Zhen-Yu Tian

Structure–activity relation of spinel-type Co–Fe oxides for low-temperature CO oxidation

A series of cobalt ferrite thin films was prepared via pulsed spray evaporation chemical vapour deposition (PSE-CVD). The samples were comprehensively characterised in terms of structure, surface, morphology, and optical and redox properties. Both X-ray diffraction (XRD) and Raman analysis show that all samples exhibited an inverse spinel structure. X-ray photoelectron spectroscopy (XPS) results indicate that the films were mainly composed of Co, Fe and O species, and an increase in the Co : Fe ratio with Fe substitution by Co was observed. Helium ion microscopy (HIM) images show the film morphology to be dependent on the Co : Fe ratio. The investigation of the optical property using ultraviolet-visible spectroscopy reveals that the increase in the Co content results in an increase in the band gap energy. In situ emission FTIR spectroscopy was used to evaluate the redox properties of the samples, and a shift of the redox temperature to higher values was observed upon increase in the Co content. The effect of Fe substitution by Co in the mixed oxide systems on their catalytic performance for CO oxidation was investigated. Co–Fe oxides exhibit substantially better catalytic performance than the single α-Fe2O3. The catalytic performance of the Co–Fe oxides towards CO oxidation was discussed with respect to the participation of surface and lattice oxygen in the oxidation process. According to XPS and temperature-programmed reduction/oxidation (TPR/TPO) results, a suprafacial mechanism where CO molecules react with surface-adsorbed oxygen functions to form CO2 was proposed as the dominant mechanism.

Mono- to Octa-Chlorinated PCDD/Fs in Stack Gas from Typical Waste Incinerators and Their Implications on Emission

Mono- to octa-chlorinated dibenzo-p-dioxins and dibenzofurans (mono- to octa-CDD/Fs) were determined in 14 stack gas samples from two municipal solid waste incinerators and two medical waste incinerators. The total PCDD/F concentrations were 5.1-390 ng/Nm(3), and the mono- to trichlorinated homologues contributed 53.2-94.5% of the total concentrations. The homologue profiles were dominated by the MoCDF, ranged from 1.51 to 113.1 ng/Nm(3), and the proportion that each PCDF homologue group contributed to the total concentration decreased with increasing chlorination level. The toxic equivalent concentrations (I-TEQs) were 0.01-2.81 ng I-TEQ/Nm(3), with 2,3,4,7,8-PeCDF being the biggest contributor, at 30.6-60.0%. Correlations were found among the PCDD/PCDF ratios (D/F ratios), the degree of chlorination, and the TEQ. Stack gases with low I-TEQs had higher proportions of the less chlorinated homologues and lower D/F ratios, which could be attributed to the removal of the more chlorinated isomers by the air pollution control systems used by the incinerators. 2,8-DiCDF, 2,4,8-TrCDF, DiCDF, and TrCDF can be used as TEQ indicators for monitoring PCDD/Fs. 2,8-DiCDF and 2,4,8-TrCDF correlated well with the TEQ because they strongly correlated with 2,3,4,7,8-PeCDF, implying possible correlations in their formation mechanisms.

Selective synthesis of α-Fe2O3 thin films and effect of the deposition temperature and lattice oxygen on the catalytic combustion of propene

Pulsed spray evaporation chemical vapour deposition (PSE-CVD), an elaborate CVD process, was employed to synthesize thin films of α-Fe2O3 for the catalytic combustion of propene. According to X-ray diffraction and Raman spectroscopy, the alpha structure type was presented as the unique phase. α-Fe2O3 with fine crystalline structure was revealed by scanning electron microscopy, and probed in depth for the first time by helium ion microscopy. Energy dispersive X-ray microscopy and X-ray photoelectron spectroscopy displayed an overview of the chemical composition of the samples. In situ emission FTIR spectroscopy was used for the accurate determination of the thermal stability of the samples at around 500 °C, and temperature-programmed reduction was performed to correlate the catalytic performance and reduction properties of the obtained α-Fe2O3 thin films. The results showed that the increase of the deposition temperature leads to significant changes of film morphology, chemical composition and reduction properties, with a direct consequence on the catalytic performance. α-Fe2O3 prepared at a low temperature (350 °C) exhibited high activity towards the deep oxidation of propene, which was attributed to its good reducibility and the plate-like structures. The alternate reduction and oxidation of the oxide surface (favored by the bulk oxygen migration towards the surface) and replenishment of bulk oxygen by gas-phase oxygen suggest that the oxidation of propene may proceed according to the Mars van Krevelen mechanism. The morphology and surface composition of the prepared samples remain the same before and after the catalytic test, demonstrating very good stability and reproducibility. We thus conclude that α-Fe2O3 effective in propene conversion can be selectively synthesized with PSE-CVD.

Identification of Combustion Intermediates in Low-Pressure Premixed Pyridine/Oxygen/ Argon Flames

Combustion intermediates of two low-pressure premixed pyridine/oxygen flames with respective equivalence ratios of 0.56 (C/O/N = 1:4.83:0.20) and 2.10 (C/O/N = 1:1.29:0.20) have been identified with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry techniques. About 80 intermediates in the rich flame and 60 intermediates in the lean flame, including nitrogenous, oxygenated, and hydrocarbon intermediates, have been identified by measurements of photoionization mass spectra and photoionization efficiency spectra. Some radicals and new nitrogenous intermediates are identified in the present work. The experimental results are useful for studying the conversion of volatile nitrogen compounds and understanding the formation mechanism of NO(x) in flames of nitrogenous fuels.

