共查询到20条相似文献,搜索用时 15 毫秒
1.
A Co-H-MFI sample has been studied through FT-IR spectroscopy of in situ adsorption and co-adsorption of probe molecules ( o-toluonitrile, CO, NO) and has been tested in the CH 4-SCR process under IR operando conditions. The o-toluonitrile ( oTN) adsorption and the oTN and NO co-adsorption, show that both Co 2+ and Co 3+ species are present on the catalyst surface. Co 3+ species are located inside the zeolitic channels while Co 2+ ions are distributed both at the external and at the internal surfaces. The operando study show the activity of Co 3+ species in the reaction. The existence of three parallel reactions, CH 4-SCR, CH 4 total oxidation and NO to NO 2 oxidation, has been confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH 4-SCR and NO oxidation, respectively. A mechanism for CH 4-SCR has been proposed. Co 2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seem not to play a key role in the SCR process. 相似文献
2.
A combined spectroscopic and catalytic study of the NO reactivity on microporous aluminophosphates, with chabasite-related structure, CoAPO-34, CuAPO-34 and CuAPSO-34, is reported. NO and CO adsorption were monitored by FTIR spectroscopy, and revealed that Co 2+/Co 3+ and Cu +/Cu 2+ redox couples, the sites responsible for the catalytic activity, are present in these catalysts. CoAPO-34 catalysts showed exceptionally high performances in the oxidation of NO to NO 2, and poor activity in other DeNO x reactions. Copper-based aluminophosphates and silico-aluminophosphates, besides good performances in the NO oxidation to NO 2, showed good activity in the N 2O decomposition even in the presence of oxygen or water in the feed. The presence of silicon has beneficial effects both on the thermal and hydrothermal stability of the zeolitic structure, as well as on the catalytic performances of the metal-aluminophosphates. 相似文献
3.
The interaction of NO with Co 2+/Co 3+ redox sites in CoAPO-18 and CoAPO-5 catalysts was studied by means of FTIR and diffuse reflectance UV–Vis spectroscopy both at 298 and 85 K. Two families of Co 2+ sites were found in the CoAPO-18 structure. (A) Ions in framework [Co 2+(OH)P], associated with Brønsted acid sites which adsorb NO to produce dinitrosyls absorbing at 1903 and 1834 cm −1; these dinitrosyl complexes are reactive, in that Co 2+ is oxidized to Co 3+ and N 2O is formed. (B) S tructural defects Co 2+ (Lewis acid sites) which stabilize dinitrosyls absorbing at 1900 and 1813 cm −1. The NO adsorption both on reduced and, more significantly, on oxidised CoAPO-18 also leads to the formation of NO 2δ+ adsorbed species. It was found that the two kinds of dinitrosyl complexes have different reactivity in presence of oxygen. Both families of sites are also present in CoAPO-5 catalysts on which, however, the redox reaction upon NO adsorption does not occur significantly. 相似文献
4.
The zeolites with MEL structure were synthesized via the hydrothermal method and the zeolites-supported catalysts, such as Cu 2+, Ga 3+, Co 3+, Ce 2+ and VO 2+/zeolites, were prepared by the incipient wetness impregnation. The structures of the synthesized zeolites were characterized by techniques of XRD, FT-IR, SIMS, 29Si and 27Al MAS NMR. The selective catalytic reduction (SCR) of NO by ammonia was carried out with a glass reactor under a downstream flow. The synthesized TS-2 showed no significant DeNO x activity, instead of catalyzing the ammonia oxidation at a high temperature. Furthermore, the catalytic activity of TS2 zeolite can be effectively modified and tuned up through incorporating second metal ion such as Fe 3+, Co 3+, and Al 3+ into the framework (i.e., [Fe,Ti]Z11, [Co,Ti]Z11, and [Al,Ti]Z11). Among the synthesized bimetallosilicates, the [Fe,Ti]Z11 zeolite is the most active catalyst for the SCR DeNO x with ammonia; the NO conversion and the N 2 yield reach around 80%. In addition, impregnating the metal ions on TS2 or bimetallosilicates is also a very effective way to improve the SCR DeNO x activity. Ga 3+/[Fe40,Ti40]Z11 and Co 3+/[Fe40,Ti40]Z11 are the most active catalysts and show a potential for the practical applications. 相似文献
5.
