/ APPLIED CATALYSIS A: GENERAL

E L S E V I E R Applied Catalysis A: General 151 (1997)443-460

Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and

power-law kinetics

F. Rosowski, A. Hornung, O. Hinrichsen, D. Herein, M. Muhler *, G. Ertl

Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany

Received 16 June 1996; revised 7 August 1996; accepted 7 August 1996

Abstract

Supported Ru catalysts for NH 3 synthesis were prepared from Ru3(CO)I2 and high-purity MgO and AIEO 3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H 2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH 3 formation was found: C s 2 C O 3 - R u / M g O > C s N O 3 - R u / M g O > R u / M g O > R u / K - A 1 2 0 3 > R u / A I 2 0 3. The Cs-promoted R u / M g O catalysts turned out to be more active than a multiply- promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10 -2 s - l for the non-aqueously prepared Cs2CO3-Ru /MgO catalyst at 588 K. The strong inhibition by H 2 was found to require a lower molar H2:N 2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH 3 mole fraction. The optimum ratio for Cs2CO3-Ru /MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH 3 mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH 3 synthesis also at high pressure.

Keywords: Ruthenium; Non-aqueous preparation; High pressure N H 3 synthesis; Kinetics

* Corresponding author.

0926-860X/97/$17.00 Copyright 1997 Elsevier Science B,V. All rights reserved. PH S0926- 860X(96)00304-3

444 F. Rosowski et al . / Applied Catalysis A: General 151 (1997) 443-460

1. Introduction

Alkali-promoted Ru-based catalysts are expected to become the second generation NH 3 synthesis catalysts [1 ]. Based on the Kellogg Advanced Ammo- nia Process (KAAP), the 600 t NH3/day Ocelot NH 3 synthesis plant started to produce NH 3 in 1992 using promoted Ru catalysts supported on carbon [2]. The Ru-based catalysts permit milder operating conditions compared with the mag- netite-based systems, such as low synthesis pressure (70-105 bar compared with 150-300 bar) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [2].

It is known that chlorine acts as severe poison in NH 3 synthesis over Ru catalysts [3-5]. The addition of chlorine to chlorine-free precursors was found to create about six activated adsorption sites for H 2 per chlorine adatom [3]. Hence, in recent kinetic studies, chlorine-free Ru complex compounds like Ru3(CO)I2 [6-8] or Ru(NO)(NO3) 3 [9] were used in the preparation. Aika et al. [8] demonstrated that MgO is one of the most effective supports for Ru. The Cs-promoted Ru on MgO (CsNO3-Ru/MgO) prepared from chlorine-free Ru3(CO)I2 was found to be as highly active for NH 3 synthesis as CsNO3-Raney Ru or as K-promoted Ru on activated carbon [8].

Alkali-promoted catalysts are usually prepared by impregnation with aqueous solutions of alkali metal salts such as CsNO 3 [8]. Datye et al. [10] showed that aqueous impregnation of MgO may lead to severe morphologic changes due to the partial transformation to the hydroxide phase. The present study reports on two novel water-free methods using alkali carbonates for the preparation of alkali-promoted Ru catalysts supported on MgO and A1203 to achieve long-term and high-temperature stability. The first method uses a solution of Cs2CO 3 in ethanol for the impregnation of R u / M g O resulting in a significantly higher catalytic activity than the conventionally prepared C s N O 3 - R u / M g O catalyst.

The second method employs a K2CO3-pretreatment of the y-alumina support prior to the impregnation with the solution of Ru3(CO)12 in THF. The pretreat- ment was carried out at 1153 K corresponding to the preparation temperature of mixed oxides of the /3-alumina type. The structure of /3-alumina consists of spinel-type (AljlOl6)+ blocks separated by mirror planes in which mobile K ions are located together with oxygen anions bridging the blocks. Compounds with the /3-alumina structure are usually non-stoichiometric having nominal compositions of e.g., K/A1 = 1 / 6 [11]. Busca et al. [12] recently performed a surface characterization of Ba-/3-alumina which is applied as support for gas- turbine combustion catalysts due to its high thermochemical stability. Their results provide evidence for a predominant basic character of the Ba-/3-alumina surface [12]. Moggi et al. [6] carried out a pretreatment of the surface of T-A1203 with KOH followed by calcination in air at 773 K for 8 h.

In the present study, information about the particle size distribution was obtained from H 2 chemisorption, transmission electron microscopy (TEM) and

F. Rosowski et al. /Applied Catalysis A: General 151 (1997) 443-460 445

X-ray diffraction (XRD). It is shown that the combined application of all three techniques is necessary for a reliable assessment of the Ru metal particle size. A modified volumetric sorption set-up was used which allows H 2 chemisorption and conversion measurements to be carded out in the same U-tube to avoid transfer artifacts due to the formation of hydroxides and carbonates in air.

In our laboratory, a systematic study is in progress aiming at a detailed understanding of the catalytic phenomena involved in the synthesis of ammonia on ruthenium. In spite of the industrial importance, relatively few studies in the catalytic literature deal with the kinetics of NH 3 synthesis over supported Ru catalysts [9,13-17]. These studies focus on the influences of PNn3' PN2 and Pn: on the rate of NH 3 formation mostly at atmospheric pressure. Alkali promotion was found to decrease the inhibition by NH 3 significantly and to give rise to a strong inhibiting effect of H e [9,15,16]. On multiply-promoted Fe catalysts, H 2 has a positive reaction order due to high coverages of adsorbed atomic nitrogen (N-* ) [18]. The reaction order for N 2 was found to be essentially in unity for both Ru and Fe catalysts indicating that the dissociative chemisorption of N 2 is the rate-determining step (rds). To our knowledge, reaction orders for N 2 and H: at high pressures have not been published yet for supported Ru catalysts.

