La2O3-Al2O3 catalysts in reduction of NO by H2

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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Effect of La203 concentration in La2Oa-A1203 supports and P d / L a 2 0 3 - A I 2 0 3 catalysts in reduction of NO by H2 N.E. Bogdanchikovaa, A. Barrerab, S. Fuentes a, G. Diaz c, A. G6mez-Cort6s c, A. Boronin d, M. H. Farias a, M. Viniegra R. b

aCentro de Ciencias de la Materia Condensada, UNAM, Apartado Postal 2681, 22800, Ensenada, B.C., M6xico bDepartamento de Quimica, UAM-Iztapalapa, C.P. 09340, M6xico D.F., M6xico ClFUNAM, A.P. 20364, 01000, D.F., M6xico dInstitute of Catalysis, 630090 Novosibirsk, Russia It was obtained that the selectivity of La203-A1203 oxides in the NO reduction by H2 depends on the La203 concentration and reaction temperature. The surface area as well as the structure of the supports is varied with the lanthana concentration. All mixed supports are highly selective to the production of N2 when the catalytic reaction is conducted at 973 K. La 3d5/2 binding energy measured for mixed supports is higher than that reported for La203 and it is comparable with value published for dispersed lanthanum phases in alumina. XRD data also show that lanthanum is highly dispersed in A1203. Lanthana concentration does not influence significantly on NO conversion with temperature neither for Pd catalysts nor for supports with an exception for 50 % LaEO3-A1203 support. Its activity is higher than activity of other supports. Application of coprecipitation method for preparation of La203-A1203 supports results in" l) significant enhance of N2 selectivity (at high temperatures) on La2Oa-A1203 supports and Pd/La203-A1203 catalysts and 2) increase of NO conversion on La203-A1203 supports compared with sol-gel method. 1. INTRODUCTION The addition of lanthanum to Pd/AI203 three-way catalysts improves their activity in the reduction of NO [ 1]. Usually Pd-lanthanum-alumina catalysts are prepared by impregnation of A1203 with lanthanum nitrate solutions and thereafter with Pd to obtain Pd/La203/A1203 catalysts [2,3]. In a previous work, we reported that Pd/La203-AI203 catalysts prepared by

1398 sol-gel method show high activity in the NO reduction by H2 and are highly selective to the production of NH3 [4,5]. At temperatures higher than 673 K moderate activity with predominant ammonia formation was detected for La203-A1203 support [5]. Comparison of data for reduction of lanthana in Pd/La203-A1203 and catalytic activity of La203-A1203 support suggests that reduced sites of lanthana are able to reduce NO in the presence of hydrogen. High ammonia selectivity of Pd/La203-A1203 catalyst was concluded to be due to high ammonia selectivity of La203-AI203 support. Coprecipitation is another known method for the synthesis of multicomponent homogeneous materials. It is simpler than sol-gel method. In the present work we study the effect of La203 concentration in La203-AI203 supports and Pd/La203-A1203, prepared by coprecipitation on the NO reduction by H2. 2. EXPERIMENTAL

2.1. Preparation Mixed La203-A1203 supports with varying concentration (2, 6, 15, 25 and 50 wt. % of La203) were synthesized by coprecipitation method. Lanthanum acetylacetonate was added to the solution of aluminum sec-butoxide in 1-butanol with following water addition. Dried samples were heated in N2 flow first at 523 K for 4 h and then at 723 K for 12 h. Preparation of another sample set included additional calcination in oven at 923 K for 4 h. A1203 support was prepared by the same method as La203-AI203 supports without Lacomponent addition. La203 support was prepared by calcination of lanthanum oxide carbonate at 923 K. Supports after calcination at 923 K were impregnated by water solution of palladium chloride, dried in N2 flow at 383 K for 12 h and then calcined at 923 K for 3 h. 2.2. Characterization Surface area measurements (SaET) were carried out by low temperature nitrogen adsorption in volumetric equipment Gemini 2600 from Micromeritics. Calculations were performed on the basis of the BET isotherm. Samples were pre-treated in argon at 473 K for 2h. X-ray diffractograms were recorded in a Philips X'pert diffractometer using the CuI~ (2, = 0.154 nm) radiation. Particle size was calculated from the full width at half maximum intensity of peaks by using the Scherrer equation and assuming shape factor value K = 0.9. X-ray photoelectron spectroscopy analyses were performed using an analysis chamber equipped with a Riber-CAMECA Mac-3 system with A1K~ radiation (hv = 1486.6 eV). Calibration of the energy position of peak maximums was made using the binding energy of carbon 1s peak at 284.8 eV. 2.3. Catalytic studies Reaction of NO with H2 was studied in a continuous flow microreactor Multipulse RIG 100, In Situ Research Instrument under atmospheric pressure. Sample weight was 100 mg.

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La203 concentration, wt. % Fig. 1. Specific surface area (BET) as a function of lanthana concentration prepared at 723 K (1) and at 973 K (2).

T~ature, K Fig. 2. NO conversion as a function of temperature for Pd catalysts with 0, 2, 6, 25, 50 % La203 (1-5) and supports with 0, 2, 6, 15 (710), 50 (6) % La203.

