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

Abstract

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 H2). 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 H2 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 H2 at an evolution rate of 555 and 41.2 µmol 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 H2 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 H2 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 H2 was reduced, whereas significantly higher CO and H2 evolved using the BRM process involving methanol. Using methanol with CO2 and H2O, 10.77- and 1.39-fold more H2 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.