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1.
Identification of active species and the rate‐determining reaction steps are crucial for optimizing the performance of oxygen‐storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce 3+ species in a ceria‐supported platinum catalyst during CO oxidation are short‐lived and therefore cannot be observed under steady‐state conditions. Using time‐resolved resonant X‐ray emission spectroscopy, we quantitatively correlated the initial rate of Ce 3+ formation under transient conditions to the overall rate of CO oxidation under steady‐state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce 3+ was present as spectators. This approach can be applied to various catalytic processes involving oxygen‐storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms. 相似文献
2.
High performance CuO-CeO 2 catalysts for selective oxidation of CO in excess hydrogen were prepared by a hydrothermal method under different preparation conditions and evaluated for catalytic activities and selectivities. By changing the n CTAB/n Ce ratio and hydrothermal aging time, the catalytic activity of the CuO-CeO 2 catalysts increased and the operating temperature window, in which the CO conversion was higher than 99%, was widened. XRD results showed no peaks of CuO x species and Cu-Ce-O solid solution were observed. On the other hand, Cu + species in the CuO-CeO 2 catalysts, which was associated with a strong interaction between copper oxide clusters and cerium oxide and could be favorable for improving the selective oxidation performance of CO in excess H 2, were detected by H 2-TPR and XPS techniques. 相似文献
3.
The HSiW( x)/Ce‐Fe catalysts were used to research the effect of silicotungstic acid contents on the catalytic activity in the selective catalytic reduction of NO x with NH 3. Doping different contents of silicotungstic acid affected surface species and redox property as well as the catalytic activity. With the increasing amount of HSiW ( x = 5%, 10% and 20%), the redox reaction between Fe 3+/Fe 2+ and Ce 4+/Ce 3+ enhanced, which could improve the ratio of Ce 3+ and Fe 3+. And then, more Ce 3+ increased the ratio of chemisorbed oxygen (O α). Besides, the type and strength of acid sites over HSiW( x)/Ce‐Fe was affected by the HSiW contents. These factors facilitated the catalytic performance. Thus, the NO x conversion of HSiW( x)/Ce‐Fe( x = 20%) was higher than 90%, which maintained in a wide temperature range between 200 and 400 °C. 相似文献
4.
A series of MnOx samples synthesized by hydrothermal methods at different temperatures were investigated as catalysts for the oxidation of toluene. The optimum oxidation performance was achieved with the catalyst prepared at 120 °C (Mn-120), for which complete conversion of toluene was attained at 250 °C. The Mn-120 sample possessed the highest concentration of Mn3+ and the highest initial H2 consumption rate, which are indicative of abundant crystal defects and superior reducibility. In addition, Mn-120 exhibited excellent oxidation ability due to the abundance of lattice oxygen species and excellent oxygen mobility. Therefore, the superior catalytic performance of Mn-120 could be attributed mainly to its redox performance and abundant crystal defects, both of which are determined by the temperature of the hydrothermal synthesis of MnOx. 相似文献
5.
采用浸渍法和共沉淀法制备了HoCeMn/TiO 2脱硝催化剂并对其结构和性能进行了表征。结果表明共沉淀法增强了活性组分和载体的相互作用,从而增加了 HoCeMnTi-C催化剂表面 Ce 3+、Mn 4+以及吸附氧的含量,使其表现出优异的低温氧化还原性能。此外,共沉淀法制备的HoCeMnTi-C具有更多的表面酸性位点及更强的表面酸性。催化剂表面酸性和氧化还原性能的提高有助于氨的吸附和活化,从而显著提高其活性。表面酸性位点的增多还抑制了 H 2O和 SO 2在催化剂表面的吸附,提升了催化剂的抗水抗硫性能。催化剂上的选择性催化还原(SCR)反应遵循Eley-Rideal(E-R)机制,催化剂硫中毒是源于形成的硫酸盐覆盖或破坏了催化剂活性位。 相似文献
6.
