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Carbon-supported promoted Ru catalyst for ammonia synthesis
Applied Catalysis A: General 185 (1999) 269–275
Carbon-supported promoted Ru catalyst for ammonia synthesis Lucio Forni a,∗ , Daniele Molinari a , Ilenia Rossetti a , Nicola Pernicone b a
Dipartimento de Chimica Fisica ed Elettrochimica, Università di Milano, Via C.Golgi, 19 20133 Milano, Italy b Via Pansa, 7 28100 Novara, Italy Received 5 March 1999; accepted 6 May 1999
Abstract A series of alkali- and/or alkali-earth-promoted, carbon-supported Ru catalysts have been prepared by impregnation from aqueous solutions of the precursors. The supports have been pretreated by heating in inert atmosphere at various temperatures, followed by partial oxidation in air at 425◦ C and then by hydrogen treatment at 900◦ C. The catalyst samples, diluted 1/22 with quartz powder, have been studied in ammonia synthesis by means of a bench-scale, downflow, continuous, tubular reactor, under standard reaction conditions (430◦ C, 100 bar total pressure, H2 /N2 = 1.5/1 feeding ratio). Caesium and Barium proved to be much more effective promoters than Potassium as promoters and the optimal temperature range for support pretreatment was found to be about 1900◦ C. The ammonia productivity, on a catalyst volume basis, of our best Ru catalyst was about twice higher than that of the most widely used Fe-based commercial catalysts. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Ammonia synthesis; Promoted Ru–C catalyst for; Effect of C pretreatment
1. Introduction During the last decades a continuous trend can be recognized towards lower pressure in ammonia synthesis process, which clearly requires more active catalysts. As the intrinsic activity of the presently used Fe catalyst was already optimized a long time ago, this requirement was firstly accomplished by decreasing catalyst particle size, through the introduction of radial reactors [1]. This was really a breakthrough in ammonia technology, but still not sufficient. So the use of Fe-based alloy catalysts was also tried (mainly Fe–Co) [2], but there remain still many doubts about their advantages. Fe catalysts preparation by magnetite precipitation, instead of melting, was tried too [3], but, in spite of the good activity, industrial production by this ∗ Corresponding author. Tel.: +39-2-266-03-289; fax: +39-2-706-38-129; e-mail: [email protected]
procedure appeared very cumbersome. So the problem seems unlikely to be resolved by the development of a modified Fe catalyst. Ru was known to be more active than Fe since a long time [2], but its high price requires the development of a supported catalyst. However, it appeared very soon that the usual oxidic carriers do not give good catalytic activity and that carbon supports are not stable under synthesis conditions. Only recently it was found that some active carbons, when subjected to special thermal treatments, can give active and stable Ru catalysts [2]. Such supports, together with proper Ru promotion, allowed to develop the first commercial non-Fe ammonia process [4,5]. However, the stability of the catalyst under high pressure hydrogen remains a key point. For a deep study of a catalytic reaction, aiming at practical application, the availability of some commercial catalysts or at least a catalyst very similar to a
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commercial one is needed. In the present case, however, the catalyst employed in industrial reactors is proprietary and not commercially available. Hence, as a first action of our research programme, the preparation method had to be studied, mainly on the basis of patent literature. Particularly, our first aim has been the study of a simple procedure for preparing an active and stable Ru/C catalyst, amenable to be produced on industrial scale.
2. Experimental
Table 1 Surface area of the supports employed Supporta
BET SA (m2 /g)
Pore volume (at P/P0 = 0.99) (cm3 /g)
G G1900 G2000 G1900/OR E E1500 E1500/OR E2000 E2700
1188 187 105 870 1253 964 1472 575 6
0.49 0.11 0.08 0.44 0.75 0.72 0.99 0.43 n.d.
a
2.1. Materials High purity (≥99.9995 vol.%) SAPIO H2 and N2 were used as reactants. Pro-analysis chemical reagents from ALDRICH or ACROS were employed as supplied for catalyst preparation and for analytical purposes. 2.2. Support Two commercial carbons were used as support: a granular (1–4 mm) charcoal (G) and an extruded (4 mm in diameter) active carbon (E). G is a very pure coconut carbon, manufactured by directly grinding and sieving the carbonised and activated raw material, while E is an extruded material, produced from pine wood sawdust and containing up to 9% of ashes. Both carbons are available from the authors. The carbons were pretreated at high temperature (from 1500◦ C to 2700◦ C for 2 h) under inert atmosphere and then ground and sieved to 0.15–0.25 mm particles. The latter were cleaned from dusty fines and subjected to an oxidising treatment in flowing air, by heating up to 425◦ C, maintained for 12 h, and then cooling down to room temperature (r.t.) The weight loss during oxidation was ca. 20%. A final reductive treatment followed, in flowing H2 , by heating up to 900◦ C, maintained for 3 h, and then cooling down to r.t. in flowing N2 . Surface area and pore volume of as supplied and of variously treated carbon samples are given in Table 1. All the N2 adsorption isotherms of these supports were of type I, except for the E2700 sample, which possesses an elevated macroporosity, precluding the determination of the specific pore volume by N2 capillary condensation.
G, E = granular, extruded carbon; 1500, 1900, 2000, 2700 = heat treated in inert gas at indicated temperature (◦ C); O = oxidised in flowing air, R = reduced in flowing H2 after oxidation (see Section 2).
