The behavior of a number of rare earth oxides as catalysts for the oxidation of graphite in air has been investigated by the methods of thermal analysis. Of the oxides studied, only CeO2 showed significant activity in accelerating the gasification of graphite by oxygen between 500 and 1000°C. Cerium salts, which decompose to a finely dispersed oxide phase at low temperatures, e.g. Ce (III) nitrate and ammonium Ce (IV) nitrate, were found to be very active catalysts. The catalytic effect may be due to a redox process involving the cyclic conversion of the oxide from the Ce (IV) to the Ce (III) oxidation state, or the oxide particles may provide sites for the dissociative chemisorption of oxygen. 相似文献
Mixed MVSb (M = Ni, Co, Bi, Sn) oxides supported by alumina were characterized and evaluated for oxidative dehydrogenations of light paraffins, ethylbenzene and ethanol. The most selective NiVSb catalyst was the most easily reduced that was interpreted on the assumption of reaction redox mechanism. The results of kinetic measurements revealed the concentration of gas-phase oxygen to be a key parameter affecting the dehydrogenation selectivity. To maintain it an optimum along the catalyst bed, a distributed oxygen feed was used. 相似文献
Cr/Sb mixed oxides with a rutile-type structure were synthesized by calcining (700°C) a mixture of oxohydrates obtained by coprecipitation from an alcohol solution containing the required amount of the components. The samples were characterized using X-ray powder diffraction (XRD), FT-IR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In the Cr/Sb/O compound prepared with Cr/Sb 1/0.8 (atomic ratio), an almost stoichiometric CrSbO4develops (no antimony or chromium oxides are found), which however is characterized by enrichment in Sb in outmost rutile atomic layers, and thus by a non-homogeneous intracrystalline distribution of the two components. For Cr/Sb ratios lower than 1, the amount of Sb in the rutile-type structure exceeds the stoichiometric value. The reactivity of Cr/Sb/O catalysts with increasing Sb contents (from Cr/Sb 1/0.8 to 1/2.8) is slightly affected by the Cr/Sb atomic ratio. In particular, the selectivity increases with increasing Sb content, while the catalytic activity is higher for the samples having higher Cr/Sb ratios. The Cr/Sb/O system presents considerable differences with respect to the V/Sb/O rutile system; these differences are discussed in reference to the properties of the transition metal components of the rutile mixed oxides. 相似文献
Interest in bifunctional catalysts, active in reactions such as hydrodesulphurisation (HDS) of hydrocarbon fractions, is growing in the last years. An improvement of CoMo/Al2O3 materials can be obtained by the introduction of other oxides during the sol–gel synthesis. This heavily affects the acid–base characteristics of the catalysts, while textural properties are less influenced. The catalytic performances change as well: a relationship between the density of acid sites and HDS activity has been found. 相似文献
Amorphous microporous homogeneously mixed oxides based on Ti or V in silica can be prepared by an acid catalyzed solgel process. The materials have been characterized by TEM, UV, IR and sorption methods. The glasses can be used for the selective epoxidation of alkenes. The catalytic properties are comparable to those of the well known crystalline Ti-containing zeolites TS-1, Ti-MCM-41 and Ti-Beta. Amorphous microporous homogeneously mixed metal oxides seem to be promising new catalytic materials supplementing the well studied selective zeolites. 相似文献
Mg/Al mixed oxides, obtained by decomposition of hydrotalcite-like precursors, represent interesting heterogeneous catalytic systems for basic-catalyzed reactions, as an alternative to environmentally unfriendly homogeneous catalysts. The reactivity of these oxides was evaluated using the methylation of m-cresol as a test reaction and relationships between catalytic performance and chemical–physical features were established. The basicity of the samples was evaluated by CO2 adsorption and thermal-programmed-desorption. The presence of Al in the mixed oxides considerably affected the density and the strength of the basic sites with respect to MgO. These basic properties in turn influenced the catalytic performance of the materials. Under the reaction conditions used in the present work, medium strength basic sites played the major role in the reaction. 相似文献
This paper describes the effect of the composition of rutile-type Sn/V/Nb/Sb mixed oxides catalysts on the catalytic performance in the gas-phase ammoxidation of propane to acrylonitrile. The variation in the atomic ratio between components in catalysts is the key for the control of activity and selectivity. In samples with atomic composition Sn/V/Nb/Sb 1/0.2/1/x (0 ≤ x ≤ 5) and 1/0.2/y/3 (0 ≤ y ≤ 3) several compounds formed, i.e., SnO2, Sb/Nb mixed oxide, Sb6O13 and non-stoichiometric rutile-type V/Nb/Sb/O; the latter segregated preferentially at the surface of the catalyst. Tin oxide provided the rutile matrix for the dispersion of the mixed oxides. The main role of Sb was shown to generate mixed oxides containing specific sites for the allylic ammoxidation of propylene intermediately formed. The presence of Nb enhanced the activity and selectivity of these sites. 相似文献
Heterobimetallic Lewis acid catalysts are broadly useful and methods to recycle them have immediate applications. However, their immobilization through covalent binding can be challenging. Non‐covalent immobilization of supported asymmetric catalysts is attractive due to ease of preparation and potential for reversible binding. We report a novel non‐covalent binding strategy for Shibasaki’s REMB framework {RE=rare earth metal; M=Li, Na, K; B=BINOL; RE:M:B=1:3:3, [M3(sol)n][(BINOLate)3RE] } and explore the reactivity of the supported catalyst.
