Vol. 17 No. 6 CHINESE JOURNAL OF CHEMlSTRY 1999

Catalytic reduction of NO, by CO on hydro- talcites-derived mixed oxides CoAlM and MgAlM

( M = C r , Mn, Fe, Co, Ni, CU)

Abstrsd Twoseriesofn~ixedoxides, GIAlMandMgAlM(M=G, Mn, Fe, GI, Ni, Cu), wereprepand by calcining their corresponding hyhtalcite-like compounds (m). The ratio of Mg: Al: M (or Co: Al: M ) was 3 : 1 : 1. The catalytic activity of all samples for the reaction of NO + CO was investigated. The results showed that the activity of CoAlM was much higher than that of MgAlM. The shucture and the pm of Fedox were characterized by XRD and HZ-TPR. The results indicated that only MgO phase was observed after calcining MgAlM hyhtalcites, &I the transition metals became more stable. 'Ihe spinel-like phase appeand in all of GIAlM samples after the calcination, and the hansition metals were changed to be more active, and easily re- duced. The activities of three series of mixed oxides CoAlCu obtained from Merent prepadon methods, Mer-

ent ratio of Co: Al: Cu and at Merent calcination temperatures. were studied in detail for p p i n g the mecha-

nism of reaction. The abiity of adsorption of NO and CO were investigated respectively for suppo&g the mecha-

nism.

Keywords metals

Hydmtalcite-like compounds, NO reduction by CO, mixed oxides MgAlhl and GIAlM, transition

Nitrogen oxides (NO,) are environmental pollutants due to their contribution to the fomtion of acid

rain. The methods for eliminating NO, from combustion exhaust gases may be clarified into two class-

es. ( i ) lowering or preventing the formation of NO, during the combustion; ( i i) selectively reducing NO, by reaction with NH3 or another reducing gases, i . e . CO and light hydmcarbons, have been pm-

posed for this purpose. A lot of catalysts, includmg perovskite-like mixed oxides4*' and metal supported catalysts ,6*7 have been extensively studied for NO, reduction by CO. but the reaction rate were too low to

have any industrial sigrUfcance.* Recently, Kannan, Swamy and Armor reported that calcined hydmtalcites containing Co, Cu or Ni

were effective catalysts for N20 decomposition.9p10 More recently, Junk0 Oi and cowoker reporting the activity of NZO decomposition over calcined ZnAlRh-HTL catalysts and the comparison of their perfor-

m c e s with CoAlRh-HTLc and W h o , l1 but there were few papem reporting about NO reduction by

Received November 4, 1998 ; Bccepted April 22, 1999.

600 Chinese Journal of Chemistry Vol. 17 No. 6 1999

CO over calcined hydmtalcites. In this paper, we wish to report the catalytic behavior of calcined MgAlM-HTLc and CoAlM-HTL (M = Cr, Mn, Fe, Co, Ni, Cu) for NO reduction by CO. XRD, TPR, NO-TF'D and CO-TPD were performed to characterize the catalysts.

Experimental

Catalyst p r e p a w n

Hydmtalcitelike compounds (HTL) were prepared by coprecipitation under low supersaturation conditions. First, an aqueous solution of metal nitrates (0.5-1.5 moVL) was mixed with an aqueou~ solution composed of NaOH and N q C 4 (2C@2-/(2[M2+] + 3[M3+ 3 ) = 1 . 3 , Na+/N@= 1.48, molar ratio) at mom temperature, vigorous stirring and pH = 8-10. Then the resulting heavy s l q was aged at 80°C for 18 h with vigorous stirring. Finally the precipitate was filtered, then washed and dried in an air oven at 80°C overnight and calcined at 450°C in air for 6 h.

For comparing the catalytic activity, two conventional methods of p r e p t i o n were employed, a) co- precipitation method: stoichiometric amounts of cobalt nitrate, copper nitrate and aluminum nitrate were coprecipitated in solution, filtered and calcined at 450°C in air for 6 h; b ) stoichiometric amounts of CQ&, CuO and y-Al20, were mechanically mixed well, then heated at 450°C in air for 6 h.

Catalyst chwmtedwn

Powder X-ray diffraction (XRD) patterns were obtained by using a X-ray diffmtometer (type D/ MAXIIB, Japan) over the range 5" < 20 < 80" at 40 kV and 10 mA , using Cu K , radiation combined with a nickel filter.

