Surface sulfur concentrations

Qualitative analysis of the spectra clearly outline the experimental dependencies on the extent of sulfur adsorption. The sulfation of ceria-only model catalysts is heavily dependent on total pressure and insensitive to temperature. The normalized sulfur transitions for exposures at 1 Torr total pressure yielded an average amplitude of 223 a.u., while exposures at 1000 Torr resulted in an average sulfur amplitude of 951 a.u. This represents an increase of 4.3 times the surface sulfur concentration for a corresponding increase of pressure by 1000 Torr. However, it should also be noted that the increase in overall pressure also represents an increase of sulfur dioxide partial pressure from 5x10 to 5x10 Torr. 

The most general observation is to be made on the basis of the tracer studies that only a part of surface sulfur is mobile or exchangeable on catalysts with sulfides and oxides. Supported Co and Ni contain only mobile sulfur, if sulfided by thiophene and contain both mobile and immobile sulfur, if they are sulfided by H2S/H2.[ 1 This is interpreted with the differences in the metal-sulfur bond strengths, as the edge S-atoms are held more weakly than the top S-atoms of the slab.I This follows from the results of some radiosulfur tracer studies of metal single crystals that different surface S-species are formed at different surface sulfur concentrations, and on different crystal faces of Pt, and... 

Internal surfaces were covered by loosely adherent corrosion product and deposit. Much of the corrosion product was cuprous oxide. Substantial amounts of iron, silicon, aluminum, zinc, and nickel were also found. Not unexpectedly, chlorine concentrations up to 2% by weight were present sulfur concentrations of about 1% were also found. 

Perdereau and Oudar s early paper20 reported LEED patterns and surface concentration data for the (111), (100) and (110) surface planes. For Ni(lll) a sequence of structures was observed starting with p(2 x 2), changing to ( /3 x v/3) 30° with increasing sulfur concentration and finally to a structure labelled SBAII , which Edmonds et al,21 identify as a (5 /3 x 2) structure. The low-concentration structures agree well with a model involving sulfur... 

The initial step of the adsorption of cyclic sulfides on a Mo(100) surface is also the formation of adsorbed thiolate groups.395-397 Adsorbed alkyl thiolates decompose to adsorbed sulfur, carbon, and hydrogen on the clean Mo surface, but once the surface is deactivated by adsorbed sulfur, alkanes and alkenes evolve from the surface. Tetrahydrothiophene (34) and trimethylene sulfide decompose on Mo(110) to alkanes and alkenes by way of a common intermediate, which is proposed to be a surface thiolate (33). The thiolate undergoes hydrogenation or dehydrogenation, depending on the surface hydrogen concentration (Scheme 4.115).398 399... 

Surface sulfide formation was attempted by Wibaut and van der Kam (122). The results were negative. However, it seems doubtful whether a sufficiently finely divided diamond powder was used. Otherwise, the analytic methods used by the authors would have been too crude for the detection of the extremely small sulfur concentrations. 

Elemental sulfur is recovered from its ore deposits found throughout the world. It is obtained commercially by the Frasch process, recovery from wells sunk into salt domes. Heated water under pressure is forced into the underground deposits to melt sulfur. Liquid sulfur is then brought to the surface. Sulfur is recovered by distillation. Often the ore is concentrated by froth flotation. 

Figure 1. Data from the literature indicate that S concentrations in surface lake sediments are poorly correlated with A, lake-water sulfate concentrations (104 lakes) B, sediment carbon content (78 lakes) or C, sediment iron content (22 lakes). Sulfur concentrations in lake sediments typically are lower than concentrations in marine sediments of comparable carbon content (upper line in B). The lower line in B represents the average C S ratio (55) reported in seston (59, 72, 27, 56, 78). Most of the lake sediments reported in the literature have more iron than sulfur. (Data are from references 24-30, 34, 48—51, 55-57, 59-61, 71, 104, 112, 199, 205, 222, and 223.)...