The Co–Mn/Ti–Ce catalyst prepared by sol–gel and impregnation method was evaluated for catalytic oxidation of Hg 0 in the simulated flue gas compared with Co/TiO 2 and Co–Mn/TiO 2. The results showed that Co–Mn/Ti–Ce catalyst exhibited higher catalytic activity (around 93% Hg 0 removal efficiency in the temperature of 150 °C with 6% O 2, 400 ppm NO, 200 ppm SO 2 and 3% H 2O) than Co/TiO 2 and Co–Mn/TiO 2. Based on the characterization results of N 2 adsorption–desorption, XRD, UV–Vis, XPS, H 2-TPR and Hg-TPD, it could be concluded that the lower band gap, better reducibility and mercury adsorption capability and the presence of Co 3+/Co 2+, Mn 4+/Mn 3+ and Ce 4+/Ce 3+ redox couples as well as surface oxygen species contributed to the excellent Hg 0 oxidation removal performance. In addition, well dispersion of active components and a synergetic effect among Co, Mn and Ce species might improve the activity further. A Mars–Maessen mechanism is thought to be involved in the Hg 0 oxidation. The lattice oxygen derived from MnO x or CoO x would react with adsorbed Hg 0 to form HgO and the consumption of lattice oxygen could be replenished by O 2. For Co–Mn/Ti–Ce, MnO x?1 could be alternatively reoxidized by the lattice oxygen derived from adjacent CoO x and CeO x which is beneficial to the Hg 0 oxidation. 相似文献
7.
Ce 1−xY xO 2 and Ce 0.85−xZr 0.15Y xO 2 mixed oxides have been prepared by 1000 °C-nitrates calcination to ensure thermally stable catalysts. The physico-chemical properties of the mixed oxides have been studied by N 2 adsorption at −196 °C, XPS, XRD, Raman spectroscopy and H 2-TPR, and the catalytic activity for soot oxidation in air has been studied by TG in the loose and tight contact modes. Yttrium is accumulated at the surface of Ce 1−xY xO 2 and Ce 0.85−xZr 0.15Y xO 2, and this accumulation is more pronounced for the former formulation than for the latter, because the deformation of the lattice due to zirconium doping favours yttrium incorporation. Yttrium and zirconium exhibit opposite effects on the surface concentration of cerium; while zirconium promotes the formation of cerium-rich surfaces, yttrium hinders the accumulation of cerium on the surface. For experiments in tight contact between soot and catalyst, all the Ce 1−xY xO 2 catalysts are more active than bare CeO 2, and Ce 0.99Y 0.01O 2 is the most active catalyst. The benefit of yttrium doping in catalytic activity of ceria can be related to two facts: (i) the Y 3+ surface enrichment hinders crystallite growth; (ii) the surface segregation of Y 3+ promotes oxygen vacancies creation. High yttrium loading ( x = 0.12) is less effective than low dosage ( x = 0.01) because yttrium is mainly accumulated at the surface of the particles and hinders the participation of cerium in the soot oxidation reaction, which is the active component. For the mixed oxides with formulation Ce 0.85−xZr 0.15Y xO 2 (operating in tight contact) the effect of zirconium on the catalytic activity prevails with respect to that of yttrium. For experiments in loose contact between soot and catalyst, the catalytic activity depends on their BET surface area, and the catalysts Ce 0.85−xZr 0.15Y xO 2 (BET = 10–13 m 2/g) are more active than the catalysts Ce 1−xY xO 2 (BET = 2–3 m 2/g). In the loose contact mode, the yttrium doping and loading have a minor or null affect on the activity, and the stabilising effect of the BET area due to zirconium doping prevails. 相似文献
8.
A series of Ce-Fe-O x catalysts prepared by the different calcination temperatures (marked as CF- X, where X represented calcination temperature) were used to the selectivity catalytic reduction of NO x by NH 3. The results explained the relationship between calcination temperature and the sulfate species over Ce-Fe-O x, and then investigated the surface acidity and catalytic performance. The large amounts of sulfate species were formed over CF-450 and CF-550 while it was decomposed with further the increasing of calcination temperature, which resulted in the loss of surface acidity, causing a decrease in the catalytic activity over Ce-Fe-O x. Thereby, the CF-450 catalyst showed the best catalytic activity and over 90% NO x conversion was obtained at 244–450 °C. Besides, the favored pore structure, more Fe 3+ active species, higher Ce 3+ concentration and the abundance of chemical adsorbed oxygen species, as well as the surface acid sites, would together contribute to the excellent catalytic activity of CF-450 catalyst. 相似文献
9.
