Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 78, No. 2, 365-371and Kluwer Academic Publishers, Dordrecht (2003)

0133-1736/2003/US$ 20.00.© Akadémiai Kiadó, Budapest.

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RKCL4195

SELECTIVE OXIDATION OF HYDROGEN SULFIDE OVERPOTASSIUM PROMOTED VANADIUM OXIDE ON AN ALUMINA

CATALYSTS

Min Woo Song*, Min Kang and Kyung Lim KimDepartment of Chemical Engineering, Yonsei University, Seoul 120-749, Korea

Received August 26, 2002Accepted October 4, 2002

Abstract

The effect of potassium addition to alumina supported vanadium catalysts on thecatalytic activity for the selective oxidation of H2S has been investigated. XRD,XPS and XANES have been used to characterize a series of K-V/Al2O3 catalysts.The enhanced sulfur yield has been correlated with the crystallinity of vanadium.

Keywords: H2S, selective oxidation, vanadium, potassium

INTRODUCTION

Most fossil fuels contain some chemically combined sulfur. Duringpurification and processing of the fossil fuels, hydrogen sulfide is a commonbyproduct. Hydrogen sulfide is removed from industrial waste gases via theClaus process [1,2]. However, the recovery of sulfur in the Claus process is notcomplete, being 3-5% due to the thermodynamic limitations. Since thefollowing catalytic step in this process is the equilibrium,

2H2S + SO2 � 3/nSn + 2H2O

a supplementary tail treatment process is required in order to match theenvironmental regulations. Recently, SuperClaus process have been developedfor treating the tail gas from the Claus process using an α-alumina supportediron-chromium oxides catalyst [3]. In this process, H2S is oxidized directly byO2 at 230oC.

366 MIN WOO SONG et al.: SELECTIVE OXIDATION

Generally, vanadium-based catalysts have been used extensively for theoxidation of organic and inorganic chemicals and proposed as selective catalystsfor the selective oxidation of H2S to sulfur. VBiO/SiO2 was used to improve thecatalytic stability and various binary oxides such as V-Mg, V-Bi, V-Sb werequite effective [4]. The catalytic behavior of VOx/Al2O3 can be modified by the addition of asecond element which acts as promoter. Potassium, as an additive, is known tobe effective for many catalytic processes [5] and the incorporation of potassiumon VOx/Al2O3 favors higher selectivity to C4-olefins during OXDH of n-butane[6]. In the present study, we investigated the promoting effect of potassium inH2S selective oxidation on VOx/Al2O3 catalyst and performed complementarycharacterization to reveal the physical and structural property.

EXPERIMENTAL

30 wt.% VOx/Al2O3 catalyst was prepared by impregnation of a �-Al2O3

support (SBET= 167.5 m2g-1) with an ammonium metavanadate aqueous solution(pH = 7). The K-doped VOx/Al2O3 catalysts were prepared by impregnation ofthe VOx/Al2O3 catalyst with an aqueous solution of potassium nitrate. Samplesare labeled as VAl for 30 wt.% VOx/Al2O3 catalyst and VKx for VOx-K/Al2O3.Catalysts, where x is the atomic ratio (K/V = 0.2-0.8). The catalysts werecalcined at 500oC for 4 h. Characterization of catalysts was performed byseveral techniques, i.e. XRD, XPS and XANES. X-ray diffraction patterns wereobtained at room temperature using Cu (K�) radiation and a Rigakudiffractometer. X-ray photoelectron spectroscopic measurement was carried outin an ESCALAB 2201-XL(Fision) equipped with the aluminium anode asunmonochromatized X-ray source (1486.6 eV) run at 10 kV and 15 mA,2∼ 8×10-8 mbar, fixed analyzer transmission. The XANES spectra were recordedat room temperature using an X-ray absorption spectrometer (PLS 3C1beamline, Pohang) with a ring current of 300 mA. Si(311) monochrometercrystals were used to enhance the resolution of edge features. Data werecollected in transmitance with upstream slit set and using N2 and Ar filledionization chambers as detector. The catalytic tests were carried out in a fixed-bed continuous flow reactor atatmospheric pressure at 250oC. The feed was a mixture of H2S-O2 diluted withHe in a molar ratio of 1 : 0.87 : 98.3 and the amount of catalysts packed in thereactor was 0.3 g. The inlet and outlet gases were analyzed online GC (HP6890)equipped with a PDD detector and 8 ft Porapak T column. Sulfur in the outletgases was collected in sulfur condenser at 120oC and all the lines were heatedabove 150oC to prevent the condensation of sulfur and water. The conversion ofhydrogen sulfide and sulfur selectivity were calculated using the equations

MIN WOO SONG et al.: SELECTIVE OXIDATION 367

100

ySelectivit S Conversion SH (%) Yield S

100reacted) SH of (moles

produced)SO of moles-reacted SH of (moles (%)y Selectivit S

100fed) SH of (moles

reacted) SH of (moles (%) Conversion SH

2

2

22

2

22

×=

×=

×=

0 2 4 6 8 10 120

20

40

60

80

100

H2S

con

vers

ion

(%)

Time (hr)

Fig. 1. H2S conversion vs time for VOx-K/Al2O3 catalysts: (���VAl; (❍ ) VK02;(�� VK04; (∇ ) VK06; (◆ ) VK08

RESULTS AND DISCUSSION

Figures 1 and 2 show the effect of K addition on the catalytic conversion ofH2S and S yield over vanadium catalysts, respectively. In case of VAl andVK02, the conversion remained stable over time with no deactivation observed.However, the conversion of VK04, VK06 and VK08 decreases sharply withtime on stream. The H2S conversion decreases with increased K loading.

