Diesel engine emissions include CO, unburned hydrocarbons (HC), NOx and particulate matter (PM), all of which have an adverse effect on the environment. In order to improve fuel-efficiency and to reduce the environmental impact, low temperature combustion (LTC) diesel engines have been developed. In addition to consuming less fuel, LTC engines effectively reduce the emission of NOx and PM;1however, at the intermediate to low temperature of LTC engine exhaust, the commonly used Pt and Pd alloy diesel oxidation catalysts (DOC) are insufficient to oxidize CO and unburned HCs. Thus, new catalysts are needed to increase the DOC activity at lower temperatures.
In this work, we study the CO and NO co-oxidation reaction on both terrace and stepped surfaces of eleven late transition metals. Existing scaling relations for CO oxidation are adopted from the literature2and supplemented with new data obtained from density functional theory (DFT) simulations. The nudged elastic band (NEB) method is used to locate transition states and provide information regarding the activation energies. These energies are found to obey linear adsorption energy and transition state scaling relations. We are also able to show empirically that only two variables – the binding energies of CO and oxygen on stepped surfaces – are needed as reactivity descriptors to predict all species’ binding energies and reaction barriers on stepped and terrace surfaces. From this result we construct a two descriptor based micro-kinetic model for CO and NO co-oxidation at different reaction conditions, from which we can obtain volcano plots showing catalytic activity trends.
As expected, Pt and Pd are shown to have the highest oxidation activity at 600 K. At lower temperatures representative of LTC emissions (425 K) the volcano top moves closer to the Ag/Au region. Our analysis suggests that alloys of Ag/Au and another precious metal may have better performance at moderate temperatures. From several alloys screened, Ag/Cu and Au/Cu alloys with a 3:1 ratio were predicted to have the best performance. Notably, if a Pt/Pd catalyst is used, any produced NO2 will immediately react with CO to generate NO and CO2. This implies a sequential oxidation of CO followed by NO, which increases the use of precious metals in the reactor and is undesired. Ag3Cu or Au3Cu alloys, on the other hand, do not efficiently catalyze the NO2 + CO → NO + CO2 reaction, but promote the simultaneous oxidation of CO and NO. Overall, our computational predictions at 425 K are that Ag3Cu and Au3Cu alloys achieve higher catalytic activity than Pt or Pd, and can also oxidize CO and NO simultaneously, which is beneficial for tuning the conditions for the downstream selective catalytic reduction (SCR) unit. Selected metal alloy catalysts are synthesized and experimentally evaluated to validate the theoretical predictions.
(1) Musculus, M. P. B.; Miles, P. C.; Pickett, L. M. Prog. Energy Combust. Sci. 2013, 39, 246–283.
(2) Jiang, T.; Mowbray, D. J.; Dobrin, S.; Falsig, H.; Hvolbæk, B.; Bligaard, T.; Nørskov, J. K. J. Phys. Chem. C 2009, 113, 10548–10553.