Volcano relationships

For the purpose of demonstrating the effects of surface coverage by Pd, 0pd, on the rate of electro-oxidation of formic acid and the ORR, Fig. 8.17 reveals that the i versus 0Pd relationship again has a volcano-like form, with the maximum catalytic activity being exhibited for 1 ML of Pd. The examples that we have given indicate that volcano relationships are the rule rather than the exception, emphasizing the importance of a systematic evaluation of the catalyst factors that control catalytic activity. A thorough... 

Plotting qOER as a function of AG2 = AG° - AG110 will therefore lead to a universal volcano relationship independent of the catalyst material (see Figure 3.2.4). This is just a convenient way of plotting the information already presented in Figure 3.2.3. 

We have summarized these developments in two recent papers the first addresses the topic of structure sensitivity in combination with BEP relationships (14) and the second addresses the Sabatier principle (27). Sabatier-type volcano relationships can be deduced for activity as a function of adsorption energy, and they can also be used to predict trends regarding deactivation of Fischer-Tropsch catalysts by C—C recombination reactions (28). We refer to these texts as background information to the material presented here. 

The anodic reaction, Hz oxidation, will be discussed in the next section. Here we will devote a few words to the cathodic reaction, Hz evolution, the main aim being to show that the effectiveness of different metals in promoting this reaction can be understood in terms of a classical volcano relationship (see Chapter 3). Generally speaking, the elementary steps in the Hz-evolution reaction are the... 

With careful experimentation one can get around these problems, however, and independent of whether the Tafel or the Heyrovsky equation obtains in a particular case, one may expect the volcano relationship to apply when we plot the exchange current density vs. the M-H bond strength for a variety of metals. This is indeed what is observed (see Fig. 5.40) at intermediate values of the M-H bond strength, the evolution of H2 is clearly the most effectively catalyzed. 

It would be wrong to suggest that no intercourse at all between the twin disciplines has taken place. The concepts of relating bulk (metallic) properties of a catalyst to the rate of a reaction at its surface, as described in detail by Bond, have been quoted and adopted by the majority of workers in the field of electrocatalysis. Concepts such as the Volcano relationship of Balandin or the Spillover of adsorbed hydrogen as suggested by Boudart have been widely and successfully transferred to electrochemical reactions. The experimental techniques employed to investigate catalytic processes have been... 

A very interesting, although not unusual feature in catalysis, is the finding that the activity of the different metals for the ORR with respect to specific properties (descriptors) such as the strength of oxygen adsorption on those metals or the Pauli electronegativity follows a volcano relationship. Both geometric aspects such as particle size, " inter-particle distance, exposed surface or electrolyte adsorption videsupra), and electronic aspects such as d-band... 

Much of the work done thus far has involved the use of Pt electrocatalysts and it should be clear from our discussion that surface adsorbates play crucial roles in a number of electrocatalytic reactions. Adsorbate (such as surface-oxide) effects in the traditional field of electrocatalysis in aqueous media have usually been tackled by developing Pt-alloy electrocatalysts (for the HOR, ORR, MOR, and COOR) and it will be interesting to see how ft-aUoy (as well as non-Pt) electrocatalysts perform during electrocatalysis in RTlLs. Compton has already made a step in this direction, comparing the activity of Pt electrocatalysts for the HER in RTlLs to that of other metals. In the case of the ORR in RTlLs, it will be interesting to explore whether Pt alloys are more active than Pt and whether a volcano relationship between the electrocatalyst composition and activity can be identified, as it has for the ORR in aqueous media. In addition, given that the COOR and MOR coincide with oxidation of ft surfaces, it may be natural to assume that inclusion of a readily oxidisable metal into the Pt electrocatalyst can aid in lowering the reaction overpotential but such work is yet to be done. 

Contemporary with the proposal of the enthalpy of the redox transition, Matsumoto and coworkers suggested the overlap between the eg orbital (or a band) and the orbital of the hydroxide ion could explain the trends in the oxygen evolution of perovskites [34, 35]. Recently, researchers predicted and verified a volcano relationship with an 6g occupancy close to unity for optimum oxygen evolution activity for perovskites [10]. In addition, they suggested that the covalency of the metal-oxygen bond could serve as a secondary predictor for the catalytic activity. 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

Sabatier and Balandin had predicted a relationship between catal)dic activity and heat of adsorption. If a solid adsorbs the reactants only weakly, it will be a poor catalyst, but if it holds reactants, intermediates or products too strongly, it wiU again perform poorly. The ideal catalyst for a given reaction was predicted to be a compromise between too weak and too strong chemisorption. Balandin transformed this concept to a semiquantitative theory by predicting that a plot of the reaction rate of a catal)Tic reaction as a function of the heat of adsorption of the reactant should have a sharp maximum. He called these plots volcano-shaped curvesl This prediction was confirmed by Fahrenfort et al." An example of their volcano-shaped curve is reproduced in Fig. 9.1. They chose the catalytic decomposition of formic acid... 

Fig. 4 Depth-age relationship of the ice cores from Fiescherhom glacier [12] and Colle Gnifetti [13, 14], Besides annual layer counting and radiocarbon ( C) dating, two types of time markers were used Saharan dust events (labeled by the year only) and volcanic eruptions (labeled by year and name of volcano). Depth is given in water equivalent. This is the amount of water contained in the ice core which is calculated using fim and ice density, respectively, both increasing with depth...

In the present chapter, we have attempted to illustrate how surface bonding and catalytic activity are closely related. One of the main conclusions is that adsorption energies of the main intermediates in a surface catalyzed reaction is often a very good descriptor of the catalytic activity. The underlying reason is that we find correlations, Brpnsted-Evans-Polanyi relations, between activation barriers and reaction energies for a number of surface reactions. When combined with simple kinetic models such correlations lead to volcano-shaped relationships between catalytic activity and adsorption energies. 

The heat of adsorption of the intermediate remains the most straightforward parameter on which we can base our reasoning. Therefore, the volcano curve is the relationship which in principle can guide in making predictions. Miles [72] sug-... 

Since in most cases adsorption heats are not available experimentally, other parameters can be correlated with the activity, e.g., the calculated heat of hydride formation. Under similar circumstances a volcano curve can still be obtained for example for intermetallic compounds of Ti [53] (Fig. 7), or a linear correlation with the activation energy can be found [226] (Fig. 8). These correlations still point to the existence of volcano-shaped relationships with a maximum around an estimated value... 

These relationships, when incorporated into microkinetics models of catalytic reaction cycles, enable remarkable new predictive insights into the control of heterogeneously catalyzed reactions. Predictive models of catalytic activity as a function of catalyst composition as well as reaction conditiorvs have been constructed (22-24). The resultant volcano curves can be considered to be an application of the Sabatier principle (25,26). 

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