Haibao Huang

Low temperature catalytic oxidation of volatile organic compounds: a review

Volatile organic compounds (VOCs) are toxic and recognized as one of the major contributors to air pollution. The development of efficient processes to reduce their emissions is highly required. Complete catalytic oxidation is a promising way to convert VOCs, especially with low concentration, into harmless CO2 and water. This reaction is highly desirable to proceed at low temperature for the consideration of safety, energy savings, low cost and environmental friendliness. Great efforts have been devoted to develop efficient catalysts in order to reduce the temperature of catalytic oxidation of VOCs. The present review highlights recent important progress in the development of supported noble metal and metal oxide catalysts in this field. We examined several typical metals that are widely adopted as essential components for catalytic oxidation of VOCs and explored the effect of some important influencing factors such as the properties of metal and support, dispersion, particle size and morphology of metals. The specific mechanism that leads to superior catalytic activity towards low temperature VOC oxidation was discussed too.

Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation

A series of highly active Pt/TiO2 catalysts were prepared by impregnation and deposition precipitation methods with different reduction processes. Their catalytic activities were evaluated by catalytic decomposition of formaldehyde (HCHO) at room temperature. The effects of reduction treatment on structural properties and catalytic activity were studied. Reduced Pt/TiO2 catalysts showed large differences in structural properties (such as particle size, oxidation state, surface content and electronic property of Pt nanoparticles, and surface oxygen) and catalytic activity for HCHO oxidation compared with the unreduced ones. Nearly 100% HCHO conversion was achieved on the former. Especially, sodium borohydride reduced Pt/TiO2 catalysts even with 0.1% Pt loading showed nearly complete oxidation of HCHO. Well-dispersed and negatively charged metallic Pt nanoparticles, and rich chemisorbed oxygen are probably responsible for their high catalytic activities.

Effect of oxygen mobility in the lattice of Au/TiO2 on formaldehyde oxidation

Two Au catalysts supported on TiO2 were prepared by impregnation method followed by sodium borohydride reduction or calcination in air (Au/TiO2-R and Au/TiO2-C, respectively). The 1 wt % Au/TiO2-R sample was found to be highly efficient for the oxidation of low concentrated formaldehyde at room temperature. A HCHO conversion of 98.5% was achieved with this catalyst, whereas the Au/TiO2-C sample showed almost no activity under the same conditions. Highly dispersed metallic Au nanoparticles with small size (∼3.5 nm) were identified in the 1 wt % Au/TiO2-R catalyst. A significant negative shift of Au4f peak in XPS spectra with respect to bulk metallic Au was observed for the 1 wt % Au/TiO2-R but no similar phenomena was found for the heat-treated catalyst. More Au nanoparticles and higher content of surface active oxygen were identified on the surface of the Au/TiO2-R in comparison with the Au/TiO2-C, suggesting that the Au/TiO2-R catalyst can enhance the amount of active sites and species involved in for HCHO oxidation. The reduction treatment by sodium borohydride promotes the formation of dispersed metallic Au nanoparticles with small size because it facilitates the electron transfer and increases the content of surface Au nanoparticles and activated oxygen. All these factors are responsible for a high activity of this catalyst in the oxidation of HCHO.

Abatement of Toluene in the Plasma-Driven Catalysis: Mechanism and Reaction Kinetics

The mechanism and reaction kinetics of toluene destruction in a plasma-driven catalysis (PDC) system were studied. The results show that the toluene removal efficiency (TRE) is greatly increased while the level of O3 by-product is significantly reduced in PDC as compared with that in nonthermal plasma (NTP). The rate constant of toluene destruction in the PDC is more than twice than that in NTP. Among the multiple reactive species responsible for toluene destruction in the PDC, hydroxyl radicals (·OH) had a small contribution, whereas energetic electrons and atomic oxygen (O) were the most important. The enhanced performance of toluene destruction by PDC was mainly due to greater amounts of O formed during the process. The catalysts improved toluene destruction by catalytic decomposition of O3 and generation of O. Essentially, better toluene abatement can be achieved by focusing on the increased energy density and improved performance of the catalyst for O3 decomposition.

Plasma-Driven Catalysis Process for Toluene Abatement: Effect of Water Vapor

Plasma-driven catalysis (PDC) was used to remove toluene in air. Water vapor is a critical operating parameter in this process. Its effect on toluene removal efficiency, carbon balance, CO2 selectivity, and outlet O3 concentration was systematically investigated. Results showed that water vapor imposed negative effect on toluene decomposition since it depressed the formation and catalytic decomposition of O3. Water vapor deposited on the catalyst would cover the catalytic active sites, resulting in the deactivation of the catalyst. There was an optimum water vapor content for the highest carbon balance and CO2 selectivity. The present paper sheds some insight into the effect of water vapor and provides a valuable basis for the application of the PDC technology.

