Insight into Cu2O/CuO collaboration in the selective catalytic reduction of NO with NH3: Enhanced activity and synergistic mechanism

Abstract

Cu-based catalysts have drawn much attention in ammonia-selective catalytic reduction (NH3-SCR) of NOx because of their outstanding low-temperature denitration (de-NOx) potential. Although satisfactory SCR performance was obtained with support from the catalytic activity of CuOx species in recent studies, the effect of the valence distribution of CuOx species on NH3-SCR reaction is still ambiguous, and elaborate exploration is necessary. Here, a series of CuAl-layered double oxide/carbon nanotubes-x (CuAl-LDO/CNTs-x) nanocatalysts with a tunable valence distribution of highly dispersed CuOx species was constructed from the topotactic transformation of CuAl-layered double hydroxide (CuAl-LDH) precursor and a controllable carbothermal reduction reaction. Multiple characterization techniques, including XRD, NH3-TPD, H2-TPR, NOx-TPD, XPS, in situ DRIFTS, and HR-TEM, were employed to elucidate the inherent relationship between the chemical properties and catalytic activity of the as-prepared catalysts. The obtained CuAl-LDO/CNTs-2 catalyst exhibited optimum catalytic performance, with NOx conversion exceeding 90% in the temperature range 180–305 °C. Such high catalytic efficiency can be attributed to the appropriate valence distribution of CuOx species, which can significantly improve the redox capacity and surface acidity of the catalysts, thereby promoting the adsorption and activation of the reactants. Combined with further theoretical calculation, a synergistic catalytic mechanism of Cu2O/CuO in the NH3-SCR reaction is tentatively proposed. Notably, the CuO active center can function as the dominant adsorption site of NO and NH3 to promote the formation of NO+ active species and the dehydrogenation activation of NH3. The Cu2O active center can act as the adsorption site for O, promoting the formation of active oxygen species O−. Consequently, the synergistic effect between Cu2O and CuO can lead to the rapid formation of reactive intermediates, proceeding via the Langmuir–Hinshelwood (reaction between adsorbed NH3 and adsorbed NOx) or Eley–Rideal (reaction between adsorbed NH3 and gaseous NO) mechanism reaction routes to complete the catalytic cycle. This work provides a fundamental understanding on Cu2O/CuO synergistic catalysis of NH3-SCR, which is propitious for the rational design and optimization of Cu-based oxide de-NOx catalysts.