Surface heterogeneity, electrocatalysts

Regarding the electrocatalyst, the similar concepts about the catalytic activity can be defined in the similar ways as Eqns (3.1)—(3.4). Actually, electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. However, in this book, we are only focused on the heterogeneous electrocatalysts. The role of electrocatalyst is to assist in transferring electrons between the electrode catalytic active sites and reactants, and/or facilitates an intermediate chemical transformation. One important difference between catalytic chemical reaction and electrocatalytic reaction is that the electrode potential of the electrocatalyst can also assist in the reaction. By changing the potential of the electrocatalyst, which is attached onto the electrode surface, the electrocatalytic activity can be enhanced or depressed significantly. 

In view of the complexity of heterogeneous systems, none of the above techniques will be able to supply, by itself, a complete atomic-level description of surface phenomena. A multi-technique approach has been perceived by many as most appropriate for fundamental studies in electrochemical surface science (30-2). Since none of the existing electrochemical laboratories are adequately equipped to perform a comprehensive experimental study, collaborative efforts between research groups of different expertise are burgeoning. Easier access to national or central facilities are also being contemplated for experiments which cannot be performed elsewhere. The judicious combination of the available methods in conjunction with the appropriate electrochemical measurements are permitting studies of electrocatalyst surface phenomena unparalleled in molecular detail. 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

In the first group belong the techniques which are also used in heterogeneous catalysis for determining the surface area of catalysts. Two such techniques are widely used The Brunauer-Emett-Teller (BET) method, based on the physical adsorption of N2 or Ar at very low temperatures [8, 44] and the H2 or CO chemisorption method [8, 44], The first method leads to the total catalyst surface area, whereas the second leads to the specific (active metal) surface area. In the case of supported electrocatalysts (e.g., Pt/C electrocatalysts used as anodes in PEM fuel cells) the two techniques are complementary, as the former can lead to the total electrocatalyst surface... 

A positive BE shift implies a more negatively charged Pt atom, and thus CO chemisorption is weaker (better elec-trocatalytic activity) on such alloy electrocatalysts. This is consistent with the predominantly electron acceptor nature of CO adsorption on most metals [23] and with the recently established rules of electrochemical promotion [23, 99] and of classical promotion in heterogeneous catalysis [100] which predict that CO binding is weakened on high work function, that is, negatively charged surfaces. 

Scherson, Palenscar, Tolmachev and Stefan provide a critical review of transition metal macrocycles, in both intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. An introduction is provided to fundamental aspects of electrocatalysis, oxygen reduction, and transition metal macrocydes. Since the theoretical and experimental tools used for investigation of homogeneous and heterogeneous electrocatalysis are considerably different, these topics are given separate discussion. The influence of the electrode surface on adsorbed macrocydes, and their influence on mechanism and rates of 02 reduction is treated in detail. Issues related to pyrolyzed macrocydes are also described. 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

Activated carbons possess high BET surface areas (400 to 2500 m /g) and micropore volumes (up to 1.2 cm /g), which makes them particularly attractive adsorbents. They are also used as supports for heterogeneous catalysts and sometimes, electrocatalysts [20]. In a number of patents it was claimed that addition of either activated carbons or activated carbon-supported Pt to the CLs composed of carbon black-supported catalysts improves cell performance [21],... 

As mentioned before, reactions at the surface of catalysts in electrochmical cells have much in common with heterogeneous chemical reactions. In both cases a mass transfer of species to and from the surface of the catalyst occurs. If more than just one reaction step has to be catalyzed, two or more different catal5dic materials are required in the form of a bi- or multifunctional electrocatalyst. 

The concept of a catalyst is quite clear in chemistry. It was first defined by Ostwald in the nineteenth century as a substance that only modifies the velocity of a chemical reaction without suffering any chemical change in the process. This definition is based on a comparison between the rates of the reaction in the presence and in the absence of the catalyst. Two different types of catalysis can be defined homogeneous catalysis, in which the catalyst and all the species involved in the reaction are in the same phase and heterogeneous catalysis, when the catalyst constitutes a different phase than that containing the reaction species and the reaction takes place on the surface of the catalyst. An inaccurate extrapolation of the definition of the catalyst to electrocatalyst would indicate that all the electrode materials are electrocatalyst since the electrode reactions are heterogeneous reactions in which an inert material (the electrode) is always present and normally does not suffer any chemical change in the... 

---