Abstract:
A process for optimizing the removal of nitrogen oxide (NO x ) and sulfur oxide (SO X ) from flue gases is provided in which the flue gases pass over a lanthanide-oxygen-sulfur catalyst. The catalyst has active sites provided on its surface which promote the dissociation of NO X  and receive and entrap oxygen released during the dissociation of the NO X . While the flue gases pass over the catalyst, a reducing gas contacts the catalyst to reduce the oxygen on the active sites of the catalyst and permit the catalyst to continue to promote the dissociation of the NO x  in the flue gas. If the flue gases contain SO X , they are then passed over a solid solution having a solvent of a first lanthanide oxide compound which crystallizes in the fluorite habit and a solute of at least one altervalent oxide of a second lanthanide. The SO X  in the flue gases reacts with the solid solution to form a sulfated lanthanide oxide which is removed from the flue gases. The sulfated lanthanide oxide may then be dissociated by raising its temperature to regenerate the lanthanide oxide.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This application is a continuation-in-part of copending application, U.S. Ser. No. 290,392, filed Dec. 29, 1988, now abandoned, which was a continuation in part of patent application 100,291 filed Sep. 23, 1987 now U.S. Pat. No. 4,885,145 which was a continuation-in-part of application Ser. No. 846,272 filed Mar. 31, 1986, now U.S. Pat. No. 4,714,598, which was a division of application Ser. No. 718,989 filed Apr. 2, 1985 now U.S. Pat. No. 4,604,268, which was a continuation-in-part of application Ser. No. 521,751 filed Aug. 8, 1983, now U.S. Pat. No. 4,507,149 which was a continuation-in-part of application Ser. No. 471,773 filed Mar. 3, 1988, now abandoned which was a continuation of application Ser. No. 174,024 filed Jul. 31, 1980 now U.S. Pat. No. 4,397,683. 
    
    
     This invention relates to a process for optimizing the removal of the oxides of nitrogen (NO x ) from gases created by the combustion of carbon and hydrocarbons. This invention further relates to a process for optimizing the removal of both NO x  and oxides of sulfur (SO x ) from gases created by the combustion of sulfur containing carbon and hydrocarbons. 
     Description of the Prior Art 
     Processes for the simultaneous removal of NO x  and SO x  from gases are known. U.S. Pat. No. 4,251,496 to Longo describes a combination process which uses cerium (one of the lanthanides) oxide for the simultaneous removal of both NO x  and SO x  from oxidizing gaseous mixtures in the presence of ammonia at temperatures ranging from 500° C. to 700° C. The process for removing both NO x  and SO x  may be conducted in one reaction zone or in a plurality of zones at temperatures of 500° C. to 700° C. FIG. 1 herein is prepared from the data in Table II of Longo. FIG. 1 shows that maximum SO x  removal is achieved when there is minimum NO x  removal and maximum NO x  removal is achieved when there is minimum SO x  removal. 
     Stelman, D., et. al., &#34;Simultaneous Removal of NO x , SO x , and Particulates From Flue Gas By A Moving Bed Of Copper Oxide&#34; U.S. Department of Energy, Pittsburgh Energy Technology Center, Contract DE-AC22-83PC60262 discloses a process for the simultaneous removal of NO x  and SO x . FIG. 12 from this report shows minimum NO x  removal when there is maximum SO x  removal and minimum SO x  removal when there is maximum NO x  removal Stelman uses copper suliate as a catalyst for NO x  and SO x  removal rather than a lanthanide based compound. 
     In addition, other prior art references describe methods of NO x  removal utilizing cerium oxide without the simultaneous removal of SO x  U.S. Pat. No. 4,115,516 to Takami et. al. describes a method for removing NO x  from compressed exhaust gas from a pressurized absorption type nitric acid plant without the precipitation of ammonium nitrate in the piping system of the process. Two of the catalysts identified as being capable of achieving these objectives are cerium oxide and cerium sulfate. However, under the conditions of the described process, the cerium sulfate is calcined to form cerium oxide before it functions as a catalyst. Takami et. al. does not indicate whether cerium oxide or calcined cerium sulfate is superior in performance as a catalyst for NO x  reduction. The gases from which Takami et. al. removes NO x  do not contain SO x . 
