Nickel catalysts presulfided

This plot of fractional H2 adsorption (H2 uptake at 300 K of catalysts presulfided in 5, 10, or 25 ppm H2S at 725 K divided by initial H2 uptake) versus mean sulfur coverage (in molecules H2S adsorbed per molecule of surface nickel), suggests a linear relationship between H2 uptake and sulfur coverage. Interestingly, the intercept at zero H2 coverage (saturation sulfur coverage) is H2S/Nis = 0.75, in excellent agreement with the adsorption stoichiometry reported by Oliphant et al. 112) for adsorption of H2S at 725 K. 

The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

SPARG A process for the C02 reforming of methane, producing carbon monoxide and hydrogen, using a partially presulfided nickel catalyst. Developed by Haldor Topsoe in the 1990s for the direct reduction of iron ore, but not commercialized. 

Catalysts are activated before use. Nickel catalysts are carefully presulfided either before or after loading to the reactor with the same organic sulfur com-poimds and procedures used for hydrotreating catalysts. Palladium catalysts are briefly activated in hydrogen at about 350°C, taking care not to overheat the bed, so that the crystallites do not sinter. 

NaY (Katalistiksj Si/Al=2 56) was used as a starting material for preparation of NH, K, Cs exchanged samples and three series of stab.lliJed Y-zeolites. Nickel was introduced before molybdenum by ion-exchange from Wi(NO )2 solution. Molybdenum was incorporated into the zeolite by sublimation of Mo(CO) in hydrogen stream at room temperature. Sulfided and non-sulfided Mo Y and Ni-Mo-Y samples were characterized with XRD, ND sorption capacity and FTIR spectroscopy. Each sample (0.25 g. 0.5-1 mm grains) was partially decarbonylated at 425 K before the WGS reaction and next presulfided with 1 2 vo1 %)/H2 mixture at 675 K, Catalysts after pre-... 

Figure 2 shows vanadium and nickel distribution for the spent NiMo/Al203 and C0M0/AI2O3 catalysts of the unsulfided and presulfided test runs. For the unsulfided catalysts, both Ni and V are observed to be more highly concentrated on the outer edges of the pellet. 

In most cases it is preferable to use an association of metals such as nickel-molybdenum or nickel-tungsten, which are more sulfur resistant. These catalysts need to be presulfided, because the metals are active in the sulfided state. 

---