In his review about "Alternative Non-iron Catalysts," Tennison [1] provides some kinetic data at high pressure illustrating that a lower PHJPN2 ratio than 3 is favorable for Ru-based catalysts under industrial synthesis conditions without, however, specifying the active metal area. Recently, Kowalczyk et al. [17] reported the results of NH 3 synthesis rate measurements at high pressures in which the active metal area was not specified either. In the latter study, it was observed that a K-promoted Ru catalyst supported on carbon is much less sensitive to high NH 3 partial pressures and consequently more active than a triply-promoted Fe catalyst under these conditions [17]. It is the main intention of the present study to present both kinetic data sets obtained at pressures up to 50 bar and the results of Ru metal area measurements from which turn-over frequencies (TOF) are derived. Moreover, power-law rate expressions r = kap p " P~H3" P~:'P~: were determined both at atmospheric pressure and at 20 bar. The kinetic data set is intended to serve as the basis for a detailed microkinetic analysis of NH 3 synthesis kinetics [ 19,20] following the concepts by Dumesic et al. [21].

2. Experimental

2.1. Preparation

The catalysts were prepared from high purity A1203 (99.99%, Johnson Matthey) or MgO (Puratronic, 99.996% metals basis, Johnson Matthey) and Ru3(CO)12 (Johnson Matthey, 99%) by wet impregnation in a rotary evaporator

446 F. Rosowski et al. /Appl ied Catalysis A: General 151 (1997)443-460

following the procedures in Refs. [8,6,22]. The achieved metal loading was about 5 wt.% Ru.

In both cases, 1.5 g of the catalyst support were heated in high vacuum at 773 K for 6 h and then dispersed in a solution of 0.157 g Ru3(CO)~2 in 60 ml THFab , for 4 h at room temperature. The impregnation was carried out under inert gas atmosphere to avoid contact with air and moisture. After evaporating the solvent at 313 K, the slightly orange powder was pressed at a pressure of 1.6 MPa into cylindrical pellets which were subsequently crushed and sieved. The 250-800 ~zm sieve fraction was slowly heated in high vacuum up to 723 K to decompose the carbonyl precursor yielding dark gray grains.

The alkali-promoted catalysts CsNO3-Ru/MgO and CsNO3-Ru/AI203 were obtained by impregnating the supported Ru catalysts R u / M g O or Ru/A1203 with solutions of CsNO 3 (99.99%, Strem) in a mixture of acetone-water -- 9 /1 (vol./vol.). Cs2CO3-Ru/MgO was prepared non-aqueously by impregnation with a solution of Cs2CO 3 (ultrapure, Johnson Matthey) in absolute ethanol. After stirring the suspension for 3 h, the solvent evaporated and the catalysts were dried in vacuum. The atomic ratios were Cs:Ru -- 1:1 for CsNO3-Ru/MgO and Cs2CO3-Ru/MgO and Cs:Ru = 3:1 for CsNO3-Ru/AI203. All catalysts were hygroscopic and were stored in a desiccator.

The Ru /K-AI203 catalyst was prepared by heating a thoroughly ground mixture of 1.385 g A1203 (99.99%, Johnson Matthey) and 0.176 g K2CO 3 (analytical grade, Merck) in a platinum crucible at 1158 K in air for 15 h following the procedure developed by Scholder and Mansmann [11 ] to produce a ternary oxide of the /3-alumina type with a stoichiometry of K 2 0 . 1 1 A1203. The reaction temperature was chosen to be close to the melting point of K2CO 3 at 1164 K. At temperatures above 1273 K, y-Al~O 3 transforms into ce-A1203 [23]. Subsequently, the wet impregnation with Ru3(CO)j2 was carried out as described above.

2.2. Characterization of the catalysts

The Ru metal area was determined by volumetric H 2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the proce- dure described in Ref. [24]. Prior to chemisorption, the catalysts were reduced by passing 80 cm 3 (STP)/min high-purity synthesis gas (Pn2/PN2 = 3 /1 ) from a connected feed system through the U-tube and heating up to 673 K for alkali-promoted catalysts or up to 773 K for alkali-free catalysts with a heating rate of 60 K/h . The NH 3 mole fraction in the effluent was analyzed at steady-state by a non-dispersive infrared detector (BINOS, Fisher-Rosemount) to determine the catalytic activity. The BET area was measured by static N 2 physisorption in the same set-up.

The TEM morphologies were analyzed in a Siemens Elmiskop 102 instru- ment at 100 keV. Samples were deposited from a slurry in hexane onto

F. Rosowski et al. /Applied Catalysis A: General 151 (1997)443-460 447

carbon-coated Cu grids and predried before insertion. The XRD data were collected in transmission with a Stoe Stadi P instrument using monochromated Cu K~I radiation and a position-sensitive detector. The full width at half maximum (FWHM) was determined by fitting pseudo-Voigt line profiles.

