Adsorption of Dinitrogen and Its Hydrogenation on a Fused Iron

Slow changes in the nitrogen location (surface N−subsurface N) have been observed. In the course of TPD-Nads tests desorption of nitrogen proceeds a...
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Langmuir 2002, 18, 10191-10195

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Adsorption of Dinitrogen and Its Hydrogenation on a Fused Iron Catalyst for Ammonia Synthesis J. Zielin´ski,*,† L. Znak,† and Z. Kowalczyk‡ Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland, and Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Received July 8, 2002. In Final Form: September 10, 2002 A triply promoted fused iron catalyst was examined by temperature-programmed desorption and temperature-programmed hydrogenation of preadsorbed nitrogen (TPD-Nads and TPH-Nads). The studies confirm the literature suggestions that both nitrogen chemisorbed on the Fe surface and subsurface nitrogen coexist in the catalyst. Slow changes in the nitrogen location (surface N-subsurface N) have been observed. In the course of TPD-Nads tests desorption of nitrogen proceeds as an irreversible process, without N2 readsorption. During TPH-Nads runs a significant amount of dinitrogen is evolved at high temperature, besides ammonia. The obtained TPH-Nads spectra are characteristic with sharp onsets of ammonia evolution. It has been demonstrated that both hydrogenation of preadsorbed nitrogen and retention of ammonia formed are responsible for the onset formation.

Introduction Fused iron catalysts are commonly used in the industrial process of ammonia synthesis. In view of the enormous scale of the NH3 production, extensive studies of the catalysts have been performed to explain their unique properties and to elaborate on new catalytic systems that would operate effectively at lower temperature and under low pressure. Nevertheless, despite the efforts, a lot of controversies and unsolved problems still remain. Reliable chemical characterization of the catalysts seems to be one such problem. Our preliminary studies have demonstrated1 that various adsorbates such as hydrogen, carbon monoxide, and nitrogen are suitable for the iron dispersion determination, provided that the experimental conditions are chosen correctly. The present paper continues that subject, giving more detailed information on the characterization of fused iron catalysts by temperature-programmed desorption and temperature-programmed hydrogenation of preadsorbed nitrogen. The surface of a fused iron catalyst is assumed to consist predominantly of Fe(111) planes.2,3 Above room temperature, the interaction of N2 with the Fe (111) plane leads to dissociative chemisorption of nitrogen molecules.3-5 At 223 °C, the initial sticking coefficient is 10-7-10-6 only, and the process is virtually nonactivated at very low, close to zero, coverage. The examinations of low-energy electron diffraction along with the thermal desorption, work function, and Auger electron spectroscopy data suggest the reconstruction of the surface to occur, resulting in the formation of “surface nitrides” with a thickness of about two atomic layers.4 Desorption of N2 is apparently a first† ‡

Institute of Physical Chemistry PAS. Warsaw University of Technology.

(1) Zielin´ski, J.; Znak, L.; Kowalczyk, Z. Pol. J. Chem. 2001, 75, 1927. (2) Jennings, J. R. In Catalytic Ammonia Synthesis, 1st ed.; Plenum Press: New York, 1991. (3) Boudart, M.; Dje´ga-Mariadassou, G. Kinetics of Heterogeneous Catalytic Reactions, 1st ed.; Princeton University Press: Princeton, 1984. (4) Bozo, F.; Ertl, G.; Grunze, M.; Weiss, M. J. Catal. 1977, 49, 18. (5) Bozo, F.; Ertl, G.; Weiss, M. J. Catal. 1977, 50, 519.

