Hydrogen Underpotential Deposition Reaction

On several noble metals (Pt, Rh, Ru, Ir, and Pd) hydrogen adsorption takes place at the potentials positive to the equilibrium potential for the hydrogen evolution reaction. This is a so-called hydrogen underpotential deposition reaction (HUPD) and indicates a strong adsorptive interaction between atomic hydrogen and the surface metal atoms. Similar UPD processes are observed for the deposition of metals on metals [237]. Certain reactions, like Cu UPD at Pt, Ru, or Rh, are used as diagnostic tools to determine the real surface area of electrocatalytic materials. 

Although the adsorption isotherms are usually complex [238], the simplest Langmuir isotherm will be presented first and later the Frumkin isotherm will be shown. The following development is similar to that in Sect. 5.1. The HUPD reaction in acid and alkaline solutions may be written as 

Assuming that the bulk and surface concentrations are the same (i.e., the hydrogen surface concentration is not affected by the passing current) the following equation is obtained  

In the case of the Frumkin adsorption isotherm, which includes lateral interactions between adsorbed hydrogen atoms, the reaction rate is described by [240] 

It is evident that an increase in the repulsion between H atoms causes a flattening of both curves and a decrease in the maximum of the pseudocapacitance. It can be added that the Frumkin isotherm was fotmd to describe HUPD at Pt(lOO) in HCIO4, Pt(llO) in H2SO4, and Pt(lll) in both acids [241]. The value of the parameter g at Pt(lll) was approximately 12. Unfortunately, the isotherms at other surfaces or metals are much more complicated. The HUPD kinetics was studied on different poly crystalline metals. It was found that the kinetics at Pt [242] was about three orders of magnitude faster than that at Ru [243], Pd [244, 245], or Rh [246] electrodes, while that at Ir was intermediate between those groups [247] (on the 

---

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. 

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]. 

The surface structure of support metal exerts a significant influence on the underpotential deposition. This phenomenon is similar to that observed for hydrogen adsorption on various noble metal surfaces. The similarity is evident as no difference can be expected between the reactions +e => and any Me" -t-e" <=> Megds- Striking differences were found, for instance, for the underpotential deposition of Cu on Pt(lll), Pt(llO), and Pt(100) crystal faces. In some cases, two (or more) distinct steps in the monolayer formation can be observed. The multistep adsorption is considered as evidence for ordered adsorption and a proof of this assumption can be obtained by ex situ electron diffraction experiments. 

Yasuyuki Ishikawa, Juan J. Mateo, Donald A. Tryk, Carlos R. Cabrera Direct molecular dynamics and density-functional theoretical study of the electrochemical hydrogen oxidation reaction and underpotential deposition of H on Pt(l 11), "Journal of Electro-analytical Chemistry", 607, 37-46 (2007). 

This type of reaction is observed for the underpotential deposition of hydrogen or metals. Unfortunately, the Langmuir isotherm is rarely found in practice, but for more complex isotherms the same impedance plots are observed only the potential dependence of the reaction parameters and Cp is different. In the presence of the... 

The major areas of application of reflectance spectroscopy have been the elucidation of reaction mechanisms, double layer studies, investigations of underpotential deposition (UPD), and studies of the electroreflectance effect (ER). This range is too large for an in depth discussion to be given here. Instead, two examples of the type of information that can be obtained will be described (a third system, hydrogen adsorption on platinum, has been discussed in Chapter 7). Those readers interested in more details are referred to a recent review [1], and the literature cited therein. 

---