Fudong Liu

Abstract: Environmental-friendly iron titanate (FeTiOx) catalyst is a potential candidate for the substitution of conventional V2O5-WO3 (MoO3)/TiO2 catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) for the NOx elimination from stationary and mobile sources for environmental protection. To understand in-depth the nature of active structure in this FeTiOx catalyst for further catalyst redesign and activity improvement, the study of X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) combined with theoretical calculation is carefully performed. Different from the crystal structure of hematite Fe2O3, homogeneous edge shared Fe3+-(O)2-Ti4+ structure in FeTiOx catalyst prepared from Ti(SO4)2 precursor is obviously formed with crystallite phase, which shows the electronic inductive effect between Fe3+ and Ti4+ species, resulting in the high NO adsorption and oxidation ability of Fe3+ species and thus high catalytic activity and N2 selectivity in the NH3-SCR reaction. In the future study, this specific edge shared Fe3+-(O)2-Ti4+ structure can be stabilized onto certain catalyst supports with large surface area for practical use, such as the catalytic removal of NOx from flue gas and diesel engine exhaust.

Abstract: A series of Fe/ZSM-5 catalysts were prepared by liquid ion exchange, incipient wetness impregnation, and solid-state ion exchange to investigate the selective catalytic reduction (SCR) of NOx by NH3 (NH3-SCR). The effect of hydrothermal deactivation of Fe-ZSM-5 catalysts prepared by different methods as a function of Fe loading was investigated. Freshly made and hydrothermal aged Fe-ZSM-5 catalysts were studied through NH3-SCR activity test and characterized using X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy. Iron species on the surface of Fe-ZSM-5 catalysts were assigned to isolated Fe3+ species, oligomeric FexOy clusters, and Fe2O3 particles based on the UV-Vis spectra. The iron species distributions in the Fe-ZSM-5 catalysts prepared by these methods were quite different, which resulted in difference in SCR activity. The NH3-SCR activity of different Fe-ZSM-5 catalysts became very similar after aging, and the activity of NH3 oxidation and NO oxidation decreased with the aged catalysts. Characterization results indicated that the activity change of the aged Fe-ZSM-5 catalysts was due to the change of iron species distribution in Fe-ZSM-5 catalysts after hydrothermal aging. The relative concentration of isolated Fe3+ species was decreased, whereas the relative concentration of oligomeric FexOy clusters and Fe2O3 particles was increased in the aged catalysts. A considerable decrease in the Brønsted acidity of catalysts was observed for the aged Fe-ZSM-5 catalysts.

Abstract: A superior Ce-W-Ti mixed oxide catalyst prepared by a facile homogeneous precipitation method showed excellent NH3-SCR activity and 100% N2 selectivity with broad operation temperature window and extremely high resistance to space velocity, which is a very promising catalyst for NOx abatement from diesel engine exhaust. The excellent catalytic performance is associated with the highly dispersed active Ce and promotive W species on TiO2. The introduction of W species could increase the amount of active sites, oxygen vacancies, and Brønsted and Lewis acid sites over the catalyst, which is also beneficial to improve the low temperature activity by facilitating �fast SCR� reaction and enhance both of the high temperature activity and N2 selectivity simultaneously by inhibiting the unselective oxidation of NH3 at high temperatures.