368 MIN WOO SONG et al.: SELECTIVE OXIDATION

Grabowski [7] reported that the decrease in activity indicates a poisoning effectof the alkaline cations on the centers for hydrocarbon activation. Also, thedeactivation is caused by the formation of less active forms of vanadium such asVOSO4. The sulfur yield of VK02 catalyst has higher sulfur yield comparedwith the VAl catalyst. Taking into account that the sulfur yield is related to thecrystallinity of vanadium, the addition of small amounts of potassium increasesthe crystallinity of vanadium. According to Shin et al. [8], the bulk phase ofvanadium is more active than the dispersed phase. XRD characteristic peakconfirms this result as shown in Fig. 3. XRD patterns of VK02 show thepresence of crystalline V2O5 phase at d = 4.38 and 3.39 Å (JCPDS No: 01-359)corresponding to 2θ at 20.3 and 26.3, respectively.

0 2 4 6 8 10 120

20

40

60

80

S Y

ield

(%

)

Time (hr)

Fig. 2. S yield vs time for VOx-K/Al2O3 catalysts: (��� VAl; (❍ ) VK02; (��VK04; (∇ ) VK06; (◆ ) VK08

MIN WOO SONG et al.: SELECTIVE OXIDATION 369

0 10 20 30 40 50 60 70 80

¡å¡å

(e)

(d)

(c)

(b)

(a)

Inte

nsity

(a.

u.)

2θ (degree)

Fig. 3. XRD diffraction patterns of VOx-K/Al2O3 catalysts: (a) VAl, (b) VK02, (c)VK04, (d) VK06, (e) VK08, (�) peaks due to V2O5

Table 1 gives the values of binding energies, BE for O1s, V2p3/2 and K2p3/2

levels, and the surface atomic ratio of K added vanadium catalysts. The bindingenergy values of V2p3/2 peak for VK02 shift to higher energy in comparisonwith VK04 and VK08. Nag and Massoth [9] reported that the binding energyfor V5+ has higher value ca. 0.7-1.5 eV than for V4+, V3+, which is reduced state.The surface atomic ratio V/Al of VK02 has slightly higher value as compared toVAl. It has been found that more vanadium is present on the surface of VK02than of unpromoted vanadium catalysts. Also, it is related to the higher S yieldfor VK02 than for VAl.

370 MIN WOO SONG et al.: SELECTIVE OXIDATION

Table 1

Binding energy for O1s, V2p3/2 and K2p3/2 levels and surface atomic ratio

Binding energy (eV) Surface atomic ratioa

Catalyst

O1s V2p3/2 K2p3/2 V/Al

VAl 531.2 517.5 - 0.192

VK02 531.1 517.5 292.8 0.203

VK04 530.9 517.2 292.8 0.176

VK08 513.0 517.1 292.7 0.182

a Corrected for atomic sensitivity factors (area) [10]

Table 2

Energy positions of the pre-edge peak and edge in V K-edge XANES spectra

Catalyst Pre-edge position(eV) Edge position (eV)a

V foil 5466.0 5474.4

NH4VO3 5472.1 5484.3

V2O5 5472.2 5480.5

VAl 5472.2 5483.4

VK02 5472.3 5484.7

VK04 5472.0 5484.0

VK06 5472.1 5483.8

VK08 5471.9 5484.1

a Determined from the position of the second maximum of the derivative of XANES curve.

Table 2 gives the energy positions of pre-edge peak and of the absorptionedge in V K-edge XANES spectra. The edge position is directly related to thebinding energy of the ejected electron during the absorption process, it moves tohigher energy with the oxidation state of the absorbing atom increases.Compared to the edge energy of VAl, VK04 and VK08, VK02 has a highervalue by 0.3-0.9 eV. These results in XANES agree with the positive shift in

MIN WOO SONG et al.: SELECTIVE OXIDATION 371

binding energy for V2p3/2 in XPS measurement. Therefore, it has beenconcluded that the oxidation state of vanadium is closely related to the activityof H2S selective oxidation.

In conclusion, the VK02 catalyst gives a higher sulfur yield compared withVAl. The addition of small amounts of K increases the crystallinity andoxidation state of vanadium. The selective oxidation of H2S is apparentlystructure-sensitive in terms of vanadium crystallinity. The decrease in activity inH2S selective oxidation indicates a poisoning effect of K+

Acknowledgments. This work has been supported in part by ElectricalEngineering & Science Research Institute grant, 01-037, which is funded byKorea Electric Power Co.

REFERENCES

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Catal., 125, 129 (1995).8. M.Y. Shin, C.M. Nam, D.W. Park, J.S. Chung: Appl. Catal., 211, 213 (2001).9. N.K. Nag, F.E. Massoth: J. Catal., 124, 127 (1990).

10. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg: Handbook of X-ray Photoelectron Spectroscopy, p. 188. Perkin-Elmer, Eden-Prairie, MN 1979.