Ind. Eng. Chem. Res. 2003, 42, 1347-1349
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Effect of an Iron Oxide Precursor on the H2 Desorption Performance for an Ammonia Synthesis Catalyst Liu Huazhang,* Liu Caibo, Li Xiaonian, and Cen Yaqing Catalysis Institute of Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China
The effects of various iron oxide precursors of an ammonia synthesis catalyst on the H2 desorption performance were studied by means of H2 temperature-programmed desorption (TPD). Among the three precursors being studied, which are Fe3O4, Fe1-xO, and their mixture, both traditional Fe3O4-based and a mixed oxide of FeO and the Fe3O4-based catalyst after reduction have three H2 adsorbed states of R1, R2, and R3 on the H2 TPD spectrum. Therein, R1 and R2 may be the H2 chemisorption states that they can react with N2 to form ammonia, and R3 should be a strong H2 chemisorption state that cannot react with N2. However, the Fe1-xO-based catalyst after reduction is in the absence of R3 adsorbed states, and R1 may be a H2 physisorption state. This is a remarkable character of the Fe1-xO-based catalyst. The order of the H2 desorption activation energy is as follows: two-phase region (Fe3O4‚FeO) > Fe3O4 > Fe1-xO. This order is just contrary to that of the catalytic activity, which is as follows: Fe1-xO > Fe3O4 > two-phase region (Fe3O4‚ FeO). The results suggest that one important reason that the Fe1-xO-based catalyst has higher activity is the absence of the high-temperature H2 adsorption state. 1. Introduction
Table 1. Chemical Composition and Crystal Phase of Samples
The Fe3O4-based ammonia synthesis catalyst has been studied deeply. Most of the former work is limited to the study of promoters and the catalyst using Fe3O4 as a precursor.1-4 In 1986, Liu et al.5-7 broke through the classic conclusion that the Fe3O4-based catalyst has the highest activity and first developed the Fe1-xObased catalyst (A301) successfully. Furthermore, the relationship called CAMEL’S hump-shape activity curve between the catalytic activity and the oxide precursor (represented as Fe2+/Fe3+) was obtained. The A301 catalyst has the highest activity among all of the ironmelting catalysts for ammonia synthesis. However, whether the oxide precursor of the catalyst is Fe3O4, Fe1-xO, or a mixture of two iron oxides such as Fe3O4 and FeO, their active state after reduction all will be R-Fe. CAMEL’S hump activity curve indicates that the activity of the iron catalyst is dependent upon the property and composition of the iron oxide precursor. So, it can be imagined that there shall be a difference in the adsorption/desorption performance of N2 or H2 on the surface of the R-Fe reduced from different iron oxide precursors. Combined with the other methods, temperature-programmed desorption (TPD) is an effective way to study gas adsorption/desorption behaviors on the catalyst surface.8-10 The results of N2-TPD on the surface of iron catalysts with different oxide precursors were reported in our previous paper.11 In this paper, the results of H2-TPD on the surface of iron catalysts with different oxide precursors are reported. 2. Experimental Section 2.1. Preparation and Analysis of the Sample. The samples were prepared by melting the mixture of magnetite, a certain amount of reducing agent (such as pure iron), and promoters (oxides, nitrates, or carbon* To whom correspondence should be addressed. Tel and Fax: 0086-571-8832-0063. E-mail:
[email protected].
sample no.
Fe2+/Fe3+
1 2 3 4 5 6 7 8 9
0.31 0.50 0.89 1.11 2.16 3.15 4.62 6.52 7.54
chemical composition (wt %)a FeO Fe2O3 20.95 29.39 42.12 47.33 62.52 70.10 76.31 80.91 82.54
72.75 65.31 52.58 47.37 32.17 24.69 18.39 13.79 12.16
crystal phase (by XRD) Fe3O4 Fe3O4 Fe3O4‚FeO FeO‚Fe3O4 FeO‚Fe3O4 FeO FeO FeO FeO
a The promoters of the samples are Al O of 2.4 wt %, K O of 2 3 2 0.7 wt %, CaO of 1.8 wt %, and SiO2 of 0.4 wt % in samples before reduction.
ates of aluminum, potassium, calcium, etc.). Adjusting the ratio of magnetite and the reducing agent controlled the chemical composition and the crystal phase (expressible as Fe2+/Fe3+) of the samples. The chemical composition was analyzed according to HG1-1430-81 Standard of China. The crystal phases of the samples were distinguished by an MXP series X-ray diffractometer (Co KR radiation, 40 kV, 25 mA, 4° min-1 continuous scanning). The chemical composition and the crystal phase of the samples are listed in Table 1. It can be seen from Table 1 that the precursor of samples 1 and 2 is Fe3O4. Fe2O3 was not detected, although it exists when Fe2+/Fe3+ < 0.5. The precursor of samples 3-5 is the mixture of Fe3O4 and FeO, whose molar ratios of FeO/Fe3O4 are 1:1.28, 1:0.82, and 1:0.30, respectively. The precursor of samples 6-9 is only FeO. The Fe3O4 phase was not detected, although it exists when Fe2+/Fe3+ > 0.5. This result indicates that the iron oxide in the precursor has completely formed to a nonstoichiometric ferrous oxide Wustite12 expressed as Fe1-xO with a defect lattice of an iron ion (x represents the defective content of the iron ion). 2.2. TPD Experiments. The H2-TPD experiments were carried out in a gas chromatograph 1104GC with
10.1021/ie0202524 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/11/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 Table 3. Ed of H2 Desorption and Catalytic Activity of Catalysts sample no.
