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.

An overview is given of the impact of nitrogen oxides on the environment and human health including the low-temperature selective catalytic reduction of NOx by ammonia (NH3-SCR) over manganese-based materials. The review gives a comprehensive overview of NH3-SCR chemistry including the impacts of NOx, stringent regulations, limitations on NOx emissions and aspects of the reaction mechanism over Mn-based catalysts using isotopic labeled and in situ FT-IR studies. The review attempts to correlate catalyst activity and stability with the acidity, manganese oxidation state, surface texture, and structural morphology. Prospectively, low-temperature SCR of NOx over Mn-based catalysts opens up novel views to mitigate NOx and help us design the next generation industrial catalysts.

Impact of nitrogen oxides on the environment and human health: Mn-based materials for the NOx abatement

Selective catalytic reduction (SCR) technology has been widely used for the removal of NOx from flue gas. However, it is still a challenge to develop novel low-temperature catalysts for SCR of NOx, especially at temperatures below 200 °C. This paper reviewed the recent progress on the Mn-based catalysts for low-temperature SCR de-NOx with NH3. Catalysts were divided into four categories, single MnOx, Mn-based multi-metal oxide, Mn-based multi-metal oxide with support, and Mn-based monolith catalyst. In the section of single MnOx, the effects of several factors, such as Mn oxidation state, crystallization state, specific surface area and morphology on catalytic activity were systematically reviewed. In the section of multi-metal oxide catalysts, the various roles played by the components of catalysts were intentionally summarized from four aspects, improving de-NOx efficiency, enhancing N2 selectivity, improving resistance to SO2 and H2O, extending operation temperature window, respectively. Moreover, the newly emerging morphology-dependent nanocatalysts were highlighted at the end of this section. In the introduction of supported metal oxide catalysts, the effects of supports were systematically analyzed according to their types, such as Al2O3, TiO2, carbon materials, etc. Considering the actual operation, Mn-based monolith catalysts were also introduced with regard to monolith supports, such as ceramics, metal wire mesh, etc. Subsequently, NH3-SCR mechanisms at low temperature, including E-R and L-H mechanisms, were discussed. At last, the perspective and the future direction of low-temperature SCR of NOx were proposed.

Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NOx with NH3: A review

A series of transition metals (Co, Cu and Fe) were selected to decorate Ce-Ti mixed oxide to elevate the low-temperature activity of selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) reaction, by adjusting the ratio of surface Ce 3+ species and oxygen vacancies. Among them, Co-Ce-Ti sample exhibited the excellent low-temperature activity and broadened temperature window, which could be attributed to the improvement of the physico-chemical properties and the acceleration of the reactions in the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Owing to the different ionic sizes of Co 2+ and Ce 4+ , the lattice distortion of Ce-Ti mixed oxide was greatly aggravated and subsequently increased the ratio of Ce 3+ and the surface adsorbed oxygen, which benefited the generation of adsorbed NO x species and improved the reaction in the L-H mechanism. Meanwhile, the coordinatively unsaturated cationic sites over the Co-Ce-Ti sample induced more Lewis acid sites and enhanced the formation of the adsorbed NH 3 species bounded with Lewis acid sites, which were considered as the crucial intermediates in E-R mechanism, and therefore facilitating the reaction between the adsorbed NH 3 species and NO molecules. The enhancements in both the reactions from L-H and E-R mechanisms appeared to directly correlated with the improved deNO x performance on the Co-Ce-Ti sample, and the L-H mechanism could be the dominate one at low temperatures due to its rapid reaction rate.

