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

First principles investigation of SO_x (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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