38
Ind. Eng. Chem. Prod. Res. Dev. 1082, 21, 38-41
Lanthanum Oxysulfide as a Catalyst for the Oxidation of CO and COS by Sop Joseph A. Bagllo GTE Laboratories, Incorporated, Waltham, Massachusetts 02254
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Experimental resub show that LacoO, and posslbly other rare earth transition metal perovskites decompose under the condkions for the reduction of 10% SO2 with 20% CO to form a transltkn metal sulfide and rare earth oxide or oxysulfide. These de~mpositionproducts are the active catalytic species for the reduction of SO2 by CO. The transition metal sumde reacts with CO to form COS as an intermediate which in turn reacts with SO2 to yield S, with the rare earth oxysLdRde as the catalyst for this latter reaction. consequentfy,the removal of COS as a poUutant can be better accomplished by eliminating the transition metal Ions in the catalyst and using rare earth oxides or oxysuifldes as the active catalytic species. Experlmental evidence has demonstrated that La202Seither atone or doped with Eu is an effective catalyst for the thermal decomposition of COS and the reduction of SOp by COS.
Introduction Carbonyl sulfide (COS)is a byproduct of petroleum refineries, stack gas purification, and many other chemical processes. This substance is highly toxic and is therefore undesirable as an effluent contaminant. Consequently, considerable interest has been expressed in the removal of carbonyl sulfide from gases (OHara et al., 1966, Sample, 1948; Huley, 1967; Richardson and Fenchan, 1954; Kiaer, 1954; Fleming and Fitt, 1950). Our interest in this area is an outgrowth of an investigation to determine the active catalytic species when LaCoo3 is used as a catalyst for the reduction of sulfur dioxide by carbon monoxide (Palilla, 1976). It has been reported that COS is generated when most of the transition metals are used as catalysts for the above reaction either alone or impregnated in commercial supports (Ham et al., 1971, 1972; Khalafalla and Ham, 1972). Happel et al. (1975) investigated binary, ternary, and quaternary oxides for SO2reduction by CO. They believed that the tendency of the oxides to form sulfides during the reaction was directly related to COS formation. In order to reduce the generation of COS, therefore, they chose those oxides with greatest stability with respect to formation of the corresponding sulfide as potential catalysts for SO2 reduction by CO. From thermodynamic data together with a consideration of stability of perovskites associated with the ionic radii of the cations, they listed LaA103, LaCo03, LaFe03, and LaTi03 as potential catalysb. From this list, they chose LaTi03 as that perovskite with greatest stability, and hence the most suitable catalyst for SO2 reduction. In this report, experimental results will be presented which demonstrate that the perovskite LaCoO3 decomposes when used as a catalyst for SO2 reduction by CO, and that the active catalytic species for this reduction is La202S and COSz. Consequently, the stability criteria formulated by Happel are not applicable for choosing the best catalyst for this process. The results of this investigation also show that COS is an important intermediate in the process and that the elimination of any unreacted COS can effectively be accomplished by using a rare earth oxide or oxysulfide alone as the catalyst. In this respect, the data also show that La202S is an effective catalyst for the thermal decomposition of COS and the reduction of SO, by COS. Experimental Section Materials Preparation. LaCoO3 was prepared by the 0196-432 118211221-QO38$01.25/0
methods described by Palilla (1976) and used in the form of a powder (- 150 to 250 pm). LazO# with average particle size of 10 bm and surface area of 0.3 m2/g was obtained from GTE Sylvania Inc. Chemical and Metallurgical Division, Towanda, Pa. La202Swith surface area of -30 m2/g was prepared by freeze-drying in a paste containing 2 g of La(N03)3and 14 g of high surface area carbon (Shawinigan black, -100 m2/g) in 100 mL of H20. After vacuum drying for 2 days the material was heated to 350 "C under vacuum for 4 h. The material was then slowly heated in air over a 6-h period up to a temperature of 700 "C so as to oxidize all the carbon to COP The resulting oxide was then converted to oxysulfide by reacting it with either wet H2S in N2 (Ormand and Banks, 1975), or with 10% COS and 5% SO2 in N2 at 750 "C. The L~,,$Q.