Bifunctional effects

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

Bifunctional effect CO oxidized by the alloying element is effective at dissociating H2O and providing OH to react with CO adsorbed on Pt and thus decreasing CO coverage. 

Bulk Pt alloys for the electrooxidation of formic acid have been less frequently studied compared to underpotential deposition (upd) modified Pt surfaces. The Pt50Ru5o surface was again found to be one of the most active Pt-Ru surfaces. Underpotentially deposited metals, such as Bi, Se, Sb, were studied as reaction modifiers for Pt surfaces and provided significant electrocatalytic activity increases. Electronic factors (ligand effects) rather than bifunctional effects were held responsible for these activity modifications, because the metal coverages that caused the activity gains were extremely small. 

Shubina and Koper also claimed that for the electrochemical oxidation of CO on bimetallic catalyst surfaces, the bifunctional effect is the most dominant mechanism. The more oxophylic element Ru or Sn seems to provide the oxygen donor, which is commonly believed to be adsorbed hydroxyl group, via activation of adsorbed water at a smaller electrode overpotential,... 

So far, the atomistic modeling on oxidation of CO and methanol has been aimed to elucidate mechanisms for (1) the bifunctional effect, in which the unique catalytic properties of each of the elements in the alloy combine in a synergetic fashion to yield a more active surface and (2) the ligand or electronic effect, in which the interaction between dissimilar atoms yield alters electronic states and hence results in a more active catalytic surface. In parallel to the study on the OER, study of oxidation of CO and methanol has seen a progress from vapor phase models to liquid phase models. However, polymer cluster has not been involved in the ab initio models. 

In conclusion, the computational study of ternary Pt-Ru-X alloys suggests that future strategies toward more active electrocatalysts for the oxidation of methanol should be based on a modification of the CO adsorption energy of Pt (ligand effect), rather than on the enhancement of the oxophilic properties of alloy components (enhanced bifunctional effect). 

The modification of platinum surfaces by foreign metal atoms promotes the oxidation of methanol either in UHV conditions or in the electrochemical environment. This promotion model has been mainly discussed in electrochemistry using the third body model [37], the ligand effect [38], or the bifunctional effect [9,39,40], A theoretical review on the inclusion of metal reaction promoters was undertaken by Anderson et al. [41] and later discussed in [42],... 

Koyama N, Koshikawa T, Morisaki N, Saito Y, Yoshida S (1990) Bifunctional effects of transforming growth factor-P on migration of cultured rat aortic smooth muscle cells. Biochem Biophys Res Commun 169 725-729... 

The CO oxidation on PtRu is enhanced primarily because of the so-called bifunctional effect, first suggested by Watanabe and Motoo [36]. Ru is supposed to act as active sites to activate water at reduced overpotential, as OH binds stronger to Ru than to Pt (see Section 16.3.3), and CO adsorbed on Pt then reacts with OH adsorbed on Ru. It is anticipated that CO mobility will play an important role in the catalysis of this reaction. In order to test this presumption, we have employed DMC simulations to model CO diffusion on PtRu bimetallic electrodes, with the specific aim to assess the role of CO diffusion in CO electrooxidation on these surfaces [37]. 

Two modes of action are expected from metals alloyed to platinum (1) the so-called bifunctional effect. When the second metal is more oxophilic it will form the oxygenated species from water at lower potentials, thus diminishing the operational potential of the anode and (2) the intrinsic effect. The second metal may diminish the electron density of the 5d band of platinum weakening the adsorption strength of poisonous species (like CO) and making them easier to oxidize. 

Additionally, Christoffersen et al. [88] observed that a weaker CO-Pt bond also makes the CO more reactive at higher overpotentials, i.e. the LH mechanism is favorable. The effect of the alloying atom (M) on mixed Pt-M metals is to promote the formation of OHs combined with a favored oxidative removal of COs and formation of COOHs. The relative contributions of ligand effect and bifunctional effect are discussed in several quantum chemistry calculations [67, 89]. A ligand effect plays some role in the oxidation of CO on Pt-Ru, but it is unimportant on Pt-Sn [67]. 

Another important effect governing the reactivity of bimetallic sirrfaces is the bifunctional effect. For example, bismuth adsorption on Pt(ll 1) produces a marked decrease of the onset potential for CO oxidation. This catalytic enhancement has been explained... 

The catalytic mechanism of PtRu has been interpreted in terms of a so-called bifunctional effect of the surface in which Pt sites adsorb and dissociate methanol-forming CO and Ru atoms adsorb and dissociate water molecules, thus providing, at low potentials, oxygen atoms needed to complete the oxidation of adsorbed CO to CO2 [75]. The facts above, showing an increased rate of adsorption of methanol in the presence of Ru, indicate that the bifunctional mechanism alone does not fully describe the catalytic action of ruthenium. 

Fuel Cell Reactions. Low temperature fuel cells such as proton exchange membrane fuel cells (PEMFC) or direct methanol fuel cells (DMFC) employ large amounts of noble metals such as Pt and Ru. There has been extensive research to replace these expensive metals with more available materials. A few studies considered transition metal nitrides as a potential candidate. In an anode reaction of DMFC, Pt/TiN displayed the electroactivity for methanol oxidation (53). Pt/TiN deposited on stainless steel substrate showed the high CO tolerance in voltammogram performed with a scan rate of 20 mV/s and 0.5 M CH3OH - - 0.5 M H2SO4 electrolyte. The bifunctional effect of Pt and TiN for CO oxidation was mentioned as observed between Pt and Ru in commercial PtRu/C catalysts. 

Bifunctional effect [45-49] Water activation is first initiated by the second component (M) to form M-OH, which then reacts with an adjacent CO adsorbed on the Pt atom to complete CO oxidation and clear the Pt surface for hydrogen oxidation. This mechanism can be expressed by equation (2.20) and equation (2.21) ... 

Barnard and Laidlaw for example have attempted to explain the mode of action of hydrolases by a bifunctional effect. One can discuss catalysis with respect to amino and carboxyl groups without implying that these groups are necessarily the ones which are responsible for the catalytic action. 

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