2.3. Kinetic measurements

The kinetic experiments were carried out in an all stainless steel microreactor flow system with three gas lines (Ar, N 2, H 2) which were operated at pressures up to 100 bar. The gases had all a purity of 99.9993%, and were further purified by means of a self-designed guard reactor [25]. The flows were controlled by electronic mass flow controllers and the temperatures by microprocessor con- trollers. The reactor consisted of a glass-lined stainless steel U-tube with an inner diameter of 3.8 mm which was placed in a copper block oven. The effluent mole fraction of NH 3 was determined by a non-dispersive infrared detector (BINOS, Fisher-Rosemount) which was calibrated by using a reference gas mixture (Linde). Limitations by heat or mass transport were avoided by using 138 mg of the 250 txm-800 p~m sieve fraction for the kinetic experiments resulting in bed heights of < 15 nun. The reduction was carried out in a flow of 80 cm 3 (STP)/min synthesis gas with a heating ramp of 60 K / h up to 673 K. The absence of poisoning by oxygen-containing compounds was tested as described in Ref. [25]. It was possible to run steady-state NH 3 synthesis at temperatures as low as 500 K. All catalysts were tested for a period of 24 days at least.

3. Results and discussion

3. I. Characterization of the catalysts

The results of the BET measurements are summarized in Table 1. As expected, y-A120 3 turned out to be the chemically and mechanically more stable support with a higher surface area than MgO. Only as a result of aqueous alkali

Table 1 Results of the BET measurements after drying the supports in high vacuum at 773 K for 6 h, after impregnation with Ru3(CO)12 and NH 3 synthesis at 773 K, and after impregnation with Ru3(CO)12, thermal decomposition in vacuum, alkali impregnation and NH 3 synthesis at 673 K

Sample BET area (m 2/g) Sample BET area (m 2/g)

MgO 51.6 A1203 110.0 Ru/MgO 25.0 Ru/Al203 104.4 CsNO 3 -Ru/MgO 23.0 CsNO 3 -Ru/Al203 70.0 CszCO 3-Ru/MgO 21.0 Ru/K-A1203 91.0

448 F. Rosowski et al. /Appl ied Catalysis A: General 151 (1997) 443-460

impregnation, the surface area was found to decrease significantly from 104 m2 /g to 70 m2/g. For MgO, 52 m2 /g were observed after drying in high vacuum at 773 K. For R u / M g O , however, the specific area was found to be only 25 m2/g after NH 3 synthesis at 773 K. After the strong initial loss in surface area, alkali impregnation led to a further decrease by only 2 m2/g. Aika et al. [8] observed a similar decrease in surface area from 90 m2/g to 36 m2 /g for a 2 wt.% R u / M g O catalyst as a result of aqueous promotion with CsNO 3 in the molar ratio of Cs:Ru = 1:1. High surface areas of MgO are obviously not stable under typical catalytic conditions.

The H 2 chemisorption results are summarized in Table 2. The H 2 monolayer capacities were used to derive Ru metal dispersions and mean particle sizes assuming spherical particles. On both MgO and A1203, the impregnation with Ru3(CO)12 resulted in mean particle sizes of about 2 nm after NH 3 synthesis at 773 K. It is remarkable that essentially the same Ru metal areas were obtained on MgO and A1203 inspite of the largely differing BET areas of the supports. For Ru /MgO, a specific Ru metal area of 14.4 m2 /g was obtained initially after NH 3 synthesis at 673 K. Increasing the NH 3 synthesis temperature to 773 K was found to decrease the metal area to 12.9 mZ/g equivalent to a particle size of 1.9 nm. For the R u / M g O catalyst impregnated with the aqueous solution of CsNO 3, the amount of chemisorbed hydrogen was found to be reduced by about a factor of two. Aika et al. [8] also observed a decrease from 136 txmol H z / g to 104 p~mol H z / g for a 5 wt.% R u / M g O catalyst as a result of impregnation with an aqueous CsNO 3 solution in the molar ratio of Cs:Ru = 1:1. These observations raise the question whether the loss in metal area is due to site blocking as suggested by Nwalor and Goodwin [9] for K - R u / S i O 2 or due to an increase in particle size.

The TEM micrographs shown in Fig. 1 reveal that the particle size distribu- tion of R u / M g O (Fig. 1A) is fairly homogeneous with an average value of about 2 nm in good agreement with the result obtained by H 2 chemisorption. The Ru metal particles are seen to be evenly distributed over the MgO particles which have a cubic morphology to some extent. However, the TEM micrograph

Table 2 Results of the H2 chemisorption measurements after NH 3 synthesis based on H:Ru = 1:1. The NH 3 synthesis was run at 773 K with R u / M g O and Ru/AI203, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assuming spherical particles

Catalyst H 2 monolayer Metal area Dispersion Mean particle (p~mol H e / g ) (me /g ) (%) size (nm)

R u / M g O 130 12.9 53 1.9 Ru/A1203 118 11.7 48 2.1 CsNO 3 - R u / M g O 69 6.8 28 3.6 CsNO 3-Ru/AI203 100 9.9 41 2.5 Cs2CO 3 - R u / M g O 57 5.6 23 4.3 R u / K - A I 2 0 3 128 12.6 52 1.9

449 F. Rosowski et al. /Applied Catalysis A: General 151 (1997) 443-460

Fig. 1. TEM micrographs after NH 3 synthesis: Ru/MgO (A, upper image), Cs2CO3-Ru/MgO (B, lower image), CsNO3-Ru/MgO (C, upper image), and CsNO3-Ru/AI203 (D, lower image). The original magnification was 300000.

