Sulfur rhodium surface

The adsorption of sulfur at platinum,40 rhodium,41 rhenium42 and ruthenium43 has been studied predominantly at fcc(lll) and hcp(0001) surfaces and shows many similar characteristics. Adsorption is initially into fee hollow sites of the fee metals and hep sites of the hep metals at higher coverages, mixed site occupancy occurs. A (2 x 2) structure is the first to be recorded appearing in the... 

LEED patterns at 0 = 1 /4, but was identified at lower coverages in islands surrounded by mobile sulfur atoms at platinum, rhodium and rhenium surfaces. Sautet and co-workers42 have analysed the statistical correlations between the intensities of sulfur features in p(2 x 2) islands on rhenium surfaces and also of streaks in areas between islands, which they attribute to sulfur atoms diffusing under the tip (Figure 10.12). 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

Pti-x ZXjc supported on carbon or alumina, Kt/Kb is proportional to x, suggesting electron transfer from platinum to zirconium, as predicted by the Engel-Brewer theory, and (2) chemisorption of sulfur on platinum has been shown to decrease electron density of the surface, while carbon has the opposite effect. The ratio Kt/Kb was very large for ruthenium, about 10 for rhodium and about unity for palladium, which may help to explain their different activities in these and other reactions. An extensive kinetic study of the hydrogenation of mixtures of benzene and toluene on NiA zeolite has however revealed a situation of some complexity, and it is not certain that the original simple concept is totally valid. 

Sulfur species may form sulfates and sulfites, which lower the catalyst surface area and cause deactivation for palladium [350]. An alumina support can trap or adsorb [351] sulfate groups. Deng and Nevell reported deactivation of alumina supported palladium, rhodium and iridium catalysts by hydrogen sulfide during methane oxidation [364]. Sulfate species were formed on the catalyst surface under the oxidising conditions. 

Fig. 7. Dependence of electrode surface charge on potential expressed, relative to the zero-charge point, on mercury (1) in 0.1 N sodium fluoride solution, on platinum (2) in 0.01 N sulfuric acid+ 0.01 N cesium sulfate solution, and on rhodium (3) in 0.01 N sulfuric acid + 1 N sodium sulfate.

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