- Email: [email protected]
Ruthenium as catalyst for ammonia synthesis
J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary
Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.
317
Ruthenium as Catalyst for Ammonia Synthesis M. Muhler*, E Rosowski, O. Hinrichsen, A. Hornung and G. Ertl Fritz-Haber-Institut der Max-Planck-GeseUschaft Faradayweg 4-6, D- 14195 Berlin (Dahlem), Germany Five Ru-based catalysts were prepared to study the effect of the support and the role of the alkali promoter in NH8 synthesis: Ru/AlzOa, Ru/MgO, Cs-Ru/A1203 and Cs(K)-Ru/MgO. The catalysts were characterized by N2 physisorption, H2 chemisorption and XPS. The absence of chlorine- and sulphur containing compounds turned out to be important for the preparation of highly active catalysts. Power law expressions were derived from conversion measurements at atmospheric pressure and at 20 bar. For all catalysts, the reaction order for H2 was found to be negative suggesting that a Pr% / PrI2 ratio in the feed gas higher than 1 / 3 would be favourable for industrial NHs synthesis at high pressure. The microkinetic analysis of the temperature-programmed desorption and adsorption of N2 and of the kinetics of isotopic exchange demonstrated the enhancing influence of the Cs promoter on the rate of N2 dissociation and recombination. XPS measurements after thorough reduction revealed a shift of the Ru 3d5/2 peak to lower binding energy by about 1 eV in the presence of Cs suggesting an electronic promoter effect. 1. Introduction
Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [ 1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NHa with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 - 105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. In spite of the industrial importance, relatively few studies in the catalytic literature deal with the kinetics of NH3 synthesis over supported Ru catalysts [4-8]. These studies focus on the inhibiting influences of PH2 and PNH8 on the rate of NH3 formation. Alkali promotion was found to decrease the inhibition by PNH8 significantly thus causing an increase in the inhibiting ' effect of PH2 [5-7]. The reaction order for N2 was found to be essentially unity indicating that the dissociative chemisorption of N2 is the rate-determining step (rds) in the overall mechanism. On multiply promoted Fe catalysts, H2 has a positive reaction order due to high coverages of adsorbed atomic nitrogen (N-.) [9]. In our laboratory a systematic study is in progress aiming at a detailed understanding of the "Corresponding author
318 catalytic phenomena involved in the synthesis of ammonia on ruthenium. The following catalysts were prepared from Rua(CO)x2 and high-purity supports: Ruthenium supported on MgO (Ru/MgO) and AlzOa (Ru/A12Oa), potassium promoted Ru/MgO (K-Ru/MgO) and cesium promoted Ru/MgO (Cs-Ru/MgO) and Ru/AIzOa (Cs-Ru/AlzOa). The catalysts were characterized by N2 physisorption (BET area), H2 chemisorption and X-ray photoelectron spectroscopy (XPS). This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which pov~er-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (IER) 2aN 2 + a~ 2 = 2 29N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [ 10] following the concepts by Dumesic et al. [ 11].
2. Experimental The catalysts were prepared from high purity A12Os (99.99%, Johnson Matthey) or MgO (Puratronic, 99.996% metals basis, Johnson Matthey) and Rua(CO)I2 (Johnson Matthey) by wet impregnation in a rotary evaporator and subsequent heating in high vacuum following the procedures in ref. [12-14]. Details of the preparation are given in ref. [15]. The achieved metal loading was 5 wt. % Ru. The cesium-promoted catalysts Cs-Ru/MgO and Cs-Ru/A12Oz were obtained by impregnating the Ru/MgO or Ru/Al2Oa catalysts subsequent to heating in vacuum to 723 K with an aqueous solution of CsNOa (99.99 %, Strem). For the preparation of K-Ru/MgO, an aqueous solution of KNOa (99.997 %, Johnson Matthey) was used. The atomic ratios were Cs(K) / Ru - 1 / 1 for Cs(K)-Ru/MgO and Cs / Ru - 3 / 1 for Cs-Ru/A12Os. The Ru metal area was determined by volumetric H2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the procedure described in ref. [ 16]. Prior to chemisorption, the catalysts were activated by passing 80 Nml/min high-purity synthesis gas (PN2 / Pn2 -" 1 / 3) from a connected feed system through the U-tube and heating to 673 K for alkali-promoted catalysts or to 773 K for alkali-free catalysts with a heating rate of 1 K/rain. The BET area was measured by static N2 physisorption in the same set-up. The kinetic experiments were carried out in an all stainless steel microreactor system with three gas lines which could be operated at pressures up to 100 bar. The gases used had the following purities: Ar 99.9993%, N2 99.9993%, H2 99.9993%. The feed gas was further purified by means of a self-designed guard reactor [17]. Gas analysis was performed by a non-dispersive infrared detector (BINOS, Fisher-Rosemount) which was calibrated by using a reference gas mixture (Linde). 138 mg of the 250/zm-800/zm sieve fraction were used for the kinetic experiments resulting in bed heights of less than 15 mm which prevented limitations by heat or mass transport. The reduction was carried out in synthesis gas using 40 Nml/min with a heating ramp of 1 K/min up to 673 K. The spectroscopic investigations were carried out in a modified LHS 12 MCD system. For the XPS measurements (Mg Kct 1253.6 eV, 240 W power) a fixed analyser pass energy of 108 eV
