The preparation of stable Ru metal clusters in zeolite Y used as catalyst for ammonia synthesis

PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

217

T h e p r e p a r a t i o n o f s t a b l e R u m e t a l c l u s t e r s in z e o l i t e Y u s e d as c a t a l y s t for a m m o n i a s y n t h e s i s u. Guntow, F. Rosowski, M. Muhler, G.Ertl and R. Schl5gl ~ ~Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany

Ru-exchanged zeolite NaY is an active catalyst sytem for ammonia synthesis in which the Ru clusters are prevented from sintering by the zeolite framework. The influence of the precursor synthesis conditions was studied by in-situ and ex-situ UV/VIS spectroscopy, AAS, and XRD. The intrinsic lability of [Ru(NH3)6]C13 against hydrolysis and oligomerisation was found to be increased in the presence of zeolite Y. The resulting free ammonium ions led to a cation-exchange of the zeolite and allowed insertion of the modified monomeric Ru complex. Oligomerisation at a single site within the zeolite formed a molecular precursor to a metallic Ru cluster. The activation procedure of the precursor yielding metallic Ru particles was studied by T P D / T P R experiments. Heating in Ar allowed to monitor dehydration, deammination, autoreduction and catalytic ammonia decomposition. The optimum catalytic NH3 synthesis activity was observed after heating the oligomeric precursor in a N2/H2 - 1/3 synthesis gas mixture. The catalytic activity was found to increase with increasing particle size providing evidence for the structure sensitivity of NH3 synthesis on Ru.

1. I n t r o d u c t i o n Ammonia synthesis is carried out in industrial practice over a promoted metallic iron catalyst with special morphological properties. This catalyst has been optimized to its limits but exhibits as one major draw-back a high sensitivity to poisoning by oxygenic compounds. The application of Ru in a suitable highly disperse form may represent a less oxygen-sensitive alternative. We are interested in finding strategies for the development of such a catalyst by merging surface analytical efforts with controlled synthetic procedures. One facet in this catalyst development is the generation of Ru clusters enclosed in a zeolite Y matrix allowing to investigate analytical and kinetic properties of well-defined small Ru particles. We have presented surface analytical and kinetic data on Ru enclosed in zeolite Y and supported on zeolite Y and A [1-3]. The present communication focusses on the preparation of the precursor intercalation compound and its activation into the active state for ammonia synthesis.

218 2. E x p e r i m e n t a l Zeolite NaY was obtained from DEGUSSA (KM-390). Its bulk elemental analysis was confirmed by RFA in wt % for Na (9.23%), A1 (11.64%) and Si (30.65%) after dehydration at 1353 K (water loss 9.27 wt%). It agrees well with the theoretical formula Na56[(A102)56(Si02)136] * 250H20 allowing to refer exchange reactions on a relative scale to a sodium content of 56 units per formula unit zeolite. A potassium form (KY) was prepared in a one-step exchange at 333K for lh with KC1. These two starting zeolites were reacted with [Ru(NH3)6]C13 either obtained from HERAEUS or made from RuC13 in an ammoxidation reaction in bi-destilled water at 333 K. The identity of the two products was checked with elemental analysis, IR and UV/VIS data. We here discuss the following four samples out of 64 preparations which were reproducible in their final catalytic behaviour and which could be scaled in batch size from 0.1 g to 50 g: KY precursor, Ru/KY, Ru/NaY, Ru/NaY(295K) prepared by reacting NaY with [Ru(NH3)6]C13 at 295 K . A typical recipe for a sample of the type of Ru/NaY is given as follows: 137 ml water and 1.73 g [Ru(NH3)6]C13 were warmed to 333 K. 5.03 g NaY were washed with 25 ml cold water into the solution which gradually turned from pale yellow to light purple. After 1 h at 333K the reaction mix was allowed to cool to room temperature for 45 min and filtered off in air. The product was dried at ambient temperature over silica gel in a dessicator. During drying the prodtict turned deep-purple. Drying in vacuum accelerated the colour change. Elemental analysis of the reaction solutions was carried out by UV/VIS for Ru and AAS for Na and K. The exchanged zeolites were analysed by RFA for Ru. Control data were obtained from AAS analysis after dissolution in concentrated HC1 up to 425 K. Powder X-ray diffraction was carried out in focussing Bragg-Brentano geometry with transmission samples with internal Si standard. Catalytic testing was carried out in an all stainlesssteel microreactor setup using an on-line IR NH3 detector (BINOS). The gases used had a purity of 99.9993% and were further purified by a self-designed purification unit [4]. The usual synthesis gas flow was 40Nml/min using 137mg sample. Temperature-programmed desorption and reduction (TPD, TPR) experiments were carried out with a conventional glass set-up with cold trap and thermal conductivity detector and an all stainless steel set-up equipped with a calibrated mass spectrometer. 3. Results and Discussion 3.1. Ru exchange and oligomerisation in zeolite Y The Ru ammine complex was chosen due to its relative high stability in water against hydrolysis and oxidative oligomerisation to ruthenium red [5]. This reaction is assumed to take place during ion exchange into the zeolite giving rise to the colour change of the zeolites from pale yellow to purple. The process was already studied in the context of Ru chemistry in zeolites intended to be used as catalysts in CO hydrogenation [6,7]. It is pointed out that the details of the preparation exert a significant influence on the stability of the final activated catalyst, in particular on the tendency of the final Ru particles to remain under reducing conditions inside the zeolite framework [2,8]. This final stability may be pre-formed already during the preparation of the precursor taking into consideration the possible cation exchange sites within the framework. The nature

