Electro-oxidation

Preparation of Pt/mesoporous carbon catalysts and their application to the methanol electro-oxidation... 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

Further convincing evidence was found by Catherino - for the formation of arsenic(IV) during the electro-oxidation of arsenic(III) in perchloric acid solution. 

In-situ ATR-FTIR spectroscopic study of electro-oxidation of methanol and adsorbed CO at Pt-Ru alloy. J. Phys. Chem. B, 108, 2654-2659. 

Another electro-oxidation example catalyzed by bimetallic nanoparticles was reported by D Souza and Sam-path [206]. They prepared Pd-core/Pt-shell bimetallic nanoparticles in a single step in the form of sols, gels, and monoliths, using organically modified silicates, and demonstrated electrocatalysis of ascorbic acid oxidation. Steady-state response of Pd/Pt bimetallic nanoparticles-modified glassy-carbon electrode for ascorbic acid oxidation was rather fast, of the order of a few tens of seconds, and the linearity was observed between the electric current and the concentration of ascorbic acid. 

The activation energy for the reaction, a, was determined for the above Pt-porous nanoparticles from the first cycle of CV measurement in the temperature range between 30 and 60 °C, Figure 13c. The activation energy was obtained from the slope, —EJR, of the Arrhenius relationship and equal to SOklmoP. This value was similar to some of those obtained for the electro-oxidation of methanol on electrodes of Pt particles dispersed in Nation [50, 51]. 

The Pt surface electro-oxidation process observed in the absence of dioxygen to form chemisorbed OH from water is driven by the potential difference at the Pt/ electrolyte interface, according to the reaction... 

Santos E, Schmickler W. 2008. Electronic interactions decreasing the activation harrier for the hydrogen electro-oxidation reaction. Electrochim Acta 53 6149-6156. 

Janik MJ, Neurock M. 2006. A first principles analysis of the electro-oxidation of CO over Pt(lll). Electrochim Acta 52 5517-5528. 

Figure 5.9 Schematic cyclic voltammogram showing the electro-oxidation of the electrode (dashed box). The curve was generated from measurements by Jerkiewicz et al. [2004] of Pt in 0.5 M H2SO4 with a reversible hydrogen reference electrode (RHE). For each separable potential range, an atomistic model of the electrode structure is shown above.

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

AU experiments to be described below are interpreted on the basis of the Langmuir-Hinshelwood (LH) mechanism for CO electro-oxidation suggested by GUman more than 40 years ago [Gihnan, 1964]. According to GUman s model, water needs to be activated on a free site on the surface, leading to surface-bonded OH ... 

Weaver and co-workers have carried out extensive smdies of CO electro-oxidation on Au single crystals [Chang et al., 1991 Edens et al., 1996]. Continuous oxidation of CO on Au starts at potentials where the formation of surface oxides or surface-bonded hydroxyl (OH) is not apparent from voltammetry. Weaver suggested the following mechanism ... 

It is well established that the main products of ethanol electro-oxidation on Pt in acidic media are acetaldehyde and acetic acid, partial oxidation products that do not require C—C bond breaking, with their relative yields depending on the experimental conditions [Iwasita and Pastor, 1994]. Apart from the loss of efficiency associated with the partial oxidation, acetic acid is also unwanted, as it constitutes a catalyst poison. 

Infrared spectroscopy has also been employed to follow the formation of acetaldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well before the onset of CO oxidation, while the onset of acetic acid formation closely follows CO2 formation [Chang et al., 1990 Xia et al., 1997]. This is readily explained by the fact that both CO oxidation and acetic acid formation require a common adsorbed co-reactant, OHads, whereas the formation of acetaldehyde from ethanol merely involves a relatively simple proton-electron transfer. 

Another metal that has attracted interest for use as electrode material is rhodium, inspired by its high activity in the catalytic oxidation of CO in automotive catalysis. It is found that Rh is a far less active catalyst for the ethanol electro-oxidation reaction than Pt [de Souza et al., 2002 Leung et al., 1989]. Similar to ethanol oxidation on Pt, the main reactions products were CO2, acetaldehyde, and acetic acid. Rh, however, presents a significant better CO2 yield relative to the C2 compounds than Pt, indicating a... 

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

Borkowska Z, Tymosiak-Ziehnska A, Nowakowski R. 2004b. High catal3rtic activity of chemically activated gold electrodes towards electro-oxidation of methanol. Electrochim Acta 49 2613-2621. 

Chen YX, Heinen M, Jusys Z, Behm RJ. 2007. Kinetic isotope effects in complex reaction networks Formic acid electro-oxidation. ChemPhysChem 8 380-385. 

Dunietz BD, Markovic NM, Ross Jr PN, Head-Gordon M. 2004. Initiation of electro-oxidation of CO on Pt based electrodes at full coverage conditions simulated by ah initio electronic structure calculations. J Phys Chem B 108 9888-9892. 

Koper MTM, Schmidt TJ, MarkovicNM, Ross PN Jr. 2001. Potential oscillations and S-shaped polarization curve in the continuous electro-oxidation of CO on platinum single-crystal electrodes. J Phys Chem B 105 8381-8386. 

Lai SCS, Koper MTM. 2009. Electro-oxidation of ethanol and acetaldehyde on platinum single-crystal electrodes. Faraday Discuss 140 399-416. 

Blais S, Jerkiewicz G, Herrero E, Feliu JM. 2002. New insight into the electro-oxidation of the irreversibly chemisorbed bismuth on Pt(lll) through temperature-dependent research. J Electroanal Chem 519 111-122. 

Hayden BE, Murray AJ, Parsons R, Pegg DJ. 1996. UHV and electrochemical transfer studies on Pt(110)-(1 X 2) The influence of bismuth on hydrogen and oxygen adsorption, and the electro-oxidation of carbon monoxide. J Electroanal Chem 409 51-63. 

For the purpose of demonstrating the effects of surface coverage by Pd, 0pd, on the rate of electro-oxidation of formic acid and the ORR, Fig. 8.17 reveals that the i versus 0Pd relationship again has a volcano-like form, with the maximum catalytic activity being exhibited for 1 ML of Pd. The examples that we have given indicate that volcano relationships are the rule rather than the exception, emphasizing the importance of a systematic evaluation of the catalyst factors that control catalytic activity. A thorough... 

Figure 8.17 Activities of Pt(l 1 l)-wML Pd electrodes from rotating disk electrode measurements, with corresponding ball models (a) electro-oxidation of formic acid in 0.1 M HCIO4 ...

Igarashi H, Fujino T, Watanabe M. 1993. Hydrogen electro-oxidation on platinum catalysts in the presence of trace carbon monoxide. J Electroanal Chem 391 119-123. 

Sugimoto W, Saida T, Takasu Y. 2006. Co-catalytic effect of nanostructured ruthenium oxide towards electro-oxidation of methanol and carbon monoxide. Electrochem Commun 8 411-415. 

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