CA1267881A - Catalyst for decreasing the content of nitrogen oxides in flue gases - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A catalyst for decreasing the content of nitrogen oxides in flue gases. The catalyst contains at least one of the metals titanium, zirconium, vanadium, tungsten, molybdenum, or cerium in the form of one or more of their oxides combined with a silicate with a three-layer structure (three-layer silicate) selected from the group consisting of illite and mica. The atomic ratio of the silicon in the three-layer silicate to the metal in the oxide is from 0.2 and 50 and preferably from 0.4 to 25.

Description

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BACKGROUND OF 'rHE INVENTION

Field of the Invention The invention concerns a catalyst for decreasing the content of nitrogen oxides in flue gases.
.

ackground Information Nitrogen oxides (NOx) are generated both from the nitrogenous constituents of the fuel and from the nitrogen in the air when fossil fuels are burned. The oxides enter the atmosphere and become extremely detrimental to the environment.
I~t is known that nitrogen oxides can be converted into N2 and H20 by NH3 and~ that the reaction is fairly selective over a wide range of temperatures, meaning that, since it proceeds in the presence of a high excess of oxygen (as is usual in flue gases) without excessive loss of ammonia as the result of oxidation, only relatively small amounts of reductants are necessary. Various catalysts for reducing NOx with ammonia are also known~
German AS 2 ~10 175, for example, discloses catalysts of this type that consist of oxides of vanadium molybdenum, and/or tungsten. the stoichiometry is V12 x yMoxWy, wherein O < x < 8.0 < y < 5 and 0.3 <
(x + y) < 8.

Furthermore, German Patent 2 458 888 discloses a method of reductively decomposing nitrogen oxides in flue gases. A mixture of gases containing nltrogen oxides, molecular oxygen, and ammonia is contacted with a catalyst composition that contains ~A) titanium in the form of oxides

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in an intlmate mixture with (B) iron or vanadium in the form of oxides~as its essential constituents.
The drawback to these catalyst~s is that th~
catalytically active constituents that they contain in the form of relatively expensive transition'metals are exploited only to a~low degree because they are not optimally distrlbuted~.~Although th~e active constltuents are extended by solid inert carrier~s,~ whi`ch does make them more~
economlcaL,~ the dilution~with inert material entails the - : :
r~sk of extensively decre~as~ng thelr c'at~alytic~activlty.
Another drawback~to these ~cata;lysts~is that~they al~so catalyze the SO2 t~hat lS often~conta~lned;in the flue gas into SO3,~whlch~,can lead~for~examp~le to deposits of~salts in the downstream~equipment of~the;~system. ;~
German OS 3 438~367~also discloses a~catalyst for decreaslng~,the content~of~nltrogen~oxide~ln~flue gases by selective reduction. Th~ cat~alys~t cons~ists~of~(~A)~80~ to 95 %
by~weight~of a~catalytic oxid;e~containing~a'~sulfur oxide and ls obtalnablé~among~other;me~thods by~heat treating an ~
aqueous' oxide compound~of tit~anium~or silicon, (B) O~to 5 %
by welght of~a~ca~talytic oxide~th~at contains vanadium~oxide, and~(Ci 1 to 15 % by weight~of~ a~cat~alytlc~oxide~ like~
tungsten oxlde.
Cons;ldered essentl~al~;~to this~ cata~lys~t is the~
formatlon of~a solld~acid~composed~o~S~lO2~and~Ti~O2, lts ~
acidity modifiéd~by~treatment,~with~;sulfuric~acid or~ammonium s~ulfate.~The~d~s~trlbu~tlon~of~the~so~lld~acld ~s consldèred~as~ ~ ;
representlng the~s~tanda,rd~for~contralling~the~adsorptlon of~
N~H3 at the~sur;Eace~of;~the cata~lyst~and henc~e~for~ improvlng its catalytic a`ct~ivity.

