Direct synthesis of H2O2 on Pd/Al2O3 contactors: Understanding the effect of Pd particle size and calcination through kinetic analysis

The direct synthesis of H2O2 on Pd-based catalysts, although recognized as a potential route for the sustainable production of H2O2, is still limited by the low catalytic selectivity and safety concerns. Here, the calcination treatment effect, with the dispersion of Pd NPs and its interaction with Al2O3, is investigated. Catalysts have been prepared by the sol-immobilization procedure (SI) on Al2O3 as asymmetric alumina membranes (AAS) and tested in both reduced and calcined form (450°C, 1°C/min, 8 h) for the direct synthesis of H2O2. Finally, the catalytic performance was compared with other catalysts, prepared by hydrazine-reduction (NRC) and impregnation-decomposition (IDC) already reported in a previous paper and calcined for a shorter time (450°C, 1°C/min, 6 h). TEM micrographs showed the formation of Pd NPs with average diameters of 12 (NR), 3.8 (IDC), and 3.3 nm (SI), respectively. The reduced SI catalyst has shown a 2-4% selectivity. However, after calcination (SIC), a 69 % selectivity to H2O2 was reached. Compared with NRC and IDC catalysts, the selectivity increased within the series NRC < IDC < SIC. The kinetic analysis of calcined catalysts showed an overall decrease of the hydrogenolysis and direct combustion routes in the series NRC > IDC > SIC. The SIC catalyst's improved performance was related to the increased interaction with the support, stabilizing Pd in its oxidized form.