Selective Catalytic Reduction Of NO By H₂ On Three–way Catalysts: A First–principles Study

With the acceleration of the urban process, motor vehicle exhaust has become one of the most important sources of pollution, of which NO_x is the main pollutant. The catalytic removal of NO_x has been one of the key subjects for research in heterogeneous catalysis because the emission of NO_x not only causes a series of environmental problems but also has adverse effect on humans. In the automotive exhausts, platinum group metals（PGMs） such as Pt, Pd, and Rh are involved in the three–way catalysts（TWCs）, which have been used for the NO reduction for several decades. Under lean–burn conditions, due to the existence of excess oxygen, traditional three way catalysts have poor effect for the reduction of NO_x. Selective catalytic reduction（SCR） of NO is an effective way to remove NO_x in the presence of excess oxygen, which selectively reduced NO_x to N₂ by employing NH₃, H₂, CO and HCx as the reducing agents. H₂–SCR has received increasing attention due to its advantage of being with high activity at low operating temperature, as a clean reductant and with being easily available by autothermal reforming of diesel fuel. In this paper, based on the periodic density functional theory（DFT）, we provided a deep insight in the detailed mechanisms of NO reduction by H₂ on TWCs, considering the effect of subsurface oxygen on Pt（100） surface. The microkinetic modeling further revealed the effect of different temperatures and pressures on reaction rates and relative product selectivity. The obtained main results are summarized as follows:1. The detailed reaction mechanisms for H₂–SCR of NO on Pd（111） surface were investigated. The results show that direct NO dissociation is highly disfavored due to high energy barrier and endothermicity; alternatively, NO dimer can be formed on Pd（111） surface at low temperature followed by the N–O bond scission to form N₂ O. The presence of H₂ promotes the NO dissociation via two abstraction reaction pathways, NO + H â†' N + OH and NO + H â†' NH + O, respectively, the energy barriers in which are much lower than that of NO direct dissociation on clean surface. The N₂ formation pathway is NO + N â†' N₂ + O rather than N + N â†' N₂. Besides, N₂ is formed preferentially than N₂ O from the coadsorbed state of NO + N. The NH₃ formation comes from the successive hydrogenation reactions of nitrogen and the NH formation is the rate–determining step. The microkinetic analysis further confirms that N₂ O is major at low temperature while N₂ becomes dominant as temperature increases. The selectivity toward N₂ and NH₃ shifts to slightly lower temperature as H₂/NO ratio increases. The present result shows that it is feasible to achieve high reduction reactivity of NO and selectivity of N₂ for the Pd catalyst by controlling reaction temperatures and H₂ pressure.2. The mechanism of H₂–SCR of NO on Pt（100） and the surface modified with subsurface oxygen atoms（Md–Pt（100）） were studied. The results show that similar catalytic activity toward NO dissociation is found on both surfaces with energy barriers of 0.86 and 0.96 e V, respectively. The pathway N + N â†' N₂ rather than NO + N â†' N₂ + O is the N₂ formation pathway on Pt（100） surface, while these two pathways are competitive on Md–Pt（100） surface. The NH₃ formation is almost negligible and reductant hydrogen can effectively remove the surface oxygen on both surfaces. The microkinetic analysis further confirms that under lower pressure, compared to the high selectivity toward N₂O（almost 100% at 300 – 500 K） on clean surface, higher N₂ low–temperature selectivity（larger than 90%） is achieved on Md–Pt（100） surface. Under the condition of intermediate pressure, N₂ O is the unique product in the 300–700 K temperature range on clean Pt（100） surface, while the selectivity toward N–containing products is almost unchanged on Md–Pt（100） surface. The present study indicates that subsurface oxygen has an enhanced effect for improving the N₂ selectivity of NO reduction on Pt catalysts.3. The detailed mechanisms of NO decomposition and reduction by H₂ on flat Rh（111） and stepped Rh（221） surfaces were investigated. The results demonstrated that stepped Rh（221） surface exhibits higher reactivity for NO reduction than Rh（111）. The NO dissociation on Rh（221） surface has almost no effect in the presence of H₂, whereas the predosed H atoms have slightly inhibited NO dissociation on Rh（111）. Microkinetic calculations further predict the product selectivity for H₂–SCR at different temperatures and pressures. It is shown that under ultrahigh vacuum（UHV） conditions, NH₃ is the only N–containing product on Rh（111）, consistent with the experimental observation; while on Rh（221） surface, the N₂ O formation is predominant at low temperatures and N₂ becomes main product above 480 K. Under near–atmospheric pressure conditions, the product selectivity on Rh（111） surface has almost no change, while N₂ O is the dominant product on Rh（221） in the whole temperature range. The present study indicates that the NO dissociation activity and product selectivity are strongly dependent on both the Rh surface structure and the experimental conditions.