Phosphorus-Doped Graphene as a Metal-Free Material for Thermochemical Water Reforming at Unusually Mild Conditions

Abstract

[EN] P-doped graphene (Phy-G) prepared by pyrolysis of phytic acid at 900 degrees C under inert atmosphere has been evaluated as a metal-free catalyst for the thermochemical water splitting. XPS, solid-state P-31 NMR, and Raman spectroscopy confirm the presence of P atoms bonded to C atoms in the graphene lattice as well as some oxygenated P groups, such as phosphates or phosphonates. HRTEM and AFM images show the characteristic sheet morphology of 2D graphene materials of several micrometers lateral size and exhibiting a high crystallinity with the characteristic hexagonal arrangement of graphenic materials. Phy-G has been submitted to consecutive oxidation/activation thermochemical cycles at 650 and 800 degrees C under H2O-saturated Ar and dry Ar atmospheres, respectively. During the oxidation periods, H-2 evolution up to 21.6 mu mol/min.g was measured. However, no O-2 evolves in the activation steps. Experimental evidence and computational calculations support the formation of P=O bonds during the oxidation steps. The computational calculations suggest that the thermocatalytic H2O splitting occurs on the P atoms of doped graphene through a stepwise process involving an intermediate with a P-OH group and a H attached to a neighboring C atom and subsequent H-2 evolution, leading to the formation of P-O bonds.