319 was used resulting in a resolution of 1.1 eV FWHM of the Ag 3d5/2 peak. The binding energy scale was calibrated using EB(AU 4fr/2) - 84.0 eV. The samples were activated in a directly attached preparation chamber (base pressure < 10-Smbar) from which the sample could be transferred into the UHV analysis chamber (base pressure 1.10 - l ~ mbar) within 1 min. The reduction was carried out in 1000 mbar synthesis gas by heating with 2 K/min to 673 K followed by 3 h NHa synthesis at this temperature. The synthesis gas mixture was replaced several times during the reduction. Charging was corrected using Mg 2s at 88.1 eV as internal standard [14]. Quantitative data analysis was performed by subtracting stepped backgrounds and using empirical cross sections [ 18]. 3. Results and Discussion
As expected, 3,-A12Os (BET area 110 mS/g) turned out to be the more stable support with a higher surface area than MgO (BET area 52 mS/g). The BET area of Ru/AlsOa was found to be 104 m2/g after NHa synthesis at 773 K which decreased significantly to 70 mS/g as a result of cesium impregnation. After NHa synthesis at 773 K, the specific area of Ru/MgO was observed to be 25 mS/g compared with 52 mS/g found for the MgO support. Cesium impregnation caused a further decrease in specific area to 23 m2/g. Table 1 Results of the H2 chemisorption measurements after NHa synthesis based on H/Ru = 1/1. NH3 synthesis was run at 773 K with Ru/MgO and Ru/AlsOa, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assuming spherical particles. Catalyst H2 monolayer Metal area Dispersion Particle size / ~mol Hs/g / mS/g /% /nm Ru/MgO 130 12.9 53 1.9 Ru/AI203 118 11.7 48 2.1 Cs-Ru/MgO 69 6.8 28 3.6 Cs-Ru/AlsOs 100 9.9 41 2.5
The H2 chemisorption results are summarized in table 1. The Hs monolayer capacities were used to derive Ru metal dispersions and mean particle sizes assuming spherical particles. On both MgO and AlsOa, the impregnation with Rua(CO)12 resulted in mean particle sizes of about 2 nm after NHa synthesis at 773 K. It is remarkable that about the same Ru metal areas were obtained on MgO and A12Oa in spite of the largely differing BET areas of the supports. For the Cs-Ru/MgO catalyst, the amount of chemisorbed hydrogen was found to be reduced by about a factor of two. XRD measurements and TEM images revealed that the decrease in metal area is indeed due to sintering of the Ru metal particles [15]. The Ru/AlsOa catalyst was not significantly affected by the impregnation with CsNOa as shown by the increase in particle size from 2.1 nm to 2.5 nm derived from H2 chemisorption. The results of the conversion measurements at atmospheric pressure using 138 mg catalyst are shown in fig. 1. The following sequence in catalytic activity was observed: Cs-Ru/MgO > Ru/MgO > Cs-Ru/AlsOa > Ru/AlsOa. It is noteworthy that the catalytic activity of the Cs-Ru/MgO catalyst exceeds significantly the catalytic activity of a multiply promoted iron
320 catalyst (trace D in fig. 1A). A recent kinetic study provides evidence that the MgO support acts as alkaline earth promoter creating promoted sites at the interface [19]. The influence of the akali promoter is shown in fig. lB. The traces were obtained with somewhat deactivated catalysts after several weeks of NHa synthesis. Cesium ttmaed out to be a better promoter than potassium in agreement with the results obtained by Aika et al. [14]. The same authors furthermore suggest that Cs promotion is less efficient on A12Os since CsOH interacts mainly with the acidic support whereas on the basic MgO support more CsOH should be in contact with the Ru metal particles [ 14].
0.8
0 >
'I 0 R u / A I 2 0 3 - 0 Ru / MgO t a C s - R u / A i 2 0 3 " n + KNO3 tt & Ru / MgO & + CsNO3 <> Fe catalyst
-A
B
Z~Cs - Ru / MgO
0.6
e'-
._o r tO ~ 0.4 0 0
-r e-
~
%%
0.2
%%%
w
,
550
l
,
600
,
,
l
l
,
650
, ,
,
i
,
700
Teml:~mture / K
,
,
l
l
,
750
I
4,
1
,
,
#
550
,
,
9
,
|
600
,
,
,
1
|
,
650
J
,
1
l
,
l
700
Temperature / K
Figure 1. NHa concentration in the reactor effluent gas using a total flow of 40 Nml/min with
PN2 / PI-I2 - 1 / 3 at atmospheric pressure. Traces A-E in fig.lA (from bottom to top) were obtained with Ru/AlzOa, Cs-Ru/AlzOa, Ru/MgO, a multiply promoted iron-based catalyst, and Cs-Ru/MgO. The corresponding NI-Ia equilibrium concentration is displayed as dashed line. Traces A-C in fig.1B (from bottom to top) were obtained with Ru/MgO, K-Ru/MgO, and Cs-Ru/MgO.
It is known that chlorine acts as severe poison for NHa synthesis [20,21]. Hence recent kinetic studies used chlorine-free Ru precursors like Rus(COh2 [8,22] or Ru(NO)(NOs)a [7]. In addition to chlorine, the presence of sulphur was found to poison Ru catalysts. Fig. 2A demonstrates that both poisons may originate from the Ru precursor. The binding energies for the C1 2p peak and of the S 2p peak observed for Ru prepared form RuO2 are typical for chloride and sulfide anions, respectively [23]. Ru prepared from Rua(CO)x2 was found to have a significantly higher purity. As shown in fig. 2B, sulphur and chlorine impurities can also originate from the support. The XPS data of MgO with a purity of 98 % reveal the presence
321 of chloride and sulphate anions which are essentially absent in MgO with a purity of 99.999 %. Hence it is mandatory to use high-purity Ru precursors and supports to prepare poison-free catalysts.
A
I XPS
C, 2 p
,- ~t~l(
Ru from
d
Ru from
I 198.6 e V RU3(00)12 , .... , . . , //L... 200 195 170
J .... 205
162.1 e V , i i , , , i .... 165 160
, 155
Binding Energy / eV
!
MgO 98%
~
MgO 99.999% ,
,
.
I
200
,
,
,
,
i
.
190
.
.
.
i
.
.
.
.
180
i
170
!
i
i
!
I
!
'
'
160
Binding Energy / eV
Figure 2. XPS C1 2p and S 2p data obtained with RuO2 and Rua(CO)x2 after reduction in synthesis gas at 1 bar and 673 K and transfer in UHV (Fig. 2A, upper half) and with two different MgO supports with purities of 98% and 99.999%, respectively (fig. 2B, lower half).