Interstellar Enols Are Formed in Plasma Discharges of Alcohols

Laboratory low-pressure cold plasma discharges, which are used to simulate some hot core environments in the star-forming region, have been investigated by employing single-photon vacuum ultraviolet (VUV) photoionization mass spectrometry. Enols with two to four carbon atoms were detected in plasma discharges of alcohols, indicating that enols can result from alcohol destruction induced by ultraviolet and cosmic radiation and accelerated electrons that are abundant in the interstellar medium. This observation, together with the detection of ethenol toward Sgr B2, suggests that larger enols, such as propenols and butenols, should be in the search list of potential molecular species to be identified in interstellar space. The laboratory experiment presented here shows that VUV photoionization sampling of plasma discharges is a valuable method for guiding the search for new interstellar molecules and helping to understand the transformation mechanism of molecular species of astrochemical significance.

Formation and contamination of PCDD/Fs, PCBs, PeCBz, HxCBz and polychlorophenols in the production of 2,4-D products

The concentrations and profiles of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), pentachlorobenzene (PeCBz), hexachlorobenzene (HxCBz) and polychlorophenols in 2,4-D were investigated in this study. Two 2,4-D acid and three 2,4-D butyl ester enterprises were selected as typical 2,4-D producers. The total concentrations of 2,3,7,8-PCDD/Fs in the 2,4-D samples ranged from 355 to 35080ngkg(-1) and the corresponding TEQ values were in the range of 13.4 and 694.6ng WHO-TEQkg(-1). The concentrations of total PCBs in the 2,4-D were in the range of 16.1 and 8023ngkg(-1), and the WHO-TEQ values of the PCBs were between 0.057 and 108.3ng WHO-TEQkg(-1), while total PCBs were between 1486 and 47342ngkg(-1). The average emission factors were 414.4μg WHO-TEQt(-1) for PCDD/Fs and 21.9μg WHO-TEQt(-1) for PCBs. The polychlorobenzenes and polychlorophenols impurities may play a key role in the PCBs and PCDD/Fs formation. The impurities of PCDD/Fs and PCBs in 2,4-D may increase the risk for the human and environmental health.

CVD synthesis of Cu2O films for catalytic application

Pure Cu2O was synthesized at 270 °C by pulsed-spray evaporation chemical vapor deposition. The results indicate that Cu2O is effective for the complete oxidation of VOCs with good reusability and reproducibility. The lattice and adsorbed oxygen as well as the hollow ball-like geometry are dedicated to the catalytic processes.

Controlled synthesis of Co3O4 spinel with Co(acac)3 as precursor

Cobalt(III) acetylacetonate (Co(acac)3) was used as a precursor to grow pure Co3O4 with pulsed-spray evaporation chemical vapor deposition (PSE-CVD). The effect of solvent and substrate temperature on the growth kinetics and morphology of the films was investigated. The obtained spinel exhibited good catalytic performance.

Polychlorinated dibenzo-p-dioxins and dibenzofurans and polychlorinated biphenyls in surface soil from the Tibetan Plateau

Concentrations of Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) in soil samples from Tibetan Plateau were determined. The average concentration of total 2,3,7,8-PCDD/Fs was (2.30±1.02) pg/g, and World Health Organization Toxicity Equivalency (WHO-TEQ) average concentration was (0.013±0.010) pgWHO-TEQ/g. The average concentration of ∑PCBs (7 indicator PCB and 12 dioxin like-PCB congeners) was (16.2±9.25) pg/g, and WHO-TEQ average concentration was 0.043±0.049pgWHO-TEQ/g. Comparing to previous studies in similar environmental conditions, PCDD/Fs and PCBs in this study showed a relatively lower concentration. The altitude dependences of PCDD/Fs and PCBs were also studied. Total organic carbon (TOC) normalized concentrations presented a quadratic relation with the altitudes, and an inflection could be found on the parabola of the total concentrations and some congeners of high concentration. The concentrations decreased with altitudes below about 4500m above sea level (a.s.l.), while they increased with altitudes above it. These phenomena indicate that cold condensation of PCDD/Fs and PCBs would happen above 4500m a.s.l, on the Tibetan Plateau.

Low temperature plasma diagnostics with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

Plasma-based technology is of great importance for a range of industrial applications. However, due to the complexity of gas-phase and surface chemical reactions, the underlying plasma chemistry is still poorly understood, leading to a growing demand for experimental setups on plasma diagnostics. In this paper, we report an apparatus that combines tunable synchrotron vacuum ultraviolet (VUV) photoionization with molecular-beam mass spectrometry for detecting ionic and neutral species formed in the processes of plasma discharge. The products including reactive intermediates such as enols can be identified unambiguously with the measurements of photoionization mass spectrometry (PIMS) and photoionization efficiency spectra. The tunable VUV PIMS has been proven as an ideal and sensitive tool for a comprehensive understanding of plasma chemistry.