2.3. Preparation and characterisation of catalysts An aqueous solution of potassium ruthentate was employed as Ru precursor. Deposition of the ruthentate on the support was done by impregnation, followed by evaporation in rotavapor and drying (90◦ C) in oven for 30 min. Reduction of Ru was made in flowing H2 + N2 (3/1, v/v) gas mixture, by heating up to 320◦ C, then maintained for 2 h, and cooling down to r.t. in flowing N2 . The sample was then carefully washed till pH = 7 of the washing solution, to eliminate any residual Potassium. The addition of promoters was carried out by impregnation from a solution of the hydroxides (K, Cs) or of the nitrate (Ba). Evaporation of water in rotavapor and drying (90◦ C) followed. The transformation of the precursors into the active promoters was obtained during the in situ activation of the catalyst, before the reaction. The determination of Ru loading on the catalyst was done spectrophotometrically [6], on a Beckman DU-68 spectrometer, by measuring the absorbance at 465 nm of the solution obtained by KNO3 /KOH attack of the solid sample. BET surface area has been measured by N2 adsorption at liquid N2 temperature, by means of a Micromeritics ASAP 2010 apparatus. The effect of high temperature treatment (graphitisation) of the support has been qualitatively analysed by XRD, on a Philips PW 1820 powder diffractometer, by comparing the collected patterns with literature data files [7]. Some relevant data of the catalysts prepared are given in Table 2.
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Table 2 Compositiona and activity of the catalysts prepared Catalyst code 9 10 11 12 13 14 15 16 17 18 19 20
Support
Ru/wt.%
K/wt.%
Ba/wt.%
Cs/wt.%
ρ B b (g/cm3 )
Activityc
G1900/OR G1900/OR G1900/OR G2000 E E1500 E1500/OR G G1900 G1900/OR E2000 E2700
17.6 17.6 17.2 12.5 13.0 12.3 13.0 11.9 11.8 13.0 11.9 11.0
– 21.1 – 6.3 6.0 6.2 5.9 6.1 6.5 6.1 11.4 5.6
– – 22.4 7.2 6.7 6.2 6.5 6.6 6.6 7.4 11.2 5.2
22.1 – – 7.7 7.0 6.9 6.8 7.2 7.1 7.3 11.5 6.2
0.78 0.75 0.68 0.70 0.67 0.74 0.72 0.74 0.87 0.77 0.80 0.82
14.3 5.8 12.1 3.8 8.9 14.7 15.2 12.4 15.0 8.8 1.0 0.8
a
The chemical composition of the catalyst is referred to the total mass of the sample. ρ B = bulk density of the catalyst bed. c Expressed as mol.% NH in effluent gas at GHSV = 30 000 h−1 . 3 b
2.4. Catalytic activity testing apparatus and procedure Testing of catalytic activity has been carried out by means of a bench-scale, continuous system, centred on a vertical, downflow, Incoloy 800 tubular reactor, 12.7 × 9.0 mm in diameter and 40 cm in length, fitted with an axial thermowell of 1.6 mm external diameter. Heating of the reactor was accomplished by an electric furnace, through a massive metal block surrounding the reactor. Reactor temperature was controlled by an Eurotherm 904 TRC through a 425S solid-state-contactor power unit. Reactant gases flow rate was regulated by means of MKS 1261C mass flow meters and the reaction pressure was regulated by means of a home-made PTFE-membrane relief valve, pressurised with nitrogen at the desired value. A high-precision manometer, a bursting-disc safety device, a drechsel gas absorber and a volumetric wet-test meter completed the apparatus. All the catalysts have been tested under standard reaction conditions at 430◦ C and 100 bar (1 bar=105 Pa) total pressure, by loading 0.3 g of fresh catalyst, uniformly diluted with quartz powder of the same particle size (0.15–0.25 mm), so to have a quartz/catalyst volume ratio of 22/1 and a bed height of 6 cm. The bed was kept at the mid-length of the reactor by flocks of quartz wool. The void space over and below the catalyst bed was filled with quartz beads, 0.25–0.85 mm in size. The dilution of the catalyst with quartz rep-
resented an acceptable compromise, allowing a sufficient bed length/diameter ratio and minimisation of the temperature gradient generated by the exothermal NH3 synthesis reaction. Before the reaction the Ru-based catalyst was activated by flowing a 1.5/1 = H2 /N2 (v/v) gas mixture at GHSV = 20 000 h−1 and 30 bar total pressure, while increasing catalyst bed temperature (5◦ C/min) from r.t. up to 450◦ C, kept for 5 h and then cooling down to the reaction temperature. The Fe-based catalyst was activated in flowing H2 /N2 = 3/1 gas mixture at 30 bar and GHSV = 20 000 h−1 by increasing temperature by 50◦ C/h from r.t. to 350◦ C, then by 10◦ C/h from 350◦ C to 500◦ C, keeping such a temperature for 12 h and then cooling down to the reaction temperature. After leading total pressure to 100 bar, the activity tests were carried out at various GHSV values, by changing the gas mixture feeding rate. After every GHSV change the steady state was always attained within a few minutes. No activity decrease was observed even after several days on-stream. The H2 /N2 ratio was usually 1.5/1, but different ratios were also tested. The equilibrium concentration of ammonia in the outcoming gas for total pressure = 100 bar, reaction temperature = 430◦ C and H2 /N2 = 3/1 or 1.5/1 have been calculated as suggested in [8] and resulted ca. 19.4 and 17.7 vol.%, respectively. The GHSV values adopted in the present investigation usually ranged from 30 000 to 200 000 h−1 . GHSV is here defined as volumetric hourly flow rate
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of reacting gas, measured under standard temperature and pressure, referred to unit volume of catalyst. The activity was expressed as vol.% of NH3 in the effluent gas at GHSV = 30 000 h−1 . Testing of the resistance of catalysts was done by monitoring by gas chromatography the formation of methane when progressively increasing the reaction temperature up to about 700◦ C. 2.5. Analysis of the effluent gas The concentration of ammonia in the gas outcoming from the reactor was determined by absorption in an excess solution of H2 SO4 , by means of the drechsel absorber, through which a given amount of the effluent gas was bubbled. The excess of acid was then titrated with NaOH solution and bromocresol green as indicator.
