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1.
Alumina or silica supported chromium oxide catalysts prepared by sol-gel technique have been investigated by X-ray diffraction, BET analysis, combined diffuse reflectance spectroscopy, EPR and reduction-extraction by ethane 1,2 diol. The results reveal the presence of Cr 3+, Cr 5+ and Cr 6+ ions. Chromium species Cr 6+ are isolated and atomically dispersed on the alumina support as free chromate CrO
4
2–
while on silica, dichromate or polychromate species like Cr 2O
7
2–
or Cr 3O
10
2–
predominate. Chromium oxide exhibits a better catalytic properties for paraxylene partial oxidation, when it is supported on alumina. The difference of oxidation states and degree of oligomerisation of chromium on the two supports affects the catalytic properties. 相似文献
2.
采用XRD、UV-vis DRS、ESR和微分吸附量热等技术,考察了铬担载量分别为2.5、5和10wt%的CrO x/SiO 2催化剂的结构、表面性质和氧化还原性能。结果表明,催化剂表面上存在多种Cr的氧化态和聚集形式。随着Cr担载量从2.5wt%到10wt%的逐渐增大,催化剂表面占主导地位的Cr物种由CrO 3单体转为多聚CrO 3和Cr 2O 3晶相。在CO 2气氛中催化剂对丙烷转化率和丙烯选择性的大小顺序为2.5wt%CrO x/SiO 2>5wt%CrO x/SiO 2>10wt%CrO x/SiO 2,反应过程中的原位ESR和UV-visDRS测定结果表明,催化剂表面的反应活性中心为Cr 5+,Cr 5+可由催化剂预处理过程中Cr 3+的氧化及丙烷反应过程中CrO 3单体的还原产生,在反应中CO 2可使Cr 3+重新氧化为Cr 5+. 相似文献
3.
A series of SBA‐15‐supported chromia‐ceria catalysts with 3% Cr and 1%–5% Ce (3Cr‐Ce/SBA) were prepared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N 2 adsorption, SEM, TEM‐EDX, Raman spectroscopy, UV–vis spectroscopy, XPS and H 2‐TPR, and their catalytic performance for isobutane dehydrogenation with CO 2 was tested. The addition of ceria to SBA‐15‐supported chromia improves the dispersion of chromium species. 3Cr‐Ce/SBA catalysts are more active than SBA‐15‐supported chromia (3Cr/SBA), which is due to a higher concentration of Cr 6+ species present on the former catalysts. The 3Cr‐3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570 °C after 10 min of the reaction. 相似文献
4.
Nanostructured copper-chromium oxides were prepared by the sol–gel process (SG) and were characterised by elemental analysis,
thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and by their activity in methane
combustion. A comparative study was made with copper chromites commercial catalysts. The as-synthesised copper chromites sample
exhibited higher specific surface area (248 m 2 g −1) with respect to commercial solids (42 m 2 g −1). The surface quantitative analysis evidenced a Cr 6+ enrichment for the SG catalyst (Cr 6+/Cr 3+=0.56) with respect to commercial sample (0.39), while the ratio of copper species Cu 2+/(Cu° + Cu +) was the same in both solids. Catalytic activity of SG solids in methane combustion was found to be comparable to that of
Pt/Al 2O 3 and superior to that of commercial copper chromites tested under the same conditions. 相似文献
5.
Rod-shaped amorphous bulk Ni–Cr–Mo-22 at.%Ta-14 at.%Nb–P alloys resistant to concentrated hydrochloric acids were prepared
by copper-mold casting. Alloys of amorphous single phase and mixture of nanocrystalline phases in the amorphous matrix were
all spontaneously passive in 6 and 12 M HCl and were immune to corrosion in 6 M HCl, although the corrosion weight loss was
detected for heterogeneous alloys in 12 M HCl. Spontaneous passivation is due to presence of stable air-formed films in which
chromium was particularly concentrated in addition to enrichment of tantalum and niobium. The angle resolved X-ray photoelectron
spectroscopy revealed that chromium and molybdenum are rich in the inner part of the film. The major molybdenum species is
in the tetravalent state, although penta- and hexavalent state molybdenum is also included. The high corrosion resistance
was interpreted in terms of the high stability of the outer triple oxyhydroxide, Cr 1−x−yTa xNb yO z(OH) 3+2x+2y−2z, and the effective diffusion barrier of the inner Mo 4+ and Cr 3+ oxide layer.
Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical
Reactivity of Metastable Materials, Warsaw, 17th-21st September, 2007. 相似文献
6.
A series of Cr/Al 2O 3 and Co/Al 2O 3 catalysts were tested in the selective ammoxidation of ethylene to acetonitrile. Catalysts were prepared either by sol–gel
method or by impregnation with chromium or cobalt acetylacetonate salts. Physicochemical properties of catalysts were accomplished
by several techniques such as chemical analysis, physisorption of N 2, X-ray diffraction (XRD), 27Al MAS NMR, UV–Visible diffuse reflectance (DRS) and Raman spectroscopy and temperature programmed reduction of H 2 (H 2–TPR). Textural analysis reveals that mesoporous materials with pronounced surface areas were obtained using sol–gel procedure
while impregnation of the support produces a moderate decrease of its surface area and pore volume. XRD analysis confirms
the presence of highly dispersed metal species which reside essentially on the surface and measure less than 4 nm. Furthermore,
27Al MAS NMR shows that for xerogels, part of metal species occupies sites on/in A1 2O 3 in close vicinity of octahedral 27Al. This, apparently, is not the case for aerogels. For Cr/Al 2O 3 catalysts, isolated Cr 6+, mono and polychromate species were identified using DRS, Raman Spectroscopy and H 2–TPR which seem to play a key role in the ammoxidation of ethylene. Furthermore, for cobalt doped catalysts, CoAl 2O 4 was identified as active phase on the basis of DRS and H 2–TPR results. From the supercritical drying, it results generally better catalysts than catalysts calcined by ordinary procedure
which leads to inactive agglomerated Co 3O 4 and CoO–Al 2O 3 phase. 相似文献
7.
In this paper, p–n junction photocatalyst NiO/ZnO was prepared by the sol–gel method using Ni (NO 3) 2 and zinc acetate as the raw materials. The structural and optical properties of the p–n junction photocatalyst NiO/ZnO were
characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscopy (SEM),
Brunauer–Emmett–Teller (BET) analysis, UV–Vis diffuse reflection spectrum (DRS) and the fluorescence emission spectra. The
photocatalytic activity of the photocatalyst was evaluated by photocatalytic reduction of Cr 2O 7
2− and photocatalytic oxidation of methyl orange (MO). The results showed that the photocatalytic activity of the p–n junction
photocatalyst NiO/ZnO is much higher than that of ZnO on the photocatalytic reduction of Cr 2O 7
2−. However, the photocatalytic activity of the photocatalyst is much lower than that of ZnO on the photocatalytic oxidation
of methyl orange. Namely, the p–n junction photocatalyst NiO/ZnO has higher photocatalytic reduction activity, but lower photocatalytic
oxidation activity. The heat treatment condition also influences the photocatalytic activity strongly, and the best preparation
condition is about 400 °C for 2 h. Effect of the heat treatment condition on the photocatalytic activity of the photocatalyst
was also investigated. The mechanisms of influence on the photocatalytic activity were discussed by the p–n junction principle. 相似文献
8.
The states of chromium in both promoted and unpromoted alumina-chromium catalysts with various chromium contents prepared
with the use of a centrifugal thermal activation product of gibbsite were studied in detail. The presence of five chromium
species was found in the catalysts of this type: two Cr 6+ and three Cr 3+ species. The concentration of each particular chromium species depends on the total chromium content of the catalyst. Based
on the experimental data, we hypothesized that highly disperse Cr 3+ particles, the formation of which was completed at a chromium content of ∼7–9 wt %, are responsible for the catalytic activity
of alumina-chromium samples in the reaction of isobutane dehydrogenation. 相似文献
9.
The chromium(III) complex [Cr III(ddpd) 2] 3+ (molecular ruby; ddpd= N, N′-dimethyl- N, N′-dipyridine-2-yl-pyridine-2,6-diamine) is reduced to the genuine chromium(II) complex [Cr II(ddpd) 2] 2+ with d 4 electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T1/2 around room temperature. The low-spin and high-spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single-crystal XRD). Excitation of [Cr II(ddpd) 2] 2+ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well-established iron(II) SCO systems. 相似文献
10.
