Mn promoted Pd/TiO2–Al2O3 catalyst for the selective catalytic reduction of NOx by H2
TL;DR: In this paper, the performances of Mn modified Pd/TiO2-Al2O3 catalysts for the selective catalytic reduction of NOx by H2 (H2-SCR) were investigated.
Abstract: The performances of Mn modified Pd/TiO2–Al2O3 catalysts for the selective catalytic reduction of NOx by H2 (H2-SCR) were investigated in this study. The added Mn not only enhanced the activity but also remarkably increased N2 selectivity above 150°C. Based on the characterization, it was found that Pd was present in the form of PdO and the synergetic effect between Pd and Mn led to more NOx adsorbed and activated over Mn promoted Pd/TiO2–Al2O3 catalyst. In situ DRIFTS studies demonstrated that the addition of Mn resulted in new types of adsorbed nitrite and nitrate species formed, both of which are reactive in the H2-SCR process. Moreover, NH3 and NHx (x = 0–2) species adsorbed on Lewis acid sites were dominantly present instead of NH4+ species on Bronsted acid sites over the Mn modified Pd/TiO2–Al2O3 catalyst. Therefore, the H2-SCR of NOx reaction was facilitated to proceed following the well-established NH3-SCR reaction route.
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TL;DR: Different available post-combustion methods of NOx removal, including selective catalytic reduction (using different types of reducing reagents, including ammonia, hydrogen, hydrocarbons, and carbon monoxide), selective noncatalytic reduction, wet scrubbing, adsorption, electron beam, nonthermal plasma, and electrochemical reduction ofNOx, are discussed.
151 citations
01 Aug 2007
TL;DR: In this paper, the selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2 -SCR) in the low-temperature range of 100-200-°C has been studied over Pt supported on a series of metal oxides (e.g., La 2 O 3, MgO, Y 2 O3, CaO, CeO 2, TiO 2, SiO 2 and Mg O-CeO 2 ).
Abstract: Abstract The selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2 -SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La 2 O 3 , MgO, Y 2 O 3 , CaO, CeO 2 , TiO 2 , SiO 2 and MgO-CeO 2 ). The Pt/MgO and Pt/CeO 2 solids showed the best catalytic behavior with respect to N 2 yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO 2 support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N 2 production (mols N 2 /g cat s). High NO conversions (70–95%) and N 2 selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h −1 with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H 2 , 5 vol% O 2 and He as balance gas. Addition of 5 vol% H 2 O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO 2 during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO 2 in the feed stream following a given support pretreatment. N 2 selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO 2 catalyst in the 100–200 °C range in the presence of water and SO 2 in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO 2 catalyst towards a new NO x control technology in the low-temperature range of 100–200 °C using H 2 as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO x species with H 2 experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO x intermediate species, NO x spillover).
123 citations
TL;DR: In this paper, the authors investigated the low-temperature performance of WO3 modified Pt/TiO2 catalysts for selective catalytic reduction of NOx by H2 (H2-SCR) in the presence of oxygen.
Abstract: Pt/TiO2 and WO3 modified Pt/TiO2 catalysts have been investigated for the selective catalytic reduction of NOx by H2 (H2-SCR) in the presence of oxygen. It was found that the addition of WO3 leads to a significant promoting effect on the low-temperature activity of Pt/TiO2 catalyst and the optimal loading is 2%. X-ray absorption near-edge structure (XANES) revealed the electron transfer from WO3 to Pt active sites, leading to the formation of metallic Pt, which is responsible for the low-temperature H2-SCR of NOx. In-situ DRIFTS demonstrated that the introduction of WO3 to Pt catalyst not only contributes to the formation of reactive adsorbed NOx species on the catalyst, but also promotes the formation of NH4+ species. All of these factors, collectively, account for the improved low-temperature activity of Pt-WO3/TiO2 catalyst.
68 citations
TL;DR: In this article, two high-activity supported-Pd catalysts (Pd/V 2 O 5 /TiO 2 /SBA-15 and Pd/v 2 o 5 / TiO 2/SBA -15) for selective catalytic reduction of NO with H 2 were studied and the role of hydrogen spillover in enhancing the catalytic activities of these two catalysts was shown.
Abstract: Two high-activity supported-Pd catalysts (Pd/V 2 O 5 /TiO 2 /SBA-15 and Pd/V 2 O 5 /TiO 2 /MCM-41) for selective catalytic reduction of NO with H 2 were studied. They differ only by the mesoporous-silica support: SBA-15 vs. MCM-41. Although the SBA-15 supported catalyst had a lower surface area, it showed a higher NO conversion (95% vs. 84%, at a high space velocity of 4.6 × 10 4 1/h). Two pieces of direct and related evidence are shown for the role of hydrogen spillover in enhancing the catalytic activities of these two catalysts. A comparison of hydrogen adsorption isotherms (via the Benson-Boudart method) show more spillover on the SBA-15 supported catalyst. The results from the transient kinetics experiments also showed that the amount of spiltover hydrogen stored on the Pd/V 2 O 5 /TiO 2 /SBA-15 under dynamic reaction conditions was significantly higher than that on Pd/V 2 O 5 /TiO 2 /MCM-41 (7.3 μmol/g vs. 2.1 μmol/g). Catalytic performance of the two catalysts for practical applications is reported. The favorable effect of hydrogen spillover on H 2 -SCR could be used as a new strategy for improving the performance of noble-metal H 2 -SCR catalysts.
55 citations
TL;DR: In this article, a novel catalytic purification process over a 3DOM catalyst, called SCRPF (Selective Catalytic Reduction and Particulate Filter), was designed and employed for the simultaneous removal of PM (particulates matter) and NOx from diesel engine exhausts.
46 citations
References
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TL;DR: In this paper, it is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements.
