Platinum catalysts adsorbed sulfur

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

Such electronic transfer induced by sulfur adsorption was also pointed out by using cinnamic acid as a probe molecule (48). The UV-visible reflexion spectra of adsorbed cinnamic acid on nonpoisoned and partly poisoned platinum catalysts shows that adsorption on pure platinum induces a shift of the peaks toward the higher wavelengths and an appearance of fine structure. Sulfurization of platinum induces a further enhancement of higher wavelength peaks. Binding energy of cinnamic acid is thus increased by sulfur adsorption on Pt catalysts. 

The amount of physical adsorption decreases rapidly as the temperature is raised and is generally very small above the critical temperatures of the adsorbed component. This is further evidence that physical adsorption is not responsible for catalysis. For example, the rate of oxidation of sulfur dioxide on a platinum catalyst becomes appreciable only above 300°C yet this is considerably above the critical temperature of sulfur dioxide (157°C) or of oxygen ( — 119°C). Physical adsorption is not highly dependent on the irregularities in the nature of the surface, but is usually directly proportional to the amount of surface. However, the extent of adsorption is not limited to a monomolecular layer on the solid surface, especially near the condensation temperature. As the layers of molecules build up on the solid surface, the process becomes progressively more like one of condensation. 

CHEMISORBED POISONS Compounds of sulfur and other materials are frequently chemisorbed on nickel, copper, and platinum catalysts. The decline in activity stops when equilibrium is reached between the poison in the reactant stream and that on the catalyst surface. If the strength of the adsorption compound is low, the activity will be regained when the poison is removed from the reactants. If the adsorbed material is tightly held, the poisoning is more permanent. The mechanism appears to be one of covering the active sites, which could otherwise adsorb reactant molecules. 

The adsorbed sulfur deactivates Pt by blocking the access to a given number of Pt atoms and modifies the electronic properties of the neighboring atoms. For example, S not only decreases the amount of CO adsorbed but also decreases the binding energy between CO and Pt (138) and also with other adsorbates (87). It has been established that certain amount of sulfur remains adsorbed on the metal, even after 30 h at 500° C in H2 flow. This is called irreversible sulfur, and this amount does not depend on the sulfiding conditions (139). This amount is approximately 0.4 atoms of sulfur for each atom of accessible platinum, in Pt, Pt-Re, and Pt-Ir catalysts. 

The catalyst of Cheekatamarla and Lane lost 75% of its original surface area when etposed for more than 50 h to the sulfur containing feed, whereas only a 50% reduction in surface area was observed for the low sulfur feed [268]. In parallel, the dispersion of the platinum decreased from 51 to 35%. These workers explained these observations by two types of sulfur species being present on the catalyst surface firstly, physically adsorbed sulfur species and, secondly, their oxidised products, which were then irreversibly bound to the catalyst sites by chemical adsorption. The activity of the catalyst could be completely re-gained when it was reduced in 20 vol.% hydrogen (with a balance of helium) for 30 min at 800 °C. 

The primary contaminants of a PEFC are carbon monoxide (CO) and sulfur (S). Carbon dioxide (CO2) and unreacted hydrocarbon fuel act as diluents. Reformed hydrocarbon fuels typically contain at least 1 percent CO. Even small amounts of CO in the gas stream, however, will preferentially adsorb on the platinum catalyst and block hydrogen from the catalyst sites. Tests indicate that as little as 10 ppm of CO in the gas stream impacts cell performance (35, 36). Fuel processing can reduce CO content to several ppm, but there are system costs associated with increased fuel purification. Platinum/ruthenium catalysts with intrinsic tolerance to CO have been developed. These electrodes have been shown to tolerate CO up to 200 ppm (37). 

Binder H, Koehling A, Sandstede G. Acceleration of the electrochemical oxidation of formic acid by sulfur and selenium adsorbed on platinum catalysts. Nature 1967 214 268-9. 

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. 

Platinum is usually used as the catalyst for converting sulfur dioxide into sulfur trioxide. The platinum speeds up the reaction by making it easier for molecules of sulfur dioxide and oxygen to collide. Sulfur dioxide molecules and oxygen molecules get adsorbed (or stuck to) the surface of the platinum. Because they are held so closely together, the sulfur dioxide and oxygen come... 

Reforming catalysts are typically platinum on a silica-alumina support. The catalysts are deactivated by feedstock contaminants such as organic nitrogen, sulfur, ammonia, and H2S. For this reason, the reformer charge is hydrotreated to remove these components. Also, any trace metal contaminants will be adsorbed onto the hydrotreating catalyst. 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

The H2 chemisorption uptakes at increasing sulfur coverage (S/Pt=0-100) of the metal are shown in Figure 3. At higher S/Pt ratios the percentage of platinum particles able to interact with the probe decreased to 40% of the oii i value of dispersion at S/Pl 92. When the catalyst was contacted with laiger quantities of H2S, the H2 chemisorption experiments showed a reverse trend i.e. increasing the S/Pt ratio, the amoimt of H2 adsorbed at room temperature increased (Fig 4). 

Figure 103. The amount of soluble organic fraction (SOF) adsorbed on an aged diesel oxidation catalyst, as a function of the washcoat formulation (monolith catalyst with 62cellscm", dedicated diesel washcoat formulations with platinum at a loading of l.76gl diesel engine bench aging for 50 h diesel fuel containing 0.15 wt. % sulfur). Reprinted with permission from ref. (68], C 1991 Society of Automotive Engineers, Inc.

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