Elsevier

Applied Catalysis B: Environmental

Volume 250, 5 August 2019, Pages 337-346
Applied Catalysis B: Environmental

Highly efficient visible-light-driven photocatalytic degradation of VOCs by CO2-assisted synthesized mesoporous carbon confined mixed-phase TiO2 nanocomposites derived from MOFs

https://doi.org/10.1016/j.apcatb.2019.03.054Get rights and content

Highlights

  • N-doped mesoporous carbon wrapped anatase-rutile phase junction TiO2 were synthesized.

  • Formed phase junction and strong interface contact promoted •O2− and •OH generation.

  • TiO2@C-N(30) exhibited 51.9% of mineralization effcicienes at 62.4% of styrene.

  • Reaction mechanism was proposed based on degradation intermediates and key radicals.

Abstract

Improving the visible light response and efficient separation of electron-hole pairs play vital roles in commonly used TiO2 photocatalyst for VOCs degradation. Herein, N-doped mesoporous carbon encapsulated anatase-rutile phase junction TiO2 (TiO2@C-N(x)) was successfully synthesized via the pyrolysis of a representative amine functionalized Ti-based MOF, NH2-MIL-125, under the atmosphere of Ar and subsequent CO2 treatment. Our synthesis stragety was based on the rational regulation of the formation of TiO2 phase junction and the decomposition of amorphous carbon onto the TiO2@C-N (without subsequent CO2 process) using CO2 as both anatase-rutile phase transformation promoter and mild oxidant. Compared with TiO2@C-N, TiO2@C-N(x) nanocomposites with subsequent CO2 process exhibit significantly improved photocatalytic activity as well as mineralization efficiencies. For example, the mineralization efficiency reached 51.9% at 62.4% of styrene degradation within 240 min of visible-light irradiation by using the optimal TiO2@C-N(30) nanocomposites as compared with only 19.7% mineralization efficiency at 31.0% of styrene degradation under the same conditions of TiO2@C-N. Furthermore, the primary radicals involved in degradation of VOCs was identified by electron paramagnetic resonance spectroscopy, and the possible degradation intermediates were also monitored by means of proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). Finally, the radicals involved degradation reaction mechanism was also tentatively proposed.

Graphical abstract

N-doped mesoporous carbon confined TiO2 nanocomposites with controllable TiO2 phase junction with strong interface interaction were synthesized for the first time via the pyrolysis of NH2-MIL-125 using CO2 as both phase transformation agent and mild oxidant. The resulting TiO2@C-N(x) nanocomposites exhibited higher degradation and mineralization effciencies as compared with TiO2@C-N in the photocatalytic degradation of gaseous styrene under vsisble light irradation.

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Introduction

Volatile organic compounds (VOCs), as a important group of pollutants in air, have imposed adverse effects on ecological environment and human health due to its toxicity and environmental persistence [1,2]. Semiconductor photocatalytic oxidation which has great potential to degrade a large variety of VOCs into innocuous CO2 and H2O under light illumination has been regarded as an attractive technology to alleviate the above issues [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. In this regard, TiO2 is one of the most extensively investigated and highly promising photocatalysts due to its earth abundance, low cost, excellent stability and relatively strong oxidative ability power [4–11]. Despite these advantages, the wide band gap (∼3.2 eV), weak adsorption of reactants and fast recombination of electron-hole of nanosized TiO2 often result in poor visible light response (only 5% of solar light) and low efficiency [5], which significantly hamper its feasible application for photocatalytic removal of VOCs. To address these issues, the development of a TiO2-based photocatalyst that combines excellent visible light absorption, large surface area and efficient electron-hole separation would be highly desirable, but exceedingly challenging.

Recently, phase junction TiO2 with same chemical composition but different crystal structure has sparked much attention to boost the photocatalytic performance [[12], [13], [14], [15], [16], [17]]. As a typical example, Degussa P25 composed of 80% anatase and 20% rutile phases usually exhibited higher photocatalytic activity compared with its pure anatase or rutile counterparts. It was also demonstrated that the formation of TiO2 phase junction by appropriate alignment of anatase and rutile phase was responsible for the improved photocatalytic activity because the phase junction at the interface can be beneficial to accelerate the charge transfer and thus reduce the electron-hole recombination rate [[12], [13], [14], [15], [16]]. However, its low surface area and weak visible light activity make it still difficult to achieve satisfactory efficiency. On the other hand, due to the large surface area, excellent light harvesting and superior electronic conductivity, the integration of TiO2 with graphitic carbon is considered as an alternative strategy to improve the photocatalytic activity [[18], [19], [20], [21], [22], [23], [24]]. This combination could not only enhance the VOCs adsorption capacity to increase the degradation chances due to large surface area, but also be likely to offer fast electron transfer channel to prevent the recombination of photogenerated electron-hole pairs. In particular, N-doping graphitic carbon can expand the light absorption region from UV to visible light while maintaining their outstanding properties [18,19]. Unfortunately, traditional photocatalysts assembled simply from TiO2 or its precursors with graphitic carbon frequently suffer from the weak interfacial contact and long electron transport distance between graphitic carbon and TiO2, leading limited electron-hole separation [20,25]. To maximize the photocatalytic performance, it is indispensable to explore an efficient strategy to integrate both the merits of the anatase and rutile phase junction TiO2 with N-doping graphitic carbon.

