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Abstract:
Herein, the development of an improved Pd on carbon nitride catalyst for the direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used for characterizing 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 O2 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 O2 to H2O2. Such low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron-withdrawing from the surrounding ligands, i.e. the N functional group on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide: higher concentration of N sites; a flexible network of π-conjugated polymeric subunits with sp3 linking subunits; a flakes-like morphology with high exposure of reactive C edge terminations. This results in a more effective kinetic stabilization of the electronically modified Pd species.
