Shijian Yang

Mechanism of N2O formation during the low-temperature selective catalytic reduction of NO with NH3 over Mn-Fe spinel.

The mechanism of N2O formation during the low-temperature selective catalytic reduction reaction (SCR) over Mn-Fe spinel was studied. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transient reaction studies demonstrated that the Eley-Rideal mechanism (i.e., the reaction of adsorbed NH3 species with gaseous NO) and the Langmuir-Hinshelwood mechanism (i.e., the reaction of adsorbed NH3 species with adsorbed NOx species) both contributed to N2O formation. However, N2O selectivity of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism was much less than that through the Eley-Rideal mechanism. The ratio of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism remarkably increased; therefore, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the decrease of the gas hourly space velocity (GHSV). As the gaseous NH3 concentration increased, N2O selectivity of NO reduction over Mn-Fe spinel increased because of the promotion of NO reduction through the Eley-Rideal mechanism. Meanwhile, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the increase of the gaseous NO concentration because the formation of NH on Mn-Fe spinel was restrained. Therefore, N2O selectivity of NO reduction over Mn-Fe spinel was related to the GHSV and concentrations of reactants.

Substitution of WO3 in V2O5/WO3–TiO2 by Fe2O3 for selective catalytic reduction of NO with NH3

To improve N2 selectivity and lower the cost, WO3 in V2O5/WO3–TiO2 was substituted by a low cost composition Fe2O3 for selective catalytic reduction (SCR) of NO with NH3. The SCR reaction over V2O5/Fe2O3–TiO2 mainly followed the Eley–Rideal mechanism (i.e. the reaction between activated ammonia species and gaseous NO). There were two active components on V2O5/WO3–TiO2 for the activation of adsorbed NH3 (i.e. V5+ and Fe3+). The acid sites on V2O5/Fe2O3–TiO2 mainly resulted from the support Fe2O3–TiO2, so the adsorbed NH3 preferred to be activated by Fe3+ rather than by V5+. V5+ on V2O5/Fe2O3–TiO2 could accelerate the regeneration of Fe3+ on Fe2O3–TiO2 due to the rapid electron transfer between V5+ and Fe2+ on the surface, so the activation of adsorbed NH3 by Fe3+ was promoted. As some NH3 adsorbed on V2O5/Fe2O3–TiO2 was not activated by Fe3+, the inactivated NH3 could then be activated by V5+ on the surface. As a result, 2% V2O5/Fe2O3–TiO2 showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 300–450 °C. Furthermore, the emission of 2% V2O5/Fe2O3–TiO2 to the fly ash can be prevented by an external magnetic field due to its inherent magnetization. Therefore, 2% V2O5/Fe2O3–TiO2 could be a promising low-cost catalyst in NO emission control.

Competition of selective catalytic reduction and non selective catalytic reduction over MnOx/TiO2 for NO removal: the relationship between gaseous NO concentration and N2O selectivity

In this work, a novel phenomenon was discovered that N2O selectivity of NO reduction over MnOx/TiO2 was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N2O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal mechanism (the reaction of over-activated NH3 with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction of adsorbed NO3− with adsorbed NH3 on the adjacent sites) could contribute to the formation of N2O. Kinetic study demonstrated that N2O selectivity would be independent of gaseous NO concentration if NO reduction over MnOx/TiO2 mainly followed the Langmuir–Hinshelwood mechanism. If NO reduction over MnOx/TiO2 mainly followed the Eley–Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more –NH2 was used to reduce gaseous NO to form N2 and the further oxidization of –NH2 to –NH was restrained, resulting in an obvious decrease of N2O selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnOx/TiO2, especially at higher temperatures. Therefore, N2O selectivity of the low temperature SCR reaction over MnOx/TiO2 decreased especially at higher temperatures after the increase of gaseous NO concentration.

The mechanism of the effect of H2O on the low temperature selective catalytic reduction of NO with NH3 over Mn–Fe spinel

H2O showed a notable inhibition on the low temperature selective catalytic reduction (SCR) reaction over Mn based catalysts. However, the mechanism of H2O effect was not clear. In this work, the mechanism of H2O effect on the low temperature SCR reaction over Mn–Fe spinel was studied using the transient reaction study and the steady-state kinetic analysis. According to the steady-state kinetic analysis, the reaction kinetic rate constants of NO reduction over Mn–Fe spinel (including the rate constants of N2 formation through the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism, and the rate constants of N2O formation) in the presence of H2O and in the absence of H2O were compared. According to the transient reaction study, the effect of H2O on the elementary reactions of NO reduction over Mn–Fe spinel through both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism was investigated. The results indicated that the effect of H2O on the low temperature SCR reaction over Mn–Fe spinel was not only attributed to the competition adsorption of H2O with NH3 and NOx, but also related to the decrease in the oxidation ability and the inhibition of the interface reaction.

A novel magnetic Fe–Ti–V spinel catalyst for the selective catalytic reduction of NO with NH3 in a broad temperature range

Fe–Ti–V spinel showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 250–400 °C, and an external magnetic field can effectively prevent the emission of a vanadium based catalyst to the environment due to its magnetization.

Comparison of preparation methods for ceria catalyst and the effect of surface and bulk sulfates on its activity toward NH3-SCR.

