Sulfato Complex Formation of V(V) and V(IV) in Pyrosulfate Melts

By combined potentiometric, electron paramagnetic resonance (EPR), and visible/near-infrared spectroscopic measurements, the coordination of SO42- to ...
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11282

J. Phys. Chem. B 1999, 103, 11282-11289

Sulfato Complex Formation of V(V) and V(IV) in Pyrosulfate Melts Investigated by Potentiometry and Spectroscopic Methods S. B. Rasmussen, K. M. Eriksen, and R. Fehrmann* Department of Chemistry B. 207 and Interdisciplinary Research Center for Catalysis (ICAT), Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark ReceiVed: July 30, 1999; In Final Form: October 19, 1999

By combined potentiometric, electron paramagnetic resonance (EPR), and visible/near-infrared spectroscopic measurements, the coordination of SO42- to V(IV) and V(V) in M2S2O7-M2SO4-V2O5 (M ) K and Cs) melts, respectively, under SO2(g) and O2(g) atmospheres at 450-470 °C has been investigated. The results for both systems are in accordance with the oxo sulfato vanadate equilibria VO(SO4)22- + SO42- a VO(SO4)34for V(IV), (VO)2O(SO4)44- + 2SO42- a 2VO2(SO4)23- + S2O72- for V(V), and 2VO2(SO4)23- + SO2 + SO42- a 2VO(SO4)34- for the V(V)-V(IV) redox reaction in melts saturated with sulfate. Constants for these equilibria have also been obtained, as well as characteristic EPR parameters and molar absorptivities for the complexes. In addition, the mole fractions of M2SO4 in the M2S2O7-M2SO4/SO2(g) systems saturated with M2SO4 were found to be 0.050 19 for M ) K at 450 °C and 0.070 02 for M ) Cs at 470 °C, respectively.

Introduction

Experimental Section

For several years we have investigated the molten salt catalyst for SO2 oxidation by numerous methods. The catalyst precursor is V2O5 dissolved in alkali pyrosulfate (mixture of Na, K, and/ or Cs), with an alkali-to-vanadium ratio of 2-4. Although a good understanding of the catalyst deactivation has been established during the past decade,1-3 less is known about the complex formation in the active catalyst. More recently the catalyst has been used also for desulfurization of power plant flue gas, where the operating conditions are significantly different from traditional sulfuric acid manufacturing,4 since the SO2 content is only around 0.2% and huge amounts of gas are handled. These conditions might result in a continuous “stripping” of SO3 from the pyrosulfate owing to the equilibrium S2O72- a SO42- + SO3, increasing the sulfate activity in the melt. Previously, we have studied the sulfato complex formation of V(V) by potentiometric, calorimetric, conductometric, spectroscopic, and NMR-spectroscopic methods,5-9 and the V(IV)V(V) redox equilibria in SO2/SO3 atmosphere by visible/nearinfrared (VIS/NIR) and electron paramagnetic resonance (EPR) spectroscopy.10 However, the formation of sulfato complexes of V(IV) in pyrosulfate melts has not been studied earlier by us. The only report in the literature11 claims the presence of the V(IV) species VOSO4 and VO(SO4)34-, but the experimental conditions were less well-defined, i.e., use of “wet” chemicals and unknown partial pressure of SO2, as pointed out earlier.10 To investigate the formation of V(IV) oxo sulfato complexes in the hygroscopic pyrosulfate melts in a controlled atmosphere, we have constructed a dynamic electrochemical gas flow cell. Furthermore, this setup leads to fast gas-liquid equilibrium and fast measurements, i.e., a few hours after addition of chemicals, compared to several weeks characteristic of the static cell earlier used.2,6 Therefore a reinvestigation of the V(V) oxo sulfato complex formation was also carried out.