order reaction4 with a mean activation energy similar to the activation energy determined for Fe4N decomposition.6,7 Adsorption of nitrogen on fused iron catalysts is an activated process with an activation energy that increases strongly with the nitrogen coverage increase.8,9 The state of adsorbed nitrogen depends on the adsorption temperature.10 Nitrogen preadsorbed at 200 °C blocks the subsequent adsorption of carbon monoxide,11 and it easily reacts with hydrogen.12 In contrast, nitrogen preadsorbed at 400 °C does not affect adsorption of carbon monoxide, and it hardly reacts with hydrogen, similarly to iron nitride. This leads to the conclusion that the former represents atomic nitrogen located on the Fe surface and the latter represents subsurface nitrogen, forming an iron nitride layer.12 The proportion between both nitrogen forms depends on temperature: at low temperature the surface nitrogen prevails, and at high temperature the underneath nitrogen dominates. Desorption of nitrogen preadsorbed on fused iron catalysts proceeds at high temperature only. The temperature-programmed desorption studies of the potassium-promoted catalyst and of that free of potassium demonstrate that the N2 desorption peak is shifted significantly to lower temperature when potassium is present in the system.9 The interaction of hydrogen with nitrogen preadsorbed on the fused iron catalysts was intensively studied by Fastrup et al.13 as a method for the active sites counting. The studies were carried out in a flow system under linear temperature rise, and they showed a broad profile of ammonia formation centered at about 140 °C. The shape of the profile provided interesting information about the surface of the catalyst and the total amount of ammonia produced was assumed by the authors13 to be a good estimation of the iron area. (6) Engelhart, G.; Wagner, G. Z. Phys. Chem. 1932, B18, 369. (7) Grabke, H. J. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 533, 541. (8) Scholten, J.; Zwietering, P.; Konvalinka, J.; De Boer, J. Trans. Faraday Soc. 1959, 55, 2166. (9) Fastrup, B. J. Catal. 1994, 150, 345. (10) Takezawa, N. J. Phys. Chem. 1966, 70, 597. (11) Takezawa, N.; Emmett, P. H. J. Catal. 1968, 11, 131. (12) Takezawa, N. J. Catal. 1972, 24, 417. (13) Fastrup, B.; Muhler, M.; Nygard Nielsen, H.; Pleth Nielsen, L. J. Catal. 1993, 142, 135.

10.1021/la020618h CCC: $22.00 © 2002 American Chemical Society Published on Web 12/02/2002

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Experimental Section Apparatus. The experiments were carried out in a glass flow system equipped with a gradientless microreactor.14,15 A temperature controller maintained the reactor temperature within 1 °C and provided linear temperature programming in the range of -200 to 800 °C. Hydrogen, helium, argon, and nitrogen were of 99.999% purity. Hydrogen was further purified by a palladium filter. Argon and helium were purified in a series of columns packed with Cu/SiO2, silica gel, and molecular sieves 5 A. The gas stream required fed the measuring system by a four-way valve, and before entering the reactor, it was additionally purified from traces of dioxygen and water by passing through a MnO/ SiO2 column. In the case of a He stream, the column was maintained at -195 °C, which lowered the content of impurities below 0.1 ppm. The composition of the gas stream leaving the reactor was monitored by means of a TCD cell, and the results were collected with a computer-controlled system. The Catalyst. A commercial triply promoted (3.0% CaO, 3.3% Al2O3, 0.65% K2O) fused iron catalyst, KM I, received from Haldor Topsoe Research Laboratories was used in the studies. The 1 g sample, of 0.5-0.8 mm grain size, was prereduced in situ in a H2 stream of 1 cm3/s. The reduction was carried out at slowly increasing temperature so that the concentration of water in the outlet gas did not exceed 1000 ppm. At the terminal temperature, 550 °C, the reduction was continued for 48 h, which lowered the H2O concentration in the outlet stream below 1 ppm. Hence, the catalyst was fully reduced when starting the chemical examinations. Moreover, because of the high temperature and long-term prereduction, the catalyst remained virtually unchanged in the course of the measurements carried out thereafter. The examinations of the catalyst by H2 chemisorption (TPD method) and CO chemisorption (pulse method) gave 25.5 and 27.6 µmol/g, respectively.1 Measurements Procedure. All the experiments were preceded by extra reduction in a H2 stream (0.5 cm3/s, 540 °C, 0.5 h). Afterward, the sample was purged from the adsorbed hydrogen in a He stream (0.5 cm3/s, 500 °C, 0.5 h), and one of the following measurements was then performed: Temperature-Programmed Desorption of Preadsorbed Nitrogen. Typically, preadsorption of nitrogen was carried out at atmospheric pressure in two steps: (1) adsorption at constant temperature of 400 °C for 0.5 h and (2) adsorption at gradually decreasing temperature (for 1 h) from the initial temperature to 20 °C. Afterward, the reactor was flushed with a He stream (0.5 cm3/s, 20 °C, 0.25 h) to remove physisorbed nitrogen, if any, and chemisorbed nitrogen was examined by the TPD or TPH methods. If not otherwise stated, the TPD-N2 runs were performed in a He stream of 0.5 cm3/s at linearly increasing temperature of 0.167 °C/s, from 20 to 550 °C, whereupon it was continued at constant temperature of 550 °C for 0.33 h. Temperature-Programmed Hydrogenation of Preadsorbed Nitrogen. As a rule, the hydrogenation was carried out in a H2 stream of 0.5 cm3/s at linearly increasing temperature of 0.167 °C/s from 20 to 550 °C, whereupon it was continued at 550 °C for 0.33 h. Since the stream leaving the reactor contained not only ammonia but also dinitrogen, occasionally two parallel experiments, A and B, were performed to distinguish between both components. In experiment A, a summed up response from both NH3 and N2 was recorded while in experiment B, NH3 was removed from the outlet stream in a trap maintained at -195 °C and the response from N2 alone was followed. Consequently, the concentration profiles of each component could be determined. Besides to the TPH-Nads runs in a H2 stream, similar experiments with an 80% H2 + Ar mixture were performed. In this case, ammonia was condensed in a column maintained at -195 °C, and the consumption of hydrogen from the stream was monitored.

