Nickel sulfur adsorption

Moreover, for coverage close to 1, a sudden decrease of the adsorption enthalpy (Fig. 1) can be explained by adsorption of species such as HS or undissociated H2S. A study of the nickel-sulfur interactions shows that the adsorbed state is energetically more stable than the bulky Ni3S2 sulfide (14). The same result was found for Ir catalysts (15). This shows that the contact of a metal with H2S will lead to a widely covered surface without any sulfur dissolution in the metal. The chemisorption energies of sulfur were also defined on Pt (16), Ir (15), Ru (17), and Fe and Co (18). For example, in the case of Pt, which is known as more resistant than Ni to sulfur poisoning, sulfur is weakly chemisorbed (16). 

Sulfur Adsorption Densities on Various Crystal Faces of Nickel"... 

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. 

Only a limited amount of thermodynamic data are available for 2-D sulfides of metals other than nickel. Data reported for Ag (57), Co (252), Cu (255), Fe (252, 154), Mo (255, 255), Ru (256), and Pt (84, 85) indicate that the heats of sulfur adsorption are generally 20-40% larger than the heats of formation of the most stable bulk sulfides. Indeed, Benard et al. (255) have shown a linear correlation between the heats of adsorption of sulfur and the... 

The deleterious effect of water vapor was speculated to be due to its inhibition of carbon formation freeing the metal surface for interaction by H2S. Thus, sulfur poisoning of nickel at high temperature (above 673 K) may be more representative of a carbon-fouled surface, whereas at low temperatures it may be more characteristic of the clean metal surface. Again, this needs to be confirmed by direct measurements of carbon and sulfur adsorption. For Ni/Al203 and Ni/ZrOz the extent of sulfur deactivation was about fiftyfold at 673 K at 523 K the extent of deactivation was about 1000-fold. However, for Raney Ni the extent of sulfur deactivation was tenfold higher at 673 K than at 523 K this difference in behavior also needs confirmation and explanation. 

Deactivation parameters obtained by plotting ln[(l — a) a)] versus time are listed in Table XIX for a number of nickel and nickel bimetallic catalysts. The fact that these plots were generally linear confirms that these data are fitted well by this deactivation model. These data, which include initial site densities for sulfur adsorption, deactivation rate constants, and breakthrough times for poisoning by 1-ppm H2S at a space velocity of 3000 hr-1 provide meaningful comparisons of sulfur resistance and catalyst life for both unsupported and supported catalysts. Table XIX shows that the... 

In studies of sulfur adsorption on polycrystalline nickel, Coad and Rividre ) detected new adsorbate levels laying on the high energy side (153.8( 23 V2 Vj) and... 

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). 

Oudar and co-workers studied the dissociative chemisorption of hydrogen sulfide at Cu(110) surfaces between 1968 and 1971.3,14 As in the case of Ni(110) described below, a series of structures were identified, which in order of increasing sulfur coverage were described as c(2 x 2), p(5 x 2) and p(3 x 2). In contrast to nickel, the formation of the latter phase is kinetically very slow from the decomposition of H2S and could only be produced at high temperatures and pressures. The c(2 x 2) and p(5 x 2) structures were confirmed by LEED,15 17 but the p(3 x 2) phase has not been observed by H2S adsorption since Oudar and colleagues work. 

Figure 10.6 STM images of the Ni(l 11) (5 /3 x 2)S phase and a model for the structure proposed to explain the decreased density of nickel within the islands, (a) 15.0 x 16.5 nm image showing the three possible domains of the (5 /3 x 2)S structure the brighter part of the image corresponds to an adlayer that has developed on top of a nickel island formed during H2S adsorption, (b) 1.8 x 2.9 nm atomically resolved image of the (5 /3 x 2)S structure, (c) Proposed clock structure for the (5 /3 x 2)S phase that accounts for the reduced nickel density in the sulfur adlayer. (Reproduced from Refs. 23 and 25).

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. 

Adsorption of benzene on sulfur- and oxygen-contaminated (110) faces of nickel revealed that the ordered layers formed differed from those obtained at the clean Ni(110) surface (23, 29). 

A part of Figure 3 in Ref. 207, reproduced on the right, reports radial EXAFS data around the S Is absorption edge for sulfur adsorbed on the (100) plane of a g nickel single-crystal surface. The top trace corresponds to the deposition of atomic S sulfur by dehydrogenation of H2S, while g, the bottom data were obtained by adsorb- M ing thiophene on the clean surface at 100 K. Based on these data, what can be learned about the adsorption geometry of thiophene Propose a local structure for the sulfur atoms in reference to the neighboring nickel surface. 

Recent studies using high resolution electron energy loss and photoelectron spectroscopy to investigate the effect of sulfur on the CO/Ni(100) system are consistent with an extended effect by the impurity on the adsorption and bonding of CO. Sulfur levels of a few percent of the surface nickel atom concentration were found sufficient to significantly alter the surface electronic structure as well as the CO bond strength. 

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