Electrooxidation catalysts

An efficient ethanol electrooxidation catalyst should combine at least two features (i) high tolerance to CO and other intermediate species generated over the surface of the electrocatalyst during alcohol electrooxidation and (ii) ability to break the C-C bond of the ethanol molecule under mild conditions. The most relevant features for the designing of CO tolerant electrocatalysts have been described above namely, Pt modification with more oxophilic metals such as Ru, Mo or Sn renders the best electrocatalysts. This is because such oxophilic atoms promote the formation of -OfT. species (involved in the CO j oxidation reaction) at potentials that are more negative than that on pure Pt (Eq. 9.17). Among those, Sn-modified Pt electrocatalysts are the most active formulations. There is also widespread consensus that the PtsSn phase is the most active one in the CO reaction and early stages of the ethanol electrooxidation process. ... 

To overcome CO deactivation, alloys of Pt with more oxophilic elements have been investigated as methanol electrooxidation catalysts. PtRu bifunctional catalysts are presently the most active for methanol oxidation. It is believed that Ru serves the role of removing COads as CO2 [93] ... 

Lowde DR, Williams JO, Atwood PA, Bird RJ, McNicol BD, Short RT. Characterization of electrooxidation catalysts prepared by ion-exchange of platinum salts with surface oxide groups on carbon. J Cbem Soc Faraday Trans 1 1979 75 2312-24. 

Moore JT, Com JD, Chu D, Jiang R, Boxall DL, Kenik EA, Lukehart CM. Synthesis and characterization of a PtsRuiA ulcan carbon powder nanocomposite and reactivity as a methanol electrooxidation catalyst. Chem Mater 2003 15 3320-5. 

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. 

Andrew, M.R., McNicol, B.D., Short, R.T., and Drury, J.S. (1977) Electrolytes for methanol-air fuel cells. I. The performance of methanol electrooxidation catalysts in sulphuric acid and phosphoric acid electrolytes. Journal of Applied Electrochemistry, 7 (2), 153-160. 

Since the initial papers on RU4, this complex has been extensively characterized and shown to function when immobilized on carbon nanotubes as a water electrooxidation catalyst [92], in solution as a homogeneous catalyst for visible-light-driven water oxidation [93], and when interfaced with [Ru(bpy)3] +-sensitized Ti02 surfaces [94], RU4 has shown no evidence of hydrolytic decomposition to the metal oxides (RUO2, WO3) in any of these studies. The mechanism of water oxidation by [Ru(bpy)] + has also been probed in some depth and the principal catalytic cycle for water oxidation involves sequential oxidation of the resting oxidation state of RU4, which is Ru(IV)4, to the oxidation state Ru(V)4 [95], followed by O2 evolution. 

This side reaction makes it difficult to control the anode reaction, and reduces the number of electrons released in BH4 electrooxidation. The reduction of hydrolysis on the anode catalyst is important for the improvement of DBFCs. Thus, the development of a new BH4 electrooxidation catalyst that does not stimulate BRt hydrolysis is required. A Rh porphyrin catalyst was investigated. Previously, it is demonstrated that carbon-supported Rh porphyrin catalysts have strong activity for the electrooxidation of several small molecules (CO, glucose, and oxalic acid) [33-36]. On the other hand, many hydride complexes of Rh have been reported [37]. Thus, it is expected that Rh porphyrin catalysts would exhibit BH4 electrooxidation activity. The catalytic activity of Rh porphyrin catalyst for BH4 electrooxidation was investigated. This new catalyst is different from other noble metal electrocatalysts such as Pt/C or PtRu/C in that the molecule is an active site. Hence, only a small amount of Rh is needed in Rh porphyrin catalysts for sufficient activity. 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

The mechanism of electrooxidation of methanol is now nearly well understood. From the considerable effort made during the past 20 years, it is now possible to propose electrocatalysts with acceptable activities for DMFCs, even though further improvement is still necessary. Despite considerable research efforts, R-Ru alloys are the only acceptable catalysts for the electrooxidation of methanol at low anode potentials. Two questions still remain unanswered ... 

