Al2O3: Nature of the active sites

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control III

Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.

591

INFLUENCE OF THE COPPER DISPERSION ON THE SELECTIVE REDUCTION OF NITRIC OXIDE OVER Cu/Al203 : NATURE OF THE ACTIVE SITES. Z. Chajara, M. Primeta, H. Praliauda, M. Chevrierb, C. Gauthierc and F. Mathisd. a Laboratoire d'Application de la Chimie dt l'Environnement, Universitd Claude Bernard LYON L 43 bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Renault Automobiles, Direction Etudes Matdriaux, 92109 Boulogne Billancourt Cedex, France.

c Renault Automobiles, Centre de Lardy, 1 allde Cornuel, 92510 Lardy, France. d Renault Automobiles, Direction de la Recherche, 9-11 Avenue du 18 Juin 1940, 92500 Rueil Malmaison, France.

ABSTRACT Cu/A120 3 solids with various Cu loadings, between 0.3 and 6.4 wt %, are used for the reduction of NO by propane in the absence and in the presence of oxygen (up to 10 vol. %) in the 423 - 773 K temperature range. At a given temperature and for high Cu loadings, the introduction of oxygen induces a decrease in the activity in nitrogen formation. For low Cu loadings the activity increases with the oxygen content in the 1-2 vol. % range, then slightly decreases for higher oxygen amounts. The nature of the Cu species accessible to CO and NO is determined by infrared spectroscopy. High Cu loadings favor the formation of bulk oxides at the surface of the support whereas low Cu loadings favor the formation of isolated Cu species. The selective reduction of NO is thus related to the presence of these isolated copper species easily reduced and reoxidized. 1. INTRODUCTION

Iwamoto et al. [1, 2, 3] and Held et al. [4] independently reported that the selective catalytic reduction of NO by hydrocarbons, such as ethene, propene,

592 propane, is possible in an oxidizing atmosphere on copper ion-exchanged ZSM-5 zeolites at temperatures as low as 473 - 673 K. Addition of oxygen to the reactants mixture is necessary to achieve the selective reduction of NO at high conversion levels. Afterwards many catalysts, zeolitic and non-zeolitic, has been found to be also active : proton-exchanged zeolites [5], cerium [6] or iron [7] or gallium [8] ion-exchanged zeolithe, alumina, silica and silica-alumina [9, 10, 11 ]. For non-zeolitic solids, the selective reduction of NO by hydrocarbons in the presence of oxygen has been considered to occur over solid acid catalysts such as alumina, silica-alumina, titania, zirconia [10] and sulfate promoted metal oxides such as titania, zirconia, ferric oxide [12]. In the case of alumina (or other simple oxides), the reaction occurs at high temperatures and under low space velocity conditions. The activity was found to be improved by the addition of platinum group metals [13, 14] and of transition metal oxides [15], especially copper [16, 17, 18]. For alumina-supported copper oxide catalysts a maximum effect has been found by the addition of 0.3 wt % Cu and it has been considered that, for higher copper contents, the formation of cupric oxide would give a solid selective for the oxidation of the hydrocarbon by oxygen [16]. In the case of alumina-supported Cu-Cs oxide catalysts the formation of an isocyanate species has been evidenced by exposition to mixtures "nitric oxide/oxygen/propene (or acetylene)" but not with propane [18, 19]. In fact the mechanism of the reaction and the nature of the active sites are still unknown. In this paper we report a study of the activity of Cu/AI20 3 solids with various copper contents towards the reduction of NO by propane in the presence of oxygen. The nature of the superficial copper species is determined by infrared spectroscopy of adsorbed CO and NO. The results are compared to the catalytic activity.

2. EXPERIMENTAL

2.1. Preparation of the catalysts Four Cu/A1203 solids with various copper loadings (0.3, 1.7, 3.2 and 6.4 wt. Cu %) were prepared by impregnation of an alumina from Degussa (BET area 100 m 2 g-1) with aqueous solutions of copper nitrate. The solids were thus dried at 383 K and calcined under oxygen at 773 K (heating rate 1 K min-1) overnight. The DRX patterns were characteristic of the gamma alumina support and the CuO phase was only detected for the solid containing 6.4 wt % copper, but not for lower copper amounts.