     U.S. Pat. No. 3,885,019 to Matsushita et. al. describes a process whereby the oxides of nitrogen in an exhaust gas are reductively decomposed over a catalyst of cerium oxide or vanadium oxide in the presence of ammonia. The cerium oxide catalyst is created by the calcination of cerium nitrate, cerium chloride, cerium sulfate, cerium ammonium nitrate or mixtures thereof. The ability of cerium oxide (CeO 2 ) alone to catalyze the reduction of NO x  by ammonia (NH 3 ) is described in Table 1 of Matsushita et. al. which shows an average reduction of NO x  of 60.8% over a temperature range from 320° C. to 440° C. Reduction as high as 95.8% of NO x  was achieved when the support on which the CeO 2  was deposited was subjected to exposure to various mineral acids before the CeO 2  was deposited on the support. The ratio of NH 3  to NO x  used for the reduction of NO x  was 1.5, which permits ammonia slip of such a magnitude that it could constitute an environmental hazard. 
     Lanthanides other than cerium have been used as catalysts. Scherzer, Julius, &#34;Rare Earths in Cracking Catalysts&#34;, Rare Earths, Extraction, Preparation and Applications, The Minerals, Metals &amp; Materials Society, 1988, describes the use of lanthanum oxide catalysts in preference to cerium oxide catalysts for cracking and hydrocracking during the manufacture of gasoline. K. Baron, A. H. Wu and L. D. Krenzke, &#34;The Origin and Control of SO x  Emissions from FCC Unit Regenerations&#34;, Symposium on Advances in Catalytic Cracking, American Chemical Society, Aug. 28-Sep. 2, 1983,  discusses the use of lanthanum oxide as a &#34;SO X  gettering&#34; catalyst in Fluid Bed Catalytic Crackers in oil refineries. 
     Church, M. L., et. al., &#34;Catalyst Formulations 1960 to Present&#34; SAE Technical Paper Series, Paper 890815, Presented Int. Cong. &amp; Exposition, Feb. 27-Mar. 3, 1989 describes the fundamentals of catalytic action. Church uses a noble metal as the catalyst for the NO x  reduction 
     Hardee, J. R. et. al., &#34;Nitric Oxide Reduction By Methane Over Rh/Al 2  O 3  Catalysts&#34;, 86 Journal of Catalysis, pages 137-146 (1984) discloses the use of reducing gases other than ammonia for the reduction of NO x . 
     SUMMARY OF THE INVENTION 
     In the prior art described above the simplistic chemical equation for the catalytic reduction of NO x  can be written: 
     
         6NO(g)+4NH.sub.3 (g)=5N.sub.2 (g)+6H.sub.2 O               (1) 
    
     However, at any temperature at which the catalytic reduction of NO x  is conducted, thermodynamic calculations indicate nearly complete dissociation of both NH 3  and NO which is the major component of NO x . Reaction (1) is kinetically controlled and can be described according to the concepts of Church, M. L., et. al. in the reference above. 
     On the basis of this concept, it can be inferred that the active sites on the catalyst are promoting the dissociation of NO (the major component of NO x ). However in the process, the active site on the catalyst responsible for promoting the dissociation is rendered inactive because the oxygen released is chemisorbed on the active site. Furthermore, when the oxygen formed by dissociation of NO is chemisorbed on the catalyst, the hydrogen released by the dissociation of the ammonia reacts preferentially with the oxygen on the catalyst instead of the oxygen in the flue gas clearing the active site so that it can against promote the dissociation of NO. 
     If, after the dissociation of the NO, the active site is rendered inactive because of the nitrogen released by the dissociation was chemisorbed on the active site, the hydrogen from the dissociation of ammonia would have to combine with the nitrogen to form NH 3  (which is the most chemically stable of the combinations of nitrogen and hydrogen). However, it has been stated above that NH 3  dissociates at the temperatures at which the catalytic reduction of NO x  occurs, therefore, the element making the active site inoperative has to be oxygen. 