采用浸渍法和共沉淀法制备了Ho Ce Mn/Ti O 2脱硝催化剂并对其结构和性能进行了表征。结果表明共沉淀法增强了活性组分和载体的相互作用,从而增加了Ho Ce Mn Ti-C催化剂表面Ce 3+、Mn 4+以及吸附氧的含量,使其表现出优异的低温氧化还原性能。此外,共沉淀法制备的HoCeMnTi-C具有更多的表面酸性位点及更强的表面酸性。催化剂表面酸性和氧化还原性能的提高有助于氨的吸附和活化,从而显著提高其活性。表面酸性位点的增多还抑制了H 2O和SO 2在催化剂表面的吸附,提升了催化剂的抗水抗硫性能。催化剂上的选择性催化还原(SCR)反应遵循Eley-Rideal(E-R)机制,催化剂硫中毒是源于形成的硫酸盐覆盖或破坏了催化剂活性位。 相似文献
10.
The large‐scale industrial production of acetic acid (HAc) from carbonylation of methanol has enabled intense research interest from direct hydrogenation of HAc to acetaldehyde (AA). Herein, a series of cerium‐iron oxide solid solution supported metallic cobalt catalysts were prepared by modified sol‐gel method and were applied in gas‐phase hydrogenation of HAc to AA. A synergistic effect between the hydrogenation metal cobalt and Ce‐Fe oxide solid solution is revealed. Specifically, oxygen vacancies provide the active sites for adsorption of HAc, while highly uniformly dispersed metallic Co adsorbs H 2 and activates the reduction of HAc into AA. Moreover, the metallic Co can also assist the cyclical conversion between Fe 3+/Fe 2+ and Ce 3+/Ce 4+ on the surface of Ce 1‐xFe xO 2‐δ supports. The unique effect substantially enhances the ability of the support material to rapidly capture oxygen atoms from HAc. It is found that the catalyst of 5% Co/Ce 0.8Fe 0.2O 2‐δ with the highest concentration of oxygen vacancy presents the best catalytic performance ( i.e. acetaldehyde yield reaches 49.9%) under the optimal reaction conditions ( i.e. 623 K and H 2 flow rate = 10 mL/min). This work indicates that the Co/Ce‐Fe oxide solid solution catalyst can be potentially used for the selective hydrogenation from HAc to AA. The synergy between the metallic Co and Ce 1‐xFe xO 2‐δ revealed can be extended to the design of other composite catalysts. 相似文献
11.
Ce 0.65Zr 0.35O 2 was prepared by co-precipitation method and a series of Mn 1-yCo y/Ce 0.65Zr 0.35O 2 catalysts with different Mn/Co molar ratio were synthesized via the co-impregnation method. These catalysts were applied for gaseous toluene oxidation, which showed that the catalytic activity was significantly improved by the addition of Mn and Co. In particular, Mn–Co(1:1)/Ce 0.65Zr 0.35O 2 with Mn/Co molar ratio of 1:1 displayed the best result with the lowest complete conversion temperature of 242 °C under a GHSV of 12,000 h ?1. The as-prepared catalysts were characterized by X-ray diffraction, H 2 temperature-programmed reduction, N 2 adsorption–desorption, X-ray photoelectron spectroscopy and O 2 temperature-programmed desorption. These characteristics revealed that the coexistence of Mn and Co could enhance the redox property and generate more surface adsorbed oxygen, thereby improving the performance of the catalysts for toluene low-temperature oxidation. The Mn–Co(1:1)/Ce 0.65Zr 0.35O 2 exhibited the best catalytic activity and high stability. The excellent catalytic activity of the Mn–Co(1:1)/Ce 0.65Zr 0.35O 2 could be ascribed to a greater amount of surface adsorbed oxygen species and Mn 4+ on the catalyst surface. 相似文献
12.
We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications.