219 of this exchange reaction in which one trivalent cation formally exchanges 3 monovalent ions is in this context of particular significance. The constitution of the Ru complex in the exchange solution needs also some consideration as possible hydrolysis products may exchange with different sites or with different kinetics as the starting Ru complex. Characteristic data of the intercalated zeolites are reported in table 1. Table 1 Characteristic data of Ru-exchanged zeolites. The ion content in solution is given as % of the total ion exchange capacity assuming an exchange ratio of Ru/Na = 3/1.

Sample prepared from % Na in solution % K in solution % Ru in solution lattice constant / ppm BET surface /

m2/g

Ru/NaY NaY 47 -

Ru/KY KY i0 33

Ru/NaY(295 NaY 26

K)

-

50

48

25

2467

2467

2467

549

517

546

The data show that there is good numerical agreement between the exchanged amount of alkali and the exchanged amount of Ru (actual content divided by 3). The absolute content of a 50% exchanged sample in Ru is about 5.6 wt%. The deliberately partially exchanged Na/K sample reacted with the Ru complex under simultaneous exchange of Na and K indicating that the kinetic preference for ion exchange is different for K and Ru. This is the first hint to the inequivalence of the exchange reactions between monovalent and polyvalent ions. The second hint arises from the purple colour pointing strongly to the presence of [(NH3)sRu+3-O-Ru+4(NH3)4-O-Ru+3(NH3)5] C16 (Ru red) in the zeolite. The complex ion is formally 6 fold positively charged. These findings are in apparent contradiction to the data in table 1 suggesting a Na / Ru exchange ratio of 3/1. In this situation the exchange isotherms determined simultaneously for the alkali ion and the polyvalent ion should indicate the nature of the exchange process. For a true ion exchange mechanism the two isotherms must exhibit the identical shapes. Exchange of Na by K satisfies this condition very accurately as shown in fig.1 (upper left isotherm). The Ru intercalation reaction exhibits, however, very different isotherms which are given in fig.1 for three reaction temperatures. The massive deviation in shapes is not due to a kinetic effect. We have determined the exchange kinetics and found complete reaction after 20 min exchange time with 85 % exchange after 2 min. The data in fig.1 indicate independent processes for the loss of Na and the incorporation of Ru into the zeolite. We assume that the exchange occurs actually between Na + and NH + liberated from the Ru complex by hydrolysis. The primary Ru ammine hydroxy chloride complex polymerises to oligonuclear compound independently from the cation exchange process. Low reaction temperatures and high Ru complex concentrations favour the formation of fewer larger Ru oligomers inside the zeolite, high temperatures polymerise the Ru compound prior to intercalation and reduce the total amount of intercalated Ru.

220 1.4

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u on n o e Na Content o f the Filtrate . . . .

