Highly Stable Photocatalytic Dry and Bi-Reforming of Methane with the Role of a Hole Scavenger for Syngas Production over a Defective Co-Doped g-C₃N₄ Nanotexture

Photocatalytic reduction of CO2 with CH4 through the dry reforming of methane (DRM) is an attractive approach to recycling greenhouse gases into valuable chemicals and fuels; however, this process is quite challenging. Although there is growing interest in designing efficient photocatalysts, they are less stable, and have lower photoactivity when employed for DRM reactions. Herein, we developed a noble metal-free hierarchical graphitic carbon nitride (HC3N4) loaded with cobalt (Co) for highly efficient and stable photocatalytic dry reforming of methane to produce synthesis gases (CO and H-2). The performance of the newly designed Co/HC3N4 composite was tested for different reforming systems such as the dry reforming of methane, bi-reforming of methane (BRM) and reforming of CO2 with methanol-water. The performance of HC3N4 was much higher compared to bulk g-C3N4, whereas Co/HC3N4 was found to be promising for higher charge carrier separation and visible light absorption. The yield of CO and H-2 with HC3N4 was 1.85- and 1.81-fold higher than when using g-C3N4 due to higher charge carrier separation. The optimized 2% Co/HC3N4 produces CO and H-2 at an evolution rate of 555 and 41.2 & mu;mol g(-1) h(-1), which was 18.28- and 1.74-fold more than using HC3N4 during photocatalytic dry reforming of methane (DRM), with a CH4/CO2 feed ratio of 1.0. This significantly enhanced photocatalytic CO and H-2 evolution during DRM was due to efficient charge carrier separation in the presence of Co. The CH4/CO2 feed ratio was further investigated, and a 2:1 ratio was best for CO production. In contrast, the highest H-2 was produced with a 1:1 feed ratio due to the competitive adsorption of the reactants over the catalyst surface. The performance of the composite was further investigated for bi-reforming methane and methanol. Using photocatalytic CO2 reduction with CH4/H2O, the production of CO and H-2 was reduced, whereas significantly higher CO and H-2 evolved using the BRM process involving methanol. Using methanol with CO2 and H2O, 10.77- and 1.39-fold more H-2 and CO efficiency was achieved than when using dry reforming of methane. The composite was also very stable for continuous synthesis gas production during DRM in consecutive cycles. Thus, a co-assisted g-C3N4 nanotexture is promising for promoting photocatalytic activity and can be further explored in other solar energy applications.