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electrocatalysis community until recently but fell short in explaining, for example, the
enhanced ORR activity of bulk Pt alloy catalysts. It is worth pointing out that the
d-band theory inherently assumes that surface reactivity is a local property, which differs
from another earlier perspective that bulk properties determine catalytic properties.
This new atomistic and mechanistic perspective rooted in theoretical solid state
physics has brought fresh fundamental insights to heterogeneous catalysis: The
activity for a number of reactions often peaks on a certain metal when correlated
with some measure of reactivity across a series of metals, thus displaying a “volcano”
trend. Using extensive density functional theory (DFT) calculations, Nørskov and co-
workers have convincingly demonstrated that this behavior is the result of key compet-
ing processes in a reaction network connected via a common intermediate [Nørskov
et al., 2002], just as described by the Sabatier principle, which observed that catalytic
activity is often maximized when the reactants interact with the catalysts with inter-
mediate strength.
Heterogeneous catalysis and electrocatalysis research has traditionally been based
on empirical approaches. These new theoretical insights, in combination with sophis-
ticated experimental techniques that give increasingly greater imaging resolution and
control over matter, and aided by ever-growing computational power, open the door to
the rational design and screening of catalytic materials for desired properties (e.g.,
activity, selectivity, and stability). Examples have already emerged in which hitherto
untested metal alloys have been identified to possess activity superior to traditional
monometallic catalysts for such reactions as ammonia synthesis, methanation,
water-gas shift, and electrocatalytic hydrogen evolution [Greeley et al., 2006;
Jacobsen et al., 2001; Knudsen et al., 2007; Sehested et al., 2007]. As demonstrated
below, this possibility has not been lost on researchers investigating ways to improve
the oxygen reduction electrocatalysts for PEMFCs.
9.2 CHEMICAL SPECIES AFFECTING THE ORR: NEW
ATOMIC-LEVEL INFORMATION
9.2.1 Adsorption of Oxygen-Containing Species
9.2.1.1 Oxygen Adsorption
Formation of adsorbed oxygen-containing
species in the ORR can occur from two sources: molecular oxygen (O
2
) and water
molecules that are oxidized above a certain potential to form hydroxyl (OH) and even-
tually atomic O. As one of the main reactants of the ORR, O
2
must of course interact
with the electrode in order for the net reaction to proceed. The thermochemical inter-
action of O
2
with Pt surfaces under ultrahigh vacuum (UHV) conditions has been
extensively studied [Avery, 1983; Gland, 1980; Gland et al., 1980; Outka et al.,
1987; Sexton, 1981]. DFT calculations have identified several adsorbed di-s O
2
states on Pt(111) (Fig. 9.1) [Eichler et al., 2000; Gambardella et al., 2001; Shao et al.,
2006b; Sljivancanin and Hammer, 2002; Xu et al., 2004]. Based on the O - O vibrational
frequency and electronic properties, the top - fcc/hcp - top (t-f/h-b) states are iden-
tified as peroxide states (O
22
), whereas the identity of the top - bridge - top (t-b-t)
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