     In accordance with this invention, there is provided a process which optimizes the catalytic dissociation of NO x  and provides a reducing gas which removes the oxygen from the active sites on the catalyst. This permits the catalyst to continue to function as a promoter of the dissociation of NO x   
     In this process, the lanthanide containing catalyst used for NO x  removal is a lanthanide oxygen sulfur compound whose ability to remove SO x  from the gaseous mixtures has been diminished from its maximum. The temperatures of the gaseous mixtures from which the NO x  is to be removed are controlled according to their SO 2  content to prevent the dissociation of the lanthanide-oxygen-sulfur compound either entirely or in part to a lanthanide-oxygen compound which is a less effective catalyst for the NO x  dissociation. Preferably, the removal of NO x  is carried out in one site and the removal of SO x  is carried out at a separate site. In such a process, the NO x  is dissociated first and the SO x  is removed later. Cerium oxide (CeO 2 ) or solid solutions of CeO 2  and other altervalent oxides which contain oxygen ion vacancies are used for the removal of SO x  from gases created by the combustion of carbon and hydrocarbons which contain sulfur. These solid solutions increase the rate and extent of SO x  removal from flue gas. 
     A further preferred embodiment of this invention uses a lanthanide-oxygen-sulfur compound whose dissociation temperature is higher than that of the cerium-oxygen-sulfur [Ce 2  (SO 4 ) 3  ] when the composition and temperature of the gases from which NO x  is to be removed are low in SO 2  and high in temperature. 
     A further preferred embodiment of this invention places the lanthanide-oxygen sulfur catalyst necessary to promote the dissociation of NO x  on a substrate, such as a pellet or zeolite, by means of an aqueous/liquid solution of the lanthanide-oxygen sulfur compound rather than to form a lanthanide-oxygen-sulfur compound from a lanthanide-oxygen precursor which is exposed to gases containing SO 2  to create the lanthanide-oxygen-sulfur compound. 
     A further preferred embodiment of this invention is when the lanthanide oxygen-sulfur catalyst of this invention becomes less effective or inoperative to promote the dissociation of NO x , the substrate being used can be recoated with another coating of the aqueous/liquid solution of the lanthanide-oxygen-sulfur compound. When the coating is dried, the catalytic action has been restored. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is prepared from the data in Table II of U.S. Pat. No. 4,251,496 which shows that maximum NO x  reduction is achieved with cerium-oxygen-sulfur compounds only when SO x  removal is greatly reduced from it maximum value. 
     FIG. 2 shows that maximum NO x  reduction is achieved with CuSO 4  only when SO x  removal is greatly reduced from its maximum value. 
     FIG. 3 shows the amount of SO 2  in a flue gas containing 3.7% O 2  in equilibrium with cerium sulfate Ce 2  (SO 4 ) 3  at various temperatures. 
     FIG. 4 is a schematic drawing showing a reactor which may be utilized to carry out the process of the present invention. 
     FIG. 5 is a schematic diagram showing the process of the present invention used for removing NO X  from gases created in a boiler or in an internal combustion engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to the removal of NO x  from oxygen containing gases resulting from the combustion of carbon and hydrocarbons, which may or may not contain sulfur, that are known generically as &#34;flue gases&#34; or &#34;exhaust gases&#34;. The terms &#34;flue gases&#34; and &#34;exhaust gases&#34; will be used hereafter to describe such gases. 
     In order to optimize the removal of NO x  and SO x  from flue gases which contain SO 2 , NO x  and O 2  resulting from the combustion of carbon and hydrocarbons, the lanthanide-oxygen-sulfur that functions as a catalyst for the dissociation of NO x  is no longer capable of achieving maximum SO 2  removal FIG. 1, which is compiled from Table II of U.S. Pat. No. 4,251,496, shows that in a process for the simultaneous removal of NO x  and SO x , the maximum reduction of NO x  (98%) was achieved after the ability of CeO 2  used for the removal of SO 2  had dropped from a high of 99% SO 2  to 76.6%. Table II in Longo further shows a maximum of 97.1% conversion of CeO 2  to Ce 2  (SO 4 ) 3  at 89 minutes into the run and maximum NO x  removal came after 155 minutes when SO 2  removal was only 76.6%. The gases represented in FIG. 1 originally contained 3000 ppm SO 2  and 225 ppm NO x . The test represented in FIG. 1 was conducted at 600° C. with a ratio of NH 3  /SO 2  of 2/1. 