593 2.2. Catalytic activity measurements Catalytic activity measurements were carried out with a fixed bed flow reactor. The reactor was a quartz tube. The flow rates were adjusted using Brooks mass flow controller units. The composition of the effluents was analyzed by gas chromatography using a dual CTR1 column from Alltech (porapak for CO2, N20, molecular sieve for 02, N2, CO) with a thermal conductivity detector and a porapak Q column with a flame ionisation detector for hydrocarbons. The mixture was analyzed every 13 minutes. NO and NO2 amounts were measured continuously on-line by means of Rosemount Infrared Analyzers. Helium was used as carrier gas as well as diluent gas. 100 mg of catalyst were charged with 400 mg of diluent (inactive low surface area ot A1203) and the total flow rate (reactants in He) was 10 dm3.h-1 (gas hourly space velocity of 50 000 h-1 for the catalyst considering a density of ca. 0.5 g cm-3). The experiments were carried out into two steps : - the calcined solids were contacted at room temperature with a C3H8 (2000 vpm) - NO (2000 vpm) mixture free of oxygen (reducing mixture). The reaction temperature was first increased from 298 to 773 K with a ramp of 4 K min-1 and then decreased from 773 to 298 K. - Oxygen was thus admitted at a given temperature, and for each temperature, the oxygen content was progressively increased in the 0.2 - 10 vol. % range.

NO may be converted into N20 , N2, NO2. In fact, substantial amounts of NO 2 were formed because of the gas phase reaction at room temeperature between NO and oxygen in the pipes of the apparatus, before the reactor and in the exit lines from the reactor, as akeady noticed [20, 21 ]. In the present study, the catalytic activity only eoncems the conversion of NO into nitrogen. 2.3. Infrared spectroscopy of CO and NO adsorptions The nature of the superficial copper species accessible to CO and NO was deduced from infrared spectroscopy measurements. The samples were pressed in order to obtain thin discs of known weight between 15 and 40 mg. The discs were placed in a sample holder made of quartz and introduced in a cell that allowed "in situ" treatments. Prior to IR measurements the samples were heated at 773 K for 1 h under oxygen (80 Torr), then dcgasscd at 773 K for 1 h. After cooling to 298 K the background spectnun was recorded, then carbon monoxide (around 50 Tort) or nitrogen monoxide (around 10 Torr) was introduced at :298 K, the contact time varying between 0 and 20 h. The gaseous phase was evacuated at the same temperature between 10 s and 3 h.

594

Infrared absorbanee spectra were recorded at room temperature on a FTIR spectrometer (I.F.S. 110 from BRUKER) with a resolution of 4 em-1.

3. RESULTS AND DISCUSSION

3.1.Activity in the absence of oxygen (C3Hs-NO-He mixture) First of all, it has been checked that the conversion does not depend on the particle size in the 20 - 500 lam range. In the absence of oxygen, the NO conversion into N2 increases with the temperature and the copper content but remains weak. It begins at around 623 K with the solid containing 6.4 wt. % Cu. It reaches 2, 11, 36 and 40 % for the solids containing 0.3, 1.7, 3.2 and 6.4 wt. % Cu respectively. The activity expressed per gram of introduced copper is approximatively independent of the copper content (between 130 * 107 and 250 * 107 moles NO converted into N2 S-1 g-1 Cu). A small activation under the reactants mixture has been observed and the values here reported are obtained during the decrease in temperature, from 773 to 298 K. CO2 and N 2 are the only products detected. The C3H8 conversion corresponds to the NO reduction according to the reaction : C3Hs+10NO

~

5N 2+3CO 2+4H20

(reaction 1).

For comparison a Cu-ZSM5 solid prepared by impregnation of a ZSM5 zeolite (Si/A1 = 18.9) by an aqueous solution of copper nitrate and containing 2.6 wt. % Cu is able to convert, at 773 K, 20 % of NO into N2, value corresponding to a rate of 173 * 107 mol. s -1 g-1 Cu (22). Let us remark that, in the absence of oxygen, the reduction of NO by C3H8 is weaker than the reduction of NO by CO. As an example, with a mixture NO (2000 vpm) - CO (2000 vpm) - He, for the 6.4 wt % Cu solid, the NO conversion reaches 100 % at 746 K and the light-off temperature is 553 K during the decrease in temperature, between 773 and 298 K. Propane is a less efficient reducing agent than carbon monoxide in spite of the more reducing character of the mixture (ratio NO/CO = 1 and ratio NO/10 C3H 8 = 0.1).

3.2.Activity in the presence of 0 2 (C3Hs-NO-O2-He mixture) 3.2.1. NO conversion into N2 The effect of the oxygen addition on the NO conversion into N 2 at 773 K is shown on the figure 1. This temperature has been selected in order to obtain a

595 good activity without oxygen. Two behaviours are observed according to the copper contents. For the solids containing 3.2 and 6.4 wt. % Cu the activity decreases as soon as oxygen is introduced and decreases continuously for higher oxygen contents.

