Non-precious Metal Catalysts for Methanol, Formic Acid, and Ethanol Oxidation

4 Non-precious Metal Catalysts for Methanol, Formic Acid, and Ethanol Oxidation 

The role of organometallie eomplexes as co-catalysts with Pt for methanol electrooxidation was mentioned earlier. Can they also act alone and replace Pt completely For the anodic oxidation of methanol, ethanol, and formic acid, studies looking at CO oxidation by various porphyrin complexes with Ir, Rh, and Co in aqueous electrolytes (both acid and alkaline) are relevant [90, 203, 204]. The mechanism proposed by Shi and Anson for the activity Co-octaethylporphyrin considers the oxidation of Co(II) to Co(III) in conjunction with coordination of CO to the Co(III) centres, followed by nueleophilic H2O attack leading to catalytic oxidation forming CO2 (see Equations 4.15-4.18) [90]. 

There have been no follow up studies on the activity of Co-oetaethylporphyrin for eleetrooxidation of methanol, ethanol, or formie aeid. Mukheijee et al. investigated pyrolized Co and Fe tetraphenylporphyrins and reported that the macroeyele N4 eentre together with the metal and metal oxide nanoeiystallites played a role in eleetroeatalysis [205]. 

Ciszevski and Milczarek reported roughly similar oxidation onset potentials for methanol and ethanol in 0.1 M NaOH using Ni(II) tetrakis(3-methoxy-4-hydroxyphenyl) porphyrin film (poly-NiTMHPP). However, they explored catalytic oxidations up to 0.5 M methanol concentration [207]. Since the alcohol electrooxidation occurs at potentials more positive than the redox potential of Ni(III)/Ni(II) (i.e., Ni(III) is the electrocatalytically active form), a direction to improve the performance would be to find macrocycles that negatively shift the Ni(III)/Ni(II) potential. Another variant would be to develop mixed macrocycle catalysts by incorporating elements that promote OHad formation, since on Ni polymer films the methanol oxidation was strongly dependent on flie OH concentration [207]. 

The other approach for the development of more economical non-noble metal electrocatalysts focuses on carbides, particularly tungsten carbides (WC and W2C). 

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