Formic acid oxidation catalyst supports

There have been a number of recent reviews of supports for fuel cell catalysts [12, 13, 81]. Although these focus on oxygen reduction and methanol oxidation, they provide an excellent overview of the breadth of support materials that are available, mechanistic information, and include some examples for formic acid oxidation. Various types of high surface area carbons have been most commonly used as supports for formic acid oxidation catalysts. However, there is now growing interest in the use of various metals, metal oxides, and conducting polymers. 

PANI-NTs synthesized by a template method on commercial carbon cloth have been used as the catalyst support for Pt particles for the electro-oxidation of methanol [501]. The Pt-incorporated PANl-NT electrode exhibited excellent catalytic activity and stabUity compared to 20 wt% Pt supported on VulcanXC 72R carbon and Pt supported on a conventional PANI electrode. The electrode fabrication used in this investigation is particularly attractive to adopt in solid polymer electrolyte-based fuel cells, which arc usually operated under methanol or hydrogen. The higher thermal stabUity of y-Mn02 nanoparticles-coated PANI-NFs on carbon electrodes and their activity in formic acid oxidation pomits the realization of Pt-free anodes for formic acid fuel cells [260]. The exceUent electrocatalytic activity of Pd/ PANI-NFs film has recently been confirmed in the electro-oxidation reactions of formic acid in acidic media, and ethanol/methanol in alkaline medium, making it a potential candidate for direct fuel cells in both acidic and alkaline media [502]. 

The most widely studied conducting polymer support is polyaniline (PANl), which has been shown to decrease the poisoning of Pt by COads [88]. Gharibi et al. have recently explored the factors responsible for the enhanced formic acid oxidation activity of Pt supported on a carbon/PANI composite [89]. They concluded that improvements in both electron and proton conductivities, as well as the increased methanol diffusion coefficient and decreased catalyst poisoning, could be involved. A carbon nanotubes/PANI composite [90], poly(o-methoxyaniline) [91], and polyindole [92] have recently been reported as effective supports for formic acid oxidation at Pt nanoparticles, while polycarbazole [93] has also been used to support PtRu nanoparticles. 

Because of its lower cost relative to Pt, there is growing interest in the development of supported Pd catalysts for formic acid oxidation. Synergies between Pd and PANl supports are well documented [94], while poly(diphenylamine-co-3-aminobenzonitrile) [95] has recently been shown to provide enhanced and more stable activities. Addition of 3-aminobenz(Miitrile to poly(diphenylamine) was found to improve the dispersion of the Pd, while both polymers eliminated the current decay seen over 1 h for carbon supported Pd. 

Considerable progress has been made in the development of supported Pt-based catalysts for formic acid oxidation, with a variety of Pt alloy, intermetallic, and surface-modified catalysts showing impressive increases in performance relative to Pt/C. The use of Au, Bi, Pb, or Sb as the second metal has been shown to be particularly beneficial, although it is not clear yet whether any of these metals combined with Pt will provide sufficient long-term durability, nor which type of modification of the Pt stmcture (alloy, intermetallic, or surface modified) is most suitable. 

Future advances in the catalysis of formic acid oxidation will benefit from further development of our understanding of the fundamental processes involved via single crystal and computational studies. Refinement of synthesis methods to produce nanoparticles with the most active and durable geometries and structures will allow fine-timing of catalysts. Continued discovery of support effects and advances in the understanding of such effects will create additional opportuitities to improve performances, lower costs, and enhance durability. 

Morales-Acosta D, Ledesma-Garcia J, Godinez LA, Rodriguez HG, Alvarez-Contreras L, Arriaga LG (2010) Development of Pd and Pd-Co catalysts supported on multi-walled carbon nanotubes for formic acid oxidation. J Power Sources 195 461-465... 

Liu Z, Zhang X (2009) Carbon-supported PdSn nanoparticles as catalysts for formic acid oxidation. Electrochem Commun 11 1667-1670... 

Morgan RD, Salehi-khojin A, Masel RI (2011) Sirperior formic acid oxidation using carbmr nanotube-supported palladiirm catalysts. J Phys Chem C 115 19413-19418... 