The quantitative XPS results obtained after reduction in synthesis gas are summarized in table 2. The observed ratios of O / Mg = 1.1 / 1 and O / A1 = 3.4 / 2 are in reasonable agreement with the stoichiometric ratios. The Ru concentration determined by XPS is somewhat higher for Ru/MgO than for Ru/A1203 in agreement with the results obtained with Hz chemisorption. Cs impregnation leads to a stronger decrease in the amount of Ru observed by XPS for Cs-Ru/MgO than for Cs-Ru/A12Oz due to the sintering of the Ru metal particles on MgO. The decrease of the Ru / support ratio observed for both catalysts may also be due to the shielding of the
322 Table 2 Surface composition determined by XPS after reduction in synthesis gas at 1 bar at 673 K and transfer in UHV. Catalyst Cs 3d O ls Ru 3p Mg 2s A1 2s Ru/MgO 50.4 3.3 46.4 Ru/A12Os 61.9 2.1 36.0 Cs-Ru/MgO 5.7 57.9 1.2 35.2 Cs-Ru/Al2Os 6.3 58.8 1.6 33.2
underlying Ru metal particle by a CsOH overlayer. Although a Cs / Ru ratio of 3 / 1 was used for the preparation of Cs-Ru/Al~Os, the ratio observed by XPS is similar to the ratio found for Cs-Ru/MgO. The latter catalyst was prepared with a Cs / Ru ratio of 1 / 1. This result indicates that the excess of Cs used for Cs-Ru/Al2Os has reacted with the bulk forming ternary phases with A12Os. Table 3 Power law exponents r - l~m,-~tt8 .PaN2.P~2 and apparent activation energies as a function of the total pressure determined in the given temperature range. The accuracy of the determination of the power law exponents and of the apparent activation energy is about q-0.1 and -4- 5 kJ/mol, respectively. Catalyst Pressure Temperature range c~(NHa) j3(N2) 7(H2) E,, / bar /K / kJ/mol Ru/MgO 1 513 - 603 -0.3 0.8 -0.3 69 20 573 - 663 -0.3 1.0 -0.5 78 Ru/AlzOs 1 593 - 663 -0.4 0.9 -0.1 70 20 573 - 688 -0.5 0.9 -0.3 76 Cs-Ru/MgO 1 498- 570 0.0 0.7 -0.7 96 20 550 - 630 0.0 0.8 -0.9 109 Cs-Ru/AlzO3 1 543 - 608 0.0 0.7 -0.6 103 20 573 - 663 0.0 0.9 -0.6 101
The results of the conversion measurements are summarized in table 3. The power law exponents and the apparent activation energies were derived following the analysis given in ref. [5]. The reaction orders of NHa and the reaction orders of N~ and H~ were determined by varying the synthesis gas flow between 40 Nml/min and 160 Nml/min and by varying the N2 / H2 ratio between 3 / 1 and 1 / 3 using a total flow of 120 Nml/min, respectively. Both determinations were carded out in the temperature range specified in table 3 ensuring the measurements to be in the kineticaUy controlled regime far from equilibrium. From the data shown in table 3, it is evident that the effect of Cs promotion on the power law kinetics is twofold: First, the reaction order for NHs is changed to essentially zero, and secondly, the apparent activation energy is higher by more than 20 kJ/mol in the presence of Cs. Contrary to the results obtained by Aika et al. [5], the reaction order for H2 was negative for all catalysts investigated. The positive reaction order for H~ reported by Aika et al. [5] for
323 Ru/Al2Oa and Ru/MgO may be due to the presence of chlorine originating from RuCla used for catalyst preparation. Fig. 3A shows the effluent NHa concentration observed for Ru/MgO as a function of reaction temperature for three different PN2 / Pn2 / P A r ratios at 20 bar total pressure. It is obvious that the reaction orders for N2 and H2 have opposite signs. Fig. 3B illustrates that the reaction orders for N2 and H2 partly compensate each other in the kinetically controlled temperature regime. Hence an increase in total pressure with a constant PN2 / Pn2 = 1 / 3 ratio does not lead to a significant increase in conversion at lower temperatures. For the application of alkali-promoted Ru catalysts under industrial synthesis conditions, it is necessary to find a compromise between kinetics and thermodynamics by increasing the PN2 / PH= ratio. The optimum observed for Cs-Ru/MgO prepared from Cs2COa at 50 bar is at about PN2 / Pn2 = 40 / 60 [15]. The high NHa concentration of about 8 % obtained with 0.138 g catalyst using a total flow of 100 Nml/min clearly shows that Ru catalysts have indeed the potential to replace Fe-based catalysts in industrial synthesis [ 15].
,, ,,
1.8 1.6 0
>
, ,,,,,,,,,,,,,,,,,,,,,
_
, ,,
A
B Ru / M g O
/P /
1
1220b~rl/min
cO
-
Ru / M g O 40 N m l / m i n
/
1.4
~ 1.2
4.5
3.5 _
N2 : H2 :Ar
//~t~
C
/
\
3
_
o ta v o
e,o o o
m 0.8 -r 7 -,-. 0.6 C :~
IJJ
3: :0
"
.
/ 7
2.5 2 _
p = 1bar p -- 9bar p = 20bar p = 50bar
1.5
"
0.4 0.2
0.5
0
i
550
600
650
700
Temperature / K
750
I
I
l
s
550
I
1
1
[
600
l
1
1
1
650
1
I
l
I
700
1
~
I
II
750
Temperature / K
Figure 3. Dependence of the NHa effluent concentrations observed for Ru/MgO on the feed gas composition (fig. 3A, left figure) and on the total pressure. In fig. 3A, trace A was obtained with PN2 / Pn2 / P A r - 1 / 1 / 2, trace B with PN2 / Pn2 / P A r = 1 / 3 / 0, trace C with PN2 / Pn2 / P A r - 3 / 1 / 0, respectively, using a total flow of 120 Nml/min at 20 bar. In fig. 3B, traces A-D (from bottom to top) were obtained at 1 bar, 9 bar, 20 bar and 50 bar, respectively, using a total flow of 40 Nml/min with PN~ / Pn2 - 1 / 3.
The reaction orders for N2 observed for all catalysts were close to 1.0 indicating that the
324 dissociative chemisorption of Nz is the rate-determining step in NHs synthesis. The kinetics of the interaction of Nz with Ru/MgO, Ru/AlzOa and Cs-Ru/MgO have been studied recently by performing N2 TPD and N2 TPA experiments and by determining the rate of isotopic exchange
28N2 + a~
- 2
2aN2 [ 2 4 ] .
Table 4 Rate constants ki - Ai- exp(-Ei / RT) for N2 + 2 , - 2 N - - , . Units of A/are ( torr- s) -~ for the forward reaction and s-1 for the reverse reaction forward rate constant reverse rate constant catalyst preexponential activation energy preexponential activation energy factor (kJ/mol) factor (kJ/tool) Cs-Ru/MgO 7.4.10 ~ 33.0 2.0.10 x~ 137.0 Ru/MgO 7.4.10 ~ 48.0 1.5.101~ 158.0 Ru/AI20s 7.4.10 ~ 60.6 1.5.101~ 158.0 .
.
.
.
.
.