Mechanistic investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation

Semiconductor-based heterojunctions, widely applied in photocatalytic solar-to-chemical energy conversion, are advantageous for synergistically expediting photocatalytic reaction beyond individual the constituent components. Here we showed new quasi-core-shell In2S3/anatase TiO2@metallic Ti3C2Tx hybrids consisting of well-designed type-II heterojunction and non-noble metal-based Schottky junction with favorable charge transfer channels for efficient photocatalysis application. The mesoporous hybrids owned pleasurable visible-light absorption property and excellent capability in photogenerated exciton separation and carrier transport. Specifically, the hybridized photocatalyst with the additive Ti3C2Tx content of 16 mg (InTi-16) had excellent visible-light photocatalytic performance towards pollutant removal in water with a degradation rate of 0.04977 min−1, which was 3.2 and 6.2 folds higher than that of pure In2S3 and pure Ti3C2Tx, respectively. What’s more, the photocatalytic degradation ability of InTi-16 had surpassed that of many other types of In2S3-based photocatalyst including In2S3/carbon nanotube (CNT), In2S3/reduced graphene oxide (rGO), In2S3/MoS2, and In2S3/TiO2 hybrids. The promising photocatalytic performance was strongly depended on the separation and diffusion of photogenerated exciton and carrier via a multitude of charge transfer channels due to the formation of double heterostructure (type-II heterojunction and Schottky junction). It had originated from the synergistic effects among the visible-light absorption of In2S3, the upward band bending of TiO2 and the favorable electrical conductivity of Ti3C2Tx. Prolonger electron lifetime favored for the generation of more strongly oxidizing radical (e.g. ·O2-) at the in-plane of Ti3C2Tx, and thus enhanced photocatalytic degradation ability. This work demonstrates that the TiO2/Ti3C2Tx can be a potentially novel platform for constructing efficient photocatalysts both for wide-ranging applications and unraveling the transfer behavior of photo-excited electrons based on charge transfer channels.

Formation of quasi-core-shell In2S3/anatase TiO2@metallic Ti3C2Tx hybrids with favorable charge transfer channels for excellent visible-light-photocatalytic performance

Anatase TiO2-supported manganese and cobalt oxide catalysts with different Co/Mn molar ratios were synthesized by a conventional impregnation method and used for selective catalytic reduction (SCR) of NOx with NH3. The catalysts were characterized by N2 adsorption/desorption, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed desorption with NH3 and NOx. Characterization of the catalyst confirmed that by using Co3O4 over Mn/TiO2, we enhanced NO oxidation ability. From in situ diffuse reflectance infrared transform spectroscopy (DRIFTs) analysis of desorption and the transient reaction, we concluded that the addition of Co could remarkably lower the activation energy of NOx chemisorption on the catalyst surface. In addition, low-temperature SCR activity mainly results from a “fast SCR” reaction. We observed four NOx species (bidentate nitrates, gaseous NO2, linear nitrites, and monodentate nitrites) on the surface of Mn/TiO2 and Co–Mn/TiO2 catalysts that all participated in the...

Mechanistic Aspects of deNOx Processing over TiO2 Supported Co–Mn Oxide Catalysts: Structure–Activity Relationships and In Situ DRIFTs Analysis

The Co3O4 and Mn-doped Co3O4 nanoparticle were synthesized by a co-precipitation method and used as selective catalytic reduction of NO with NH3 (NH3-SCR) catalysts. After the doping of manganese oxides, the NH3-SCR activity of Mn0.05Co0.95Ox catalyst is greatly enhanced. The NO oxidation ability of two catalysts is compared, and the X-ray diffraction results demonstrate that Mn has been successfully doped into the lattice of Co3O4. The X-ray photoelectron spectroscopy and temperature-programmed reduction with H2 results confirmed that there is a strong interaction between Mn and Co in the Mn0.05Co0.95Ox catalyst. Their adsorption and desorption properties were characterized by temperature-programmed desorption with NH3 or NO + O2 and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs). These results indicated that the doping of manganese could provide more acid sites on the catalysts, and bidentate nitrates species originated from NOx adsorption are obviously activated on...

In Situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism over Mn-Doped Co3O4 for the Selective Catalytic Reduction of NOx with NH3

Manganese based catalysts are highly active in the NH3-SCR reaction for NOx removal. Unfortunately, manganese oxides can be easily deactivated by sulfur dioxide in the flow gas, which has become the main obstacle for their practical applications. To address this problem, we presented a green and facile method for the synthesis of multi-shell Fe2O3@MnOx@CNTs. The morphology and structural properties of the catalysts were systematically investigated. The results revealed that the MnOx@CNT core-shell structure was formed during the chemical bath deposition, while the outermost MnOx species were transformed to Fe2O3 after the galvanic replacement reaction. The formation of the multi-shell structure induced the enhancement of the active oxygen species, reducible species as well as adsorption of the reactants, which brought about excellent de-NOx performance. Moreover, the Fe2O3 shell could effectively suppress the formation of the surface sulfate species, leading to the desirable SO2 resistance to the multi-shell catalyst. Hence, the synthesis protocol could provide guidance for the preparation and elevation of manganese based catalysts.

