Catalysts for methanol oxidation

The development of highly efficient methanol fnel cells depends on a nnmber of scientific aspects (1) the development of more highly active catalysts for methanol oxidation at temperatnres not over 60 to 70°C (desirable in cells without ruthenium, which is in short supply) (2) the development of selective catalysts for the oxygen electrode (i.e., of catalysts insensitive to the presence of methanol) and (3) the development of new membrane materials having a lower methanol permeability. 

It was seen when studying mixed systems Pt-WOj/C and Pt-Ti02/C that with increasing percentage of oxide in the substrate mix the working surface area of the platinum crystallites increases, and the catalytic activity for methanol oxidation increases accordingly. With a support of molybdenum oxide on carbon black, the activity of supported platinum catalyst for methanol oxidation comes close to that of the mixed platinum-ruthenium catalyst. 

Rao V, Simonov PA, Savinova ER, Plaksin GV, Cherepanova S V, Kryukova GN, Stimming U. 2005. The influence of carbon support porosity on the activity of PtRu/Sibunit anode catalysts for methanol oxidation. J Power Sources 145 178-187. 

Drew, K., Girishkumar, G., Vinodgopal, K., and Kamat, P.V. (2005) Boosting fuel cell performance with a semiconductor photocatalyst Ti02/Pt-Ru hybrid catalyst for methanol oxidation. Journal of Physical Chemistry B, 109 (24), 11851-11857. 

The electrocatalytic activity of the nanostructured Au and AuPt catalysts for MOR reaction is also investigated. The CV curve of Au/C catalysts for methanol oxidation (0.5 M) in alkaline electrolyte (0.5 M KOH) showed an increase in the anodic current at 0.30 V which indicating the oxidation of methanol by the Au catalyst. In terms of peak potentials, the catalytic activity is comparable with those observed for Au nanoparticles directly assembled on GC electrode after electrochemical activation.We note however that measurement of the carbon-supported gold nanoparticle catalyst did not reveal any significant electrocatalytic activity for MOR in acidic electrolyte. The... 

Choisnet, J Abadzhieva, N Stefanov, P Klissurski, D Bassat, JM Rives, V Minehev, L. X-ray photoeleetron spectroscopy, temperature-programmed desorption and temperature-programmed reduction study of LaNiOs and La2Ni04+s catalysts for methanol oxidation. J. Chem. Soc., Faraday Transactions, 1994, Volume 90, 1987-1991. 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

The most active catalysts for methanol oxidation are presently based on bifunctional systems such as Pt-Ru. Yet the evaluation of a metal-support interaction would require the analysis of a simpler catalytic system (i.e., a monometallic catalyst) this would avoid the interference of all those aspects such as degree of alloying, changes in crystallographic parameters, chemical state of the promoting element, which significantly affect the activity, and thus a comprehensive interpretation of the data actually available. 

As in the case of catalysts for oxygen reduction, the surface properties of Pt catalysts for methanol oxidation are governed by both the preparation method and the nature of carbon support. As discussed above, the presence of surface groups such as carboxylic, carbonyl, phenolic, lactone, and pyrone functionalities determine... 

The growth of Ru islands on Pt(hkl) was found to be substrate-dependent, so that the Ru layer is almost completely in the fonn of a monolayer on Pt(llO), whereas the two- and three-dimensional growth is facilitated on the other two low-index Pt surfaces, especially on Pt(lll). The Ru-Pt(lll) is more effective catalyst for methanol oxidation " than the other two surfaces decorated with Ru, because the edge of a Ru island is the active site in methanol oxidation therefore, controlling the extent of the multidimensional islands is of a particular importance for fuel cell catalysis. 2 ... 

Control of reaction paths on catalyst surfaces by optimizing the structure and electronic properties is a key issue to be solved in surface science. Iron/molybdenum oxides are used as industrial catalysts for methanol oxidation to form formaldehyde selectively. The iron /molybdenum oxide catalyst consists of Fe2(Mo04)3 and M0O3, and shows kinetics and selectivity similar to those of M0O3 for methanol oxidation [Ij. It suggests that Mo-O sites play an important role in the reaction. M0O3 has a layered structure along a (010) plane, but the (010) surface is not reactive because it has no unsaturated Mo site [1]. On Mo metal surfaces such as (100) [2,3] and (112) [4], major products in methanol reactions were H2 and CO. Therefore, we considered that partial oxidation of Mo sites is needed for the selective oxidation of methanol. We have reported that methanol reaction pathways on Mo(l 12) could... 

A COMPARISON OF IRON MOLYBDATE CATALYSTS FOR METHANOL OXIDATION PREPARED BY COPRECIPTATION AND NEW SOL-GEL METHOD... 