Fig. 1. Catalytic activity versus reactants feeding ratio. GHSV = 60 000 h−1 , T = 430◦ C, P = 100 bar. (䉬) catalyst 17 and (䊏) catalyst 10 (Table 2).
3. Results and discussion 3.1. Activity tests The experimental variables of an activity test for ammonia synthesis catalysts are: temperature, pressure, feed composition, GHSV, catalyst particle size, catalyst dilution (optional). To choose the values to be used for each of them in our standard test, the following criteria have been adopted. 3.1.1. Temperature and pressure For an applied study values as close as possible to the industrial ones are recommended. Hence, we chose 430◦ C and 100 bar, respectively. In fact, tests at atmospheric pressure are not acceptable for the characterisation of ammonia synthesis catalysts, except for purely academic investigations. 3.1.2. Feed composition It is known [9] that, as to kinetic behaviour, Ru is different from Fe, for which a stoichiometric H2 /N2 mixture is the best choice. So we did experiments at different H2 /N2 ratios (Fig. 1). While the highest reaction rate corresponds to a ratio of about 1.0, the value of 1.5 was adopted in all our tests, in order to operate closer to the industrial conditions.
Fig. 2. Effect of alkaline or alkali-earth promoters on activity. T = 430◦ C, P = 100 bar and H2 /N2 = 1.5 (vol.). (䉬, N, 䊏): catalysts 9,11,10 (Table 2), respectively.
3.1.3. GHSV Inspection of typical conversion curves (Fig. 2) shows that at any GHSV value catalysts with different activity can be easily differentiated. A value of 30 000 h-1 was then chosen for our activity comparison standard tests, aiming at approaching, even if very partially, the industrial ones. 3.1.4. Catalyst particle size In order to have a value of the effectiveness factor very close to unity, a very small particle size was chosen (average diameter 0.2 mm). This is very different from the usual size (typically around 3 mm) of the industrial catalyst, but the adverse effect on activity can be minimised through a suitable external distribution of Ru. 3.1.5. Catalyst dilution As it is convenient to carry out the standard test as isothermally as possible, the exothermicity of the reaction forced us to strongly dilute the catalyst (1/22
L. Forni et al. / Applied Catalysis A: General 185 (1999) 269–275
in volume with quartz), so to keep the temperature gradient within a few degrees. The standard activity test so developed proved to allow the collection of very reliable data for catalyst comparison purposes.
3.2. Choice of precursors and effect of promoters There is evidence [10] that chlorine has an adverse effect on Ru catalysts for ammonia synthesis, but, in spite of this, RuCl3 has been mostly used for catalyst preparation. It is very difficult to control the residual amount of Cl in the catalyst, so lack of reproducibility may occur. On this respect, K2 RuO4 appears to be the most convenient precursor. For the same reason hydroxides or nitrates were used to introduce the promoters. On the basis of patent literature [4,5], K, Cs and Ba were chosen as promoters and their total loading was fixed to about 20 wt.%. Their relative promoting effect was checked by adding each of them alone on the same support (Fig. 2). They were found to rank in the following order: Cs>Ba K. The much higher promoting effect of Ba with respect to K has been reported also by Kowalczyk et al. [11], who found also a stabilising effect of Ba against catalyst overheating at 520◦ C. As for Cs, the same authors previously found [12] only a slightly higher promoting activity, with respect to K, contrary to our data and to those reported by Tennison [2]. The use of chlorinated precursors could well give misleading results, at least in some literature data.
3.3. Effect of active carbon Our two active carbons (G and E) were used without any pretreatment for the preparation of K–Ba–Cs-promoted catalysts. Catalyst composition and activity data are given in Table 2, samples 13 and 16. The lower purity of E could be the reason for the lower activity of catalyst 13, with respect to 16. Experimental data on different active carbons as supports for ammonia synthesis catalysts are not available in open literature, so no comparison can be made with previous data.
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3.4. Effect of carbon pretreatment It is well known that heat treatment of carbon at high temperature under inert atmosphere brings about a more or less deep graphitisation, independently of the nature of the original carbon. To make graphitisation, we treated our two active carbons at different temperatures, within the 1500–2700◦ C range. The XRD analysis showed that the typical reflections of the graphite crystalline structure (d = 3.368, 2.838 and 1.678 Å) became progressively more intense with increasing the pretreatment temperature. Furthermore, such a treatment produced a drop in surface area (Table 1), the more evident, the higher the preheating temperature, with a definite lower decrease for carbon E. On some graphitised carbons an oxidation–reduction treatment was performed (see Section 2), which allowed to roughly recover (carbon G) or even to overcome (carbon E) the original porosity (Table 1). Treatment at 2700◦ C showed clearly excessive, so the corresponding support was not investigated any more. The effect of heat treatment of the support on the activity of the final catalyst can be seen by comparing the data (Table 2) of samples 16, 17, 12, prepared on carbon G, with those of samples 13, 14, 19, prepared on carbon E. All these samples have a similar composition, differing only for the support nature and pretreatment. An increase of activity occurs on passing from catalyst 16 (untreated support) to catalyst 17 (support treated at 1900◦ C). However, the treatment at 2000◦ C (catalyst 12) led to a dramatic decrease of activity. A similar, but higher increase of activity may be noticed for catalysts prepared on carbon E, on passing from catalyst 13 (untreated support) to catalyst 14 (support treated at 1500◦ C), while carbon treated at 2000◦ C and 2700◦ C (samples 19, 20) again led to very low activity. It is likely that the high activity increase coming from graphitisation of carbon E is connected with simultaneous removal of impurities, as a drastic change in the porous texture (carbon G and G-1900) gives a much lower activity increase (Tables 1 and 2). On the basis of our present data, the oxidation– reduction treatment, following heating to high temperature, did not prove to be particularly useful. Indeed, on going from catalysts 14 to 15 (Table 2) the activity did not change substantially, while on going from catalysts 17 to 18 an appreciable decrease was observed.
