Fig. 6. (a) Example absorption spectra of the reoxidation of pre-reduced HPA-Nb at 120 °C (the blue spectra represents t = 0 min). (b) Normalized concentration–time profiles at 750 nm of the reoxidation of pre-reduced HPA-Nb catalyst. Reaction conditions: 80–140 °C, 5 bar synthetic air saturation, 11 μM pre-reduced HPA-Nb initial concentration, LCWM reactor.