ABSTRACT
ABSTRACT The pathways for the electro-oxidation of methanol on platinum and platinum-alloy electrodes are explored in some detail, with evidence from a wide variety of physical and electrochemical techniques summarized. Chemi-sorption of methanol on platinum itself takes place very rapidly on the bare electrode surface through a succession of intermediates derived from methanol by successive scission of C—H bonds and formation of Pt—C bonds. On low-index single-crystal surfaces in moderate to high concentrations of methanol, the final adsorption products are linearly and bridge-bonded CO, with the adsorption of methanol inhibited by adsorbed HSO 4 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203750469/7b216794-c74b-4b63-98ec-850a4e8d1fd1/content/in47_1.tif"/> , especially on Pt(111). At potentials below 0.4 V vs. RHE, little further oxidation of this final chemisorbate takes place, and the electrode surface becomes essentially poisoned. Above this potential, some oxidation of the adsorbate can take place to an extent that increases strongly with potential, especially on the Pt(100) surface. It should be emphasised that the adsorbate layer formed on chemisorption of methanol is not identical to that formed on adsorption of CO in solution: Coverages are lower in the first instance, and there is evidence for coadsorption of anions and other carbon-containing species. Under conditions of low methanol concentration, there is now quite strong evidence that one of these species is the triply bonded Pt3C—OH. The mechanism of oxidation of methanol on promoted platinum has also been extensively studied, particularly for Pt/Ru alloys, which show a far higher activity for methanol oxidation. There appear to be at least two effects operating here: Ru promotes chemisorption of methanol at lower potentials by providing a number of kinetically more facile sites for chemisorption, though the final coverage of methanol does decrease with increasing surface Ru content at room temperature. A second and more significant effect is that Ru can be oxidized at much lower potentials than Pt itself, and the Ru—OHads species that form on the surface can promote oxidation of chemisorbed fragments to CO2. At relatively low Ru coverage, it would appear that migration of COads to Ru—OHads sites is rate limiting, but at higher Ru coverages, the oxidation of COadS becomes limiting. The optimal Ru:Pt ratio remains somewhat controversial, particularly on well-defined surfaces at lower temperatures. However, at higher temperatures and methanol concentrations and for particulate catalysts, a molar ratio approaching 1:1 Pt:Ru appears best.
