(735b) A Detailed Microkinetic Model for SO2 Oxidation From Diesel Engine Exhaust

Hom N. Sharma¹, Ashish B.
Mhadeshwar^1,x
and Rampi Ramprasad^{1, 2,*}

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

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

*rampi@ims.uconn.edu

^xCurrently
at ExxonMobil Research & Engineering, Annandale, NJ

Currently used
ultra-low sulfur diesel (USLD) contains up to 15 ppm sulfur, consistent with
the United States Environmental Protection Agency (US-EPA) regulations [1]. In
typical diesel engine exhaust, this results into ~1 ppm of sulfur in the oxide
form [2], which can deactivate the diesel oxidation catalyst (DOC) after long
term exposure as well as result in increased particulate matter (PM) emissions
[3-4]. The noble metals (Pt/Pd) based DOC is one of the most expensive
components of the emissions aftertreatment system. Despite the extensive
research conducted on DOC performance, the interaction of DOC with sulfur
oxides in the exhaust is not clearly understood. In this work, first we will
discuss the development of a detailed 24 elementary-step microkinetic model for
Pt-DOC that accounts for SO₂ oxidation. The detailed mechanism
development is carried out using several parameter estimation techniques, such
as semi-empirical Unity Bond Index-Quadratic Exponential Potential (UBI-QEP),
Transition State Theory (TST), quantum mechanical Density Functional Theory
(DFT), and temperature programmed experiments in literature. Model predictions
for catalytic oxidation of SO₂ in fixed bed and monolith reactors
under practically relevant operating conditions will be presented (see Figure).
The microkinetic model is validated against experimental data and reaction
pathways are analyzed. Next, we explore the SO_x (x = 0 ─ 4)
interactions and reaction pathways of SO₂ oxidation on Pt and Pd
surfaces using first principles DFT. Our results show good agreement with the
experimental observations. The elementary-step surface reaction mechanism and DOC-SO_x
interactions could be utilized for improved design and control of the
aftertreatment system as well as for appropriate DOC sizing.

Figure: Panel a: performance
of the microkinetic model [5] for SO₂ oxidation on Pt. Symbols
represent experimental data; solid lines represent our simulations; and dashed
lines represent the calculated equilibrium conversion. Panel b: simulated axial
coverage profiles at 350 °C.  Panel c:  simulated axial coverage profiles at
550 °C.

References

[1]
  Environmental Protection Agency, Federal Register. Vol. 71, No. 83 (2006).

[2]
  O. Kröcher, M. Widmer, M. Elsener, D. Rothe, Ind. Eng. Chem. Res. 48 (2009)
9847-9857.

[3]
  M. Lemmetty, T. Rönkkö, A. Virtanen, J. Keskinen, L. Pirjola, Aerosol Sci.
Technol. 42 (2008) 916-929.

[4]
  T. Kolli, M. Huuhtanen, A. Hallikainen, K. Kallinen, R. Keiski, Catal. Lett.
127 (2009) 49-54.

[5]
  H.N. Sharma, S.L. Suib, A.B. Mhadeshwar, in: Novel Materials for Catalysis
and Fuels Processing, ACS, 2013, In press.