     The same relationship between SO x  and NO x  removal is shown in FIG. 2 representing the removal of SO x  from flue gas and the catalytic dissociation of NO x  with ammonia after the copper oxide is no longer capable of maximum removal of SO 2  The fact that both the sulfates of cerium and copper can catalyze the dissociation of NO x  is evidence that other sulfates should be equally capable of catalyzing the reduction of NO x . 
     The stability of the lanthanide-oxygen-sulfur compound which controls the extent of NO x  dissociation is in turn from which the NO x  is to be removed. FIG. 3 presents the results of equilibrium calculations which describe the concentration of SO 2  in a flue gas containing 3.5% O 2  that is required to prevent the dissociation of Ce 2  (SO 4 ) 3  as a function of temperature. When the integrity of the Ce 2  (SO 4 ) 3  has been preserved, it is then capable of achieving maximum NO x  removal as shown in FIG. 1. 
     Maximum NO x  dissociation at any temperature and particularly at higher temperatures is achieved when the NO x  is removed from the flue gases first and any SO 2  in the gas serves to prevent the dissociation of the lanthanide-oxygen-sulfur compounds which serves as a catalyst for the dissociation of NO x   
     When NO x  removal from flue gases is required at high temperatures from gases which do not contain sufficient SO 2  to prevent the dissociation of the cerium-oxygen sulfur compound necessary to catalyze the dissociation of NO x  with a reducing gas, a lanthanide-oxygen-sulfur compound must be used which has a higher dissociation temperature than Ce 2  (SO 4 ) 3 . 
     The reduction of oxygen from the active sites of the catalyst which permits the continuation of the dissociation of NO x  can be achieved with reducing gases such as H 2  which results from the dissociation of gases rich in hydrogen such as ammonia (NH 3 ) and methane (CH 4 ) These reducing gases may be separately added to the flue gases or, if the gas containing carbon and hydrocarbons is burned in an appropriate manner, may be a constituent of the flue gases. 
     The lanthanide oxides used for the removal of SO 2  crystallize in the fluorite habit. When altervalent anions of other lanthanide oxides or oxides of the alkaline earth element are in solid solution in lanthanide oxides which crystallize in the fluorite habit, oxygen ion vacancies are created in the solid solution which enhance its ability to remove SO 2  from flue gases. 
     It is well known to those skilled in the art that pellets, granules or coatings on substrates may be damaged or destroyed if the pellets, granules or coatings on a substrate undergo a change in composition, density, crystal structure, or size of the crystal lattice between the compounds before and after reaction. As an example, if cerium oxide (CeO 2 ) is used as a precursor to create pellets, granules or coatings of cerium sulfate (Ce 2  (SO 4 ) 3  ) on a substrate, the CeO 2  deposited on the substrate has a density of 7.123 g/cc and its crystal habit is fluorite. However, Ce 2  (SO 4 ) 3  has a density of 3.192 g/cc and its crystal habit is either monoclinic or rhombic. Therefore, the integrity of a coating of Ce 2  (SO 4 ) 3  on a substrate created by applying Ce 2  (SO 4 ) 3  from an aqueous/liquid solution of the compound to the substrate should be better than a coating of Ce 2  (SO 4 ) 3  on the substrate created by coating the substrate with CeO 2  and exposing it to gases containing SO 2  and O 2  which would convert it to Ce 2  (SO 4 ) 3 . 
     Cerium has been used to illustrate the principles of the present invention. The use of cerium in the explanation of the present invention does not preclude the use of other lanthanides in the present invention in place of all or part of the cerium. 
     The reaction for the removal of NO x  created by the combustion of carbon and hydrocarbons which may or may not contain sulfur which is catalyzed by the lanthanide-oxygen-sulfur compounds is: 
     
         2NO(g)=N.sub.2 (g)+O.sub.2 (g)                             (2) 
    
     Thermodynamic calculations predict that NO is unstable at all temperatures below that at which it is formed. However, the kinetics of that reaction are such that the dissociation does not take place unless it is catalyzed. When properly catalyzed, reaction (2) takes place rapidly with N 2  being released into the gas stream from which it came, but the oxygen is retained on the active sites of the catalyst. Therefore, a reducing gas must be added to the gas stream which is capable of reducing the oxygen on the active sites of the catalyst so those sites are again operative in the promotion of the dissociation of NO x . Ammonia has been the common source of the hydrogen used for the selective catalytic reduction of NO x  because it dissociates almost completely at the low temperatures conventionally used for the selective catalytic reduction of NO x . Because the oxygen on the active sites of the catalyst shares bonds with the catalyst, it is more easily reduced by the hydrogen than the oxygen that is in the flue gas. 