Abstract: Selective catalytic reduction of NOx with NH3 or urea (NH3/Urea-SCR) as reducing agents over catalytic materials in oxygen-rich conditions is one of the most efficient and widely-used techniques for the removal of NOx from stationary and mobile sources. The most important catalyst system for NH3-SCR process is vanadium-based catalyst. In this review, the composition and NH3-SCR performance, the activity improvement of vanadium-based catalysts and the corresponding NH3-SCR reaction mechanisms are summarized. The possible developing orientations in the field of NH3-SCR technique are also previewed. The conventional V2O5-WO3 (MoO3)/TiO2 catalyst and corresponding improved vanadium-based catalysts usually show excellent deNOx efficiency and SO2 durability in the medium temperature range. On these catalysts, the highly dispersed V5+ species and poly-vanadate species are confirmed to be the active phases for NH3-SCR reaction. Over vanadium-based catalysts prepared by different methods or with different compositions, a majority of researchers consider that the NH3-SCR reaction follows an Eley-Rideal (E-R) mechanism and some researchers prefer to a Langmuir-Hinshelwood (L-H) mechanism, which might be related to the vanadium loading amount and reaction temperature. During the subsequent work in further study, the researchers should combine multiple characterization methods aiming at different catalyst systems, and pay more attention to the influence of temperature on the reaction mechanism together with the effect of surface acid/basic property on the adsorption and activation of NH3/NOx. Accordingly, much more comprehensive and authentic reaction mechanism can be concluded. The systematic understanding of the research progress in vanadium-based catalysts is beneficial to the development of highly efficient, stable vanadium-based SCR catalytic converters at the present stage, and also important for the design and synthesis of novel, efficient, environmental-friendly vanadium-free SCR catalysts with high resistance to poisoning.

Abstract: Selective catalytic reduction (SCR) of nitrogen oxides (NOx) under oxygen-rich conditions is a research hotspot in the field of environmental catalysis, of which the core problem is to develop environmental-friendly, highly effective and stable SCR catalyst systems. At present, the NH3-SCR technique using urea/NH3 as reducing agent has already been widely applied for stationary flue gas denitrogenation and diesel engine exhaust purification. The HC-SCR technique using hydrogen carbons (HC) as reducing agent is also promising for practical application. For urea/NH3-SCR system, in this paper, the research progress of iron titanate catalyst, cerium-based oxide catalysts and small-pore zeolite catalysts is comprehensively summarized, including the catalyst structure, the SCR reaction mechanism, the improvement of low temperature activity and also the poisoning resistance performance. For HC-SCR system, herein, the SCR of NOx by higher HCs/diesel, and the achievements in the mechanism study of HC-SCR are summarized systematically, which can provide new guideline for the development and application of diesel-SCR technique.

Abstract: Addition of alkali metal ions significantly promotes the activity of the Pt/TiO2 catalyst for the HCHO oxidation reaction by stabilizing an atomically dispersed Pt-O(OH)x alkali metal species and opening a new low-temperature reaction pathway. The atomically dispersed Na-Pt-O(OH)x species can effectively activate H2O and catalyze the facile reaction between surface OH and formate species to total oxidation products.

Abstract: A Ce-Ti based (CeO2-TiO2) catalyst prepared by an optimized homogeneous precipitation method showed excellent NH3-SCR activity, high N2 selectivity, broad operation temperature window, and high resistance to space velocity (even under a high gas hourly space velocity (GHSV) of 500,000 h�1). Compared with V2O5-WO3/TiO2 and Fe-ZSM-5 catalysts, the CeO2-TiO2 catalyst showed better catalytic performance for NH3-SCR. Under a more realistic condition of simulated diesel engine exhaust, the monolith catalyst of CeO2-TiO2 showed over 90% NOx conversion from 250 to 450 °C under a GHSV of 20,000 h�1 in the presence of H2O, CO2, and C3H6. The high dispersion of active CeO2 on TiO2 in the process of homogenous precipitation and the synergistic effects between CeO2 and TiO2 in CeO2-TiO2 are important reasons for the high NH3-SCR activity.

Abstract: A novel Ce�W mixed oxide catalyst prepared by homogeneous precipitation method presented nearly 100% NOx conversion in a wide temperature range from 250 to 425 °C for the selective catalytic reduction of NOx with NH3 under an extremely high GHSV of 500000 h�1.

