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

• Published in: Surface science 2005-12, Vol.598 (1), p.156-173
• Main Authors: Jaksic, J.M., Vracar, Lj, Neophytides, S.G., Zafeiratos, S., Papakonstantinou, G., Krstajic, N.V., Jaksic, M.M.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electrocatalytic super-activity; Exact sciences and technology; Hypo-hyper-d-interelectronic bonding; Intermetallic bifunctional electrocatalytic synergism; Mo,Pt-acac (acetylacetonate) precursors; Physics; Primary oxide (Pt–OH, MoO 2OH) spillover; SMSI (strong metal–support interaction); Tafel catalytic RDS
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2005.08.036

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 η versus log j 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 MoO 3 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 (MoO 3 free) MoPt 3 and MoPt 4 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 (Mo 3Pt 2) 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 MoO 2OH, both being imposed by adsorptive dissociation of water upon titania (TiO 2). 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.