Chemicals. Pure and dry M2S2O7 were made by thermal decomposition of their corresponding peroxodisulfates, M2S2O8, obtained as earlier described.1,5,12 Cs2SO4 and K2SO4 were from Merck (Pro analysi, >99.5%). Commercial gases SO2 (>99.9%), O2 (99.8% + 0.2% N2 and Ar), and N2 ( ∼0.33. For the model equilibria proposed in Table 2, the position of the equilibria and thereby also the coordination number of SO42to V(V) at fixed p(SO42-) will only for models 1 and 2 be independent of the total concentration of V(V), cV(V). As it appears from Figures 3 and 4, n˜ is clearly dependent on cV(V). The experiments have been carried out by the addition of both V2O5 and M2S2O7 in proportions that maintained cV(V) close to either 0.05 or 0.20 M. The variances given in Table 2 are the results of least-squares fit of the experimental coordination number, n˜ , to the one that can be calculated for the particular model equilibrium, using the equilibrium constant as parameter. An F-test on the variances of the two best models for the K and Cs systems show that model 3 gives a significantly better fit (i.e., on a >99.9% level for both K and Cs) than any of the other models. Thus the following reactions apply very well for both systems:

Dissolution reaction: V2O5 + 2S2O72- a (VO)2O(SO4)44Complex reaction: (VO)2O(SO4)44- + 2SO42- a 2VO2(SO4)23- + S2O72This conclusion is in full agreement with the recent Raman spectroscopic investigations,16,17 where the dimeric V(V) complex (VO)2O(SO4)44- was found to dominate up to XV2O5 ) 0.33 and that VO2(SO4)23- and S2O72- were formed by addition of SO42- to the dimer. The structure of the dimer complex in the solid has been known for some years24 from single-crystal X-ray investigations of the compound Cs4(VO)2O(SO4)4, isolated from Cs2S2O7-V2O5 melts. Similar structures are found for the K and Rb analogues.25

J. Phys. Chem. B, Vol. 103, No. 51, 1999 11285

Figure 5. Coordination number of SO42- to V(IV) for the model VO(SO4)22- + SO42- a VO(SO4)34- calculated for the equilibrium constant K ) 12.76. Range of cV(IV) ) 0.01-0.32 M.

Figure 6. Coordination number of SO42- to V(IV) for the model VO(SO4)22- + SO42- a VO(SO4)34- calculated for the equilibrium constant K ) 7.01. Range of cV(IV) ) 0.05-0.35 M.

M2S2O7-M2SO4-V2O5/SO2(g) System (M ) K and Cs). The potentials of the M2S2O7-M2SO4-V2O5 melts have been measured in SO2(g, 1 atm) by bubbling the melts in both chambers by SO2 (Figure 1). Earlier attempts to perform potentiometric measurements in SO2(g) in the sealed static cell5 led to continuous (catalytic?) formation of bubbles on the gold electrode surface, making the potentials unmeasurable. This experimental problem was the driving force for developing the gas flow cell used here, where possible bubbles formed on the gold electrodes are swept away continuously by the gas flow agitated melt. The measured compositions, cell potentials, and calculated values of n˜ of 11 different compositions in the K system at 450 °C and 8 in the Cs system at 470 °C are given in Tables S5 and S6 of the Supporting Information. Applying eq 6 to the measured potentials, as the system in O2(g), the experimental coordination numbers of SO42- to V(IV) have been calculated. In this connection cV(IV) has been assumed equal to c°V(V), since the spectrophotometric measurements (see below) showed a reduction degree >96% for V(V) at PSO2 ) 1 atm. The results are displayed in Figures 5 and 6. Despite the large variation of cV(IV), no jumps of n˜ are observed. Again, limiting the possible V(IV) species to be monomeric or dimeric gives rise to the four

TABLE 2: Vanadium (V) Model Equilibria. Coordination of SO42- to V(V) in the M2S2O7-M2SO4-V2O5/O2(g) Molten System (M ) K (450 °C) and M ) Cs (470 °C)), Equilibrium Constants (K) and Variances (σ) K

Cs

model reaction

K

σ

K

σ

1. (VO)2O(SO4)44- + SO42- a (VO)2O(SO4)562. VO2SO4- + SO42- a VO2(SO4)233. (VO)2O(SO4)44- + 2SO42- a 2VO2(SO4)23- + S2O724. 2VO2SO4- + SO42- + S2O72- a (VO)2O(SO4)56-

>109 6.32 3.53 2.48

4.1 0.0206 0.000971 0.228

>109 7.39 2.88 2.30

2.4 0.0174 0.00115 0.328

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Rasmussen et al.

