Catalytic activity of Cu-MeAlPO-11 in NO decomposition

6.ENVIRONMENTAL

ELSEVIER

Applied Catalysis

B: Environmental

15 (1998) 233-240

Catalytic activity of Cu-MeAlPO-11

in NO decomposition

Jii? DgdeEek, JiK cejka, Blanka WichterlovB* J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences

of the Czech Republic, DolejjiGova3, CZ-182 23 Prague 8, Czech Republic

Received 6 April 1997; received in revised form 13 June 1997; accepted

13 June 1997

Abstract Cu ions were incorporated into MgAlPO- 11 and ZnAlPO-11 molecular sieves by ion exchange procedure from the Cu acetate solution. The number of the Cu ions in MeAlPO-11 corresponded roughly to the number of the negative charges in the framework produced by the substitution of trivalent aluminium by divalent Mg or Zn cations. Both Cu-MeAlPOs exhibited constant conversion in NO decomposition in time-on-stream (T-O-S). The turn-over-frequency (T-O-F) values of NO per Cu atom of Cu-MeAlPO-11 at 770 K are comparable to those of the Cu ions implanted in the cationic sites of high silica ZSM-5 matrix. 0 1998 Elsevier Science B.V. Keywords:

Nitric oxide decomposition; Copper; Aluminophosphate molecular sieve; AEL

1. Introduction Nitrogen oxides produced during high temperature combustion processes now-a-days represent a major air pollutant. Decomposition or selective reduction of NO by hydrocarbons to nitrogen in a non-reducing atmosphere has became, therefore, a main target to be achieved in environmental catalysis, and the NO decomposition to molecular nitrogen and oxygen would represent the simplest and most attractive approach to NO, pollution control. Nevertheless, a lot of effort was devoted to the development of a suitable catalytic material which should exhibit high and stable activity in NO decomposition, such properties have been observed only for Cu ions implanted at the cationic sites of aluminosilicate zeolite matrices [l-6]. For various metal oxide *Corresponding

author.

0926~860)3/98/$19.00 fc 1998 Elsevier Science B.V. All rights reserved. PII SO926-3373(97)00050-7

materials, like Cos04, Mn203, Sr/La*Os [7,8], the decomposition activity was substantially lower or diminished already after a short reaction run. Cu ions exchanged in the ZSM-5 matrix exhibit unique and stable activity among Cu-zeolites in contrast to the Cu ions exchanged in Y and mordenite structures [3,5]. Particularly the Cu-ZSM-5 with overexchanged level (Cu2+/A1>0.5) reaches very high decomposition activity. This finding was brought about following assumptions on the structure of the Cu active site. There is no doubt that the NO decomposition is a redox process [9]. Finally the monomeric Cu site was suggested as an active site [lO-141. In a detailed spectral analysis we have shown [14-161 that the Cu site active in NO decomposition is that one possessing low positive charge on the divalent Cu cation, which is easily reduced and exhibits open, close to planar coordination sphere, and prevails in population at high Cu loadings. Nevertheless that the

234

J. DSdeFek et al. /Applied

Catalysis B: Environmental

15 (1998) 233-240

800

600 >r .X E a, E

400

200

0

0

20

30 2

Fig.

Table 1 Composition

of Cu-MeAlPO-I

40

50

theta (“)

1. XRD powder pattern of MgAlPO-11.

1

Sample

cu (wt.%)

Mg (wt.%)

Zn (wt.%)

Al (wt.%)

P (wt.%)

Cu-MgAIPO-1 l/a Cu-MgAlPO- 11/b Cu-ZnAlPO-11

3.56 1.24 4.04

1.50 0.49 0.00

0.00 0.00 3.12

16.2 19.1 14.3

19.0 22.0 19.0

distribution of aluminium in the framework of high silica zeolites is not known, a high relative population of this Cu site in ZSM-5 at loadings close to theoretical exchange and at overexchange levels, and in the frameworks with high Si/Al ratio led us to suggest that this active Cu ion is adjacent to a single Al framework characteristic atom [ 11,141. The above mentioned spectral and structural features are also in agreement with the sorption and redox properties of this Cu center [17]; it has a high tendency to preserve a monovalent state even in the presence of oxygen at temperature as high as 570 K, and to prefer formation of dinitrosyl complexes instead of mononitrosyl ones compared to the other Cu ions. Similar behaviour of the NO decomposition activity and reducibility of the Cu-ZSM-5 zeolites depending on the Cu/Al/Si composition ratios implied suggestion [16,17] that both

