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Photocatalysis is regarded as one of the most promising technologies for indoor volatile organic compounds (VOCs) elimination due to its low cost, safe operation, energy efficiency, and high mineralization efficiency under ambient conditions. However, the practical applications of this technology are limited, despite considerable research efforts in recent decades. Until now, most of the works were carried out in the laboratory and focused on exploring new catalytic materials. Only a few works involved the immobilization of catalysts and the design of reactors for practical applications. Therefore, this review systematically summarizes the research and development on photocatalytic oxidation (PCO) of VOCs, with emphasis on recent catalyst's immobilization and reactor designs in detail. First, different types of photocatalytic materials and the mechanisms for PCO of VOCs are briefly discussed. Then, both the catalyst's immobilization techniques and reactor designs are reviewed in detail. Finally, the existing challenges and future perspectives for PCO of VOCs are proposed. This work aims to provide updated information and research inspirations for the commercialization of this technology in the future.

Highlights • Pure CeO 2 can achieve both photocatalysis and catalytic ozonation in VUV-PCO system. • High toluene removal, mineralization and ozone removal can be achieved by pure CeO 2. • Amount of Ce3+ plays a critical role in toluene removal. • Differences in photocatalytic activity were explained by electrochemical behaviors. Abstract Volatile organic compounds (VOCs) are one of the biggest concerns in the ambient environment that have attracted a great number of studies related to its remediation. Vacuum ultraviolet (VUV)-based photocatalytic oxidation (PCO) is a promising technology in controlling VOCs. CeO 2 is found to be a promising catalyst for the VUV-PCO technology possessing catalytic capacities in both the photocatalysis and ozonation processes. In this study, three typical kinds of pure CeO 2 with various nanostructures were synthesized and creatively applied for VUV-photocatalytic oxidation of toluene. Material characterizations, including BET, SEM, TEM, XRD, Raman, XPS, H 2 -TPR and DRS, were conducted to characterize different CeO 2 samples. Results indicated that synthetic CeO 2 samples have a much more remarkable enhancement in degrading toluene compared to commercial ones. Higher COx generation and mineralization rates were obtained with the use of all the synthetic CeO 2 samples. Compared to commercial CeO 2 , the synthetic samples have much better capacity towards removing ozone and the best activity in ozone decomposition was observed in CeO 2 Nanorods catalysts. Both the effects of photocatalysis and catalytic ozonation exist in all the CeO 2 catalysts. The amount of Ce3+ plays a critical role in both the catalytic activity of photocatalysis and ozonation that higher ratio of Ce3+ leads to better performance in toluene removal. The differences in photocatalytic activity were further explained by the results of electrochemical behaviors of different CeO 2 samples which showed varying responses to UV irradiation. [ABSTRACT FROM AUTHOR]

• Toluene was efficiently removed over F-TiO 2 nanosheets under VUV-PCO process. • The role of F ion on photocatalytic toluene degradation was systematically explored. • α-MnO 2 nanowire was excellent for residual ozone removal. • Different substrates were studied for potential industrial application. The particular role of fluorine (F) doping in the photocatalytic activity of nanocrystalline titanium dioxide (TiO 2) towards toluene degradation was systematically studied. TiO 2 nanosheet catalysts doped with different amounts of F− (denoted by F-TiO 2) were synthesized by hydrothermal method. For comparison, calcination was performed on the catalyst to remove F-. The photocatalytic oxidation (PCO) of toluene over F-TiO 2 was found to be much higher than those of TiO 2 and commercial P25 under the irradiation of vacuum ultraviolet (VUV) light, with a high toluene degradation efficiency of about 90 %. Moreover, the toluene degradation efficiency increased to 96 % with the addition of only 1 wt.% α-MnO 2 nanowires, and the residual ozone (O 3) in the system can be completely eliminated. In addition, four different substrates were explored for catalyst coating to facilitate the operation and minimize catalyst loading, which can provide a promising strategy for practical volatile organic compounds (VOCs) abatement in industrial applications. [ABSTRACT FROM AUTHOR]

The optimized C-USTiO 2 photocatalysts on carbon cloth for a toluene degradation test at continuous toluene gas flow under LED light irradiation showed a superior toluene removal efficiency. • USTiO 2 with abundant active sites was obtained by a facile acid assisted solvent method. • Light absorption ability of C-USTiO 2 was promoted through a carbon doping process. • Photo-generated electrons and holes of C-USTiO 2 were separated efficiently. • Optimized C-USTiO 2 exhibited superior toluene removal rate and robust stability under visible LED light irradiation. Photocatalytic activity triggered by visible light from light emitting diode (LED) is highly restricted by limited active sites, poor light absorption, and sluggish photo-generated carrier separation of the photocatalysts. Particularly for continuous degradation reaction of volatile organic compounds (VOCs), confined contact between flowing air pollutant gas and photocatalysts renders the degradation ineffective within a transient retention period. Considering the restriction of photocatalytic reaction occurred on semiconductor surface, ultra-small TiO 2 (USTiO 2) nanoparticles were fabricated by a facile acid-assisted solvent method, which possess copious active sites. A simple heating process was further applied for carbon doping on USTiO 2 (C-USTiO 2) surface. As a result, the obtained C-USTiO 2 exhibited high visible light absorption, rich photo-generated carrier density, and its rapid carrier separation performance. In this study, optimized C-USTiO 2 photocatalyst was deposited on carbon cloth and tested for continuous toluene degradation under LED light irradiation. Results showed that the toluene removal efficiency can reach higher than 80% with high concentration of CO 2 generated and an excellent cycle stability of over 180 min. [ABSTRACT FROM AUTHOR]