of CsNO3-Ru/MgO (Fig. 1C) displays a rather non-uniform particle size distribution with particle sizes ranging from 3 nm to 10 nm. The largest particles appear to be hexagonally shaped. Fig. 1D shows a TEM micrograph of CsNO3-Ru/A1203. The contrast of the metal particles in Fig. 1D is signifi- cantly lower compared with the MgO-based catalysts. However, the absence of larger Ru particles as seen in Fig. 1C for CsNO3-Ru/MgO can clearly be confirmed. The estimated particle size of about 2-3 nm demonstrates that the Ru/A1203 catalyst was not significantly affected by the impregnation with C s N O 3 in agreement with the minor increase in particle size from 2.1 nm to 2.5 nm derived from H 2 chemisorption.

The increase in size of the Ru metal particles on MgO due to Cs impregnation is further corroborated by the XRD results shown in Fig. 2. The diffraction pattern of Ru /MgO after NH 3 synthesis at 773 K (upper trace in Fig. 2) reveals only the presence of highly crystalline MgO. This is in agreement with the Ru particle size of about 2 nm determined by means of H 2 chemisorption and TEM since such a particle size lies below the detection limit of the diffractometer. For the CsNOa-Ru/MgO catalyst, an additional weak diffraction pattern is ob- served which originates from Ru metal. From the widths (FWHM) of the diffraction peaks, the dimensions of the Ru crystallites are derived applying the Scherrer equation to the three most intense peaks. The (100), (002) and (101)

450 F. Rosowski et al. / Applied Catalysis A: General 151 (1997) 443-460

15

~o 10

E - 5

15

10

"E 5

Ru/MgO

[ i i i i

(200)

(220)

Y I I i r t I i

C s N O 3 - R u / MgO

XRD

I I I

,loo, i/ I i i i I i i i

30 40 I

5O 2 0

(102) .)

L i J t I i

60

(110)

i i I

70

Fig. 2. XRD patterns after NH 3 synthesis of Ru/MgO (upper pattern) and of CsNO 3 R u / M g O (lower pattern).

dimensions listed in Table 3 are in good agreement with the TEM results and indicate rather isotropic Ru crystallites. The XRD patterns of the 3/-A1203 supported catalysts confirmed the poor crystallinity of the support in agreement with the TEM micrograph shown in Fig. 1D. Diffraction peaks due to Ru were not detected for Ru/A1203, CsNO3-Ru/AI203, and Ru/K-A1203.

In order to avoid the evident disadvantages of aqueous alkali impregnation for Ru/MgO, Cs2CO 3 was chosen as alkali precursor, which is soluble in ethanol

Table 3 XRD results: Ru crystallite dimensions of the CsNO 3 - R u / M g O and Cs2CO 3-Ru/MgO catalysts after NH 3 synthesis obtained by applying the Scherrer equation

Diffraction peak CsNO 3 - R u / M g O Cs2CO 3 - R u / M g O (hkl) D (nm) D (nm)

100 10 9 002 8 6 101 9 6

F. Rosowski et al. /Applied Catalysis A: General 151 (1997)443-460 451

contrary to CsNO 3. To our knowledge, Cs2CO 3 has not yet been used for this purpose presumably due to its high decomposition temperature of 880 K. The reduction of Cs2CO 3 under NH 3 synthesis conditions below 773 K should occur only in close contact with Ru metal particles which can provide atomic hydrogen. This assumption is supported by a significantly lowered onset temper- ature of CsNO 3 reduction in the presence of Ru powder or RuC13 as observed by Aika et al. [26].

The H 2 chemisorption results shown in Table 2 indicate that apparently the non-aqueous impregnation with Cs2CO 3 led to even larger Ru particles than the aqueous process with CsNO 3. The TEM micrograph of the Cs2CO3-Ru/MgO catalyst shown in Fig. 1B indeed reveals a non-uniform Ru particle size distribution. However, a considerable fraction of the 2-3 nm Ru particles is seen to be still present after Cs2CO 3 impregnation as confirmed by the comparison with the TEM micrograph of Ru /MgO (Fig. 1A). Furthermore, the mean size of the larger Ru particles detected by XRD is found to be considerably smaller as shown in Table 3. Thus, both TEM and XRD results indicate a significantly higher Ru metal area for the Cs2CO3-Ru/MgO catalyst compared with the CsNO3-Ru/MgO catalyst. The H 2 chemisorption at room temperature may be blocked by the presence of Cs2CO 3 species thus yielding only a lower limit to the actual Ru metal area. This conclusion is further supported by the signifi- cantly higher catalytic activity of the Cs 2CO3-Ru/MgO catalyst compared with the CsNO3-Ru/MgO catalyst as will be discussed further.

For the Ru/K-A1203 catalyst, a BET surface area of 91 m2/g (Table 1) was observed after NH 3 synthesis. The Ru metal area of 12.6 m2/g is even somewhat higher than the Ru metal area of Ru/AI203. The XRD patterns of the T-A1203 support, the Ru/A1203 and the Ru/K-A1203 catalysts were found to be essentially identical within the experimental resolution. In order to prepare crystalline K-/3-alumina, higher amounts of K2CO 3 (K:A1 = 1:6) and longer periods of calcination are necessary which, however, would presumably lead to a significant loss in specific surface area.

