Junliang Wu

Decomposition of Toluene in a Plasma Catalysis System with NiO, MnO2, CeO2, Fe2O3, and CuO Catalysts

The performance of NiO, MnO2, CeO2, Fe2O3, and CuO catalysts on alumina in removing toluene from a gas stream was studied in a plasma catalysis system. The NiO catalyst performed better than the other catalysts, generating more toluene-destroying oxygen species by decomposing ozone. The optimum nickel loading in the NiO/γ-Al2O3 catalyst was approximately 5 wt%, close to the monolayer dispersion threshold of NiO on γ-Al2O3. The presence of water vapor had a negative effect on catalytic performance due to its quenching of high speed electrons and its competition with toluene for adsorption sites. Water vapor also reduced the outlet ozone concentration by inhibiting the production of key intermediate in the ozone formation process.

Enhancement of the non-thermal plasma-catalytic system with different zeolites for toluene removal

Based on the important effect of catalyst on the plasma-catalytic system, various types of zeolites (5A, HZSM-5, Hβ, HY and Ag/HY) were chosen as catalysts to remove toluene under non-thermal plasma conditions in this work. The results showed that all the zeolites, with or without toluene adsorption abilities, significantly enhanced the toluene removal efficiency in the plasma discharge zone. Moreover, the carbon balance and CO2 selectivity showed the same tendency of Ag/HY > HY > Hβ (HZSM-5) > 5A, which was basically consistent with toluene adsorption ability, while being opposite to the ozone emission. Loading silver on the zeolite greatly decreased organic byproduct emission, and further improved the mineralization of toluene oxidation. At the same time, the intermediates including ring-opening products on the catalyst surface were identified, and the pathways of toluene decomposition were proposed.

Controllable synthesis of 3D hierarchical Co3O4 nanocatalysts with various morphologies for the catalytic oxidation of toluene

Three-dimensional (3D) hierarchical Co3O4 nanocatalysts with different morphologies and various exposed crystal planes were synthesized via a hydrothermal process without the use of a cobalt surfactant precursor and subsequent direct thermal decomposition. The morphologies obtained include 3D hierarchical cube-stacked Co3O4 microspheres (C sample), 3D hierarchical plate-stacked Co3O4 flowers (P sample), 3D hierarchical needle-stacked Co3O4 double-spheres with an urchin-like structure (N sample), and 3D hierarchical sheet-stacked fan-shaped Co3O4 (S sample), which exhibit high efficiency for the total oxidation of volatile organic compounds (VOCs). Among them, the C sample exhibits the best activity with the temperature required for achieving a toluene conversion of 90% (T90%) of approximately 248 °C and the activity energy (Ea) of 80.2 kJ mol−1, which is at least 32 °C lower than that of the S sample with a higher Ea of 114.9 kJ mol−1 at a space velocity (WHSV) of 48 000 mL g−1 h−1. The effects of morphology on the physicochemical properties and catalytic activity of the Co3O4 catalysts are investigated using numerous analytical techniques. It is concluded that the large specific surface area, highly defective structure with abundant surface adsorbed oxygen species and rich high valence Co ions in the C sample are responsible for its excellent catalytic performance. Moreover, no significant decrease in catalytic efficiency is observed over 120 h at 255 °C on the C sample, which indicates that it exhibits excellent stability for toluene oxidation. Therefore, it shows potential as a non-noble catalyst in practical applications.

Carbon dioxide hydrogenation to methanol over Cu/ZrO2/CNTs: effect of carbon surface chemistry

Methanol synthesis from CO2 hydrogenation in a fixed-bed plug flow reactor was investigated over Cu–ZrO2 catalysts supported on CNTs bearing various functional groups. The highest methanol activity (turnover frequency 1.61 × 10−2 s−1, space time yield 84.0 mg gcat−1 h−1) was obtained over the Cu/ZrO2/CNTs catalyst (CZ/CNT-3) with CNTs functionalized by nitrogen-containing groups and Cu loading only about 10.3 wt% under the reaction conditions of 260 °C, 3.0 MPa, V(H2) : V(CO2) : V(N2) = 69 : 23 : 8 and GHSV of 3600 h−1. The catalysts were fully characterized by N2 physisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR) and temperature-programmed desorption of H2 (H2-TPD) techniques. The excellent performance of CZ/CNT-3 is attributed to the presence of nitrogen-containing groups on the CNTs surface, which increase the dispersion of copper oxides, promote their reduction, decreases the crystal size of Cu, and enhances H2 and CO2 adsorption capability, thus leading to good catalytic performance towards methanol synthesis.

