626 research outputs found
Enhancing of catalytic properties of vanadia via surface doping with phosphorus using atomic layer deposition
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Vac. Sci. Technol. A 34, 01A135 (2016) and may be found at https://doi.org/10.1116/1.4936390.Atomic layer deposition is mainly used to deposit thin films on flat substrates. Here, the authors deposit a submonolayer of phosphorus on V2O5 in the form of catalyst powder. The goal is to prepare a model catalyst related to the vanadyl pyrophosphate catalyst (VO)2P2O7 industrially used for the oxidation of n-butane to maleic anhydride. The oxidation state of vanadium in vanadyl pyrophosphate is 4+. In literature, it was shown that the surface of vanadyl pyrophosphate contains V5+ and is enriched in phosphorus under reaction conditions. On account of this, V2O5 with the oxidation state of 5+ for vanadium partially covered with phosphorus can be regarded as a suitable model catalyst. The catalytic performance of the model catalyst prepared via atomic layer deposition was measured and compared to the performance of catalysts prepared via incipient wetness impregnation and the original V2O5 substrate. It could be clearly shown that the dedicated deposition of phosphorus by atomic layer deposition enhances the catalytic performance of V2O5 by suppression of total oxidation reactions, thereby increasing the selectivity to maleic anhydride.DFG, 53182490, EXC 314: Unifying Concepts in Catalysi
A new model to describe the physics of VOPO
In the past different models for the magnetic salt vanadyl pyrophosphate
(VOPO) were discussed. Neither a spin ladder nor an alternating chain are
capable to describe recently measured magnetic excitations. In this paper we
propose a 2D model that fits better to experimental observations.Comment: 4 pages, 6 figures include
Selective oxidation of n-butane to maleic anhydride under oxygen-deficient conditions over V-P-O mixed oxides
The selective oxidation of n-butane to maleic anhydride over V-P-O mixed oxides was studied under oxygen deficient conditions. The mixed oxides were prepared with P/V atomic ratios ranging from 0.7 to 1.0. Catalysts with P/V <1.0 did not show any selectivity to maleic anhydride formation, regardless of whether or not (VO)2P2O7 was present. For catalysts with P/V = 1.0, containing (VO)2P2O7 and/or the so-called. Ī²-phase, the selectivity was strongly influenced by the actual surface V5+/V4+ ratio. This ratio is determined by the temperature, the crystal phases present in the catalyst and the composition of the gas mixture. Optimal selectivity was obtained at 425Ā°C with 15% butane in air and a butane/oxygen ratio of 0.9
Inelastic Neutron Scattering from the Spin Ladder Compound (VO)2P2O7
We present results from an inelastic neutron scattering experiment on the
candidate Heisenberg spin ladder vanadyl pyrophosphate, (VO)2P2O7. We find
evidence for a spin-wave excitation gap of meV, at a
band minimum near . This is consistent with expectations for
triplet spin waves in (VO)2P2O7 in the spin-ladder model, and is to our
knowledge the first confirmation in nature of a Heisenberg antiferromagnetic
spin ladder.Comment: 11 pages and 2 figures (available as hard copy or postscript files
from the authors, send request to [email protected] or
[email protected]), TEX using jnl, reforder and eqnorder, ORNL-CCIP-94-05
/ RAL-94-04
1-Butanol dehydration and oxidation over vanadium phosphate catalysts
The transformation of 1-butanol into either butenes or maleic anhydride was carried out both with and without oxygen, using V/P/O catalysts. With vanadyl pyrophosphate prepared by coprecipitation, at temperature lower than 240 ā¦C and without oxygen, selectivity to butenes was higher than 90%, but a slow deactivation took place.
At temperature higher than 300 ā¦C and in the presence of air, maleic and phthalic anhydrides were the prevailing products, with selectivity of 60% and 14%, respectively. Catalytic performance was affected by crystallinity and acidity. Ī±I-VOPO4 showed a poor performance in the absence of air, with a quick deactivation due to coke
accumulation; but it displayed an excellent selectivity to butenes (close to 98%) at temperatures lower than 320 ā¦C in the presence of air, with stable performance. At temperature higher than 360 ā¦C, Ī± I-VOPO4 was reduced to vanadyl pyrophosphate and catalyzed the direct oxidation of 1-butanol into maleic anhydride, but with 35% selectivit
The Effects of Dopants on The Physico-Chemical Properties of Vanadyl Pyrophosphate Catalysts
Vanadium phosphorus oxide (VPO) is a commercial catalyst for selective oxidation
of butane to maleic anhydride. The nature of the oxidant of the doped and undoped
(VOhP207 catalysts derived from (i) VPO, reaction of V20s with H3P04 in isobutanol
and (ii) VPD, reaction of VOP04Ā·2H20 with isobutanol were investigated. Metal
cations, namely of sodium, potassium, magnesium and bismuth, were added as dopants into (VO)2P2O7 lattice.