Novel heterogeneous catalysts were prepared using immobilization of bis(2‐decylsulfanylethyl)amine–CrCl 3 (Cr‐SNS‐D) on various supports, namely commercial TiO 2, Al 2O 3 and magnetic Fe 3O 4@SiO 2 nanoparticles, to yield solid catalysts denoted as support@Cr‐SNS‐D. The structure of the catalysts was confirmed on the basis of spectroscopic analyses, N 2 adsorption–desorption and inductively coupled plasma (ICP) analysis. The surface areas of Al 2O 3@Cr‐SNS‐D, Fe 3O 4@SiO 2@Cr‐SNS‐D and TiO 2@Cr‐SNS‐D catalysts were determined to be 70, 23 and 41 m 2 g ?1, respectively. A decrease in surface area from that of the supports clearly establishes accurate immobilization of Cr‐SNS‐D catalyst on the surface of the parent carriers. The loading of Cr was determined to be 0.02, 0.16 and 0.11 mmol g ?1 for Cr‐SNS‐D supported on TiO 2, Al 2O 3 and Fe 3O 4@SiO 2, respectively, using ICP analysis. After preparation and full characterization of the catalysts, ethylene trimerization reaction was accomplished in 40 ml of dry toluene, at 80°C and 25 bar ethylene pressure and in the presence of methylaluminoxane (Al/Cr = 700) within 30 min. The supported chromium catalysts were found to be efficient and selective for the ethylene trimerization reaction. The highest activity (74 650 g 1‐hexene g Cr?1 h ?1), as well as no polyethylene formation during reaction processes, was observed when TiO 2 was used as the catalyst support. 相似文献
11.
Catalytic properties of -Al 2O 3 -supported complex vanadium and chromium oxides V 2–x
Cr
x
O 5– (0 < × s 1.3), amorphous to X-rays, in the reduction of nitrogen oxides with ammonia were studied. Vanadium exists in these catalysts mostly in a pentavalent state and chromium exists as Cr 3+ and Cr 6+. As the content of chromium in the catalysts increases, the optimal temperature of the process decreases, and the degree of conversion of nitrogen oxides increases.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 339–342, February, 1996. 相似文献
12.
Full control on the valence of the active ions in solids to improve properties is the central topic of chemistry and materials. Cr 3+ and Cr 4+ ions generally emit wavelength-different near-infrared (NIR) light. Here, we have developed a chromium valence-controllable single-phase phosphor, Mg 2GeO 4:Cr 3+,Cr 4+ to achieve super-broad NIR luminescence. High Li + content charge compensators can stabilize Cr 3+, whereas high-temperature sintering tends to facilitate the formation of Cr 4+. Through fine adjusting the synthesis conditions, pure Cr 3+ or Cr 4+ luminescence can be obtained with peak emission locating at 935 nm and 1190 nm, respectively. Super broad band dual emission spanning from 650 nm to 1600 nm is realized via fully controlling the concentration ratio of Cr 3+ to Cr 4+ in a single host. By measuring the transmission spectra of several foodstuff illuminated by our phosphors, non-destructive analysis in food safety areas can be realized. This study provides a new strategy for exploiting super broad band NIR luminescent materials. 相似文献
13.
Three dendritic PNP ligands with ethylenediamine, 1,4‐butanediamine, 1,6‐diaminohexane as bridged groups are synthesized in good yields, respectively. Three dendritic PNP chromium complexes ( C1 – C3 ) are prepared with the ligands and chromium(III ) chloride tetrahydrofuran complex (CrCl 3(THF ) 3) as materials. The dendritic PNP ligands and the synthetic chromium complexes are fully characterized by spectroscopic and analytical methods. All chromium complexes activated with methylaluminoxane (MAO ) exhibited moderate activities on ethylene oligomerization (7.90 × 10 4 – 2.15 × 10 5 g (mol Cr h) −1] and had better selectivity for C 6 and C 8 oligomer, reaching up to 81%. The chromium complex ( C1 ) activated with diethylaluminium chloride (Et 2AlCl) has higher catalytic activity than the chromium complex C1 activated with MAO , although the chromium complex ( C1 ) activated with Et 2AlCl had lower selectivity for C 6 and C 8 oligomer. The effects of solvent and reaction parameters on ethylene oligomerization are also studied using the chromium complex C1 as pre ‐ catalyst and MAO as co ‐ catalyst. Under optimized conditions ([complex] = 2 μmol, Al/Cr = 500, 25 °C, 0.9 MP a ethylene, 30 min), the catalytic activity of complex C1 in toluene is 2.15 × 10 5 g (mol Cr h) −1 and the selectivity for C 6 and C 8 oligomer is 36.76%. In addition, the structure of complexes significantly affects both the catalytic activity and the selectivity on ethylene oligomerization. 相似文献
14.