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The SiO2 is employed in the form of a silica sol, It is known that silica gels distinguished both by high BET
surfaces~and by high porosity can be obtained from SiO2 sols, although the percentage of macropor~es is low, which has a del'eterious effect on material transport and hence on catalytic activity.
German O~S~2 748~4~71,~finally,~dlscloses a,ca*alyst compositi~on to~be employed in~the vapor-phase reduction of nitrogen oxides w1th~ammonla,~espec~ia1~1y~to~reduce~the~
con~tent of~nitrogen oxides in~flue~ gas~es.~'~This~catalyst conslsts~essentially of~an~ox~ide~or~sul~fate~ of~one of~the~
metals copper~, vanadlum,~chromlum,;~molybdenum, tungsten, manganese~ iron, or cerium~ on~;a shaped~carrier that contains titanium~;oxide and~a~minor~port~ion~of~a~cl~ay~minera~l with a mean partlcle size of 0.1 to 100 ~m.~Clay minerals, inter al~ia~,~of~`the~th~ree-layer~t,y'pe ~(thre~e-lay~er~;~sillcatés~)~, can be~employed~ U~p to 15~;%;by w~ei'ght of~these;~clay mlnerals is c~laimed to~lncrease only the~st'a~bi~ ty~-o~~the catalys~t;~. The additives~have no~slgnlficaAt~e~ffect~oA~ca'~talyt~lc ~act,lvi~ty~
at these~levels~and larger,~a~o~un,ts~ev~en have a~negativ~e effect in~thls~,respect. Due~ to thelr~chemical compos1t'ion, they~also~dmcrease the~catalyst's~ré~mistanc~to 1ue~gas~es t h a t con~aln~SO~

SUMMA~Y OP,TH~
It~has~been~dlscover`e~d~that~ te~or'~mica t at;~
teract~'syn~erglstlcally with~othqr catalytlc'cons~tit'uents can ba~emp~loyèd~to~obtaln~cata~lysts~of~the~ qformsald geAus wlth lmproved~ activity.~The~'lat~ter~catalysts~can~be~ employed n tura to extenslvely~d;ecr~éase,the~cont~eAt~of~nitrogeA~

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~2G7881 oxide in flue gases, while simultaneously exploiting the expensive oxidic constituents of the catalyst and the reductants. In spite of thelr~ relatively high alkali metal content these catalysts have a high~resistance to S0x.
Illite and mlca belong to the three-layer silicates.
The object of the invention is accordingly to provide s~ca~talyst for decreaslng the~content of nitrogen ; oxldes in flue~gases~ and~;contalnlng~at~least~one of the metals~tltanlum, zirconlu~m~, vanadium,~tungsten,~molybdenum, or~cerium;~in~the~form~of one~or more of~the~i;r ox~;ides combined wlth a sllica;te~with~a three-layer~structure (three-laye~silicate).
The~;ca~talyst~ls~cha~racterized~in that a)~ the~three-l~ay~er~si~l~icat~e~ s~;~or~comprise~s:~a~
member of~th~è ~roup consisting~o~f illi~te~and~mica.
b~ the~abomlc~ratio~o*~the~sllicon tn~the~acld-act~ivated~th~ree-la~yer sill~càte~to~the~met~al~s) in the;~
oxide(s) ls~from 0.2 and~S0 and preferably from~0.4 t~o~25.

DETAILED DESCRIPTION OF THE INVENTION:
Although the illi*e~ or~mica~are not specially ~
treated, there~is obviously~a~syner~giistic~interactLon~with t`he~metal oxides. Although~no~unambiguous;~mechanl~sti e~xplanatlon of~this~synerglsm` n~ provlded~ at~ he present,~ th~e-p~a~r~t~icu~larly f;~avora~ble~ si~licate~;la~er~str~u ture o;f~the~llllte or mlcà;~would~ ~t be~
Thè~catalys in acco c~e 1th the~ mventl ~are;~
also outstanding~;ln~their~re~sls~tance~to~sul~fur~oxidès and sulfuric aci~d~althoug~h~the~ alkali~mé~tal~content; i~not~
redùced by an~ac~ld trea~ment~of~th~ lLllte or~mica.

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The starting compounds employed for the metal-oxide constituents of the catalyst in accordance with the invention are on the one hand the correspond~ing metal oxldes and on the other the suhstances that can be converted into the metal oxides, e.g. the metals and hydroxides and especially salts, complex compounds, and/or oxygen acids or salt5 derived from the last. They can be emp~loyed if necessary in conjunction with an addltive~that functions as a reductant and/or complexing agent.~
Cer;lum can~ for ex~ample be employed~;in the for~m of 3, Ceo2~ ce~so4~)2~and;ce2~(c2o4~3~ Appropriat~e;
star~ting~materlals for zirconium oxlde are~ in additlon~ to the oxidé~hyd~r~ates, for~example, the zlrconium and zlrconyl salt 9 llke Zr~504)2, Zrcl4~ zrocl2~and~zr~c2o4)2-Approprlate starti~ng~substan~ces~for~the~tungstenconst~ituten~ts are, ;for~example,~ tungsten~oxlde.s~like~W03, W10029, W40~ and W02 and~mono-~and~polytungstic acid~s, ;heteropoly;acids, tungstates,~and~tun~gstlc~halides~ and~
oxyhalides.~Molybdenum compol~nds can ~also~ be~employed~lnstead of the~analogous tungsten~compounds.
kppropriate vanadi~um starting compound9 include V205, V02, V203, ~and~ VO along;~wlth~ortho- and~polyvanadic~ acids or~
vanadates,;vanadium hali~des~and oxyhal~ldes~lik~e VOC13~, for example,~and~various~vanadium~or~ vsnsdyl~salts. ~
Approprlate~;tltanlum~compounds~are, ln~addltion ~to the oxldes and~oxlde~hyd~ra~tes,~the~tltan~lum and~tltanyl ~
salts, expecl~ally~the~halides~and~sul~fates~ Although ~tit;anyl sulfate is preferable~from the~ec~onom~ic~polnt of~vlew,~ metal~
organic oompounds llke~tltsna;tes, lsopropyl~tltsnate~for exa~ple,~can also~be employéd.~

