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

Abstract

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 thoroughly 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 (vs. log) 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 electrocatalysis. As the main observation 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 (MoPt3) features pronounced electrocatalytic properties for the HER.

Spillover Phenomena and Its Striking Impacts in Electrocatalysis for Hydrogen and Oxygen Electrode Reactions

“…Due to its chemically unstable reactive properties and consequently so fast decomposition recombination, leading to the irreversible and stable surface oxide (M=O) formation, the corresponding reversible peak usually extends in mineral acids and alkaline solutions within a narrow potential range, except when interactively supported on hypo-d-oxides and the latter being continuously supplied by water moisture. Similar behavior is a feature of some other transition metal primary-type oxides of common formulas and structure, like MoO(OH), NiOOH, and WO 2 (OH), and can be identified by corresponding peaks within their potentiodynamic and XPS spectra [39,40]. The point is that all transition metals of any, or in particular high, altervalent capacity, afford the reversible primary-oxide-type states, usually of pronounced catalytic activity and high electronic conductive properties (WO(OH), MoO 2 (OH), Pt-OH, Au-OH), but unfortunately, in their oxidation, sequences end up sweep rate between hydrogen and oxygen potential evolving limits.…”

“…Although their existence is clearly indicated by XP spectra [39,40], under electrode polarization primary oxides feature some unique specific fluent (jellium [42] or fluid) structure of dipoles with noticeable Pauli's repulsion (the spillover precondition) amongst adjacent M-OH species (Scheme 1). The latter causes their rearrangement into an antiparallel structure (OH-adsorbate being oriented in the upright position, with the O-atoms closest to the metal plane [31]), and, consequently, there is no surface change monitoring in the course of the in situ STM scans, as might be expected along with the corresponding charge transfer for their adsorptive generation within the reversible potentiodynamic peak [41].…”

“…During cathodic polarization, H-adatom intermediates keep the metallic surface free from oxides [47,58] and feature maximal synergistic electrocatalytic activities [52,59]. Figure 7 in [40] correspondingly reveals such polarization properties for Pt-Mo intermetallic phases. Meanwhile, since hypo-d-metals are hydrophilic elements and thereby sensitive on water and oxides, hypo-hyper-d-d-intermetallic catalysts feature two different electrode properties.…”

“…The first component at binding energy 71.3 eV is attributed to metallic platinum. The peak at 72.7 eV is assigned to Pt(OH) x (2 ≤ x ≤ 1), due to combination of Pt with OH in the primary oxide (M-OH type), or some other congenial O-species, while the third component at 74.9 eV is ascribed to PtO x [40], finally ending up as the stable surface oxide (PtO 2 ). The percentage contribution of each Pt-oxy-component to the total peak area is equivalently shown for both specimens in Table 1.…”

“…In particular, the CO L area decreases steadily without featuring any initial increasing stage or any inflection point, while complete desorption of CO is observed already at 370 K. In addition, the profiles obtained from the various catalysts with different Mo/Pt atomic ratios are quite similar, indicating that this factor is not decisive for the CO L desorption profiles. Meanwhile, it would be noteworthy that, in hydrogen gas stream, the lowest desorption temperature features the sample with the lowest Mo atomic percentage (MoPt 4 /TiO 2 ) [39] or the most active intermetallic phase (MoPt 4 ) for the HER [40]. As a difference with He atmosphere, H-adatoms consume corresponding amount of the primary oxide (Pt-OH), enabling thereby the higher coverages by CO, and consequently the desorption temperature of the latter increases, while CO tolerance proportionally decreases.…”