Amorphous saturated cerium–tungsten–titanium oxide nanofiber catalysts for NOx selective catalytic reaction

Abstract

Herein for the first time, Ce_0.184W_0.07Ti_0.748O_2−δ nanofibers are prepared by electrospinning to serve as a catalyst in the selective catalytic reduction (SCR) process. The addition of cerium is proven to inhibit crystallization of TiO₂, yielding an amorphous TiO_x-based solid solution stable up to 500 °C in air, with supersaturated substitutional Ce. At higher temperatures, anatase phase (titanium oxide) is then observed along with fluorite (cerium oxide). Tungsten is instead demonstrated to promote the reduction of the Ce⁴⁺ to Ce³⁺ with the formation of oxygen vacancies (δ). Catalytic experiments under the best working conditions (dry and in the absence of SO₂) are performed to characterize the intrinsic catalytic behavior of the new catalysts. At a temperature lower than 300 °C, superior NO_x conversion properties of the amorphous TiO_x nanofibers over the crystallized TiO₂ (anatase) nanofibers are observed and attributed to higher specific surface area (SSA), larger amount of oxygen vacancies, and higher amount of Ce³⁺ over Ce⁴⁺. Comparison with literature data for ceria–tungsten-based nanoparticles also points out higher catalytic performances for the developed nanofibers at the lowest temperatures (<300 °C). This is mainly attributed to the unique nanofibrous morphology and to the doping approach. The stability of the amorphous Ce–W–TiO_x nanofibers over time (120 h) and over a number of cycles (5) is demonstrated. Yet, superior catalytic performances of the developed catalysts in a wide range of temperatures (200–500 °C) over state-of-the-art material V–W–titania nanoparticles and nanofibers are also proven.