Abstract: Selective catalytic reduction of NO with NH3 (NH3-SCR) is a well-proven technique for the removal of NOx from stationary sources such as coal-fired power plants, and is also one of the most promising techniques for the NOx elimination from diesel exhaust under oxygen-rich conditions. Due to some inevitable disadvantages of the present V2O5-WO3 (MoO3)/TiO2 catalyst for industrial use, many researchers focus on the development of novel, highly efficient, stable, environmental-friendly and vanadium-free NH3-SCR catalysts. The research progress in the field of vanadium-free NH3-SCR catalysts is reviewed, including zeolite catalysts (such as Fe-zeolite catalysts and Cu-zeolite catalysts) and oxide catalysts (such as Fe-based oxide catalysts, Mn-based oxide catalysts, and other vanadium-free oxide catalysts). Several aspects in this field, including the evaluation of catalytic performance, the structure analysis of active sites, the improvement of low temperature activity, the study of NH3-SCR reaction mechanism, the enhancement of H2O/SO2 durability, and the feasibility analysis for industrial use, have been discussed in detail. The possible developing orientation and research interests in the field of vanadium-free NH3-SCR catalysts are previewed.

Abstract: The reaction mechanism of the selective catalytic reduction (SCR) of NOx with NH3 over environmental-friendly iron titanate catalyst (FeTiOx) was investigated in detail. Over the iron titanate crystallite with specific Fe�O�Ti structure, both Brønsted and Lewis acid sites were present and involved in the SCR reaction. NH3 mainly adsorbed on titanium sites in the form of ionic NH4+ and coordinated NH3, while NOx mainly adsorbed on iron sites in the form of monodentate nitrate. In a relatively low temperature range (<200 °C), the SCR process mainly followed the Langmuir�Hinshelwood (L�H) mechanism, in which the formation of monodentate nitrate from NO oxidation by O2 over Fe3+ was the rate-determining step. In contrast, in a relatively high temperature range (>200 °C), the SCR process mainly followed the Eley-Rideal (E�R) mechanism, in which the formation of NH2NO intermediate species following the H-abstraction of NH3 by neighboring Fe3+ was the rate-determining step.

Abstract: The effects of adding CeO2 to Ag/Al2O3 on the selective catalytic oxidation of ammonia to nitrogen were investigated by activity test and N2 physisorption, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis diffuse-reflectance spectroscopy, high resolution transmission electron microscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy of NH3 adsorption, and O2-pulse adsorption. Adding a suitable amount of CeO2 improved the catalytic activity of Ag/Al2O3 for NH3 oxidation at temperatures below 160 oC, and slightly influenced N2 selectivity by improving the catalyst�s ability to adsorb and activate O2 and the adsorption and activation of NH3.

Abstract: A Ce-Ti mixed oxide catalyst prepared by homogeneous precipitation method presented excellent NH3-SCR activity with nearly 100% NOx conversion in a temperature range of 200-350 oC (the main temperature range of diesel engine exhaust) at a space velocity of 50,000 h-1 and prominent performance even under a high space velocity of 150,000 h-1.

Abstract: Direct sulfation of Zr�Co hydroxides provides a significant advantage over traditional impregnation by conserving Brønsted acid sites for the introduction of a second active component and also for subsequent catalytic reactions. After Pd loading, sulfated Zr�Co exhibits excellent activity, selectivity, and durability for the selective catalytic reduction of NOx by methane. Using Co K-edge X-ray absorption spectroscopy, we provide the direct evidence of the entrance of Co into the lattice of cubic zirconia to form a substitutional solid solution.

Abstract: The influence of calcination temperature on the microstructure, redox behavior, reactant adsorption capability and catalytic activity of iron titanate catalyst (FeTiOx) for the selective catalytic reduction (SCR) of NOx with NH3 was investigated in detail using various characterization methods. After high temperature calcination (above 600 °C), the apparent SCR activity of FeTiOx catalyst obviously decreased, which was mainly due to decrease of surface area, pore volume, mobility of lattice oxygen and reactant adsorption capability. However, well crystallized pseudobrookite Fe2TiO5 showed much higher intrinsic SCR activity after normalization by surface area. This is mainly owing to the formation of larger proportion of monodentate nitrate, which is the real reactive nitrate species in the NOx reduction process, among total nitrate species on the surface of FeTiOx catalysts with higher crystallization degree. This implies a possibility that the FeTiOx catalyst after high temperature calcination (600 or 700 °C) with higher thermal stability could be loaded onto porous support with large surface area to further improve its dispersion and thus the apparent SCR activity for practical use, such as the DeNOx process for diesel engines.