First a surface reaction involving sulfur and oxygen atoms may occur and decrease the surface oxygen concentration and consequently the above discussed effect upon the prenol selectivity ... 

Fig. 57. Surface reaction between sulfur and oxygen. The relative concentration of adsorbed sulfur is shown as a function of time for constant substrate temperature and oxygen coverage. R refers to the initial sulfur concentration (209).

Equations (4.1) through (4.18) are supplemented in each cell of the spatial division of the ocean surface with initial conditions (Table 4.3). The boundary conditions for Equations (4.11) through (4.18) are zero. The calculation procedure to estimate sulfur concentration consists of two stages. First, at each time moment th for all cells Qiy, Equations (4.1)-(4.18) are solved by the quasi-linearization method, and all reservoirs of sulfur are estimated for ti+x = tf + At, where time step At is chosen from the convergence state of the calculation procedure. Then, at moment t(+1 using the climate unit of the global model these estimates are specified with account of the atmospheric transport and ocean currents over time At. 

Auger electron spectroscopy (AES) is particularly suited for surface analysis (depth 0.5-1 nm). AES depth profile analysis was employed to determine the thickness and composition of surface reaction layers formed under test conditions in the Reichert wear apparatus in the presence of four different ZDDPs additives at different applied loads (Schumacher et al., 1980). Using elemental sensitivity factors the concentration of the four elements (S, P, O, C) was determined at three locations corresponding to a depth of 1.8, 4.3, and 17 nm. No significant correlation between wear behavior and carbon or oxygen content of the reaction layer was observed. A steady state sulfur concentration is reached after a very short friction path. Contrary to the behavior of sulfur, phosphorus concentration in the presence of ZDDPs increases steadily with friction path, and no plateau value is reached. 

AO and AS = the increase in the oxygen and sulfur concentration on the rubbing surfaces from the virgin surfaces cps = counts per second 02 = oil samples with blow-in oxygen... 

The surface concentration of carbon in the used catalysts is consistent with the bulk carbon analysis, while surface carbon on the fresh and regenerated samples seems disproportionately high. Surface sulfur is consistently lower than the bulk values of Table I indicating the presence of metallic sites on the surface, since these levels on the used catalysts are incomplete for total sulfiding of Co and Mb. 

To enable quantitative determination of rates of sulfur deactivation, of extents of sulfur deactivation at very low gas-phase sulfur concentrations, of true dynamic equilibrium between gas-phase sulfur concentration and metal surface, and/or of the amount of sulfur adsorbed on the surface, the following requirements must be satisfied in the design of experimental apparatus ... 

Fig. 35. Sulfur coverage of the nickel surface of a reforming catalyst in a typical naphtha-based ammonia plant as a function of distance through the reactor and inlet sulfur concentration (ppm) (Ref. 237b). Assumed profiles of temperature and hydrogen flows ...

The values shown in the table indicate that the material compacts to the desired level with a 5-ton vibrating compactor. Surface skid resistance of the sections was measured using a California portable skid tester in accordance with California test method 342 D. The skid values are comparable with published values for fresh-open or dense-graded asphaltic concretes. The friction values increased with increasing sulfur concentration. 

Deactivation by sulfur has been explained by the withdrawing of electrons from the catalyst surface. It has also been shown that sulfur inhibits the dissociation of CO on iron surfaces l]. The deliberate partial poisoning of iron/manganese cataly.sts with sulfur has been used to shift the product selectivity towards short-chain hydrocarbons. At higher sulfur concentrations (0.7 mg S/g catalyst) the activity is significantly decreased and the olefin selectivity increased [82]. Sulfur poisoning of nickel catalysis has recently been shown to inhibit the chemisorption of hydrogen 83.84). 

Like ozone, sulfur dioxide is subject to deposition into the oceans, with no re-emission. This arises from the high reactivity of the gas in seawater, which ensures its rapid destruction in the water and effective zero surface-water concentration driving the one-way flux (Liss, 1971). The high solubility and aqueous reactivity of SO2 makes its exchange subject to gas phase control (see Section 6.03.2.1.1). 

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