The NO, NO/O 2, and NO/O 2/H 2O adsorption on MnO 2/NaY (5 and 15 wt.% MnO 2) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O 2 on NaY and MnO 2/NaY proceeds via oxidation of NO forming NO 2− and NO 3− species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO 2. Dimerisation of NO 2 to N 2O 4 and following disproportionation of the latter causes the formation of NO + and NO 3− species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn 2+ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O 2 adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO 2 dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn 4+ is reduced to Mn 2+ as evidenced by in situ EPR measurements. 相似文献
6.
Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl 2 vapor is deposited. TPR shows peaks for Co 2+ ions at 695–705°C and for Co 3O 4 at 385–390°C, but a peak in the 220–250°C region appears to indicate Co 2+ oxo-ions. The catalysts have been tested for the selective reduction of NOx with iso-C4H10 under O2-rich conditions and in the absence of O2, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co3O4 clusters first oxidize NO to NO2, which is chemisorbed as NOy (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N2 yield 90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co2+ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O2-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co2+ ions without intermediate formation of NO2. The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO2 formation. 相似文献
7.
Nanometer perovskite-type oxides La 1−xSr xMO 3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m 2 g −1, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn 4+ and Mn 3+ in La 1−xSr xMnO 3−δ and Co 3+ and Co 2+ in La 1−xSr xCoO 3−δ, Sr substitution induced the rises in Mn 4+ and Co 3+ concentrations; adsorbed oxygen species (O −, O 2−, or O 22−) were detected on the catalyst surfaces. The O 2-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H 2-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h −1, the catalytic activity (as reflected by the temperature ( T100%) for EA complete conversion) increased in the order of LaCoO 2.91 ( T100% = 230 °C) ≈ LaMnO 3.12 ( T100% = 235 °C) < La 0.6Sr 0.4MnO 3.02 ( T100% = 190 °C) < La 0.6Sr 0.4CoO 2.78 ( T100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn 4+/Mn 3+ and Co 3+/Co 2+ ratios), and rich lattice defects of the nanostructured La 1−xSr xMO 3−δ materials. 相似文献
8.
Mixed oxides of the general formula La 0.5Sr xCe yFeO z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3−x and oxides -Fe 2O 3 and CeO 2 depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4+O 2 (“lean-NO x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5Sr xCe yFeO z materials to reduce NO by CO or by CH 4 under “lean-NO x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s −1) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al 2O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5Sr xCe yFeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5Sr 0.2Ce 0.3FeO z and La 0.5Sr 0.5FeO z solids. 相似文献
9.
The influence of NO on the adsorption and desorption of NO 2 on BaO/TiO 2 has been studied under lean conditions. The adsorption of NO 2 involves the disproportionation of NO 2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio, Three different nitrate species form on the catalyst: surface nitrates on the TiO 2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD). The reverse reaction of the NO2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO2. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NOx storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO2 as an intermediate product. This NO2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period. 相似文献
10.
Pd n+/Ce n+/Na +/γ-Al 2O 3-type materials used as FCC additives for CO/NO x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H 2 at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO x species on the catalyst surface. However, the Na + species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm −1. Finally, hydroxyls from the support are also actively participating in the formation of HNO x type compounds with characteristic stretching vibrations in the 3500–3572 cm −1 region. 相似文献
11.
