Ramanathan Sh. (Ed.) Thin Film Metal-Oxides: Fundamentals and Applications in Electronics and Energy

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314 T.P. Holme et al.

Table 10.2 O

net charge and bond lengths of O–O and O–M in adsorbed states on varying sites

in metal clusters. Energies in electron volt, distances in

A, and charge in e

Cluster Site E

ads

O–O length O–M length O

net charge

Twofold 0.74 1.336 2.207 0.3384

Threefold 0.43 1.336 2.177 0.3339

Twofold 0.91 1.328 2.232 0.2907

Fivefold 0.88 1.324 2.227 0.2713

Fourfold 0.25 1.296 2.453 0.1059

Fivefold 0.22 1.291 2.509 0.0856

fcc 0.91 1.493 2.271 0.6404

Onefold 1.39 1.349 1.975 0.2234

Bridge 1.15 1.448 2.046 0.4540

Threefold 1.09 1.446 1.986 0.4729

Threefold 0.79 1.344 2.015 0.2571

Ag 0.30 1.308 2.349 0.1750

atop

1.25 1.457 2.06 0.5366

bridge

1.32 1.410 2.081 0.3665

bridge

1.72 1.417 2.049 0.3972

atop

0.84 1.345 2.005 0.3110

bridge

1.98 1.426 2.065 0.3735

Oxygen Dissociation

States and energies of oxygen dissociated on Ag clusters and on Pt are shown in

Fig. 10.7. In all cases excluding Ag

, oxygen dissociation on the catalyst cluster

is found to be energetically favored. Thus there is a thermodynamic driving force

for oxygen to dissociate on metal clusters, though the process may be kinetically

limited at some temperatures.

By comparing geometries of dissociated states to those of associatively adsorbed

states, it can be seen that oxygen reacts strongly with the metal cluster in the disso-

ciation process, signiﬁcantly altering the shape of the cluster, particularly in the case

of silver clusters. One driving force for this process is that as the adsorbed oxygen

gains a partial negative charge, the oxygen atoms repel each other.

For platinum clusters, the dissociation energy does not seem to have a clear

dependence on cluster size, but for silver clusters, oxygen dissociation is more

energetically favorable on larger clusters. Atomic oxygen is experimentally found

to be bound to silver surfaces with energies between 0.7 and 1.8 eV as found by

Stegelmann and Stoltze [27] and Campbell [23], respectively. These calculations

show that oxygen is weakly bound to silver at high coverage (approximated by

ads

and O

ads

), but at lower coverage the binding energy is 0.9 and

1.1 eV/atom on Ag

and Ag

, respectively, within the experimental range. This ev-

idence could be interpreted to show that the larger clusters are a better model for

silver, as expected from the electronic structure calculations.

Experimentally, atomic oxygen adsorbed on Pt has an adsorption enthalpy of

1.7 eV in the high coverage regime, increasing to 4.8 eV for low coverages [25].

10 Design of Heterogeneous Catalysts and Application to Oxygen Reduction Reaction 315

Fig. 10.7 Stable energies and structures of O

.ads/

for oxygen dissociated and adsorbed at

different sites on (a)Agclustersand(b) Pt clusters. E D 0 is deﬁned for each system to be O

and M

separated at inﬁnity. Ag atoms in white, Pt in gray, and O in black

The oxygen-to-Pt ratio in these small clusters is high enough to fall in the high

coverage regime, so the calculated binding energy falls within this broad range sug-

gested by experiment.

Activated States

Transition states for oxygen dissociation were found on all Pt clusters and the Ag

cluster. The energy of the stable states for these systems is shown in Fig. 10.8.The

PES in this ﬁgure is a schematic – only the energy of stationary states has been

calculated, and a line has been drawn between them to indicate the reaction path.

316 T.P. Holme et al.

Fig. 10.8 Energy (in electron volt) of stationary states on the oxygen dissociation reaction pathway

on Pt clusters and Ag

. Inset text gives the activation energy barrier

The activation energy of oxygen dissociation decreases with the size of the Pt clus-

ter, and is signiﬁcantly lower on Pt clusters than Ag

. The Sabatier principle, which

indicates a correlation between activation energy and the stability of products, is

generally followed in these systems, with the exception of the very strong binding

of atomic O by Pt

. In all cases, the energy cost to break the O–O bond is much

lower than the bond energy of oxygen of 9.66 eV calculated by B3LYP/LANL2DZ

showing the good catalytic ability of these metals.

10.5.1.2 Cluster Composition

Energy of adsorption and dissociation of oxygen on alloyed clusters is shown in

Fig. 10.9. Adsorption is more favorable on Pt sites than on Ag sites. From Mulliken

population analysis, Pt atoms in the alloy become negatively charged, which makes

them attractive adsorption sites for O, as these negatively charged Pt atoms are more

able to donate charge to O.

The Sabatier correlation between activation energy and bond strength of the dis-

sociated product atoms again holds. The Ag

cluster has the lowest activation

energy for oxygen dissociation and binds the products most strongly, whereas the

cluster binds the products least strongly and has the largest activation energy.