End‐functionalized polyesters have been synthesized by ring‐opening polymerization (ROP) of ?‐caprolactone (CL) initiated with five different ruthenium derivatives in the presence of a series of alcohols as transfer agents. Mechanistic studies were performed for ROP of CL with RuCl 2(PPh 3) 3 ( I ), TpRuCl(PPh 3) 2 ( II ), and TpRuCl(PHPh 2)(PPh 3) ( III ) as catalysts in the presence or absence of benzyl alcohol (BzOH). Obtained molecular weights are proportional to CL/BzOH ratio, but there is not a direct relationship with CL/ruthenium complex ratios. 1H and 13C NMR spectroscopy revealed the existence of benzyl ester end‐groups. Catalysis involves (a) dissociation of ruthenium complexes, (b) coordination of the lactone CL, (c) coordination of the BzOH with the formation of a metal alkoxide, (d) transfer from the alkoxyl ligand to the coordinated lactone, and (e) ring‐opening of CL by oxygen‐acyl bond cleavage. The proposed mechanism is supported by 1H, 13C, and 31P NMR, gel permeation chromatography (GPC), and MALDI‐TOF analysis of the polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6926–6942, 2006 相似文献
15.
Nanocavity microreactor containing 15- and 16-membered diaza dioxa Schiff-base cobalt(II) complexes “[Co(Et[15]N 2O 2)] 2+, [Co(Pr[16]N 2O 2)] 2+, [Co(Ph[15]N 2O 2)] 2+ and [Co(Ch[15]N 2O 2)] 2+” have been prepared by the template synthesis of diamine (1,2-diaminoethane, 1,3-diaminopropane, 1,2-diaminobenzene or 1,2-diaminocyclohexane) with [(1,3-bis(2-carboxyaldehydephenoxy)propane)cobalt] 2+;[Co(BCAPP)] 2+@NaY within the pores of zeolite-Y. The nanosized cobalt(II) complex were entrapped in the supercage of Y-zeolite by a three-step process in the liquid phase: (i) exchange of Co(II) ions with NaY in water solution, (ii) reaction of Co(II)–NaY with excess BCAPP in methanol; [Co(BCAPP)] 2+@NaY; (iii) template synthesis of [Co(BCAPP)] 2+@NaY with diamine. The new nanosized complex entrapped in the nanocavity of zeolite Y was characterized by several techniques: chemical analysis and spectroscopic methods (FT-IR, UV–Vis, XRD, BET, DRS, XPS, TGA). 相似文献
16.
Three complexes of composition [Cr L(X) 3], where L = 4′-(2-pyridyl)-2,2′:6′,2″-terpyridine and X = Cl −, N 3−, NCS − are synthesized. They are characterized by IR, UV–Vis, fluorescence, EPR spectroscopic, and X-ray crystallographic studies. Structural studies reveal that the Cr(III) ion is coordinated by three N atoms of L in a meridional fashion. The three anions occupy the other three coordination sites completing the mer-N 3Cl 3 ( 1) and mer-N 3N 3 ( 2 and 3), distorted octahedral geometry. The Cr–N2 has a shorter length than the Cr–N1 and Cr–N3 distances and the order Cr–N(NCS −) < Cr–N(N 3−) < Cr–Cl is observed. They exhibit some of the d–d transitions in the visible and intra-ligand transitions in the UV regions. The lowest energy d–d transition follows the trend [Cr LCl 3] < [Cr L(N 3) 3] < [Cr L(NCS) 3] consistent with the spectrochemical series. In DMF, they exhibit fluorescence having π → π ∗ character. All the complexes show a rhombic splitting as well as zero-field splitting (zfs) in X-band EPR spectra at 77 K. 相似文献
17.