A series of CeO2 catalysts prepared with sulfate (S) and nitrate (N) precursors by hydrothermal (H) and precipitation (P) methods were investigated in selective catalytic reduction of NOx by NH3 (NH3-SCR). The catalytic activity of CeO2 was significantly affected by the preparation methods and the precursor type. CeO2-SH, which was prepared by hydrothermal method with cerium (IV) sulfate as a precursor, showed excellent SCR activity and high N2 selectivity in the temperature range of 230-450 °C. Based on the results obtained by temperature-programmed reduction (H2-TPR), transmission infrared spectra (IR) and thermal gravimetric analysis (TGA), the excellent performance of CeO2-SH was correlated with the surface sulfate species formed in the hydrothermal reaction. These results indicated that sulfate species bind with Ce(4+) on the CeO2-SH catalyst, and the specific sulfate species, such as Ce(SO4)2 or CeOSO4, were formed. The adsorption of NH3 was promoted by these sulfate species, and the probability of immediate oxidation of NH3 to N2O on Ce(4+) was reduced. Accordingly, the selective oxidation of NH3 was enhanced, which contributed to the high N2 selectivity in the SCR reaction. However, the location of sulfate on the CeO2-SP catalyst was different. Plenty of sulfate species were likely deposited on CeO2-SP surface, covering the active sites for NO oxidation, which resulted in poor SCR activity in the test temperature range. Moreover, the resistance to alkali metals, such as Na and K, was improved over the CeO2-SH catalyst.

A highly efficient CeWOx catalyst for the selective catalytic reduction of NOx with NH3

In this study, two methods were used to prepare Ce–W oxide catalysts. The CeWOx catalyst prepared by the homogeneous precipitation method showed excellent NH3-SCR performance, with over 80% NOx conversion obtained from 225 to 450 °C under a high GHSV of 300 000 h−1. Characterization revealed that the homogeneous precipitation method can achieve highly dispersed active species and intense interaction between the Ce and W species on the CeWOx catalyst, and thus result in enhanced charge imbalance, superior redox functions, and outstanding adsorption and activation properties for the reactants, which are the main reasons for the highly efficient NOx abatement of the CeWOx catalyst.

Separation of phenol from aqueous solutions by polymeric reversed micelle extraction

Abstract Polyoxyalkylene block copolymers consisting of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO) or poly(butylene oxide) (PBO) have not attracted much attention until recent years. Variations of the molecular characteristics (PPO/PEO ratio, molecular weight) of the copolymer during the synthesis allows the production of molecules with optimum properties that meet the specific requirements in different areas. Our present interest is focused, on the formation of the reversed polymeric micelles, which are formed in ‘oil’ phases with PEO–PPO–PEO-type as well as PPO–PEO–PPO-type triblock copolymers. A novel process of polymeric reversed micelle extraction is subsequently proposed based upon the concepts of reversed micelle extraction and polymeric micelle extraction. Extraction equilibrium partition of phenol between polymeric reversed micelle solutions was investigated to study the extraction behaviors. PPO content, copolymer and co-surfactant types affected the extraction process.

Novel approach for a cerium-based highly-efficient catalyst with excellent NH3-SCR performance

A highly-efficient NH3-SCR catalyst, CeO2/WO3–TiO2, was prepared by a novel stepwise precipitation approach. This catalyst showed excellent catalytic performance, with superior low-temperature activity, high N2 selectivity and a broad operational temperature window. Furthermore, the CeO2/WO3–TiO2 catalyst could perform well even under an extremely high space velocity condition of 1 000 000 h−1.

Promotion mechanism of CeO2 addition on the low temperature SCR reaction over MnOx/TiO2: a new insight from the kinetic study

CeO2 addition showed a notable improvement on the low temperature selective catalytic reduction (SCR) performance of MnOx/TiO2. In this work, a new insight into the promotion mechanism was established from the steady state kinetic study. The kinetic rate constants of the SCR reaction through the Eley–Rideal mechanism, those of the SCR reaction through the Langmuir–Hinshelwood mechanism, and those of the non-selective catalytic reduction (NSCR) reaction over MnOx/TiO2 and MnOx–CeO2/TiO2 were obtained according to the steady state kinetic analysis. After comparing the reaction kinetic constants of NO reduction over MnOx/TiO2 and MnOx–CeO2/TiO2, the mechanism of the addition of CeO2 for NO reduction over MnOx/TiO2 was discovered according to the relationship between the reaction rate constants and the catalyst properties. Because the oxidation ability of MnOx/TiO2 increased, the rate constant of the SCR reaction over MnOx/TiO2 increased remarkably after CeO2 addition, resulting in a notable promotion of N2 formation. The oxidation of NH3 to NH over MnOx/TiO2 required two Mn4+ cations on the adjacent sites. However, the probability of two Mn4+ cations occurring on the adjacent sites on MnOx/TiO2 obviously decreased after CeO2 addition, although the oxidation ability of MnOx/TiO2 increased. Therefore, the rate of N2O formation during NO reduction over MnOx/TiO2 did not vary notably after CeO2 addition. As a result, both the SCR activity and N2 selectivity of NO reduction over MnOx/TiO2 improved after CeO2 addition.