40

6.4 wt. % Cu

Z

* l

/

__ /

om

1.7 wt. % Cu

20 0.3 wt. % Cu

f Z

3.2 wt. % Cu 0

2

4

6

8

10

0 2 content voi. (%) Figure 1. Conversion of NO into nitrogen at 773 K as a function of the oxygen content for the three Cu/Al20 3 catalysts

On the contrary, for the solids containing 0.3 and 1.7 wt. % Cu the NO conversion into N 2 is enhanced by oxygen addition. It goes through a maximum in the 1-2 vol. % 02 range, but remains relatively important even with a large excess of oxygen. With 10 vol. % 02 and with the 1.7 and 0.3 wt % Cu solids, the NO conversion is equal to 19 and 7 % respectively. This enhancement is lower than the one observed with Cu-ZSM5 catalysts [22]. Let us recall that, at 623 K, for a Cu-ZSM5 solid prepared by impregnation and containing 2.6 wt. %

596 Cu, the NO conversion into N 2 reached 78 % in the presence of 0.5 vol. % 0 2 and is equal to 25 % in the presence of 10 vol. % 02. It may be noticed that, with CO as a reducing agent, the introduction of oxygen leads to a drastic decrease in NO conversion whatever the copper content. 3.2.2. Activity for the N 2 formation expressed per gramm of copper. The activities expressed per mole of NO converted into N2 per gramm of copper and per second are reported in the figure 2. The solid with the lower copper content is the more active one. 3.2.3. Main products and C3H 8 conversion The main products are N2, NO 2 and CO2., N20 was never detected. The conversion of NO into NO2 increases significantly with the oxygen content and is performed by homogeneous reaction in the pipes of the apparatus. Let us recall that with 10 vol. % 02 this conversion into NO2 reaches 25 % in the absence of catalyst [21]. In the presence of the catalysts the conversion of NO into NO2 varies between 25 and 17 % in the presence of 10 vol. % 0 2. The lowest value is obtained with the 1.7 wt. % Cu solid for which the conversion of NO into N 2 is the highest. For this solid, the residual NO partial pressure, in the pipes at the exit of the reactor, is lowered by the NO reduction into N 2. With the solids containing 1.7, 3.2 and 6.4 wt. % Cu, propane is fully oxidized into CO 2. With the 0.3 wt. % Cu solid a small part of C3H8 is oxidized into CO at 773. For instance the conversion of C3H8 into CO reaches 4 % for an overall conversion of 19 % in the presence of 10 vol. % 02. The C3H8 conversion continuously increases with the temperature and the oxygen concentration. Propane conversion and NO reduction begin at the same temperature. However this C3H 8 conversion exceeds the conversion which would be due to the reaction with NO (reaction 1). For instance in the presence of 10 vol. % 02, with the 0.3 and 1.7 wt. % Cu solids, when the conversion of NO into N2 is equal to 7 and 19 %, the C3H 8 overall conversion reaches 19 and 60 %, respectively, showing the importance of the following reaction: C3H 8 + 5 02

~

3 CO 2 + 4 H20 (reaction 2).

For the 6.4 wt % Cu solid the C3H 8 conversion reaches 89 %.

597

v,u

.g

10

0.3 wt. % Cu

!

"7 r~

6.4 wt. % Cu

m

ou

1.7 wt. % C u

fq

Z om

3.2 wt. % Cu I.

0

Z

0

2

4

6

8

10

0 2 content voi. (%)

Figure 2. Rate of reduction of NO into N2 (mol. S-1 g-I Cu) at 773 K as a function of the oxygen content for the three Cu/AI203 catalysts.

The C3H8 conversion is never complete. However the maximum in NO conversion observed when the oxygen content increases is not due to a deactivation of the catalyst because additional runs with low oxygen contents lead to NO conversion similar to those of the first rim. The decrease may be due to the influence of the propane partial pressure.

