Visible Light-Driven Reforming of Lignocellulose into H₂ by Intrinsic Monolayer Carbon Nitride

Abstract

The photoreforming of lignocellulose is a novel method to produce clean and sustainable H 2 energy. However, the catalytic systems usually show low activity under ultraviolet light; thus, this reaction is very limited at present. Visible light-responsive metal-free two-dimensional graphite-phased carbon nitride (g-C 3 N 4 ) is a good candidate for photocatalytic hydrogen production, but its activity is hindered by a bulky architecture. Although reported layered g-C 3 N 4 modified with active functional groups prepared by the chemical exfoliation enhances the photocatalytic activity, it lost the intrinsic structure and thus is not conducive to understand the structure−activity relationship. Herein, we report an intrinsic monolayer g-C 3 N 4 (∼0.32 nm thickness) prepared by nitrogenprotected ball milling in water, which shows good performance of photoreforming lignocellulose to H 2 driven by visible light. The exciton binding energy of g-C 3 N 4 was estimated from the temperature-dependent photoluminescence spectra, which is a key factor for subsequent charge separation and energy transfer. It is found that monolayer g-C 3 N 4 with smaller exciton binding energy increases the free exciton concentrations and promotes the separation efficiency of charge carriers, thereby effectively improving its performance of photocatalytic reforming of lignocellulose, even the virgin lignocellulose and waste lignocellulose. This result could lead to more active catalysts to photoreform the raw biomass, making it possible to provide clean energy directly from locally unused biomass.

Ultrathin 2D g‐C3N4 nanosheets for visible‐light photocatalytic reforming of cellulose into H2 under neutral conditions

“…16,17 However, up to now, only a few studies of g-C 3 N 4based PR of cellulose into H 2 have been reported. [18][19][20] These works also achieved their PR reactions by dissolving cellulose using strong alkaline conditions, confirming that achieving PR of cellulose under neutral conditions using g-C 3 N 4 as photocatalyst is desirable but challenging. It is documented that bulk g-C 3 N 4 (BCN) can be fabricated directly by thermal polymerization of various inexpensive precursors, but it has a relatively small surface area and low charge separation efficiency, thereby significantly limiting its photoreactivity.…”

“…Graphitic‐like carbon nitride (g‐C 3 N 4 ), a typical metal‐free semiconductor that originally emerged for photocatalytic H 2 production from water in 2009, 15 has been considered as a desirable candidate for PR because of its visible‐light response, nontoxicity, excellent stability and tunable optical features 16,17 . However, up to now, only a few studies of g‐C 3 N 4 ‐based PR of cellulose into H 2 have been reported 18‐20 . These works also achieved their PR reactions by dissolving cellulose using strong alkaline conditions, confirming that achieving PR of cellulose under neutral conditions using g‐C 3 N 4 as photocatalyst is desirable but challenging.…”

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

“…NCN CN x Lignin Simulator –/100 25 1.6 0.16/4.3 M, KPi 40.8 – – [ 52 ] Pt-. NCN CN x Lignin Simulator –/100 25 1.6 0.16/10 M, KOH 14.5 – – [ 52 ] Pt/g-C 3 N 4 Lignin Xe 300/100 5 5 5/pH = 10 20.75 – [ 53 ] NiS/CdS Lignin Xe 300/ 25 1 0.1/lactic acid 147.6 44.9/–λ ≥ 400 nm [ 51 ] Pt/TiO 2 Poplar wood Xe 300/– – 0.4 4 26 – – [ 47 ] Pt/TiO 2 Pinewood Simulator –/200 160 30.0 …”

“…Visible light-responsive photocatalysts such as sulfides ( E bg = ⁓2.4 eV CdS [ 38 , 50 , 51 ]) and carbon nanomaterials ( E bg = ⁓2.7 eV for graphitic carbon nitride [ 52 , 53 ]) have also been developed for H 2 generation from photo-reforming of lignocellulosic substrates. 1D CdS nanowires (NWs) were prepared and loaded with NiS 2 by a two-step hydrothermal method.…”

“…Cellulose, lignin, and raw biomass substrates were photo-reformed to H 2 over NCN CN x (NiP as the co-catalyst) under the irradiation of a solar simulator, which demonstrates nontoxic, noble-metal free, and visible-light-promoted photo-reforming of lignocellulosic materials. Rao et al [ 53 ] reported a monolayer g-C 3 N 4 prepared by nitrogen-protected ball-milling of bulk g-C 3 N 4 in water, which showed activity towards photo-reforming of lignin and hemicellulose to H 2 driven by visible light; the r H 2 was 60 (hemicellulose) and 20.8 (lignin) µmol h −1 g cat −1 , with Pt as the co-catalyst. For 2D layered carbon-based materials, a thinner layered structure could result in larger surface area and more active sites, which would enhance activity for producing H 2 from photocatalytic reaction [ 54 ].…”