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.

A series of Mn–M′/TiO2 (M′ = Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) catalysts were prepared using incipient wetness technique and investigated for the low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. A combination of various physico-chemical techniques was used to investigate the influence of co-doped metals on the characteristics of Mn/TiO2 catalyst. The catalytic performance of these materials was compared with respect to the M′/Mn atomic ratio in order to examine the correlation between physicochemical characteristics and reactivity of optimized materials. XRD results suggest that metal oxide species exist in a highly dispersed state at a certain level called two-dimensional monolayer coverage. Our XPS results illustrated that the MnO2 phase is extremely dominant over the Mn2O3 phase (Mn4+/Mn3+ = 22.31, 96%) in the Mn–Ni(0.4)/TiO2 anatase catalyst, whereas Mn2O3 phase is in competition with MnO2 in other catalysts (Mn4+/Mn3+ = 1.34–12.67). H2-TPR data results are in good accordance with XPS results that the absence of the high-temperature (736 K) peak indicates that the dominant phase is MnO2 in Mn–Ni(0.4)/TiO2 catalyst. This increase in reducibility of manganese and dominant MnO2 phase seems to be the reason for the superior activity and time on stream patterns of our best catalyst. BET and pore volume measurements revealed high surface area and pore volume of the Mn–Ni/TiO2 catalyst. The in situ NH3-FTIR results suggest that the manganese (Mn/TiO2) and manganese–nickel (Mn–Ni/TiO2) surface sites have only Lewis acidity, with no apparent Bronsted acidity. The catalytic performance of various Mn–M′/TiO2 (M′/Mn = 0, 0.2, 0.4, 0.6, 0.8) catalysts was studied for the low-temperature SCR reaction at a range of temperatures (160–240 °C) under industrial conditions using GHSV = 50,000 h−1. The intrinsic activity of Mn–Ni/TiO2 catalyst with M′/Mn = 0.4 atomic ratio measured under differential reaction conditions, was found to be highly active, selective and broadening the temperature window for this reaction.

Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures

Low-temperature NH 3 -SCR is an environmentally important reaction for the abatement of NO x from stationary sources. In recent years FeMnO x has attracted significant attention as a potential catalyst for this process, however its catalytic activity and SO 2 resistance require further improvement. In this contribution FeMnO x has been modified to examine the effect of ceria on catalytic activity and SO 2 resistance in the low temperature region. Preparation of catalysts via the citric acid method generate modified materials that exhibit enhanced NO turnover compared to FeMnO x . A reduction in NH 3 adsorption (NH 3 -TPD) and a suitable ratio between NO sites (NO-TPD) and chemisorbed surface oxygen (XPS) are beneficial for the promotion of fast-SCR. A comparative SO 2 resistance study of FeMnO x and Ce(12.5) showed that the latter exhibited improved stability in the presence of SO 2 , as indicated by the retention of pore volume (N 2 adsorption) and surface composition (XPS). In-situ DRIFTS demonstrated that chemisorbed N-containing species on Ce(12.5) were much more stable in the presence of SO 2 compared to FeMnO x , which resulted in the formation of significantly less metal sulphates and NH 4 HSO 4 .

Ceria modified FeMnO x -Enhanced performance and sulphur resistance for low-temperature SCR of NOx

Highly visible-light active C- and V-doped TiO2 for degradation of acetaldehyde

SiO2/V2O5/TiO2 catalysts were synthesized for removing elemental mercury (Hg0) from simulated coal-combustion flue gas. Experiments were carried out in fixed-bed reactors using both pellet and powder catalysts. In contrast to the SiO2-TiO2 composites developed in previous studies, the V2O5 based catalysts do not need ultraviolet light activation and have higher Hg0 oxidation efficiencies. For Hg0 removal by SiO2-V2O5 catalysts, the optimal V2O5 loading was found between 5 and 8%, which may correspond to a maximum coverage of polymeric vanadates on the catalyst surface. Hg0 oxidation follows an Eley-Rideal mechanism where HCI, NO, and NO2 are first adsorbed on the V2O5 active sites and then react with gas-phase Hg0. HCI, NO, and NO2 promote Hg oxidation, while SO2 has an insignificant effect and water vapor inhibits Hgo oxidation. The SiO2-TiO2-V2O5 catalysts exhibit greater Hg0 oxidation efficiencies than SiO2-V2O5, may be because the V-O-Ti bonds are more active than the V-O-Si bonds. This superior oxidation capability is advantageous to power plants equipped with wet-scrubbers where oxidized Hg can be easily captured. The findings in this work revealed the importance of optimizing the composition and microstructures of SCR (selective catalytic reduction) catalysts for Hg0 oxidation in coal-combustion flue gas.

Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas.

TiO 2 -SiO 2 with various compositions prepared by the coprecipitation method and vanadia loaded on TiO 2 -SiO 2 were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH 3 and in the undesired oxidation of SO 2 to SO 3 , using BET, XRD, XPS, NH 3 -TPD, acidity measurement by the titration method and activity test. TiO 2 -SiO 2 , compared with pure TiO 2 , exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V 2 O 5 . The SCR activity and N 2 selectivity are found to be more excellent over vanadia loaded on TiO 2 -SiO 2 with 10–20 mol% of SiO 2 than over that on pure TiO 2 , and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH 3 adsorbed on the catalysts. With increasing SiO 2 content, the remarkable activity decrease in the oxidation of SO 2 to SO 3 , favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V 4+ on TiO 2 -SiO 2 , while V 5+ exists on TiO 2 , according to XPS. It is concluded that vanadia loaded on Ti-rich TiO 2 -SiO 2 with low SiO 2 content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO x activity but also the low SO 2 oxidation performance.

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

The V2O5 catalysts supported on TiO2–SiO2–MoO3 (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH3 at low temperatures. The V2O5/TSM catalyst with 7–13 wt% SiO2 was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO3 to TiO2–SiO2 and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO2 oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.

Motonobu Kobayashi

Papers

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3