     Although ammonia has been the preferred source of hydrogen for the reduction of oxygen on the active sites of catalysts, hydrocarbons which dissociate at the temperatures at which catalytic reduction of NO x  takes place are available. The data presented in FIG. 1 was obtained at a temperature of 600° C. (1112° F.) At that temperature, 98% NO x  removal was achieved from simulated flue gases whose SO 2  concentration was 0.03% or 3000 ppm. The data in FIG. 3 indicates that the catalyst, Ce 2  (SO 4 ) 3  , will not dissociate under these conditions. Typically, catalysts for NO x  reduction have operated at temperatures between 304° C. and 398° C. (580° F. and 750° F.) because of the possibility of physical and chemical changes to the catalyst that could reduce its ability to promote the dissociation of NO x  if operated outside of that temperature range. Equilibrium calculations indicate the composition of gases resulting from the dissociation of methane are: (1) at  1200° F. 75% of the methane would have dissociated, and the hydrogen content of the resulting gases would be 50%; and (2) at 580° F. (the lowest operating temperature of conventional catalysts) there would only be 66% dissociation of the NH 3  and the hydrogen content of the resulting should increase with increasing temperature. 
     Although the ammonia may be preferred as a reductant for the oxygen remaining on the catalyst because it dissociates more completely, the cost of an equal amount of H 2  from dissociation of CH 4  would be lower. 
     When the source of hydrocarbon is coal, the typical composition of the gases resulting from its combustion in the boiler of a power plant are: CO 2  13.21%, H 2  O 9.21%, N 2  73.48%, SO 2  0.35%, O 2  3.74%, HCl 0.01%, and NO x  0.05%. When the hydrocarbon is natural gas or methane the typical analysis of the flue gas resulting is: NO x  96 ppm, CO 100 ppm, CO 2  8.15%, O 2  0.63%, H 2  O 16.3%, N 2  72.56%, and SO 2  2 to 3 ppm. All of the catalysts for the reduction of NO x  must operate within this kind of chemical environment. 
     In many instances, the catalysts used for the dissociation of NO x  are based on either the oxides of the metals or the sulfates of the metals. The curves in FIGS. 1 and 2 show that maximum NO x  dissociation is achieved when the catalyst is a sulfate of the metal. Therefore the reaction of the catalyst with the SO 2  of the gases and the dissociation temperature of the resulting metal sulfate is of extreme importance. As an example the equation for the formation of Ce 2  (SO 4 ) 3  from a flue gas containing SO 2  and O 2  may be written: 
     
         1/3CeO.sub.2 (s)+SO.sub.2 (g)+1/30.sub.2 (g)=7/8Ce.sub.2 (SO.sub.4).sub.3 (s)                                                       (3) 
    
     
         ΔG°=-63000+50.0T                              (4) 
    
     Equilibrium calculations using the thermodynamic information in equation (4) can determine the amount of SO 2  required in flue gases to prevent the dissociation of Ce 2  (SO 4 ) 3 . The results of these calculations are shown graphically in FIG. 3. At any temperature between 400° C. and 800° C., the SO 2  concentration of flue gas containing a normal amount of O 2  (approximately 3-4%) must be equal to or greater than the equilibrium value shown on this curve to prevent the dissociation of Ce 2  (SO 4 ) 3 . Therefore, any attempt at simultaneous NO x  and SO x  removal of 95% or greater is impossible. This principle is illustrated in FIG. 1 which shows: 
     1. when there is maximum SO 2  removal, NO x  dissociation is limited (less than 85%) because the CeO 2  has not been converted completely to Ce 2  (SO 4 ) 3 . 
     2. maximum NO x  dissociation (approximately 98%) is attained when the CeO 2  has been almost completely converted to Ce 2  (SO 4 ) 3  which is the most effective cerium-containing catalyst, but its ability to remove SO 2  has been lowered to less than 80%. U.S. Pat. No. 4,251,496 states that the compound formed when CeO 2  is exposed to SO 2  is cerium oxysulfate, but there is no thermodynamic information to substantiate the formation of cerium oxysulfate. 