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Fig. 3. Methane formation at increasing reaction temperature. Figures refer to catalyst code (Table 2). P = 100 bar, H2 /N2 = 1.5 (vol.). Behaviour of catalyst 19 (not reported in the figure) practically overlaps catalyst 12.
Unfortunately, no data are available in the open literature on this subject. 3.5. Catalyst stability The risk of support degradation by Ru-catalysed methanation under ammonia synthesis conditions has been recently pointed out [11]. This phenomenon could have dramatic consequences in industrial reactors, should the catalyst not be sufficiently resistant to methanation. Therefore, it is absolutely necessary to evaluate this property on a quantitative basis through a specially designed standard test. The test we have developed for this purpose consists in monitoring the methane formation during ammonia synthesis at 100 bar and temperature up to 700◦ C. This test appears to be more exhaustive than that used by Kowalczyk et al. [11], carried out at atmospheric pressure in hydrogen at 520◦ C. Our data of methane formation as a function of temperature are shown in Fig. 3. They refer to a group of catalysts of practically identical composition, except for the support, and show that catalysts 13, 14, 15, 16, prepared with untreated carbons or after heat treatment at 1500◦ C, started to form methane at ca. 500◦ C and that methane formation increased considerably with increasing temperature. Therefore, catalysts prepared on such supports cannot be taken into account for practical purposes. By contrast, when the carbons were treated at temperatures of at least 1900◦ C (catalysts 17, 18, 20), methane formation dropped dramatically at least up to 600◦ C, and grew slowly beyond this temperature.
Fig. 4. Comparison of Fe-based catalyst (䉬) with catalyst 17 (䊏) (Table 2). Reaction conditions as for Fig. 2 for the Ru-catalyst. For the Fe-based catalyst H2 /N2 = 3 (vol.).
The oxidation–reduction treatment (catalysts 14, 15 and 17, 18) does not seem to have an influence on methane formation. Therefore, it appears that the degree of carbon graphitisation is a primary factor for catalyst stability. In particular, heat treatment of the active carbon at 1500◦ C is insufficient to provide stability. 1900◦ C results to be an optimal temperature, as higher temperatures, while further increasing catalyst stability, have an adverse effect on activity. Of course, other variables in catalyst composition and preparation, which are currently being investigated, could appreciably influence catalyst stability. This kind of knowledge is of utmost importance for the development of industrial Ru-based ammonia synthesis catalysts. 3.6. Comparison with a commercial Fe-based catalyst Our presently best-performing catalyst as to both activity and stability (catalyst 17, Table 2) has been compared with one of the most widely used Fe-based commercial catalysts, under the optimal reaction conditions for each of the two samples. The results are shown in Fig. 4. On a volume basis the ammonia productivity, expressed as kg NH3 /(h m3 catalyst), of catalyst 17 is about twice that of Fe catalyst, in rough agreement with data reported by Kowalczyk et al. [12]. This activity ratio rises to about 8 when the two catalysts are compared on a weight basis and to about 80 when they are compared on a metal weight basis.
L. Forni et al. / Applied Catalysis A: General 185 (1999) 269–275
4. Conclusions The most important results of the present investigation may be summarised as follows: (i) a chlorine-free procedure, easily amenable to scale up, has been set up for the preparation of a stable carbon-supported Ru-based catalyst, much more active for the synthesis of ammonia than the usual Fe-based one; (ii) the much higher promoting effect of Ba and Cs, with respect to K, has been confirmed; (iii) the beneficial effect of heat pretreatment of the carbon has been outlined in terms of both activity and stability of catalyst and (iv) the heat pretreatment to ca. 1900◦ C allows to obtain a very active catalyst, in the absence of any further oxidation–reduction treatment. References [1] C.W. Hooper, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Plenum Press, New York, 1991, p. 253.
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[2] S.R. Tennison, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Plenum Press, New York, 1991, p. 303. [3] F. Montino, L. Balducci, F. Ferrero, N. Pernicone, US Patent No. 5,221,657, 22 June 1993, to MURST, Rome. [4] US Patent No. 4,163,775, 7 August 1979, to BP Co. [5] US Patent No. 4,568,532, 4 February 1986, to Kellogg, M.W. Co. [6] R.E. Connick, C.R. Hurley, J. Am. Chem. Soc. 74 (1952) 5012. [7] Selected Powder Diffraction Data, JCPDS, Swarthmore, PA, 1981, File No. 23–64. [8] E.W. Comings, High Pressure Technology, McGraw-Hill, New York, 1956, p. 360. [9] F. Rosowski, A. Hornung, O. Hinrichsen, D. Herein, M. Muhler, G. Ertl, Appl. Catal. A: General 151 (1997) 443. [10] N.M. Dobrynkin, P.G. Tsyrulnikov, A.S. Noskov, N.B. Shitova, I.A. Polukhina, G.G. Savelieva, V.K. Duplyakin, V.A. Likolobov, in: B. Delmon, P.A. Jacobs, R. Maggi, J.A. Martens, P. Grange, G. Poncelet (Ed.), Preparation of Catalysts VII, Elsevier, Amsterdam, 1998, p. 213. [11] Z. Kowalczyk, S. Jodzis, W. Raròg, J. Zielinski, J. Pielaszek, Appl. Catal. A: General 173 (1998) 153. [12] Z. Kowalczyk, J. Sentek, S. Jodzis, E. Mizera, J. Gòralski, T. Paryjczak, R. Diduszko, Catal. Lett. 45 (1997) 65.
