Surface metal modifiers for methanol electrooxidation on platinum, molybdenum and tungsten

Abstract

New promising results on the electrocatalysis of methanol electro-oxidation on platinum surfaces modified with molybdenum and tungsten deposition were found by combining different methods (spontaneous, potentiostatic and potentiodynamic). A quasi-reversible process involving MoO2 and MoO3 inter-conversion characterizes platinum/molybdenum interaction, the MoO2 formation on platinum being favoured according to the calculated equilibrium constant. The maximum surface excess for molybdenum species was 1.83x10-4 molcm-2 considering a quasi-reversible charge transfer under langmuirian conditions for the adsorbates. Tungsten preferentially occupies platinum (111)-stepped sites exhibiting a complex redox process between WO2-4 and WO3-4 soluble species, being more remarkable with lower values of the upper potential limit. The analysis of methanol oxidation was accomplished by using linear sweep voltammetry in a bulk methanol solution or by stripping voltammetry of adsorbed residues on smooth and columnar polycrystalline platinum after deposition of molybdenum and/or tungsten. The platinum surface modified by potentiodynamic deposition of molybdenum followed by double deposition of tungsten presents the best performance upon methanol electrooxidation, recording at 0.7 V, a 50% higher current density than on bare platinum. It was also found that platinum/tungsten surfaces prepared by potentiostatic or potentiodynamic deposition exhibit the lowest amount of carbon monoxide residues, i.e., 43% of coverage on platinum/tungsten against 81% on bare platinum. Tafel slopes for methanol oxidation on pure platinum and double molybdenum and tungsten depositions are ca. 120 mV dec-1 suggesting a first monoelectronic charge transfer as rate-determining step. For the simultaneous deposition of molybdenum and tungsten, higher Tafel slopes (as 200 mV dec-1) were calculated, but for platinum surfaces modified by single metal deposition, Tafel slopes slightly higher than 2RT/F were observed.