Pd Supported on Carbon Nitride Boosts the Direct Hydrogen Peroxide Synthesis

Herein, the development of an improved Pd on carbon nitride catalyst for direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used to characterize the chemical speciation of the Pd-selective and unselective sites. Selectivity trends among the samples suggest that a bare metal surface with a differential heat of CO chemisorption ranging between 140 and 120 kJ mol(-1) is responsible for the total O-2 hydrogenation, while a maximum threshold value of differential heat of CO chemisorption of approximately 70 kJ mol(-1) is necessary for the partial hydrogenation of O-2 to H2O2. Such a low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron withdrawal from the surrounding ligands: i.e., the N functional groups on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide the following: a higher concentration of N sites, a flexible network of pi-conjugated polymeric subunits with spa linking subunits, and a flakelike morphology with high exposure of reactive C edge terminations. This results in a more effective kinetic stabilization of the electronically modified Pd species.