Table 4 summarizes the rate constants ki - - A i 9 exp(-E//RT) for the forward and the reverse reaction derived from our microkinetic analysis of the steady-state and transient experiments with the three catalysts, i.e. Cs-Ru/MgO, Ru/MgO, and Ru/A12Os catalyst [24]. The rate constants in table 4 for Ru/A12Os should be considered as initial rate constants since it was not possible to achieve a higher coverage of N - - , than 0.25. Furthermore, it was not possible to detect TPA peaks for Ru/AlzOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of N2 due to the presence of promoted active sites which dominate under NHa synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [ 19]. The N~ TPD, N2 TPA and IER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N2 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with N2. On polycrystalline Ru samples, IR measurements by Aika and Tamaru [25] revealed the influence of the alkali promoter on the stretching frequency of N2 -- * which was interpreted in the frame of a charge transfer mechanism. XPS should be the appropriate technique to detect charge transfer from the promoter to the Ru metal clusters. The Ru 3d spectrum of the Ru/MgO precursor after heating in vacuum to 723 K in order to decompose the adsorbed Ru carbonyl compounds is shown as lower trace in fig.4. The binding energy of the Ru 3ds/~ peak indicates that Ru is not yet reduced to the metallic state. Furthermore, the intensity ratio of the Ru 3da/2 and Ru 3d5/2 peaks shows that significant amounts of carbon compounds are present giving rise to overlapping C ls peaks at about 285 - 290 eV. After reduction (trace in the middle), the binding energy of the Ru 3d5/2 peak was found to be 280.0 eV indicating complete reduction to Ru metal. After reduction of the Cs-Ru/MgO catalyst, the Ru 3d speaks were observed to be shifted by 1 eV to lower binding energy (top trace in fig.4). It has to be noted that the Mg 2s peak had to be used as internal standard (EB(Mg 2s) - 88.1 eV) to correct for charging. However, the MgO bulk should not be affected by cesium impregnation. The XPS shift is influenced by many factors like the extraatomic relaxation energy which might change due to the presence of a Cs+O coadsorbate layer resulting from the decomposition of presumably CsOH as mobile
325
1 eV "~/~
XPS Ru3d
8000
r r
6OOO
r r-
-'-" C: 4000
2000
I
0 295
290
285
' x_..,
I ,
280
275
Binding Energy / eV
Figure 4. XPS Ru 3d data observed for the Ru/MgO catalysts. The Ru 3d spectra (from bottom to top) were obtained with the precursor after heating in high vacuum to 773 K , after reduction in 1 bar synthesis gas up to 773 K, and after impregnation with aqueous CsNOa solution and subsequent reduction in synthesis gas up to 673 K.
species. Since this shift was only observed after thorough reduction in the directly attached preparation chamber with rapid transfer in UHV, it seems plausible to assume that the treatment at 673 K in 750 mbar H2 (purity 99.9999 %) caused a partial reduction of the Cs+O coadsorbate layer thus creating oxygen vacancies which might serve as electron-donating adsorption sites for N2. Further studies are in progress to clarify this hypothesis. 4. Conclusions The preparation of Ru-based catalysts from high-purity supports using Rus(CO)xz followed by impregnation with aqueous Cs solution was shown to result in stable and active NHs synthesis catalysts. Cs-Ru/MgO was found to have a higher catalytic activity at atmospheric pressure than a multiply promoted Fe-based catalyst. Power law expressions were derived from conversion measurements at atmospheric pressure and at 20 bar. For all catalysts, the reaction order for H2 was found to be negative suggesting that a higher PN2 / Px2 ratio in the feed gas than 1 / 3 would be favourable for industrial NHa synthesis at high pressure. Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NHa synthesis without alkali
326 promotion. Ru/MgO turned out to be a heterogeneous system with respect to the adsorption and desorption of N2 due to the presence of promoted active sites which dominate under NHa synthesis conditions. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with N2. XPS results provide evidence for an electronic promoter effect. REFERENCES 1. S.R. Tennison, in Catalytic Ammonia Synthesis, Plenum Press, New York, (Ed. J.R. Jennings), 1st. ed. (1991 ) 303. 2. EJ. Shires, J.R. Cassata, B.G. Mandelik, C.E van Dijk, U.S. Patent, 4479925 (1984) Oct. 30. 3. T.A. Czuppon, S.A. Knez, R.V. Schneider IN, G. Worobets, Chem. Engineering, March
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
18. 19. 20. 21. 22. 23. 24. 25.
1993, presented at the 1993 AICHE Ammonia Safety Symposium, Sept. 1993, Orlando, Florida, 100, No.3 (1993) 19. ER. Holzman, W.K. Shiflett, J.A. Dumesic, J. Catal., 62 (1980) 167. K. Aika, M. Kumasaka, T. Oma, O. Kato, H. Matsuda, N. Watanabe, K. Yamazaki, A. Ozaki, T. Onishi, Appl. Catal., 28 (1986) 57. H. Bails, M. Glinski, J. Kijenski, A. Wokaun, A. Baiker, Appl. Catal., 28 (1986) 295. J.U. Nwalor, J.G. Goodwin Jr., Topics in Catal., 1 (1994) 285. E Moggi, G. Albanesi, G. Predieri, G. Spoto, Appl. Catal. A, 123 (1995) 145. L.M. Aparicio, J.A. Dumesic, Topics in Catal., 1 (1994) 233. O. Hinrichsen, E Rosowski, M. Muhler, G. Ertl, Chem. Eng. Sci., Proc. of the 14th Int. Symposium on Chem. React. Engineering, (1996) accepted. J.A. Dumesic, D.E Rudd, L.M. Aparicio, J.E. Rekoske, A.A. Trevino, The Microkinetics of Heterogeneous Catalysis, ACS Professional Reference Book, Washington, DC, (1993). H. Kn6zinger, Y. Zhao, B. Tesche, R. Barth, R. Epstein, B.C. Gates, J.E Scott, Faraday Discuss. Chem. Soc., 72 (1982) 53. E Moggi, G. Predieri, G. Albanesi, S. Papadopulos, E. Sappa, Appl. Catal., 53 (1989) L 1. K. Aika, T. Takano, S. Murata, J. Catal., 136 (1992) 126. E Rosowski, A. Hornung, O. Him'ichsen, D. Herein, M. Muhler, G. Ertl, Appl. Catal., (1996) in preparation. R.A. Dalla Betta, J. Catal., 34 (1974) 57. B. Fastrup, H.N. Nielsen, Catal. Lett., 14 (1992) 233. Practical Surface Analysis, John Wiley, Chichester, (Ed. D. Briggs, M.E Seah) , 2nd ed. (1994). E Rosowski, O. Hinrichsen, M. Muhler, G. Ertl, Catal. Lett., (1996) accepted. W.K. Shiflett, J.A. Dumesic, Ind. Eng. Chem. Fundam., 20 (1981 ) 246. S. Murata und K.-I. Aika, Appl. Catal. A, 82 (1992) 1. Y. Kadowaki, S. Murat~ K.-I. Aika, Stud. Surf. Sci. Catal., Elsevier Science Publishers, (Ed. L. Guczi, E Solymosi, E Tetenyi), 75 (1993) 2055. J.E Moulder, W.E Stickle, EE. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, (1992). O. Hinrichsen, E Rosowski, A. Hornung, M. Muhler, G. Ertl, J. Catal., (1996) submitted. K. Aika, K. Tamaru, in Ammonia: Catalysis and Manufacture, Springer-Verlag, Berlin, (Ed. A. Nielsen), 1st ed. (1995).
Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.
317
Ruthenium as Catalyst for Ammonia Synthesis M. Muhler*, E Rosowski, O. Hinrichsen, A. Hornung and G. Ertl Fritz-Haber-Institut der Max-Planck-GeseUschaft Faradayweg 4-6, D- 14195 Berlin (Dahlem), Germany Five Ru-based catalysts were prepared to study the effect of the support and the role of the alkali promoter in NH8 synthesis: Ru/AlzOa, Ru/MgO, Cs-Ru/A1203 and Cs(K)-Ru/MgO. The catalysts were characterized by N2 physisorption, H2 chemisorption and XPS. The absence of chlorine- and sulphur containing compounds turned out to be important for the preparation of highly active catalysts. Power law expressions were derived from conversion measurements at atmospheric pressure and at 20 bar. For all catalysts, the reaction order for H2 was found to be negative suggesting that a Pr% / PrI2 ratio in the feed gas higher than 1 / 3 would be favourable for industrial NHs synthesis at high pressure. The microkinetic analysis of the temperature-programmed desorption and adsorption of N2 and of the kinetics of isotopic exchange demonstrated the enhancing influence of the Cs promoter on the rate of N2 dissociation and recombination. XPS measurements after thorough reduction revealed a shift of the Ru 3d5/2 peak to lower binding energy by about 1 eV in the presence of Cs suggesting an electronic promoter effect. 1. Introduction
Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [ 1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NHa with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 - 105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. In spite of the industrial importance, relatively few studies in the catalytic literature deal with the kinetics of NH3 synthesis over supported Ru catalysts [4-8]. These studies focus on the inhibiting influences of PH2 and PNH8 on the rate of NH3 formation. Alkali promotion was found to decrease the inhibition by PNH8 significantly thus causing an increase in the inhibiting ' effect of PH2 [5-7]. The reaction order for N2 was found to be essentially unity indicating that the dissociative chemisorption of N2 is the rate-determining step (rds) in the overall mechanism. On multiply promoted Fe catalysts, H2 has a positive reaction order due to high coverages of adsorbed atomic nitrogen (N-.) [9]. In our laboratory a systematic study is in progress aiming at a detailed understanding of the "Corresponding author
318 catalytic phenomena involved in the synthesis of ammonia on ruthenium. The following catalysts were prepared from Rua(CO)x2 and high-purity supports: Ruthenium supported on MgO (Ru/MgO) and AlzOa (Ru/A12Oa), potassium promoted Ru/MgO (K-Ru/MgO) and cesium promoted Ru/MgO (Cs-Ru/MgO) and Ru/AIzOa (Cs-Ru/AlzOa). The catalysts were characterized by N2 physisorption (BET area), H2 chemisorption and X-ray photoelectron spectroscopy (XPS). This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which pov~er-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (IER) 2aN 2 + a~ 2 = 2 29N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [ 10] following the concepts by Dumesic et al. [ 11].
2. Experimental The catalysts were prepared from high purity A12Os (99.99%, Johnson Matthey) or MgO (Puratronic, 99.996% metals basis, Johnson Matthey) and Rua(CO)I2 (Johnson Matthey) by wet impregnation in a rotary evaporator and subsequent heating in high vacuum following the procedures in ref. [12-14]. Details of the preparation are given in ref. [15]. The achieved metal loading was 5 wt. % Ru. The cesium-promoted catalysts Cs-Ru/MgO and Cs-Ru/A12Oz were obtained by impregnating the Ru/MgO or Ru/Al2Oa catalysts subsequent to heating in vacuum to 723 K with an aqueous solution of CsNOa (99.99 %, Strem). For the preparation of K-Ru/MgO, an aqueous solution of KNOa (99.997 %, Johnson Matthey) was used. The atomic ratios were Cs(K) / Ru - 1 / 1 for Cs(K)-Ru/MgO and Cs / Ru - 3 / 1 for Cs-Ru/A12Os. The Ru metal area was determined by volumetric H2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the procedure described in ref. [ 16]. Prior to chemisorption, the catalysts were activated by passing 80 Nml/min high-purity synthesis gas (PN2 / Pn2 -" 1 / 3) from a connected feed system through the U-tube and heating to 673 K for alkali-promoted catalysts or to 773 K for alkali-free catalysts with a heating rate of 1 K/rain. The BET area was measured by static N2 physisorption in the same set-up. The kinetic experiments were carried out in an all stainless steel microreactor system with three gas lines which could be operated at pressures up to 100 bar. The gases used had the following purities: Ar 99.9993%, N2 99.9993%, H2 99.9993%. The feed gas was further purified by means of a self-designed guard reactor [17]. Gas analysis was performed by a non-dispersive infrared detector (BINOS, Fisher-Rosemount) which was calibrated by using a reference gas mixture (Linde). 138 mg of the 250/zm-800/zm sieve fraction were used for the kinetic experiments resulting in bed heights of less than 15 mm which prevented limitations by heat or mass transport. The reduction was carried out in synthesis gas using 40 Nml/min with a heating ramp of 1 K/min up to 673 K. The spectroscopic investigations were carried out in a modified LHS 12 MCD system. For the XPS measurements (Mg Kct 1253.6 eV, 240 W power) a fixed analyser pass energy of 108 eV