598

3.3. Superficial copper ions accessible to CO 3.3.1. FTIR spectra of adsorbed CO The admission of CO at 298 K onto the Cu/A1203 solids calcined and evacuated at 773 K results in the appearance of IR absorption bands whose number and positions are function of the Cu loading (figure 3). The solid containing 6.4 wt % Cu is characterized by an intense IR v CO band at 2125 cm -1 and a weak shoulder at 2190 cm -1. With the contact time with CO (between 0 and 20 hours) the intensity of the 2125 cm -1 band increases by a factor 1.3. Upon evacuation at 298 K the species show a poor stability since the intensity is strongly lowered. After evacuation for 1 h the spectrum shows a unique and weak band at 2120 cm-1. The spectnnn of CO adsorbed on the solid containing 3.2 wt % Cu is similar (intense band at 2125 cm -1 and weak shoulder at 2190 cm-1). The spectrum of the solid containing 1.7 wt % Cu shows, besides the 2125 cm-1 vCO band and the shoulder at 2190 cm -1 another intense band at 2110 cm -1 and weak shoulders at 2160 and 2165 cm -1. With the contact time with CO, between 0 and 20 hours, the intensity increases by a factor 1.7. After 1 hour evacuation the spectrum shows only the 2120 and 2110 bands whose intensities are strongly decreased. The spectrum of the 0.3 wt % Cu is more complex showing the presence of an intense band at 2160 cm-1 besides the bands at 2110, 2125, 2135 cm -1 and the shoulder at 2200 cm-1. The 2110 cm -1 band is the more intense one. With the contact time with CO the intensity increases by a factor 1.1. Upon evacuation at 298 K the intensity of all the bands is strongly decreased. After 1 hour evacuation the spectrum shows bands at 2145, 2155 and 2110 cm- 1, the 2110 cm- 1 band remains the more intense one.

599 ~t 2110 F----

2160 t

JJ

i

2125

2125 2125

2135

t_

2_

6.4 wt. % C u

1.7wt.%Cu I

2200

2100

,

2200 2100

0.3wt.%Cu II

i

1

2 2 0 0 2100

cm-I Figure 3. FTIR spectra o f CO adsorbed on Cu/AI20 3 samples (previously calcined under 02 and evacuated at 773 K). 50 Tort CO pressure, 1 h at 298 K. 6. 4 wt. % Cu : 38 mg pellet 1.7 wt. % Cu : 32 mg pellet O.3 wt. % Cu : 39 mg pellet

600 3.3.2. Nature of the species adsorbing CO Let us recall that the admission of CO at 298 K on Cu-ZSM5 zeolithes treated in the same conditions as the Cu/A120 3 solids leads to the appearance of a unique and very intense band at 2150-60 cm -1 band that we have ascribed to CO adsorbed onto isolated Cu + ions [22, 23]. From literature data [24-32] it is possible to consider that the 2190-2200 cm -1 band is associated with CO adsorbed on isolated Cu 2+ ions or to A13+ surface ions. The 2135, 2125 and 2110 cm -1 bands may be due to CO adsorbed on Cu + and Cu 0 ions arising from the partiel reduction of an CuO oxide ie., on non-isolated copper Cu + and (or) Cu 0 ions. CO is more strongly adsorbed on Cu 0 and Cu + than on Cu ++. The high temperature treatment at 773 K under vacumn is at the origin of the generation of Cu + ions according to a reduction process of some Cu 2+ ions. The number of reduced copper ions decreases with the temperature of evacuation. For instance for the 6.4 wt. % Cu solid, after 1 h of contact with CO, the intensity of the 2125 em -1 is lowered by a factor 2.1 if the evacuation is performed at 423 K instead of 773 K. Furthermore the probe molecule itself induces a reduction of the remaining Cu 2+ ions. When the contact time with CO increases from 1 minute to 20 hours the intensity of the 2125 em -1 increases by a factor 1.35 after evacuation at 773 K and by a factor 1.8 atter evacuation at 423 K. 3.3.3. In conclusion the solid containing a low amount of copper is characterized by the appearance of a ~ CO band at 2160 cm -1 assigned to CO adsorbed onto isolated Cu + ions. The spectrum of the solid containing a high amount of copper, for which CuO has been detected by DRX, is characteristic of CO adsorbed on Cu + arising from the reduction of bulk CuO. CO appears to be able to reduce Cu ++ ions at room temperature.

3.4. Superficial copper species adsorbing NO 3.4.1. FTIR of adsorbed NO The admision of NO on the solids calcined and evacuated at 773 K results in the appearance, besides the bands of gaseous NO (1875 cm-1), of gaseous N 2 0 (2215 and 2240 cm-1) and of nitrates and nitrites on the support (1480-1665 cm-1 range) of two absorption doublets, a intense one (1890-95 and 1870-60 cm-1 bands) and a weak one (1765-60 and 1740 cm -1 bands) due to NO adsorbed onto Cu n+ ions. Figure 4 shows the spectra of NO adsorbed at 298 K on the two solids 6.4 and 0.3 wt. % Cu calcined and evacuated at 773 K, after substraction of the gas phase.