Abstract: Iron titanate catalyst (FeTiOx) is a potential candidate for the substitution of conventional V2O5�WO3 (MoO3)/TiO2 and Fe/Cu-zeolite catalysts for the selective catalytic reduction (SCR) of NOx with NH3 because of its high SCR activity and N2 selectivity in the medium temperature range. Due to the presence of small amount of SO2 in typical diesel exhaust derived from combustion of sulfur-containing fuels, it is very important to investigate the influence of sulfation on SCR activity, catalyst structure and reaction mechanism. After sulfation under the SCR condition, the surface area and pore volume of FeTiOx catalyst decreased to a certain extent due to the formation of sulfate species. According to the characterizations of FeTiOx catalyst using X-ray diffraction, X-ray absorption fine structure spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy of SO2 + O2 treatment, the sulfate species mainly formed on iron sites in a chelating bidentate conformation, resulting in the enhancement of Brønsted acidity and Lewis acid strength simultaneously. NH3 adsorption was greatly enhanced in the high temperature range, while NOx adsorption was severely inhibited due to the stronger acidity of sulfate species. The operation temperature window of the sulfated catalyst shifted ca. 50 °C towards high temperature range accordingly. The reaction mechanism study shows that the Langmuir�Hinshelwood reaction pathway was cut off by the sulfation process, resulting in the activity loss at low temperatures; only Eley�Rideal reaction pathway between adsorbed NH3 species and gaseous or weakly adsorbed NO dominated in the SCR reaction, which made this catalyst resistant to SO2 poisoning at relatively high temperatures.

Abstract: A novel iron titanate catalyst prepared by conventional co-precipitation method showed excellent activity, N2 selectivity and H2O/SO2 durability in the selective catalytic reduction (SCR) of NO with NH3. The influence of precursors and preparation methods on the catalyst structure and activity was comprehensively investigated. Iron titanate catalyst prepared using titanium sulfate as Ti precursor was favorable for the high activity and selectivity, comparing with that using titanium tetrachloride as precursor and Fe2O3/TiO2 loaded type catalyst. Especially, the best iron titanate catalyst showed good activity in a temperature window of 200�350 °C with the NOx conversion above 90% in the absence of H2O, which was 50�150 °C lower than those of other known Fe-based catalysts. Iron titanate crystallite with specific Fe�O�Ti structure was found to be the main active phase. The interaction between iron and titanium species in atomic scale led to an enhancement of oxidative ability of Fe3+, which was beneficial to the SCR reaction.

Abstract: A series of iron titanate catalysts, FeaTibOx, with different Fe�Ti molar ratios are synthesized via a facile coprecipitation method and tested for the selective catalytic reduction (SCR) of NOx with NH3. The structural properties and redox behavior of the serial catalysts are comprehensively characterized. Comparing with pristine TiO2 and Fe2O3, the coexistence of iron and titanium species is favorable to form crystallites with specific Fe�O�Ti structure, which is highly active for the NH3-SCR reaction. Fe4Ti4Ox catalyst with a Fe�Ti molar ratio of 1:1 shows the highest intrinsic activity, due to its smallest particle size, enhanced oxidative ability of Fe3+, highest mobility of lattice oxygen, and abundant acid sites. The correlation between catalytic performance and reactant adsorption capability/conformation is also studied, indicating that an appropriate method to improve the low temperature SCR activity of iron titanate catalyst is to enhance the adsorption ability of NOx as monodentate nitrate on catalyst surface.

Abstract: Selective catalytic reduction (SCR) of NO with NH3 over Mn substituted iron titanate catalyst (Fe0.75Mn0.25TiOx) was fully investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy. At relatively low temperatures, both ionic NH4+ and coordinated NH3 contributed to the SCR reaction, and both bridging nitrate and monodentate nitrate were confirmed to be the reactive nitrate species. In the SCR reaction condition, surface NH4NO3 species was formed as intermediate species and its reactivity was also proved. An NH3-SCR mechanism over Fe0.75Mn0.25TiOx at low temperatures was proposed accordingly, in which the reduction of NH4NO3 by NO was possibly the rate-determining step. Due to the mild and reversible inhibition effect of H2O on NH3/NOx adsorption, the SCR activity decline in the presence of H2O was also slight and recoverable; however, the inhibition effect of SO2 was much more intense and irreversible, because the formation of nitrate species was totally inhibited by the formation of sulfate, resulting in the cut-off of the SCR reaction pathway at low temperatures.