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It has been discovered that expecially pr~ctical results can be achieved when the metal oxides are individually present in the following ranges of concentration (by weight):

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TiO2 =10 - 80 Gew.-%
W03~and/or MoO3 ~ 25 Gew.-%
; ~V25~ ~ =0,~1 - 25 Gew.-%
CeO2 ~ 25~ Gew.

with the~ llite~or mic~a~accounting~for~,the rest of the activ~e constituents.~
The metal ox;~des~are pres~ent~in~the preeerred catalysts~ in a binary~ especially~ter~nary combination.
When~ present -n a~tern~ary~comb~Lnation, the~,me~al oxides~aré~present in a~'preferred~catalyst~in one~of the ol~10wing~per~centages~by~weight'~

a) (Ti~o2 ~ W03 and/or ~oO3 ~ V205) = 10 - 80, (b (T12~+ CeO2 +~v2os ~
cj (TlO2 +~ZrO2 + V205)~ 0 - 8~0 d) ~W03~and/~r~M03 +~Ceo2~v2 5~
e) ~wo3~and/or uoo~3 + zro2 ;+~v2os) wi~th~the~ ite~or~mlca accounting~for the~rest~oE~thè

a~ctlve~onst~ltuents.~
The ratios between~-thè~we'ights o~ the metal oxides present ln a~t~ernary~combination~in~a~ preferred~catalyst are ~ ~ ~7 783!3~L

(a) ~O3 and/or MoO3:TiO2 = 0.01 - 0.25 V2O5:TiO2 = 0.01 - 0.11 (b) CeO2:TiO2 = 0.05 - 0.23 V2OS:TiO2 = 0.01 - 0.11 : (c) ZrO2:TiO2 = 0.01 - 0.24 V2O5:TiO2 = 0.01 - 0.11 :
(d) CeO2:WO3 and/or MoO3 = 0.1 - 5.0 : V2O5:WO3 and~/or MoO3 = 0.1 - 2.5 (e) V2O5:WO3 and/or MoO3 = 0.1 - 2.5 ~ ZrO :WO3 andjor MoO3 The catalysts in ~accordance with the invention can be :~ ~: obtained, for example, by impregnating the illite or mica ~: with a solution containing one or more of the :
aforesaid metals in the form of salts and/or complex compounds and calcining it.
In another variant, the catalyst can be obtained by mechan:ically mixing the illite or mica with an oxide or salt of one or more of the aforesaid metals (by grind.ing ln a ball mill for~example),:~lmpregnating the mixture, if~
necessary,~wlth a solutlon contain1ng one or~:more of the aforesaid metals in the form of salts and/or complex compounds,~and calclning it. ~
The~catalysts 1n accordan~ce with the ~nvention can also be o~tained by precipitating~:or reprecipitating at;
: ~
~ : least one~compound containing one:or more of -the aforesaid - :

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metals in the presence of a suspension o~ the illite or mica, washing out the foreign ions, and calcining.
The compound or compounds containing one or more of the aforesaid metals can also be precipitated or reprecipitated in the presence of a mixture of suspensions of the illite or mica and of an oxide or salt of one or more of the aforesaid metals. This stage is followed by washing out the foreign ions and calcining.
The result of these procedures is an almost optimally intimate mlxture of the oxldic metal constituents with the illite or mica.
If the oxidic metal constituents consist of several metal oxides, the particular starting compounds can either be precipitated together or one after another in several stages, with the sequence of precipitation stages generally affecting the catalytic activity and needing to be optimized individually. It can of course turn out to be practical to impregnate the illite or mica, subsequent to one or more precipitaton stages, if necesssary, with a solution of a corresponding transition compound. Impregation can occur either before or after shaping and calcinlng the catalyst~
The catalyst in accordance with the invention can also contain an inert~ carrier. The catalyst is usually present in the from of molded shapes, expecially balls, tablets; extruded shapes, oblong or flat honeycombs, (called "channel grlds") platesj rods, tubes, rings, wagon wheels, or saddles.
The shapes can be obtained for exa~ple by table~ing or extruding the catalyst composition, with additives also mixed in, if necessary, to facilitate shaping. Such lZG~788~