Temperature p m m e d reduction (H2-TPR) was carried out on a homemade apparatus. 50 mg

sample was treated at 500°C in helium for an hour and cooled to room temperatux under the same atmo- sphere and then reduced by a reducing gas with a flow rate of 35 mL'min at a heating rate of 200C/min. The composition of reducing gas was 5%H2 + 95 %N2.

Temperature p'ogrammed desorption of NO or CO (NO-TPD and CO-TPD) were carried out on a homemade apparatus, 100 mg sample was treated at 500°C in helium for an hour and cooled to room tem- perature under the same atmosphere then purged with helium at a rate of 35 U m i n . Finally the sample was heated at a rate of 35"c/min in helium for recording the WD spectra. The TCD was used as the de- tector for TPD and TPR measurements.

Catalytic activity memluemer~

The NO reduction by CO was carried out in a flow reactor by feeding a gas mixture of NO (8000 ppm 1 , CO (8500 ppm) and He (balance) at a flow rate of 22.5 cm3/min over 50 mg catalyst ( W/F = 1 . 3 4 g-s. cm") . The gas composition was analyzed before and after the reaction by an on line gas chro- matography (Pye Panchromato-graph , UK) using 0.5 nm molecular sieve column for NO, N2 and 0 2 , and

LIU et al. Hydmtalcite-like compounds 60 1

Porapak Q column for N20 and C G , respectively

Results and discussion

Structure and composition o f c d y s t s

The detailed structural investigations on I s had been well done by many researchers.'2-'4 The XRD pattern of hydmtalcites of MgAl (3 : 1 ) calcined at the different temperam were investigated in de- tail. It can be found that the main transformations occur between 400 and 500"C, where the phase of H"Ls disappears and MgO begins to appear in XRD patterns. Between 500 and 600°C small variation in XRD pattern is observed. High surface area MgO and, possibly, amophous phases containing aluminum ions are formed fmrn the MgAlCQ-HT decomposition. Table 1 summaries the surface area of samples cal-

cined at different tempemhue. From Table l , it can be found that the surface area increases when the structure of €IT is decomposed, and changes little between 500 and 6OOT.

Table 1 surface ~ ~ e a s of MgAlC@-HTLcs after calcination at merent t e m p -

In the XRD patterns of the products of MgAlM-HTLcs calcining at 5009=, only MgO could be ob- served after calcination for 6 h. A 1 2 4 and other transition metal oxides are not detected. It indicates that A 1 2 4 and other transition metal oxide are highly dispersed in MgO. Due to the complexity of valence of

Mn ion, the phase of manganese oxide is observed in the case of MgAlMn . The spinel-like phase is presented in the XRD patterns of the products of calcining CoAlM-HTLcs.

But some kinds of spinel could be formed, e. g . CoM2O4, COCCQO~ or MA.lzO4 in this system, l2 it is hardly to ascertain the products of calcining CoAlM-HTLcs is a monophasic product or a mixture.

Catalytic activity for NO + CO reaction

Efect of prepa~aion metM Fig. 1 illustrates the Merent catalytic activities of the samples CoAlCu (7: 1 : 1) whose preparation methods were different. 'Ihe methods include: 1 ) hydmtalcites as precursor; 2) copmipitation method; and 3) calcination of the mixed oxides. 'Ihe results of XRD show that the phase of spinel appears in all of samples prepared from Merent methods.

The conversion of NO reaches 100% at very low temperature (about 120°C) with the catalyst which comes from hydmtalcite. The sample which comes from the mechanical mixing of the oxides performs the lowest activity for reaction.

Effect of the ratio of Co: Al: Cu Fig. 2 gives the catalytic activity of NO reduction over samples

602 Chinese Journal of Chemistry Vol. 17 No. 6 1999

with different Co: Cu : Al ratio ( 7 : 1 : 1 , 3 : 1 : 1 , 1 : 1 : 1 ) and samples are composed of two components (Co-Al,Cu-Al, Co ( o r C u ) : A l = 3 : 1 ) .