TABLE 3: Vanadium (IV) Model Equilibria. Coordination of SO42- to V(IV) in the M2S2O7-M2SO4-V2O5/SO2(g) Molten System (M ) K (450 °C) and M ) Cs (470 °C)), Equilibrium Constants (K) and Variances (σ) K model reaction

K

1. (VO)2(SO4)4 + SO4 a (VO)2(SO4)5 2. VO(SO4)22- + SO42- a VO(SO4)343. (VO)2(SO4)44- + 2SO42- a 2VO(SO4)344. 2VO(SO4)22- + SO42- a (VO)2(SO4)564-

2-

6-

>108 12.76 18.15 >107

Cs σ

K

σ

560 0.00494 0.0208 0.0452

3.04 7.01 6.08 >107

0.485 0.000217 0.0109 0.0459

Figure 7. Room-temperature EPR spectra of frozen melts of the indicated systems for c°V(V) ) 0.1 M, quenched from 450 °C. The calculated g-components for the possible species VO(SO4)34- formed in solution (A) do not fit the g-components of the species VO(SO4)23possibly formed in solution (B).

possible model equilibria listed in Table 3. Of those only model 1 and 2 are expected to give values of n˜ independent of cV(IV) at fixed p(SO42-). However, as can be seen in Table 3, model 2 has the lowest variance and model 3 has the next lowest in both the K and the Cs system. Applying an F-test, models other than model 2 can be ruled out on a level >99.9%, except for model 3 in the K system, where the level is only >97.5%. However, the following EPR spectroscopic measurements will add to clear up this matter. EPR Spectra of the M2S2O7-M2SO4-V2O5/SO2(g) Systems (M ) K and Cs). EPR spectra on molten and quenched mixtures of these systems in quartz ampules, sealed under 0.8 atm SO2, have been recorded. In Figure 7 are shown the roomtemperature spectra of the K system for c°V(V) ) 0.1 M, quenched in an ice bath from the molten state at 450 °C. The indicated perpendicular and parallel components of the g-factor have been calculated on the basis of the nuclear spin 7/2 for 51V (abundance 99.75%) and using the iterative procedure of Chasteen.26 In Figure 7 it is seen that these components calculated for the complex formed in the sulfate saturated system significantly do not fit the complex formed in melts without sulfate. Thus the spectra show that we are dealing with two different vanadyl sulfate complexes, here given the formulas of model reaction 2 (Table 3). The choice of monomeric complexes is justified from the number of g-tensor components where the degeneracy of the eight gx and gy components into eight gz components further indicate a high axial symmetry of the complex, i.e., highly symmetric coordination in the horizontal plane of the VO2+ complex. Further support for the complexes to be monomeric comes from Figure 8. Here the series of spectra obtained on stepwise heating of the quenched sulfate saturated sample shows increasingly line broadening due to the change in Boltzman distribution by temperature, and finally at 475 °C, the characteristic eightline feature of a monomeric V(IV) complex in solution. No sign

Figure 8. EPR spectra of the quenched sample from the K2S2O7-K2SO4(sat.)-V2O5/SO2(g) system after reheating to the indicated temperatures.

of line splitting due to coupling by a close V(IV) neighboring atom is seen. Very similar EPR spectra and conclusions have been obtained for the Cs system and in addition for the Na and Rb analogue systems as well.27 Thus model reactions including dimeric V(IV) complexes (Table 3) are very unlikely, leaving model reaction 2 as the only reaction, fitting the combined potentiometric and EPR spectroscopic investigations in both the K and the Cs system. The most probable reactions due to reduction of V(V) complexes to V(IV) and sulfate complex formation of V(IV) are therefore:

Reduction in melts at low sulfate concentration: (VO)2O(SO4)44- + SO2 a 2VO(SO4)22- + SO3 Reduction in sulfate sat. melts: 2VO2(SO4)23- + SO2 a VO(SO4)22- + VO(SO4)34Complex formation: VO(SO4)22- + SO42- a VO(SO4)34The characteristic EPR parameters obtained for the K and Cs systems at sulfate saturation are given in Table 4. It can be concluded that the parameters, within the experimental error, are identical for the complexes in the liquid and frozen states, indicating that the geometry of the proposed complex VO(SO4)34is essentially maintained by quenching.