319 was used resulting in a resolution of 1.1 eV FWHM of the Ag 3d5/2 peak. The binding energy scale was calibrated using EB(AU 4fr/2) - 84.0 eV. The samples were activated in a directly attached preparation chamber (base pressure < 10-Smbar) from which the sample could be transferred into the UHV analysis chamber (base pressure 1.10 - l ~ mbar) within 1 min. The reduction was carried out in 1000 mbar synthesis gas by heating with 2 K/min to 673 K followed by 3 h NHa synthesis at this temperature. The synthesis gas mixture was replaced several times during the reduction. Charging was corrected using Mg 2s at 88.1 eV as internal standard [14]. Quantitative data analysis was performed by subtracting stepped backgrounds and using empirical cross sections [ 18]. 3. Results and Discussion
As expected, 3,-A12Os (BET area 110 mS/g) turned out to be the more stable support with a higher surface area than MgO (BET area 52 mS/g). The BET area of Ru/AlsOa was found to be 104 m2/g after NHa synthesis at 773 K which decreased significantly to 70 mS/g as a result of cesium impregnation. After NHa synthesis at 773 K, the specific area of Ru/MgO was observed to be 25 mS/g compared with 52 mS/g found for the MgO support. Cesium impregnation caused a further decrease in specific area to 23 m2/g. Table 1 Results of the H2 chemisorption measurements after NHa synthesis based on H/Ru = 1/1. NH3 synthesis was run at 773 K with Ru/MgO and Ru/AlsOa, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assuming spherical particles. Catalyst H2 monolayer Metal area Dispersion Particle size / ~mol Hs/g / mS/g /% /nm Ru/MgO 130 12.9 53 1.9 Ru/AI203 118 11.7 48 2.1 Cs-Ru/MgO 69 6.8 28 3.6 Cs-Ru/AlsOs 100 9.9 41 2.5
The H2 chemisorption results are summarized in table 1. The Hs monolayer capacities were used to derive Ru metal dispersions and mean particle sizes assuming spherical particles. On both MgO and AlsOa, the impregnation with Rua(CO)12 resulted in mean particle sizes of about 2 nm after NHa synthesis at 773 K. It is remarkable that about the same Ru metal areas were obtained on MgO and A12Oa in spite of the largely differing BET areas of the supports. For the Cs-Ru/MgO catalyst, the amount of chemisorbed hydrogen was found to be reduced by about a factor of two. XRD measurements and TEM images revealed that the decrease in metal area is indeed due to sintering of the Ru metal particles [15]. The Ru/AlsOa catalyst was not significantly affected by the impregnation with CsNOa as shown by the increase in particle size from 2.1 nm to 2.5 nm derived from H2 chemisorption. The results of the conversion measurements at atmospheric pressure using 138 mg catalyst are shown in fig. 1. The following sequence in catalytic activity was observed: Cs-Ru/MgO > Ru/MgO > Cs-Ru/AlsOa > Ru/AlsOa. It is noteworthy that the catalytic activity of the Cs-Ru/MgO catalyst exceeds significantly the catalytic activity of a multiply promoted iron
320 catalyst (trace D in fig. 1A). A recent kinetic study provides evidence that the MgO support acts as alkaline earth promoter creating promoted sites at the interface [19]. The influence of the akali promoter is shown in fig. lB. The traces were obtained with somewhat deactivated catalysts after several weeks of NHa synthesis. Cesium ttmaed out to be a better promoter than potassium in agreement with the results obtained by Aika et al. [14]. The same authors furthermore suggest that Cs promotion is less efficient on A12Os since CsOH interacts mainly with the acidic support whereas on the basic MgO support more CsOH should be in contact with the Ru metal particles [ 14].
0.8
0 >
'I 0 R u / A I 2 0 3 - 0 Ru / MgO t a C s - R u / A i 2 0 3 " n + KNO3 tt & Ru / MgO & + CsNO3 <> Fe catalyst
-A
B
Z~Cs - Ru / MgO
0.6
e'-
._o r tO ~ 0.4 0 0
-r e-
~
%%
0.2
%%%
w
,
550
l
,
600
,
,
l
l
,
650
, ,
,
i
,
700
Teml:~mture / K
,
,
l
l
,
750
I
4,
1
,
,
#
550
,
,
9
,
|
600
,
,
,
1
|
,
650
J
,
1
l
,
l
700
Temperature / K
Figure 1. NHa concentration in the reactor effluent gas using a total flow of 40 Nml/min with
PN2 / PI-I2 - 1 / 3 at atmospheric pressure. Traces A-E in fig.lA (from bottom to top) were obtained with Ru/AlzOa, Cs-Ru/AlzOa, Ru/MgO, a multiply promoted iron-based catalyst, and Cs-Ru/MgO. The corresponding NI-Ia equilibrium concentration is displayed as dashed line. Traces A-C in fig.1B (from bottom to top) were obtained with Ru/MgO, K-Ru/MgO, and Cs-Ru/MgO.
It is known that chlorine acts as severe poison for NHa synthesis [20,21]. Hence recent kinetic studies used chlorine-free Ru precursors like Rus(COh2 [8,22] or Ru(NO)(NOs)a [7]. In addition to chlorine, the presence of sulphur was found to poison Ru catalysts. Fig. 2A demonstrates that both poisons may originate from the Ru precursor. The binding energies for the C1 2p peak and of the S 2p peak observed for Ru prepared form RuO2 are typical for chloride and sulfide anions, respectively [23]. Ru prepared from Rua(CO)x2 was found to have a significantly higher purity. As shown in fig. 2B, sulphur and chlorine impurities can also originate from the support. The XPS data of MgO with a purity of 98 % reveal the presence
321 of chloride and sulphate anions which are essentially absent in MgO with a purity of 99.999 %. Hence it is mandatory to use high-purity Ru precursors and supports to prepare poison-free catalysts.
A
I XPS
C, 2 p
,- ~t~l(
Ru from
d
Ru from
I 198.6 e V RU3(00)12 , .... , . . , //L... 200 195 170
J .... 205
162.1 e V , i i , , , i .... 165 160
, 155
Binding Energy / eV
!
MgO 98%
~
MgO 99.999% ,
,
.
I
200
,
,
,
,
i
.
190
.
.
.
i
.
.
.
.
180
i
170
!
i
i
!
I
!
'
'
160
Binding Energy / eV
Figure 2. XPS C1 2p and S 2p data obtained with RuO2 and Rua(CO)x2 after reduction in synthesis gas at 1 bar and 673 K and transfer in UHV (Fig. 2A, upper half) and with two different MgO supports with purities of 98% and 99.999%, respectively (fig. 2B, lower half).
