Wenpo Shan

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.

NH3-SCR performance of fresh and hydrothermally aged Fe-ZSM-5 in standard and fast selective catalytic reduction reactions.

Hydrothermal stability is one of the challenges for the practical application of Fe-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with NH3 (NH(3)-SCR) for diesel engines. The presence of NO(3) in the exhaust gases can enhance the deNOx activity because of the fast SCR reaction. In this work, a Fe-ZSM-5 catalyst was prepared by a solid-state ion-exchange method and was hydrothermally deactivated at 800 °C in the presence of 10% H(2)O. The activity of fresh and hydrothermal aged Fe-ZSM-5 catalysts was investigated in standard SCR (NO(2)/NOx = 0) and in fast SCR with NO(2)/NOx = 0.3 and 0.5. In standard SCR, hydrothermal aging of Fe-ZSM-5 resulted in a significant decrease of low-temperature activity and a slight increase in high-temperature activity. In fast SCR, NOx conversion over aged Fe-ZSM-5 was significantly increased but was still lower than that over fresh catalyst. Additionally, production of N(2)O in fast SCR was much more apparent over aged Fe-ZSM-5 than over fresh catalyst. We propose that, in fast SCR, the rate of key reactions related to NO is slower over aged Fe-ZSM-5 than over fresh catalyst, thus increasing the probabilities of side reactions involving the formation of N(2)O.

Novel MnWOx catalyst with remarkable performance for low temperature NH3-SCR of NOx

A novel W promoted MnOx catalyst (MnWOx) was used for the selective catalytic reduction (SCR) of NOx with NH3 at low temperatures, with high deNOx efficiency from 60 to 250 °C under relatively high space velocity. The MnWOx catalyst showed a unique core–shell structure with Mn3O4 covered by Mn5O8 while Mn4+ species at the outer surface served as a real active phase for NH3-SCR. The W doping resulted in the smaller particle size of MnOx active phase, increased the surface acidity and facilitated the NO/NH3 oxidation, thus enhancing low temperature deNOx efficiency by promoting both Langmuir–Hinshelwood and Eley–Rideal reaction pathways. This novel catalyst is promising to be used in the deNOx process for flue gas after dust removal and desulfurization.

Catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

Selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperature is a major challenge in environmental catalysis. In recent years, great efforts have been devoted to the development of low-temperature SCR catalysts for both stationary sources and diesel engines. Mn-based catalysts have attracted great attention due to their excellent low-temperature activity. However, vulnerability to SO2 and H2O poisoning and preference for N2O formation make these catalysts still far away from industrial application. V2O5 loaded on carbon materials has shown both high SCR activity and SO2 tolerance at low-temperature. This type of catalyst is very promising for applications in low-temperature SCR for stationary sources. Recently, Cu-containing small pore zeolites, such as Cu-SSZ-13 and Cu-SAPO-34 with a CHA structure and Cu-SSZ-39 and Cu-SAPO-18 with an AEI structure, were shown to have very high activity at low temperature and excellent hydrothermal stability at high temperature and thus received much attention for applications on diesel vehicles. In this review, we will focus on the recent studies on low-temperature NH3-SCR catalysts. In addition, the future directions of low-temperature SCR development will also be discussed.

The use of ceria for the selective catalytic reduction of NOx with NH3

The selective catalytic reduction of NOx with NH3 (NH3-SCR) is one of the widely used NOx control strategies for stationary sources (particularly for power plants) and mobile sources (particularly for diesel vehicles). The application is aimed at meeting the increasingly stringent standards for NOx emissions. Recently, ceria has attracted much attention for its applications in NH3-SCR catalysts owing to its unique redox, oxygen storage, and acid-base properties. In this article, we comprehensively review recent studies on ceria for NH3-SCR catalysts when used as support, promoter, or the main active component. In addition, the general development of ceria for NH3-SCR catalysts is discussed

Surface ozone and meteorological condition in a single year at an urban site in central-eastern China

Surface ozone and some meteorological parameters were continuously measured from June 2003 to May 2004 at urban Jinan, China. The levels and variations of surface ozone were studied and the influences of meteorological parameters on ozone were analyzed. Annual and diurnal ozone variation patterns in Jinan both show a typical pattern for polluted urban areas. Daytime ozone concentrations in summer were the highest in the four seasons. However, during nighttime from 2100 to 0600 hours ozone concentrations in spring was higher than that in summer. Daily averaged ozone showed negative correlation with pressure and relative humidity and positive correlation with temperature, total solar radiation, sunshine duration and wind speed during the study period. Further studies show that, solar radiation is a primary influence factor for the daytime variations of ozone concentrations at this site; transport of pollutants by wind could enhance the pollution at this site; precipitation has a significant influence on decreasing surface ozone. A multi-day ozone episode from 16 to 21 June 2003 was observed at this site. Surface meteorological data analysis and backward trajectory computation show that the episode is associated with the influence of typhoon Soudelor, attributing to both local photochemical processes and transport of air pollutants from southeastern coastal region, especially Yangtze River Delta region.

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.

DRIFTS study of a Ce–W mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

A systematic study has been conducted on the reactivity of the selective catalytic reduction of NOx with NH3 (NH3-SCR) in a wide range of NO/NO2 feed ratios (from 0 : 1 to 1 : 0) over a CeWOx catalyst prepared by a homogeneous precipitation method. By using in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), the roles of NO and NO2 at low temperatures during the fast SCR reaction have been revealed. NO2 adsorption results in the formation of surface nitrates, which participate in the NH3-SCR reaction through two pathways: one path where the nitrates react with NH3 to form ammonium nitrate (NH4NO3), then NO reduces NH4NO3 below its melting point to form N2 (the NH4NO3 path); the other path where the surface nitrates are reduced by NO to form active nitrite species that further react with NH3 to produce N2 (the nitrate path). “The NH4NO3 path” and “the nitrate path” contributed simultaneously to the standard and the fast SCR reaction at low temperatures. Both the surface nitrates and NO in the gas phase were suggested to be necessary for the excellent performance of the fast SCR reaction over the CeWOx catalyst.

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.

Low-temperature and low-pressure non-oxidative activation of methane for upgrading heavy oil

It is highly desirable to upgrade viscous heavy oil, such as bitumen extracted from Canadian oil sand, to be transportable by pipeline. Conventionally, this is achieved by expensive catalytic hydrogenation under a hydrogen pressure of 15–20 MPa. In this study, it is reported that by using zinc and silver cation-modified HZSM-5 as the catalyst, methane can be activated at a low temperature of 380 °C and a pressure of 5 MPa to efficiently upgrade heavy oil, leading to the formation of partially upgraded crude oil which is more desirable for pipeline transportation and downstream refining. In addition, methane activation and its participation in the upgrading process were further evidenced by employing butylbenzene, styrene, and benzene as model compounds to represent heavy oil. This study opens a door for upgrading heavy oil with natural gas under fairly mild operation conditions instead of expensive hydrogen under rather stringent ones.