Soghomon Boghosian

Progress on the mechanistic understanding of SO2 oxidation catalysts

Abstract For almost a century vanadium oxide based catalysts have been the dominant materials in industrial processes for sulfuric acid production. A vast body of information leading to fundamental knowledge on the catalytic process was obtained by Academician [G.K. Boreskov, Catalysis in Sulphuric Acid Production, Goskhimizdat (in Russian), Moscow, 1954, p. 348]. In recent years these catalysts have also been used to clean flue gases and other SO − 2 containing industrial off-gases. In spite of the importance and long utilization of these industrial processes, the catalytic active species and the reaction mechanism have been virtually unknown until recent years. It is now recognized that the working catalyst is well described by the molten salt/gas system M 2 S 2 O 7 –MHSO 4 –V 2 O 5 /SO 2 –O 2 –SO 3 –H 2 O–CO 2 –N 2 (M=Na, K, Cs) at 400–600°C and that vanadium complexes play a key role in the catalytic reaction mechanism. A multiinstrumental investigation that combine the efforts of four groups from four different countries has been carried out on the model system as well as on working industrial catalysts. Detailed information has been obtained on the complex and on the redox chemistry of vanadium. Based on this, a deeper understanding of the reaction mechanism has been achieved.

Selective catalytic reduction of NO with NH3 over mesoporous V2O5-TiO2-SiO2 catalysts

Mixed vanadia-titania-silica catalysts (3 or 6 wt% V2O5, and 16-34 wt% TiO2) were one-pot prepared by sol-gel and hydrothermal methods in the presence of surfactants. Sodium silicate (25.5-28.5% silica) or tetraethylorthosilicate was used as a precursor for silica; tetraisopropylorthotitanate or titanyl acetylacetonate, for titania; and vanadyl sulfate or vanadium acetylacetonate, for vanadia. Cetyltrimethylammonium bromide, octadecyltrimethylammonium bromide, or dodecylamine was used as a surfactant. The catalysts were characterized by adsorption and desorption curves of N-2 at 77 K, NH3-DRIFTS, H-2-TPR, XRD, in situ Raman spectroscopy, XPS, and TEM. The catalysts were tested in NO reduction with ammonia using a total flow rate of 100 ml/min and a feed composition of nitric oxide 0.1 vol%, ammonia 0.1 vol%, oxygen 3 vol%, in helium. Vanadia was found to be entrapped in these catalysts as V(V) species in which the population of V=O monomeric bonds strongly depended on the dispersion. Titanium also existed in a very oxidated state, and for high dispersions it adopted a tetrahedral coordination. These structures led to surfaces on which mainly Lewis acid sites are effective under reaction conditions. Under such conditions, the dominant route followed an Eley-Rideal mechanism, yielding in such a way very high activity and selectivity. A comparison with a conventional V2O5-TiO2 catalyst led to the conclusion that the intrinsic activity of one-pot prepared polymeric sol-gel catalysts is higher

Formation of Crystalline Compounds and Deactivation During SO2 Oxidation in V2O5-M2S2O7 (M = Na, K, Cs) Melts

The formation of low-valence crystalline vanadium compounds was studied in the V2O5M2S2O7 (M = Na, K, Cs) unsupported melt systems in the temperature range 350–480 °C during SO2 oxidation with unconverted 10% SO2, 11% O2, and 79% N2 as the feed gas. A gas-molten-salt reactor system was built to provide the possibility of isolating the crystalline precipitates under operating conditions at any temperature by filtering the catalyst melts. Both V(IV) and V(III) crystalline compounds were formed under different process conditions. The V(IV) compounds K4(VO)3(SO4)5, Na2VO(SO4)2, and Cs2(VO)2(SO4)3 and the V(III) compounds KV(SO4)2 NaV(SO4)2, and CsV(SO4)2 were isolated from the melts. A drop in the catalytic activity was observed at temperatures where these compounds started to precipitate. For the first time it has been possible to observe the drop in catalytic activity and the formation of low-soluble vanadium compounds simultaneously. It was also found that (i) high alkali-to-vandium ratios, large alkali cation promoters, or mixing of the alkali promoters caused the precipitation and the steep activity drop to occur at lower temperatures, and (ii) the crystalline precipitates of V(IV) and V(III) could be redissolved by a heat treatment at/or above 470 °C or by purging the melts with N2. The thermal stability of the V(IV) compounds has been investigated by means of DTA. Furthermore the decomposition rate of KV(SO4)2 and K4(VO)3(SO4)5 during isothermal heating at different temperatures in the range 470–510 °C has been measured and IR spectra of the decomposition products have been recorded and interpreted. The results indicate that a heat treatment of melts containing large amounts of V(IV) and V(III) precipitates leads to reoxidation of vanadium to the +V oxidation state. Certain conditions required for reactivation of deactivated catalysts are pointed out and are discussed in relation to the dissolution of the precipitates in the melts.

