Hydrogen volcano curve

Fig. 9-1 Hydrogen volcano curve logarithm of exchange current density of H2 reaction vs. enthalpy of hydrogen adsorption at different metals. (Courtesy V. M. Schmidt, Mannheim, Germany)...

The volcano curve is bounded by the rate of dissociative adsorption of CO and hydrogenation of adsorbed carbon. This is illustrated in Figure 1.9. 

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)... 

A complete theory of electrocatalysis leading to volcano curves has been developed only for the process of hydrogen evolution and can be found in a seminal paper by Parsons in 1958 [26]. The approach has shown that a volcano curve results irrespective of the nature of the rate-determining step, although the slope of the branches of the volcano may depend on the details of the reaction mechanism. 

The apparently contradictory behavior of Fe, Co and Ni is in fact explained by their ability to absorb atomic hydrogen, which reduces the strength of H adsorption. The shift of Fe, Co and Ni from the descending to the ascending branch of the volcano curve indicates that D(M-H) > V2l3(H-H) in the gas phase but < /2D(H-H) in solution under H2 evolution. These phenomena of H absorption are presumably responsible for the time-dependent properties of some electrodes during H2 discharge. 

Hydrogen evolution is the only reaction for which a complete theory of electrocatalysis has been developed [33]. The reason is that the reaction proceeds through a limited number of steps with possibly only one type of intermediate. The theory predicts that the electrocatalytic activity depends on the heat of adsorption of the intermediate on the electrode surface in a way giving rise to the well known volcano curve. The prediction has been verified experimentally [54] (Fig. 2) and the volcano curve remains the main predictive basis on which the catalytic activity is discussed [41, 55],... 

The volcano curve is based on the performance of simple metals, and the heat of adsorption is the factor which turns out to be responsible for the change in activity from metal to metal. However, the difficult issue from a theoretical point of view is the identification of the properties of metals which govern the magnitude of the adsorption heat. A number of correlations have been proposed, based on different electronic or structural properties of metals [56-65]. The aim has been to establish a reactivity scale from which predictions on the behaviour of unknown materials could be made. It appears that the correlation between the electron work function and the activity of metals for hydrogen evolution is very likely to be the most reliable [54,66], since it has been verified several times by different authors independently [57,62,67],... 

Predictions based on the volcano curve do not show any general validity since only a few combinations give more active materials. In particular, it is to be noted that Ni, Co and Fe appear on the right branch of the volcano curve if the A//ad measured in the gas phase is used [54], Thus, they would be on the same branch as Mo. They appear on the left branch of the curve [66] if the heat of adsorption is derived in situ from electrochemical measurements [80]. This indicates that these metals are modified by hydrogen discharge, probably because of hydrogen absorption which renders the surface adsorption bond weaker. The same possibility has been pointed out for Pt [81]. Thus, the verification of the prediction based on the volcano curve does not appear to be entirely convincent. 

Jaksic has tried to propose a predictive basis for the hydrogen reaction beyond the volcano curve. The idea is based on Brewer-Engel s theory [82] for bonding in metals and intermetallic phases. According to this theory, a maximum in bond strength and stability of the intermetallic phases is expected as a metal lying at the beginning of a transition period in the Periodic Table (e.g., Ti or Zr) is combined with... 

It thus appears that there may be a basis for some predictions which can guide in the selection of components for composite materials, but the theoretical basis for discussions goes always back to the principles of the volcano curve in the sense that a relative increase or decrease in activity is customarily explained by recurring to the features of such a curve [92], Therefore, a theory is needed to describe the dependence of the adsorption strength of hydrogen on the electronic properties of composite materials. However, before a sound theory can be proposed, it is necessary that the experimental picture be freed from the many obscurities, ambiguities and irreproducibilities due to the scarce characterization of the surface of various materials, and to the insufficient identification of various factors which can influence electrode kinetics. 

FIGURE 2 Schematic representation of normalized methanation rate rcH, as a function of X. The Sabatier maximum in the volcano curve results from the competition between the increase in rate of Cf hydrogenation and the decrease of 6 when X increases. Adapted from Ref (53). 

It should be recognized that while the adsorption of the reactants on the catalyst surface is a necessary feature in catalytic processes, adsorption, of itself, does not necessarily lead to a catalyzed reaction. For a reaction between adsorbed species to take place the adsorption of the reactants cannot be too strong nor too weak as illustrated by the volcano curve in Fig. 2.4. When the adsorption is too weak the amount of adsorbed species is too low to sustain the reaction. When strong adsorption occurs the substrate cannot leave the surface and the catalyst becomes poisoned for further reaction. For instance, hydrogen can be adsorbed on almost all metals and ethylene on most of them yet only a few are eapable of promoting the hydrogenation of ethylene. On metals such as Ti, V, Cr, Mo or W,... 

Bulk (Unsupported) M0S2 and H 52- Promoted. The promoted catalysts display the phenomenon of synergy. The catalytic activity in hds and also in hydrogenation of olefines and aromatics is greater than the sum of the activities of the individual components and reaches a maximum value at a particular catalyst composition for both bulk and supported catalysts. An activity-composition plot is typically a volcano curve. The composition of the catalyst is expressed preferably as mole fraction of promoter (x = [M] / ([M] + [M ]) where M is the promoter and M is Mo or W. Synergy is discussed further in connection with supported catalysts in the following Section. 

The catalytic reactivity of a material can be described by Sabatier s principle [41, 87], It states, that catalytic reactions proceed best if the interaction between reactant/adsorbate and surface is neither too strong, nor too weak ° thus the optimum reactivity is related to the heat of adsorption. Sabatier s principle is reflected in volcano curves [88], where the reactivity of different elements towards a particular reaction is plotted as a function of its position in the periodic table, and thus its elec-tron(ic) configuration [87]. As a result of experimental and theoretical observations plotted as volcano curves, often Pt turns out to be the optimum catalyst material [89]. This is the reason for the choice of Pt in this thesis with respect to CO oxidation [1, 20] and for the hydrogenation of ethene [21, 35], where Pt is known to be ideal. The optimum reactivity of Pt (compared to other al-metals) is further well described using the popular d -band model [18-21]. The model describes trends in the interaction between an adsorbate and a fil-metal surface to be governed by the coupling to the metal rf-bands [90]. 

Figure 1.10 An example of the volcano curve for the hydrogenation of ethylene. Open points, evaporated metal films filled points, silica-supported metals circles, first row transition metals squares, second row transition metals triangles, third row transition metals. (After G.C Bond 1974)...

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.

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