     Regeneration of the Ce 2  (SO 4 ) 3  formed by the desulfurization of flue gases is achieved by increasing the temperature and removing any SO 2  from contact with the Ce 2  (SO 4 ) 3  which permits reaction (3) to reverse with the formation of CeO 2  , SO 2  , and O 2  It has been determined experimentally that dissociation of Ce 2  (SO 4 ) 3  occurs rapidly at temperatures greater than 780° C. (1436° F.). 
     The information shown in FIG. 2 indicates that CuSO 4  is not as effective a catalyst for the dissociation of NO x  as Ce 2  (SO 4 ) 3  CuSO 4  catalyzes the dissociation of NO x  to achieve just over 90% NO x  removal compared to Ce 2  (SO 4 ) 3  which catalyzes the dissociation to achieve 98% NO x  removal. Although the CuSO 4  is not as effective a catalyst as Ce 2  (SO 4 ) 3  , the fact that both of these sulfates do catalyze the reduction of NO x  indicates that other sulfates are candidates as catalysts for NO x  removal. 
     The dissociation temperature for various sulfates and oxy-sulfates has been calculated based on thermodynamic data similar to those shown for equation (4). For these calculations the dissociation temperature has been defined as the temperature at which the pressure of the gases released by dissociation is one atmosphere. The dissociation temperature of some sulfates and oxy sulfates is shown below: 
     
                       TABLE I______________________________________DissociationCompound     Temperature °C.______________________________________La.sub.2 O.sub.2 SO.sub.4        1670Pr.sub.2 O.sub.2 SO.sub.4        1578Nd.sub.2 O.sub.2 SO.sub.4        1567Sm.sub.2 O.sub.2 SO.sub.4        1525CaSO.sub.4   1183MgSO.sub.4   1014Ce.sub.2 (SO.sub.4).sub.3         922CuSO.sub.4    650______________________________________ 
    
     Calculations performed in the same manner and with the same assumption of 3.7% O 2  composition in flue gases indicate that CuSO 4  requires more SO 2  to be in equilibrium with it to prevent dissociation than is required to keep Ce 2  (SO 4 ) 3  from dissociation. However, the higher the dissociation temperature the smaller should be the amount of SO 2  in contact with the sulfate necessary to prevent dissociation. 
     Calculations performed in the same manner and with the same assumption of 3.7% SO 2  composition in the flue gas indicate that La 2  O 2  SO 4 , which is the most likely lanthanum sulfate to form from lanthanum oxide in the presence of flue gases, would require little SO 2  in the gas to prevent dissociation. The result of these calculations are shown in TABLE II: 
     
                       TABLE II______________________________________     Amount SO.sub.2 Required     To PreventTemperature     Dissociation of La.sub.2 O.sub.2 SO.sub.4______________________________________1027° C.     p SO.sub.2 = 8.60 × 10.sup.-7 = 0.86 ppm SO.sub.2 927° C.     p SO.sub.2 = 2.16 × 10.sup.-8 = 0.02 ppm SO.sub.2 827° C.     p SO.sub.2 = 2.80 × 10.sup.-10 = 2.77 × 10.sup.-4     ppm SO.sub.2______________________________________ 
    
     Lanthanum oxide has been used as a &#34;SO x  gettering&#34; catalyst in Fluid Bed Catalytic Crackers (FCC) in oil refineries for many years and the preference for lanthanum oxide for this type of catalyst is described in the paper by Scherzer, described above. 
     When the pSO 2  necessary to prevent dissociation of Ce 2  (SO 4 ) 3  shown in FIG. 3 is compared with pSO 2  necessary to prevent the dissociation of La 2  O 2  SO 4  listed in Table II, it can be seen that it requires several orders of magnitude lower pSO 2  to prevent dissociation of La 2  O 2  SO 4  than it does to prevent the dissociation of Ce 2  (SO 4 ) 3 . Therefore, in situations where there is little or no SO 2  in the flue gases or NO x  reduction is required at high temperatures, La 2  O 2  SO 4  may be the preferred catalyst. 
     The phase stability diagram for the La-O-S system indicates that La 2  O 2  SO 4  is the most likely sulfate to form when La 2  O 3  is exposed to flue gases. 
