Kinetics volcano plot

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance. 

No catalyst has an infinite lifetime. The accepted view of a catalytic cycle is that it proceeds via a series of reactive species, be they transient transition state type structures or relatively more stable intermediates. Reaction of such intermediates with either excess ligand or substrate can give rise to very stable complexes that are kinetically incompetent of sustaining catalysis. The textbook example of this is triphenylphosphine modified rhodium hydroformylation, where a plot of activity versus ligand metal ratio shows the classical volcano plot whereby activity reaches a peak at a certain ratio but then falls off rapidly in the presence of excess phosphine, see Figure... 

Sabatier-type volcano plots have been constructed for a number of different commercially relevant systemsl l. A simple kinetic expression that simulates the Sabatier result is found when one realizes that the decomposition of molecules requires a vacant site for molecular fragments to adsorb on. For instance, in the N2O decomposition reaction, the dominant surface species (most abundant reaction intermediate) is atomic oxygen (O), which is in equilibrium with the gas phase. When the slow step in the reaction is dissociative adsorption of N2O, the mean-field kinetic rate expression for N2O decomposition, normalized per unit surface area of catalyst, becomes ... 

Varying the metal acetate in the precursor, the obtained ORR activity was studied by He et al. [187]. It was found that the kinetic current density increases in the order Ni (3d 4s) Cu (3d ° 4s) < Mn (3d 4s ) < Cr (3d 4s) Co (3d 4s ) < Fe (3d 4s ). If the ORR activity is plotted versus the 3d orbital occupation a volcano plot with a maximum at 3d will be obtained (For the ease of comparison, the electron configuration of the metals is given in brackets). It should be pointed out that this order nearly reflects the relation between ORR onset potentials and the number of 3d electrons as shown for non-pyrolyzed porphyrins in Fig. 16.7. 

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

It was noted early (16) that there was a periodic relation of I o or Ini o to properties of metals across the periodic table (Fig. 15), such as lattice energy and electron work function, O. This is illustrated in one of such plots, versus O, shown in Fig. 2 for the kinetics of cathodic Hj evolution on various electrode metals. Early plots of In I o versus gave two or three distinguishable regions with an apparent maximum in 4> for increasing In Iq values (75), that is, a volcano relation lying on its side (see further comments below). 

The activity for the ORR of Pt monolayers, deposited on different single-crystal surfaces, using the Cu UPD technique [14], were investigated in acid and in alkaline electrolytes [7, 8]. Figure 5.1a show the typical ORR curves obtained for pure Pt/C and Pt monolayer on Pd/C nanoparticles, and Fig. 5.1b shows the plot of ORR activity versus Pt d-band center on different surfaces [7]. As can be seen, the ft monolayer electrocatalysts exhibited support-induced tunable activity by arising either by structural and/or electronic effects. It can be observed that the most active of all surfaces is PtML/Pd(lll), and the least active is PtML/Ru(0001). The plots of the kinetic current on the platinum monolayers on various substrates at 0.8 V as a function of the calculated d-hznA center, e, generated a volcano-like curve, with PtML/Pd(lll) showing the maximum activity (Fig. 5.1a). 

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