Boosting C₃H₆ Epoxidation via Tandem Photocatalytic H₂O₂ Production over Nitrogen-Vacancy Carbon Nitride

Abstract

Propylene oxide (PO) is an important raw material in the plastics industry. The propylene epoxidation using O 2 without H 2 and noble metals is considered a fantasy reaction. Here we constructed a tandem system for photocatalytic in situ H 2 O 2 production and propylene epoxidation. In this tandem transformation system, we prepared carbon nitride containing N 3C vacancies in one step by the synergistic action of argon pyrolysis and a supramolecular self-assembly precursor. The introduction of N 3C vacancies significantly promoted the photogenerated electron−hole separation and enhanced the adsorption of H + and O 2 for excellent photocatalytic H 2 O 2 performance (5775 μmol/g/h) via the dual paths involving • O 2 − and 1 O 2 . To avoid the separation and purification of H 2 O 2 , a compatible solvent system consisting of methanol and water was utilized to promote the production of both H 2 O 2 and PO. Benefiting from them, the PO production efficiency came to 5515 μmol/g/h with a selectivity of 99.1%. This work not only opened up an idea for propylene epoxidation using O 2 but also provided a reference for creating more propylene epoxidation systems in series with photosynthetic H 2 O 2 .

Nitrogen‐Based Imperfections in Graphitic Carbon Nitride – New Trend for Enhancing Photocatalytic Activity?

“…Additionally, there was a contraction in interlayer π-π stacking distances from 0.326 to 0.324 nm, accompanied by a slight shift in the (002) plane. [46] Similar results were obtained for the CN synthesized under argon [59,75] / nitrogen [76] /hydrogen gas [77] flow during thermal treatment while air/steam flow [75,78,79] was found to sharpen the (002) peak, indicating the increasing order of CN structure (Figure 3a, b and c). The existence of oxygen gas molecules during synthesis generates polarization on the surface owing to the electronegativity of oxygen atoms, thereby intensifying the interaction between the layers and consequently reducing the interplanar spacing.…”

“…The existence of oxygen gas molecules during synthesis generates polarization on the surface owing to the electronegativity of oxygen atoms, thereby intensifying the interaction between the layers and consequently reducing the interplanar spacing. [75,78,80] Likewise, Shi et al noted a reduction in the distance between CN layers, accompanied by an upward shift in the 2θ angle, with the increased addition of tartaric acid (Figure 3d). [81] Using HNO 3 and H 2 O 2 as additives with the purpose of oxidation were also an enhanced prominence of the (100) peak in CN, signifying an increase in plane size and expanded properties.…”

“…For example, the decrease intensity of peak at 156-157 ppm belonging to C i is attributed to the created N 2C vacancies in the structure [116,121,126] or the increased area of C e is interpreted as the addition of H-bond containing functional groups might strength the area of that corresponding signals [59] and those peaks were compared in some cases (Figure 5c). [75,83,112,121,126,127] In addition to that, the broadness and the new appearing shoulders on these characteristic peaks show the disordered melem units in CN structure after the introduction of Nvacancies. [77,99,100,128] The alteration in the chemical shift of those related signals towards downfield or upfield provides an additional information about the origin of the generated defects in the structure.…”

“…[99,100] While theoretical calculations indicate that removing NH x requires less energy compared to N 2C , the majority of studies have consistently reported the preferential loss of N 2C . [101] Certain reaction conditions, like the previous one, [64] using CaCO 3 template, [102] calcination in the presence of air [75] and the use of HNO 3 , [83] resulted in the removal of N 3C lattice sites. Many strategies alike replacing N atom with an element (P, O and S) or inducing vacancy in the presence of other dopants (Se, Mn, Cu and Mg) contributed to the determination Figure 3.…”

“…[79,84,134] Most of the studies including ours reported similar morphology changes after the thermal treatment of CN (Figure 6b). [57,59,63,75,135] The utilization of a template during synthesis [85,112] resulted in a thinner structure with homogeneously distributed pores matching with the size of template, whereas diverse conditions yielded varying morphologies such as coral-like, [88] rod-like, [83,136] and spherical [48] structures (Figure 6c). For instance, Nguyen et al discussed the effect of reaction atmospheres including air, N 2 , and CO 2 on the synthesis of CN and they reported the formation of smaller pores under CO 2 atmosphere while larger ones in the case of N 2 environment.…”