Hydrogen coverage

The UCKR.ON test problem assumes the simplest uniform surface implicitly, because adsorbed hydrogen coverage is directly proportional to the partial pressure of gaseous hydrogen and adversely affected by the partial pressure of the final products. Such a simple mechanism still amounts to a complex and unaccustomed rate expression of the type solved by second order algebraic equations. 

The slope of the Tafel curve drj/d log / is only one of the criteria that are required to determine the mechanism of the h.e.r., since different mechanisms, involving different r.d.s. often have the same Tafel slope. Parameters that are diagnostic of mechanism are the transfer coefficient, the reaction order, the stoichiometric number, the hydrogen coverage, the exchange current density, the heat adsorption, etc. 

Apparent Changes in Reaction Selectivity at Low Hydrogen Coverage... 

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

Faghoni F, Goddard WA. 2005. Energetics of hydrogen coverage on group VIII transition metal surfaces and a kinetic model for adsorption/desorption. J Chem Phys 122 014704. 

Figure 3.17 Computational high throughput screening for 736 pure metals and surface alloys. The rows indicate the identity of the pure metal substrates, and the columns indicate the identity of the solute embedded in the surface layer of the substrate. The solute coverage is (a) ilVIL, (b) ML, and (c) 1 ML, and the adsorbed hydrogen coverage is also jML. The diagonals of the plots correspond to the hydrogen adsorption free energy on the pure metals. Adapted from [Greeley et al., 2006] see this reference for more details.

The compressive stress in PS decreases with annealing temperature and is reversed for temperatures above 350 °C, indicating that the PS is under tension [Su4], These changes are reversible if the hydrogen coverage is restored by a short HF dip however, at annealing temperatures above 500 °C the PS microstructure is irreversibly changed [Ha4],... 

Figure 26. Comparison of the resonance scattering from H atoms or H+ obtained by fitting the Fano line shape in HCIO4 (open squares) and H2SO4 (closed squares) with the adsorbed hydrogen coverage (closed circles) and sulfate adsorption (open circles) obtained by cyclic voltammetry. (Reproduced with permission from ref 50. Copyright 2001 The Electrochemical Society, Inc.)...

Fio. 10. High-resolution spectra of (a) adsorbed methane, (b) gaseous methane, and (c) adsorbed hydrogen. Coverage for methane 0.08 and for hydrogen 0.2. Weak and irregular absorption bands are present because of atmospheric water vapor (133). 

S. Cristol, J. F. Paul, E. Payen, D. Bougeard, S. Clemendot, and F. Hutschka, Theoretical Study of the MoS2 (100) Surface A Chemical Potential Analysis of Sulfur and Hydrogen Coverage. 2. Effect of the Total Pressure on Surface Stability, J. Phys. Chem. B 106 (2002), 5659. 

The adsorbed hydrogen coverage ( H(a)) is at present unknown. The activation energy and preexponential factor can be taken from the work of Christmann et al. (396a). These are 9 kcal mol-1 and 0.075 s 1, respectively. 

CC-AI2O3 catalyst was attributed to a decrease in hydrogen coverage brought about by an alloying effect.396... 

In order to explain this hydrogen effect it can be supposed that i) the hydrogen coverage in normal conditions is not sufficient to maintain the catalyst in the adequate reduced state, ii) the excess of hydrogen inhibits the formation of carbonaceous deposits (and the modification of the catalyst) or the strong adsorption of some reagents and products... 

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