Structure of Vanadium Oxosulfato Complexes in V₂O₅−M₂S₂O₇−M₂SO₄ (M = K, Cs) Melts. A High Temperature Spectroscopic Study

Abstract

The V V and V IV oxosulfato complexes formed in V 2 O 5 -M 2 S 2 O 7 -M 2 SO 4 (M ) K, Cs) melts under SO 2 (g) or O 2 (g) atmosphere have been studied by electronic absorption (VIS/NIR) and Raman spectroscopy at 450 °C. VIS/NIR spectra have been obtained at 450 °C for V 2 O 5 -K 2 S 2 O 7 molten mixtures in SO 2 atmosphere (P SO 2 ) 0-1.2 atm). The data are in agreement with the V V T V IV equilibrium: (VO) 2 O(SO 4 ) 4 4-(l) + SO 2 (g) T 2VO(SO 4 ) 2 2-(l) + SO 3 (g). SO 2 does not coordinate to the V V complex but starts significantly to coordinate to V IV for P SO 2 > 0.4 atm according to VO(SO 4 ) 2 2-(l) + SO 2 (g) T VO(SO 4 ) 2 SO 2 2-(l). The Raman spectral features and the exploitation of the relative Raman intensities indicate that the (VO) 2 O(SO 4 ) 4 4dimeric complex unit, possessing a V-O-V bridge, is formed in the V 2 O 5 -M 2 S 2 O 7 binary mixtures. The spectral changes occurring upon interaction of the binary V 2 O 5 -K 2 S 2 O 7 mixtures with SO 2 or addition of M 2 SO 4 to the binary V 2 O 5 -M 2 S 2 O 7 mixtures indicate a cleavage of the V-O-V bridge and formation of the V IV O(SO 4 ) 2 2or V V O 2 (SO 4 ) 2 3monomeric complex units, respectively. The most characteristic bands due to the various complexes in the melts have been assigned. The spectral data are discussed in terms of possible structures. For the first time, high-temperature vibrational spectroscopy has been used to study the structural and vibrational properties of V 2 O 5 -K 2 S 2 O 7 and V 2 O 5 -K 2 S 2 O 7 -K 2 SO 4 melts. The results are valuable for the mechanistic understanding of SO 2 oxidation at the molecular level.

Vanadia?silica and vanadia?cesium?silica catalysts for oxidation of SO2

“…This is well established by comparing the Raman spectra of the Cs-promoted catalysts to unsupported model molten salt/gas systems [14,16,17,44,64].…”

“…Furthermore a weak band at 1078 cm −1 due to the ν 1 (S 2 O 7 2− ) mode appears, indicating a slight excess of molten pyrosulfate. Moreover, SO 3 and S 2 O 7 2− react with the V V oxosulfato complexes and/or the residual vanadium(V) oxides found at the catalyst surface according to [14,16]: identified as the active species in the catalytic cycle of SO 2 oxidation in V 2 O 5 -based supported molten salt catalysts [15,17]. The lower signal-to-noise ratio in spectra 10c and 10d is due to the much darker (dark brown) color of the V 2 O 5 -Cs 2 S 2 O 7 melts relative to the color of V 2 O 5 -Cs 2 SO 4 melts in oxygen atmosphere [16].…”

“…The formation of molten vanadium oxosulfato complexes in Cs-promoted vanadia catalysts is crucial for the catalytic performance and the structural properties of the active melt have been thoroughly characterized [11][12][13][14][15][16][17][18] and the results of this study suggest that the catalytic activity is related to the formation of the same molten vanadium oxosulfato complexes. This is well established by comparing the Raman spectra of the Cs-promoted catalysts to unsupported model molten salt/gas systems [14,16,17,44,64].…”

“…Actually, the industrial catalyst is a unique supported liquid-phase catalyst (vanadium oxides with alkali promoters forming a pyrosulfate melt supported on kieselghur). Under reaction conditions, 693-823 K, the SO 2 oxidation proceeds on active sites located in the interior of the liquid film and recently, the V(V) dimeric complex (VO) 2 O(SO 4 ) 4 4− has been identified as the catalytically active species [11][12][13][14][15][16][17]. Detailed investigations of the effect of the promoters (Na, K, Cs) showed that the activity increases with increasing alkali atomic number, and the presence of Cs as a promoter improves the low-temperature activity with the deactivation occurring at temperatures below 673 K [18][19][20][21].…”

Oxidation of sulfur dioxide over supported solid V2O5/SiO2 and supported molten salt V2O5?Cs2SO4/SiO2 catalysts: molecular structure and reactivity

“…Following a landmark article [6] initiating a systematic study of complex formation of V V in pyrosulfate melts (without the presence of the catalyst carrier), a number of methods including potentiometry, cryoscopy, spectrophotometry, calorimetry, conductometry, cyclic voltammetry, NMR, and Raman spectroscopy have been used to study the V V complexes in V 2 O 5 -M 2 S 2 O 7 -M 2 SO 4 melts [6][7][8][9][10][11][12][13][14]. Significantly, the catalytic cycle involves only V V species (binuclear/dimeric complexes) [5,15], while side reactions leading to reduction of V V to the catalytically inactive V IV (or V III ) are recognized as deactivation routes.…”