Hydrogen adsorption on nickel

The effect of other surface impurities may be more severe than that of oxygen. For instance, adsorbed sulfur strongly inhibits hydrogen adsorption on nickel 58), while chlorine adsorbed on nickel is also likely to be a tenaciously held surface contaminant. 

Results of previous investigations 23,110, 111, 113, 141, 157-165) show that hydrogen adsorption on nickel at room temperature is lowered by preadsorbed sulfur. Moreover, the fraction by which hydrogen adsorption is reduced in polycrystalline and supported nickel catalysts is generally proportional to the mean fractional coverage of sulfur. This is illustrated by data in Fig. 16 from Bartholomew and co-workers 112, 113, 141, 157-162). 

This view is supported by the statistical-mechanical interpretation of the adsorption isotherm at such low coverages that the differential heat of adsorption of hydrogen is nearly constant at 26 kcal./mole. From the agreement between the surface fraction calculated by means of equation (18) with n = 2, and the experimentally found value 0ob., for the hydrogen adsorption on nickel, Kwan et al. concluded that the surface of reduced nickel is homogeneous, or that every surface element is equally available for the chemisorption of hydrogen. 

It has been suggested that the rate limiting step in the mechanism is the chemisorption of propionaldehyde and that the hydrogen undergoes dissociative adsorption on nickel. Determine if the rate expression predicted by a Hougen-Watson model based on these assumptions is consistent with the experimentally observed rate expression. 

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

In the first example (46), hydrogen adsorption on different sites of a nickel crystal was studied by considering the interaction of a hydrogen atom with a limited number of metal atoms. The nickel crystal was truncated to obtain the clusters shown in Fig. 31, containing 13,10, 9 and 8 nickel atoms, and representing models for the bulk crystal and for the (111), (100) and (110) surfaces respectively. The nearest-neighbour distance in all clusters was... 

Galea NM, Kadantsev ES, and Ziegler T. Studying reduction in solid oxide fuel cell activity with density functional theory-effects of hydrogen sulfide adsorption on nickel anode surface. JPhys Chem C 2007 111 14457-14468. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

In other cases the adsorption of hydrogen atoms results in surface hydride dipoles pointing with their negative ends away from the metal, as is observed in the adsorption on nickel,... 

Fig. 12.9. Relative extent of hydrogen adsorption on copper-nickel...

Cadenhead, D.A. and Wagner, N.J. "Low-temperature hydrogen adsorption on copper-nickel alloys."/. Phys. Chem. 72 2775-2781 1968. 

Fig. 27. Alternative model for hydrogen adsorption on (110) nickel. It is considered less reasonable than that of Fig. 26b. Slight displacements of top layer Ni atoms may perhaps account for the one-half order beams of Fig. 26a. Illustration to scale shows H atoms of diameter 0.74 A bonded to two top layer Ni atoms which have been pulled out of their normal lattice sites along vector displacements marked by the small arrows. View is along close-packed surface direction. Nearest neighbor Ni—Ni bonds joining atoms such as labelled 1 and 2 need to be stretched 8%.

Another method for determination of the nickel area is based on chemisorption of hydrogen sulphide [389], Hydrogen sulphide is the stable sulphur compound at conditions for tubular reforming. It is the most severe poison for the reaction (refer to Section 5.4). The adsorption of hydrogen sulphide on nickel is rapid even below room temperature [376] [381]. At temperatures of industrial interest hydrogen sulphide is chemisorbed dissociatively on nickel. 

Studies on HOR electrocatalysis have built up a foundation for all modem electrocatalysis. Since the pioneering work of the 1960s, when flie dependence of hydrogen adsorption upon the crystallographic orientation of the platinum surface was discovered, the study of the relationship between electrochemical activity and surface structure has been the main theme of electrochemical research [50]. In this section, we first introduce the electrocatalysis of the HOR on Pt and Pt-alloy electrodes, with detailed discussion of the stmcture-sensitivity of hydrogen adsorption on well-defined Pt surfaces. Then we discuss two types of non-noble catalysts for the HOR carbides and Raney nickels. 

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). 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Figure 3.2. Potential energy diagram of chemisorption for the adsorption of hydrogen on nickel (after Le Page, 1987).

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