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The Mechanism of the Catalytic Oxidation of Hydrogen Sulfide II

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The Mechanism of the Catalytic Oxidation of Hydrogen Sulfide II JOURNAL OF CATALYSIS 42, 87-95 (1976) The Mechanism of the Catalytic Oxidation of Hydrogen Sulfide II. Kinetics and Mechanism of Hydrogen Sulfide Oxidation Catalyzed by Sulfur 1 M. STEIJNS, F. DERKS, A. VERLOOP, AND P. MARS Department of Chemical Engineering, Twente University of Technology, Enschede, The Netherlands Received July 4, 1975 The kinetics of the catalytic oxidation of hydrogen sulfide by molecular oxygen have been studied in the temperature range 20-250°C. The primary reaction product is sulfur which may undergo further oxidation to SO9 at temperatures above 200°C. From the kinetics of this autocatalytic reaction we derived an oxidation-reduction mecha- nism. The two rate influencing steps are the chemisorption of oxygen and the reaction between dissociatively chemisorbed HsS and chemisorbed oxygen. The high activation energy for the formation of SO2 (120 kJ mol-1) explains the high selectivity towards sulfur, although SO2 is thermodynamically the most favored product. At temperatures above 3OO”C, where the formation of SO2 occurs readily, the SO2 may be an in- termediate in the reaction of H2S with 02 leading to S and HzO. INTRODUCTION kinetic data on activated carbon into a In Part I (1) we concluded that sulfur is mechanism. Cariaso concluded that the a catalyst for the oxidation of hydrogen chemisorption of oxygen by the carbon sur- sulfide. Sulfur deposits in the pores of face is the first step in the mechanism, but porous materials and causes an autocataly- that the second step, the impact of H2S tic effect. On the basis of some ESR mea- with chemisorbed oxygen, determines the surements we proposed that sulfur radicals overall rate. Sreeramamurthy supposed are the sites for oxygen chemisorption. that HzS and O2 both chemisorb on the From kinetic data we obtained evidence for carbon surface. The chemisorption of oxy- a dissociative adsorption of hydrogen gen is dissociative. The reaction between sulfide. the chemisorbed species leading to sulfur Kinetic data on this reaction are scarce. and water is rate determining. A survey of the more important publica- The possibility that SO2 is an inter- tions is given in Table 1. Some of the work mediate was also discussed by some authors. (Z-5) could be affected by the large exo- The opinion of Brodsky and Pagny (3) is thermicity of the reaction at high concen- that SO2 is not an intermediate ; Prettre tration and conversion levels. Sreerama- and Sion (4), on the other hand, asserted murthy (6) and Cariaso (7) tried to fit their the contrary. Cariaso (7) excluded SO2 as an intermediate on the basis of thermo- 1 This publication is the second in a series on the mechanism of hydrogen sulfide oxidation; see (1) dynamic calculations assuming that Sz is for Part I. the predominant species in the gas phase at 87 Copyright 0 1976 by Academic Press, Inc. All rights of reproduction in any form reserved. 88 STEIJNS ET AL. TABLE 1 Survey of Literature Data on the Kinetics of HIS Oxidation Catalyst Temp range Activation energy Remarks (“C) ; concn Em (kJ mol-1); reaction (vol%) orders in On and HzS 1. Brodsky and Pagny (3) a. Pumice a. T. 260-420 a. En = 22.634.4 a. At high temperatures HzS, 2.5-3.0 noa = 0.0-0.1 (3OOOC) and high oxygen con- 02. 1.5-7.9 centration formation of SOP occurs b. Pumice + b. T, 200-300 b. Ea = 27.2 b. Impregnation with iron iron oxide H,S, 2.5-3.0 oxide leads to an in- OS, 1.5-16.0 crease of the activity with a factor 10 2. Prettre and Sion (4) ~-Al~O~ T, 4Ck150 E- = 33.5 (T < 120°C) No SO1 is formed even at SBET = 220 Is/g HzS, 0.5* En = 14.7 (T > 120°C) Oz/HzS = 3. VP = 0.15 cd/g 02, 0.2-1.5 IZHB = 0.5 nap = 0.85 3. Puri et al. (6) a. Active charcoal a. T, 12&240 a. I.+ = 25.1 a + b. A part of the sulfur SBET = 30&400 m*/g HtS, 5* no2 = 0.1 formed is bound irreversi- 02, 5-80 bly by the carbon b. Carbon black b. T, 120-240 b. Ea = 12.6-16.8 SBET = 25-45 ma/g HzS, 5b 02, 30 4. Sreeramamurthy (6) Active carbon T. 70~100 En = 6.7-29.3 Low activation energy is SBET = 320 d/g HzS. 4s no2 = 0.5 caused by diffusional in- 02, 2a TL”29 = 1 fluence according to the author 5. Cariaso (7) Carbon molecular T, 100-160 Measured, I+ = 20.1 Diffusional limitations’ sieve (CMS) HtS, 0.25-0.5 Caled, h’a = 40.2 02, 1.0 ml28 = 1 no* = 0 0 Energy of activation calculated from reaction rates and not from rate constants. b The concentration is varied around this value. c We calculated that the rates were largely determined by the diffusion rate of HxS through the hangdown tube of the thermo- balance to the catalyst sample. 