Good catalysts must compromise between the extremes of low activation energy

versus high stability of reaction products, so the composition of the best catalyst is

one that should have intermediate properties. The optimal catalyst composition is a

function of the temperature of catalyst operation: at higher temperatures, a greater

driving force for adsorption must be supplied to counteract the greater gas phase en-

tropy. Therefore, for higher temperature operation, a catalyst with greater Pt content

10 Design of Heterogeneous Catalysts and Application to Oxygen Reduction Reaction 317

Fig. 10.9 Calculated energy for oxygen adsorption and dissociation on AgPt alloys of varying

composition. The text gives the activation energy barrier

is predicted to show better performance, whereas at lower temperatures the catalyst

should contain more Ag.

Comparison with Fig. 10.8 shows that the activation energy for oxygen dissocia-

tion on alloyed clusters is lower than on Pt clusters for a given number of Pt atoms.

The activation energy on the Ag

cluster is lower even on a per-atom basis, show-

ing that an AgPt alloy is predicted to be more catalytically active than pure Pt.

The intermediate electronic structure of PtAg compounds leads to higher

predicted catalytic activity for dissociative adsorption of oxygen, as shown in

Fig. 10.10. The compounds have a lower activation energy for dissociative adsorp-

tion than the pure components, in agreement with Huang’s experimental results [28].

10.5.2 Slab Model

The following sections detail the results of electronic structure calculations and oxy-

gen adsorption on Pt and Ag. Slabs were terminated with low energy (111) surfaces.

To study the ability to ﬁne-tune "

, and therefore the strength of interactions with

oxygen, the electronic structure of both bilayers and random alloys were studied.

A Pt monolayer over bulk Ag was studied as a representative bilayer, and for com-

parison, random alloys Pt

1x

were studied for 0:125  x  0:5.

Different sites were evaluated for Pt monolayers on Ag, and the fcc site was

found to be the most energetically favorable.

318 T.P. Holme et al.

0 20 40 60 80 100

1.0

1.5

2.0

2.5

E [eV]

diss

act

Ag at%

EIS experimental results

QS simulation results

Fig. 10.10 Activation energy of oxygen dissociative adsorption on Ag

1x

clusters (open

squares) versus x. Experimental data (ﬁlled circles) from electrochemical impedance spec-

troscopy [29]

Fig. 10.11 Density of states for slabs of pure Ag, a PtAg

random alloy, a bilayer of Pt

/Ag, and

pure Pt. The horizontal line marks the Fermi energy. Inset ﬁgures show the slab geometry of the

alloys, with Ag atoms in white and Pt in gray

10.5.2.1 Electronic Structure

The work function for Ag and Pt was calculated to be 4.5 and 5.8 eV, respectively;

corresponding experimental values are 4.7 and 5.9 eV, respectively [29].

The density of states of Ag, Pt, and intermediate PtAg compounds is shown in

Fig. 10.11. Ag has a low and relatively narrow band of states, whereas the band for

10 Design of Heterogeneous Catalysts and Application to Oxygen Reduction Reaction 319

Fig. 10.12 DOS of the Pt/Ag bilayer (left panel), showing the Fermi level as a dashed line, and of

a weighted sum of Ag and Pt slabs (right panel)

Pt is broad and closer to the Fermi level. The intermediate compounds do have an

electronic structure in some sense between the pure components. The Fermi level

lies between the pure components, and the breadth of the band of states is broader

than Ag but narrower than Pt. The Fermi level of the Pt/Ag bilayer is very close to

the Fermi level of pure Ag, and the DOS is quite similar to Ag, with one feature

just below the Fermi level that may be identiﬁed with the peak in the Pt DOS at the

Fermi level at the same energy.

To demonstrate the similarity of the DOS of the bilayer with the individual com-

ponents, Fig. 10.12 shows the bilayer DOS and the DOS of a simple weighted sum

of Ag and Pt slabs.

The simple prediction of mixing without interactions qualitatively reproduces

the features of the compound, particularly near the important d-band. Therefore, as

a ﬁrst approximation, a bilayer DOS can be estimated from elemental DOS, sim-

plifying the combinatorial nature of computing DOS for any bilayer of interest to a

matter of addition based upon preexisting calculations.