Petrini, G., F.Gaibasa, M. Petrera and N. Pemicone, Study of Iron(II) Molybdate as Precursor of Catalysts for Methanol Oxidation to Formaldehyde , in Chemistry and Uses of Molybdetmm, Ed. H. F. Barry and P. C. Mitchell, p.437. Climax Molybdenum Company, Ann Arbor, Michigan, USA (1982). 

N. Burriesci, F. Garbassi, M. Petrera, and G. Petrini, "Solid State Reactions in Fe-Mo Oxide Catalysts for Methanol Oxidation During Aging in Industrial Plants", in Catalyst Deactivation, eds. Delmon Froment, Elsevier, Amsterdam, 1980, pp. 115-126. 

V vs RHE(Reversible Hydrogen Electrode)) in Pt catalysts is well defined, while for PtRu catalysts it is less defined, because the adsorption/desorption hydrogen peaks are not developed on Ru. The double layer region of the PtRu/C catalysts is larger than for Pt/C, both because of the presence of more oxygenated species and also of a larger surface area due to a smaller particle size. Furthermore, the performance of the PtRu/C catalysts, for methanol oxidation, shows a superior activity as compared to the Pt catalysts in the... 

The catalysts for methanol oxidation and oxygen reduction were 40% Pt-20% Ru/Vulcan XC 72 (E-TEK Inc.) and 20% Pt/Vulcan XC 72 (Electrochem Inc.), respectively. The reaction layers were prepared by directly mixing in an ultrasonic bath a suspension of Nafion ionomer in water with the catalyst powder the obtained paste was spread on carbon cloth backings. Pt loading was 2 mg cm in both electrodes. The MEA was manufactured by pressing the electrodes onto the composite membrane at 130° and 50 atm [22]. 

As discussed above, Ru is an excellent promoter of Pt catalysts for methanol oxidation and Pt nanoparticles decorated with Ru show better performance in the oxidation process than do commercially available Pt/Ru (1 1) alloy nanoparticles. In order to compare the electronic properties of these two systems, we undertook further "C NMR investigations of the Pt/Ru alloy nanoparticles covered by CO, and have obtained new insights into the electronic alterations caused by Ru addition to Pt surfaces. 

Much research has been carried out on catalysts for methanol oxidation (see also Section 9.3.4) to find a catalyst which can avoid the poisoning effect of the CO species. Several promoters have been found to increase the activity of the Pt catalyst. One of the most important and most investigated promoter is Ru. A bimetallic alloy consisting of Pt and Ru supported on carbon has thus far been one of the major research interests in DMFC technology. The action of Ru can be explained as follows. The adsorption of H2O molecules at Ru surfaces takes place with lower overvoltages (Eq. 9-35). 

Y-M. Tsou, L. Cao, E. De Castro, Novel High Performance PtRu Alloy Catalysts for Methanol Oxidation and CO Tolerance Applications, Abs. 675,205th Meeting, 2004 The Electrochemical Society, Inc., 2004. 

B. Rajesh, K. R. Thampi, J.M. Bonard, AJ. McEvoy, N. Xanthopoulos, H.C. Mathieu, and B. Viswanathan, Pt particles supported on conducting polymeric nanocones as electro-catalysts for methanol oxidation, J. Power Sources, 133,155-161 (2004). 

Starting point for the emergence of electrocatalysis was the discovery that hydrocarbons could be oxidized at low temperatures (this fact had not been a part of the Ostwald scenario). Then it was discovered that synergistic effects were operative in the use of ruthenium-platinum catalysts for methanol oxidation, and that compounds such as platinum-free metalloporphyrins were useful catalysts for certain electrochemical reactions in fuel cells. Hopes were expressed that in the future expensive platinum catalyst could be replaced. Again, in the attempts of commercial realization of these discoveries considerable difficulties were encountered, which led to a period of disenchantment and pessimism in 1970s and 1980s. It had been demonstrated beyond doubt that, fundamentally, hydrocarbons could be oxidized at low temperatures, but practical rates that could be achieved were unrealistically low. It had also been demonstrated that fuel cells could be made to work without... 

For further improvement of fuel cell performance, first of all obvious problems mentioned in the chapters of this book need to be solved, such as raising the activity of catalysts for methanol oxidation, lowering the sensitivity of oxygen-electrode catalysts toward methanol, and building new improved membranes. In addition, a number of fundamental problems need to be solved in the field of electrocatalysis ... 

Therefore, a catalyst for methanol oxidation should be able to (a) dissociate the C-H bond and (b) facilitate the reaction of the resulting residue with some O containing species, especially CO, to form On a pure Pt electrode, which is known to be a good cat-... 

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