A method for kinetic study of methanol oxidation at Pt electrodes by electrochemical in situ infrared spectroscopy

In this contribution, we describe a method to estimate the kinetics of the indirect pathway in methanol oxidation reaction (MOR) at Pt electrode by using electrochemical in situ infrared spectroscopy combined with a thin-layer flow cell. Based on a quantitative relationship between the coverage of adsorbed carbon monoxide (CO_ad) and its infrared band intensities determined experimentally, CO_ad coverages during MOR are estimated from the simultaneously recorded time-resolved IR measurements. By assuming that CO _ad oxidation goes through Langmuir-Hinshelwood mechanism, its rate constant is estimated in pure supporting electrolyte under otherwise identical condition. Based on the result, the rates of CO_ad formation and CO_ad oxidation during MOR are deduced from the CO_ad coverage-time curve. The methodology is exemplified with MOR on Pt electrode at +0.6 V (versus RHE) in 0.1 M HClO₄ with 2 M methanol. It is found that under forced-flow condition: (i) the maximum reaction rate for CO _ad oxidation is ca. 0.004 molecule site^-1 s^-1 which is 100 times smaller than the maximum rate for CO_ad formation from methanol dehydrogenation; (ii) with increase in CO_ad coverage from zero to 0.5 ML, the current efficiency of the indirect pathway for MOR increases and reaches ca. 17% under steady state. The general applicability of such a method is shortly discussed.