From the Scanning Electron Microscopy (SEM) and Transmission Electron
Microscopy (TEM) analysis, the catalysts which have gone through a series of redox
reaction do not produce the original morphology of (VO)2P207.
The results indicated that the addition of dopants lowered the Brunauer-Emmet-Teller
(BET) surface area of vanadyl pyrophosphate catalysts, except Bi-doped VPD
catalysts. All the doped VPO and VPD catalysts have been shown a vanadyl
pyrophosphate phase with three distinct peaks at 22.9Ā°, 28.4Ā° and 29.3Ā° in X-ray
Diffraction (XRD) analysis. The Bi-doped VPO and VPD catalysts had significantly shifted the first reduction peak to a lower temperature in H2-TPR analysis. The
introduction of Bi have modified the (VOhP207 matrix and mobility of these catalyst
increased. It can be seen from TPRn analysis where VPDBil increased the selectivity
of butene and butadiene (selective products) and decreased the selectivity of CO and
C02 (unselective products)
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory
We have studied the magnetic structure of the high
symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = ā156.8 K and J_O = ā68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging āOāPāOā unit. In contrast, J_O results from a direct overlap of 3d_(x^2 ā y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in VāO bond length as a function of strain along a, we found that the VāO bonds of Vā(OPO)_(2)āV are covalent and rigid, whereas the bonds of Vā(O)_(2)āV are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2ā300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride
Intercalation of Co-complex into the layered structure of VOPO4.2H2O for the preparation of vanadyl pyrophosphate, (VO)2P2O7 catalyst
Layered vanadyl phosphate dihydrate, VOPO4Ā·2H2O is one of the precursor to vanadyl pyrophosphate (VPO) catalyst which is the sole catalyst used industrially for the partial
oxidation of n-butane to maleic anhydride. With a basal spacing of 0.74 nm, layered VOPO4Ā·2H2O was used as the host and Co-complex (Co(acac)2) as a guest The obtained
precursor, VOHPO4Ā·0.5H2O was confirmed by XRD and were activated in a reaction flow of n-butane/air mixture (0.75% n-butane/air) to form vanadyl pyrophosphate catalyst ((VO)2P2O7) at 460oC for 18 h. Both catalysts were characterised by using several methods i.e. X-ray Diffraction (XRD), Braunner Emmer Teller (BET) surface area and Temperature Programmed Reduction (TPR), Redox titration and Scanning
Electron Microscopy (SEM). Co-complex was succesfully intercalated into the layer and as proven by XRD with a presence of a new peak appeared at 2Īø = 6.8Āŗ and another new
peak was also observed at 2Īø = 13.5 Āŗ. TPR studies of Co intercalated VPO shows a sharp peak come with larger area (compared to unintercalated catalyst) which correspond
to the removal of oxygen species associated to V4+ phase. Another peak at lower temperature which corresponds to the oxygen species released from V5+ phase. An improved of n-butane conversion is expected due to the increment of the active oxygen species (O-) which responsible to the activation of n-butane. Higher amount of oxygen linked to V5+ also will contribute to the activity of the Co-intercalated catalyst
The susceptibility and excitation spectrum of (VO)PO in ladder and dimer chain models
We present numerical results for the magnetic susceptibility of a Heisenberg
antiferromagnetic spin ladder, as a function of temperature and the spin-spin
interaction strengths and . These are contrasted with new
bulk limit results for the dimer chain. A fit to the experimental
susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate,
(VO)PO, gives the parameters meV and meV. With these values we predict a singlet-triplet energy gap of
meV, and give a numerical estimate of the ladder triplet
dispersion relation . In contrast, a fit to the dimer chain model
leads to meV and meV, which predicts a gap of meV.Comment: 16 pages, 6 figures available upon request, RevTex 3.0, preprint
ORNL-CCIP-94-04 / RAL-94-02
- ā¦