Carbon-supported promoted Ru catalyst for ammonia synthesis Lucio Forni a,∗ , Daniele Molinari a , Ilenia Rossetti a , Nicola Pernicone b a
Dipartimento de Chimica Fisica ed Elettrochimica, Università di Milano, Via C.Golgi, 19 20133 Milano, Italy b Via Pansa, 7 28100 Novara, Italy Received 5 March 1999; accepted 6 May 1999
Abstract A series of alkali- and/or alkali-earth-promoted, carbon-supported Ru catalysts have been prepared by impregnation from aqueous solutions of the precursors. The supports have been pretreated by heating in inert atmosphere at various temperatures, followed by partial oxidation in air at 425◦ C and then by hydrogen treatment at 900◦ C. The catalyst samples, diluted 1/22 with quartz powder, have been studied in ammonia synthesis by means of a bench-scale, downflow, continuous, tubular reactor, under standard reaction conditions (430◦ C, 100 bar total pressure, H2 /N2 = 1.5/1 feeding ratio). Caesium and Barium proved to be much more effective promoters than Potassium as promoters and the optimal temperature range for support pretreatment was found to be about 1900◦ C. The ammonia productivity, on a catalyst volume basis, of our best Ru catalyst was about twice higher than that of the most widely used Fe-based commercial catalysts. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Ammonia synthesis; Promoted Ru–C catalyst for; Effect of C pretreatment
1. Introduction During the last decades a continuous trend can be recognized towards lower pressure in ammonia synthesis process, which clearly requires more active catalysts. As the intrinsic activity of the presently used Fe catalyst was already optimized a long time ago, this requirement was firstly accomplished by decreasing catalyst particle size, through the introduction of radial reactors [1]. This was really a breakthrough in ammonia technology, but still not sufficient. So the use of Fe-based alloy catalysts was also tried (mainly Fe–Co) [2], but there remain still many doubts about their advantages. Fe catalysts preparation by magnetite precipitation, instead of melting, was tried too [3], but, in spite of the good activity, industrial production by this ∗ Corresponding author. Tel.: +39-2-266-03-289; fax: +39-2-706-38-129; e-mail: [email protected]
procedure appeared very cumbersome. So the problem seems unlikely to be resolved by the development of a modified Fe catalyst. Ru was known to be more active than Fe since a long time [2], but its high price requires the development of a supported catalyst. However, it appeared very soon that the usual oxidic carriers do not give good catalytic activity and that carbon supports are not stable under synthesis conditions. Only recently it was found that some active carbons, when subjected to special thermal treatments, can give active and stable Ru catalysts [2]. Such supports, together with proper Ru promotion, allowed to develop the first commercial non-Fe ammonia process [4,5]. However, the stability of the catalyst under high pressure hydrogen remains a key point. For a deep study of a catalytic reaction, aiming at practical application, the availability of some commercial catalysts or at least a catalyst very similar to a
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 4 4 - 1
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commercial one is needed. In the present case, however, the catalyst employed in industrial reactors is proprietary and not commercially available. Hence, as a first action of our research programme, the preparation method had to be studied, mainly on the basis of patent literature. Particularly, our first aim has been the study of a simple procedure for preparing an active and stable Ru/C catalyst, amenable to be produced on industrial scale.
2. Experimental
Table 1 Surface area of the supports employed Supporta
BET SA (m2 /g)
Pore volume (at P/P0 = 0.99) (cm3 /g)
G G1900 G2000 G1900/OR E E1500 E1500/OR E2000 E2700
1188 187 105 870 1253 964 1472 575 6
0.49 0.11 0.08 0.44 0.75 0.72 0.99 0.43 n.d.
a
2.1. Materials High purity (≥99.9995 vol.%) SAPIO H2 and N2 were used as reactants. Pro-analysis chemical reagents from ALDRICH or ACROS were employed as supplied for catalyst preparation and for analytical purposes. 2.2. Support Two commercial carbons were used as support: a granular (1–4 mm) charcoal (G) and an extruded (4 mm in diameter) active carbon (E). G is a very pure coconut carbon, manufactured by directly grinding and sieving the carbonised and activated raw material, while E is an extruded material, produced from pine wood sawdust and containing up to 9% of ashes. Both carbons are available from the authors. The carbons were pretreated at high temperature (from 1500◦ C to 2700◦ C for 2 h) under inert atmosphere and then ground and sieved to 0.15–0.25 mm particles. The latter were cleaned from dusty fines and subjected to an oxidising treatment in flowing air, by heating up to 425◦ C, maintained for 12 h, and then cooling down to room temperature (r.t.) The weight loss during oxidation was ca. 20%. A final reductive treatment followed, in flowing H2 , by heating up to 900◦ C, maintained for 3 h, and then cooling down to r.t. in flowing N2 . Surface area and pore volume of as supplied and of variously treated carbon samples are given in Table 1. All the N2 adsorption isotherms of these supports were of type I, except for the E2700 sample, which possesses an elevated macroporosity, precluding the determination of the specific pore volume by N2 capillary condensation.
G, E = granular, extruded carbon; 1500, 1900, 2000, 2700 = heat treated in inert gas at indicated temperature (◦ C); O = oxidised in flowing air, R = reduced in flowing H2 after oxidation (see Section 2).