601

II 1870

1890

V-

r"

[ 0 . 3 wt. % C u 9

1895 r

L.

1920

A~ ~g

_/

1860

,

6.4wt.%Cu

1740

]

i i

1900

,

'll 1700

i

1900

,

,

i,

,, 1700

cm-I

Figure 4. FTIR spectra o f NO adsorbed on Cu/AI20 3 samples (previously calcined under 0 2 and evacuated at 773 K). 10 Torr NO at 298 K, the gas phase has been substracted 6.4 wt. % Cu 9 30 mgpellet 0.3 wt. % Cu 9 24 mgpellet

602 For the 6.4 wt % Cu solid the 1870 cm -1 band is more intense than the 1890 cm -1 one and the 1740 cm-1 band appears only as a weak shoulder on the 1765 cm-1 band. The spectnma shows also a weak shoulder at 1920 cm-1. With the contact time with NO, between 1 mn and 1 h, the intensity of the 1765 cm-1 band decreases by a factor of ca. 5. The other bands are apparently not modified. Upon evacuation at 298 K the intensities of all the bands are lowered by a factor ~ 10. NO is not strongly held on the Cu n+ ions. The specmun of NO adsorbed on the 1.7 wt. % Cu is similar but the intensity of the 1740 cm -1 band has increased and has become comparable to the intensity of the 1760 cm- 1 band. The spectnnn of NO adsorbed on the 0.3 wt. % Cu solid is modified. In particular the 1895 cm -1 band becomes more intense than the 1860 cm -1 one and the 1760 cm -1 band pratically disappears. 3.4.2. Nature of the species adsorbing NO According to literature data [24, 28, 33-36], the bands at 1890-95 and 186070 cm -1 are due to NO adsorbed on Cu ++ ions : on isolated Cu ++ ions (1890-95 cm-1) and on Cu ++ ions belonging to bulk CuO (1860-70 cm ~ respectively. The bands at 1760-65 and 1740 cm -1 are characteristic of NO adsorbed onto Cu + ions : isolated Cu + ions (1740 cm -1) and Cu + ions belonging to bulk Cu20 (1760-65 cm-1). From the results exposed above, when the copper content decreases, the intensities of the bands due to isolated copper species (NO on Cu ++ at 1890-95 cm-1 and NO on Cu + at 1740 cm-1) become higher than the intensities of the bands assigned to the oxides (CuO at 1860-70 cm-1 and Cu20 at 1760-65 cm-1). This phenomenon is especially marked with the 0.3 wt % Cu solid. It may be noticed that, at 298 K and whatever the copper content, the NO probe molecule oxidizes the Cu + reduced species created during the evacuation at 773 K. In effect when the contact time with NO increases from 1 min to 20 h, the intensity of the 1890 cm -1 band increases whereas the intensity of the 1760 cm ~ band decreases. All the species possess a very poor stability upon evacuation at 298 K. NO adsorbed on Cu ++ belonging to CuO is the more stable species. 3.4.3. In conclusion the presence of isolated copper species is favoured by low copper amounts. The NO adsorption corroborates the results deduced from the CO adsorption. NO is able to reoxidize Cu + at room temperature.

603 4. CONCLUSION

The infrared spectroscopy of adsorbed CO and NO has allowed us to determine the nature of the copper species at the surface of the alumina support. High copper loadings favor the formation of bulk oxides on this surface. At the same time with high copper contents the introduction of oxygen induces a decrease in the reduction of NO into nitrogen by propane. The presence of bulk copper oxides leads to the formation of a catalyst very active for the oxidation of the hydrocarbon molecule by oxygen. Because of the decrease in the reductant partial pressure NO cannot be converted into nitrogen. Low copper loadings favors the formation of isolated copper ions less active in the oxidation of propane by gaseous oxygen. In addition, the activity toward the NO reduction into nitrogen enhanced by the presence of oxygen. The selective reduction of NO is thus related to the presence of isolated copper ions at the surface of the support. Because of the presence of various copper containing species in the studied catalysts, it is not possible to quantify the amounts of copper belonging to every species : isolated Cu 2+ of Cu + ions, bulk oxides (CuO and Cu20). Furthermore the oxidation state of copper is easily changed, Cu ++ being reduced into Cu + by the CO probe molecule and Cu + being oxidized by the NO probe molecule even at 298 K. ACKNOWLEDGEMENTS

The authors are very grateful to Mr. J. Billy for his assistance in FTIR experiments. REFERENCES

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