Cycled storage-discharge (CSD) plasma catalytic removal of benzene over AgMn/HZSM-5 using air as discharge gas

Cycled storage-discharge (CSD) plasma catalytic removal of benzene (C6H6) using air as the discharge gas over AgMn/HZSM-5 (AgMn/HZ) catalyst is reported in this study. The properties of AgMn/HZ catalyst were compared with HZ, Mn/HZ, and Ag/HZ catalysts in investigations of C6H6 storage capacity and plasma catalytic oxidation of stored C6H6. Among HZ, Mn/HZ and Ag/HZ catalysts, the AgMn/HZ catalyst possessed the highest breakthrough capacity of C6H6, which is almost twice than that of the unsupported HZ zeolite. For the AgMn/HZ catalyst, stored C6H6 is oxidized completely to CO2 due to the promoting effect of Ag and MnOx. On the other hand, this accelerates the oxidation rate of stored C6H6 during plasma oxidation of stored C6H6. The effects of discharge parameters on plasma oxidation of stored C6H6 over the AgMn/HZ catalyst are discussed. During the CSD process, under conditions of ∼20000 mL h−1 g−1 space velocity, 6 W of input power, 0.4 vol% of absolute humidity and 24 min of discharge, the stored C6H6 conversion increased rapidly with cycle number during the first three cycles and nearly all stored C6H6 could be oxidized into CO2 thereafter. In addition, CO2 selectivity was maintained at around 100% and only a small amount of N2O (40–50 ppm) was detected during all five cycles. Temperature-programmed desorption of adsorbed C6H6 on the used AgMn/HZ catalyst indicates that the minor Ag sites, which are highly active, cannot be renewed by the air plasma, but the major Ag sites, which are normally active, are renewable. This explains the variation of stored C6H6 conversion with cycle number.

Formation of willow leaf-like structures composed of NH2-MIL68(In) on a multifunctional multiwalled carbon nanotube backbone for enhanced photocatalytic reduction of Cr(VI)

Efficient separation and transfer of photogenerated electron/hole as well as enhanced visible light absorption play essential roles in photocatalytic reactions. To promote the photocatalytic reduction of Cr(VI), a toxic heavy metal ion, multiwalled carbon nanotube (MWCNT) was introduced as an electron acceptor into NH2-MIL-68(In). This led to the growth of a willow leaf-like metal-organic framework (MOF) on an MWCNT backbone forming MWCNT/NH2-MIL-68(In) (PL-1), which showed a highly efficient transfer of photogenerated carriers. Moreover, MWCNT incorporation introduced more mesopores for Cr(VI) diffusion and enhanced the visible light adsorption without lowering the conduction band position. As a result, the photocatalytic kinetic constant of PL-1 was found to be almost three times higher than that of the parent NH2-MIL-68(In). Thus, growing MOFs on MWCNTs provides a facile and promising solution for effective remediation of environmental pollution by utilizing solar energy. This work provides the first example of using MWCNT/MOF composites for photocatalytic reactions.

Adsorption of VOCs on reduced graphene oxide

A modified Hummers method was adopted for the synthesis of graphene oxide (GO) and reduced graphene oxide (rGO). It was revealed that the modified method is effective for the production of GO and rGO from graphite. Transmission electron microscopy (TEM) images of GO and rGO showed a sheet-like morphology. Because of the presence of oxygenated functional groups on the carbon surface, the interlayer spacing of the prepared GO was higher than that of rGO. The presence of OH and CO groups in the Fourier transform infrared spectra (FTIR) spectrum and G-mode and 2D-mode in Raman spectra confirmed the synthesis of GO and rGO. rGO (292.6m2/g) showed higher surface area than that of GO (236.4m2/g). The prepared rGO was used as an adsorbent for benzene and toluene (model pollutants of volatile organic compounds (VOCs)) under dynamic adsorption/desorption conditions. rGO showed higher adsorption capacity and breakthrough times than GO. The adsorption capacity of rGO for benzene and toluene was 276.4 and 304.4mg/g, respectively. Desorption experiments showed that the spent rGO can be successfully regenerated by heating at 150.0°C. Its excellent adsorption/desorption performance for benzene and toluene makes rGO a potential adsorbent for VOC adsorption.