150°C. It is known, however, that under under near differential conditions ; the Cariaso’s conditions Ss is the main S-species highest conversions of H&S or O2 were 0.25. (8). Summarizing, it can be concluded that Also the sulfur, deposited during reaction, from the literature data it is not possible was equally distributed over the catalyst to prefer one mechanism. bed. The uniform yellow color of the bed in The present paper describes experiments the case of a zeolite catalyst pointed to this on concentration and temperature depen- equal distribution of sulfur. Temperature dences of the H&S oxidation rate on different effects were negligible because the maxi- porous materials. A mechanism is proposed mum (i.e., adiabatic) increase in tempera- which covers most of the kinetic data. The ture amounts to only 3°C in the case of 10% oxidation of sulfur was studied to investi- conversion of 0.5 ~01% HZS. gate the consecutive reaction leading to SO2 and SO3. The role of SO2 as an intermediate Catalysts is also discussed. In Table 2 the catalysts used are listed and some properties relating to their pore METHODS structure are given. The surface area was Apparatus calculated by applying the BET equation The fixed-bed reactor system and the to argon adsorption data obtained at thermobalance have already been described -196°C. The pore volume was calculated (1). All our experiments were carried out from the difference between the apparent HYDROGEN SULFIDE OXIDATION. II 89 TABLE 2 Catalyst Data Catalyst Manufacturer Surface area Pore vol W g-9 (cm3 g-l) Molecular sieve 13X Union Carbide 470 0.24 ~-&OS Peter Spence 190 0.34 Degussa 78 0.78 Carbon molecular sieve (CMS) Seifert K. G. 1000 - Activated carbon RBWI” Norit 950 0.81 hwr) Our lab. 1050 0.45 Q This catalyst was extracted with dilute sulfuric acid to remove the inorganic impurities. catalyst volumes in mercury and methanol unfilled and from the outer surface of the at room temperature. part’icles. In case .a material possesses a Carbon molecular sieve (CWS) is a very high intrinsic activity compared to the pure carbon with a pore radius mainly in autocatalytic activity, deposition of sulfur the range of lo-12 A. The material is pre- lowers the activity and branch A is not pared by carbonization of vinylidene chlo- observed. ride polymer. For experiments in the regions A or B where the activity did not attain a station- RESULTS ary level as a function of reaction time, the Kinetic experiments were carried out not rate data are always compared at the same only on different materials (Table 2) but sulfur content. The sulfur content is cal- also at different sulfur content,s of these culated by integration of the HZS oxidation porous materials. As we already pointed out rate in the fixed-bed experiments. The de- in Part I (I) the oxidation rate of H2S can sorption rate of sulfur is in this case much be strongly dependent on the amount of smaller than the oxidation rate. sulfur deposited in the micropores. For At a stationary activity level (region C), materials which have a low intrinsic oxida- the sulfur content of the material does not tion activity (= initial oxidation rate), the change further. For some materials it is characteristic course of the activity as a even possible to vary concentrations and function of sulfur uptake by the material temperature in a limited region without is given in Fig. 1. There are three distinct ranges, A, B, and C. In range A the activity increase wit.h reaction time is due to the autocatalytic activity of the sulfur de- posited in the pores. In range B a decrease of the activity is observed because pores become filled up with sulfur which results in a considerable decrease of the specific surface area of sulfur. A stat,ionary activity level is reached in range C, when the amount of sulfur formed equals the amount FIG. 1. Catalytic activity as a function of the of sulfur desorbed from t’he large pores left amount of sulfur deposited in the porous material. 90 STEIJNS ET AL. TABLE 3 Rate Dependence on the Oxygen Concentration Catalyst SCg(g T (“C) Vol 7; Reaction order cat)-‘] HzS 02 MS 13X 0.08 130 1.27 0.087-0.403 1.05 0.16 130 1.27 0.087-0.403 1.02 MS 13X5 0.304” 151 1.20 0.11 -0.78 0.83 MS 13X 0.304 22 0.25 1.04 -5.78 0.60 Norit RBWI 0.12 180 1.0 0.15 -0.63 70 02 > 0.35, 0.0 % 02 < 0.35, 0.85 CMS 0.15 200 0.75 0.075-0.375 > % 02 > 0.15, 0.5 0.30 200 0.75 0.075-0.375 % 02 < 0.15, 0.75 Carbon (sugary 0.70s 130-150 > 0.30 0.65 -1.76 0.23 f 0.10 0.7w 170-190 1.68 0.07 -0.57 0.79 f 0.10 y-Al203 (Degussa) 0.05-0.40 80 1.0 0.17 -0.50 0.95 (Peter Spence) 0.40 115 0.74 0.10 -0.60 1.0 n Stationary activity level of branch C in Fig.
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