First in situ Raman study of vanadium oxide based SO2 oxidation supported molten salt catalysts

In situ Raman spectroscopy at temperatures up to 500 °C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO2 oxidation. Vanadia/silica catalysts impregnated with Cs2SO4 were exposed to various SO2/O2/SO3 atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported “model” V2O5–Cs2SO4 and V2O5–Cs2S2O7 molten salts. The data indicate that (1) the VV complex VVO2(SO4)23− (with characteristic bands at 1034 cm−1 due to ν(V=O) and 940 cm−1 due to sulfate) and Cs2SO4 dominate the catalyst surface after calcination; (2) upon admission of SO3/O2 the excess sulfate is converted to pyrosulfate and the VV dimer (VVO)2O(SO4)44− (with characteristic bands at 1046 cm−1 due to ν(V=O), 830 cm−1 due to bridging S–O along S–O–V and 770 cm−1 due to V–O–V) is formed and (3) admission of SO2 causes reduction of VV to VIV (with the ν(V=O) shifting to 1024 cm−1) and to VIV precipitation below 420 °C.

Vanadium (V) complexes in molten salts of interest for the catalytic oxidation of sulphur dioxide

High temperature Raman spectroscopy is used for the first time for establishing the structural and vibrational properties of VV complexes in V2O5- Cs2S2O7 (0≤ XV2O50≤ 0.24) and V2O5- Cs2S2O7- Cs2SO4 (0≤ XV2O50≤ 0.25) molten salt mixtures at 450°C under static equilibrium conditions. Based on Raman band intensity correlations and band assignments it is found that the VV complex present in V2O5- Cs2S2O7 molten mixtures has a dimeric (VO)2O(SO4)44-configuration containing a V-O-V bridge. Addition of Cs2SO4 in V2O5- Cs2S2O7 mixtures results in the reaction of the VV dimer with sulfate ions and the spectral data obtained are accounted for by the following reaction scheme: (VO)2O(SO4)44-+2SO42-→ 2VO2(SO4)23-+ S2O72-. The results are of value for the progress on the mechanistic understanding of the SO2 oxidation at the molecular level.

On the configuration, molecular structure and vibrational properties of MoOx sites on alumina, zirconia, titania and silica

The article addresses the critical molecular structural issue of differentiating between the mono-oxo (MoO) and di-oxo [Mo(O)2] configurations as well as the most plausible structures for the oxo-molybdenum [(MoOx)n] sites (including aspects related to coordination number of Mo and extent of association/polymerization) deposited on typical catalyst supports such as γ-Al2O3, monoclinic ZrO2, TiO2-anatase and SiO2. The issue is of topical character and has been the subject of persistent post-2005 research endeavors comprising both theoretical (mainly DFT) work as well as careful experimental/spectroscopic studies (Raman, IR, DR-UV/Vis) that in some cases have also been combined with isotopic labeling experiments. The pertinent vibrational properties are discussed in relation to site configuration (mono-oxo vs. di-oxo), structure and extent of association/polymerization of dispersed oxomolybdates. Vibrational isotope effects and mechanisms for 18O/16O exchange at the molecular level are given special attention.

Characterization of vapour complexes over molten POCl3-MCl3 (M = Al, Ga) mixtures : Raman spectra and thermodynamics

Abstract Raman spectra were obtained at temperatures up to 800 K and pressures up to 6 atm. for vapours over POCl3-AlCl3 and POCl3-GaCl3 molten mixtures under static equilibrium conditions. Spectra are also measured for molten POCl3-AlCl3 and POCl3-GaCl3 salt mixtures. A comparison of the spectral features in the POCl3-MCl3 (M = Al, Ga) vapours with the spectra of the 1 : 1 and 1 : 2 POCl3-MCl3 molten mixtures indicates that the vapour complexes have a 1 : 1 stoichiometry and that complexing occurs through oxygen bridging. Gaseous POCl3·AlCl3 remains stable with no sign of decomposition at temperatures up to 825 K, while the POCl3·GaCl3 vapour complex vaporizes dissociatively. The spectra are interpreted in terms of a Cs symmetry for POCl3·AlCl3 with a bent POAl bridge (nine polarized and eight depolarized bands assigned) and C3v symmetry for POCl3·GaCl3, with a straight POGa bridge (five polarized and six depolarized bands assigned). In the spectra of POCl3-MCl3 (M = Al, Ga) melts several bands are interpreted to account for POCl3(AlCl3)2(l) (four polarized and six depolarized bands assigned) and POCl3(GaCl3)2(l) (two polarized and three depolarized bands assigned) species. Accurate relative Raman intensity measurements are used for determining the thermodynamic functions of the reactions: (a) POCl3(g)+nGa2Cl6(g) = POCl3 (GaCl3)2n(g), in the temperature range 600–800 K and pressures up to 6 atm.; it is shown that the predominant vapour complex formed has n = 0.5 (POCl3·GaCl3) and the thermodynamic functions according to this reaction (n = 0.5) are determined from the Raman experiments as ΔH° = −37±0.8 kJ mol−1 and ΔS° = −44±1.1 J mol−1 K−1. (b) Ga2Cl6(g) = 2GaCl3(g) in the temperature range 400–800 K and pressures up to 2.5 atm., where the thermodynamic functions are determined as ΔH° = 83±1.3 kJ mol−1 and ΔS° = 131±2.2 J mol−1 K−1.

