Pt surface sites

The voltammograms are unique for each basal plane and are often used as a diagnostic tool for the identification of primary Pt-surface sites [2-6,54]. The structure-sensitivity of the voltammogram serves as a means to characterize the preferential orientation of a given sample [55]. The voltammogram for platinum nanoparticles obtained by different preparation procedures is shown in Figure 6.13. The difference in voltammetric behavior displays the influence of preparation procedures on the fraction of (100), (110) and (111) atomic sites on the surface of the nanoparticles. 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

If a catalyst mass contains only one type of catalytic site we shall call it a monofunctional catalyst. By one type is meant that every catalytic site or surface exhibits the same qualitative catalytic property as to what reaction or reaction steps it can catalyze. We shall concern ourselves only, of course, with reaction steps which are thought to be relevant to the reaction examined. For example, we normally assume that platinum/charcoal is a monofunctional catalyst in the hydrogenation of olefins. (For the present purpose we need not be concerned about the quantitative equivalence of every Pt-surface site, i.e., whether or not there is uniformity or a spectrum of catalytic effectiveness for the same reaction among different platinum sites.)... 

We have applied our Multitrack reactor, an advanced TAP-like reactor system,23 to evaluate the number of redox-active Pt surface sites (Ptsurf redox) on Pt/A Os catalysts as a function of Pt dispersion. The number of Ptsurf redox sites was compared with the total number of Ptsurf sites determined by conventional volumetric CO chemisorption. 

The supporting electrolyte type and concentration of formic acid impact the observed overpotentials. The two most commonly used supporting electrolytes are either H2SO4 or HCIO4. Specific bisulfate anion adsorption onto Pt surface sites from H2SO4 adversely increases the onset potential of formic acid electrooxidation. The top of Fig. 3.8 shows an unfavorable increase in the onset potential for OHads in the anodic cycle by 0.1 V on a Pt ( 2.3 nm)/C catalyst in the presence of 0.1 M H2SO4 versus 0.1 M HCIO4 [65]. In the presence of 0.5 M formic acid, the initial response in the forward anodic sweep at potentials below 0.4 V versus SCE is... 

A smaller percentage of Pt surface sites participate in the dehydrogenation pathway, limiting activity to high overpotentials that promote OHads formation to complete the dehydration reaction. 

To strongly accelerate process (27) further to secure a larger fraction of CO-free Pt surface sites and thereby a higher rate of (25) + (26), a large jump in anode overpotential, which results in the fall in cell voltage by 0.3-0.4 V seen in Figs 32 and 33, is required. 

In this case, the rate law has been experimentally determined to be first order with respect to CO and also first order with respect to the Pt surface sites available for reaction (second order overall). Since we would like to know how fast the Pt surface is poisoned, we write the rate law in terms of the CO surface coverage, 3> ... 

This gas-solid reaction is a self-limiting process that leads, at most, to a monolayer of CO gas coverage on the catalyst. Once all of the available Pt surface sites have reacted, no further CO will adsorb on the surface of the Pt. This is expressed by the fact that when the CO surface coverage (O) goes to 1, the reaction rate (dO/df) goes to zero. 

Thus, even for a relatively low level (100 ppm) of CO impurity in a flowing gas stream, this Pt surface would be 80% poisoned within about 1 h Its effectiveness as a catalyst would then be greatly diminished as the adsorbed CO would block other gases from accessing catalytic Pt surface sites. 

The effect of NH3 on the hydrogen oxidation reaction (HOR) has not been fully identified. Uribe et al. [25] reported that NH3 did not adsorb significantly on Pt surface sites, thus not affecting the HOR. But Halseid et al. [27] observed that the presence of NH3 shifted the potentials of the hydrogen adsorption process on a Pt surface in aqueous solutions. Clearly, more work is required to understand the effect of NH3 on the HOR. 

In catalysis prepared on supports containing sulfonic surface species, the interaction of adsorbates such as D2 or CO with them has been evidenced by means of TPD tests of adsorbed probes. Co-adsorption of both CO and D2 reveals that pre-adsorbed CO blocks Pt surface sites for D2 chemisorption on them, but the pathways for surface diffusion of D2 toward the support remain operative. 

---