Fifteen catalysts were prepared by intermittently impregnating alumina washcoats with water solutions containing La 3+, Co 2+ and PdCl 2−4 ions/complex and calcining them at 550–820°C. The catalysts were evaluated with respect to light-off performance, at stationary and transient feed gas stoichiometry, respectively, and redox characteristics, using NO/CO/C 3H 6/O 2/N 2 gas mixtures to simulate car exhaust. Alumina supported Pd exhibited three-way activity, i.e., simultaneous oxidation of CO and C 3H 6 and reduction of NO in a narrow interval around stoichiometric composition of the feed gas. Compared to Pd alone, addition of La or Co caused a widening of the interval under net reducing conditions. Addition of Co to Pd caused a significant increase in the activities for oxidation of CO and C 3H 6 under stoichiometric conditions. The conversions of CO and C 3H 6 started at about 100 degrees lower temperatures over Co-promoted Pd compared to unpromoted Pd. A marked increase in the activity for the reduction of NO at transient conditions was observed over Co-promoted Pd compared to unpromoted Pd. The catalysts were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy combined with energy-dispersive spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements. Only Co 2+ could be detected by XPS in the surface layers of the Co-containing sample. A significant part of the cobalt is present in forms which can be oxidized and reduced under synthetic car exhaust conditions. These oxidizable/reducible cobalt sites are predominantly active for oxidation of CO and C 3H 6, hence promoting the reduction of NO over Pd by initiating these exothermic reactions in the catalyst. 相似文献
12.
Molecular DFT modeling combined with computational spectroscopy (EPR and IR) were applied for analysis of the NO bond breaking and NN and OO bond making in the context of deNO x and deN 2O reactions. Interaction of NO, N 2O and NO 2 with cationic (transition metals) and anionic (surface O 2− ions) centers was explored at the molecular level. The elementary events such as reactant coordination, charge and spin redistributions, which are principal molecular constraints for efficient decomposition of the nitrogen oxides (N 2O and NO) were discussed. Particular attention was paid to dynamics of the NO bond cleavage in N 2O molecule through electron and oxygen atom transfer routes, evaluation of preferable coordination modes of NO, discrimination between inner- and outer-sphere mechanism of NN bond formation, and the influence of spin and electronic redistribution on the reaction course (spin catalysis). Owing to their simplicity and well known surface chemistry, model systems selected for studies of such processes include MoO x/SiO 2, MgO and ZSM-5 zeolite exchanged with various transition metal ions (TMI) of different electron configuration and spin multiplicity: Mo 5+ (d 1, 2D) Fe 3+, Mn 2+, Cr + (d 5, 6S), Fe 2+ (d 6, 5D), Co 2+ (d 7, 4F), Ni 2+ (d 8, 3F), Cu 2+ (d 9, 2D) and Cu +, Zn 2+ (d 10, 1S). 相似文献
13.
The effects of Zr doping on the existence of Cu and the catalytic performance of Ce 0.7−xZr xCu 0.3O 2 for CO oxidation were investigated. The characterization results showed that all samples have a cubic structure, and a small amount of Zr doping facilitates Cu 2+ ions entering the CeO 2 lattice, but excessive Zr doping leads to the formation of surface CuO crystals again. Thus, the number of oxygen vacancies caused by the Cu 2+ entering the lattice (e.g., Cu 2+–□–Ce 4+; □: oxygen vacancy), and the amount of reducible copper species caused by CuO crystals, varies with the Zr doping. Catalytic CO oxidation tests indicated that the oxygen vacancy and the reducible copper species were the adsorption and activation sites of O 2 and CO, respectively, and the cooperative effects between them accounted for the high CO oxidation activity. Thus, the samples x = 0.1 and 0.3, which possessed the most oxygen vacancy or reducible copper species, showed the best activity for CO oxidation, with full CO conversion obtained at 110 °C. The catalyst is also stable and has good resistance to water during the reaction. 相似文献
14.