The quantitative XPS results obtained after reduction in synthesis gas are summarized in table 2. The observed ratios of O / Mg = 1.1 / 1 and O / A1 = 3.4 / 2 are in reasonable agreement with the stoichiometric ratios. The Ru concentration determined by XPS is somewhat higher for Ru/MgO than for Ru/A1203 in agreement with the results obtained with Hz chemisorption. Cs impregnation leads to a stronger decrease in the amount of Ru observed by XPS for Cs-Ru/MgO than for Cs-Ru/A12Oz due to the sintering of the Ru metal particles on MgO. The decrease of the Ru / support ratio observed for both catalysts may also be due to the shielding of the
322 Table 2 Surface composition determined by XPS after reduction in synthesis gas at 1 bar at 673 K and transfer in UHV. Catalyst Cs 3d O ls Ru 3p Mg 2s A1 2s Ru/MgO 50.4 3.3 46.4 Ru/A12Os 61.9 2.1 36.0 Cs-Ru/MgO 5.7 57.9 1.2 35.2 Cs-Ru/Al2Os 6.3 58.8 1.6 33.2
underlying Ru metal particle by a CsOH overlayer. Although a Cs / Ru ratio of 3 / 1 was used for the preparation of Cs-Ru/Al~Os, the ratio observed by XPS is similar to the ratio found for Cs-Ru/MgO. The latter catalyst was prepared with a Cs / Ru ratio of 1 / 1. This result indicates that the excess of Cs used for Cs-Ru/Al2Os has reacted with the bulk forming ternary phases with A12Os. Table 3 Power law exponents r - l~m,-~tt8 .PaN2.P~2 and apparent activation energies as a function of the total pressure determined in the given temperature range. The accuracy of the determination of the power law exponents and of the apparent activation energy is about q-0.1 and -4- 5 kJ/mol, respectively. Catalyst Pressure Temperature range c~(NHa) j3(N2) 7(H2) E,, / bar /K / kJ/mol Ru/MgO 1 513 - 603 -0.3 0.8 -0.3 69 20 573 - 663 -0.3 1.0 -0.5 78 Ru/AlzOs 1 593 - 663 -0.4 0.9 -0.1 70 20 573 - 688 -0.5 0.9 -0.3 76 Cs-Ru/MgO 1 498- 570 0.0 0.7 -0.7 96 20 550 - 630 0.0 0.8 -0.9 109 Cs-Ru/AlzO3 1 543 - 608 0.0 0.7 -0.6 103 20 573 - 663 0.0 0.9 -0.6 101
The results of the conversion measurements are summarized in table 3. The power law exponents and the apparent activation energies were derived following the analysis given in ref. [5]. The reaction orders of NHa and the reaction orders of N~ and H~ were determined by varying the synthesis gas flow between 40 Nml/min and 160 Nml/min and by varying the N2 / H2 ratio between 3 / 1 and 1 / 3 using a total flow of 120 Nml/min, respectively. Both determinations were carded out in the temperature range specified in table 3 ensuring the measurements to be in the kineticaUy controlled regime far from equilibrium. From the data shown in table 3, it is evident that the effect of Cs promotion on the power law kinetics is twofold: First, the reaction order for NHs is changed to essentially zero, and secondly, the apparent activation energy is higher by more than 20 kJ/mol in the presence of Cs. Contrary to the results obtained by Aika et al. [5], the reaction order for H2 was negative for all catalysts investigated. The positive reaction order for H~ reported by Aika et al. [5] for
323 Ru/Al2Oa and Ru/MgO may be due to the presence of chlorine originating from RuCla used for catalyst preparation. Fig. 3A shows the effluent NHa concentration observed for Ru/MgO as a function of reaction temperature for three different PN2 / Pn2 / P A r ratios at 20 bar total pressure. It is obvious that the reaction orders for N2 and H2 have opposite signs. Fig. 3B illustrates that the reaction orders for N2 and H2 partly compensate each other in the kinetically controlled temperature regime. Hence an increase in total pressure with a constant PN2 / Pn2 = 1 / 3 ratio does not lead to a significant increase in conversion at lower temperatures. For the application of alkali-promoted Ru catalysts under industrial synthesis conditions, it is necessary to find a compromise between kinetics and thermodynamics by increasing the PN2 / PH= ratio. The optimum observed for Cs-Ru/MgO prepared from Cs2COa at 50 bar is at about PN2 / Pn2 = 40 / 60 [15]. The high NHa concentration of about 8 % obtained with 0.138 g catalyst using a total flow of 100 Nml/min clearly shows that Ru catalysts have indeed the potential to replace Fe-based catalysts in industrial synthesis [ 15].
,, ,,
1.8 1.6 0
>
, ,,,,,,,,,,,,,,,,,,,,,
_
, ,,
A
B Ru / M g O
/P /
1
1220b~rl/min
cO
-
Ru / M g O 40 N m l / m i n
/
1.4
~ 1.2
4.5
3.5 _
N2 : H2 :Ar
//~t~
C
/
\
3
_
o ta v o
e,o o o
m 0.8 -r 7 -,-. 0.6 C :~
IJJ
3: :0
"
.
/ 7
2.5 2 _
p = 1bar p -- 9bar p = 20bar p = 50bar
1.5
"
0.4 0.2
0.5
0
i
550
600
650
700
Temperature / K
750
I
I
l
s
550
I
1
1
[
600
l
1
1
1
650
1
I
l
I
700
1
~
I
II
750
Temperature / K
Figure 3. Dependence of the NHa effluent concentrations observed for Ru/MgO on the feed gas composition (fig. 3A, left figure) and on the total pressure. In fig. 3A, trace A was obtained with PN2 / Pn2 / P A r - 1 / 1 / 2, trace B with PN2 / Pn2 / P A r = 1 / 3 / 0, trace C with PN2 / Pn2 / P A r - 3 / 1 / 0, respectively, using a total flow of 120 Nml/min at 20 bar. In fig. 3B, traces A-D (from bottom to top) were obtained at 1 bar, 9 bar, 20 bar and 50 bar, respectively, using a total flow of 40 Nml/min with PN~ / Pn2 - 1 / 3.
The reaction orders for N2 observed for all catalysts were close to 1.0 indicating that the
324 dissociative chemisorption of Nz is the rate-determining step in NHs synthesis. The kinetics of the interaction of Nz with Ru/MgO, Ru/AlzOa and Cs-Ru/MgO have been studied recently by performing N2 TPD and N2 TPA experiments and by determining the rate of isotopic exchange
28N2 + a~
- 2
2aN2 [ 2 4 ] .
Table 4 Rate constants ki - Ai- exp(-Ei / RT) for N2 + 2 , - 2 N - - , . Units of A/are ( torr- s) -~ for the forward reaction and s-1 for the reverse reaction forward rate constant reverse rate constant catalyst preexponential activation energy preexponential activation energy factor (kJ/mol) factor (kJ/tool) Cs-Ru/MgO 7.4.10 ~ 33.0 2.0.10 x~ 137.0 Ru/MgO 7.4.10 ~ 48.0 1.5.101~ 158.0 Ru/AI20s 7.4.10 ~ 60.6 1.5.101~ 158.0 .
.
.
.
.
.
