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 MoO 2 and MoO 3 inter-conversion characterizes platinum/molybdenum interaction, the MoO 2 formation on platinum being favoured according to the calculated equilibrium constant. The maximum surface excess for molybdenum species was 1.83 × 10 −4 mol cm −2 considering a quasi-reversible charge transfer under langmuirian conditions for the adsorbates. Tungsten preferentially occupies platinum (1 1 1)-stepped sites exhibiting a complex redox process between WO 4 2 − and WO 4 3 − 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 electro-oxidation, 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 2 RT/ F were observed.