The oxidative dehydrogenation of ethane into ethylene by CO 2 over a series of silica-supported chromium oxide catalysts was investigated. The results showed that the catalysts were effective for the reaction and CO 2 in the feed promoted the catalytic activity. The 5%Cr/SiO 2 catalyst exhibited the excellent performance with 30.7% ethane conversion and 96.5% ethylene selectivity at 700 oC. ESR and UV-DRS were used to probe the active sites and the species with high valent states (Cr 5+ and/or Cr 6+) were found to be important for the reaction. 相似文献
18.
Supported copper catalysts on low surface area silica were prepared by several methods and characterized by AAS, XRD, N 2 adsorption, SEM, H 2-TPR, N 2O titration, TGA-DTA, UV–Vis techniques. Their hydrogenating properties were examined in the gas-phase hydrogenation of benzaldehyde. The analysis of characterization results revealed that the choice of preparation method affected the texture, composition, and structure of the calcined and reduced Cu/SiO 2 catalysts. The dispersion and size distribution of copper species was present in different forms in the catalysts that exhibited low specific surface areas. In gas-phase hydrogenation of benzaldehyde to benzyl alcohol, the catalysts tested at the reaction temperatures of 160 and 200 °C were stable and conducted to a good catalytic activity and benzyl alcohol selectivity ranging between 5 and 39 µmol min ?1 g ?1 and 0–95%, respectively. The activity of the catalysts in gas-phase hydrogenation also depended on the particle size and the nature of copper species formed on low surface area silica. 相似文献
19.
A series of catalysts based on Mn-Fe loaded zeolites was prepared by impregnation and their activity in the selective catalytic reduction of NO with ammonia (NH3-SCR) was investigated. The highest catalytic conversion was recorded for MnFe-ZSM-5 (MnFe-Z), followed by MnFe-BEA (MnFe-B) and MnFe-MOR (MnFe-M), while MnFe-FER (MnFe-F) showed a very poor activity over the entire temperature range. In order to evidence a correlation between the structure and acidity of the zeolites and NO conversion, the prepared samples were characterized by various techniques (ICP-AES, N2 physisorption at 77 K, XRD, 27NMR, Raman, FTIR spectroscopy of adsorbed ammonia, H2-TPR, DRS UV–Vis, EPR and XPS). The superior catalytic activity of MnFe-Z at low temperature is attributed to the abundance of Mn4+ concentration as revealed by XPS, the highest NH3-L/NH4+ ratio indicative of the contribution of metals in generating Lewis acidic centers as evidenced by IR-NH3, and the better reducibility of manganese and iron on ZSM-5 which increases the kinetics for red-ox cycles as confirmed in TPR analysis. Fe3Mn3O8 mixed oxide phase is also detected by XRD and XPS and can be associated with the high reducibility of MnFe-Z which generates a high oxidation ability favoring NO to NO2 oxidation. Raman spectroscopy was also used to confirm the existence of a strong synergy between metals and ZSM-5 support revealed by the shift in the signal position and the decrease in band intensities. The results showed that the zeolite framework and acidity generate catalysts with different textural and structural properties which influence the metal dispersion and speciation and hence influence the catalytic performances. 相似文献
20.
Characteristics of electroreduction of Cr 3+ and Cr 2+ ions on the dropping mercury (literature data) and indium cathodes are analyzed as a function of pH of the medium. A hypothesis that polymeric hydroxocompounds of chromium, which are present in different aggregate states, represent the main or partial reactants in the $ Cr\left( {III} \right)\xrightarrow{{ + e}}Cr\left( {II} \right)\xrightarrow{{ + 2e}}Cr Characteristics of electroreduction of Cr3+ and Cr2+ ions on the dropping mercury (literature data) and indium cathodes are analyzed as a function of pH of the medium. A hypothesis
that polymeric hydroxocompounds of chromium, which are present in different aggregate states, represent the main or partial
reactants in the
stages is additionally substantiated.
Original Russian Text ? V.N. Korshunov, V.A. Safonov, L.N. Vykhodtseva, 2008, published in Elektrokhimiya, 2008, Vol. 44,
No. 3, pp. 275–285. 相似文献
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