these properties of the individual Cu sites are controlled by the local and total negative framework charge balancing the corresponding Cu sites. Pirone et al. [18,19] reported probably another type of the Cu sites in Cu-ZSM-5 exhibiting stable activity in NO decomposition. The turn-over-frequency (T-OF) values per Cu ion in their highly overexchanged ZSM-5 (Si/A1=80) increased with increasing overexchange level up to &/Al ranging from 2 to 3. Their T-O-F values (ca. 0.06-0.4 s-l at 770 K, T-O-F= number of NO molecules converted per Cu ion per second) in their extremely highly overexchanged CuZSM-5 are slightly lower than those of the Cu ions exchanged in ZSM-5 as described above (with T-O-F ranging from 0.2 to 1.3 s-l at 670 K [11,14]). The activity of the highly overexchanged Cu-ZSM-5, reported in Refs. [18-211 for NO decomposition

.I. Dt?deEek et al. /Applied Catalysis B: Environmental 15 (1998) 233-240

and also for selective catalytic reduction of NO by hydrocarbons, was suggested to be connected with the defect sites or with polymeric Cu-0 oxidic species, i.e. not with true exchangeable single Cu ion sites balanced by framework aluminium. It should be pointed out that a positive effect of extremely high Cu loading was observed up to now only with one sample of the ZSM-5 zeolite with Si/Al=80. Therefore, contribution of this particular zeolite to this effect needs further investigation. Finally, it has to be stressed here that all other attempts to find out some other inorganic matrix as a carrier for the highly active Cu ions or another composite oxidic material, exhibiting such high and stable decomposition activity, have not been successful up to now. This contribution presents the results on the activity in NO decomposition of the Cu ions Incorporated from Cu acetate solution into crystalline metalloaluminophosphate (Mg or Zn) molecular

235

sieves of AlPO-11 structure (AEL). The obtained T-O-F values of NO decomposition per Cu ion are close to those of the Cu ions at exchangeable sites of aluminosilicate ZSM-5 matrix, and the overall activity of these materials is much higher in comparison with that consisting of the metal oxide based materials.

2. Experimental 2.1. Material

synthesis

MgAlPO- 11 and ZnAlPO- 11 molecular sieves were synthesized in the following way: 12.31 g of pseudoboehmite (Catal B, Vista) was added, under vigorous stirring, to a mixture of 19.77 g of H3P04 (85%) and 50.23 g H20. The gel formed was stirred for at least 2 h at ambient temperature. After that 9.00 g of di-npropylamine in 3.20 g of Hz0 was added and the gel

0.3

'1.2

0.2 7 0.c r 0.1

0.0 I

7500

15~00

wavenumber Fig. 2. VIS-NIR spectra of hydrated Cu-MgAlPO-1 acetate (------) and MgAlPO-11 (- . . -).

l/a (-

22koo

(cm-l)

. -), Cu-MgAlPO-1 I/b (- -),

Cu.ZnAlPO-11

(*

- a), 0.1 M solution of Cu

236

.I. DZde&k et al./Applied Catalysis B: Environmental 15 (1998) 233-240

75'00

wavenumber Fig. 3. Normalized

VIS-NIR

22500

15000

spectra of hydrated

was stirred for 2 h. Then a solution of a given amount of Mg or Zn acetate in 4.5 g of water was added and the resulted gel stirred for additional 2 h. After such mixing the gel was transferred into teflon lined autoclaves (90 ml) and heated at 470 K under autogeneous pressure and fast agitation for 16 h. After cooling, the sample was recovered by filtration, washed with deionized water and dried at 353 K overnight. Activation of the molecular sieve at 470 K removed organic template as evidenced spectroscopically. Characteristic XRD pattern for MgAlPO- 11 (recorded on a Seifert 3000P difractometer) is presented in Fig. 1 and is consistent with that published in Ref. [22]. Together with SEM confirms high crystallinity of the synthesized MeAlPO-11 samples (crystal size S-10 pm). Cu-MgAlPO- 11 and Cu-ZnAlPO- 11 with Cu concentration ranging from 1.24 to 4.04 wt% were pre-