Design of multi-shell Fe2O3@MnO(x)@CNTs for the selective catalytic reduction of NO with NH3: improvement of catalytic activity and SO2 tolerance.

In this work, we have rationally designed and originally developed a novel monolith de-NOx catalyst with nickel foam as the carrier and three dimensional hierarchical MnO2@NiCo2O4 core–shell nanowire arrays in situ grown on the surface via a two-step hydrothermal process with a post calcination treatment. The catalysts were systematically examined by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, elemental mapping, ion sputtering thinning, X-ray photoelectron spectroscopy, inductively coupled plasma atomic emission spectroscopy, H2 temperature-programmed reduction, NH3/NO + O2 temperature-programmed desorption measurements and catalytic performance tests. The results indicate that the nanowire is composed of hollow NiCo2O4 spinel as the core and MnO2 nanoparticles as the shell layer. By ingeniously making the hierarchical Ni–Co oxide nanowires as the support for manganese oxides, the MnO2@NiCo2O4@Ni foam catalyst not only takes advantage of the high surface area of Ni–Co nanowires to achieve high loading amounts as well as high dispersion of manganese oxides, but also makes use of the synergistic catalytic effect between Ni, Co and Mn multiple oxides, and exhibits excellent low-temperature catalytic performance in the end. In addition, with the structure and morphology well maintained under long term steady isothermal operation, the catalyst is able to sustain high NO conversion and exhibits superior catalytic cycle stability and good H2O resistance. Considering all these favorable properties, the MnO2@NiCo2O4@Ni foam catalyst could serve as a promising candidate for the monolith de-NOx catalyst at low temperatures and the rational design of in situ synthesis of 3D hierarchical monolith catalysts also puts forward a new way for the development of environmental-friendly and highly active monolith de-NOx catalysts.

Rational design and in situ fabrication of MnO2@NiCo2O4 nanowire arrays on Ni foam as high-performance monolith de-NOx catalysts

In this work, we developed a novel zirconium doped CeVO 4 to form Ce 1− x Zr x VO 4 ( x  = 0.05, 0.10, 0.15, 0.20, 0.30, 0.50, 0.70, 0.80) solid solution as a low-temperature catalyst for the selective catalytic reduction (SCR) of NO x with NH 3 . The optimized catalysts showed excellent performance at low temperature. The light-off temperature (the temperature at which the conversion of NO reaches 50%) was down to about 125 °C, while the temperature window (the NO conversion is above 80%) ranged from 150 to 375 °C. The selectivity was kept close to 100% during the whole temperature range. Furthermore, the catalysts also exhibited good H 2 O/SO 2 durability and fascinating performance at high gas hourly space velocity of 400,000 h −1 . Hydrogen temperature-programmed reduction, X-ray photoelectron spectroscopy, ammonia and nitrogen oxides temperature-programmed desorption and in-situ diffuse reflectance infrared Fourier transform experiments were performed to study the influence of Zr doping on the SCR performance. It was found that the introduction of Zr in CeVO 4 with a proper amount could significantly increase the surface area, oxidative ability, active oxygen species and especially surface acid sites of the catalysts, which were beneficial to the promotion of SCR performance.

Hang Hu

Papers

Mechanistic Aspects of deNOx Processing over TiO2 Supported Co–Mn Oxide Catalysts: Structure–Activity Relationships and In Situ DRIFTs Analysis

In Situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism over Mn-Doped Co3O4 for the Selective Catalytic Reduction of NOx with NH3

Design of multi-shell Fe2O3@MnO(x)@CNTs for the selective catalytic reduction of NO with NH3: improvement of catalytic activity and SO2 tolerance.

Rational design and in situ fabrication of MnO2@NiCo2O4 nanowire arrays on Ni foam as high-performance monolith de-NOx catalysts

Promotional effects of zirconium doped CeVO4 for the low-temperature selective catalytic reduction of NOx with NH3