Abstract: Selective catalytic reduction (SCR) of NO with NH3 over manganese substituted iron titanate catalysts was fully studied. The low temperature SCR activity was greatly enhanced when partial Fe was substituted by Mn, although the N2 selectivity showed some decrease to a certain extent. The Mn substitution amounts showed obvious influence on the catalyst structure, redox behavior and NH3/NOx adsorption ability of the catalysts. Among FeaMn1�aTiOx (a = 1, 0.75, 0.5, 0.2, 0) serial catalysts, Fe0.5Mn0.5TiOx with the molar ratio of Fe:Mn = 1:1 showed the highest SCR activity, because the interaction of iron, manganese and titanium species in this catalyst led to the largest surface area and the highest porosity, the severest structural distortion and most appropriate structural disorder, the enhanced oxidative ability of manganese species, the highest mobility of lattice oxygen, the proper ratio of Brønsted acid sites and Lewis acid sites together with the enhanced NOx adsorption capacity.

Abstract: An iron titanate catalyst with a crystallite phase, prepared by a co-precipitation method, showed excellent activity, stability, selectivity and SO2/H2O durability in the selective catalytic reduction of NO with NH3 in the medium temperature range.

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Abstract: With dimethyl phthalate as the model pollutant and Ru/Al2O3 as catalyst, this paper systemically investigates the removal of total organic carbon (TOC) of system. Our results have confirmed that Ru/Al2O3 can significantly increase the effect of ozonation. TOC removal in 120 min can reach 72% while only 24% with ozone alone. The optimal catalyst preparing condition was 0.1 wt% Ru content, 600 °C calcination temperature, 0.5�1.0 mm particle diameter, which is characterized by a high surface area and a large population of surface active sites. The contrasting experiments of ozone alone, catalyst adsorption after ozonation, and catalytic ozonation confirmed that catalytic reaction was the most important process to TOC removal in system with Ru/Al2O3 as catalyst.

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Abstract: Selective catalytic reduction of NOx with NH3 (NH3-SCR) is one of the most promising NOx control technologies for lean burn engines, particularly for diesel engines, to meet the increasingly stringent standards for NOx emission worldwide. Due to some inevitable disadvantages of the well-established vanadium-based catalyst (V2O5-WO3 (MoO3)/TiO2) for NH3-SCR of NOx from stationary sources, including the narrow operation temperature window, the large N2O formation at high temperatures, the easy sublimation and biological toxicity of vanadium pentoxide, environmentally-benign and vanadium-free catalysts for NH3-SCR reaction are requisite to be developed urgently. Recently, cerium-based oxides with high oxygen storage capacity and excellent redox property attracted much attention for their use as environmentally-benign NH3-SCR catalysts. In this chapter, we will comprehensively introduce our recent research results about the cerium-based catalysts for NH3-SCR process, including the supported type CeOx/TiO2 catalyst prepared by conventional impregnation method and mixed oxide CeTiOx catalyst prepared by a novel homogeneous precipitation method. The effect of WOx addition into the CeTiOx catalyst will also be studied in detail, which can improve the NH3-SCR activity in the whole temperature range and enhance the N2 selectivity in the high temperature range simultaneously. The total substitution of titanium species by tungsten species in CeWOx catalyst can further improve the SCR activity and N2 selectivity to a certain extent, with nearly 100% NOx conversion in a wide temperature range from 250 to 425 oC even under an extremely high gas hour space velocity (GHSV) of 500,000 h-1. The excellent catalytic performance of this catalyst is attributed to a synergistic effect between Ce and W species. The cerium-based catalysts introduced in this chapter can be used as potential candidates for practical application in the deNOx process for diesel engines, which is an important developing direction for the technological applications of cerium materials.

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