High surface area CeO2–ZrO2 mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H2, TPO, magnetic susceptibility measurements to obtain the Ce3+ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides.
All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides. 相似文献
(MoVW)5O14-type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested
to play an important role as a structural promoter in the formation and stabilization of this oxide. Vanadium is responsible
for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for
optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation
could be considerably improved, when the amount of the (MoVW)5O14-type oxide was increased by thermal annealing. A model is proposed on the basis of the correlation between Raman wavenumber
and bond order and degree of reduction, which explains the observed different selectivities of MoO3−x and the (MoVW)5O14-type oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. According to
this model, the (MoVW)5O14 mixed oxide catalyses partial oxidation because of its intermediate C–H activation and oxygen releasing oxygen functionalities.
However, these (MoVW)5O14-type industrial oxidation catalysts are heterogeneous and highly complex systems. Their physicochemical characterization
also revealed that their chemical bulk and surface compositions vary with thermal activation and oxygen potential. A core-shell
model is suggested to describe the active catalyst state, the shell providing a high number of active centers, the core high
electronic conductivity and ion mobility. The fact that the surface composition of such catalysts is considerably different
from their bulk compositions, most probably implies that the “molecular structure” at their surface differs too considerably
from their bulk crystal structure. Hence, the posed question about the active catalyst structure and its relation to its catalytic
performance cannot unambiguously be explained by the crystallographic structure, but still remains unsolved. 相似文献
Highly selective reduction of nitrogen oxides to dinitrogen occurs to a high level in oxygen-rich atmospheres by using a small amount of propane as a reducing agent over alumina, silica-alumina, titania and zirconia catalyst. Judging from the data of activity and ammonia TPD measurement on a series of silica-alumina catalysts, acidity is suggested to be one of the main factors that determine catalytic activity. 相似文献
MnOx/C composites prepared by the reduction of KMnO4 by active carbon black and doped with Ca(II), Mg(II), Ni(II), Bi(III), and Cr(III) ions were tested as catalysts of oxygen reduction in alkaline electrolyte. The polarisation curves were analysed by logarithmic wave analysis. MnOx doped with the transition metal salts are slightly more active than those with Ca(II) and Mg(II). 相似文献
Catalysts, which were recently developed by Toyota for the control of automobile exhaust, are reviewed. (1) For use in low emission vehicles, a CeO2-ZrO2 solid solution (CZ) with both high oxygen storage capacity and high heat resistance was developed as a support for a high performance three-way catalyst (TWC). (2) A novel three-way catalyst named the NOx storage-reduction catalyst (NSR) was developed for automotive lean-burn engines. The NSR catalyst can store NOx in an oxidizing atmosphere and then reduce stored NOx at stoichiometric or reducing conditions. Also, it has high tolerance to sulfur poisoning which is the most stringent problem for the NSR catalyst. 相似文献
Mo–V–Nb–O mixed metal oxides, obtained by heat-treatment in N2 at 425 °C, have been studied as catalysts in the oxidative dehydrogenation of ethane. They present higher catalytic activity, while maintaining the same selectivity to ethylene, than the corresponding metal oxides calcined under air. Both amorphous and crystalline phases are present on active and selective catalysts. The implications of the presence of these phases as well as their physicochemical characteristics on the nature of active and selective sites are discussed. 相似文献
This paper describes the role of Sb and Nb, components of Sn/V/Nb/Sb mixed oxides catalysts for the gas-phase ammoxidation of propane to acrylonitrile. In samples without Nb and with atomic ratios Sn/V/Sb 1/0.2/x (x = 0 to 3), Sb in the form of amorphous oxide is necessary in order to obtain an active and selective catalyst. However, during reaction the dispersed Sb oxide segregates to α-Sb2O4, and the yield to acrylonitrile decreases considerably. The addition of Nb gives rise to the formation of Nb-containing SbOx and non-stoichiometric rutile-type V/Nb/Sb mixed oxides. The presence of these compounds enhances the catalytic activity and the selectivity to acrylonitrile. Moreover, the catalyst shows a stable catalytic performance, with no segregation of α-Sb2O4. 相似文献