Highlights • Toluene removal efficiency was greatly improved in the presence of Mn-TiO 2 /CeO 2 catalyst. • Ozone could be completely utilized and removed by compositing Mn-TiO 2 with CeO 2. • CeO 2 improved the generation of hydroxyl radicals (OH) which played a significant role in toluene degradation. Abstract Volatile organic compounds (VOCs) are attracting more and more attention among all the air pollutants. Photocatalytic oxidation under vacuum ultraviolet (VUV) irradiation presents a novel and promising process for VOC degradation while also generating ozone as a byproduct. To improve the degradation efficiency of VOCs and remove the residual ozone Mn-TiO 2 /mesoporous CeO 2 composite catalysts were developed and used as a catalyst in a VUV-PCO system for toluene degradation. Characterizations including SEM, TEM, BET, XRD, XPS, Raman and H 2 -TPR, were conducted to examine the physical and chemical features of the synthetic samples. Results of a toluene degradation test showed that the Mn-TiO 2 /CeO 2 catalyst had superior activity in removing toluene and ozone to Mn-TiO 2 and pure CeO 2. The toluene removal efficiencies of the catalysts with a different molar ratio of Ti/Ce were found to be in the order of Mn-TiO 2 /CeO 2 (1:1) > Mn-TiO 2 /CeO 2 (1:2) > Mn-TiO 2 /CeO 2 (2:1). The mineralization rate increased in the order of Mn-TiO 2 /CeO 2 (1:1) > Mn-TiO 2 /CeO 2 (2:1) ≈ Mn-TiO 2 > Mn-TiO 2 /CeO 2 (1:2) > CeO 2 > VUV photolysis. High ozone removal rates of over 99.9% were obtained for all the doped samples with Mn, resulting in a residual ozone lower than 0.12 ppm. With the use of the scavenger tert-butanol (TBA) and the electron spin resonance (ESR) test, hydroxyl radicals (OH) were found to play a significant role in toluene oxidation in the VUV-PCO process and ozone is also involved in the formation of OH. [ABSTRACT FROM AUTHOR]

[Display omitted] • Surface grafted Mn2+ speeds up the photo-generated carrier separation of ultra-small TiO 2 (USTiO 2). • Mn2+ decomposes the surplus ozone effectively through the sustainable valence state shift. • Mn2+ functionalized USTiO 2 loaded on carbon cloth exhibits effective toluene degradation activity and robust stability in a flow reactor. • Porous carbon cloth substrate with appropriate structure provides numerous active sites to allow VUV light penetration for toluene degradation with the low amount of catalyst usage. Vacuum ultraviolet (VUV) light-induced photocatalytic degradation of volatile organic compounds (VOCs) is a promising strategy to solve many air pollution problems. However, the effectiveness of the process depends on the photocatalyst adopted. In this study, ultra-small TiO 2 (USTiO 2) nanoparticles were first synthesized through an acid-assisted solvent method; then, Mn2+ was added to form a Mn2+/USTiO 2 composite, which was then deposited onto a carbon cloth substrate by a facile dropping-drying process. Photo-generated carrier separation of Mn2+/USTiO 2 was enhanced and electron recombination was suppressed ascribed to the Mn2+ grafting. Residual O 3 produced by the VUV light was also thoroughly decomposed via the Mn valence state shift, forming reactive oxygen species to further enhance toluene degradation activity. Under VUV light illumination, the optimized 2.0% Mn2+/USTiO 2 on carbon cloth showed a superior toluene degradation efficiency of 98%, with high mineralization efficiency up to 50%. This hybrid structure exhibited robust stability of over 400 min. [ABSTRACT FROM AUTHOR]

• VUV-PCO displayed higher toluene removal and mineralization rate than VUV photolysis. • O 2 enabled greater enhancement for toluene degradation than H 2 O introduced. • Toluene and O 3 was reduced as H 2 O participating in aerobic VUV based processes. • The intermediates were affected by the comprehensive effect of O 2 , H 2 O and catalyst. Volatile organic compounds (VOCs) exists ubiquitously in chemical industries and were regarded as major contributors to air pollution, which should be strictly regulated. Vacuum ultraviolet irradiation coupled with photocatalytic oxidation (VUV-PCO) has been considered as an efficient approach to VOCs removal due to high-energy photons which could break down VOCs directly and be absorbed by photocatalysts to generate free radicals for further oxidation. However, the photochemical transformation mechanisms of VOCs have not been fully revealed. Herein, we systematically analyzed the intermediates using proton-transfer-reaction mass spectrometer (PTR-MS) to explore the transformation mechanisms of toluene degradation in VUV and VUV-PCO processes. VUV-PCO process displayed superior toluene degradation efficiency (50 %) and mineralization efficiency (65 %) compared with single VUV photolysis (35 %) and UV photocatalysis (5 %). TiO 2 was deeply involved into CO 2 generalization by amplifying the advantages of VUV system and further mineralizing the intermediates. In VUV and VUV-PCO processes, O 2 participation changed the intermediates distribution by increasing multiple oxygenated products, while the introduction of water contributed to the formation and degradation of most intermediates. A possible degradation mechanism of toluene under VUV irradiation combined with TiO 2 was proposed. This study provides a deep mechanistic insight into VOCs degradation by VUV-PCO process. [ABSTRACT FROM AUTHOR]