(-

(cm-j)

-_) and dehydrated

(----

) Cu-MgAlPO-1

l/a.

pared by stirring 3 g of MeAlPO-11 in 150 ml of 0.1 M Cu acetate aqueous solution at room temperature for 3 h. The solids were then filtered, thoroughly washed by distilled water and dried at ambient temperature. The chemical composition of the CuMeAlPO-11 molecular sieves was determined by atomic absorption spectroscopy after their dissolution, and is given in Table 1. 2.2. Spectroscopy The diffuse reflectance spectra (DRS) of hydrated and dehydrated samples in the VIS-NIR region were recorded on a Perkin-Elmer Lambda 19 spectrometer equipped with an integrated sphere coated with BaS04 collecting the reflected light to the detector from perpendicularly irradiated sample. The absorption

J. DZdeEek et al./Applied Catalysis B: Environmental 15 (1998) 233-240

0

I

I

20

40

231

I

I

60

T-O-S (min) Fig. 4. Dependence He flow at 810 K.

of NO concentration

intensities were evaluated Munk equation, F(R,) R, is diffuse reflectance nite layer and F(R,) is coefficient.

on the reactor outlet on the reaction time-on-stream.

by the Schuster-Kubelka= (1 - &J2/2R,, where measured from a semi-infiproportional to absorption

2.3. Catalytic activity The catalytic activity of Cu-MeAlPO-11 was tested in a through-flow reactor with an inlet NO of 4000 ppm in helium and total feed of 100 ml min-‘, a catalyst weight of 300 mg and in the temperature range 470-800 K. The samples were activated in a helium stream (99.996%) with an increase of 5 K min-’ to the temperature (470 or 570 K), held for 1 h and then the reaction of NO decomposition was started. NO and NOz were analyzed with the accuracy of 0.5% at the inlet and outlet of the reactor by a chemiluminescence analyzer Vamet 138 (CZ). No NO2 was detected in the products (detection limit 5 ppm). Only traces of N20 were observed by mass spectrometry (Hewlett Packard, 5971A).

Cu-ZnAlPO-11,

810 K, sample preheated

in a

3. Results and discussion The VIS-NIR spectra of the hydrated Cu-MgAlPO11, Cu-ZnAlPO-11 and of parent MeAlPO- 11 are shown in Fig. 2. The spectrum of copper acetate solution used for the ion exchange is also given for comparison in Fig. 2. The metalloaluminophosphates after the treatment with Cu acetate solution were light blue and exhibited an absorption band in the NIR region with maximum at 12 500-13 000 cm-‘. This spectrum is similar to those well-known spectra of the Cu2+ ions in aqueous solutions of various Cu2+ salts [23]. It corresponds to the d-d* transition of the Cu2+ ion in distorted octahedral symmetry of the water ligands. After heating of Cu-MeAlPO-11 in vacuum at or above 470 K the integral intensity of the Cu2+ absorption increased by a factor of 1.6 and the maximum of the Cu2+ absorption band was shifted to lower frequencies as clearly seen particularly from a shift of the band edge at low frequency side of the band (see Fig. 3). Simultaneously, the combination vibration band (S+v) at 5220 cm-’ [24], reflecting the presence

238

.I. DtTdeEeket al./Applied Catalysis B: Environmental I5 (I998) 233-240

500

600

700

temperature

(K)

Fig. 5. Temperature dependence of NO conversion on Cu-MeAlPO-11. Cu-MgAlPO-1 I/b (-_O-_) before first NO decomposition testing at 470 K (solid points) or at 570 K (open points).

