Metal surface, hydrogen dissociation

The catalytic reduction of a particular intermediate involves the addition of hydrogen. Just as in oxidation catalysis, there is the issue of whether one or two hydrogen atoms from H2 are incorporated into the substrate through identical intermediate hydrogen atoms. In heterogeneous catalysis, this question translates into whether dissociation occurs ho-molytically or heterolytically. On a transition-metal surface H2 dissociation generates two equivalent hydrogen atoms such as we have seen in Chapter 3. As discussed in Chapter 4, however, Hg can dissociate heterolytically H2 H + H+. 

At temperatures above 200 °C, unalloyed steels are attacked by pressurized hydrogen. This kind of attack is different from low temperature hydrogen embrittlement. Hydrogen is adsorbed at the metal surface and dissociates into atomic hydrogen, which becomes dissolved in the steel. There it reacts with cementite Fe3C according to... 

In this case. Fig. 4.14 exhibits the surface atomic stmcture of metallic site Me°, representing two atoms of metal and hydrogen dissociated (H) [2] ... 

Dissociation of molecular hydrogen gas under pressure at container metal surfaces... 

The gases that have been used most often are hydrogen, carbon monoxide, and oxygen. Hydrogen is by far the most useful, and it has the best established adsorption mechanism. It dissociates at room temperature on most clean metal surfaces of... 

A general conclusion regarding H2 adsorption on alkali modified metal surfaces is that alkali addition results in a pronounced decrease of the dissociation adsorption rate of hydrogen as well as of the saturation coverage. 

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). 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

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