The Adsorption of Hydrogen

Various examples of chemisorption of hydrogen on metals or on oxides have already been mentioned in Secs. V,8,b through 11. In this section we shall discuss some problems related to the transformation of physically adsorbed hydrogen to chemisorbed hydrogen and the relations between various forms of chemisorbed hydrogen. We shall restrict ourselves to some selected problems and refer to the articles of Eley (142) and of Beeck (143) in this series for many other questions concerning chemisorption. 

This chemisorption probably does not lead to the formation of covalent bonds between C atoms and H atoms such bonds are formed at higher temperature, hence by a chemisorption process with an appreciably high activation energy and a far higher heat of adsorption. 

Burstein (145) in an extensive study of the exchange between H2 and Da on charcoal found that this process is governed by the dissociative adsorption of hydrogen in the range of temperatures between 500°K. and 

Another possible explanation was recently offered by Zwietering (151), who discusses the possibility that type-A adsorption is due to hydrogen atoms having a dipole with the negative pole pointing away from the surface, and that type-B represents the type where the dipole has a reversed direction. Type-A adsorption is comparable to chemisorption on pure-metal surfaces in the form of wires or of films obtained by sublimation the sign of the dipoles of the hydrogen chemisorption at very low 

Photoelectric measurements at higher temperatures have often shown that chemisorbed hydrogen forms dipoles pointing with their positive ends away from the surface, also in those cases where in later investigations at low temperatures (liquid-air temperatures) contact potential measurements revealed dipoles of reversed direction. It is, therefore, not to be excluded that both types of chemisorption may occur at the same metal surfaces and it is to be expected then that the latter type, which in the case of iron is called 4-type chemisorption  

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Miehelsen H A, Rettner C T and Auerbaeh D J 1993 The adsorption of hydrogen at eopper surfaees A model system for the study of aetivated adsorption Surface Reacf/onsed R J Madix (Berlin Springer) p 123... 

Catalysis and the Adsorption of Hydrogen on Metal Catalysts Otto Beeck... 

As the adsorption of hydrogen is rather weak, the corresponding term in the denominator may be omitted. The rate expression shows that the reaction is suppressed by H2S. Hence, the most active catalysts (which appear at the top of the volcano curve of Fig. 9.7)... 

Such information can be obtained from cyclic voltammetric measme-ments. It is possible to determine the quantity of electricity involved in the adsorption of hydrogen, or for the electrooxidation of previously adsorbed CO, and then to estimate the real surface area and the roughness factor (y) of a R-C electrode. From the real surface area and the R loading, it is possible to estimate the specific surface area, S (in m g ), as follows ... 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Figure 3.2. Potential energy diagram of chemisorption for the adsorption of hydrogen on nickel (after Le Page, 1987).

Eeulner P, Menzel D. 1985. The adsorption of hydrogen on Ru(OOOl) Adsorption states, dipole moments and kinetics of adsorption and desorption. Surf Sci 154 465. 

Electrode processes can be retarded (i.e. their overpotential is increased) by the adsorption of the components of the electrolysed solution, of the products of the actual electrode reaction and of other substances formed at the electrode. Figure 5.43 depicts the effect of the adsorption of methanol on the adsorption of hydrogen at a platinum electrode (see page 353). 

The adsorption of hydrogen, ethylene, and CO on Pt(l 1 1) was extensively studied. Molecular hydrogen dissodatively adsorbs on the catalytic Pt(l 1 1) surface... 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

Fig. 8. The adsorption of hydrogen on copper-nickel catalysts as a function of the copper content. The circles represent the total amount of hydrogen adsorbed at room temperature at 10-cm pressure. The triangles represent the amount of strongly adsorbed hydrogen, i.e., the amount not removed by a 10-min evacuation at room temperature following the completion of the initial adsorption isotherm. The amount of strongly adsorbed hydrogen is determined as the difference between the initial isotherm and a subsequent isotherm obtained after a 10-min evacuation (74).

Studies on the adsorption of hydrogen from the gas phase had provided strong evidence for the existenee of two forms of adsorbed hydrogen and the AC impedance studies were supported by the results of the new LSV and CV techniques. The early measurements using the voltammetry methods were hampered by the use of impure electrolytes which resulted in ill-defined hydrogen adsorption and desorption peaks but the realisation of the need for a clean electrochemical system soon resulted in the routine observation of the now familiar twin Hads peaks. 

Hence, it appeared that the story was complete and the adsorption of hydrogen on platinum understood. However, considerable controversy was arising even as the definitive experiments were being performed. In 1959, Schuldiner carried out kinetic measurements on the H2-evolution reaction... 

Hydrogen has a low solubility in solvents for example, at ambient conditions, only 0.018 and 0.078 mL of gaseous H2 dissolves into each milliliter of water and ethanol, respectively. However, the solubility is much more pronounced in metals. Palladium is particularly notable in this respect, which dissolves about 1000 times its volume of the gas. The adsorption of hydrogen in steel may cause "hydrogen embrittlement," which sometimes leads to the failure of chemical processing equipment [4],... 

Square lattice gases with two- and three-body interactions a model for the adsorption of hydrogen on Pd(I00) ... 

The applied electrode potential has been shown to have an effect on both the XANES and EXAFS of PtRu catalysts. The variations of the Pt d band vacancy per atom, (/7j)t,s, with potential over the range 0.0—0.54 V vs RHE for both the poorly mixed 1 1 PtRu/C catalyst investigated by McBreen and Mukerjee ° and a well mixed 1 1 PtRu/C catalyst studied by Russell et al. were less than that for a pure Pt/C catalyst. McBreen and Mukerjee attributed this difference to a reduction in the adsorption of hydrogen on the Pt sites of the alloy catalyst. The results also provide evidence of an electronic effect upon alloying Pt with Ru. The effects on the Ru XANES were much less significant, but some evidence of a change to a higher oxidation state at potentials above 0.8 V was observed. ... 

The Adsorption of Hydrogen on Metal Films Partially Covered with... 

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