Coverage, atomic hydrogen

Determine the coverages of atomic hydrogen and CO for the following three gas mixtures ptot = 1 bar and pco= 1,10,100 ppm. The operational temperature of the fuel cell is 80 °C and the following information on the adsorption... 

It is not a trivial point that 0fj vs. E curves are practically linear. In a reversible system the electrode potential can be linked to the activities (concentrations) of the potential-determining substances. In the system being discussed, this substance is atomic hydrogen. According to the Nemst equation we have E = const - (RTIF) X In Cjj. It follows that the degree of coverage, 0, is linearly related to the logarithm of concentration c in the solution ... 

Jachimowski TA, Meng B, Johnson DP, Weinherg WH. 1995. Thermal desorption studies of high-coverage hydrogen overlayers on Ru(OOl) created with gas-phase atomic hydrogen. J Vac Sci Technol A 13 1564. 

In the Cu/Ru system, ruthenium may function as a reservoir for atomic hydrogen, which is accessible via spillover to neighboring copper. Kinetically controlled spillover of hydrogen from ruthenium to copper (5) is consistent with the observed optimum reaction rate at an intermediate copper coverage. 

The rate enhancement observed for submonolayer Cu deposits may relate to an enhanced activity of the strained Cu film for this reaction due to its altered geometric and electronic properties. Alternatively, amechansim whereby the two metals cooperatively catalyze different steps of the reaction may account for the activity promotion. For example, dissociative Hj adsorption on bulk Cu is unfavorable due to an activation barrier of approximately 5 kcal/mol . In the combined Cu/Ru system, Ru may function as an atomic hydrogen source/sink via spillover to/from neighboring Cu. A kinetically controlled spillover of Hj from Ru to Cu, discuss above, is consistent with an observed optimum reaction rate at an intermediate Cu coverage. 

Fig. 11.11. Surface coverage with dangling bonds (6>db), hydrogen-terminated (cross linked) sites (6 n) and methyl-terminated sites (6 113) as a function of the atomic hydrogen flux density...

In Fig. 11.11, model results of the surface coverages 0db, 0 and 0H3 are shown as a function of the atomic hydrogen flux density. The important point here is that a substantial fraction of the surface is dynamically covered with methyl groups (<9h3)- In the framework of our model, these need to be cross linked with each other before they can be activated for further chemisorption events. Not visible from the logarithmic scaling is the fact that at zero hydrogen flux, 6 113 is equal to one. It is this dynamical surface cov-... 

As explained above, the requirement of low anodic overpotential implies that 1, co. and thus the steady-state coverage by hydrogen atoms under such... 

McCabe and Schmidt note that the P state for H/Pt(lll) could equally well be described by second-order kinetics or by first-order kinetics with a variable E. However, H2/D2 exchange experiments confirm that P2 and p- are both atomic states. Adsorbed atomic hydrogen could still desorb as molecular hydrogen with first-order kinetics if only a single surface site is involved in the desorption. This seems to be the case for H on Pd at high surface coverages (see later), but is unlikely to be important for the 03-state which only desorbs when 0 < 0.5. 

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