Metal-organic frameworks (MOFs) are an emerging family of porous materials constructed from inorganic metal ions and organic ligands through coordination bonding [26,27]. Recently, MOFs have proved to be an ideal platform for synthesizing a variety of carbon confined metal oxide or metal nanoparticles because the metal nodes and organic ligands in MOFs can be in situ transformed to metal oxide or metal nanoparticles and porous carbon depending on the matrix MOF structure and pyrolysis conditions [[28], [29], [30]]. The resulting nanocomposites are able to partly inherit the intriguing structural features of parent MOFs, including large surface area, uniform heteroatom doping (e.g., N, S), highly dispersed active sites and controllable compositions [28,29,[31], [32], [33]]. More importantly, obviously different from previous methods involving TiO2 or its precursors deposited onto carbon, the coordination bondings between metal nodes and organic ligands in MOFs are capable of providing strong interactions between the in situ formed metal oxide and carbon during the pyrolysis process, which would guarantee the excellent interfacial contact. Additionally, it has demonstrated that the phase transformation between anatase and rutile TiO2 can occur by adjusting the reducibility and oxidizability of the calcination atmosphere [34,35]. Therefore, accurate selection of MOF and its pyrolysis conditions could obtain N-doped mesoporous carbon confined anatase-rutile phase junction TiO2 (TiO2@C-N(x)) with well synergistic effects and gain optimal photocatalytic performance for VOCs efficient degradation.

Bearing the aforementioned considerations in mind, a facile synthetic strategy was developed to achieve TiO2@C-N(x) nanocomposites by direct pyrolysis of a representative amine functionalized Ti-based MOF, NH2-MIL-125, under the treatment of Ar and subsequent CO2 atmosphere. Interestingly, simultaneous control over the ratio of anatase-rutile TiO2 and surface properties of carbon shell were acquired by the use of CO2 as both phase transformation agent and mild oxidant. Moreover, the introduction of CO2 could enlarge the existing pores and even create new pore by oxidative decomposition of the amorphous carbon derived from MOF under Ar atmosphere while the more stable graphitic carbon would be retained. Such regulations were conducive to accelerate the electron transfer and shorten the transport distance between the photogenerated carriers and reactants, leading to efficient photogenerated electron-hole separation and then improved photocatalytic activity. Remarkably, the TiO2@C-N(x) nanocomposites with CO2 treatment exhibited extraordinary high photocatalytic effciecies and the mineralization efficiency.

Section snippets

Chemicals

Tetrapropyl orthotitanate (98%, Ti-(OC3H7)4), 2-aminoterephthalic acid (98%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from J&K Scientific Ltd. Methanol (A.R.), N,N-dimetylformanide (A.R.) and styrene (A.R.) were purchased from Sinopharm Chemical reagent Co., Ltd. and used without any purification.

Synthesis of NH2-MIL-125

NH2-MIL-125 was prepared by a solvothermal method according to the previously reported procedures [36]. Briefly, tetrapropyl orthotitanate (2.4 mL), 2-aminoterephthalic acid (2.2 g), DMF

Characterization of photocatalysts

In this work, N-doped mesoporous carbon encapsulated anatase-rutile phase junction TiO2 was synthesized by direct pyrolysis of NH2-MIL-125 under Ar and subsequent treatment of CO2, and the synthesis strategy was illustrated in Scheme 1. The crystallographic structure of the NH2-MIL-125, TiO2@C-N and TiO2@C-N(x) samples was examined by the XRD patterns. As showed in Fig. S1, the XRD peaks of the as-synthesized NH2-MIL-125 matched well with the simulated XRD patterns, demonstrating the formation

Conclusions

In summary, a facile strategy was developed to construct the N-doped mesoporous carbon confined anatase-rutile phase junction TiO2 composites via direct pyrolysis of NH2-MIL-125 under Ar and subsequent CO2 atmosphere. The inert Ar atmosphere offered a benign environment for pyrolyzing MOF to form TiO2@C-N. Subsequently, the weak oxidative CO2 served as an anatase-rutile phase transformation promoter and favored decomposing the amorphous carbon of TiO2@C-N to improve the ratio of graphitic

Acknowledgments

This work was supported by National Natural Science Foundation of China (21406075, 41425015, and 41731279), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01Z032), Pearl River S&T Nova Program of Guangzhou (201806010177), Guangdong Special Support Plan for Science and Technology for Innovation leading scientists (2016TX03Z094 and 2016TQ03Z291) and the Innovation Team Project of Guangdong Provincial Department of Education (2017KCXTD012).

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