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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.