3.2. Kinetics at atmospheric pressure

The results of the initial steady-state conversion measurements at atmospheric pressure are summarized in Fig. 3. The catalysts with MgO as support were found to have a significantly higher catalytic activity than the ones with A1203 as support. On both supports, alkali promotion increased the catalytic activity strongly. The non-aqueously prepared catalyst CszCO3-Ru/MgO turned out to be almost twice as active than the CsNO3-Ru/MgO catalyst prepared by aqueous impregnation. The Ru /K-AI203 catalyst was found to be nearly as active as CsNO3-Ru/AI203. The activity of the Cs2CO3-Ru/MgO catalyst exceeded that of a multiply-promoted Fe-based catalyst significantly. Based on 114 ~ m o l / g Ru surface atoms (Ru:H -- 1:1) and a NH 3 synthesis rate of 1.23

452 F. Rosowski et al . /Applied Catalysis A." General 151 (1997) 443-460

\ = Cs2CO 3 - RU / MgO 0.8 / , , ' ~ ~ ~ CsNO 3 - RU / MgO

o~ m / ' / ~ ~ Ru / MgO o I:: # ~f-"~" + CsNO3- Ru / AI203

0.6 \ A Ru / K - AI20 3 ~=o / / " s ~ R u / A I 2 0 3

~= / / D \ \ ~ thermodynamic / / _ ~ . equilibrium "1-

0.2

$

0 t I I I I I i I I I I k I I I I I I I ~ I I I

550 600 650 700 750 Temperature / K

Fig. 3. NH 3 mole fractions in the reactor effluent observed subsequent to reduction for 138 mg of catalyst using a total flow of 40 cm 3 (STP)/min with XH[XN2 ~ 3 :1 at atmospheric pressure. Traces A - F (from bottom to top) were obtained with R u / A I 2 0 3 , Ru/K-A1203 , CsNO3-Ru /AI203 , Ru/MgO, C s N O 3 - R u / M g O and Cs2CO 3 - R u / M g O , respectively. Trace G was obtained with a multiply-promoted Fe catalyst (KMI, Haldor Topscie). The corresponding NH 3 equilibrium mole fractions at atmospheric pressure are displayed as dashed line.

txmol/(s g) at 588 K using a synthesis gas flow of 40 cm 3 (STP)/min, a TOF of about 10 - 2 S-1 is derived for C s 2 C O 3 - R u / M g O which is one of the highest values reported so far in the literature.

The values of the TOF at atmospheric pressure and at 20 bar derived from the steady-state kinetic measurements after at least 14 days on stream are summa- rized in Table 4. For the calculation of the TOF, the Ru metal areas specified in Table 2 were used. The alkali-promoted catalysts were found to deactivate to some extent initially. It is evident that alkali promotion is mandatory to obtain highly active NH 3 synthesis catalysts. Comparing the TOFs at 588 K reveals that the difference between the least active Ru /AI203 catalyst and the most active Cs2CO3-Ru /MgO catalyst is almost two orders of magnitude.

Fig. 4 displays the effluent NH 3 mole fraction (XNH ~) as a function of temperature for the feed gas compositions XH=: XN2: XAr = 1 : 1:2 (trace A), 3:1:0 (trace B), and 1:3:0 (trace C). Traces C and B illustrate the enhancing effect of a higher mole fraction of N 2 and the inhibiting effect of a higher mole fraction of H 2 in the feed gas, respectively. All reaction orders and apparent activation energies derived from the steady-state kinetic experiments following the analysis given in Ref. [15] are summarized in Table 5. The reaction orders of NH 3 were determined by varying the synthesis gas flow between 40 c m 3 (STP)/min and 160 cm 3 (STP)/min. The reaction orders of N 2 and H 2 were derived by varying

F. Rosowski et al. /Appl ied Catalysis A: General 151 (1997) 443-460 453

Table 4 Turn-over frequencies obtained from steady-state kinetic measurements after at least 14 days on stream using a

_ z s u r f a c e X synthesis gas composition of XH2.XN2 -- 3:1. The Ru metal areas tnRu ) were derived from H 2 chemisorp- tion (Table 2) using a stoichiometry of H:Ru = 1:1. The TOF values have been calculated according to

XNH3" Q TOF =

22414cm 3 (STP) /mol . n~Uu rfac~

Catalyst P (bar) Q (cm 3 (STP)/min) TOF (s- 1) at

588 K 623 K 673 K

Ru/MgO 1 120 7.5.10 -4 1.6.10-3 3.7.10- 3 20 120 1.1.10 3 3.0.10-3 8.8.10-3

Ru/AI203 1 120 1 /0 .10 -4 2.5.10 -4 6.5.10 -4 20 120 2.5-10 -4 7.5.10 -4 2.5-10- 3

CsNOs-Ru /MgO 1 (initial) 40 5.3.10 -3 equi. equi. 1 120 2.9.10 -3 8.3.10 -3 equi. 20 120 3.0.10 -3 1.0.10 -2 4.8.10-2

CsNO 3-Ru/AI203 1 120 1.7- 10 4 5.9.10-4 2.8- 10-3 20 120 2.8.10 -4 8.3.10 -4 3.6.10 -3

CseCO 3 - R u / M g O 1 (initial) 40 1.0.10 -2 equi. equi. 1 120 6.6.10 3 1.3.10 -2 equi. 20 40 5.8.10 -3 1.9.10 -2 - 50 100 5.8.10 -3 1.9.10 2 8.9.10-2

Ru/K-AI203 1 120 1.3.10 -4 2.9.10 -4 1.1 10-3

the H2:N 2 ratio between 3:1 and 1:3 using a total flow of 120 cm 3 (STP)/min with Ar as balance gas. Both determinations were carded out in the temperature range specified in Table 5 ensuring the measurements to be performed in the kinetically controlled regime far from equilibrium [27].