Results and Discussion TPD of Preadsorbed Nitrogen. Figure 1 presents TPD-Nads spectra corresponding to various preadsorption (14) Zielin´ski, J. J. Catal. 1982, 76, 157. (15) Zielin´ski, J. React. Kinet. Catal. Lett. 1981, 17, 69.

Figure 1. Effect of preadsorption conditions on the TPD-Nads spectra.

Figure 2. Effect of preadsorption conditions on the nitrogen uptake.

procedures; the amounts of nitrogen adsorbed are shown in Figure 2. As seen, interaction of dinitrogen with fused iron catalysts proceeds with a measurable rate at above 150 °C, leading to N2 dissociative adsorption.4,5 Nitrogen is strongly fixed on ironsthe N2 desorption starts above about 350 °C (Figure 1). The TPD-Nads spectra expand to lower temperature, and the uptake of nitrogen considerably grows with increasing the initial temperature of N2 adsorption (Figures 1 and 2). A similar effect was observed when the prolonged period of equilibration, 15 h, was applied (trace 5 in Figure 1). These results demonstrate that the amount and the state of nitrogen adsorbed on the catalyst surface are a complex function of the N2 adsorption procedure. Generally, the TPD-Nads spectra obtained in this work are similar, but not identical, to the spectra reported by Fastrup,9 Muhler et al.,16 and Dahl et al.17 The differences in the shape and peaks positions result, very likely, from two reasons: (i) the samples subjected to the examinations might be different in contents of the potassium promoter that strongly affects desorption of nitrogen,9 and (ii) the procedures of nitrogen adsorption were not identical. In the cited papers,9,16 the adsorption of nitrogen was carried out directly after ammonia synthesis, and the resultant TPD-Nads spectra could be influenced by the intermediate NHx species retaining on the surface after the synthesis (16) Muhler, M.; Rosowski, F.; Ertl, G. Catal. Lett. 1994, 24, 317. (17) Dahl, S.; To¨rnqvist, E.; Jacobsen, C. J. H. J. Catal. 2001, 198, 97.

Fused Iron Catalyst

Figure 3. Effect of NHx species on the TPD-Nads profile.

Figure 4. Effect of the He flow rate on the rate profiles of nitrogen desorption.

