Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.

Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects.

Adsorption isotherm models: Classification, physical meaning, application and solving method

A review of functionalized carbon nanotubes and graphene for heavy metal adsorption from water: Preparation, application, and mechanism.

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. In this review, we provide an up to date overview of the complex colloidal nanostructures that could be prepared so far by cation exchange. At the same time, the review gives an account of the fundamental thermodynamic and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.

Forging Colloidal Nanostructures via Cation Exchange Reactions

Ceria (CeO2) showed a poor activity for the selective catalytic reduction (SCR) of NO with NH3, while CeO2 showed an excellent SCR activity in the presence of SO2 at 300–500 °C. The promotion of SO2 on the SCR reaction over CeO2 was mainly due to the sulfation of CeO2. The SCR reaction over CeO2 and that over sulfated CeO2 both followed the Eley–Rideal mechanism (i.e. the reaction between activated NH3 with gaseous NO). Meanwhile, the catalytic oxidization of NH3 to NO could simultaneously happen during the SCR reaction, resulting in a drop of NOx conversion at high temperatures. The adsorption of NH3 on CeO2 was obviously promoted after the sulfation, resulting in an obvious promotion of the Eley–Rideal mechanism. Meanwhile, the adsorption sites for NH2 adsorption and the oxidization agents for NH2 oxidization on CeO2 were separated after the sulfation. Thus, the probability of the collision between NH2 and Ce4+ on sulfated CeO2 was much less than that on CeO2, resulting in an obvious repression of the catalytic oxidization of NH3 to NO. As a result, the SCR activity of sulfated CeO2 was much better than that over CeO2.

Novel effect of SO2 on the SCR reaction over CeO2: Mechanism and significance

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH3, the catalytic combustion of volatile organic compounds, Hg0 oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the rol...

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review.

In order to facilitate the removal of elemental mercury (Hg0) from coal-fired flue gas, catalytic oxidation of Hg0 with manganese oxides supported on inert alumina (α-Al2O3) was investigated at lower temperatures (373−473 K). To improve the catalytic activity and the sulfur-tolerance of the catalysts at lower temperatures, several metal elements were employed as dopants to modify the catalyst of Mn/α-Al2O3. The best performance among the tested elements was achieved with molybdenum (Mo) as the dopant in the catalysts. It can work even better than the noble metal catalyst Pd/α-Al2O3. Additionally, the Mo doped catalyst displayed excellent sulfur-tolerance performance at lower temperatures, and the catalytic oxidation efficiency for Mo(0.03)−Mn/α-Al2O3 was over 95% in the presence of 500 ppm SO2 versus only about 48% for the unmodified catalyst. The apparent catalytic reaction rate constant increased by approximately 5.5 times at 423 K. In addition, the possible mechanisms involved in Hg0 oxidation and the ...

Catalytic Oxidation of Elemental Mercury over the Modified Catalyst Mn/α-Al2O3 at Lower Temperatures

Capture of gaseous elemental mercury from flue gas using a magnetic and sulfur poisoning resistant sorbent Mn/γ-Fe2O3 at lower temperatures.

MnOx/graphene composites were prepared and employed to enhance the performance of manganese oxide (MnOx) for the capture of elemental mercury (Hg0) in flue gas. The composites were characterized using FT-IR, XPS, XRD, and TEM, and the results showed that the highly dispersed MnOx particles could be readily deposited on graphene nanosheets via hydrothermal process described here. Graphene appeared to be an ideal support for MnOx particles and electron transfer channels in the catalytic oxidation of Hg0 at a high efficiency. Thus, MnOx/graphene-30% sorbents exhibited an Hg0 removal efficiency of greater than 90% at 150 °C under 4% O2, compared with the 50% removal efficiency of pure MnOx. The mechanism of Hg0 capture is discussed, and the main Hg0 capture mechanisms of MnOx/graphene were catalytic oxidation and adsorption. Mn is the main active site for Hg0 catalytic oxidation, during which high valence Mn (Mn4+ or Mn3+) is converted to low valence Mn (Mn3+ or Mn2+). Graphene enhanced the electrical conduct...

Zan Qu

Papers

Novel effect of SO2 on the SCR reaction over CeO2: Mechanism and significance

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review.

Catalytic Oxidation of Elemental Mercury over the Modified Catalyst Mn/α-Al2O3 at Lower Temperatures

Capture of gaseous elemental mercury from flue gas using a magnetic and sulfur poisoning resistant sorbent Mn/γ-Fe2O3 at lower temperatures.

MnOx/Graphene for the Catalytic Oxidation and Adsorption of Elemental Mercury.