-il P

Fig. 1 Activity of samples CoAlCu ( 7 : 1 : 1) which come h m Merent method of preparation for

NO reduction by CO [ (1) : Method of hydro- talcites as precursor, (2 ) : Method of copre- cipitation, ( 3 ) : Method of sintering the mix-

ture of metal oxides j .

r - - = - * _ la, t 0

1 ...... ) f

Fig. 2 NO conversion in NO-CO reaction on catalysts with different composition [ ( 1 ) : Co: Al: Cu =7:1:1; (2 ) : CO:Al :Cu=3: l : l ; (3): C0:M: c u = 1: 1: 1 ; ( 4 ) : cu: Al = 3 : 1; ( 5 ) : Co:Al=3:1)].

From Fig. 2, it can be found that samples containing Co, Cu, Al have higher activity than those containing only Co , Al or Cu , Al . And when the concentration of Co ion increases in these samples, the catalytic activity increases. It indicates that Co ion has an important role in the reaction. The Cu ion as a

promoter plays an obvious effect on the catalytw activity.

Effect of the calcining temperatwe Fig. 3 illustrates the conversion of NO at Merent tempera- ture on CoAlCu samples (7 : 1 : 1 ) which are calcined at different temperatures.

From Fig. 6 it can be found that the two samples, which are respectively calcined at 350% and 45091, have similar catalpc activity for NO reduction by CO, and their activities are higher than othen. The activity of sample, which is calcined in 800% , is the lowest one among al l samples. It indicates that the activity decreases rapidly, with the calcination temperature increasing.

Comparison of the activities of MgALM and CoAlM Fig. 4 illustrates the conversion of NO as a function of temperature on MgAlM hydrotalcite catalysts which were calcined at 500°C.

This catalytic activity of MgAlM catalysts could be grouped into three classes: 1 ) no activity below 4OOT. This class includes two samples: MgAl and MgAlNi; 2) no activity below 300°C. This class in- cludes four samples, MgAlCr, MgAlMn , MgAlFe , and MgAlCo. These catalysts exhibit catalytic activity at 325-425°C. But the activity difference is small. 3 ) Only MgAlCu has good activity for NO reduction by CO at ZOT, the conversion of NO to N2 approaches 90% at 250%.

LlU e t a ! . Hydrotalcite-like mpunds 603

................. * 1 2 , ..................... 3 - ................. 4, ............. '/ 5*.. 4 ......

d 150 a00 250 300 350 400 '

t (-3 D

Fig. 3 Conversion of NO in NO-CO reaction over samples CoAlCu (7: 1: 1) which calcined at

Merent temperature (Temperature of calci-

nation: 1: 45O'C; 2: 3509:; 3: 6009:; 4: 8009:).

F&. 4 Conversion of NO us. reaction temperatwe in NO reduction by CO over MgAlM ( M = hansi- tion metals) ( 1 : Mg-Al-Cu; 2: Mg-Al-Mn; 3: Mg-Al-Fe; 4: Mg-Al-G; 5: MgAl-Co; 6: Mg-Al-Ni; 7: Mg-A).

Fig. 5 illustrates the catalytic activity of CoAlM ( 3 : 1 : 1 ) catalysts for the NO reduction by CO.

Fig. 5 Conversion of NO us. d o n temperature in NO reduction by CO over CoM ( M = transition metals)

(1: CeAl-G; 2: GAl-Cu; 3: GAl-Mn; 4: GAl-Fe; 5: G A l - N i ; 6: GAl).

All these catalysts consisted of three components show appreciable conversion even at about 150"c, and reached 100% conversion at 180°C. The CoAl sample shows very low activity below 200T, and reaches about 50% conversion at 250°C.

Obviously, the catalytic activity of CoAlM derived from hydrotalcites is much higher than that of

604 Chinese Journal of Chemistry Vol. 17 No. 6 1999

MgALM for the reduction of NO by CO. Fmm the above discussion, it can be concluded that the samples fmm hydmtalcite have activity for

NO reduction by CO. The activity can be attributed to the “template effect” when the hydmtalcite is used as precursor. After the samples were calcined at high temperature, it performed less activity for its sinter- ing. The difference between the activity of MgAlM and CoAlM could be explained by the results of TPR study.

H,-TF’R s t d ~ of MgAlM and CoAlM

Fig. 6 shows the H2-TPR profiles of mixed oxides derived from MgAlM hydmtalcites after He pre- treatment.

