Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 281

9.3 IMPROVING ORR KINETICS AND PLATINUM UTILIZATION
9.3.1 Platinum Alloy Electrocatalysts
It was known as early as the 1970s that some Pt bulk alloys possessed higher activity
for catalyzing the ORR. Typically, one of the 3d transition metals (including Cr, Mn,
Fe, Co, and Ni) was alloyed with Pt at an atomic concentration of less than 50%, as
higher concentration diminished the beneficial effects. Enhancements of ORR kinetics
of from 2 - 3 times [Mukerjee et al., 1995; Paulus et al., 2002; Stamenkovic et al.,
2002] up to 10 - 25 times [Toda et al., 1999a, b] were reported. The original expla-
nation of the alloying effect came in the form of the hypothesized dual-site mechanism
for breaking the O - O bond: Jalan and Taylor suggested that the distance between
neighboring Pt atoms was too large for the dissociative adsorption of O 2 species
[Jalan and Taylor, 1983]. Alloying with base metals was beneficial, they argued,
because it caused the Pt lattice constant to contract, a phenomenon well documented
with X-ray diffraction (XRD), while maintaining the generally favorable properties of
Pt. The strong correlation between Pt - Pt distance and ORR activity was repeatedly
confirmed [Mukerjee et al., 1995].
Subsequently, it was discovered by using surface-sensitive techniques such as X-ray
photoelectron spectroscopy (XPS) that Pt alloy catalysts lost all base metal content from
their surfaces in electrochemical experiments, leaving behind a pure Pt shell a few atomic
layers thick [Igarashi et al., 2001; Paffett et al., 1988; Toda et al., 1999a, b; Wan et al.,
2002; Watanabe et al., 1994]. Because of the strong tendency of Pt to segregate to the
surface when alloyed with most metals [Ruban et al., 1999], annealing the alloy catalysts
at high temperature also resulted in a very thin outer shell of pure Pt, most likely only one
atomic layer thick (Pt “skin”) [Atli et al., 1994; Stamenkovic et al., 2002].
The discovery of the Pt “skin” led some researchers to reject the geometric argu-
ment [Toda et al., 1998] and instead explain the enhanced ORR activity of the
alloys in terms of the bulk electronic structure. Interpretation of in situ XANES and
EXAFS results suggested 10 - 20% greater Pt d-band vacancy for the alloys compared
with pure Pt catalysts [Mukerjee et al., 1995]. It was thus suggested that increased Pt
d-band vacancy strengthened the metal - O 2 interaction and thus facilitated O - O bond
scission [Toda et al., 1998, 1999a, b]. Others emphasized effects on surface-blocking
species such as OH and anions [Arenz et al., 2003; Mukerjee et al., 1995; Stamenkovic
et al., 2007a, b] and pointed out the importance of high surface area [Paffett et al.,
1988; Stamenkovic et al., 2007a, b] and specific crystalline facets [Markovic and
Ross, 2002]. It became apparent that the way in which the ORR was affected by alloy-
ing Pt with the base metals could not be unequivocally explained with current exper-
imental techniques.
To shed light on the origin of the enhanced ORR activity, Xu and co-workers per-
formed extensive DFT calculations to investigate the reactivity of the Pt skin [Xu et al.,
2004], in particular how oxygen interacts in vacuum with the ordered Pt 3 Co alloy and
with a monolayer of Pt formed on the alloy as a model for Pt skin. Figure 9.10 ident-
ifies the various adsorption sites for O and O 2 . Experiments have shown that up to four
layers of Pt could sustain a 2.5% compressive strain without creating any surface