run. In our experiments (Figure 1), this could not be the case. The effect of NHx species on the N2 desorption has been documented in Figure 3, which illustrates how the TPD-Nads spectrum is changed by a small amount of ammonia (0.9 µmol) adsorbed onto the catalyst precovered with nitrogen (54.2 µmol of Nads). The examination shows that during the TPD run 0.5 µmol of NH3 is desorbed to the gas phase, but the other part (0.4 µmol NH3) lowers considerably the rate of N2 desorption, as evidenced by the shift in the TPD-Nads peak to higher temperature (see Figure 3). This result appears to indicate that a small number of surface sites controls the rate of N2 desorption. The above suggestion is in agreement with the recent findings of Dahl et al.,17 who stated that only about 5% of the catalyst sites participated in the processes of N2 adsorption and desorption. Figure 4 presents the effect of the He flow rate on the rate of nitrogen desorption. In the beginning of the examination, below ca. 450 °C, the rate of nitrogen evolution is independent of the flow of helium. Hence, the readsorption of nitrogen does not affect the N2 evolution. At higher temperature, in the range of 450-550 °C, the readsorption of nitrogen suppresses slightly the N2 desorption rate, but the effect may be considered rather as negligible in relation to the significant, 9-fold change in the He flow. At the end of the examinations, at constant temperature of 550 °C, the rate of nitrogen desorption is, within experimental error, still independent of the He flow rate. Concluding, the results presented in Figure 4 demonstrate that, at first approximation, desorption of

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Figure 5. TPH-Nads examinations of the fused iron catalyst: (A) a summed up response both from NH3 and N2; (B) a response from N2.

nitrogen from the iron catalyst may be considered as an irreversible process, without N2 readsorption. The TPD-Nads spectra presented in Figures 1, 3, and 4 indicate a considerable amount of nitrogen to evolve at a temperature even as high as 550 °C. The possibility that nitrogen from the iron phase evolves at high temperature can be ruled out since the solubility of nitrogen at 400 °C and at 1 atm of N2 pressure is only 0.3 µmol N/g18 compared to 54.2 µmol of N adsorbed on the catalyst surface. It is supposed that nitrogen strongly bound to the near surface Fe atoms desorbs at high temperature, at the end of the TPD-Nads experiments. TPH of Preadsorbed Nitrogen. Figure 5 shows the original results of TPH-Nads measurements performed with various heating rates. The spectra marked as A represent summed up responses both from NH3 and N2 whereas the spectra marked as B correspond to the responses from N2 alone. The presence of nitrogen in the outlet gas during TPH-Nads experiments has not been reported so far. According to the similar studies of Fastrup,13 nitrogen preadsorbed on fused iron catalysts is almost fully hydrogenated to NH3 below 300 °C, and consequently, the integrated NH3 signal has been used to determine the amount of preadsorbed nitrogen.13,19 In contrast, our measurements demonstrate that in the course of TPHNads tests only about half of the preadsorbed nitrogen is converted to ammonia below 300 °C, and above 350 °C a significant amount of nitrogen evolved, besides ammonia. The spectra presented in Figure 5 were used to determine the concentration profiles of ammonia and dinitrogen in the exit stream. Subsequently, these values were used to calculate the quotient (Q) for the reaction of ammonia synthesis, N2 + 3H2 ) 2NH3, defined as

Q)

pNH32 pN2pH23

where pNH3, pN2, and pH2 are the partial pressures of ammonia, nitrogen, and hydrogen in the gas mixture. Figure 6 shows the NH3 and N2 concentration profiles as well as the results of Q calculations for the TPH-Nads run performed with a 0.167 °C/s heating rate. As seen, at high temperature, above 500 °C, the reaction quotient is close (18) Grabke, H. J. Z. Phys. Chem. (Munich) 1976, 100, 185. (19) Kowalczyk, Z.; Sentek, J.; Jodzis, S.; Muhler, M.; Hinrichsen, O. J. Catal. 1997, 169, 407.

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Figure 6. Analysis of the TPH-Nads examination at a temperature ramp of 0.167 °C/s.

Figure 7. TPH-Nads tests for the samples with low coverage of preadsorbed nitrogen.

to the equilibrium constant of the reaction K,20 thus indicating the composition of the reacting mixture to be close to that in the equilibrium state. At lower temperature, however, the quotient Q is by an order of magnitude higher than the equilibrium value. This means, in agreement with previous suggestions, that desorption of dinitrogen from the catalyst is a relatively slow process. The TPH-Nads spectra obtained in this work (Figures 5 and 6) exhibit a characteristic sharp onset of the ammonia evolution. The appearance of that feature is not clear. Fastrup et al.13 found the onset very abrupt for K-free iron catalysts but it was rather smooth for K-promoted samples. Out of accord, Kowalczyk et al.19 recorded the onset sharp for both doubly and triply promoted samples. On the basis of detailed examinations, Fastrup et al.13 ascribed that feature to the autocatalytic mechanism of nitrogen hydrogenation, according to which ammonia desorption creates free iron sites for hydrogen adsorption. Once the NH3 desorption has started, the number of iron sites accessible to hydrogen increases, so a self-accelerating effect is observed. To verify the above hypothesis, we performed a series of the TPH-Nads experiments (see Figure 7), in which, contrary to the examinations illustrated in Figures 5 and 6, the samples with low coverage of nitrogen were examined. The low nitrogen coverage was achieved (20) Stull, D. R.; Westrum, E. F.; Sinke, G. C. In The Chemical Thermodynamics of Organic Compounds; John Willey & Sons: New York, 1969.