~O~Ssamples were prepared in essentially the same manner. COS2was prepared by reading metallic cobalt and sulfur powders in sealed evacuated quartz ampules a t 650 "C (Kuznetsov et al., 1965). The surface area of the resulting material was varied from 0.1 m2/g to 1.0 m2/g by grinding the product and sieving into batches of different particle size. The types and concentrations of impurities that were found in these materials are not expected to strongly influence their catalytic behavior. For example, emission spectrographicanalyses of LaC0O3showed 50.05% Ni and Al, 5200 ppm Ba and Ca, and 520 ppm of Cr, Cu, Fe, Mg, Mn, Pb, and Si. No precious metals were found in any of these materials. Except for phase analysis by X-ray diffraction techniques, no compositional analyses were made. The composition of L~,,$Q.~O~Sis therefore nominal. The reactant gases (SO2, CO, Ha,C02, and CHJ v d diluents (N2and Ar) were chemically pure (>99.5% mm purity). Carbonyl sulfide was 97.5% minimum purity. All gases were obtained from commercial suppliers. Experimental Equipment. Flow reactors were employed for catalyst evaluation. They were all fabricated from quartz and heated by electric resistance furnaces. In order to eliminate the catalytic effects of platinum or rhodium, the thermocouples were located on the outside surface of the reactors. The reactor shown in Figure l a allowed the quenching of samples from 650 to 25 "C. The design facilitated the controlled atmospheric transfer of samples to either the electron or X-ray diffractometer. Figure l b shows the design of a two stage reactor in which sequential gas chromatographic analyses can be obtained
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0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 3Q
Table I. Details of Chromatographic Conditions for Analyses of Effluent Gases column A column B COS, SO,,H,S, gas analyzed CO,N, column packing
Molecular Sieve 5A packing size S O / l O O mesh column 6 ft x in. SS helium flow rate, 20
FURNACE SAMPLE ON QUARTZ WOOL
cm3/min
column
temperature
14
@
VALVES VALVEDSAMPLING PORTS 10
Figure 1. A, Reactor used for quenching experiments; B, two-stage reactor for mechanism studies.
R
TO L I h $ C * R O Y * T c a R U *
COI
=7
Figure 2. Experimental flow diagram.
from input and output sections of each stage. The schematic flow diagram of the experiments is shown in Figure 2. Tylan linear mass flow controllers were used to meter and control the gas flows. A Hewlett-Packard gas chromatograph Model 5710A equipped with a thermocouple and flame detector was used for gas analyses. Details of the chromatographic columns and operating conditions are listed in Table I. In all cases, premixed gases were used as calibration standards. X-ray and electron diffractometers equipped with provisions for the introduction of inert gases over the sample were used for structural identifications of catalytic species. Thermodynamic calculations were accomplished by the use of the NASA Chemical Equilibrium Computer Program (Gordon and McBride, 1976). Results A. Identification of Carbonyl Sulfide as the Intermediate in CO-S02 Reactions. Experimental. Samples of -0.3 m2/g LaCo03were used to catalyze the reduction of SO2 to S2 from a stream containing only 20% CO, 10% SO2,and He. The gas hourly space velocity (ghsv = volume of gas over volume of catalyst in one hour) was 3200 ghsv. After functioning for 24 h with a 98% removal of the SO2, the samples were quenched from 650 "C to room temperature. Analyses of the catalytic material by the methods of X-ray and electron diffraction revealed that La202Sand Cos2are the major phases with substantially smaller amounts of Co& and C Q 8 present. Other samples of LaCoO3were subjected to a gas stream containing 7.3% SO2 in CHI, H2, H20, COz, 02,CO, and N2 with a ghsv of 2000 and reaction temperature of 650 OC. These conditions are representative of those found in the exhaust
COO.N, Porapak QS
80/1UO mesh 6 ft x in. glass
50
90 "C
90 "C
of a certain coal gasifier. Analyses of the decomposed catalysts again revealed Cos2 and La202Sas the major phases present. At this point of the investigation, a series of SO2-CO reactions were investigated using Cosz and La202Sas catalysts, individually and as mixtures. The results described below permit the assignment of distinct roles to both Cos2 and La202S. A 0.2 mL volume of Cos2 (0.46 g) with a surface area of 0.25 m2/g was reacted at 650 OC with a gas stream containing 10% SO2, 20% CO, and 70% N2 with a gas hourly space velocity of 15000 ghsv. Under these conditions, a significant amount of carbonyl sulfide was formed during the early stages of the reaction (