CsNO 3 - Ru / MgO f E 1 bar r 4 / Q . . o. 3000 120 cm3(STP) / min /

"='~o H2:N2:Ar o 1 : 1 : 2 Z~ I \ _ .OO0o / j

E

z

1000 = : : :

U J

0 I i I I i I i i i i I J i I I I

500 550 600 650 Tempera tu re / K

Fig. 4. Dependence of the NH 3 effluent mole fraction on the feed gas composition observed for CsNO 3 - Ru/MgO. Trace A was obtained with XH2:Xr%:XAr = 1:1:2, trace B with XH2:XN2:XAr = 3:1:0, trace C with XH2:XN2:XAr = 1:3:0, respectively, using a total flow of 120 cm 3 (STP)/min.

454 F. Rosowski et al. / Applied Catalysis A: General 151 (1997) 443-460

Table 5

Power-law exponents r = kapp'PNH3"PN," P~t 2 and apparent activation energies as a function of the total pressure determined in the given temperature range using a total flow of 120 cm 3 (STP) /min . The latter were

derived from Arrhenius plots at constant flow rate. The accuracy of the determination of the power-law

exponents and of the apparent activation energy is about _+ 0.1 and _+ 5 k J / m o l , respectively

Catalyst P (bar) T range (K) a ( N H 3) /3(N 2) y (H 2) E a ( k J / m o l )

R u / M g O 1 513-603 - 0.3 0.8 - 0.3 69

20 573-663 - 0.3 1.0 - 0.5 78

R u / A l z O 3 l 593-663 - 0 . 4 0.9 - 0 . 1 70

20 573-688 - 0 . 5 0.9 - 0 . 3 76

CsNO 3 - R u / M g O 1 498-570 0.0 0.7 - 0.7 96 20 550-630 0.0 0.8 - 0.9 109

C s N O 3 - R u / A I 2 0 3 1 543 608 0.0 0.7 - 0 . 6 103

20 573-663 0.0 0.9 - 0.6 101

C s e C O 3 - R u / M g O 1 493-543 0.0 0.7 - 0 . 6 88

R u / K - A I 2 0 3 , Tm~ x = 673 K 1 588-633 0.0 1.0 - 0 . 5 125

R u / K - A I 2 0 3 , T, nax = 773 K I 558-633 - 0 . 2 0.9 - 0 . 3 105

The effect of alkali promotion on the power-law kinetics is threefold: first, the reaction order for NH 3 is changed to essentially zero, second, the inhibiting effect of H e is stronger, and third, the apparent activation energy is higher by more than 20 kJ/mol . The reaction order of N 2 is found to be close to 1.0 reflecting that the dissociative chemisorption of N 2 is the rate-determining step of NH 3 synthesis for all catalysts under all experimental conditions studied. Contrary to the results obtained by Aika et al. [15] and Baris et al. [16], the reaction order for H e was negative for all catalysts investigated. The positive reaction order for H 2 reported by Aika et al. [15] and Baris et al. [16] for Ru/A1203 and R u / M g O may be due to the presence of chlorine originating from RuC13 used for the catalyst preparation.

The reaction orders indicate a rather low fractional coverage of atomic nitrogen (O N ) under synthesis conditions which is further decreased in the case of alkali-promoted catalysts. On Fe-based catalysts, the catalytic activity is significantly reduced at high XNH 3 leading to high ON blocking further N 2 dissociation [18]. This effect is obviously absent on Ru surfaces. The interaction of N 2 with supported Ru catalysts has been studied recently in great detail [28,29].

In our recent publication about the interaction of N 2 with Ru/MgO, it was assumed that a small fraction of the total Ru metal area dominated the rate of NH 3 synthesis, and that such promoted sites might originate from the interaction with the MgO support [29]. Comparing the power-law rate expressions of R u / M g O and Ru /AlzO 3 listed in Table 5 reveals that the rate of NH 3 synthesis over R u / M g O is somewhat less inhibited by NH 3 and somewhat stronger inhibited by H 2. However, the differences are rather small and do therefore not allow to identify R u / M g O as alkaline-earth promoted catalyst. Following the approach by Holzman et al. [14], Arrhenius plots at constant XNH 3

F. Rosowski et aL / Applied Catalysis A: General 151 (1997)443-460 455

were used to derive apparent activation energies Ua x from conversion measure- ments at atmospheric pressure. For Ru/A1203, a value of 100 kJ/mol was obtained with 150 ppm < XNH 3 < 600 ppm which is significantly higher than the value of 70 kJ/mol obtained at constant flow. For Ru/MgO, a value of 68 kJ/mol was obtained with 500 ppm

456 F. Rosowski et al. / Applied Catalysis A: General 151 (1997) 443-460

E Q.