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Concentrations [mmol Ru/l]

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Concentrations [mmol Ru/i]

Figure 1. Potassium and Ruthenium exchange isotherms of zeolite NaY in aqueous solution.

The following observations support this interpretation of the "ion exchange" which is better refered to as intercalation process. We tried several Ru compounds which form monomeric complexes in water without ammine ligands as exchange reagents and all failed to incorporate into the zeolite. In-situ observation of the exchange solution by UV/VIS spectroscopy showed that the reaction solution underwent a chemical reaction after the addition of the zeolite optically visible by a pale purple colouring. In fig.2 spectra of Ru red, [Ru(NH3)6]C13 and the reaction solution of Ru/NaY are compared. The optical colour change is not indicative of oligomerisation to Ru red but to the formation of a new mono-nuclear complex in solution. The shifted absorption maximum from 265 nm to 294 nm is consistent with [Ru+3(NH3)5OH] 2+ (see data in ref. [5] and references therein) in full agreement with the interpretation of the isotherm data. The broad shape of the 265 nm absorption of the pure Ru compound in solution implies that the compound undergoes hydrolysis already without the zeolite present. This observation gave rise to a systematic study of the starting complex in solution as function of time, temperature and pH. At 333 K the compound is stable in acidic and neutral media for 24 hours. The compound hydrolyses to a small extent above pH of 2.5 but the product which is not identifiable from

221

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,

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,

700

,

,

,

,

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800

Wavelength / nm

Figure 2. UV/VIS spectra in aqueous solution of [Ru(NH3)6]C13, Ru red and of the reaction solution of Ru/NaY

literature spectra remains at a steady concentration of below 10% of the starting solution assuming comparable molar extinctions. If the starting complex is intercalated into NaY at pH 3 or below, a pale yellow solid is obtained which converts, however, after drying either in air or under highly anaerobic conditions (Ar, less than 1 ppm oxygenates) within a period of days into the usual purple compounds. The oligomerisation reaction occurs thus as consequence of the drying process which seems to remove additional ligands from the primary intercalation product. The time and temperature evolution of the stability of the Ru complex at pH 6 was further investigated by in-situ UV/VIS spectroscopy yielding a rational limit for the reaction conditions if the presence of a mononuclear starting complex is required for the desired zeolite product. At 300 K the solution is stable within the definition given above. At 323 K significant reaction sets in at about 8 h in solution, at 333 K this is the case after 1.5 hours. These data led us to 333K and 1 hour reaction time. At longer times and higher temperatures the Ru complex hydrolyzes to an unidentified intermediate in equilibrium with Ru red as final product under these conditions. The reaction sequence is independent from the concentration of the solution within our limits of observation (upper limit about 0.05 mol/1) Powder X-ray diffraction gave the following valuable diagnostic information for the successful preparation of a stable catalytic precursor material. Stable final catalysts proved to exhibit a characteristic change in the intensity distribution of the diffraction pattern. Typical data are displayed in fig.3. The exchange of Na by K leads to no significant changes in the intensity profile. We note a systematic loss in crystallinity of a fraction of the material. The Ru exchanged samples show a reproducible and characteristic modifi-

222 cation of the intensity distribution. Most sensitive are the reflections (220), (311), (333), (440) which give an interchange in their relative intensities easily seen in fig.3. All other intensities are also changed in a way not to be accounted for by the changed X-ray absorption coefficient of the sample caused by the presence of the heavy scatterer Ru. The data imply a defined location of the Ru inside the zeolite such that all structure factors of the framework are affected.

NaY

KY

5

ffl e-

Ru/NaY i

0

e-

-

Ru/NY

.

9

.