Zielin´ ski et al.

Figure 8. Interaction of hydrogen with preadsorbed nitrogen.

by three ways: (1) preadsorption of nitrogen at low temperature of 200 °C, (2) preadsorption at 200 °C with a subsequent preheating at 380 °C in a He stream (i.e., without desorption of nitrogen), and (3) preadsorption at 400 °C followed by preheating of the sample up to 539 °C in a He stream to desorb a large part of nitrogen. The studies clearly demonstrate that at low coverages the onsets are much the same as those for the sample saturated with nitrogen (compare Figure 5 and Figure 6). This evidences the autocatalytic mechanism proposed by Fastrup13 not to be essential for the sharp onset observed. Closer inspection of the spectra presented in Figure 7 shows that (i) a large part of nitrogen preadsorbed according to procedure 1 is hydrogenated to ammonia, forming a prominent NH3 peak at 160 °C, while the size of this peak decreases by half, on account of the N2 peak at 550 °C, when the sample with preadsorbed nitrogen was preheated in helium up to 380 °C and (ii) in the case of N2 preadsorption procedures 2 and 3, the proportion between the amount of ammonia evolved and the amount of desorbed nitrogen is the same. On the basis of the above observations, one may conclude that nitrogen preadsorbed at 200 °C is located on the iron surface mainly. During the heating at 380 °C as well as during the TPH-Nads run, nitrogen migrates to the subsurface regions, thus becoming hardly accessible to hydrogen (hardly reducible). At low temperature, below 200 °C, the rate of the migration is expected to be low. Indeed, a complementary test has shown that N2 preadsorbed at 200 °C is almost completely hydrogenated to ammonia when the hydrogenation was carried out at a constant temperature of 156 °C for a long period of time (5 h). At high temperature, however, the nitrogen migration may be fast, thus resulting in a specific proportion between subsurface nitrogen and nitrogen adsorbed on the Fe surface. Figure 8 summarizes the interaction of hydrogen with nitrogen preadsorbed on an iron catalyst. The rates of the reaction are expressed as consumption or evolution of individual reactants divided by the stoichiometric coefficient of the reaction N2 + 3H2 ) 2NH3. The obtained results indicate that hydrogen consumption begins already at about 50 °C (curve 3). Having in mind that preadsorbed nitrogen effectively blocks the adsorption of hydrogen,21 this result indicates that the consumption of hydrogen is connected with hydrogenation of preadsorbed nitrogen to various -NHx species. Ammonia, the final product of nitrogen hydrogenation, initially is adsorbed (retained) (21) Ertl, G.; Huber, M.; Lee, S. B.; Paa´l, Z.; Weiss, M. Appl. Surf. Sci. 1981, 8, 373.

Fused Iron Catalyst

on the acidic sites of the catalyst. When the sites are fully covered, ammonia appears in the gas phase forming a more or less sharp onset of ammonia formation. At higher temperature, when adsorption of ammonia does not play a significant role (in the range 150-350 °C), the evolution of ammonia parallels the consumption of hydrogen (see curves 1 and 3 in Figure 8). Over 350 °C the profile of H2 consumption could not be measured by the method used in this work since the response from H2 consumption overlapped with that from N2 evolution.

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The proposed interpretation of the TPH-Nads run (Figure 8) explains the shape and position of ammonia onset reported previously for various iron catalysts.13,19 It was observed that in the case of K-free catalysts, which adsorb strongly a large amount of ammonia, the onset of ammonia is really sharp while for K-promoted samples, which adsorb strongly only a small amount of ammonia, the onset is moderately sharp or even quite smooth.13 LA020618H