9000

E 8000 O ".= 7000 O

6000

5000 O

E 4000

3~ 3000 Z

2000

~ 1000

W 0

C s 2 C O 3 - Ru / M g O

i . . . . i J . . . . 'l L I

t l , l l J , I , , i J , i , l l , , J l l t l ,

100 104 108 112 116 120 124

3000

2500

2000

1500

1000

500

0 7 7 3

7 2 3

6 7 3

523

588

Ru / K - A I203

i r

144148152156160164168

7 7 3

7 2 3 ~ "1

6 7 3

E I -

Time on stream / h

Fig. 5. Influence of heating to 773 K on the NH 3 effluent mole fractions (solid lines) as a function of time on stream observed for C s 2 C O 3 - R u / M g O (left) and for R u / K - A I 2 0 3 (right) using a total f low of 40 cm 3 (STP)/min with xH2:xN2 = 3:1 at atmospheric pressure. The corresponding temperature profiles are shown as dashed lines.

heating both catalysts up to 773 K. The decrease of XNH 3 from 4750 ppm to 3600 ppm observed at 588 K for the Cs2CO3-Ru/MgO catalyst after 8 h at 773 K indicates that the catalyst indeed deactivated to some extent due to the heat treatment. However, after the second heat treatment, no further changes of the catalytic activity were found until the end of the run after 42 days on stream. For R u / K - A 1 2 0 3, the deactivation from XNU 3 = 1350 ppm to 1190 ppm observed at 673 K was less strong, and the catalytic activity was found to be stable already after the first heat treatment at 773 K.

The reaction orders of R u / K - A I 2 0 3 derived both prior and subsequent to ammonia synthesis at 773 K (Table 5) reveal that the degree of interaction of the K + O promoter with the Ru particles changed significantly. The increased inhibition by NH 3, the decreased inhibition by H 2 and the decrease in apparent activation energy indicate a transition from an efficiently alkali-promoted state to a less promoted state. It is plausible to assume that the K + O promoter was removed from the Ru metal surfaces at 773 K by desorption and diffusion to the support. Hence, only the interface between the Ru metal particles and the K-doped A1203 support should be promoted corresponding to Ru/MgO. This assumption is supported by the similar reaction orders of R u / M g O and R u / K - A120 3 after ammonia synthesis at 773 K. The greater affinity of Cs + O to the acidic Al203 support is presumably also the reason why the promotion with Cs is much more efficient on the basic MgO support (Fig. 3). This conclusion is supported by the results of recent X P S / I S S measurements [30].

F. Rosowski et al. / Applied Catalysis A: General 151 (1997) 443-460

3.4. Kinetics at high pressure

457

The reaction orders of N 2 and H 2 for alkali-promoted catalysts were found to compensate each other almost completely in the kinetically controlled tempera- ture regime both at atmospheric pressure and at 20 bar. Thus, an increase in total pressure from 1 bar to 50 bar (traces A-D in Fig. 6) with a constant synthesis gas composition of XH2:XN2 = 3:1 did not lead to a significant increase in conversion for CsNO3-Ru/A1203 at low temperatures.

The analysis of the power-law kinetics therefore raises the question whether alkali-promoted Ru catalysts can be applied under industrial high-pressure synthesis conditions. Due to the inhibiting effect of XH2, and the accelerating effect of XN~, it is obvious that the 3:1 ratio of XH2:XN2 has to be decreased in order to obtain higher rates of NH 3 formation. However, non-stoichiometric feed-gas compositions are less favorable because of thermodynamic reasons. Hence, the Xla2:XN2 ratio was varied between 5:95 and 75:25 at 50 bar using C s 2 C O 3 - R u / M g O to find a compromise between kinetics and thermodynamics. The resulting steady-state XNH 3 as a function of temperature using a total flow of 100 cm 3 (STP)/min is shown in Fig. 7. The ratio of XNH:XN: = 5:95 (trace A) leads to a significant rate of NH 3 formation at temperatures as low as 520 K. When the temperature was higher than about 570 K, XNH ~ was controlled by thermodynamic equilibrium leading to a decrease in XNH ~. The maximum conversion was achieved at 720 K by using a ratio of XH2:XN2 = 60:40 (trace G)

3.5 CsNO 3 - Ru / AI20 3 /

--. 3 40 cm3(STP) / min / / ~ D

._~_o 2 .5 O p = 50 bar ~ ~ 2 [ ] p = 2 0 b a r / ~ C

E o ,', p = 5 bar / / 1,5 0 p = 1 bar ~

"-r / z

2

o.5 A

- 0 ~ I ~ " 1 I I I I I h I L J I I I I i i I I

6 0 0 6 5 0 7 0 0 7 5 0

T e m p e r a t u r e / K

Fig. 6. Dependence of the NH 3 effluent mole fraction on the feed gas composition observed for CsNO 3 - R u / A I 2 0 3 as a function of total pressure, Traces A - D (from bottom to top) were obtained at 1 bar, 5 bar, 20 bar, and 50 bar, respectively, using a total flow of 40 cm 3 (STP) /min with XH2:XN2 = 3:1.