of water molecules in the samples disappeared. It indicates that most of the copper ions are implanted in metalloaluminophosphates in the hydrated Cu2+ complexes, which change their environment after the material dehydration. This change in the d+d* transition under dehydration indicates accessibility of water or other molecules to the coordination sphere of the cation, being important for catalysis. The chemical analysis of Cu-MeAlPOs (Table 1) showed that roughly one divalent Cu ion, if the Cu2+ ion exchange is assumed, is balanced by one negative framework charge formed due to substitution of one trivalent aluminium for a divalent Zn or Mg cation in the AlPO framework (some excess of the Cu ions above the ratio Cu/Me=l is observed with CuZnAlPO). The Cu/Me ratio close to 1 in aluminosilicates [4,9,14,18] indicates that a similar situation for the charges balancing the Cu ions in MeAlPO-11 can occur as for the Cu ions at cationic sites in overexchanged ZSM-5.

800 and Cu-ZnAlPO-11

(-_O-)

calcined

The conversion of NO in its decomposition over Cu-MgAlPO- 11 and Cu-ZnAlPO- 11 was constant with time after some initial period. Fig. 4 depicts dependence of NO concentration on the reactor outlet depending on the reaction time-on-stream (T-O-S). At first a high, sharp decrease in NO concentration is observed followed by levelling off at the constant concentration of NO, reflecting constant conversion. The constant conversion was reached within 2 h of T-O-S for all samples and significant differences were not observed between Cu-MgAlPO-11 and Cu-ZnAlPO- 11. E.g. with Cu-ZnAlPO- 11 (preheated in a He flow at 810 K) the initial NO conversion at 810 K (after T-O-S of 10 min for equilibration of the through-flow system) was 20%. The conversion values after T-O-S of 1, 2, 6 and 24 h were 11.2, 10.4, 10.8 and 10.5, respectively, i.e. indicating constant conversion within an experimental error. NO conversion into nitrogen over Cu-MgAlPO-11 and Cu-ZnAlPO-11 after T-O-S of 2 h depending on

.I. Dt?de&k et al/Applied

Catalysis B: Environmental 15 (1998) 233-240

temperature is given in Fig. 5. Two sets of experiments differing in the initial calcination temperature were carried out. In the first set Cu-MeAlPOs were calcined in a helium stream at 470 K followed by NO decomposition tested at this temperature for 2 h. Then the sample was calcined in a helium stream under increasing temperature up to 520 K (step of 50 K) for 30 min followed by a catalytic test carried out at 520 K. This procedure with catalytic testing at 50 K steps was employed up to 800 K. In the second experiment Cu-MeAlPOs were calcined in a helium stream at first at 570 K and then procedure as given above for catalytic testing of NO decomposition with 50 K temperature steps was carried out. The NO conversion of both Cu-metalloaluminophosphates increased steadily with temperature. Surprisingly, if the NO decomposition reaction was started at 470 K, the activity of both Cu-MeAlPO samples was substantially higher compared to those calcined first at 570 K. It indicates that some development of the Cu site environment took place, depending on the heat treatment. A low temperature treatment of the catalyst in NO atmosphere during catalyst testing at 470 K might affect the Cu site structure. However, an explanation of this effect requires further studies. The catalytic activity of the Cu-MeAlPO- 11 s in NO decomposition at 770 K (data from Fig. 5, solid lines) given in Table 2 are compared with the activity of the CuNa-ZSM-5 zeolites (measured at comparable conditions, see Table 2, and with the same experimental set-up) at temperature of 720 K at which maximum conversion was reached, and with the data of Hall et al. [4,9] and Pirone et al. [18]. It is seen that the T-O-F values of Cu-metalloaluminophosphates are close to