-._ * _ _ _ _ _ -_ - -. - ........... . . .

.......... Mg-AI-Fe & ~ _ _ _ _ _ _ _ _ _ _ _ - _ _ - -----------’ .............................. e

Mg-AI 2 _,_*_._.__-.-.-I- Mg-AICo .I” ‘. *.,

Mg-AI -N i

_L.._ V

..-. - _____._._ .............................................. .. 100 200 300 400 5 0 0 600 700 100 200 300 400 500 600 700

t (“C) (“C)

Fig. 6 H2-TPR profiles of mixed oxides MgAlM ( M = Cr, Mn, Fe, Co, Ni, Cu) derived h m hydmtalcites.

No reduction peaks were observed for MgAl hydrotalcite sample. Therefore, the reduction peaks oh- served for the other samples should be attributed to the reduction of transition metal oxides. The H2-TPR profiles could be classified into 3 groups: 1) ( > 700“c, ) , it includes two samples respectively derived from MgAl and M g W i hydmtalcites, 2) the reduction temperatures are at the range between 400 and 550“1:, this p u p includes four samples derived from MgAlM ( M = Cr, Mn, Fe, Co) hydmtalcites. 3) the reduction temperaturn are below 4OOT, this group includes only one sample: MgAlCu, its reduction peak appears at about 300%. It indicates that the mixed oxide derived from MgAlCu hydrotalcite is easily reduced, and it also shows the highest activi-

tY ‘ The result of TPR of samples MgAlM coincides with their catalytic activities for NO + CO reaction.

The lower the reduction temperature, the higher the catalytic activity for NO + CO reaction. It indicates that h e catalytic pmperty of samples is effected by the pmperty of redox.

Fig. 7 illustrates the TPR profiles of the series of samples dprived from CoAlM hyhtalcites .

the reduction temperature of sample is very high

LIU et al. Hydrotalcite-like compounds 605

n i!N .- - n .._

. . E CO-Al-Mn 2 : '., ,' %*

I. . . . . . . . . .......... 5

Co-a-Fe .... .____. - , . . . . . : __.-.

! .: ....... . . . . . _. Co-AI-Ni .......... %- ... 1 ........ _..- .... ...... .....

100 200 300 400 500 600 700 100 200 300 400 500 600 700

I ("C) t ("C)

Fig. 7 HZ-TF'R profiles of mixed oxides COAlM (M = G, Mn, Fe, GI, Ni, Cu) derived from hydrotalcites.

The TPR profiles of CoAlM are more complicated than these of MgAlM . In the case of CoAl sample, the reduction appearing as a bmad profile centered at about 500°C and a much larger peak on much high- er temperature, up to 800°C , corresponding to the processes : Co3+ +Co2 + , and GI2 + +Co + or Co . Af- ter the transition metals were dopped into CoAl sample, the reduction peaks of Cu ions and G ions can be obviously observed, but that of other transition metals cannot be observed because they can' t be separated from the reduction peak of GI ions. However, it can be found that the reduction temperature of Co ion obviously decreases, it indicates that Co ion is activated after the transition metals are doped into CoAl.

By comparing Fig. 6 and Fig. 7, it can be easily found that the reduction temperatures of CoAlM are far lower than that of MgAlh4 . The phase of spinel appears after calcination of CoAlM hydrotalcites . In the structure of spinel, due to the merent property of attracting electron of Co and M ions, the oxygen in the bond Co-0-M is activated, . After calcination of MgAlM-HTLcs , transition metal oxides are highly dispersed on the surface of MgO-AlZO3, and they can be regarded as transition metal oxides supported by MgO-Al203. The supported transition metal oxides become more stable. So, the series of &AlM could be reduced more easily than MgAW. It also results in that the catalytic activities of b A l M are better than that of MgAlM.

NO-TPD and CO-"I'D

Figs. 8a and 8b show the profiles of NO-TPD and CO-TPD for the samples ( Co: Cu : Al = 7 : 1 : 1 ) which come from different synthesis methods.