additives include, for example, graphite and aluminum stearate. Additives to improve the surface structure can also be mixed in. These include, for example, organic substances that will burn up and leave a porous structure during the subsequent calcination.
It is not absolutely necessary to employ additives to facilltate shaping because the starting material is plastically deformable even when lntlmately mixed with the metal constituents. Neutral bentonites or other binders like kaolin or cement can, however, also be added. The material is generally shaped wi~th water or organic solvents like monovalent or polyvalent alcohols added.
The catalysts ln accordance with the invention are usually dried after being shaped, and calclned at temperatures of 200C to 700C and preferably 300C to :
550C. Inorganic fibrous materials can also be added before shaping to improve strength. Calclnation activates the catalyst, which accordingly obtains its practical properties,~ expecially if the aforesaid tsmperature ranges are malntained.
The examples hereinbelow specify typlcal procedures for manufacturing the catalysts in accordance with the invention.
Another object of the invention is the use of the catalysts io accordance with the~in~ention for reductlvely decreasing ~the conten~t of nitrogen oxide in flue gases that contain, in addition~to the usual constituènts, sulfur oxides ~50x)' whereby NH3 is employed as a reductant.
In reduclng with NH3, the content of nitrogen oxides in the flue gases lS decreased due to the formation of N2 ~Z678~
and H~O. Although nitrogen oxides (NOX) are any compound of nitrogen and oxygen like NO, N2O3, NO2, and N2O5, the most important in the present context are NO and NO2, mainly the former.
The concentration of NOX in the flue gases that are to be cleaned can vary widely, generally ranging from 100 ppm by volume to 5 % by volume. The molar ratio of NH3 to NOX is generally 0.3 to 3, preferably 0.6 to 1.5, and can be regulated by controls technology to obtain maximum NOX
conversion at the minimum possible NH3 slippage. The NH3 can be added either in the form of a gas or in aqueous solution.
The catalysts in accordance with the invention are distinguished beyond known catalysts by a very extensively selective conversion of the ammonia that is preferred for reducing the nitrogen oxides. In conventional methods, expecially at high operating temperatures, a consi.derable amount of the ammonia does not get consumed duri.ng the desired NOX removal, but oxidizes due to the oxyger1 present in the flue gas. This leads to additional nitrogen formation or decrease.s the conversion o NOX observed between the entra~ce into and exit from the reactor, leading to unnecessary consumption of NH3.
Any of the reactors employed for heterogeneous catalyzed gas-phase reactions are appropriate for the NOX
reduction if their design allows high volumetric flue-gas currents in relation to output. Permlssible space velocities are in the range of 500 to 20,000 and preferabIy 1000 and 15,000 liters of gas per hour and liters of catalyst in terms of a gas to 0 C and 1 bar~ Space ve~ocity will be designated as the dimension h 1 in what follows for the sake ~;~6~78~3~

of simplicity. Appropriate reactlon temperatures range rom approximately 200C to 600C and preferably 250 to 430 C.
If the temperatures are much higher the ammonia can be oxidized by the oxygen in the flue gas, removing the ammonia from the reaction along with the nitrogen oxides and allowing the degree of NOX reduction to drop. This undesirable effect, however, is not as powerful with the catalysts in accordance with the invention as with known catalysts.
Typical examples of the manufacture and use of the catalysts in accordance with the invention will now be specified.
The effectiveness of the catalysts with respect to eliminating nitrogen oxides from mixtures of gases that contain, among other substances, oxygen and sulfur oxides is determined by contacting the catalyst with a stream of gas flowing through a tube packed with the catalyst and electrically heated from outside. The mixture of gases i5 composed of:

2 3 % by volume H20 1 0 "
NO 750 ppm by volume NO2 SO~ppm by volume : NH3 800 ppm by volume S2 950 ppm by volume SO3 50 ppm by volume and N2 to make up 100 % by volume.

:
The concentration of NOX in the mixture was measured ~2~7~1131 before and after it traveled through the catalyst packing by an appropriated analyzer (chemoluminescence). The level of NOX converted subsequent to establishment of a stationary state and as defined by the equation cNO _ cA
NOX conversion (CNo ) x x 100 (%) NOx was selected as the measure for the effectiveness of the catalysts in reducing the nltrogen oxldes. CNO represents the concentrations of NO and NO2, and the superscripts E and A the state of the mixture of gases before and after traveling~through the catalyst.