Table 2 A comparison

of NO conversion

into nitrogen on Cu-MeAIPO-11

239

that of CuNa-ZSM-5 with Si/Al= 14.1, but lower than that with Si/A1=22.5 (cf. Refs. [11,14]). However, the Cu ions in metalloaluminophosphates exhibit a higher activity compared to the Cu ions in extremely highly overexchanged Cu-ZSM-5 (cf. Ref. [ 181). The authors in Refs. [ 18-211 consider that the active sites (for NO decomposition as well as for selective catalytic reduction of NO by paraffins) in their highly overexchanged zeolitic matrix of ZSM-5 structure are the Cu ions forming oxidic polymeric species or represent the Cu ions implanted in the defect sites. It has to be noted that the initial T-O-S behaviour of NO conversion over Cu-AlPO-11 is similar to that transient behaviour of NO decomposition observed on reduced Cu-ZSM-5 as given in Ref. [19]. It might indicate that also in Cu-MeAlPO- 11 the active site is a monovalent copper. According to our preliminary results, the copper in MeAlPO- 11 is much less reducible to Cu+ compared to Cu-ZSM-5 as indicated by Cu+ luminescence measurements of variously treated CuMeAlPOs (for Cu-ZSM-5 see Ref. [ 151). It can explain why for attaining the same activity in NO decomposition over CuMeAlPOs much higher temperature is required in comparison with Cu-ZSM-5. Table 3 compares the decomposition activity related to the grams of the catalyst, i.e. Cu-molecular sieves and bulk oxide materials, like Mnz03, Co304 and Sr/LazOj (Refs. [7,8]). It is clearly seen that if the Cu ions are balanced by negative framework charge regardless of the matrix then they are superior catalysts for NO decomposition. However, it should be stressed that both the composition of the matrix as well as the local Cu ion environment play a decisive role in the activity of the individual Cu ions.

and Cu-ZSM-5

Catalyst

cu (wt.%)

Concentration

Cu-MgAlPO-1 l/a Cu-MgAlPO- 1 l/b Cu-ZnAlPO- 11

3.56 1.24 4.04

0.4 0.4 0.4

770 710 710

0.3 0.6 0.2

This work This work This work

Cu(Na)-ZSM-5 Si/Al 14.1

1.10-3.93

0.4

720

0.2-0.8

[11,141

Cu(Na)-ZSM-5 Si/Al 22.5

1.10-2.00

0.4

720

0.7-1.3

u1,141

Cu-ZSM-5” Cu-ZSM-Sa

1.58-5.21 0.38-3.24

4.0 0.5

800 770

2.9-14.1 o.om.4

[4>91 Cl81

aNote different NO concentrations

and temperature.

of NO in He (%)

Temperature

(K)

TOFx10m3

(s-l)

Ref.

240

J. DtTdeEek et al/Applied

Table 3 A comparison of the rate of NO conversion into nitrogen molecular sieves with metal oxide catalyst at 770 K

Catalysis B: Environmental

on Cu

Catalyst

Rate (molecules NO/g.s)

Ref.

Cu-MgAlPO- 11 a Cu-MgAlPO-11” CU-ZSM-~“.~ Cu-ZSM-Sb SrlLaaOs’ La20sC MnZOsC coso4c

2.0-2.5x 10’7 2.0x 10’7 2.2-8.4x 10” 0.08-1.4x 10’9 1.7x 10’5 9.6x lOI 4.8~10’~ 3.1x10’6

This work This work

Note different reaction concentration: a 4000 ppm. b 40 000 ppm. ’ 20 000 ppm. d Temperature 720 K.

conditions

[ll M. Iwamoto, S. Yokoo, K. Sakai, S. Kagawa, J. Chem. Sot.

NO

However, it has to be pointed out that the CuMeAlPO catalysts still do not achieve the level of activity necessary for technology applications. Moreover, for evaluation of their suitability, the resistance of the Cu ions as well as the matrix itself to the presence of water, oxygen and eventually SO2 should be the most important requirements.

4. Conclusions It has been shown that copper ions exchanged in MgAlPO-11 and ZnAlPO-11 molecular sieves at the degree that corresponds roughly to one Cu ion, which is charge balanced by one framework negative charge, exhibits stable (more than 24 h) and selective activity in NO decomposition to molecular nitrogen and oxygen. The activity in turn-over-frequency of NO molecules per Cu ion is comparable to those values obtained for the Cu ions exchanged in cationic sites of aluminosilicate MFI framework.

Acknowledgements Financial support of Grant Agency Republic (project no. 203/1996/1089)

Agency of Academy of Sciences (project no. A4040707) for J.C. is highly acknowledged. The authors thank CONDEA Vista Co. for providing a pseudoboehmite for metalloaluminophosphate synthesis.

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