Adsorption sulfur on metals

Thermodynamics of Sulfur Adsorption 1. Affinity of Sulfur for Metals 

Adsorption of sulfur on monocrystalline, polycrystalline, and supported metallic catalysts has been extensively studied. For all adsorption reactions, thermodynamics states that A//llds = Ea - Ed (where A//ads is the enthalpy of adsorption, Ea is the activation energy of adsorption, and Ed 

Such techniques were used by Benard and Oudar and co-workers (5) on polycrystalline Cu catalysts (9), on monocrystals of Ag (10), and on other metals (//). On the low-index Ag monocrystals, the bonding energy of chemisorbed sulfur follows the sequence (110) (100) (111), showing that the sulfur affinity changes with respect to the crystallographic orientation under consideration and therefore on the basis of the coordination number. 

On nickel catalysts, which have been extensively studied (12-14), McCarty and Wise (14) showed that in a temperature range from 373 to 873 K, sulfur coverage of nearly half a monolayer can be reached with H2S partial pressures as low as 1-10 ppb. Such results, indicating that the equilibrium H2S = H2(g) + S(a) is totally displaced toward the right side, show the very high affinity of sulfur for transition metals and thus the difficulties in avoiding sulfur poisoning in metallic catalysis. 

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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What is the mechanistic nature of sulfur adsorption on metal surfaces and what are the surface structures formed ... 

Only a few kinetic studies of the rate of sulfur adsorption on metals have been made. They reveal that rates of adsorption of H2S on metals are generally very rapid, the high sticking probability suggesting no barrier to adsorption and dissociation until saturation is approached. In the case of Pt and Cu (83, 92), two adsorption regimes are observed (1) at 0 < 0.25-0.3, the adsorption of sulfur occurs with a high sticking coefficient ( 1.0) and... 

Among relevant studies one can cite the investigations of sulfur adsorption on molybdenum " ), silicon ), copper " ), gold ), vanadium ) and iron " ). Such examples may be found in very large number in the literature we shall try and set in evidence the general trends observed in structures resulting from sulfur adsorption on metallic surfaces. 

THERMODYNAMICS OF SULFUR ADSORPTION ON METAL SURFACES IN WATER... 

Thermod5mamics of Sulfur Adsorption on Metal Surfaces in Water.409... 

Nevertheless, two factors strongly influence the heat of sulfur chemisorption on metal surfaces relative coverage and crystallographic structure. Thus sulfur chemisorbs at high coordination sites and, as a result, a selective poisoning of structure-sensitive reactions, preferentially catalyzed by these sites, may occur. Such a simple geometrical model can be used to explain change in selectivities induced by sulfur adsorption. 

As demonstrated in the case of H2S on Ni, thermodynamic data such as AG° and A//ad are of great value in understanding the extent and reversibility of sulfur poisioning. Clearly, more thermodynamic data for adsorption of sulfur are needed for important metals of catalytic interest such as Co, Fe, Mo, Pd, Pt, Re, Rh, Ru, etc. Since sulfur adsorption on Co, Pd, Rh, and Ru... 

Since preadsorbed sulfur generally blocks the adsorption of other molecules, it would only be logical to expect that it would also prevent the adsorption of H2S or S2. Previously discussed studies of sticking coefficients (73, 83, 92, 99, 101) and H2S adsorption on metals (57, 106, 112-115) provide evidence that the sticking coefficient and heat of adsorption for H2S or S2 decrease with increasing coverage. Thus, rates and strengths of sulfur adsorption on sulfur-saturated metal surfaces are clearly lower than those on a clean metal surface. 

Metal-sulfur bonds of adsorbed sulfur are significantly stronger than metal-sulfur bonds in bulk metal sulfides. Heats of H2S adsorption on metals are 80-100% larger than heats of formation of the bulk sulfides. 

Prior to H2S introduction to the reactor, the bypass was filled with H2S by flowing fresh H2S through the U-lube for a half hour. Then a pulse of M2 was carried to the reactor by switching and flowing the feed gas thtough bypass for a half hour. During the H2S pulses, H2S concentration in the effluent stream was monitored and no sulfur breakthrough was detected except fcM 0.5-1 ppm H2S within the first five minutes. This was probably due to sulfur adsorption on the metal surfaces in the system for this reason sulfur content in the used catalysts was analyzed. 

Different catalysts were sulfided at 500°C by the hydrogen sulfide/hydrogen mixture and then treated at the same temperature under pure hydrogen for 10 h. After such a treatment, only sulfur strongly bound to the metallic surface remains adsorbed on the catalyst since sulfur adsorption on the alumina support is wholly reversible under hydrogen at 500°C [19]. The sulfur coverages (9s) calculated from the amounts of irreversible sulfur are reported in Table 3. 

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. " Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that... 

Finally, wc note that sulfur adsorption on all the low Miller index surfaces tabulated involves bonding of the adatom to a metal atom in the top layer. There are two exceptions, Rh(lIO) and Ir(110), surfaces which are sufficiently open for the S atom to bond directly to a second layer metal atom. 

Sydberger, T., and Vannerberg, N. G. (1972). Influence of the relative humidity and corrosion products on the adsorption of sulfur dioxide on metal surfaces. Corros. Sci., 72(10), nS-lM. 

Figure 5.26. Hydrogen adsorption on metal—sulfur pairs. Adapted from A. Travert et al.l" l.

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

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