Photocatalytic bireforming of methane over silver-lanthanum modified graphitic carbon nitride with titania nanocomposite in monolith photoreactor

Abstract

Photocatalytic conversion of carbon dioxide (CO2) and methane (CH4) offers a solution of greenhouse gas mitigation with alternative energy supply. The objective of this study is to design and fabricate photoreactor system and to synthesize silver (Ag) and lanthanum (La) modified protonated carbon nitride (pCN) coupled titanium dioxide (TiO2) photocatalysts for enhanced photocatalytic CO2 reduction with CH4 in the presence of water to fuels. The ternary Ag-La/pCN-TiO2 composite catalysts were synthesized through sonicated assisted hydrothermal and sol-gel methods. The performance of nanomaterials was investigated using photocatalytic bireforming of methane (BRM), dry reforming of methane (DRM), steam reforming of methane and steam reforming of carbon dioxide in a fixed-bed and monolith photoreactors under UV and visible light irradiations. Ag/La-loaded protonated carbon nitride nanotubes (pCNNT) produced both carbon monoxide (486 µmol g-cat-1 h-1) and hydrogen (79 µmol g-cat-1 h-1) under visible light irradiations, while productivity was highest in BRM process, which was further improved in a monolith photoreactor with CO and H2 production rate of 770 and 891 µmol g-cat-1 h-1, respectively. Furthermore, using pCN-TiO2 composite loaded with La, higher amount of CO was obtained, while production of H2 had increased with Ag-loading. More importantly, a remarkable improvement in productivity of both CO and H2 with H2/CO ratio greater than one was obtained using Ag-La co-loaded pCN-TiO2 composite catalyst. The highest CO and H2 production rate of 2105 and 2387 µmol g-cat-1 h-1, respectively, were obtained using BRM process in a monolith photoreactor. The performance of monolith photoreactor was 1.4 and 3.2 fold higher for CO and H2 rich synthesis gas (syngas) production than using fixed-bed reactor over the composite catalyst under UV-light irradiations. The reaction mechanism based on Z-scheme system for DRM and BRM was successfully developed under UV light irradiation, while direct electron transfer was observed under visible light irradiations. The quantum efficiency of 4.07 % and 4.624 % was achieved for CO and H2 production, respectively in a monolith photoreactor, while it was only 1.144 % and 0.548 % in a fixed-bed photoreactor during BRM under UV-light irradiations. Among the operating parameters, feed ratio was the influential parameter to maximize yield and selectivity. The stability test revealed prolonged life and reusability of Ag-La/pCN-TiO2 composite photocatalyst in three cyclic runs. The Langmuir-Hinshelwood model confirms surface reactions due to efficient sorption process in a monolith photoreactor over composite catalysts. In conclusion, Ag-La loaded pCN-TiO2 composite catalyst and monolith photoreactor via BRM provided an ideal system to get hydrogen enrich syngas production for renewable fuels productions.