2.3. Preparation and characterisation of catalysts An aqueous solution of potassium ruthentate was employed as Ru precursor. Deposition of the ruthentate on the support was done by impregnation, followed by evaporation in rotavapor and drying (90◦ C) in oven for 30 min. Reduction of Ru was made in flowing H2 + N2 (3/1, v/v) gas mixture, by heating up to 320◦ C, then maintained for 2 h, and cooling down to r.t. in flowing N2 . The sample was then carefully washed till pH = 7 of the washing solution, to eliminate any residual Potassium. The addition of promoters was carried out by impregnation from a solution of the hydroxides (K, Cs) or of the nitrate (Ba). Evaporation of water in rotavapor and drying (90◦ C) followed. The transformation of the precursors into the active promoters was obtained during the in situ activation of the catalyst, before the reaction. The determination of Ru loading on the catalyst was done spectrophotometrically [6], on a Beckman DU-68 spectrometer, by measuring the absorbance at 465 nm of the solution obtained by KNO3 /KOH attack of the solid sample. BET surface area has been measured by N2 adsorption at liquid N2 temperature, by means of a Micromeritics ASAP 2010 apparatus. The effect of high temperature treatment (graphitisation) of the support has been qualitatively analysed by XRD, on a Philips PW 1820 powder diffractometer, by comparing the collected patterns with literature data files [7]. Some relevant data of the catalysts prepared are given in Table 2.
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Table 2 Compositiona and activity of the catalysts prepared Catalyst code 9 10 11 12 13 14 15 16 17 18 19 20
Support
Ru/wt.%
K/wt.%
Ba/wt.%
Cs/wt.%
ρ B b (g/cm3 )
Activityc
G1900/OR G1900/OR G1900/OR G2000 E E1500 E1500/OR G G1900 G1900/OR E2000 E2700
17.6 17.6 17.2 12.5 13.0 12.3 13.0 11.9 11.8 13.0 11.9 11.0
– 21.1 – 6.3 6.0 6.2 5.9 6.1 6.5 6.1 11.4 5.6
– – 22.4 7.2 6.7 6.2 6.5 6.6 6.6 7.4 11.2 5.2
22.1 – – 7.7 7.0 6.9 6.8 7.2 7.1 7.3 11.5 6.2
0.78 0.75 0.68 0.70 0.67 0.74 0.72 0.74 0.87 0.77 0.80 0.82
14.3 5.8 12.1 3.8 8.9 14.7 15.2 12.4 15.0 8.8 1.0 0.8
a
The chemical composition of the catalyst is referred to the total mass of the sample. ρ B = bulk density of the catalyst bed. c Expressed as mol.% NH in effluent gas at GHSV = 30 000 h−1 . 3 b
2.4. Catalytic activity testing apparatus and procedure Testing of catalytic activity has been carried out by means of a bench-scale, continuous system, centred on a vertical, downflow, Incoloy 800 tubular reactor, 12.7 × 9.0 mm in diameter and 40 cm in length, fitted with an axial thermowell of 1.6 mm external diameter. Heating of the reactor was accomplished by an electric furnace, through a massive metal block surrounding the reactor. Reactor temperature was controlled by an Eurotherm 904 TRC through a 425S solid-state-contactor power unit. Reactant gases flow rate was regulated by means of MKS 1261C mass flow meters and the reaction pressure was regulated by means of a home-made PTFE-membrane relief valve, pressurised with nitrogen at the desired value. A high-precision manometer, a bursting-disc safety device, a drechsel gas absorber and a volumetric wet-test meter completed the apparatus. All the catalysts have been tested under standard reaction conditions at 430◦ C and 100 bar (1 bar=105 Pa) total pressure, by loading 0.3 g of fresh catalyst, uniformly diluted with quartz powder of the same particle size (0.15–0.25 mm), so to have a quartz/catalyst volume ratio of 22/1 and a bed height of 6 cm. The bed was kept at the mid-length of the reactor by flocks of quartz wool. The void space over and below the catalyst bed was filled with quartz beads, 0.25–0.85 mm in size. The dilution of the catalyst with quartz rep-
resented an acceptable compromise, allowing a sufficient bed length/diameter ratio and minimisation of the temperature gradient generated by the exothermal NH3 synthesis reaction. Before the reaction the Ru-based catalyst was activated by flowing a 1.5/1 = H2 /N2 (v/v) gas mixture at GHSV = 20 000 h−1 and 30 bar total pressure, while increasing catalyst bed temperature (5◦ C/min) from r.t. up to 450◦ C, kept for 5 h and then cooling down to the reaction temperature. The Fe-based catalyst was activated in flowing H2 /N2 = 3/1 gas mixture at 30 bar and GHSV = 20 000 h−1 by increasing temperature by 50◦ C/h from r.t. to 350◦ C, then by 10◦ C/h from 350◦ C to 500◦ C, keeping such a temperature for 12 h and then cooling down to the reaction temperature. After leading total pressure to 100 bar, the activity tests were carried out at various GHSV values, by changing the gas mixture feeding rate. After every GHSV change the steady state was always attained within a few minutes. No activity decrease was observed even after several days on-stream. The H2 /N2 ratio was usually 1.5/1, but different ratios were also tested. The equilibrium concentration of ammonia in the outcoming gas for total pressure = 100 bar, reaction temperature = 430◦ C and H2 /N2 = 3/1 or 1.5/1 have been calculated as suggested in [8] and resulted ca. 19.4 and 17.7 vol.%, respectively. The GHSV values adopted in the present investigation usually ranged from 30 000 to 200 000 h−1 . GHSV is here defined as volumetric hourly flow rate
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of reacting gas, measured under standard temperature and pressure, referred to unit volume of catalyst. The activity was expressed as vol.% of NH3 in the effluent gas at GHSV = 30 000 h−1 . Testing of the resistance of catalysts was done by monitoring by gas chromatography the formation of methane when progressively increasing the reaction temperature up to about 700◦ C. 2.5. Analysis of the effluent gas The concentration of ammonia in the gas outcoming from the reactor was determined by absorption in an excess solution of H2 SO4 , by means of the drechsel absorber, through which a given amount of the effluent gas was bubbled. The excess of acid was then titrated with NaOH solution and bromocresol green as indicator.