10

20

30

40

20

Figure 3. XRD transmission patterns of zeolite NaY, K-exchanged zeolite Y, Ru/NaY and Ru/KY

Other intensity distributions were not reproducible for identical preparations and always resulted in instable final catalysts. The observation indicate that under conditions of the presence of a defined starting complex intercalation of Ru into an ammonium exchanged zeolite Y occurs in one prefered site. As only these single-site intercalated samples yield stable catalysts it is suggested that this site is the supercage of the zeolite out of which the resulting Ru cluster cannot migrate during controlled activation. The oligomerisation reaction of a mixed Ru ammine hydroxy complex inside the zeolite during either drying or thermal activation is the key to the formation of a Ru cluster at an exchange state where

223 less than all possible exchange sites carry intercalated Ru species: would they be present as mononuclear metal species then diffusion of mobile Ru species and agglomeration inside the zeolite during activation would have to occur which may be difficult to discriminate from diffusion of the Ru species out of the zeolite crystal and formation of a large metallic particle on the outside of the zeolitic support (see for discrimination of the two situations ref. [8,2,9]). 3.2. A c t i v a t i o n of t h e R u - e x c h a n g e d z e o l i t e Y p r e c u r s o r The controlled activation of the precursor into a metallic Ru cluster species inside the zeolite framework is of vital importance to maintain the stability of the activated catalyst. Two pathways leading to metallic Ru clusters within the zeolite matrix are further investigated in the following. The Ru-exchanged precursor may either be heated in an inert carrier gas or in vacuum leading to autoreduction of the Ru ions by the ammine ligands, or the precursor is reduced by heating in a N2/H2 - 1/3 synthesis gas mixture.

2000

Ru/NaY

I

TPD

NH3 E

1500

to .m t.-

H2

1000

r O

500

I

300

I

400

I

i

i

I

i

I

500

i

I

600

i

700

Temperature / K

Figure 4. TPD profile of Ru/NaY obtained by heating in Ar with 10 K/min.

Temperature-programmed heating in Ar as shown in fig.4 allows to monitor dehydration, deammination, autoreduction and catalytic ammonia decomposition. The formation of N2 starting at 330 K unambiguously monitors the autoreduction of the Ru complex which passes through a maximum at 590 K. At about 600 K a drop in the concentration of NH3 and a simultaneous increase in the concentration of H2 occurs pointing to the presence of Ru metal particles since Ru metal is known to catalyse NH3 decomposition. During the subsequent TPR experiment no consumption of H2 is observed within the experimental error proving essentially complete autoreduction to Ru metal [10].

224

TPR RuO2

5 tO .==.

Ru/NaY

o. E

:3 r

o 0 oJ "I-

j,

I

Ru(NH3)6CI3

Ru red =, i

300

,

4110

,

I

;

I

I

i

500

i

600

,

i

I

',

i

700

Temperature / K

Figure 5. TPR profile of RuO2, Ru/NaY, [Ru(NH3)6]C13 and Ru red obtained by heating in 20% H2 in Ar with 10 K/min.

Heating RuO2 in H2 gives rise to a single, rather symmetric TPR peak at about 400K. The TPR profile of the Ru/NaY precursor is significantly more complex consisting of roughly three peaks at 400 K, 440 K and 500 K with an onset of reduction at about 370 K. Contrary to the Ru/NaY precursor the TPR profiles of the reference compounds [Ru(NH3)~]C13 and Ru red exhibit a single peak at about 530 K and 510 K, respectively. The TPR results obtained with the reference compounds indicate that the Ru-O bond is obviously easier to reduce than the Ru-NH3 bond. Hence the complex shape of the Ru/NaY TPR profile ranging from 370 K to 540 K points to the presence of various hydrolyzed Ru ammine complex compounds within the zeolite matrix. In the search for optimum catalytic activity the two different activation procedures were applied to Ru/NaY and Ru/NaY(295K). The on-line monitored NH3 concentration in the reactor exit gas is displayed in fig.6 while cycling the temperature between 664 K, 724 K, and 784K. Trace A was obtained after heating Ru/NaY with 10 K/min in synthesis gas to 844 K. The activity was found to increase during the first three cycles, and already during the fourth cycle steady state NH3 production was achieved. At 784 K the NH3 concentration is limited by thermodynamic equilibrium. When applying the autoreduction procedure to Ru/NaY following the recipe given by Cisneros and Lunsford [8], the catalyst was much less active as shown in fig.6B demonstrating a peculiar transient behaviour as response to temperature changes. Trace C resulted after heating the Ru/NaY(295K) precursor prepared at 295 K to 844 K in synthesis gas. The catalytic activity was observed to increase over a period of 80 h finally reaching the same activity as Ru/NaY.

225

1250

A

1000 750

E

Q.

