Alloy catalysts adsorption energies

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Table 2 Activation energies (kJ moP ) corresponding to the adsorption, Eai. desorption, E -i, and disproportionation reaetion, 2, of carbon monoxide over silica-supported Pt, Rh, and Pto.5o-Rho.5o alloy catalysts...

An even more pronounced difference in light-off performance between pre-oxidation and pre-reduction is seen for C0/AI2O3 (see below). The reason that pre-reduced Pd/Co/AbOs and Pt/Co/Ab03 start to react at lower temperatures than the pre-oxidised samples (see Figure 5) is most likely due to that the initial reaction on cobalt sites starts at lower temperatures for the pre-reduced samples than for the pre-oxidised catalysts. The pre-reduction may also induce alloy formation with lower adsorption energy of CO compared with the noble metal [18]. It is also possible that 0-vacancies form on the cobalt oxide which promote the dissociative adsorption of O2. Reaction with CO will then proceed on the noble metal or at the interface between Pt (or Pd) and cobalt oxide [9-10]. 

As a result of a DFT calculations of a variety of model ternary PtRuM alloy catalysts yielded detailed adsorption energies and activation... 

With Equation 5.24, we can calculate a useful approximation of the reversible potential on a specific catalyst surface using the adsorption energies of each species involved in the reaction, if we know the reversible potential of a redox reaction in aqueous solution, or we can calculate the shift of the onset reversible potential between two unlike catalyst surface compositions if the absolute reversible potential in aqueous phase for a specific redox reaction is not known. For example, the change in onset reversible potential due to a Pt surface and an alloy will be... 

Catalytic layers in the anode and cathode are also critical components in a PEMFC. Platinum (Pt) or a Pt-Ru alloy supported by nano-carbon particles is often used as the catalyst in the anode [1-6]. Favorable electronic properties have been suggested by density-functional theory studies, which have shown fliat flie CO adsorption energy is the lowest on the Pt monolayer located above Ru compared with the adsorption energies on pure Pt, pure Ru, and a Pt-Ru mixed surface layer over Pt [7, 8], The bifunctional mechanism [4] suggests that Ru provides an active surface for oxidative removal of adsorbed CO at the neighboring Pt sites. Thus, a... 

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