Example 1 (a) 180 g titanyl sulfate (TiOSO4) is stirred into a suspension of 400 g dioctahedral illite h~ving a BET
surface area of 41 m2/g and the chemical composition listed in Table I. The batch is neutralized with ammonla. The solids are suctioned-off, washed free of sulfates, dried for 15 hours at 120 C, and kneaded into a solution of 10.5 g of ammonium metatungstate in water and into a solution obtained by reducing 1.3 g ammonium metavanadate with a 1.6-fold~
excess of oxalic-acid dlhydrate~. Th;e amounts of solvents were selected to ensure easy-to-knead pastes. The;dried composition lS pressed into shaped bodies which are calcined at 500 C for 5 hours. ~ ;

, Example 2 400 g potassium mica (muscovite) are reduced to an average particle size of 40 ~m by grinding. The chemical composition is listed in Table I.
The finely divided mica is impregnated with titanyl sulfate~ammonium metatungstate and ammonium metavanadate.
The dried cOmpQSition is pressed into shaped bodies which are calcined at 500 C for 5 hours.
The composition of the catalysts, the reaction temperatures and the NOX conversions at a space velocity of 6400 h are listed in Table II. The above-mentioned gas mixture was used for carrying out the reactions; the NOX
conversions were calculated according to the above-mentioned formula.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A catalyst for decreasing the content of nitrogen oxides in flue gases comprising at least one metal selected from the group consisting of titanium, zirconium, vanadium, tungsten, molybdenum and cerium in the form of one or more of their oxides and a silicate with a three-layer structure selected from the group consisting of illite and mica, the atomic ratio of the silicon in the three-layer silicate to the metal in the oxide being between 0.2 and 50.

2. A catalyst according to claim 1, wherein the atomic ratio of the silicon in the three-layer silicate to the metal in the oxide is from 0.4 to 25.

3. A catalyst according to claim 1, wherein the metal oxides are individually present in the following ranges of concentration:
TiO2: 10-80% by weight, WO3 and/or MoO3: 1-25% by weight, V2O5: 0.1-25% by weight and CeO2: 1-25% by weight with the illite or mica accounting for the rest of the active constituents.

4. A catalyst according to claim 1, wherein when present in a ternary combination, the metal oxides are present in one of the following percentages by weight:
(a) (TiO2 + WO3 + and/or MoO3 + V2O5) = 10-80 (b) (TiO2 + CeO2 + V2O5) = 10-80 (c) (TiO2 + ZrO2 + V2O5) = 10-80 (d) (WO3 and/or MoO3 + CeO2 + V2O5) = 10-25 (e) (WO3 and/or MoO3 + ZrO2 + V2O5) = 10-25 with the illite or mica accounting for the rest of the active constituents.

5. A catalyst according to claim 4, wherein the ratios between the weights of the metal oxides present in the ternary combination are:
(a) WO3 and/or MoO3: TiO2 = 0.01-0.25 V2O5:TiO2 = 0.01-0.11 (b) CeO2:TiO2 = 0.05-0.23 V2O5:TiO2 = 0.01-0.11 (c) ZrO2:TiO2 = 0.01-0.24 V2O5:TiO2 = 0.01-0.11 (d) CeO2:WO2 and/or MoO3 = 0.1-5.0 V2O5:WO3 = 0.1-2.5 and (e) V2O5:WO3 and/or MoO3 = 0.1-2.5 ZrO2:WO3 = 0.1-10

6. A catalyst according to claim 1, wherein the catalyst is in the form of shaped bodies, oblong or flat honeycomb, plates, rods, tubes, rings, wagon wheels or saddles.

7. A catalyst according to claim 1, wherein the catalyst is produced by impregnating the illite or mica with a solution containing one or more of the metals in the form of salts and/or complex compounds and conducting calcination.

8. A catalyst according to claim 1, wherein the catalyst is produced by mechanically mixing the illite or mica with an oxide or salt of one or more of the metals, impregnating the resultant mixture with a solution containing one or more of the metals in the form of salts and/or complex compounds, and conducting calcination.

9. A catalyst according to claim 1, wherein the catalyst is produced by precipitating or reprecipitating at least one compound containing one or more of the metals in the presence of a suspension of the illite or mica, washing out the foreign ions, and calcining.

10. A catalyst according to claim 1, wherein the catalyst is produced by preciptitating or reprecipitating at least one compound containing one or more of the metals in the presence of a mixture of suspensions of the illite or mica and of an oxide or salt of one or more of the metals, washing out the foreign ions, and calcining.