Engineering hydrogenation active sites on graphene oxide and N-doped graphene by plasma treatment

Abstract

Graphene oxide (GO) and N-doped graphene [(N)G] graphenes were submitted to H2 glow discharge under different discharge regimes, in both the negative glow and positive column plasma regions. The resulted catalysts were fully characterized using several techniques such as Raman, DRIFT and XPS spectroscopy, powder X-ray diffraction, H2 pulse chemisorption and H2-, CO2- and NH3-TPD experiments. Density functional theory calculations were performed taking a slab model of graphene sheet with an optimized CC bond length (1.426 Å) and a 16 Å vacuum layer between sheets. An overview of these characterizations showed that the O/C atomic ratio of GO is influenced by the plasma regime, indicating the occurrence of O removal, as also predicted by DFT calculations. In the case of (N)G, the plasma treatment also removes pyridinic N with an increase of the C/N ratio. The efficiency of the plasma modification has been checked through catalytic tests in hydroisomerization of 1-octene and hydrogenation of α-methyl-styrene. Contrarily to classical thermal activation requiring high temperatures, the generation of the defects by treating with plasma occurs at voltages in the range of 2−5 kV. In consequence, the hydrogenation and isomerization of alkenes resulted with high yields and good selectivities. Graphene prepared from sodium alginate from brown algae was considered as reference in these investigations.