     The rate of desulfurization of flue gases with doped and undoped cerium oxide (CeO 2 ) has been investigated. The results of this investigation are shown in Table III. For these experiments the granules of doped and undoped CeO 2  were prepared by the Marcilly technique which utilizes the formation of the sorbent from aqueous solutions of a water soluble salt of the lanthanide oxide and citric acid. The solutions are evaporated to the consistency of a thick sugar syrup and are then evaporated to dryness in a vacuum oven operating at approximately 25° C. at 25 inches of vacuum. After evaporation the dried sorbent was pyrolyzed at 400° C. to produce a material in which the dopants are in solid solution in the CeO 2 . 
     The doped and updoped CeO 2  were then exposed to synthetic flue gases containing 3000 ppm SO 2 , 3.5% O 2 , 22% CO 2  and 74% for a period of one hour. The weight gained by the sorbents is due to reaction (4) described above. The sorbents with highest rate of weight gain and the greatest weight gain are superior to the ones with lower rates of weight gain and lower total weight gain. 
     
                                           TABLE III__________________________________________________________________________CALCULATED RATE OF WEIGHT GAINAND TOTAL WEIGHT GAIN AFTER EXPOSUREOF DOPED AND UNDOPED SORBENTS TO FLUE GAS AT 550° C.        RATE OF*               TOTAL % INCREASEDOPANT   CODE        WT GAIN               WT GAIN                     RATE OF WT GAIN                                 TOTAL WT GAIN__________________________________________________________________________None     6211-8        2.5    3.0 mg                     --          --None(Duplicate)    6211-8        2.5    3.0 mg                     --          --5 m/oCaO 6211-1        4.0    4.5 mg                     60.0        50.010 m/oCaO    6211-2        4.9    5.0 mg                     96.0        66.75 m/oLa.sub.2 O.sub.3    6211-9        3.0    3.0 mg                     20.0         0.010 m/oLa.sub.2 O.sub.3    6211-4        4.2    5.0 mg                     68.0        66.75 m/oSrO 6211-5        4.2    4.5 mg                     68.0        50.010 m/oSrO    6211-6        5.6    7.5 mg                     124.0       150.0__________________________________________________________________________ *mg/min/gm Surface area of sorbents predicted to be 20 m.sup.3 /gm 
    
     Table III above clearly shows the superiority of doped CeO 2  to undoped CeO 2  for the removal of SO 2  from the flue gas streams. 
     The lanthanide-oxygen-sulfur compound to be used as a catalyst can be impregnated onto the substrate of pellets, granules, Raschig rings, honeycombs, zeolites, or other substrates known to those skilled in the art prior to their installation into ducts through which the gases from which the NO x  is to be removed pass. If and when this catalyst becomes inoperative for any reason, these substrates may be recoated with the aqueous/liquid solutions of the preferred catalyst, and their ability to catalyze the reduction of NO x  will be restored. 
     EXAMPLE I 
     A typical analysis of the flue gas from a pulverized coal fired boiler is: 3000 ppm SO 2  , 13.21% CO 2 , 3.7% O 2 , 9.2% H 2  O, 73.48% N 2 , and 500 ppm NO x . This flue gas may be exposed to Ce 2  (SO 4 ) 3  on a substrate which has been immersed in an aqueous solution containing Ce 2  (SO 4 ) 3  and subsequently dried at a temperature sufficiently low to prevent the dissociation of the Ce 2  (SO 4 ) 3 . Based on the data presented in FIG. 3, when a flue gas containing 3000 ppm SO 2  to which at least 750 ppm NH 3  has been added is exposed to the substrate containing the Ce 2  (SO 4 ) 3   catalyst at a temperature of less than 600° C., 95% reduction of NO x  to N 2  is expected. 
     EXAMPLE II 
     A typical analysis of the flue gas from a boiler fired with natural gas is: 2-3 ppm SO 2 , 14.1% CO 2 , 0.6% O 2 , 82.1% N 2 , and 100 ppm NO x  This flue gas, which would also contain 100 ppm CO, may be exposed to a La 2  O 2  SO 4  coating on a substrate. Because of difficulty of dissociation of La 2  O 2  SO 4  at temperatures as high as 1227° C., the chemical composition of the La 2  O 2  SO 4  is expected to be little changed after long time exposure to such flue gases. It is expected that the substrate catalyzes the reduction of the NO x  to N 2  as long as its composition was essentially La 2  O 2  SO 4 . 