The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al 2O 3 on the selective catalytic reduction (SCR) of NO x with CH 4 under dry and wet reaction stream. After being reduced in H 2 at 350°C, the PtCo bimetallic zeolites showed higher NO to N 2 conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH 4 = 1000 ppm, O 2 = 2%), the NO conversion dropped to zero over Co 2.0Mor at 500°C and GHSV = 30,000 h −1, whereas it is 20% in Pt 0.5Co 2.0Mor. In Pt/Co/Al 2O 3 the NO x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10 −4 s −1 for Pt 0.5Co 2.0Fer, 13 × 10 −4 s −1 for Pt 0.5Co 2.0Mor, 4.33 × 10 −4 s −1 for Pt 0.5Co 2.0ZSM-5 and 0.5 × 10 −4 s −1 for Pt/Co/Al 2O 3. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co 2+ and Pt 2+ at exchange positions. Co° seems not to participate as an active site in the SCR of NO x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO 2 reaction, the activation of CH 4 and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites. 相似文献
15.
FeO x/ZrO 2 samples, prepared by impregnation with Fe(NO 3) 3, were characterised by means of DRS, XRD, FTIR, redox cycles and volumetric CO adsorption. Volumetric CO adsorption, combined with FTIR, showed that 45% of iron in the sample containing 2.8 Fe atoms nm −2 was capable of forming iron carbonyls. DRS evidenced Fe 2O 3 on samples with Fe-content≥2.8 atoms nm −2. The selective catalytic reduction of NO with C 3H 6 in the presence of O 2 was studied with a reactant mixture containing NO=4000 ppm, C 3H 6=4000 ppm, O 2=2%. The dependence on iron-content suggests that only isolated iron, prevailing in dilute FeO x/ZrO 2, is active for NO reduction, whereas iron on the surface of small oxide particles, prevailing in concentrated FeO x/ZrO 2, is active for C 3H 6 combustion. 相似文献
16.
为了制备低成本、高色度的钴蓝颜料,本文以高岭土为载体,以Al 2O 3和Co 3O 4为主要原料,通过引入ZnO、CaO及MgO不同金属氧化物,采用固相法制备了高色度(Co xM 1-xAl 2O 4)/高岭土复合颜料(M为Ca 2+、Mg 2+或Zn 2+)。系统考察了研磨时间、煅烧温度、煅烧时间和不同金属氧化物掺量对复合颜料呈色性能的影响规律。研究表明,在煅烧温度1 200 ℃、研磨时间12 h和 n(Co 2+)/ n(M 2+)为3:2时,制得的复合颜料具有最好的呈色性能( L*=53.68,a*=7.58,b*=-62.89)。同时,引入不同的金属元素,可实现对复合颜料颜色的调控,引入Ca 2+后,所制备的Co xCa 1-xAl 2O 4复合颜料偏红相,而引入Zn 2+后,所制备的Co xZn 1-xAl 2O 4复合颜料偏绿相。通过相关表征,提出了复合颜料的呈色机理,在颜料制备过程中,引入与Co 2+离子半径接近的Mg 2+或Zn 2+,Mg 2+或Zn 2+可进入CoAl 2O 4的四面体配位中,部分替代Co 2+,形成MgAl 2O 4-CoAl 2O 4或ZnAl 2O 4-CoAl 2O 4的固溶体,而引入离子半径较大的Ca 2+,形成CaAl 2O 4和CoAl 2O 4的均相混合物。最后,将制得复合颜料应用到有机硅耐热涂料中,可以明显提高有机硅涂料的热稳定性。 相似文献
17.
The pathway for selective reduction of NO x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH 4 oxidation, NO 2 reduction) with that of the combined reaction (NO + O 2 + CH 4). Co (+2) was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co (+2) ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co (+2) ions in H-MOR and Na-MOR, but the rate of Co (+2) ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O 2 is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO 2 reduction to N 2 is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O 2 + CH 4 indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO 2. For these catalysts, the rate (and selectivity) for formation of N 2 is identical from NO + O 2 + CH 4 and NO 2 + CH 4. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N 2 formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N 2 formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N 2. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N 2 is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N 2. 相似文献
18.