From Fig. 8a, it can be found that there exists large difference among the profiles of NO-TPD . The ability of NO adsorption on the sample from hydmtakite is much stronger than the others, and moreover, the desorption of NO begm to appear at lower temperatures ( < 1009:). At such a low temperature,

606 Chinese Journal of Chemistry Vol. 17 No. 6 1999

there is no desorption peak observed in the other samples. The results of analysis show that the product at Lower temperature (about 1OO"C) is nitrogen gas, following the increase of temperature, the product is NO. It indicates that not only has the sample from hydrotalcite better ability of NO adsorption, but also it can weaken the bond of N-0, whch can result in NO decomposition. Similar to the result of NO-TPD, the profiles of CO-TPD shows this sample from hydrotalcite has better ability of CO adsorption than other two samples. There is no desorption peak observed when the product gas flows through some amount of Ca (OH), before it is analyzed by chromatogram analyzer. It shows that the desorbed gas is C@, not CO. This result indicates there are many active oxygen on the surface of sample, which can interact with CO a d s o d d to result in the formation of C@ , following the increase of temperature.

-~

a

1 100 200 300 400 450 30 100 200 300 400

I ("C) t ("C)

0

Fig.8 NO-lPD(a) a n d ( b ) CO-TPDcurvesofcoAlCu(7:1:1) samplespreparedbydifTerentmethods(1. Method of hydmtalcites as precursor; 2. Melhod of copmipitation; 3 . Method of sintering the mixtum of

metal oxides).

Figs. 9a and 9b illustrate the profdes of NO-TPD and CO-TPD of the samples ( Co : Al : Cu = 7 : 1 : 1 ) calcined at different temperatures.

It can be obviously observed that the desorptions of N2 appears for two samples which are calcined at lower temperam (350°C , 450T) but the desorption of N2 doesn' t appear for the other two samples. The samples become less active for adsorption and decomposition of NO when the temperature calcination increaes .

Being similar to the ability of NO adsorption, the sample' s ability for CO adsorption rapidly de- crease, following the calcination temperature increasing. The w o n maybe is the amount of active oxygen on the surface and sdace area decrease. Comparing the results of NO-TPD with CO-TPD, it can be found that the ability of CO adsorption changes larger than that of NO adsorption. Similar result can be

LlLT et al. Hyhtalcite-like compounds 607

found in Figs. 8a and 8b.

0

t ("C)

J

j0 100 200 300 400 4

t ("C)

Fig. 9 NO-TPD (a) and CO-TPD (b) curves of &AlCu (7: 1 : 1) samples calcined at different temperahue

( 1 : 3509:; 2: 4509:; 3: 6009:; 4: S O O ~ ) .

Reaction mechanism

Fig. 10 shows the conversion of NO and CO and the on entration of each product gas in reaction system. The catalyst CoAlCu (7 : 1 : 1 ) was prepad from calcination of hyhtalcite .

8 8 I: B 0

Fig. 10 Species content and conversion of NO and CO in reaction system catalyzed on CoAlCu (7 : 1 : 1 ) (0: Conv. of NO; 0 : Conv. of CO; A: &nt. of Cq; A : Cont, of N20; +: Cont. of Nz) .

608 Chinese Journal of Chemistry Vol. 17 No. 6 1999

Thruugh comparing the concentration of each product, it can be easily found that the reaction o c c m by two steps: First, CO is oxidized to C@ and NO is converted into N20; secondly, at higher tempem- ture , N 2 0 is reduced to N2 by CO. A tentative mechanism of NO reduction by CO over this series of cata- lysts is p r o p e d as follows:

co + 0-Cat - C Q + 0 - c a t NO + 0 - C a t + NO-Cat iiNO-Cat) - N 2 0 + 0-Cat + 0 - C a t (lower Temp.) N 2 0 + 0 - C a t .-+ NZO-Cat

N20-Cat + N2 + 0-Cat (high Temp. ) 2(N,O-Cat) + N2 + 2(0-Cat) (high Temp.)

where O-cat is the lattice oxygen in catalyst, C]-Cat is the oxygen vacancy. The results obtained from the study of NO-TPD and CO-TPD could support this mechanism. When

the .sample has good ability for CO adsorption, a large number of oxygen vacancy will be formed easily with the temperature increasing. It is favorable for the reaction: NO + 0 - C a t -+ NO-Cat. So their catalytic activities for NO reduction by CO are higher than these samples which have less ability for CO adsorption.

References

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10. 1 1 . 12 I 13. 14.

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(SONG, J . P . ; LING, J . )