     EXAMPLE III 
     The exhaust gases from a gasoline burning internal combustion engine can contain as much 0.70% CO, 0.22% NO x , 0.015% hydro-carbons, and 0.36% O 2 . If such a gas were passed over a La 2  O 2  SO 4  catalyst on a substrate in the exhaust system of an internal combustion engine, the promoting effect of the catalyst could cause the dissociation of the NO x  to N 2  and oxygen. The CO could reduce the oxygen on the active sites of the catalyst making it capable of continuously catalyzing the dissociation of NO x . 
     However, if insufficient reducing gases are contained in the exhaust gases of the internal combustion engine because of previous catalytic reduction of the reducing agents or the operating parameters of the engine have been controlled to preclude the formation of sufficient amount of reducing gases, additional reducing gases may be added to the exhaust gases to increase their reducing power sufficiently that, when in contact with a lanthanide-oxygen-sulfur compound, the dissociation of the NO x  present in the exhaust gases achieves a NO x  level sufficient to meet present and future requirements for NO x  emissions from internal combustion engines. 
     FIG. 4 shows a schematic of a reactor which may be used with the process of the present invention. FIG. 4 shows a reactor 10 having separate NO x  removal unit 12 and SO x  removal unit 14. NO X  removal unit 12 and SO X  removal unit 14 can utilize any one of a fixed bed, moving bed or fluidized bed construction. NO x  removal unit 12 includes a bed of catalyst 16 over which the flue gases pass. If needed, a reducing gas can also be introduced to NO x  removal unit 12. The flue gases which pass over catalyst 16 are introduced into SO x  removal unit 14 where they pass over lanthanide oxide bed 18. The solid solution of the lanthanide oxide containing altervalent oxides which crystallize in the fluorite habit in lanthanide oxide bed 18 reacts with the flue gases to form a sulfated lanthanide oxide compound. The sulfated lanthanide oxide is regenerated to lanthanide oxide in regeneration unit 20. 
     FIG. 5 is a schematic of the process for removing NO x  from gases created either from a boiler of a power plant or from an internal combustion engine. When the stack gases 30 contain enough reducing gas to react with the oxygen that accumulates on the active sites of the lanthanum-oxygen sulfur catalyst in NO X  removal unit 32, no additional reducing gas is required. 
     When the stack gases 30 do not contain enough reducing gas to remove sufficient NO X  to meet environmental requirements, additional reducing gas may be added in excess of the stoichiometric amount necessary to meet the environmental requirements. The reducing gas is added to stack gases 30 at or before NO X  removal unit 32. Excess reducing gas may be removed from the gas stream in the oxidation catalysis unit 34 before the exhaust gases 36 go up the stack. 
     When there is an excess of reducing gases in the stack gases 30, NO X  is removed by the lanthanum-oxygen-sulfur catalyst in NO X  removal unit 32. The excess reducing gas is removed in oxidation catalysis unit 34. 
     The present invention can be used in an automotive exhaust control system. In the automotive exhaust system, exhaust gases containing CO, hydrocarbons, and NO x  with a 2-15% level of O 2  exit an internal combustion engine. The exhaust gases pass over a lanthanide-sulfur oxygen catalyst which removes the NO x  present in the exhaust gases. The exhaust gases then pass through a conventional CO and hydrocarbon oxidation unit. The resulting exhaust gas is low in CO, NO x  and hydrocarbons. 
     Church et. al. teaches the removal of NO x  using conventional noble metal catalysts. However, these catalysts are readily poisoned by NO and O 2  and do not function in the oxidizing atmosphere of a more efficient lean burning engine. The lanthanide sulfur-oxygen catalysts of the present invention do not encounter the drawbacks experienced by the conventional catalysts. Experimental data indicate that NO x  concentration in the exhaust gas can be decreased by greater than 90% when a stoichiometric amount of NH 3  is used with a lanthanide-sulfur oxygen catalyst in the presence of flue gases containing 4% O 2  and 10% H 2  O. 
     Various embodiments and modifications of this invention have been described in the foregoing description and examples, and further modifications are included within the scope of the invention as described by the following claims.