In many macroscopic systems, the massive behavior is a convoluted answer to many microscopic features of the system. For example, the catalysis of the electrooxidation of an organic molecule may be generated by some local arrangement of atoms on a catalyst, defined at the atomic level. If some hypotheses are available to explain the enhancement of the reaction, this can be checked by inserting these hypotheses in the model. In a first approximation, a qualitative explanation is often sought. If this is... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

Arenz M, Mayrhofer KJJ, Stamenkovic V, Blizanac BB, Tomoyuki T, Ross PN, Markovic NM. 2005. The effect of the particle size on the kinetics of CO electrooxidation on high surface area Pt catalysts. J Am Chem Soc 127 6819-6829. 

Demarconnay L, Brimaud S, Coutanceau C, Leger JM. 2007. Ethylene Glycol electrooxidation in alkaline medium at pluri-metalhc FT based catalysts. J Electroanal Chem 601 169-180. 

Dubau L, Coutanceau C, Gamier E, Leger JM, Lamy C. 2003a. Electrooxidation of methanol at platinum-mthenium catalysts prepared from colloidal precursors Atomic composition and temperature effects. J Appl Electrochem 33 419-429. 

Schmidt TJ, Gasteiger HA, Behm RJ. 1999. Methanol electrooxidation on a colloidal PtRu-alloy fuel-cell catalyst. Electrochem Commun 1 1-4. 

Waszczuk P, Solla-Gull6n J, Kim HS, Tong YY, Montiel V, Aldaz A, Wieckowski A. 2001a. Methanol electrooxidation on platinum/ruthenium nanoparticle catalysts. J Catal 203 1-6. 

Andreaus B, Eikerling M. 2007. Active site model for CO adlayer electrooxidation on nanoparticle catalysts. J Electroanal Chem 607 121-132. 

Gavrilov AN, Savinova ER, Simonov PA, Zaikovskii VI, Cherepanova SV, TsirUna GA, Parmon VN. 2007. On the irrfluence of the metal loading on the stmcture of carbon-supported PtRu catalysts and their electrocatal3ftic activities in CO and methanol electrooxidation. Phys Chem Chem Phys 9 5476-5489. 

Jusys Z, Kaiser J, Behm RJ. 2001. Electrooxidation of CO and H2/CO mixtures on a carbon supported Pt catalyst—A kinetic and mechanistic study by differential electrochemical mass spectrometry. Phys Chem Chem Phys 3 4650-4660. 

Jusys Z, Kaiser J, Behm RJ. 2002a. Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation. A DBMS study. Electrochim Acta 47 3693-3706. 

Lima A, Coutanceau C, Leger J-M, Lamy C. 2001. Investigation of ternary catalysts for methanol electrooxidation. J Appl Electrochem 31 379-386. 

Wang H, Jusys Z, Behm RJ. 2004. Ethanol electrooxidation on a carbon-supported Pt catalyst Reaction kinetics and product yields. J Phys Chem B 108 19413-19424. 

Wang H, Jusys Z, Behm RJ. 2006. Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRuo,2 catalysts A quantitative DEMS study. J Appl Electrochem 36 1187-1198. 

Frelink T, Visscher W, vanVeen JAR. 1995. Particle-size effect of carbon-supported platinum catalysts for the electrooxidation of methanol. J Electroanal Chem 382 65-72. 

Wang et al240 reported the electrooxidation of MeOH in H2S04 solution using Pd well-dispersed on Ti nanotubes. A similar reaction was studied by Schmuki et al.232 (see above), but using Pt/Ru supported on titania nanotube which appear a preferable catalyst. Only indirect tests (cyclic voltammetry) have been reported and therefore it is difficult to understand the real applicability to direct methanol fuel cell, because several other aspects (three phase boundary to methanol diffusivity, etc.) determines the performance. 

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