Enhanced H₂O₂ Production via Photocatalytic O₂ Reduction over Structurally-Modified Poly(heptazine imide)

Abstract

Solar H 2 O 2 produced by O 2 reduction provides a green, efficient, and ecological alternative to the industrial anthraquinone process and H 2 /O 2 direct-synthesis. We report efficient photocatalytic H 2 O 2 production at a rate of 73.4 mM h –1 in the presence of a sacrificial donor on a structurally engineered catalyst, alkali metal-halide modulated poly(heptazine imide) (MX → PHI). The reported H 2 O 2 production is nearly 150 and >4250 times higher than triazine structured pristine carbon nitride under UV–visible and visible light (≥400 nm) irradiation, respectively. Furthermore, the solar H 2 O 2 production rate on MX → PHI is higher than most of the previously reported carbon nitride (triazine, tri-s-triazine), metal oxides, metal sulfides, and other metal–organic photocatalysts. A record high AQY of 96% at 365 nm and 21% at 450 nm was observed. We find that structural modulation by alkali metal-halides results in a highly photoactive MX → PHI catalyst which has a broader light absorption range, enhanced light absorption ability, tailored bandgap, and a tunable band edge position. Moreover, this material has a different polymeric structure, high O 2 trapping ability, interlayer intercalation, as well as surface decoration of alkali metals. The specific C≡N groups and surface defects, generated by intercalated MX, were also considered as potential contributors to the separation of photoinduced electron–hole pairs, leading to enhanced photocatalytic activity. A synergy of all these factors contributes to a higher H 2 O 2 production rate. Spectroscopic data help us to rationalize the exceptional photochemical performance and structural characteristics of MX → PHI.

Hydrophobized poly(heptazine imide) for highly effective photocatalytic hydrogen peroxide production in a biphasic fatty alcohol–water system

“…A comparative analysis of surface XPS and bulk SEM-EDX elemental analyses indicates that the surface-to-bulk ratio of K + is below 1 for most of the samples. This is in agreement with other report on PHIs, 25 and suggests that the potassium is likely to be found in the bulk of the material, supposedly being a structural part of poly(heptazine imide) network, while in case of KPHI-OTS the K + ions are also present in form of separated KCl (Table S1). The higher surface content of C than that of N can be explained i) by the formation of surface carbonate species in the parent material or the carbon atoms of the modifiers (Table S1), and ii) by the fact that since the samples are prepared in air, there are definitely some adsorbed hydrocarbons on the sample, which will be visible in the surface sensitive XPS.…”

“…13,17 The ionic variants of carbon nitride typically show improved photogenerated charge separation and effective accumulation and storage of photogenerated electrons, [21][22][23][24] and exhibit an improved activity in photocatalytic H 2 O 2 evolution compared to the conventional, non-ionic, melon polymers. 12,13,25 Although, the superior activity of PHI can also be explained, among other factors, by partial substitution of the terminal NH 2 -groups for -N − -C^N species in PHI, resulting in fewer hydrogen bond interactions and better dispersibility of the material in polar solvents. 26 A major challenge in carrying out such reactions in suspension is the effective separation of the dissolved reduction and oxidation products for their subsequent use, in order to avoid the decomposition of H 2 O 2 on the surface of the photocatalyst by photoredox decomposition or by its disproportionation in the dark.…”

Structure and Optical Properties of Polymeric Carbon Nitrides from Atomistic Simulations

“…2.8 eV. 14,15 Typical PCN materials obtained by straightforward thermal synthesis are semicrystalline and contain a multitude of microstructural motifs. 16−20 This lack of crystallinity complicates structural characterization.…”

Synergistic effect of exfoliation and substitutional doping in graphitic carbon nitride for photocatalytic H2O2 production and H2 generation: a comparison and kinetic study

“…9 In this regard, photocatalytic H 2 O 2 production using semiconductor catalysts from water and O 2 through twoelectron O 2 reduction has received tremendous attention worldwide. [10][11][12] However, due to poor visible light harvesting, faster charge carrier recombination, and a deficit of effective surface-active sites, the photocatalytic H 2 O 2 yield is far from the practical application requirement. 13 Among numerous semiconductor photocatalysts, graphitic carbon nitride (g-C 3 N 4 ) is extremely sought after and has received remarkable interest from the scientific community.…”