Structural effects on kinetic properties for hydrogen electrode reactions and CO tolerance along Mo-Pt phase diagram

The effect of structural and surface versus bulk properties of Mo-Pt alloys and intermetallic phases taken along their phase diagram upon kinetic and electrocatalytic features for the cathodic hydrogen evolution (HER) has been investigated and displayed. All specimens along Mo-Pt phase diagram in broader reversible potential range feature Volmer-Tafel mechanism with the catalytic recombination of Tafel as the rate determining step (RDS), while further polarization plot in semi-logarithmic eta versus logj system shows Volmer-Heyrowski mechanism with the electrochemical desorption of Heyrowski reaction being the RDS: the extension of the former depends on the degree of MoO3 coverage and blocking active centers of electrode surface. XPS, UPS, XRD and work function characterization of all specimens revealed congenial volcano plots relative to the same dependence in electrocatalytic activity. As the main observation and rule, the most stable and prevailing Pt content specimens feature the best electrocatalytic and kinetic properties. Activated (MoO3 free) MOPt3 and MOPt4 catalysts feature all along the Tafel plot reversible Tafel catalytic reaction as the RDS, and create properties of super-activity within a broader current density range. It has been pointed out that an intermetallic phase with prevailing Mo atomic percentage (Mo3Pt2) features pronounced electrocatalytic properties for the HER. XPS measurements of nanostructured intermetallic phases of the same nominal composition, revealed that oxide species of Mo form solid solution with Pt, while significant portion of Pt surface atoms become screened by two-dimensional Mo oxide clusters. Such a structural modification alters the chemisorptive properties of Pt, so that XPS measurements reveal its partial oxidation, the oxide state being the primary oxide (Pt-OH), that spillovers along with MoO2OH, both being imposed by adsorptive dissociation of water upon titania (TiO2). It has been inferred that such structural and surface state a priori defines the overall behavior of composite Mo-Pt electrocatalysts. BRIEFS: Brewer hypo-hyper-d-interelectronic (average) configurational, Interionic and crystal structural effects of composite Laves type intermetallic phases and alloys taken along Mo-Pt phase diagram on kinetic, Synergistic electrocatalytic and super-active properties for hydrogen electrode reactions (HELR) and CO tolerance in macro-size and nanostructured electrodes. (c) 2005 Elsevier B.V. All rights reserved.