Sulfato Complex Formation of V(V) and V(IV)

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TABLE 4: EPR Parameters for the Quenched and Molten M2S2O7-M2SO4 (sat.)-V2O5/SO2(g) Systema M

g|

g⊥

a| (Gauss)

a⊥ (Gauss)

(2g⊥ + g|)/3

giso

(2a⊥ + a|)/3

aiso

K Cs

1.930(3) 1.936(3)

1.980(3) 1.981(3)

203.5(10) 194.8(10)

71.7(5) 65.1(5)

1.963 1.966

1.970(5) 1.968(5)

115.7 108.3

108.3(10) 107.6(10)

a

Quenched melts: g|, g⊥, a|, a⊥ at room temperature. Molten state: giso and aiso at 470 °C (Cs) and 450 °C (K).

TABLE 5: Applied Model Reactions and Variances for the Spectrophotometric Series of Measurements, Performed on the M2S2O7-K2SO4(sat.)-V2O5/SO2(g) System at 450 °C (M ) 20% Na + 80% K) σ model reaction

series

1. (VO)2O(SO4)44- + SO2 + 2SO42- a (VO)2(SO4)56- + S2O722. (VO)2O(SO4)44- + SO2 + 3SO42- a 2 VO(SO4)34- + S2O723. 2VO2(SO4)23- + SO2 + SO42- a 2VO(SO4)344. 2VO2(SO4)23- + SO2 a (VO)2(SO4)56a

1a

series 2b

0.565 0.438 0.197 0.221

0.745 0.413 0.158 0.369

11 spectra in the pressure range PSO2 ) 0.006-1.57 atm (Figure 9). b 14 spectra in the pressure range PSO2 ) 0.001-1.33 atm.

TABLE 6: Comparison of Characteristic EPR and VIS/NIR Parameters for V(V) and V(IV) Complexes Formed in the Melt-Gas Systems K2S2O7-V2O5/SO2-SO3-N2a and the M2S2O7 (M ) 20% Na + 80% K)-K2SO4(sat.)-V2O5/SO2(g) or O2(g)b at 450 °C oxid. state

V

IV

V

IV

species

(VO)2O(SO4)44-, a

VO(SO4)22-, a

VO2(SO4)23-, b

VO(SO4)34-, b

1.57(2)

1.979(5) 111(5) 18.91(8)

1.91(5)

1.970(5) 108(5) 19.3(5)

giso aiso 730 nm isosbestic Kredox a

15.0(1) 1.45 × 10-2 (eq 11)

14.1(2) 1.8(2) × 103 (eq 10)

Reference 10. b This work.

Spectrophotometric Investigations on the M2S2O7-M2SO4V2O5/SO2(g) Systems. (M ) 20% Na + 80% K). The solutions of vanadium complexes change color from red brown in O2(g) to green and finally blue by increasing the SO2(g) partial pressure stepwise up to 1.6 atm in SO2/N2 gas mixtures. Thus the V(V)-V(IV) redox reaction seems well-suited for spectrophotometric measurements as performed earlier10 on the K2S2O7-V2O5 system in SO2/SO3 gas mixtures. Two series of VIS/NIR spectra have been obtained at 450 °C on the M2S2O7 (M ) 20% Na + 80% K)-K2SO4(sat.)-V2O5 system for, respectively, c°V(V) ) 0.2 and 0.5 M, in oxidizing (O2) and reducing (SO2/N2) gases at equilibrium. This Na-K mixed cation system corresponds to the composition of a widespread industrial SO2 oxidation catalyst, where we earlier28 have investigated the M2S2O7-V2O5 phase diagram. However, the absorption spectra of vanadium complexes are not expected to change significantly by the relatively small addition of Na2S2O7 to the K2S2O7 melt. The spectra of one of the series are shown in Figure 9. At the highest SO2 partial pressures, a band maximum is found at 730-740 nm in both series, slightly different from the maximum found10 at 730 nm for this band in melts with low SO42- concentration. Furthermore, the isosbestic point found in both series at 622-630 nm with a molar absorptivity  ) 14.1 ( 0.2 L‚mol-1‚cm-1 is also slightly different from the one found10 at 615 nm and  ) 15.0 ( 0.1 in the previous investigation. The presence of the isosbestic points is a strong indication of redox equilibria involving only two absorbing species in both the previous10 and present work, however different couples of species. This is in accordance with the potentiometric and EPR work described above. Applying Beer’s law on the absorbance at 730 nm leads to the equation