Removal of formaldehyde using highly active Pt/TiO2 catalysts without irradiation

Formaldehyde (HCHO) is one of the major indoor air pollutants. TiO2 supported Pt catalysts were prepared by sol-gel method and used to eliminate HCHO at room temperature without irradiation. The reduced Pt/TiO2 catalyst (denoted as Pt/TiO2-H2) showed much higher activity than that calcined in air (denoted as Pt/TiO2-air). More than 96% of the conversion of HCHO was obtained over 0.5 wt% Pt/TiO2-H2, on which highly dispersed metallic Pt nanoparticles with very small size (~2 nm) were identified. Metallic Pt rather than cationic Pt nanoparticles provide the active sites for HCHO oxidation. Negatively charged metallic Pt nanoparticles facilitate the transfer of charge and oxygen species and the activation of oxygen.

Recent Development of VUV-Based Processes for Air Pollutant Degradation

As air pollution become more and more serious nowadays, it is essential to find out a way to efficiently degrade the air pollutants. Vacuum ultraviolet (VUV)-based processes are an emerging and promising technologies for environmental remediation such as air cleaning, wastewater treatment and air/water disinfection. With VUV irradiation, photolysis, photocatalyst is and ozone-assisted oxidation are involved at the same time, resulting in the fast degradation of air pollutants because of their strong oxidizing capacity. The mechanisms of how the oxidants are produced and reacted are discussed in this review. This paper mainly focuses on the three VUV-based oxidation processes including VUV photolysis, VUV combined with ozone-assisted oxidation and VUV-PCO with emphasis on their mechanisms and applications. Also, the outlooks of these processes are outlined in this paper.

Photocatalytic Oxidation of Gaseous Benzene under 185nm UV Irradiation

Benzene is a toxic air pollutant and causes great harm to human being. Photocatalytic oxidation (PCO) has been frequently studied for benzene removal, however, its PCO efficiency is still very low and the photocatalysts are easy to be deactivated. To improve the efficiency and stability of PCO, UV lamps with partial 185 nm UV irradiation were used to activate photocatalysts (denoted as 185-PCO). Cobalt modified TiO2 (Co-TiO2) was developed to improve the PCO activity and eliminate ozone generated from 185 nm UV irradiation. Results show that benzene removal efficiency of PCO with 254 nm UV irradiation (denoted as 254-PCO) is only 2.1% while it was greatly increased to 51.5% in 185-PCO. 185-PCO exhibited superior capacity for benzene oxidation. In the 185-PCO process, much ozone was left in case of TiO2 as photocatalysts while it can be nearly eliminated by 1% Co-TiO2.

A Photocatalytic Rotating Disc Reactor with TiO2 Nanowire Arrays Deposited for Industrial Wastewater Treatment

A photocatalytic rotating disc reactor (PRD-reactor) with TiO2 nanowire arrays deposited on a thin Ti plate is fabricated and tested for industrial wastewater treatment. Results indicate that the PRD-reactor shows excellent decolorization capability when tested with methyl orange (>97.5%). Advanced oxidation processes (AOP), including photocatalytic oxidation and photolytic reaction, occurred during the processing. Efficiency of the AOP increases with reduction in light absorption pathlength, which enhanced the photocatalytic reaction, as well as by increasing oxygen exposure of the wastewater thin film due to the rotating disc design. It is found that, with a small dosage of hydrogen peroxide, the mineralization efficiency of industrial biodegraded wastewater can be enhanced, with a superior mineralization of >75% total organic carbon (TOC) removal. This is due to the fact that the TiO2 photocatalysis and hydrogen peroxide processes generate powerful oxidants (hydroxyl radicals) that can strongly improve photocatalytic oxidation efficiency. Application of this industrial wastewater treatment system is benefited from the TiO2 nanowire arrays, which can be fabricated by a mild solvothermal method at 80 °C and under atmospheric pressure. Similar morphologies and microstructures are found for the TiO2 nanowire arrays deposited on a large metal Ti disc, which makes the wastewater treatment process more practical and economical.

Novel urchin-like Fe2O3@SiO2@TiO2 microparticles with magnetically separable and photocatalytic properties

Novel urchin-like microparticles with photocatalytic activity and magnetically separable properties were prepared by a layer-by-layer assembly process. Photocatalytic degradation was investigated using two types of mercury vapor lamps: an ozone generating lamp emitting at both 254 nm and 185 nm as well as a germicidal lamp emitting at 254 nm only. This novel photocatalyst demonstrated superior photocatalytic activity in the mineralization of phenol under UVC illumination compared with the commercial P25, Degussa TiO2, especially in repeated usage. Importantly, this photocatalyst can be quickly separated for reuse simply by using a magnet. The merits of 3D spiny nanostructured TiO2 microparticle photocatalysts are high specific surface area, good permeability, reduced charge recombination rate and high catalytic activity while the incorporation of magnetically separable properties enables rapid and easy retrieval of the suspended photocatalyst after use.