(216g) Novel Low Temperature Nox Removal for Diesel Exhaust

One
major obstacle preventing widespread introduction of diesel engines is the lack
of a suitable catalyst/reductant combination for this process. Hydrocarbons
present in engine exhaust are generally less active and selective compared to
ammonia. Although the NH₃-SCR is very effective, the requirement of
a separate NH₃ source and injection system, combined with issues
relating to NH₃ slip, make this approach unappealing. Another
effective reductant, urea, also has its shortcomings such as its high freezing point
and the lack of urea distribution infrastructure. However, recent studies
indicate that hydrogen is a promising reductant that achieves high NO_x
conversion over a wide temperature window^a.

We
are currently investigating a new strategy to achieve high NO_x
removal at low temperatures in an oxygen rich atmosphere. This approach
involves a coupled system: an ethylene glycol (EG) reforming unit to convert a
mixture of ethylene glycol and water into hydrogen and CO followed by a H₂-deNO_x
unit. The group of Dumesic has found that aqueous phase reforming of ethylene
glycol with high H₂ selectivity can be achieved using a 3%
Pt/Alumina or Raney Ni-Sn catalyst at 500K^b. Their experiments are
conducted at higher pressures, near the bubble point of the solution, whereas
our gas phase reforming is carried out near atmospheric pressure. Therefore,
our low temperature reforming unit requires simple equipment that would be easy
to operate.

We
have studied the performance of modified Pt supported catalyst for gas phase EG
reforming at 230^oC. Prior to testing, the samples were calcined in
air followed by reduction in H₂ at 250^oC. Modification of
the Pt supported catalysts with Na produce a twofold effect, an increase in the
reforming activity and also stability of the catalyst. At a low EG concentration
of 420 ppm, our results indicate complete conversion of ethylene glycol into
hydrogen, mainly via decomposition. At higher EG concentrations (2.15% EG in
the feed), oxidative reforming produced 1.4 H₂/EG.

We
plan to couple this EG reforming unit to a de-NO_x unit. Based on our
calculations, 0.551 kg of reforming catalyst would be needed to deliver the
amount of H₂ for complete NO_x removal^a. Under
realistic engine operating conditions, the amount of EG required for 100% NO_x
removal would be 1.8 l/hr, which suggests that such a coupled system would be
feasible for practical application.

^a
N. Macleod, R. Cropley, J.M. Keel, R.M. Lambert, J. Catal. 221 (2004) 20-31.

^b
G.W. Huber, J.W. Shabaker, J.A. Dumesic, Science, 300 (2003) 2075.