Nickel catalysts adsorbed sulfur

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

Mozingo (20) has shown with a wide variety of compounds that Raney nickel catalyst in the presence of a solvent and with only its adsorbed hydrogen can cleave either reduced or oxidized sulfur from the remainder of the molecule at a moderate temperature. Two courses for the reaction can be postulated ... 

TPH tests with pure alumina (alpha) indicated that sulfur was not adsorbed on this material during fixed-bed poisoning tests, although sulfur adsorbed on nickel catalysts supported on alumina, thus indicating it adsorbs on the surface of nickel only. Tests with a pure alumina (alpha) bed also indicated that hydrogen was not adsorb on it at the conditions for nickel surface area measurements by hydrogen. 

It can be seen from the Fig. 2 that sulfur desorption from the catalyst beds (Catalysts A C) which had been poisoned at 800-900°C under 1 and 20 bar pressure begins when the temperature of the bed is above 400°C, the most part being desorbed rapidly between 500-700°C. On the other hand, when the HjS concentration of the bed is sufficiently high for bulk nickel sulfide formation, the desorption of sulfur in the atmospheric tests occurs at the same temperamre as the catalyst which has been treated in fixed-bed poisoning tests. However, in the 20 bar tests the desorption of sulfur begins at about 650°C. This temperature is lower than the bulk nickel sulfide formation temperature of about 900°C but higher than the above mentioned desorption temperature of adsorbed sulfur species. Therefore, some adsorbed sulfur, in addition to bulk nickel sulfide, may have been present on the catalyst. Fig. 2 shows that the desorption of sulfur in the case of bulk sulfide occurs more slowly than in the case of chemisorbed sulfur. 

With an H2S/H2 ratio in the gas of 1 ppb, the equilibrium surface coverage of nickel at 500°C is around 70%. This means that all sulfur in the feed is quantitatively adsorbed on the nickel catalyst of a prereformer. The result is not only the deactivation of the prereformer catalyst even at very low sulfur levels, but also the protection of downstream catalysts from poisoning. Sulfur uptake on the catalyst will initially take place as shell poisoning and because of pore diffusion restrictions, it may take years before sulfur reaches the center of the particle. ... 

In the following, examples of the ensemble control by means of adsorbed poisons are discussed with the the emphasis on steam reforming of methane on sulfur passivated nickel catalysts. The conclusions for ensemble control will be compared with data for catalytic reforming on PtRe(S) catalysts and for the impact of chlorine on partial oxidation of methane on Pd-catalysts. 

In the first step, the unshared electron pair of the sulfur is attracted to a low-valent nickel atom of the Raney nickel catalyst surface. The compound is adsorbed on the surface through the unshared electron pair of sulfur, which acts as an electron donor. Therefore, the carbon-sulfur bond is weak-... 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

Based on elemental analyses and microprobe tracing (Dautzenberg et al., 1978), metal deposits appear to be present in sulfide forms and not as adsorbed porphyrin-type compounds or as metals in the elemental or metallic state. Takatsuka et al. (1979) and Rankel and Rollmann (1983) have reported direct linear correlations of the spent catalyst sulfur content with the deposited metal content. The sulfide forms of nickel and vanadium are consistent with expectations based on thermodynamics for the conditions typically encountered in residuum hydroprocessing units (600-800°F, 1000-2200 psig, H2/H2S environment). 

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). 

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