Surface coverage, with hydrogen

In order to reveal the nature of deactivation, the potential of the catalyst slurry was continuously measured during the partial oxidation of alcohols. Cyclic voltammetric measurements [16] were also performed in the same aqueous alkaline solution with model (unsupported) catalysts for the interpretation of the potential values. The experiments revealed that the oxidation of alcohols may be divided into three groups. The basis of classifying is the oxidation state of proroot-ed catalyst and its surface coverage with hydrogen or oxygen (OH) during reaction. 

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...

Fig. 3.4 Characterization of Pt Niilll) surface red curves) compared to Pt(lll) blue curves) (a) In situ SXS profiles (confirmed existence of (lll)-skin structure as weU as improved stability over Pt(l 11) in designated potential range) (a ) Segregation profile (between 0.05 and 1.0 V first atomic layer 100 at.%, second 48 at.%, third 82 at.% and fourth 75 at.% of Pt) (b) Cyclic voltammograms and (c) surface coverage by hydrogenated species and oxide species. Reprinted with permission from [23], copyright 2007 by American Association for the Advancement of Science...

Naito et al. studied hydrogenation with use of adsorption measurements, mass spectrometry, and microwave spectroscopy for product analysis. In the room temperature deuteriation of propene, butene, and 1,3-butadiene, the main products were [ H2]-propane, [ H2]-butane, and l,2-[ H2]-but-l-ene, respectively. They showed, using mixtures of H2 and D2, that deuterium was added in the molecular form and at a rate proportional to the partial pressure of D2, as opposed to D surface coverage the reaction rates were zero order in hydrocarbon. They proposed, therefore, in contrast to the model of Dent and Kokes for ethene (but note in this case that reaction rate was 0.5 order in hydrogen pressure and proportional to ethene surface coverage), that hydrogenation proceeded by interaction of adsorbed hydrocarbon with gas-phase D2, that is by an Eley-Rideal mechanism. 

The data points to the importance of two parameters, that of adhesive interactions between adsorbate and surface, and lateral interactions between adsorbate molecules on the surface. In general, the free energy of adsorption (at zero coverage) is of the order of 10-30 kJ mol for alcohols, carboxylic acids and esters on ferrous surfaces, consistent with hydrogen bonding between surface and adsorbate. 

This expression takes into account that the products of an elementary act and the reaction as a whole are different, namely, an adsorbed hydrogen atom and an H2 molecule in the gaseous phase, as well as that adsorbed particles participate in the elementary act. For gaseous H2, Th2 = 1 for mercury-type cathodes, for which the surface coverage with adsorbed hydrogen is quite small, yn, = 1. If y for adsorbed H3O and H2O, as well as the same particles in the bulk are made equal, the kinetic activity coefficient will be equal to unity. Even if this relation is not strictly accurate, the value will be close to unity and only a minor variation with a change in the solution composition is likely ... 

Figure 21. Relationship between hydrogen overpotential shift on lead electrode and surface coverage with adsorbed...

Stonehart and Kohlmayr investigated the HER rate on a rotating Pt bead electrode as a function of surface coverage with CO. They concluded that the electrocatalytic activity of Pt for the hydrogen ionization is a quadratic, but not a linear, function (1 - poison) of the free surface fraction, thus suggesting a rate-determining dual-site step, probably the Tafel reaction. However, this correlation was questioned later by Breiter. ... 

Hydrogen dissociates on all metals already at low temperatures only on copper is the dissociation activated. The metal-hydrogen bond strength varies little with the substrate metal, differences in surface structure are more important. As the recom-binative desorption of hydrogen atoms is a rapid process, the surface coverage of hydrogen is usually low at typical reaction temperatures. 

There is still a lack of experiments on the hydrogen cathodic reactions with well-controlled surface coverages by hydrogen, oxygen, and adsorption blockers, characterizing particularly the influence of the blockers on the occupancy of the subsurface sites. Such studies are needed to fully understand the surface processes involved in H-induced cracking of metals and alloys. 

The effect of the surface coverage with adsorbed hydrogen (0) on the reaction rate is considered only through its effect on the fraction of the free surface. We thus assume that the Langmuir isotherm is valid. This simplification is admissible since we are primarily interested in cathodes which are poor adsorbers of hydrogen, i.e. the cathodes with 6 1. 

Nishihara C and Nozoye H 1995 influence of underpotentiai deposition of copper with submonolayer coverage on hydrogen adsorption at the stepped surfaces Pt(955), Pt(322) and Pt(544) in sulfuric acid solution J. Electroanal. Chem. 396 139-42... 

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