IR spectra ofWO3, In2O3, Co3O4 and their compositions: (a) WO3, annealing at 600 °С (2 h), (b) WO3, annealing at 600 °С (2 h), after exposure to a gas-air mixture containing 3.6 ppm NO2 (T = 187 °С, t = 30 min.), (c) In2O3, annealing at 600 °С (2 h), (d) In2O3, annealing at 600 °С (2 h), after exposure to a gas-air mixture containing 3.6 ppm NO2 (T = 187 °С, t = 30 min.), (e) Сo3O4, annealing at 450 °С (2 h), (f) Сo3O4, annealing at 600 °С (2 h).