Hydrogen nickel surface covering

Fig. 2. Typical curves of the relative changes of the electrical resistance of nickel films as a function of time (a) adsorption of one dose of hydrogen on the surface, partially covered by preadsorbed oxygen (b) adsorption of one dose of oxygen on the surface, covered by preadsorbed hydrogen (both at 300°K).

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

Fig. 14. Heat of adsorption at 23°C. of hydrogen on evaporated nickel films as a function of surface covered. ( unoriented films O oriented films.)...

The spectra of the adsorbed butenes, pentenes, and hexenes discussed above were obtained by chemisorbing on a hydrogen-covered surface at 35° C. The results show that some dehydrogenation must occur under these conditions, since it is impossible to get four-point adsorption without having some dissociation. When higher-molecular-weight olefins (or paraffins) are chemisorbed on a bare nickel surface, spectra similar to A of Fig. 3 are obtained, and no distinguishing characteristics are observed. 

Under the conditions of the experiments, the nickel surfaces may be considered as fully covered by hydrogen 0H = 1. Now the temperature-independent rate constants for the three mechanisms can be calculated from the power rate law as given in IV, 2. According to (29), the exponent of the hydrogen pressure in the rate equation will be... 

These apparent discrepancies can be resolved as follows. First, the values at lower temperatures (S/Nis = 0.25 and 0.33) (96, 104, 111) are smaller because hydrogen atoms from the dissociative chemisorption of H2S remain adsorbed on the nickel surface blocking sites for further sulfur adsorption. At higher temperatures hydrogen desorbs allowing sulfur atoms to cover most or all of the nickel sites, and thus higher S/Nis ratios (e.g., 0.6-0.7) are observed. 

A reactor constructed of stainless steel 410 was used for pyrolysis since it contained no nickel. The coke layer formed during pyrolysis was usually thin and greyish. Less frequently, a piece of black coke was found on the surface. The metal surface (Surface C) was always grey. Figure 5 shows the two types of coke formed at Surface A in the stainless steel 410 reactor. The black (less frequent) coke appeared to be a floe of fine filaments, about 0.05 / m in diameter, with occasional 0.4- m filaments. The predominant deposit seems to be platelets of coke that include metal crystallite inclusions, the lighter area. The metal particles in the coke deposits, as detected by EDAX, were chromium rich compared with the bulk metal, as reported in Table III. Some sulfur also was present in the deposit the sulfur was present, no doubt, because of the prior treatment of the surface with hydrogen sulfide. Surfaces B and C for the stainless steel 410 reactor are also shown in Figure 6. Surface B indicated porous coke platelets. Surface C was covered mostly with coke platelets, and cavities existed on the surface. Metal crystallites rich in iron apparently were pulled from the metal surface and were now rather firmly bound to Surface B. Surface C was richer in chromium than the bulk metal. 

Cyclohexene was adsorbed by a nickel catalyst powder (surface area 15 m g ) at 300 K at 1 mbar and 10 mbar pressure and on a nickel catalyst covered with hydrogen [82]. INS spectra are shown in Fig. 7.19. We note (a) The INS spectrum of cyclohexene adsorbed by nickel at low pressure was different from the spectrum of solid cyclohexene. The spectrum was assigned to adsorbed benzene (peaks near 900, 1140 cm )... 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

A paper by 0. Beeck (33d) has only recently come to the author s notice. Working with evaporated films of metal of area of the order of 10,000 cm. Beeck has confirmed all Roberts results on the speed, extent, and heat of adsorption as a function of surface covered, for the adsorption of hydrogen on tungsten. Nickel shows a similar behavior. This makes extremely likely the view that adsorption on tungsten powder is complicated in some fashion, if not by solution, as suggested above, then by some kind of physical or chemical heterogeneity of the surface, as in the Halsey and Taylor picture. 

The nickel catalyst surface is reduced after 4.5 h at a current density of 5 mAm. The electrodes are not expected to be reduced after a short activation time because the catalysts are still significantly covered by the PTFE film, observed with XPS and specific surface area measurements. Perforations in the PTFE film are sufficient to allow contact between the electrolyte and the nickel catalyst and therefore to reduce the nickel oxide. During the activation process, the perforations in the PTFE film are increased or a new contact between the electrolyte and the metal catalyst is formed. The contact area may be created by the hydrogen that is formed on the nickel surface and that separates the PTFE film from the catalyst surface. Therefore, the initial nickel signal of the activated electrode (observed by XPS) is increased. The specific surface area increases without the complete removal of the PTFE film. 

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. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

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