In this comprehensive investigation, a series of Ce
x
Zr 1−x
O 2/Al 2O 3, Ce
x
Zr 1−x
O 2/SiO 2 and Ce
x
Zr 1−x
O 2/TiO 2 composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence
of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical
characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy
(XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate
the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The
XRD analyses revealed the formation of Ce 0.75Zr 0.25O 2, Ce 0.6Zr 0.4O 2, Ce 0.16Zr 0.84O 2 and Ce 0.5Zr 0.5O 2 phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration
with XRD results suggested enrichment of zirconium in the Ce
x
Zr 1−x
O 2 solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects
and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce
x
Zr 1−x
O 2 phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment
at 1073 K. The XPS studies revealed the presence of cerium in both Ce 4+ and Ce 3+ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization
techniques indicated absence of pure ZrO 2 and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found
to stabilize more effectively the nanosized Ce
x
Zr 1−x
O 2 oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly,
the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these
findings are consolidated in this review. 相似文献
13.
研究了纳米Ce 1—xMn xO 2上乙醇催化氧化发光特性, 重点考察了反应温度和催化剂组成(Ce/Mn比)对发光强度的影响规律. 为研究催化发光机理, 在相近的反应条件下考察了纳米Ce 1—xMn xO 2上乙醇催化氧化反应的活性和选择性. 结果表明: 催化发光强度与催化反应中生成CH 3CHO的产率有很好的顺变关系, 表明CH 3CHO是导致C 2H 5OH分子在纳米Ce 1—xMn xO 2催化剂上氧化发光的“活性分子”. 相似文献
14.
The catalytic performance of tetra‐ n‐butylammonium salts of Keggin and Wells–Dawson sandwich‐type polyoxotungstates, [M 4(PW 9O 34) 2] m? and [M 4(P 2W 15O 56) 2] n? (M = Mn 2+, Fe 3+, Co 2+, Ni 2+, Zn 2+), in the oxidation of cyclooctene and cyclohexene with 30% hydrogen peroxide under various conditions was investigated. In comparison, Wells–Dawson sandwich‐type polyoxometalates were found to be less active than Keggin ones. In both of them, those containing Zn and Fe gave higher conversions for different oxidation conditions. These catalysts showed very good reusability in the oxidation reaction. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
15.
用共沉淀法制得一系列铈锆比不同的Ce 0.3+xZr 0.6-xY 0.1O 1.95储氧材料, 并用于制备了一系列低贵金属Pt+Rh/Ce 0.3+xZr 0.6-xY 0.1O 1.95+Al 2O 3三效催化剂. 用比表面、程序升温还原以及X射线衍射对该系列催化剂进行表征, 结果发现, 催化剂的活性与催化剂中贵金属的还原性能密切相关, 低铈储氧材料比高铈储氧材料更有利于促进贵金属还原, 因而含低铈储氧材料催化剂的活性明显优于含高铈储氧材料催化剂的活性, Pt+Rh/Ce 0.35Zr 0.55Y 0.1O 1.95+Al 2O 3的活性最佳, 对HC, CO和NO的起燃温度最低分别为: 235, 175, 200 ℃. 样品经1000 ℃水热老化之后, 贵金属Pt被烧结而发生迁移, 使得催化剂的活性及还原性能变差, 含低铈材料的催化剂的抗老化性能优于含高铈材料的催化剂, 其中Pt+Rh/Ce 0.35Zr 0.55Y 0.1O 1.95+Al 2O 3的抗老化性能最好. 相似文献
16.
Ce
x
Sn 1−x
O 2 metal oxides were prepared by a citrate method and used as supports for CuO/Ce
x
Sn 1−x
O 2 catalysts. The as-prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy, high-resolution
TEM, and temperature-programmed reduction techniques. XRD analysis indicated that the Ce
x
Sn 1−x
O 2 samples maintain the fluorite structure and form a solid solution when x = 0.9 or 0.8. TPR analysis revealed that two kinds of copper species are present on the surface of the Ce
x
Sn 1−x
O 2 support. The catalytic properties of the CuO/Ce
x
Sn 1−x
O 2 catalysts were evaluated for low-temperature CO oxidation using a microreactor-GC system. The effects of Ce/Sn ratio and
CuO loading on the catalytic performance were investigated. The results showed that these CuO/Ce
x
Sn 1−x
O 2 catalysts are very active for low-temperature CO oxidation. The 650 °C calcined 7 wt%-CuO/Ce
x
Sn 1−x
O 2 catalyst exhibited the highest catalytic activity. 相似文献
17.
