Mechanism of the Alloying Effect on Anode Catalysts

It is known that the electrochemical oxidation of methanol on Pt involves several intermediate steps, such as dehydrogenation, CO-like species chemisorption, OH species adsorption, chemical interaction between adsorbed CO and OH compounds, as well as CO2 evolution. The overall oxidation process of methanol to carbon dioxide proceeds through a six-electron transfer process [46]  

Much work has been put into discovering the mechanism of the promoting effect of the alloying elements (such as Ru, Sn, W, etc.) on the activity of Pt catalysts towards the MOR. Although the nature of these binary alloys remains elusive, three major theories, i.e., the bi-functional effect, the electronic (ligand) effect, and the spillover effect, were established to explain the role of the second elements on the activity improvement of Pt binary alloys. 

Watanabe and Motoo [47] proposed a bi-functional mechanism to elucidate the MOR activity enhancement of Ru ad-atoms on Pt catalyst. The bi-functional mechanism says that the unique catalytic properties of each element in the alloy combine in a synergetic fashion to yield an alloy surface that is more active than each element alone. For the MOR on a Pt-Ru catalyst, the chemisorption of methanol on Ru sites is significantly less favored than on Pt sites, while the 

The bi-functional mechanism, although simple, can explain very well the promoted MOR activity of Pt-Ru alloy catalysts. This mechanism is also well adapted by other binary alloys such as Pt-Sn [48]. It has been identified that CO does not bind to the Sn sites, with the result that OH can more easily adsorb on the Sn sites without competition from CO. The synergetic effect on Pt and Sn sites gives rise to Pt-Sn, a very active CO electrooxidation catalyst. However, the strong adsorption of OH species on Sn sites, particularly at high potentials, makes the Pt-Sn catalyst inferior to the Pt-Ru catalyst for the methanol oxidation reaction. 

The second mechanism is the electronic (or ligand) effect. It says that the additional element can modify the Pt surface electronically [50]. Chemisorption of CO on Pt involves the donation of an electron pair from the o anti-bonding orbital of CO to the unfilled 5rf-orbitals of Pt. A back donation of electrons from the Pt metal to the CO-orbitals further stabilizes their interaction. X-ray absorption spectroscopy studies [51 ] showed that alloying Sn with Pt caused partial filling of the Pt 5c -bands, resulting in a weaker chemisorption of CO. Combined with the easier adsorption of OH species on the Sn sites at low electrochemical potentials, the Pt-Sn alloy would enhance CO electrooxidation, and thus promote the 

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