A/l ) cV(V)‚V(V) + cV(IV)‚V(IV)

(9)

where l is the optical path length (cm), cV(V) and cV(IV) are the equilibrium concentrations (mol‚L-1) of the V(V) and V(IV) complexes, and V(V) and V(IV) are their molar absorptivities (L‚mol-1‚cm-1). The initial concentration of vanadium is c°V(V) ) cV(V) + cV(IV). Four possible models of redox equilibria are given in Table 5. From the spectra in O2(g), V(V) was found to be 1.91 (5) L‚mol-1‚cm-1. For all measured spectra (except spectrum L and M in Figure 9), the initial value c°V(V) and PSO2 at equilibrium are known. For a particular redox reaction (Table 5) and a given value of the equilibrium constant (K), it is then possible to calculate cV(V) and cV(IV) for all investigated melts. Then, applying eq 9, it is possible to calculate a value of V(IV) that gives rise to the smallest deviation between measured and calculated molar absorptivities (A/L‚c°V(V)) at 730 nm for all the obtained spectra. This deviation is then minimized by variation of K. From this procedure values of K and V(IV), leading to the smallest variances for the particular model, have been found. They are listed in Table 5 for the applied models and experimental series performed. Applying an F-test on the obtained variances show that model reaction 3 on a 99% level is significantly better than the other models, except for model 4 applied on the measurements in series 1, where the level is below 90%. However, the best overall fit to the spectrophotometric data is the redox reaction

2VO2(SO4)23- + SO2 + SO42- h 2VO(SO4)34- (10) involving the V(V) and V(IV) complexes VO2(SO4)23- and VO(SO4)34-, respectively, which also gave the best fit to the potentiometric and EPR spectroscopic measurements. The equilibrium constant at 450 °C for eq 10 is calculated to be 1.8(2) × 103 as an average for both spectrophotometric series.

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Rasmussen et al. The previous investigations5,6 came to somewhat different conclusions for V(V), possibly owing to the different experimental technique and the fact that none or only very few experiments were done at varying concentrations of V(V). However, the cryoscopic measurements5 point to monomeric V(V) formed in dilute sulfate-poor solutions. This discrepancy compared to the present work, favoring dimeric V(V), is for the moment not well understood. However, similar measurements29 on the Cs system point to partly dimerized V(V) complexes formed. The composition range of industrial SO2 oxidation catalysts corresponds to XV2O5 ) 0.20-0.30, and it operates in SO2O2-SO3-N2 atmospheres, i.e., in SO3 containing gases. Therefore, low concentrations of SO42- are expected (due to the reaction SO42- + SO3 a S2O72-), and the V(V) complexes formed are most probably dominated by the dimer (VO)2O(SO4)44-, as confirmed by the recent Raman spectroscopic measurements.16,17 Concerning the V(IV) complexes at these high vanadium concentrations, the equilibrium nVO(SO4)22- a VO(SO4)2n2n- between monomeric and polymeric complexes seems to be important as judged from previous in situ EPR measurements on industrial catalysts.2 Acknowledgment. The Danish Natural Science Research Council and ICAT (Interdisciplinary Research Center for Catalysis) sponsored by the Danish Research Councils through the “Chemistry Programme” have supported this investigation. Supporting Information Available: Tables S1-S6 giving experimental mole fractions, molar concentrations, and measured potentials for all measured systems (6 pages). See any current masthead page for ordering and access information. References and Notes

Figure 9. VIS/NIR spectra of the mixture M2S2O7 (M ) 20% Na + 80% K)-K2SO4(sat.)-V2O5/SO2(g) at 450 °C, c°V(V) ) 0.5 M, equilibrated at different SO2 partial pressures (atm): 1.57 (A), 1.52 (B), 0.0559 (C), 0.0375 (D), 0.0315 (E), 0.0184 (F), 0.0127 (G), 0.009 57 (H), 0.007 47 (I), 0.006 12 (J), 0.006 00 (K),