The cognition of active sites in the Ni-based catalysts plays a vital role and remains a huge challenge in improving catalytic performance of low temperature CO 2 dry reforming of methane (LTDRM). In this work, typical catalysts of SiO 2 and γ-Al 2O 3 supported Ni and Ni-Ce were designed and prepared. Importantly, the difference in the chemical speciations of active sites on the Ni-based catalysts is revealed by advanced characterizations and further estimates respective catalytic performance for LTDRM. Results show that larger[Ni n0] particles mixed with [Ni-O-Si n]) species on the Ni/SiO 2(R) make CH 4 excessive decomposition, leading to poor activity and stability. Once the Ce species is doped, however, superior activity (59.0% CH 4 and 59.8% CO 2 conversions), stability and high H 2/CO ratio (0.96) at 600?℃ can be achieved on the Ni-Ce/SiO 2(R), in comparison with other catalysts and even reported studies. The improved performance can be ascribed to the formation of integral ([Ni n0]-[Ce III-□-Ce III]) species on the Ni-Ce/SiO 2(R) catalyst, containing highly dispersed [Ni n0] particles and rich oxygen vacancies, which can synergistically establish a new stable balance between gasification of carbon species and CO 2 dissociation. With respect to Ni-Ce/ γ-Al 2O 3(R), the Ni and Ce precursors are easily captured by extra-framework Al n-OH groups and further form stable isolated ([Ni n0]-[Ni-O-Al n]) and [Ce III-O-Al n] species. In such a case, both of them preferentially accelerate CO 2 adsorption and dissociation, causing more carbon deposition due to the disproportionation of superfluous CO product. This deep distinguishment of chemical speciations of active sites can guide us to further develop new efficient Ni-based catalysts for LTDRM in the future. 相似文献
19.
Infrared and kinetic studies were conducted for NiO-Al 2O 3 aerogel catalysts to determine the types and relative numbers of Ni sites responsible for catalytic activity and selectivity in the nitroxidation of propene to 2-propenenitrile. The evaluated catalysts had Ni to Al ratios of 0.04/1.0, 0.4/1.0 and 1.0/1.0. The effect of catalyst preparation on active Ni sites was determined by studying pairs of catalysts with the same Ni/Al ratio. The infrared studies were carried out using CO and NO as molecular probes, and by varying the temperature of pretreatment ( calcination and reduction). Bands for adsorbed CO were seen for all catalysts corresponding to weakly held CO on Ni 2+ and Ni1+ sites. Both linear and bridged adsorption of CO on reduced Ni O sites were also evident on catalysts reduced in H 2 at 550°C. All catalysts showed bands arising from NO strongly held on Ni sites. The CO and NO bands varied in intensity depending on the temperature of pretreatment. Catalytic activity depends on both the number of exposed Ni sites and a catalyst's ability to be easily oxidized and reduced without loss of sites. Ni crystal size was found to affect selectivity as well as activity 相似文献
20.
The adsorption of CO and its reaction with NO in the 400–600 °C temperature range on Ce n+/Na +/γ-Al 2O 3 and Pd n+/Ce n+/Na +/γ-Al 2O 3 type materials used commercially as FCC additives were monitored by FTIR spectroscopy. Exposure of both types of samples to CO leads to the formation of carboxylates and carbonates. The concentration of these species was higher in samples containing Pd, indicating that palladium catalyzes their formation. The Pd n+ cations initially present in these samples undergo partial reduction to form metallic Pd in the presence of CO even at room temperature. More complete reduction of Pd, along with some aggregation, was observed after exposure to CO at elevated temperatures. Exposure of both types of samples to NO/CO mixtures in the 400–600 °C temperature range leads to the formation of surface isocyanate species. Both Na + and Ce n+ promote the formation of such NCO species. However, surface isocyanate species were formed with substantially higher rates in the presence of palladium. The formation of the isocyanate species strongly correlates with changes observed in the νOH region, indicating that hydroxyls actively participate in the surface chemistry involved and are capable of protonating the NCO species. The isocyanates are also reactive towards O 2 and NO yielding CO 2 and N 2. These results suggest that isocyanates are possibly involved as intermediates in the CO–NO reaction over the materials examined. 相似文献
|