Non-catalytic gas-phase NO oxidation in the presence of decane
Chih-Han Liu, University at Buffalo
Chih-Han Liu1, Kevin Giewont1, Todd J. Toops2, Eric A. Walker3, Caitlin Horvatits1 and Eleni A. Kyriakidou1,*
1Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA
2National Transportation Research Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
3Institute of Computational and Data Sciences, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA
Gas phase oxidation reaction of NO to NO2 is facilitated by radicals formed during hydrocarbon (HC) oxidation, also known as “mutually sensitized” oxidation of NO and HCs [1-3]. This reaction may occur in a diesel vehicle aftertreatment system due to the co-existence of O2, HCs and NO. Specifically, the low temperature oxidation catalyst test protocol defined by US DRIVE recommends a low temperature combustion of diesel (LTC-D) gas composition of 12% O2, 6% H2O, 6% CO2, 400 ppm H2, 2000 ppm CO, 100 ppm NO, 500 ppm C2H4, 300 ppm C3H6, 100 ppm C3H8 and 2100 ppm n-C10H22 (HCs in C1 basis) . Therefore, studies that use the LTC-D protocol should not attribute all reactions to the catalyst under study, because such a misattribution could be misleading. HC oxidation can take place in an empty reactor under LTC-D conditions forming ·RO2 radicals. These radicals can be reactive for the oxidation of NO to NO2 as follows: NO + ·RO2 → NO2 + ·RO [2,5].
Herein, we report the effect of HCs (C2H4, C3H6, C3H8 and n-C10H22) on NO oxidation under LTC-D conditions. These experimental results are achieved by flowing the LTC-D gas mixture (333 ml/min) through an empty reactor at a ramping temperature (100-500oC). Gas phase NO oxidation was not observed when only O2 and NO were present in the reactor feed (Fig. 1a). The contribution of C2H4, C3H6, C3H8 and n-C10H22 in gas phase oxidation of NO was evaluated. The results indicated that C3H6 and C3H8 did not facilitate NO oxidation. However, NO oxidation was observed in the presence of C2H4 at temperatures greater than 470 oC. Specifically, 35% NO conversion was observed at 487 oC when NO and C2H4 were present in the reactor feed. Moreover, 100% NO conversion was achieved at 330 oC in the presence of n-C10H22 (Fig. 1b). The gas phase reaction of NO to NO2 in the presence of n-C10H22 led to the formation of HNCO, indicating that NO was able to directly or indirectly interact with n-C10H22. Density functional theory calculations were conducted to investigate thermodynamically-possible initiating elementary steps in the reaction network of mutually-sensitized oxidation of NO and n-C10H22. Unimolecular decomposition (n-C10H22 decomposes by itself) is not thermodynamically feasible. A thermodynamically-favorable mechanism for mutually sensitized oxidation is the formation of a gas-phase radical, peroxy decyl (·C10H21O2). ·C10H21O2 is produced via reaction of n-C10H22 with O2 (recombination of decyl radicals ·C10H21 with O2) (Fig. 1c) . ·C10H21O2 then oxidizes NO to NO2 with a change in free energy of -0.95 eV at 330 oC (Fig. 1c). Future work is to investigate why NO does not oxidize by itself (thermodynamically downhill) but NO does oxidize via other pathways when in the presence of HCs and oxygen. Overall, this work illustrates that the gas phase oxidation reaction of NO to NO2 was facilitated in the presence of n-C10H22 under LTC-D conditions without a catalyst. A candidate intermediate for oxidizing NO is ·C10H21O2 radical, although other peroxy intermediates are possible.
Figure 1. NOx (NO and NO2) and n-C10H22 concentration profiles at the outlet of an empty reactor as a function of temperature in the (a) absence and (b) presence of n-C10H22. Gas composition: 100 ppm NO, 0 or 2100 ppm n-C10H22, 12% O2, balance Ar. (c) Proposed initial elementary reaction steps for the mutually sensitized oxidation of NO and n-C10H22 obtained through density functional theory calculations.
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