Underpotential deposited hydrogen

Figure 8.13 In situ electrochemical SXS characterization of PtsNi) 11) and Pt(l 11) surfaces (a)XRV measurements forPtsNitlll) at the (0, 0, 2.7) (filled squares) andPt(lll)at (1, 0, 3.6) (open triangles) (b) surface coverage by underpotentially deposited hydrogen (Hupd) and hydroxyl species (OHad) calculated from the cyclic voltammograms (c) segregation profile ascertained from the SXS measurements. (Reprinted with permission from Stamenkovic et al. [2007a]. Copyright 2007. American Association for the Advancement in Science.)...

MoPt3 -H pd (underpotentially deposited hydrogen) 66.7 279 Electrochemical 2004JAK/KRA... 

The spectra show two resonances located around 2065 cm and 1965 cm . The first corresponds to linearly bound CO, the latter was assigned to CO in a bridge-bound position [786]. The conversion of CO in a bridging location into species atop has been identified with SFG [794]. The combination of the high light intensity (e.g. free-electron lasers) and the possibility to obtain absolute spectra without any modulation technique has allowed the detection of generally weak bands like those of over- and underpotential deposited hydrogen on a platinum electrode [795, 796]. Studies of coadsorption of cyanide anions and cetylpyridinium cations on Au(l 11) and Au(210) revealed marked differences [797]. CN is bound... 

In electrochemistry, adsorbed hydrogen is denoted as either the underpotentially deposited hydrogen, Hypd, that is the H adlayer formed under thermodynamic equilibrium conditions where the coverage is changed reversibly with the potential applied or the over-potentially deposited hydrogen. Hqpd. as defined by Conway and co-workers [102] for the... 

It has been established that H d is the reactive intermediate in the HOR, and therefore the kinetics of the HOR is mainly determined by the interaction between H d and the Pt surface atoms. There are two different possible states of adsorbed hydrogen. One is the Hupd (the underpotentially deposited hydrogen), which is the strongly adsorbed stated formed on the surface at potentials more positive than the Nernst potential, and the other is the H pd (the overpotentially deposited hydrogen), which is the weakly adsorbed state formed close to or negative with respect to the Nernst potential. ... 

The electrocatalysis of HCOOH oxidation on Pd constitutes a special case and represents an exception when compared to CH3OH and C2H5OH. Some of the early work on this topic has been presented in Section 4.1. The cyclic voltammogram of Pd(lll) in 0.1 M H2SO4 is shown by Figure 4.32 [167], Key features are i) pronunced underpotentially deposited hydrogen peaks (adsorption/desorption) around -0.05 V vs. SCE, ii) ordered SOd adlayer formation at +0.1 V vs. SCE, and iii) Pd surface oxidation at 0.8 V vs. SCE and reduction of the surface oxides at 0.4 V on the cathodic scan, generating irreversible surface defects [167]. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

A cyclic voltammogram ofZn underpotential deposition on Pt(llO) in sulfuric acid solution is more reversible than that in perchlorie acid solution with respect to the change of potentials, as shown in Fig. 27. The was evaluated as about 1 V. Since Zn underpotential deposition in acidic solutions was observed in the adsorbed hydrogen region, the pH was increased to separate Zn underpotential deposition from the hydrogen adsorption region. At pH 4.6, Zn underpotential deposition was observed, as shown in Fig. 29. The different cyclic voltammogram appearances of Zn underpotential deposition on Pt(lll) at different pHs is discussed in Section n.4. 

In this chapter, hydrogen adsorption, particularly observed on Pt electrodes, was not treated as an underpotential deposition phenomenon. However, from a theoretical point of view, it may provide a breakthrough insight into underpotential deposition. Since the underpotential deposition ofM on M is quite characteristic among different combinations of M and M, together with a change in the kind of anions in the solution, a theoretical approach, which requires simplification, is still limited, and more experimental clarification is needed for theoretical work. However, the jellium model was successfully used to describe the lattice contraction of adsorbate T1 and Pb on Ag(lll) as underpotential deposition. ... 

Ft partial and full monolayers have been deposited onto alternative metal particle cores. Brankovic, Wang, and Adzic found Ft spontaneously deposited onto hydrogen-reduced Ru particles at a surface coverage of 0.11 and 0.50. An alternative method of depositing Ft onto Fd, Fd alloy, and Au particles has been developed by Sasaki et al. and Zhang et al. This involves the deposition of a Cu overlayer by underpotential deposition followed by galvanic displacement by Ft. Complete monolayers of Ft have been claimed and the technique has been extended to preparation of mixed monolayers of Ft and other precious metals. ... 

Underpotential deposition 339 van der Waals force 56, 175 in the hydrogen molecular ion problem 175 tunneling, in 186... 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Discharge of H+ ions reveals the role played by the structure of Ag surface in the underpotential deposition (UPD) of hydrogen. Figure 4 shows that in the acidic medium, the activity of the electrode surfaces toward H+ discharge decreases in the series Ag(lll) > Ag(poli) > Ag(lOO) > Ag(llO), following the decrease in the surface density of atoms [48,189]. 

Underpotential deposited layers have a strong effect on the electro-catalytic properties of electrodes for surface-sensitive reactions such as organic oxidations, hydrogen evolution, oxygen reduction, etc. A review on this subject has recently been published by Adzic [131a, b]. 

Fig. 14. Activity of Pt for hydrogen evolution in 1 M HC104 as a function of the coverage with underpotential deposited Pb. A change in mechanism with Pb coverage is indicated. After ref. 303, by permission of Pergamon Press.

The importance of electronic interaction as the basic mechanism leading to synergetic effects is also suggested by results showing that the simultaneous underpotential deposition of Cd and Tl on Ag(lll) results in an enhancement of the hydrogen evolution rate while inhibitory effects are shown by the two kinds of adatoms separately [317]. 

Hydrogen-terminated boron-doped diamond (BDD) has emerged as a unique electrode material displaying, among other characteristics, extraordinary low chemical and electrochemical reactivity over a wide potential range allowing studies to be performed in PEO/Li salt solutions without interference from underpotential deposition, alloy formation and/or intercalation processes [67],... 

---