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1.
Schubert U.-A. Anderle F. Spengler J. Zühlke J. Eberle H.-J. Grasselli R.K. Knözinger H. 《Topics in Catalysis》2001,15(2-4):195-200
Titania-supported vanadia catalysts were modified by addition of antimony oxide for application in o-xylene selective oxidation to phthalic anhydride. It was shown that active and selective catalysts can be prepared by ball-milling mixtures of powders of TiO2, V2O5and Sb2O3followed by calcination. X-ray photoelectron spectroscopy proves the formation of highly dispersed overlayers of vanadium oxide and antimony oxide, in which V5+is partially reduced to lower oxidation states and Sb3+is partially oxidized to Sb5+. Antimony oxide segregated into the outermost surface layers. It is therefore inferred that the presence of the antimony oxide modifier spatially separates V–O species and leads to site isolation which may be responsible for the positive effect of the modifier for the catalyst's selectivity. 相似文献
2.
Thin films can make a useful link between single crystal and supported vanadium oxide. The deposition of vanadium oxide thin films with physical vapour deposition techniques ensures clean and highly controllable synthesis. The resulting material is easily accessed with surface sensitive techniques. On flat TiO2 anatase substrates, XPS–XPD and UPS indicated that the vanadia deposition was epitaxial, and fully oxidised if performed in situ. A step closer to typical industrial catalysts was achieved by sputter deposition onto sub-millimetre inert particles. In addition to surface characterisation, these model particle catalysts allow use in reactors for catalytic testing under relevant process conditions. On both silica and titania supports, sputter deposited vanadia of varying thickness proved to be equally well dispersed. Oxidative dehydrogenation (ODH) activity was higher over vanadia/titania (anatase) than over vanadia/silica, demonstrating the synergetic interaction between anatase and vanadia. Highest activity in alkane ODH was observed for vanadia a few monolayers thick, supported on titania-coated particles. 相似文献
3.
The performance of V–Ca–O catalysts for the oxidative dehydrogenation of propane has been studied for the first time, being
compared with that of similar V–Mg–O catalysts, and their differences are interpreted in terms of their physico-chemical properties.
The VCaO catalyst showed the same initial selectivity to propene as the most selective VMgO composition, but it decreased
faster with increasing propane conversion. Vanadium species present in the catalyst surface were different for the two supports:
isolated V5+ tetrahedra on CaO and magnesium orthovanadate and pyrovanadate on MgO. The intrinsic activity of isolated vanadium centres
in the surface of the V–Ca–O catalyst was more than one order of magnitude higher than that of dimeric or polymeric tetahedral
species in the magnesium-containing catalysts.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH3 only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst: calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings. 相似文献
5.
The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated
that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination
and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated
with the fully oxidized surface (V5+) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm−1) and bridging V–O–V (∼900–940 cm−1) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface
V5+ LMCT band (>25,000 cm−1) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates
and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are
greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V5+) species to reduced phases (V3+/V4+) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift
in UV-Vis DRS edge energy did not exhibit any bands from reduced V3+/V4+ phases in the d–d transition region (10,000–30,000 cm−1), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V5+ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle,
although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces
remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy
intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region
(2800–3000 cm−1) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon
which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity
of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
Ch. Schild J. Engweiler J. Nickl A. Baiker M. Hund M. Kilo A. Wokaun 《Catalysis Letters》1994,25(3-4):179-189
A model vanadia/silica interface has been prepared by vapour deposition of vanadium-oxy-triisopropoxide onto the native oxide layer of a silicon wafer. The VO
x
/SiO2/Si model system has been used to investigate the morphological and chemical changes occurring with vanadia/silica catalysts when they are exposed to different gas atmospheres. Atomic force microscopy and X-ray photoelectron spectroscopy have been used to follow these changes. The studies show that exposure of the vanadia/silica interface to conditions prevailing during the selective reduction of NO by NH3 results in temperature dependent morphological changes, while the oxidation states of the vanadia species reflected by the ratio V(III)/V(V) change only little during the surface reconstruction. 相似文献
7.
Herman Richard G. Sale John W. Stenger Harvey G. Lyman Charles E. Agogliatti John E. Cai Yeping Ramachandran Bala Choi Sukwon 《Topics in Catalysis》2002,18(3-4):251-257
Titania/vanadia, zeolite, and noble metal catalysts are utilized for selective catalytic reduction (SCR) of NO
x
using ammonia as the reductant in different temperature ranges. Studies of aging have been carried out to probe deactivation rates and mechanisms. Periodic laboratory testing of samples of NO
x
reduction catalysts from multi-layer reactors, such as those utilized at electric power plants, allows prediction of catalyst life-times. Testing has been carried out under protocol conditions with monolith, plate-type, and pelleted catalysts so that relative NO reduction rates can be compared, with or without the presence of SO2. The catalysts were analyzed by surface analysis techniques, including electron microscopy and X-ray photoelectron spectroscopy, to probe surface morphology, loss of active components, presence of poisons, and blocking of pores and active sites by ammonium bisulfate to determine the dominant mode(s) of gradual deactivation. 相似文献
8.
Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH4 species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH3 species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH4 species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed. 相似文献
9.
Changes of the V2O5/Al3O3 catalyst aged for up to 10 years under real conditions of the selective catalytic reduction of NO
x
by ammonia (SCR) at the tail gases of the nitric acid plant were characterized by51V NMR spectroscopy, porosimetry, temperature programmed reduction (TPR) and catalytic activity measurements. The catalytic activity and the redox properties of the catalyst were found intact. Only small variations of the ratio of the octahedral and tetrahedral vanadia species were documented by51V NMR on aged catalyst. 相似文献
10.
The effect of magnesia loading and preparation procedure of vanadia on
titania catalysts on the physicochemical characteristics and the
performance in propane oxidative dehydrogenation were investigated. A
series of magnesia promoted vanadia catalyst (5 wt% V2O5) with varying amounts of MgO (1.9--10 wt%) were synthesized by
synchronous and sequential deposition on titania support. The catalysts
were characterized using several techniques (BET, XRD, H2-TPR and NH3)-TPD). Both MgO loading and preparation procedure affect the catalyst surface properties and the behavior in the oxidative
dehydrogenation of propane. Magnesia addition results in drastic
increase in propene selectivity, while the effect on activity is
negative. The activity is inversely related with the magnesia loading.
Deposition of V2O5 on previously prepared MgO/TiO2 presents a beneficial effect in the activity of the sample. The role of acidity and reducibility is explored. There is no correlation between
reducibility and activity of the catalysts, whereas the acidity seems to influence the catalytic performance. Catalyst containing 5 wt% V2O5 and 1.9 wt% MgO prepared by sequential deposition of
V2O5 on already doped with MgO titania exhibits the most interesting results. 相似文献