Fig. 1. Catalytic activity versus reactants feeding ratio. GHSV = 60 000 h−1 , T = 430◦ C, P = 100 bar. (䉬) catalyst 17 and (䊏) catalyst 10 (Table 2).
3. Results and discussion 3.1. Activity tests The experimental variables of an activity test for ammonia synthesis catalysts are: temperature, pressure, feed composition, GHSV, catalyst particle size, catalyst dilution (optional). To choose the values to be used for each of them in our standard test, the following criteria have been adopted. 3.1.1. Temperature and pressure For an applied study values as close as possible to the industrial ones are recommended. Hence, we chose 430◦ C and 100 bar, respectively. In fact, tests at atmospheric pressure are not acceptable for the characterisation of ammonia synthesis catalysts, except for purely academic investigations. 3.1.2. Feed composition It is known [9] that, as to kinetic behaviour, Ru is different from Fe, for which a stoichiometric H2 /N2 mixture is the best choice. So we did experiments at different H2 /N2 ratios (Fig. 1). While the highest reaction rate corresponds to a ratio of about 1.0, the value of 1.5 was adopted in all our tests, in order to operate closer to the industrial conditions.
Fig. 2. Effect of alkaline or alkali-earth promoters on activity. T = 430◦ C, P = 100 bar and H2 /N2 = 1.5 (vol.). (䉬, N, 䊏): catalysts 9,11,10 (Table 2), respectively.
3.1.3. GHSV Inspection of typical conversion curves (Fig. 2) shows that at any GHSV value catalysts with different activity can be easily differentiated. A value of 30 000 h-1 was then chosen for our activity comparison standard tests, aiming at approaching, even if very partially, the industrial ones. 3.1.4. Catalyst particle size In order to have a value of the effectiveness factor very close to unity, a very small particle size was chosen (average diameter 0.2 mm). This is very different from the usual size (typically around 3 mm) of the industrial catalyst, but the adverse effect on activity can be minimised through a suitable external distribution of Ru. 3.1.5. Catalyst dilution As it is convenient to carry out the standard test as isothermally as possible, the exothermicity of the reaction forced us to strongly dilute the catalyst (1/22
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in volume with quartz), so to keep the temperature gradient within a few degrees. The standard activity test so developed proved to allow the collection of very reliable data for catalyst comparison purposes.
3.2. Choice of precursors and effect of promoters There is evidence [10] that chlorine has an adverse effect on Ru catalysts for ammonia synthesis, but, in spite of this, RuCl3 has been mostly used for catalyst preparation. It is very difficult to control the residual amount of Cl in the catalyst, so lack of reproducibility may occur. On this respect, K2 RuO4 appears to be the most convenient precursor. For the same reason hydroxides or nitrates were used to introduce the promoters. On the basis of patent literature [4,5], K, Cs and Ba were chosen as promoters and their total loading was fixed to about 20 wt.%. Their relative promoting effect was checked by adding each of them alone on the same support (Fig. 2). They were found to rank in the following order: Cs>Ba K. The much higher promoting effect of Ba with respect to K has been reported also by Kowalczyk et al. [11], who found also a stabilising effect of Ba against catalyst overheating at 520◦ C. As for Cs, the same authors previously found [12] only a slightly higher promoting activity, with respect to K, contrary to our data and to those reported by Tennison [2]. The use of chlorinated precursors could well give misleading results, at least in some literature data.
3.3. Effect of active carbon Our two active carbons (G and E) were used without any pretreatment for the preparation of K–Ba–Cs-promoted catalysts. Catalyst composition and activity data are given in Table 2, samples 13 and 16. The lower purity of E could be the reason for the lower activity of catalyst 13, with respect to 16. Experimental data on different active carbons as supports for ammonia synthesis catalysts are not available in open literature, so no comparison can be made with previous data.
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3.4. Effect of carbon pretreatment It is well known that heat treatment of carbon at high temperature under inert atmosphere brings about a more or less deep graphitisation, independently of the nature of the original carbon. To make graphitisation, we treated our two active carbons at different temperatures, within the 1500–2700◦ C range. The XRD analysis showed that the typical reflections of the graphite crystalline structure (d = 3.368, 2.838 and 1.678 Å) became progressively more intense with increasing the pretreatment temperature. Furthermore, such a treatment produced a drop in surface area (Table 1), the more evident, the higher the preheating temperature, with a definite lower decrease for carbon E. On some graphitised carbons an oxidation–reduction treatment was performed (see Section 2), which allowed to roughly recover (carbon G) or even to overcome (carbon E) the original porosity (Table 1). Treatment at 2700◦ C showed clearly excessive, so the corresponding support was not investigated any more. The effect of heat treatment of the support on the activity of the final catalyst can be seen by comparing the data (Table 2) of samples 16, 17, 12, prepared on carbon G, with those of samples 13, 14, 19, prepared on carbon E. All these samples have a similar composition, differing only for the support nature and pretreatment. An increase of activity occurs on passing from catalyst 16 (untreated support) to catalyst 17 (support treated at 1900◦ C). However, the treatment at 2000◦ C (catalyst 12) led to a dramatic decrease of activity. A similar, but higher increase of activity may be noticed for catalysts prepared on carbon E, on passing from catalyst 13 (untreated support) to catalyst 14 (support treated at 1500◦ C), while carbon treated at 2000◦ C and 2700◦ C (samples 19, 20) again led to very low activity. It is likely that the high activity increase coming from graphitisation of carbon E is connected with simultaneous removal of impurities, as a drastic change in the porous texture (carbon G and G-1900) gives a much lower activity increase (Tables 1 and 2). On the basis of our present data, the oxidation– reduction treatment, following heating to high temperature, did not prove to be particularly useful. Indeed, on going from catalysts 14 to 15 (Table 2) the activity did not change substantially, while on going from catalysts 17 to 18 an appreciable decrease was observed.
