pt surface was lifted upon Cu adsorption. The system was then heated to different temperatures and the formation of different ordered surface alloys was evidenced by... 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

In some cases, the catalyst is a solid substance on whose surface a reactant molecule can be held (adsorbed) in a position favorable for reaction until a molecule of another reactant reaches the same point on the solid. Metals such as iron, nickel, platinum and palladium seem to act in this way in reactions involving gases. There is evidence that in some cases of surface adsorption, bonds of reactant particles are weakened or actually broken, thus aiding reaction with another reactant particle. 

Vitreosil filtering, 103 see also Platinum apparatus Crushing and grinding 155 Crystal violet indicator, 308 Cupferron 170, 440, 471, 474 Cupron 442, 473 Current adsorption, 616 catalytic, 616 condenser, 595 diffusion, 592, 596 efficiency, 504 kinetic, 616... 

A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BET surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 

---