The incorporation of H MoOs as co-catalyts with Pt in the PAni support matrix prepared by electrodeposition was investigated for methanol, formic acid, and formaldehyde oxidation [323]. In addition to surface area enhancement-related increase in the oxidation current densities for the three Cl molecules, both Pt/PAni and Pt-HxMoOs/PAni revealed true catalytic effects as well, according to Wu et al. [323]. Figure 4.68 illustrates the case of formic acid oxidation. Compared to Pt/Pt, the polyaniline support brought about an increase of both peak currents on the forward scans, at 0.3 and 0.6 V vs. SCE, followed by a pronounced expansion of the oxidation wave on the return scan. The presence of HxMoOs in the catalyst formulation increased the formic acid electrooxidation rate, especially in the low potential region of the forward scan, between 0 and 0.5 V vs. SCE (Figure 4.68), whereas on the return sweep the oxidation wave was close to the Pt/PAni case. 

Figure 4.68. The effect of PAni support and HxMoOj co-catalyst on formic acid oxidation on Pt. 1) Pt/Pt, 2) Pl/PAni, and 3) Pt-H MoGj/PAni. 0.1 M HCOOH - 0.5 M H2SO4, 100 mV s [323]. (Reprinted from Journal of Power Sources, 145(2), Wu YM, Li WS, Lu J, Du JH, Lu DS, Fu JM, Elecfrocatal5hic oxidation of small organic molecules on polyanQine-Pt-HxMo03, 286-91,2005, with permission from Elsevier.)...

The method of catalyst preparation (especially in the case of alloys) and the catalyst interaction with the polymer support matrix play important roles in determining the resultant electrocatalytic effect. The catalyst/support couple PtSn/PAni (0.5 pm thickness) with PtSn synthesized by electroreduction at 0.1 V vs. RHE was found to be an effective catalytic system for formic acid oxidation, lowering the anode potential by over 100 mV compared to pure Pt/PAni and PtRu/PAni [324]. Moreover, the oxidation of formic acid on PtSn/PAni commences at low potentials, in the hydrogen adsorption region, around 0.1-0.2 V vs. RHE. 

Poly-3-methylthiophene (P3MT) was produced by electropolymerization on graphite and utilized as a Pt and PtPb catalyst support for formic acid oxidation [337]. In chronoamperometry experiments, after 300 s at a constant potential of 0.8... 

The present chapter has presented a comprehensive review of electrode kinetic and catalytic aspects associated with methanol, ethanol, and formic acid oxidation. The prevalent point of view in selecting and organizing the vast amount of information in this area was that of practical applicability in order to advance the technology of direct fuel cells. Emphasis was placed on the catalytic system , starting with catalyst preparation methods and focusing on the interaction of catalyst/support/ionomer/chemical species. The development of catalytic systems was followed, from fundamental electrochemical and surface science studies to fuel cell experiments (whenever experimental data was available). Advances in both fundamental electrocatalysis and electrochemical engineering hold promise for the development of high-performance and cost-effective direct liquid fuel cells. 

A number of authors have investigated the use of Pt/Au alloy and Pt-decorated Au (shell-core) nanoparticles as catalysts for formic acid oxidation in acid solution. Park et al. [94] described chemical reduction techniques for preparing Pt/Au alloy, pure Au, and Pt-modified Au nanoparticles on carbon supports. The Pt/Au alloy and Pt-modified Au nanoparticles showed higher activities for formic acid oxidation than pure Pt, especially at low potentials in the region of 0.2 V. These results of Park et al. [94] are supported by the data of Kristian et al. [95]. Although the detailed mechanism of operation of these new electrocatalysts remains to be fully clarified, the combination of improved performance, combined with a substantial reduction in the use of Pt, are attractive features of Pt-decorated Au nanoparticle electrocatalysts from a fuel cell viewpoint. 

Labeling studies indicated that the obtained formic acid was originated from both substrate and solvent. When the catalyst was supported onto silica to provide a heterogeneous catalyst, methane is oxidized at 80 °C and 32 bars CH4 to CH2O (up to 1.1 TON) and HCOOH (up to 27.3 TON). 

Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crys-tallographically ordered a-Al203, and the porous amorphous t]- and y-Al203. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of phenol or the dehydration of formic acid. 

Methanol, Formaldehyde, and Formic Acid Adsorption/Oxidation on a Carbon-Supported Pt Nanoparticle Fuel Cell Catalyst A Comparative Quantitative OEMS Study... 

The adsorption and oxidation of the Ci molecules methanol, formaldehyde, and formic acid over a carbon-supported Pt/C fuel cell catalyst under continuous electrolyte flow have been investigated in a quantitative, comparative online DBMS study. 

Precious metals (Pt, Pd, Ru) deposited on supports have been reported to be active for catalytic wet air oxidation (CWAO). Gallezot et al [9] have shown that platinum catalysts supported on carbon could decompose formic, oxalic and maleic acids very easily, at... 

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