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Fig. 3. Methane formation at increasing reaction temperature. Figures refer to catalyst code (Table 2). P = 100 bar, H2 /N2 = 1.5 (vol.). Behaviour of catalyst 19 (not reported in the figure) practically overlaps catalyst 12.
Unfortunately, no data are available in the open literature on this subject. 3.5. Catalyst stability The risk of support degradation by Ru-catalysed methanation under ammonia synthesis conditions has been recently pointed out [11]. This phenomenon could have dramatic consequences in industrial reactors, should the catalyst not be sufficiently resistant to methanation. Therefore, it is absolutely necessary to evaluate this property on a quantitative basis through a specially designed standard test. The test we have developed for this purpose consists in monitoring the methane formation during ammonia synthesis at 100 bar and temperature up to 700◦ C. This test appears to be more exhaustive than that used by Kowalczyk et al. [11], carried out at atmospheric pressure in hydrogen at 520◦ C. Our data of methane formation as a function of temperature are shown in Fig. 3. They refer to a group of catalysts of practically identical composition, except for the support, and show that catalysts 13, 14, 15, 16, prepared with untreated carbons or after heat treatment at 1500◦ C, started to form methane at ca. 500◦ C and that methane formation increased considerably with increasing temperature. Therefore, catalysts prepared on such supports cannot be taken into account for practical purposes. By contrast, when the carbons were treated at temperatures of at least 1900◦ C (catalysts 17, 18, 20), methane formation dropped dramatically at least up to 600◦ C, and grew slowly beyond this temperature.
Fig. 4. Comparison of Fe-based catalyst (䉬) with catalyst 17 (䊏) (Table 2). Reaction conditions as for Fig. 2 for the Ru-catalyst. For the Fe-based catalyst H2 /N2 = 3 (vol.).
The oxidation–reduction treatment (catalysts 14, 15 and 17, 18) does not seem to have an influence on methane formation. Therefore, it appears that the degree of carbon graphitisation is a primary factor for catalyst stability. In particular, heat treatment of the active carbon at 1500◦ C is insufficient to provide stability. 1900◦ C results to be an optimal temperature, as higher temperatures, while further increasing catalyst stability, have an adverse effect on activity. Of course, other variables in catalyst composition and preparation, which are currently being investigated, could appreciably influence catalyst stability. This kind of knowledge is of utmost importance for the development of industrial Ru-based ammonia synthesis catalysts. 3.6. Comparison with a commercial Fe-based catalyst Our presently best-performing catalyst as to both activity and stability (catalyst 17, Table 2) has been compared with one of the most widely used Fe-based commercial catalysts, under the optimal reaction conditions for each of the two samples. The results are shown in Fig. 4. On a volume basis the ammonia productivity, expressed as kg NH3 /(h m3 catalyst), of catalyst 17 is about twice that of Fe catalyst, in rough agreement with data reported by Kowalczyk et al. [12]. This activity ratio rises to about 8 when the two catalysts are compared on a weight basis and to about 80 when they are compared on a metal weight basis.
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4. Conclusions The most important results of the present investigation may be summarised as follows: (i) a chlorine-free procedure, easily amenable to scale up, has been set up for the preparation of a stable carbon-supported Ru-based catalyst, much more active for the synthesis of ammonia than the usual Fe-based one; (ii) the much higher promoting effect of Ba and Cs, with respect to K, has been confirmed; (iii) the beneficial effect of heat pretreatment of the carbon has been outlined in terms of both activity and stability of catalyst and (iv) the heat pretreatment to ca. 1900◦ C allows to obtain a very active catalyst, in the absence of any further oxidation–reduction treatment. References [1] C.W. Hooper, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Plenum Press, New York, 1991, p. 253.
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[2] S.R. Tennison, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Plenum Press, New York, 1991, p. 303. [3] F. Montino, L. Balducci, F. Ferrero, N. Pernicone, US Patent No. 5,221,657, 22 June 1993, to MURST, Rome. [4] US Patent No. 4,163,775, 7 August 1979, to BP Co. [5] US Patent No. 4,568,532, 4 February 1986, to Kellogg, M.W. Co. [6] R.E. Connick, C.R. Hurley, J. Am. Chem. Soc. 74 (1952) 5012. [7] Selected Powder Diffraction Data, JCPDS, Swarthmore, PA, 1981, File No. 23–64. [8] E.W. Comings, High Pressure Technology, McGraw-Hill, New York, 1956, p. 360. [9] F. Rosowski, A. Hornung, O. Hinrichsen, D. Herein, M. Muhler, G. Ertl, Appl. Catal. A: General 151 (1997) 443. [10] N.M. Dobrynkin, P.G. Tsyrulnikov, A.S. Noskov, N.B. Shitova, I.A. Polukhina, G.G. Savelieva, V.K. Duplyakin, V.A. Likolobov, in: B. Delmon, P.A. Jacobs, R. Maggi, J.A. Martens, P. Grange, G. Poncelet (Ed.), Preparation of Catalysts VII, Elsevier, Amsterdam, 1998, p. 213. [11] Z. Kowalczyk, S. Jodzis, W. Raròg, J. Zielinski, J. Pielaszek, Appl. Catal. A: General 173 (1998) 153. [12] Z. Kowalczyk, J. Sentek, S. Jodzis, E. Mizera, J. Gòralski, T. Paryjczak, R. Diduszko, Catal. Lett. 45 (1997) 65.