Figure 10.13 shows the level of the centroid of the d-electrons referenced to

the Fermi level for different compositions of Pt

1x

. The chemically important

d-electrons in Ag have a centroid "

D 3:9 eV below the Fermi level, whereas the Pt

d-electrons lie closer to the Fermi level, only 2.2 eV below. The electrons in Pt closer

to the Fermi level are more reactive, in agreement with results of the cluster model

showing that Pt donates more charge to adsorbed O, leading to stronger adsorption

on the right side of the optimum. On the left side of the optimum in binding energy

predicted by the Hammer-N¨orskov model, Ag has d-electrons that are too far from

320 T.P. Holme et al.

Fig. 10.13 Centroid of the d-band, referenced to the Fermi level, of Pt

1x

as a function of x

(squares, left axis) and adsorption energy of atomic O on the metal slab (triangles, right axis)

the Fermi level to form ﬁlled antibonding orbitals, which results in stronger O–Ag

bond energy. Because of the relation derived in the introduction for temperature-

dependant optimal adsorption strength of molecular oxygen, the calculation may

predict that the Ag

0:5

alloy will be the optimal catalyst for oxygen dissociation

at approximately 200

10.5.2.2 Oxygen Adsorption

Among all sites tested for adsorption at 0.25 ML coverage, O

preferentially adsorbs

in a peroxo- conﬁguration on fcc sites on Ag and Pt as shown in Fig. 10.14. In agree-

ment with Parker, oxygen prefers to adsorb on threefold hollow sites on Pt (111)

[30]. Adsorption on Pt is more exothermic than on Ag, and is more site-dependant

on Pt than on Ag. This accounts for the orders of magnitude higher diffusivity on

Ag than on Pt (3  10

14

=sat900

ConPt,and2  10

5

=sat700

Con

Ag, from Velho [31] and Sunde [32]), as O diffusion on Pt requires hopping over

much larger energy barriers. The bond strength between oxygen and Ag is closer to

the range derived for optimal bond strength (1.1–1.6eV) than the O–Pt bond.

Figure 10.15 shows the correlation between M–O bond strength and O–O bond

length. Generally, stronger M–O bonds (measured by adsorption energy) translate

to weaker O–O bonds (measured by bond length). A stronger M–O bond results in

greater electron transfer to O

. These electrons are backdonated into 



orbitals,

which weakens the O–O bond, accounting for the catalytic effect of adsorption

on metal surfaces. Adsorption on the most energetically favored Ag site results in

greater O–O bond stretching than any site on Pt, further conﬁrming the high cat-

alytic activity of Ag.

10 Design of Heterogeneous Catalysts and Application to Oxygen Reduction Reaction 321

Fig. 10.14 AdsorptionenergyforO

at varying sites on Pt (o) and Ag .x/. Energy for all

superoxo- sites on the left, and two peroxo- states on the right. Data are for surface coverage of

 D 0:25

Fig. 10.15 Adsorption energy and O–O bond length at varying sites on Pt (o) and Ag .x/

The peroxo-adsorbedoxygen on Pt is relatively strong for the more modest weak-

ening of the O–O bond, presumably for geometric reasons.

The stronger interaction of O with Pt than Ag can be explained by the O–M bond-

ing model proposed by Hammer and Nørskov [1] whereby oxygen forms bonding

and antibonding interactions with metal d-electrons. The energies of the bonding

322 T.P. Holme et al.

and antibonding states are related to the position of "

.ForAg,"

is 2 eV lower

than Pt, so more antibonding states fall below the Fermi level. The occupation of

antibonding states weakens the Ag–O bond. In addition, more electron donation to

accounts for the weaker O–O bond on Ag than on Pt.

10.5.3 Experiments

Experimental data for the oxygen reduction reaction on dense M/YSZ electrodes

(M D Ag, Pt, Ag

,orAg

;YSZD yttria-stabilized zirconia) from

Ref. [29] is shown in Fig. 10.16.

The exchange current density, a measure of the reaction speed, increases in the

order Ag

< Ag

< Pt << Ag. Diffusion of oxygen in metal is critical to

the reaction speed on a dense electrode; due to the fast diffusion of oxygen in silver,

silver exhibits the highest exchange current density.

Fig. 10.16 Tafel plot showing activation overpotential as a function of log of current density on

dense M/YSZ electrodes (M D Ag, Pt, Ag

,orAg

). The exchange current density j

indicated in the ﬁgure. Data from Ref. [28]

10 Design of Heterogeneous Catalysts and Application to Oxygen Reduction Reaction 323

10.5.3.1 Stress

XRD stress analysis shows that the Ag lattice contracts signiﬁcantly with alloying,

but the Pt lattice does not expand greatly. Figure 10.17 shows that the Ag lattice

contracts by 1.4% with 25% Pt added, and by 1.8% with 40% Pt. In contrast, the

Pt lattice hardly expands: 0.23% with 60% Ag, and 0.13% with 75% Ag. Silver is

not very soluble in Pt and, therefore, does not signiﬁcantly expand the Pt lattice. On

the contrary, Pt is more soluble in Ag, so the Ag lattice shrinks by a greater degree

upon alloying. An alternate phase, assigned to be Ag

from the phase diagram

by Okamoto [21], appears in the Pt

0:25

0:75

scan. The results cohere with the phase

diagram by Okamoto, as the range of solubility of Pt in Ag is much higher than Ag

in Pt.

Fig. 10.17 Results of strain in (a)Ptand(b) Ag from XRD. Lower panel shows the Ag–Pt phase

diagram from Ref. [21], reprinted with permission of ASM International