Table 4 summarizes the rate constants ki - - A i 9 exp(-E//RT) for the forward and the reverse reaction derived from our microkinetic analysis of the steady-state and transient experiments with the three catalysts, i.e. Cs-Ru/MgO, Ru/MgO, and Ru/A12Os catalyst [24]. The rate constants in table 4 for Ru/A12Os should be considered as initial rate constants since it was not possible to achieve a higher coverage of N - - , than 0.25. Furthermore, it was not possible to detect TPA peaks for Ru/AlzOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of N2 due to the presence of promoted active sites which dominate under NHa synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [ 19]. The N~ TPD, N2 TPA and IER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N2 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with N2. On polycrystalline Ru samples, IR measurements by Aika and Tamaru [25] revealed the influence of the alkali promoter on the stretching frequency of N2 -- * which was interpreted in the frame of a charge transfer mechanism. XPS should be the appropriate technique to detect charge transfer from the promoter to the Ru metal clusters. The Ru 3d spectrum of the Ru/MgO precursor after heating in vacuum to 723 K in order to decompose the adsorbed Ru carbonyl compounds is shown as lower trace in fig.4. The binding energy of the Ru 3ds/~ peak indicates that Ru is not yet reduced to the metallic state. Furthermore, the intensity ratio of the Ru 3da/2 and Ru 3d5/2 peaks shows that significant amounts of carbon compounds are present giving rise to overlapping C ls peaks at about 285 - 290 eV. After reduction (trace in the middle), the binding energy of the Ru 3d5/2 peak was found to be 280.0 eV indicating complete reduction to Ru metal. After reduction of the Cs-Ru/MgO catalyst, the Ru 3d speaks were observed to be shifted by 1 eV to lower binding energy (top trace in fig.4). It has to be noted that the Mg 2s peak had to be used as internal standard (EB(Mg 2s) - 88.1 eV) to correct for charging. However, the MgO bulk should not be affected by cesium impregnation. The XPS shift is influenced by many factors like the extraatomic relaxation energy which might change due to the presence of a Cs+O coadsorbate layer resulting from the decomposition of presumably CsOH as mobile
325
1 eV "~/~
XPS Ru3d
8000
r r
6OOO
r r-
-'-" C: 4000
2000
I
0 295
290
285
' x_..,
I ,
280
275
Binding Energy / eV
Figure 4. XPS Ru 3d data observed for the Ru/MgO catalysts. The Ru 3d spectra (from bottom to top) were obtained with the precursor after heating in high vacuum to 773 K , after reduction in 1 bar synthesis gas up to 773 K, and after impregnation with aqueous CsNOa solution and subsequent reduction in synthesis gas up to 673 K.
species. Since this shift was only observed after thorough reduction in the directly attached preparation chamber with rapid transfer in UHV, it seems plausible to assume that the treatment at 673 K in 750 mbar H2 (purity 99.9999 %) caused a partial reduction of the Cs+O coadsorbate layer thus creating oxygen vacancies which might serve as electron-donating adsorption sites for N2. Further studies are in progress to clarify this hypothesis. 4. Conclusions The preparation of Ru-based catalysts from high-purity supports using Rus(CO)xz followed by impregnation with aqueous Cs solution was shown to result in stable and active NHs synthesis catalysts. Cs-Ru/MgO was found to have a higher catalytic activity at atmospheric pressure than a multiply promoted Fe-based catalyst. Power law expressions were derived from conversion measurements at atmospheric pressure and at 20 bar. For all catalysts, the reaction order for H2 was found to be negative suggesting that a higher PN2 / Px2 ratio in the feed gas than 1 / 3 would be favourable for industrial NHa synthesis at high pressure. Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NHa synthesis without alkali
326 promotion. Ru/MgO turned out to be a heterogeneous system with respect to the adsorption and desorption of N2 due to the presence of promoted active sites which dominate under NHa synthesis conditions. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with N2. XPS results provide evidence for an electronic promoter effect. REFERENCES 1. S.R. Tennison, in Catalytic Ammonia Synthesis, Plenum Press, New York, (Ed. J.R. Jennings), 1st. ed. (1991 ) 303. 2. EJ. Shires, J.R. Cassata, B.G. Mandelik, C.E van Dijk, U.S. Patent, 4479925 (1984) Oct. 30. 3. T.A. Czuppon, S.A. Knez, R.V. Schneider IN, G. Worobets, Chem. Engineering, March
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
18. 19. 20. 21. 22. 23. 24. 25.
1993, presented at the 1993 AICHE Ammonia Safety Symposium, Sept. 1993, Orlando, Florida, 100, No.3 (1993) 19. ER. Holzman, W.K. Shiflett, J.A. Dumesic, J. Catal., 62 (1980) 167. K. Aika, M. Kumasaka, T. Oma, O. Kato, H. Matsuda, N. Watanabe, K. Yamazaki, A. Ozaki, T. Onishi, Appl. Catal., 28 (1986) 57. H. Bails, M. Glinski, J. Kijenski, A. Wokaun, A. Baiker, Appl. Catal., 28 (1986) 295. J.U. Nwalor, J.G. Goodwin Jr., Topics in Catal., 1 (1994) 285. E Moggi, G. Albanesi, G. Predieri, G. Spoto, Appl. Catal. A, 123 (1995) 145. L.M. Aparicio, J.A. Dumesic, Topics in Catal., 1 (1994) 233. O. Hinrichsen, E Rosowski, M. Muhler, G. Ertl, Chem. Eng. Sci., Proc. of the 14th Int. Symposium on Chem. React. Engineering, (1996) accepted. J.A. Dumesic, D.E Rudd, L.M. Aparicio, J.E. Rekoske, A.A. Trevino, The Microkinetics of Heterogeneous Catalysis, ACS Professional Reference Book, Washington, DC, (1993). H. Kn6zinger, Y. Zhao, B. Tesche, R. Barth, R. Epstein, B.C. Gates, J.E Scott, Faraday Discuss. Chem. Soc., 72 (1982) 53. E Moggi, G. Predieri, G. Albanesi, S. Papadopulos, E. Sappa, Appl. Catal., 53 (1989) L 1. K. Aika, T. Takano, S. Murata, J. Catal., 136 (1992) 126. E Rosowski, A. Hornung, O. Him'ichsen, D. Herein, M. Muhler, G. Ertl, Appl. Catal., (1996) in preparation. R.A. Dalla Betta, J. Catal., 34 (1974) 57. B. Fastrup, H.N. Nielsen, Catal. Lett., 14 (1992) 233. Practical Surface Analysis, John Wiley, Chichester, (Ed. D. Briggs, M.E Seah) , 2nd ed. (1994). E Rosowski, O. Hinrichsen, M. Muhler, G. Ertl, Catal. Lett., (1996) accepted. W.K. Shiflett, J.A. Dumesic, Ind. Eng. Chem. Fundam., 20 (1981 ) 246. S. Murata und K.-I. Aika, Appl. Catal. A, 82 (1992) 1. Y. Kadowaki, S. Murat~ K.-I. Aika, Stud. Surf. Sci. Catal., Elsevier Science Publishers, (Ed. L. Guczi, E Solymosi, E Tetenyi), 75 (1993) 2055. J.E Moulder, W.E Stickle, EE. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, (1992). O. Hinrichsen, E Rosowski, A. Hornung, M. Muhler, G. Ertl, J. Catal., (1996) submitted. K. Aika, K. Tamaru, in Ammonia: Catalysis and Manufacture, Springer-Verlag, Berlin, (Ed. A. Nielsen), 1st ed. (1995).