Raman spectroscopic studies of vapor complexation in the MCl4-POCl3 and MCl4-AlCl3 (M = Zr or Hf) binary systems

Raman spectra of vapor mixtures ZrCl4-POCl3, HfCl4-POCl3, ZrCl4-AlCl3 and HfCl4-AlCl3 are obtained in the temperature range 300–500°C. The spectra consist of superposition of bands due to the component gases plus a few new bands which were attributed to vapor complexation. A comparison of the spectra features in the systems MCl4-POCl3 (M = Zr or Hf) with the spectra of the 1 : 1 liquid and glass MCl4 · POCl3 (M = Zr or Hf) with the spectra of the 1 : 1 liquid and glass MCl4 · POCl3 indicate that the vapor complexes also have a 1 : 1 stoichiometry and that complexing occurs through oxygen bridging. The strongest Raman bands for the Zr-O-P and the Hf-O-P complex are at 508 and 512 cm−1, respectively. For the MCl4-AlCl3 systems two possible vapor complexes, MAl2Cl10 and MAlCl7, are considered. Temperature dependence measurements of the Raman spectra of the ZrCl4-AlCl3 systems indicate that the predominant vapor complex is probably 1 : 1 and permit the estimation of the enthalpy of the reaction: ZrCl4(g) + 12Al2Cl6(g) = ZrAlCl7(g); ΔH0 = −34.9 ± 2.5 kJ mol−1. The strongest Raman bands for the Zr-Cl-Al and the Hf-Cl-Al complex are at 401 and 391 cm−1, respectively, and is assigned to M-Cl (terminal) stretching.

Raman Spectroscopic Study of Tungsten(VI) Oxosulfato Complexes in WO3―K2S2O7―K2SO4 Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structure

The dissolution reaction of WO3 in pure molten K2S2O7 and in molten K2S2O7-K2SO4 mixtures is studied under static equilibrium conditions in the XWO3(0) = 0-0.33 mol fraction range at temperatures up to 860 °C. High temperature Raman spectroscopy shows that the dissolution leads to formation of W(VI) oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the W=O stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(=O)2 configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO3-K2S2O7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO3 + nS2O7(2-) --> C(2n-). It is found that n = 1; therefore, the reaction WO3 + S2O7(2-) > WO2(SO4)2(2-) with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K2SO4 in WO3-K2S2O7 melts point to a WO3 · K2S2O7 · K2SO4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO3-K2S2O7-K2SO4 system according to WO3 + S2O7(2-) + SO4(2-) --> WO2(SO4)3(4-). The coordination sphere of W in WO2(SO4)2(2-) (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO2(SO4)2}2(μ-SO4)2](8-) configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of aWO2(SO4)3(4-) moiety resulting in two bridging sulfates. The most characteristic Raman bands for the W(VI) oxosulfato complexes pertain to W(=O)2 stretching modes (i) at 972 (polarized) and 937 (depolarized) cm(-1) for the ν(s) and ν(as) W(=O)2 modes of WO2(SO4)2(2-), and (ii) at 933 (polarized) and 909 (depolarized) cm(-1) for the respective modes of [{WO2(SO4)2}2(μ-SO4)2](8-).

Vapour complexation and thermochemistry over NaI-TbI3 mixtures: a mass spectrometric investigation

Abstract High-temperature Knudsen effusion mass spectrometry was used for studying the equilibrium vapours over equimolar NaI-TbI3 mixtures in the temperature range 765–885 K. The data prove the existence of the sodium tetraiodoterbate (NaTbI4) vapour complex, which is shown to undergo ionization by deriving the full series of NaTbI+n (n = 1,2,3,4) fragments upon electron impact. The detection of an Na2TbI+4 fragment with very low ion intensity provides evidence for the formation of a second, larger vapour complex molecule [Na2TbI5 (g) or Na2Tb2I8(g)] to a much lesser extent. Ion intensity data are used for calculating the equilibrium partial pressures of gaseous species and for determining the thermodynamic functions of the reaction: NaI(g) + TbI3(g) ⇌ NaTbI4(g); ΔH° = −225±3 kJ mol−1, ΔS° = − 144±4 J mol−1 K−1. The equilibria 2NaI(g) + TbI3(g) ⇌ Na2TbI5(g) and 2NaTbI4(g) ⇌ Na2Tb2I8(g) are also considered and their thermodynamic functions are determined and discussed with reference to their consistency. The volatility enhancement of TbI3, which is mainly due to NaTbI4(g) formation, ranges between 35 at 885 K and 95 at 765 K.