Removal of toluene in adsorption–discharge plasma systems over a nickel modified SBA-15 catalyst

An atmospheric-pressure dielectric barrier discharge (DBD) has been used to investigate the destruction of low concentrations of toluene with nickel loaded SBA-15 (Ni–SBA) catalyst in adsorption–discharge plasma system. The adsorption capacity and catalytic activity of SBA and Ni–SBA were studied. The experimental results showed that the incorporation of Ni into the ordered hexagonal mesopores of SBA afforded a remarkable enhancement in the catalytic activity and CO2 selectivity. However, the presence of the metal oxide lowered the specific surface area and decreased the adsorbed amount of toluene. In situ FTIR was used to explore catalysts adsorption process, and revealed that incorporation of Ni resulted in partial oxidation of toluene over Ni–SBA. The catalysts, after several adsorption-plasma process cycles, were characterized by XRD and TEM techniques. The results indicated that the plasma treatment tended to decrease the size of the metal oxides and increase their dispersion in the support surface and influence the adsorption and catalytic performance. In addition, GC-MS spectra were used to analyze the by-products and speculate the reason of catalysts deactivation.

Leaf-like Co-ZIF-L derivatives embedded on Co2AlO4/Ni foam from hydrotalcites as monolithic catalysts for toluene abatement

Herein, a series of distinctively monolithic catalysts were first synthesized by decorating leaf-like Co-ZIF-L derivatives on Co2AlO4 coral-like microspheres from CoAl layered double hydroxides (LDHs), which were coated on three-dimensional porous Ni foam. As a proof of concept application, toluene was chosen as a probe molecule to evaluate their catalytic performances over the as-synthesized catalysts. As a result, the L-12 sample derived from Co2AlO4@Co-Co LDHs displayed an excellent catalytic performance, cycling stability and long-term stability for toluene oxidation (T99 = 272 °C, 33 °C lower than that of Co2AlO4 sample), where leaf-like Co-ZIF-L served as a sacrificial template to synthesize Co-Co LDHs. The improved catalytic performance was attributed to its distinctive structure, in which leaf-like Co-ZIF-L derivatives on Co2AlO4 resulted in its higher specific surface area, lower-temperature reducibility, rich surface oxygen vacancy and high valence Co3+ species. This work thus demonstrates a feasible strategy for the design and fabrication of hybrid LDHs/ZIFs-derived composite architectures, which is expected to construct other novel monolithic catalysts with hierarchical structures for other potential applications.

Macroporous Ni foam-supported Co 3 O 4 nanobrush and nanomace hybrid arrays for high-efficiency CO oxidation

Herein, we reported the synthesis of well-defined Co3O4 nanoarrays (NAs) supported on a monolithic three-dimensional macroporous nickel (Ni) foam substrate for use in high-efficiency CO oxidation. The monolithic Co3O4 NAs catalysts were obtained through a generic hydrothermal synthesis route with subsequent calcination. By controlling the reaction time, solvent polarity and deposition agent, these Co3O4 NAs catalysts exhibited various novel morphologies (single or hybrid arrays), whose physicochemical properties were further characterized by using several analytical techniques. Based on the catalytic and characterization analyses, it was found that the Co3O4 NAs-6 catalyst with nanobrush and nanomace arrays displayed enhanced catalytic activity for CO oxidation, achieving an efficient 100% CO oxidation conversion at a gas hourly space velocity (GHSV) 10,000hr-1 and 150°C with long-term stability. Compared with the other Co3O4 NAs catalysts, it had the highest abundance of surface-adsorbed oxygen species, excellent low-temperature reducibility and was rich in surface-active sites (Co3+/Co2+=1.26).