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. β-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 425°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 $E_{gap} = 3.7\pm 0.2$ meV, at a band minimum near $Q=0.8 A^{-1}$. 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 ◦C and without oxygen, selectivity to butenes was higher than 90%, but a slow deactivation took place. At temperature higher than 300 ◦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. αI-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 ◦C in the presence of air, with stable performance. At temperature higher than 360 ◦C, α 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 VOP04·2H20 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 22.9°, 28.4° and 29.3° 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) = −156.8 K and J_O = −68.6 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 –O–P–O– unit. In contrast, J_O results from a direct overlap of 3d_(x^2 − 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 V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)_(2)–V are covalent and rigid, whereas the bonds of V–(O)_(2)–V 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 2–300 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, VOPO4·2H2O 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 VOPO4·2H2O was used as the host and Co-complex (Co(acac)2) as a guest The obtained precursor, VOHPO4·0.5H2O 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 2θ = 6.8º and another new peak was also observed at 2θ = 13.5 º. 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)2_2P2_2O7_7 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 $J_\perp$ and $J_{||}$. 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)$_2$P$_2$O$_7$, gives the parameters $J_\perp = 7.82$ meV and $J_{||} = 7.76$ meV. With these values we predict a singlet-triplet energy gap of $E_{gap} = 3.9$ meV, and give a numerical estimate of the ladder triplet dispersion relation $\omega(k)$. In contrast, a fit to the dimer chain model leads to $J_1=11.11$ meV and $J_2=8.02$ meV, which predicts a gap of $E_{gap} = 4.9$ meV.Comment: 16 pages, 6 figures available upon request, RevTex 3.0, preprint ORNL-CCIP-94-04 / RAL-94-02