NO reduction over an Al-embedded MoS2 monolayer: a first-principles study

Abstract

Converting toxic air pollutants such as nitric oxide (NO) and carbon monoxide (CO) into less harmful gases remains a critical challenge for many industrial technologies. Here, by performing first-principles calculations, we introduce a cheap, stable and novel catalyst for the conversion of NO and CO molecules into N₂O and CO₂ using Al-doped MoS₂ (Al-MoS₂). According to our results, dissociation of NO molecules on Al-MoS₂ has a large energy barrier (3.62 eV), suggesting that it is impossible at ambient temperature. In contrast, the coadsorption of NO molecules to form (NO)₂ moieties is characterized as the first step of the NO reduction process. The formed (NO)₂ is unstable on Al-MoS₂, and hence it is easily decomposed into N₂O molecules, and an oxygen atom is adsorbed onto the Al atom (O_ads). This reaction step is exothermic and needs an activation energy of 0.37 eV to be overcome. Next, the O_ads moiety is removed from the Al atom by a CO molecule, and thereby the Al-MoS₂ catalyst is recovered for the next round of reaction. The side reaction producing NO₂ via the reaction of NO with the O_ads moiety cannot proceed on Al-MoS₂ due to its large activation energy.

Fig. 1. (a) The relaxed geometry, (b) charge density difference and (c) TDOS/PDOS plot of Al–MoS₂. The red and violet regions in charge density difference map indicate the charge loss and accumulation regions, respectively. The dashed line in the TDOS/PDOS plot indicates the Fermi level (set to be zero).

Fig. 2. The geometry configuration, PDOS and EDD plots of (a) NO, (b) ON and (c) CO adsorbed over Al–MoS₂. The dashed line in the PDOS plots shows the Fermi level (set to be zero). The green and violet colors in the EDD plots indicate electron density accumulation and loss regions, respectively.

Fig. 3. The Gibbs free energy profile for the dissociation of (NO)₂ moiety on Al–MoS₂. All bonds distances are in Å.

Fig. 4. The Gibbs free energy profile and relaxed stationary points for the coadsorption of (NO)₂ moiety to form (a) D1 and (b) D2 complexes on Al–MoS₂. All bonds distances are in Å.

Fig. 5. The Gibbs free energy profile and relaxed stationary points for the dissociation of (NO)₂ on Al–MoS₂. All bonds distances are in Å.

Fig. 6. The Gibbs free energy profile and relaxed stationary points for the removal of O_ads by (a) CO and (b) NO molecule. All bonds distances are in Å.