458 F. Rosowski et al. /Appl ied Catalysis A: General 151 (1997) 443-460

8

8 6

=73 z 2

C s 2 C O 3 - Ru / MgO F ' , ' G 50 bar ' x / 100 cma(sTP)/min

H2 : N2 D ~ , 75:25 , , ~ " , , "., 60 : 40 50:50 C -..... ",, ~7 40 : 60 / ~ ~ ~ ~ - O 30 : 70 A / "- .. A 20 :80 " . / , " . . [] l o : 9 o B ~ - e " o

540 570 600 630 660 690 720 750 Temperature / K

Fig. 7. Dependence of the NH 3 effluent mole fraction on the feed gas composition observed for Cs2CO 3- R u / M g O at 50 bar using a total flow of 100 cm 3 (STP)/min. Traces A - H (from bottom to top) were obtained with XHz:XN2, ratios of 5:95, 10:90, 20:80, 30:70, 40:60, 50:50, 60:40 and 75:25, respectively. The corresponding NH 3 equilibrium mole fractions at 50 bar are displayed as dashed lines.

equal to 3:2. Correspondingly, lower values than 3:1 can be found in patents by the M.W. Kellogg Company [31,32].

Finally, the activity of Cs2CO3-Ru/MgO under these optimized conditions is compared in Table 6 with the kinetic data published by Nielsen [33] using a multiply-promoted Fe catalyst (KMIR). It is evident that a few percent lower effluent XNH 3 can be obtained at about the same space velocity and temperature

Table 6 Experimental high-pressure data observed for the Fe-based KMIR catalyst [33] and the present Cs2CO 3- R u / M g O catalyst. For the KMIR, the catalyst bed density was 2.5 g / c m 3, and the catalyst weight was 3.125 g resulting in a catalyst volume in the reactor of 1.25 cm 3. The space velocity (SV) is based on the inlet flow rate Q at standard temperature and pressure (STP) and on the catalyst volume. For the CszCO 3 - R u / M g O catalyst, the catalyst volume in the reactor was 0.142 cm 3 using 0.138 g. Both reactors used were shown to operate as isothermal plug-flow reactors. The rate of ammonia synthesis under these conditions was not limited by transport phenomena as shown by Nielsen [33]

T (K) p (bar) SV (h- ] ) Q (cm a (STP)/min) Xn2:XN2 XNH 3 (%) Catalyst

723 107 11600 221 3:1 13.9 718 50 11531 25 3:2 10.2 723 107 16000 305 3:1 13.2 696 50 18 450 40 3:2 9.7 723 107 47100 899 3:1 9.8 718 50 46123 100 3:2 7.8

KMIR Cs2CO 3 - R u / M g O KMIR Cs2CO 3 - R u / M g O KM1R Cs2CO 3 - R u / M g O

F. Rosowski et al. /Applied Catalysis A: General 151 (1997) 443-460 459

conditions with, however, a pressure of 50 bar only instead of 107 bar necessary for the Fe-based KMIR catalyst.

4. Conclusions

The preparation of Ru catalysts from Ru3(CO)12 and high-purity supports resulted in long-term stable catalysts for NH 3 synthesis. For the reliable assessment of the Ru particle size, the combined application of H 2 chemisorp- tion, TEM and XRD was found to be necessary yielding about 2 nm as average Ru particle size on MgO and A1203. Impregnating R u / M g O with aqueous solutions of CsNO 3 was observed to cause a significant increase of the Ru particle size contrary to Ru/A1203 with essentially unchanged Ru particle sizes. The non-aqueous impregnation of R u / M g O with Cs2CO 3 in ethanol was found to be less detrimental to the Ru metal dispersion resulting in the most active catalyst with an initial TOF at 588 K of about 10 -2 s -1.

On Ru/MgO, promotion with Cs was found to result in a higher increase in TOF than on Ru/A1203 presumably due to a stronger degree of interaction between the Cs promoter and the acidic A1203 support. The determination of the apparent activation energy at constant xN~ 3 provided additional evidence that the higher TOF of R u / M g O compared with the TOF of Ru/A1203 originates from the promotion by the alkaline earth support. Unpromoted catalysts are hardly active for NH 3 synthesis, whereas the catalytic activity of efficiently Cs-promoted catalysts exceeds that of a multiply-promoted Fe cata- lyst at atmospheric pressure.

The rate of NH 3 formation over all catalysts was found to be inhibited by H 2. The power-law rate expressions derived at atmospheric pressure and at 20 bar provided three criteria for the effect of alkali promotion in addition to the increase in TOF by almost two orders of magnitude: First, the reaction order of NH 3 is changed to essentially zero, second, the reaction order of H 2 is more negative, and third, the apparent activation energy determined at constant flow is increased.

These criteria can be used as tools to elucidate the degree of interaction of the alkali promoter with the Ru metal particles. According to the power-law kinetics, pretreating A1203 with K2CO 3 prior to impregnation with Ru3(CO)12 resulted in a promoted catalyst which changed to a less promoted state when heating up to 773 K in synthesis gas.

Due to the inhibiting effect of H 2, a lower XH2:XN2 ratio has to be used under high pressure NH 3 synthesis conditions. For Cs2COa-Ru/MgO at 50 bar, the optimum ratio was found to be around 3:2, leading to an effluent XNH 3 which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar with roughly the same space velocity at nearly the same temperature. Hence, the alkali-promoted Ru catalysts can indeed be developed into a low-pressure alternative to the conventional Fe catalysts.

460 F. Rosowski et al. /Appl ied Catalysis A: General 151 (1997) 443-460

Acknowledgements

The authors benefited from discussions with K.-I. Aika, B. Fastrup, S.R. Tennison and R. Schl~Sgl and are grateful to W. Mahdi, J. Schiitze, M. Wesemann, U. Klengler, and N. Pffinder for technical assistance, and to Haldor TopsCe A / S for supplying the iron catalyst.

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