Reactions at metal

Secondary Ion Mass Spectroscopic Studies of Adsorption and Reaction at Metal Surfaces Correlations with Other Surface-Sensitive Techniques... 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

It follows from the Franck-Condon principle that in electrochemical redox reactions at metal electrodes, practically only the electrons residing at the highest occupied level of the metal s valence band are involved (i.e., the electrons at the Fermi level). At semiconductor electrodes, the electrons from the bottom of the condnc-tion band or holes from the top of the valence band are involved in the reactions. Under equilibrium conditions, the electrochemical potential of these carriers is eqnal to the electrochemical potential of the electrons in the solution. Hence, mntnal exchange of electrons (an exchange cnrrent) is realized between levels having the same energies. 

The reorganization energy of the slow polarization for the reactions at metal electrodes can be calculated with the use of Eqs. (34.11). For a spherical model of the reacting ion, it is equal approximately to... 

KoperMTM, Voth GA. 1998. A theory for adiabatic bond breaking electron transfer reactions at metal electrodes. Chem Phys Lett 282 100-106. 

The topic of this review, reactions at metal surfaces, has been in general treated in a similar way to gas-phase reactivity. High level ab initio electronic structure methods are used to construct potential energy surfaces of catalytically important surface reactions in reduced dimensions. Once a chemically accurate potential surface is available, quantum or classical dynamics may be carried out in order to more deeply understand the microscopic nature of the reaction. 

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

Fig. 4. Accumulating evidence is starting to show that molecules which undergo large amplitude vibration can interact strongly with metallic electrons in collisions and reactions at metal surfaces. This suggests that the Born-Oppenheimer approximation may be suspect near transition states of reactions at metal surfaces.

Although it falls somewhat out of the scope of this paper and has furthermore been reviewed comprehensively recently,16 it would be remiss to overlook the exciting new work on chemicurrents. As we have seen for vibrational energy transfer, it is also observed that dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabati-cally by creation of excited phonons or nonadiabatically by excitation of... 

The basic model presented above is applicable to hydrogen ion discharge reactions at metals. A characteristic feature of these... 

VIII. Formation of Free Di- and Poly-ynes by Reactions at Metal Centers. 224... 

FORMATION OF FREE Dl- AND POLY-YNES BY REACTIONS AT METAL CENTERS... 

Ill Coupling Reactions at Metal Centres A. Some General Remarks... 

Turning now to the applications considered in Secs. IX, X, and XI, we note that the activation energies of desorption and migration are conveniently measured by S.P. methods, while the results obtained for the adsorption of N2O on Pt show that there is considerable scope for the application of work-function measurements in investigating catalytic reactions at metal surfaces. 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

Tafel plots have been used successfully for evaluation of slow electrochemical reactions at metal electrodes. Their application to electrochemical sensors is somewhat limited because of the mass transport boundary condition imposed by the nature of the Buttler-Volmer equation. Nevertheless, because it is simple and inexpensive, it should be always tried as the first approach, but bearing in mind its limitations. 

Reactions at metal surfaces. They have developed ReaxFF for Pt/C/H/O, Ru/H/O, and Ni/C/H/O interactions in order to enable a large-scale dynamical description of the chemical events at the fuel cell metal anode and cathode. 

Electron-Transfer Reactions at Metal and Semiconductor Electrodes... 

Chemical reaction at metal 2Fe2+ + S03 + l/202 — S042 + 2Fe3+... 

---