Pretreatment of catalysts before the catalytic experiments consisted in heating in H2 flow at 673 K for 1 h. The concentration of the reaction mixture in helium was 5% with NO/I-I2 molar ratio equal to 0.5. The flow rate of reaction mixture was 120 cm3/min (at room temperature and under atmospheric pressure). The reaction products were detected by gas chromatography using a Tremetrics Gas Chromatograph model 9001. 3. RESULTS AND DISCUSSION 3.1. BET surface a r e a (SBET) Figure 1 shows that SBETof LaEO3-AI203 supports prepared at 723 and 923 K depend on La203 concentration IFig. 1). In the range 5.8 - 25 wt. % of La203 SBETreaches a maximum 390 and 215 - 290 m g for preparation temperatures 723 and 923 K, respectively. Obtained results (Fig. 1) show that La203 is textural promoter in the range ca. 2-25 % and 2-15 % of lanthana for preparation temperatures 723 and 923 K, respectively. In contrast, lanthana is textural inhibitor for higher La203 concentrations.

3.2. X-ray diffraction (XRD) Diffraction patterns show a structure typical of )'-A1203 for supports with lanthana concentration from 0 to 15 wt. %. An average crystallite size for these supports is ca. 3.7 nm.

1400 XRD patterns of supports with 25 and 50 wt. % of La203 do not contain any well-pronounced peaks. This implies that particles of these samples are amorphous. On the other hand, the results of SBETmeasurement show that SBETof supports with La203 concentration < 25 wt. % are higher than SBET of supports with large La203 concentration (Fig. 1). Hence, according to SBET data, average particle size of supports with low La203 concentration is higher than that one of supports with large La203 concentration. These XRD and SaEx data can be observed when the particles of the supports with 25 and 50 wt. % of La203 are large but amorphous. Amorphous character of supports with La203 concentration > 25 wt. % could be a result of the disruption of y-AI203 spinel structure as suggested by Ledford et al [6]. The lack of peaks corresponding to lanthanum phases in XRD patterns of all lanthana-alumina supports suggests the presence of dispersed lanthanum species within the alumina. The same suggestion was made by Haack et al. [7]. 3.3. XPS data Maximum peak position for La 3d5/2 binding energy for La203 is at 835.1 eV. It was deconvoluted into two peaks at 833.9 and 835.6 eV those are typical for La203 and slightly (0.5 eV) shifted towards higher energy as compared with La203, respectively [8]. For La203-A1203 supports with 2, 6, 25 and 50 wt. % of La203, peak maximums for La 3d5/2 binding energies are in the range 835.8-836.1 eV with average at ca. 835.9 eV. That is 0.8 eV higher than were measured for pure La203. Thus, XPS spectra show that the state of lanthanum in mixed La203-A1203 supports differs from the state of lanthanum in La203. This La 3d5/2 binding energy is very close to value (836.2 eV) for dispersed lanthanum--containing phase in alumina after calcination at 873 K measured by Ledford et al. [6]. 3.4. Catalytic data Activity of Pd catalysts is significantly higher than that one of supports (Fig. 2). It can be seen in Fig. 2 that La203 concentration does not influence significantly on NO conversion with temperature neither for Pd catalysts nor for supports with an exception for 50 % La2OaA1203 support. Its activity is higher than activity of other supports. All studied supports are not selective to N20 (Figs. 3). Ammonia is the main product only for supports with > 2 % of La203 at 773 K. At 873 and 973 K nitrogen is the main product on all supports (Fig. 3). The supports with 2 and 5.8 wt. % of La203 seem to be suitable supports in order to obtain high NO conversion into N2 with minimum production of NH3. In contrast, on La203-AI203 supports prepared by sol-gel method in this temperature range N2 selectivity was low (20 %) [4]. All Pd/La203-A1203 catalysts at 473-573 K behave similar to Pd/AI203 catalyst (Fig.3). (Data at 423 K we do not consider because at this temperature the activity increases and selectivity changes very rapidly with temperature.) Higher 573 K the behavior of Pd/La203A1203 catalysts with 2 % and 6-50 % La203 become different (Fig. 3). The behavior of Pd/2 %-La203-A1203 at higher temperatures is still very similar to Pd/AI203 catalyst. Hence, 2 % of La203 is concentration not enough to change activity and selectivity of Pd/A1203 catalyst,

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Fig. 3. Dependence of the selectivity of Pd catalysts (A) and supports (B) on La203 concentration at different temperatures" circles - NH3; triangles - N20; squares - N2.

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1402 while it is enough to change activity and selectivity of A1203 support and chemical state of lanthanum in the support. At high temperatures the behavior of Pd/La203-AI203 catalysts with 6-50 % La203 becomes similar to behavior of supports (N2 selectivity reaches 80 % at 973 K). Selectivity results suggest that in Pd/La203-AI203 catalysts with 2 % La203 catalytic reaction occurs on Pd in the wide temperature range (423-973 K). NO conversion on La203-A1203 supports prepared by coprecipitation is considerably higher than on supports prepared by sol-gel method [4]. At 973 K NO conversion on majority of supports prepared by coprecipitation is 100 % (Fig. 2) while on sol-gel support it is only 40 % [4]. NO conversion on Pd/La203-A1203 catalysts prepared by coprecipitation (Fig. 2) is similar to that one on catalysts prepared by sol-gel method [4]. Hence, application of coprecipitation method for preparation of La203-AI203 supports results in: 1) significant enhance of N2 selectivity (at high temperatures) on La203-A1203 supports and Pd/La203-AI203 catalysts and 2) increase of NO conversion on La203-A1203 supports compared with sol-gel method.

Acknowledgements The authors thank M.E. Aparicio and G. Soto for technical assistance in experiments and Dr. V. Petranovskii for fruitful discussions. This work was supported by CONACYT grant No 25381-A.

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