(361a) First Principles Investigation of SOx (x = 0 - 4) Chemistry On Pt (111) and Pd (111)

Hom N. Sharma¹, V. Sharma²,
Ashish B. Mhadeshwar^1,3 and R. Ramprasad^{1, 2,*}

¹Department of Chemical and Biomolecular Engineering,
University of Connecticut, Storrs, CT

²Material Science and
Engineering, University of Connecticut, Storrs, CT

³Currently
at ExxonMobil Research & Engineering, Annandale, NJ

*rampi@ims.uconn.edu

Sulfur oxides
(SO_x (x = 0 ─ 4)) in the diesel engine exhaust are one
of the major factors contributing toward deactivation of the noble metals
(Pt/Pd) based emissions aftertreatment catalysts [1, 2]. Interactions
of SO_x with the catalyst metals and supports are associated with
changes in various structural, morphological, and electronic properties, which can
lead to sulfate formation [3, 4].  Despite many experimental
and computational studies, SO_x interactions with metal surfaces and their
impact on catalytic activity are not clearly understood. In the present study,
we explore the SO_x (x = 0 ─ 4) chemistry in terms of interactions
and oxidation pathways on Pt(111) and Pd(111) surfaces using first principles
density functional theory (DFT) [5]. We have
identified various stable configurations (figure 1), which reveal the charge
redistribution patterns, and molecular orbital interactions of SO_x
on both surfaces during adsorption. The possible reaction pathways and
estimated activation barriers for SO_x oxidation on the metal
surfaces ? identified using climbing image nudged elastic band (CI-NEB) method [6] ? are found to
be in good agreement with the experimental observations. This comprehensive and
comparative study provides important insights about SO_x chemistry on
Pt and Pd surfaces, which can be useful to design and improve sulfur resistance
catalyst materials.

Figure 1: Representative stable configurations
of SO_x molecules on Pt(111): (a) S (fcc) η¹ S_f
, (b) SO (fcc) η¹-S_f , (c) SO (fcc) η²
S_bO_a, (d) SO₂ (fcc) η² S_bO_a,
(e) SO₂ (fcc) η³ S_aO_aO_a,
(f) SO₂ (bridge) η¹ S┴, (g) SO₃
(fcc) η³ O_aO_aO_a, (h) SO₃
(fcc) η³ S_aO_aO_a, and (i) SO₄
(fcc) η³ O_aO_aO_a. The
number super-scripted to η represents the number of atoms in a molecule
coordinated to the metal surface; and the subscripts a, b, and f
represent the atop, bridge, and fcc hollow sites, respectively.

References

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[3]   J.A. Rodriguez, J. Hrbek, Acc.
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[4]   H.N. Sharma, S.L. Suib, A.B.
Mhadeshwar, in: Novel Materials for Catalysis and Fuels Processing, ACS, 2013,
In press.

[5]   G. Kresse, J. Furthmüller,
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[6]   G. Henkelman, B.P. Uberuaga,
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