Volmer mechanism

It is of interest to calculate the variation of delayed fluorescence efficiencies of donor and acceptor as a function of acceptor concentration as follows. The donor triplet will be quenched by the normal Stern-Volmer mechanism and its lifetime will therefore be given by ... 

The concept of the Stern-Volmer mechanism and the resolution of that mechanism can also be applied to the donor. In Figure 6, the mechanistic quantum yield for the emission step 41 is given by... 

STERN-VOLMER MECHANISM a simple set of steps describing the deactivation of an excited-state molecule by first-order (and pseudo-first-order) steps and by quenching steps. 

The hydrogen oxidation reaction (HOR) at the anode proceeds on Pt-based catalysts and is one of the simplest reaction systems. ° Nonetheless, fundamental information of the mechanism and kinetics of HOR is still lacking. The most common mechanisms are the so-called Heyrovsky-Volmer and Tafel-Volmer mechanisms involving the following steps ... 

The difference between the Volmer equation and the Langmuir equation is that while the affinity constant remains constant in the case of Langmuir mechanism, the apparent affinity constant in the case of Volmer mechanism decreases with loading. This means that the rate of increase in loading with pressure is much lower in the case of Volmer compared to that in the case of Langmuir. 

Hydrogen under electrochemical conditions was investigated very recently [222, 223]. Santana et al. investigated the electro-oxidation of molecular hydrogen at the Pt(110)-water interface [222]. The Tafel-Volmer mechanism with a homolytic H-H bond cleavage followed by the formation of adsorbed terminal hydrogen atoms and further oxidation of the H atoms was observed by the authors. Furthermore, Santana et al. found the potential dependent activation energies for this process to be in accordance with experimental results. 

HOR studies on smooth noble metal surfaces in acidic media show that platinum is the most active. The reaction mechanism on bulk polycrystalline platimun electrode is usually assumed to proceed through a Tafel-Volmer mechanism via reactions (4) and (5), the dissociative adsorption of hydrogen being the rate-determining step (r.d.s.) [51]. In alkaline electrolytes, HOR on polycrystalline Pt follows a Tafel-Volmer sequence and the r.d.s. is the dissociative hydrogen adsorption given by reactions (4) and (6) [52]. 

CO-poisoned Pt electrodes confirm again the Tafel (r.d.s.)-Volmer mechanism for H2 oxidation on platinum. The proportionality shown in equation (37) is consistent only with the sequence proposed. 

HOR on Pt-Ru/C anodes has also been studied in a symmetrical H2IH2 polymer electrolyte membrane fuel cell, because the polarisations involved in this reaction are small [70], This cell contained a MEA in which a proton exchange membrane is sandwiched between a Pt-Ru catalysed anode and a Pt catalysed cathode. The anode (working electrode) was then fed with H2, as well as the cathode, which was used as counter and reference electrode. The analysis of the anodic polarisation scans together with the dependence of the exchange current density on the partial pressme of hydrogen allowed concluding that the Pt-Ru catalysed anode follows the Tafel-Volmer mechanism. 

HOR on a Pt/C electrode in alkaline solutions has also been studied by us in a three-electrode arrangement [71]. A semi-empirical equation in agreement with the Tafel-Volmer mechanism was proposed to quantitatively explain the current vs. potential curves obtained in a variety of experimental conditions ... 

Let us consider III as a rate-determining step (so-called Volmer mechanism). We have... 

Based on the Tafel-Heyrovsky-Volmer mechanism [5-7], there are three elementary reaction steps for the reaction, H2 2H -1- 2e , on Pt catalysts ... 

In the past, the majority of the basic laws and concepts in electrode kinetics were developed and verified by Tafel [11], Volmer [12], and Frumkin [13] using the hydrogen electrode. Two important reaction mechanisms are well recognized and experimentally validated. The first is the Volmer-Tafel mechanism, shown in Equations 3.11-3.13. The other, which is more important for the hydrogen electrode, is the Hyrovsky-Volmer mechanism, expressed in Equations 3.14-3.17. 

In this section, the reaction kinetics of the anode catalyst layer in the presence of carbon dioxide are established. These kinetics can be used in the simulation of mass transport in a similar manner to the CO kinetics. The reaction kinetics formulation given below is the same as that used in the models developed in Janssen [106] and Minutillo and Perna [107]. The Tafel-Volmer mechanism, as shown in equation (7.92) and equation (7.93), is used to describe the hydrogen oxidation. 

In 1992, Kakiuchi et al. showed, also by impedance measurements, that the apparent rate constants for anions were even faster than for cations of relatively equal radius [100] (Table 1.2). This led the authors to conclude that dielectric friction at the interface must be considered. They also showed than the Goldman-type current-potential relationship was more appropriate than the Butler-Volmer mechanism (see Section 1.3.3.1) [101]. 

When at eqnilibrium the bulk concentrations are equal, the potential difference is eqnal to the formal potential for the ion-transfer reaction, and the global activation energy barrier is symmetrical [91]. As the potential is varied, the overall driving force zF (A"(l)-A"( ) ) partially lowers the activation energy barrier and, as in the Butler-Volmer mechanism, the current is then given by... 

The HOR in acid solution is considered to proceed through either the Tafel-Volmer or Heyrovsky-Volmer mechanisms, depending on the nature of the adsorption step. If it is a purely chemical process, the mechanism is Tafel-Volmer and if it is a combined chemical and electrochemical process, it is Heyrovsky-Vohner [3]. 

The mechanism of HOR is thought to follow a similar Tafel-Volmer or Heyrovsky-Volmer mechanism, as discussed in Section 1.2.3, except with OH -mediated reactions ... 

The other studies of nonbulk metal HOR in alkaline come from Cabot et al. [25,44] who used a Pt-containing gas diffusion electrode (GDE) to closely represent the electrodes of a fuel cell in their RDE experiments. They concluded that at low overpotentials (near the OCV). the Tafel reaction is the rate-determining step in a Tafel-Volmer mechanism, with the diffusion of H2 becoming rate determining at higher overpotentials. These studies also showed that the exchange current density for HOR is lower in alkaline media for GDE. 

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