Silver-montmorillonite modified titanium dioxide assisted carbon nitride nanocomposites for photocatalytic hydrogen production through water splitting

Abstract

Photocatalytic hydrogen (H2 ) generation is one o f the most promising solutions to convert solar power into clean energy to replace non-renewable fossil fuel. The objective of this study is to investigate montmorillonite (MMT) dispersed and silver (Ag)-bridged protonated carbon nitride/titanium dioxide (pCN/TiO2) Zscheme heterojunction composite for stimulating photocatalytic H2 evolution under UV and visible light in different photocatalytic reactor systems. The newly designed MMT-Ag/pCN-TiO2 composite photocatalysts were fabricated through a sol-gel assisted hydrothermal method and were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, energy-dispersive X-ray mapping, transmission electron microscopy, Brunauer-Emmett-Teller, ultraviolet-visible (UV-vis) spectroscopy and photoluminescence spectroscopy. The photocatalytic activity was tested using slurry, fixed bed and monolith photo-reactor systems for continuous H2 production. Using slurry system, MMT-Ag/pCN-TiO2 photo-catalyst produced 667 |imol h-1 of H2 which is 8.41 and 9.66 times higher than pCN/TiO2 and TiO2 samples, respectively. The efficiency was improved due to formation of heterojunction with faster charges separation, whereas, Ag provides hot photo-generated electrons by surface plasmon resonance and MMT traps electrons for H2 production. Optimization reveals that the highest production of H2 was obtained at pH 7, glycerol concentration of 5 wt. % and 0.15 g of catalyst loading using slurry reactor. Furthermore, by applying an engineering approach MMT-Ag/pCN-TiO2 showed H2 production rate was increased to 8230 prnol h- 1 using a monolith reactor, which are 9.01 and 12.34 times higher than fixed-bed and slurry photo-reactors. The monolith honeycomb reactor exhibited a higher apparent quantum yield and space yield of 39.85 % and 54.86 |imol h-1cm"3 compared to slurry (22.36 %, 5.13 |imol h-1cm"3) and fixed-bed reactors (4.42 %, 6.09 |imol h-1cm"3). The superior performance of a monolith reactor was due to higher photon flux utilization, large illuminated surface area and processing volume. The schematic of type II heterojunction and Z-scheme mechanism of MMTAg/ pCN-TiO2 were developed and the photocatalytic performance was compared in all types o f systems. In conclusion, excellent performance o f composite catalyst using a monolith reactor compared to a slurry and fixed-bed reactor for H2 production would offer a new opportunity in engineering approach for renewable fuels applications.