Synthesis and characterization of Ba(Fe,Zr,Ni)O3 perovskites for potential application in electrochemical NOx decomposition

Abstract

Emissions of nitrogen oxides remain a significant problem associated with the use of fuel-efficient lean combustion in transportation (diesel engines) and the passivation of catalytic converters by excess oxygen in exhaust gasses. One possible way to achieve high-efficiency NOx conversion is electrochemical reduction under anodic polarization in symmetrical solid oxide cells. Recent research was focused on improving efficiency in the presence of 1-15 vol.% oxygen: seeking for NOx selective electrodes based on mixed-conducting transition metal oxides, impregnations with basic components (BaO, BaCO3, K2O) to enhance adsorption of acidic NOx or pre-oxidation of NO to NO2 which is more readily adsorbed on the electrode surface. The present work is focused on the synthesis of Ba-rich Ba(Fe,Zr,Ni)O3 perovskites and characterization of their electrical properties, phase stability, and thermomechanical behavior with the aim to assess the prospects of their use as electrocatalysts for NOx elimination. Ba(Fe1-x-yNixZry)O3-d (x = 0.05–0.5, y = 0–0.4) ceramics were synthesized by glycine-nitrate combustion technique. Single-phase perovskites with a minor NiO impurity were found to form only for prevailing Fe content and with the total Ni and Zr content x+y ≤ 0.3 at 1100°C. Sintering at T ≥ 1200°C leads to segregation of secondary phases. The electrical conductivity of single-phase ceramics is moderate and reaches 3.5 S/cm at 400-500°C; this may limit electrochemical performance. The perovskite phase is demonstrated to be stable in reducing conditions down to pO2 ~ 10-16 atm. The dilatometric studies show unusual and nonlinear behavior of thermal expansion coefficients, with the absolute values varying in the range of 15-50 ppm/K at 250-950°C.