Abstract: Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts. It is concluded, after the investigation of hundreds of catalyst formulations, that many of the fundamental questions relating to lean deNOx reactions have been addressed and the main boundary conditions have been established. It seems clear that catalysts with sufficient activity, selectivity or stability to satisfy the demanding conditions that appertain in automotive applications are still far away. The rapidly growing interest in NOx storage systems reflects this situation, and it now seems to be the case that acceptable direct NOx reduction catalysts may be very difficult to find for lean-burn applications.
827 citations
TL;DR: Tungstation was beneficial for the formation of Ce(3+), which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction, resulting in the better activity of CeO (2)-WO(3)/TiO(2) compared to that of CeCe(2)/Ti O(2).
Abstract: CeO(2)/TiO(2) and CeO(2)-WO(3)/TiO(2) catalysts prepared by impregnation method assisted with ultrasonic energy were investigated on the selective catalytic reduction (SCR) of NO(x) (NO and NO(2)) by NH(3). The catalytic activity of 10% CeO(2)/TiO(2) (CeTi) was greatly enhanced by the addition of 6% WO(3) in the broad temperature range of 200-500 °C, the promotion mechanism was proposed on basis of the results of in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH(3) was introduced into both catalysts preadsorbed with NO + O(2), SCR would not proceed except for the reaction between NO(2) and ammonia. For CeO(2)/TiO(2) catalysts, coordinated NH(3) linked to Lewis acid sites were the main adsorbed ammonia species. When NO + O(2) was introduced, all the ammonia species consumed rapidly, indicating that these species could react with NO(x) effectively. Two different reaction routes, L-H mechanism at low temperature ( 200 °C), were presented for SCR reaction over CeO(2)/TiO(2) catalyst. For CeO(2)-WO(3)/TiO(2) catalysts, the Lewis acid sites on Ce(4+) state could be converted to Bronsted acid sites due to the unsaturated coordination of Ce(n+) and W(n+) ions. When NO + O(2) was introduced, the reaction proceeded more quickly than that on CeO(2)/TiO(2). The reaction route mainly followed E-R mechanism in the temperature range investigated (150-350 °C) over CeO(2)-WO(3)/TiO(2) catalysts. Tungstation was beneficial for the formation of Ce(3+), which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce(3+) state resulted in the better activity of CeO(2)-WO(3)/TiO(2) compared to that of CeO(2)/TiO(2).
618 citations
TL;DR: In this article, a series of cerium modified MnOx/TiO2 catalysts were prepared by sol-gel method and used for low-temperature selective catalytic reduction (SCR) of NOx with ammonia.
538 citations
TL;DR: In this paper, the authors classified the selective catalytic reduction (SCR) of NOx by hydrocarbon into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation reduction mechanism.
Abstract: The catalytic removal of nitrogen oxide (NOx) under lean‐burn conditions is one of the most important targets in catalysis research. Some lean‐NOx control technologies such as the direct decomposition of NOx, NOx storage‐reduction (NSR), and selective catalytic reduction (SCR) using different reducing agents (diesel soot, NH3, or hydrocarbon) are described. The reaction mechanism of NSR, which is the most promising technology, together with some novel NSR catalysts is discussed. Some mechanisms of SCR of NOx by hydrocarbon (HC‐SCR) were classified into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation‐reduction mechanism. Based on the discussion of the reaction mechanism, the influence of some factors (catalyst support, metal loading, calcination temperature, catalyst preparation method, oxygen, reducing agents, water, and sulfur) on the activity of HC‐SCR catalyst is discussed. It seems that Ag/Al2O3 catalyst offers the most promising for SCR of NOx by hydrocarb...
484 citations
TL;DR: In this article, a series of TiO 2 supported manganese oxide catalysts were prepared by wet-impregnation method for the low temperature selective catalytic reduction (SCR) of NO with ammonia as a reductant.
Abstract: A series of TiO 2 supported manganese oxide catalysts were prepared by wet-impregnation method for the low temperature selective catalytic reduction (SCR) of NO with ammonia as a reductant. A combination of various physico-chemical techniques such as N 2 physisorption, O 2 chemisorption, TPR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman were used to characterize the chemical environment of these catalysts. O 2 chemisorption and XRD results suggest that Mn exist in a well-dispersed state at below 16.7 wt.% of Mn on TiO 2 anatase (Hombikat), 7.5 wt.% on TiO 2 rutile (Kemira) and P-25 (80% anatase + 20% rutile), and in microcrystalline phase above these loading levels on respective support materials. These results also reveal that Mn interacts very well with pure anatase phase compared to rutile. XPS results of Mn/TiO 2 anatase (Hombikat) catalysts illustrated the presence of MnO 2 as a major phase (peak at 642.0 eV) along with Mn 2 O 3 as the minor phase at lower loadings. The presence of Mn 2 O 3 disappears at higher loadings. The characterization results indicated that the manganese oxide exists as an isolated species at very low loadings, highly dispersed state probably as two dimensional monolayer species at intermediate loadings, polymeric or microcrystalline form of manganese oxide at higher (above monolayer capacity) loadings was envisaged. The catalytic performance of various amounts of Mn loaded on different TiO 2 supported catalysts for low temperature SCR reaction at catalyst bed temperature 175 °C under power plant conditions using GHSV = 50,000 h −1 was studied. The catalyst with 16.7 wt.% Mn/TiO 2 anatase (Hombikat) was found to be highly active and selective catalyst for this reaction. The Raman studies acted as complimentary tool to XPS in order to characterize the manganese oxides (MnO, Mn 2 O 3 , Mn 3 O 4 , MnO 2 ). Raman data show that there is a strong interaction between the Mn oxides and the support, which is responsible for the impressive catalytic performance in comparison with other systems we investigated.
461 citations