Design of Diesel Oxidation Catalysts with Improved Activity at Reduced Temperature Using Computational Methods

Low temperature combustion (LTC) diesel engines have higher fuel efficiency and produce less NOx and particulate matters (PM) compared to traditional diesel engines. However, this benefit comes at the cost of higher concentrations of CO and unburned hydrocarbons (HC) emissions.1 Meanwhile the low exhaust temperature results in a reduction in the activity of commonly used diesel oxidation catalysts (DOC). Thus, improved DOC with superior activity at reduced exhaust temperatures are needed.

In this work, we use a combination of density functional theory and descriptor-based microkinetic modeling to computationally screen a large number of bimetallic alloys. This approach enables us to quickly search for promising DOC under different reaction conditions.2 Based on the computational results, we propose that better low temperature DOC can be made by the alloys of coinage metals with Pt or Pd. Compared to the commonly used DOC which are Pt and Pd alloys, these coinage metal alloys will not only achieve higher activity at the reduced temperature, but also prevent the mutual inhibition between CO and NO oxidations. Besides, manufacturing cost of the catalytic converter is reduced by using coinage metals. These promising alloys have been synthesized and their activity has been experimentally tested using temperature programmed oxidation.

In addition, we build reactor models to optimize the DOC efficiency and catalysts loading. Since under real working conditions, the exhaust CO, NO and HC are oxidized along the DOC reactor and different reaction conditions require different catalyst compositions, the overall DOC performance can be improved by designing a DOC reactor with metal concentration gradients, where the local alloy formulation is optimized for the local temperature and exhaust gas compositions. To this end, we have built and optimized a preliminary reactor model when only CO is present. The optimal catalyst for each position is identified from microkinetic models. Our results show the reactor length is reduced by 70% by using the non-uniform metal distributed DOC, which implies the use of metal catalysts is significantly decreased. Our ongoing efforts
aim towards our ultimate goal of optimizing the DOC composition in the presence of CO, NO and HC. With this optimized DOC, we are hoping to lead to significant advances to meet tailpipe emission regulations at the low exhaust temperatures of a LTC engine, which will enable the widespread use of LTC engines.

References:
1. Musculus, M. P. B., Miles, P. C., and Pickett, L. M. Prog. Energy Combust. Sci. 39, 246 (2013).
2. Nørskov, J. K., Bligaard, T., Rossmeisl, J., and Christensen, C. H. Nat. Chem. 1, 37 (2009).