Study on the mechanism of synthetic (Ce,La)CO3F sulfuric acid acidification and NH3-SCR loaded with Mn and Fe

Abstract

A hydrothermal method was used to synthesise (Ce,La)CO₃F grain simulated minerals, in accordance with the Ce–La ratio of bastnaesite in the mineralogy of the Bayan Ebo process. The NH₃-SCR catalytic activity of the synthesised (Ce,La)CO₃F was improved by loading transition metals Mn and Fe and sulphuric acid acidification treatments. The activity test results showed that the catalysts which were simultaneously acidified with sulphuric acid and loaded with transition metals Mn and Fe had a NO_x conversion of 92% at 250 °C. XRD, SEM, XPS and in situ Fourier transform infrared spectroscopy (FTIR) were used to investigate the physical phase structure, surface morphology, reaction performance and mechanism of the catalysts, to provide theoretical guidance for the specific reaction path of cerium fluorocarbon ore in the NH₃-SCR reaction. The results showed that the introduction of transition metals and sulphuric acid greatly increases the proportion of adsorbed oxygen (O_α) and facilitates the adsorption of NH₃ and NO. The catalyst surface metal sulphate and metal oxide species act as the main active components on the catalyst surface to promoted the reaction, and cracks and pores appear on the surface to facilitate the adsorption of reactive gases. The reaction mechanism of the SO₄²⁻–Mn–Fe/(Ce,La)CO₃F catalyst, and characterisation of the adsorption and conversion behaviour of the reactive species on the catalyst surface, were investigated by Fourier transform infrared spectroscopy (FTIR). The results showed that the catalyst follows the E–R and L–H mechanisms throughout the reaction, with the E–R mechanism being the main reaction. The reaction species were NH₄⁺/NH₃ species in the adsorbed state and NO. The NH₃(ad) species on the Lewis acidic site is the main NH₃(g) adsorbed species for the reaction, bonded to Ce⁴⁺ in the carrier (Ce,La)CO₃F to participate in the acid cycle reaction, and undergo a redox reaction on the catalyst surface to produce N₂ and H₂O. The SO₄²⁻ present on the catalyst surface can also act as an acidic site for the adsorption of NH₃. The above results indicated the excellent performance of the SO₄²⁻–Mn–Fe/(Ce,La)CO₃F catalyst, which provided a theoretical basis for the high value utilization of bastnaesite.