Mn xO y/SBA-15 catalysts were prepared via the impregnation method and utilized for toluene removal in dielectric barrier discharge plasma at atmospheric pressure and room temperature. The catalysts were characterized by X-ray diffraction, N 2 adsorption–desorption, Raman spectroscopy, X-ray photoelectron spectroscopy, H 2 temperature-programmed reduction, and O 2 temperature-programmed desorption methods. The characterization results indicated that manganese loading did not influence the 2D-hexagonal mesoporous structure of SBA-15. The catalyst had various oxidation states of manganese (Mn 2+, Mn 3+, and Mn 4+), with Mn 3+ being the dominant oxidation state. Toluene removal was investigated in the environment of pure N 2 and 80 % N 2 + 20 % O 2 plasma, showing that the toluene removal efficiency and CO 2 selectivity were noticeably increased by Mn xO y/SBA-15, especially in the presence of 5 % Mn/SBA-15. This activity was closely related to the high dispersion of 5 % Mn on SBA-15 and the lowest reduction temperature exhibited by this catalyst. Mn loading increased the yield of CO 2 in the N 2 plasma and promoted the deep oxidation of toluene. During toluene oxidation, oxygen exchange might follow a pathway, wherein bulk oxygen was released from the Mn xO y/SBA-15 surface; gas-phase O 2 subsequently filled up the vacancies created on the oxide. Each of the manganese oxidation states played an important role; Mn 2O 3 was considered as a bridge for oxygen exchange between the gas phase and the catalyst, and Mn 3O 4 mediated transfer of oxygen between the catalyst and toluene. 相似文献
18.
用沉积沉淀法合成两种不同系列的CeO2-ZrO2-La2O3混合氧化物(ZrO2和La2O3沉积CeO2粒子(标记为A-x)以及CeO2和La2O3沉积ZrO2粒子(标记为B-x)),并用作Rh催化剂的载体。XRD、拉曼、TPR、XPS和O2脉冲等表征结果显示出不同的沉积顺序将导致不同的结构和氧化还原性能,且B-x具有更高的氧迁移性、储氧能力和表面Ce浓度。当其负载Rh后,Rh/B-x催化剂具有更高的NO和CO转化率及N2选择性,且Ce的最佳含量为50at%。这可能归因于Rh负载于富铈表面形成更多有利于NO分解的表面Ce3+活性位。 相似文献
19.
Catalytically active MnO x species have been reported to form in situ from various Mn‐complexes during electrocatalytic and solution‐based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnO x catalysts form without supports in situ under photocatalytic conditions. Our most active 4MnO x catalyst (~0.84 mmol O 2 mol Mn ?1 s ?1) forms from a Mn 4O 4 bearing a metal–organic framework. 4MnO x is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR‐TEM as a disordered, layered Mn‐oxide with high surface area (216 m 2g ?1) and small regions of crystallinity and layer flexibility. In contrast, the SMnO x formed from Mn 2+ salt gives an amorphous species of lower surface area (80 m 2g ?1) and lower activity (~0.15 mmol O 2 mol Mn ?1 s ?1). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p 3/2 core levels detects enriched Mn 3+ and Mn 2+ content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria. 相似文献
20.
A series of the solid‐solution phosphors Lu 3?x?yMn xAl 5?xSi xO 12: yCe 3+ is synthesized by solid‐state reaction. The obtained phosphors possess the garnet structure and exhibit similar excitation properties as the phosphor Lu 3Al 5O 12:Ce 3+, but with an effectively improved red component in the emission spectrum. This can be attributed to the energy transfer from Ce 3+ to Mn 2+. Our investigation reveals that electric dipole–quadrupole interactions dominate the energy‐transfer mechanism and that the critical distance determined by the spectral overlap method is about 9.21 Å. The color‐tunable emissions of the Lu 3?x?yMn xAl 5?xSi xO 12: yCe 3+ phosphor as a function of Mn 3Al 2Si 3O 12 content are realized by continuously shifting the chromaticity coordinates from (0.354, 0.570) to (0.462, 0.494). They indicate that the obtained material may have potential application as a blue radiation‐converting phosphor for white LEDs with high‐quality white light. 相似文献
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