Abstract
A comprehensive understanding of NO catalytic oxidation on different La-based perovskites LaBO3 (B=Mn, Fe, Co) enables to ultimately utilize the catalyst in the lean-burn NOx after treatment system. Here, we report a comparative study of the NO oxidation on LaBO3 (B = Mn, Fe and Co) surfaces by first-principles calculations though density functional theory (DFT). Based on the adsorption of NOx (x=1, 2 and 3) on the LaO and BO2 terminations of (001) surface, we find that the NOx adsorbates are bound stronger on the LaO terminations than BO2 ones. Infrared vibrational spectra and the NO oxidation reactions calculations suggest that BO2 surfaces are more active compared to LaO ones. The primary step for NO oxidation is the desorption of NO2* from the BO2 surfaces with a sequence of barrier 1.43eV, 1.60eV, 1.68 eV for CoO2, MnO2, and FeO2 terminations, respectively. Fundamentally, least charge transfer from CoO2 surface to NO2 ensures its smallest activation energy in contrast to the other two BO2 terminations. These findings provide insights into the influence of B-site transition metal and different terminations on NO oxidation activity of La-based perovskites which might be extended to design of other NO oxidation catalysts.
Highlights
The NO catalytic oxidation is a critical step in the learn-burn NOx after treatment technologies.[3,4,5]
In the NOx storage/reduction (NSR) technology,[4] NOx is firstly adsorbed by the lean NOx trap (LNT) in the fuel lean period and reduced to N2 in subsequent fuel rich period during which the oxidation of NO determines the LNT’s trapping efficiency.[6]
In the surface reaction pathways of NO catalytic oxidation, the trends of energy changes of reaction steps on LaO terminations of three perovskites are similar and the generated NO2∗ adsorbates are energetically unfavorable for desorption from these terminations, leading to the formation of the stable nitrite by the reduced NOx adsorbates and the surface La cations
Summary
The lean-burn diesel engine system (air/fuel ratio >20) displays significant fuel efficiency over the gasoline engine due to the higher compression ratios.[1,2] The NO catalytic oxidation is a critical step in the learn-burn NOx after treatment technologies.[3,4,5] In the NOx storage/reduction (NSR) technology,[4] NOx is firstly adsorbed by the lean NOx trap (LNT) in the fuel lean period and reduced to N2 in subsequent fuel rich period during which the oxidation of NO determines the LNT’s trapping efficiency.[6]. Development of catalyst to oxidize NO is crucial to achieve the high efficiency of NOx removal. The ternary transition perovskite oxides (TMOs) ABO3 (A-sites with rare-earth metal or alkaline earth metal cations and B-sites with transition metal cations) have recently attracted great attention to replace Pt in the NOx after treatment systems due to their low cost, high catalytic activities, and good thermal durability.[6,12,13,14,15,16,17] Different single phase metallic and binary oxide surfaces, the ternary oxides tend to have complicate terminations even on the same surfaces. Chen et al.[18] applied first-principles phase diagram calculations to demonstrate the energetic preference of the termination LaO-(001) p(1 × 1) surface of perovskite LaCoO3.
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