500 250

~

tO t._

0 500

r (D 0 tO 0

250

x

1000

03

-1-z

0

750 500 250 0 _

0

10

20

30

40

50

60

70

80

784 724 664

E

{D

90

T i m e on s t r e a m / h

Figure 6. NH3 exit concentration as a function of time cycling between 664 K, 724 K, and 784 K. A: Ru/NaY after heating with 10 K/min to 844 K in synthesis gas. B: Ru/NaY after stepwise heating in Ar (lh at 373 K, lh at 473 K, lh at 573 K) to 724 K followed by heating in H2 from 300 K to 724 K. C: Ru/NaY(295K) after heating with 10 K/min to 844 K in synthesis gas.

TEM investigations after NH3 synthesis [11] revealed that the autoreduction procedure resulted in average particles sizes of about 1 nm in agreement with Cisneros and Lunsfords [8] observations. The reduction in synthesis gas, however, produced Ru clusters in the range from 2 nm to 3 nm. These observations support the postulated structure sensitivity of NH3 synthesis on Ru yielding increasing catalytic activity with increasing particle size [12,8]. Obviously, the Ru/NaY precursor prepared at 333 K provided larger oligomers resulting in larger clusters during the initial reduction compared with Ru/NaY(295K) prepared at 295 K. The increasing catalytic activity with time as shown in fig.6 is therefore attributed to the growth of the Ru clusters within the zeolite matrix reaching the maximum size of 3 nm after reduction in synthesis gas.

226 4. Conclusions The intrinsic lability of [Ru(NH3)s]C13 against hydrolysis and oligomerisation is increased in the presence of a zeolite. The resulting free ammonium ions are used to cation-exchange the zeolite and allow insertion of the modified monomeric Ru complex. Oligomerisation at a single site within the zeolite forms a molecular precursor to a Ru cluster large enough that under controlled activation which removes all ligands and reduces the Ru to the formal oxidation state zero the Ru is retained in the supercages of the zeolite. Furthermore, the zeolite matrix offers the opportunity to prepare Ru clusters with well-defined particle size distributions yielding supporting evidence for the structure sensitivity of NH3 synthesis on Ru. The present study shows that it is useful to accompany all steps of a synthetic route to a catalyst by analytical data in order to find for each reaction step chemically motivated reaction parameters and to reduce the number of experiments in the multi-dimensinal parameter space characteristic of even such a simple reaction as the "cation exchange" process of a zeolite. REFERENCES

1. J. Wellenb/ischer, U. Sauerlandt, W. Mahdi, G. Ertl and R. SchlSgl, Surf. Interf. Anal. 18 (1992) 650 2. W. Mahdi, U. Sauerlandt, J. Wellenb/ischer, J. Sch/itze, M. Muhler, G. Ertl, and R. SchlSgl, Catal. Lett. 14 (1992) 339 3. J. Wellenbiischer, M. Muhler, W. Mahdi, U. Sauerlandt, J. Sch/itze, G. Ertl and R. SchlSgl, Catal. Lett. 25 (1994) 61 4. B. Fastrup and H.N. Nielsen, Catal. Lett. 14 (1992) 233 5. J.N. Armor, H.A. Scheidegg'er, H. Taube, J. Am. Chem. Soc. 90 (1968) 5928 6. J.J. Verdonk, R.A. Schoonheydt, and P.A. Jacobs, J. Phys. Chem. 85 (1981) 2393 7. J.J. Verdonk, R.A. Schoonheydt, and P.A. Jacobs, J. Phys. Chem. 87 (1983) 683 8. M.D. Cisneros and J.H. Lunsford, J. Catal. 141 (1993) 191 9. J. Wellenbiischer, F. Rosowski, U. Klengler, M. Muhler, G. Ertl, U. Guntow and R. SchlSgl, Proc. 10th Int. Zeolite Conf. (1994) 10. J.J. Verdonk, P.A. Jacobs, M. Genet, and G.J. Poncelet, J. Chem. Soc. Faraday Trans. 1, 76 (1980)403 11. B.Tesche, to be published 12. S.R. Tennison, in: Catalytic Ammonia Synthesis, ed. J.R. Jennings (Plenum Press, NY, 1.Edition 1991) p. 303