R1
Ed (kJ mol-1) R2
1 2 3 4 5 6 7 8 9
27.80 78.50 103.2 82.78 58.49 30.00 21.90 14.50 9.10
99.30 96.90 121.4 81.40 72.00 60.76 62.03 61.86 61.10
R3 155.9 177.1 141.4 143.0 156.2
activitya (mmol g-1 h-1) 84.97 85.82 76.04 78.12 81.60 87.61 96.50 96.16 95.28
a Activity test conditions: H :N ) 3:1, volume ) 2 cm3, SV ) 2 2 30 000 h-1, p ) 15 MPa, T ) 673 K, particle size ) 1.0-1.4 mm, -3 packing density ) 2.88 g cm unreduced.
Figure 1. H2-TPD spectra when β ) 20 °C min-1. The numbers on the right indicate the code of the samples as described in Table 1. Table 2. H2-TPD Data of r1, r2, and r3 β (°C min-1) sample no. 10 1 2 3 4 5 6 7 8 9
187 213 211 198 191 134 123 123 90
T1(R1) (°C)
T2(R2) (°C)
T3(R3) (°C)
20
30
40
10
20
30
40
10
20
30
40
220 229 224 216 206 158 141 169 143
245 240 231 223 223 174 176 205 168
263 245 238 238 230 193 190 225 216
302 291 293 285 282 258 275 290 282
316 310 307 305 301 281 297 316 308
332 321 316 317 314 299 313 335 324
337 327 322 326 328 308 326 345 338
525 527 538 541 555 543
544 551 561 560 578 559
562 560 578 583 591
569 567 589 589 605
a quartz reactor of 5 mm internal diameter and using 0.4 g of catalyst of 200-300 standard mesh. The catalyst was reduced by pure hydrogen at 500 °C for 5 h, then cooled slowly down to 40 °C in 2 h to make H2 adsorption to saturation, and subsequently purged with Ar for 2 h to remove the nonchemical adsorbed hydrogen on the catalyst surface at that temperature. Eventually, the catalyst was heated at heating rates (β) of 10, 20, 30, and 40 °C min-1 respectively in the range of 40-700 °C at 16.7 mL min-1 of Ar and 101 kPa of pressure, and the TPD profiles were recorded with a thermal conductivity detector. 3. Results and Discussion 3.1. H2 Adsorbed State on the Surface of Iron Catalysts with Different Oxide Precursors. The H2-TPD data are listed in Table 2. Figure 1 gives the H2-TPD spectra when β ) 20 °C min-1. From Figure 1, it can be seen that there are three peaks in the H2 TPD spectrum. They indicate three H2 adsorbed states of R1, R2, and R3. The desorption temperature of R1 is somewhat different because of the different precursors, and the average temperatures are 242.5, 222, and 180 °C corresponding to FeO‚Fe3O4, Fe3O4, and Fe1-xO precursors, respectively. R2 is masked by R1, and it displays a small outgrowth. Thus, R2 has a character similar to that of R1. With the increase of
Fe2+/Fe3+, the R2 peak becomes big first and then becomes small. The desorption temperature of R2 is 258-345 °C. It is remarkable that there is a close relationship between R3 and the iron oxide precursor. With the increase of Fe2+/Fe3+, the peak R3 becomes weak from strong, and when the precursor becomes complete Wustite (Fe1-xO, Fe2+/Fe3+ > 3.15), the peak R3 disappears. This indicates that there are only two H2 adsorbed states of R1 and R2, while R3 does not exist on the surface of the reduced Fe1-xO-based catalyst. This is a remarkable character of the Fe1-xO-based catalyst. 3.2. Desorption Activation Energy Ed. According to the TPD theory,13 under the conditions that the inner and outer diffusions are eliminated and no readsorption takes place (the H2 desorption activation energy Ed is independent of the coverage (θ) of H2), the following equation can be deduced:
2 log Tm - log β )
Ed Ed 1 + 1og 2.303R Tm νnRnθn-1 m
(1)
where Tm is the peak temperature (K), β is the heating rate (°C min-1), Ed is the desorption activation energy (kJ mol-1), θm is the coverage at peak maximum, νn is a preexponential factor (s-1), and n is an order of desorption. If we maintain a constant initial coverage of the adsorbate in each experiment, it can be seen that θm is almost the same in each desorption. So, the last item on the right-hand side of eq 1 turns into a constant. From plots 2 log Tm - log β vs 1/Tm, Ed is calculated by the slope of the line, listed in Table 3. It can be seen from Table 3 that the desorption activation energy Ed of R1 has significant differences for the three iron catalysts. The order of H2 desorption activation energy Ed is as follows: two-phase region (Fe3O4‚FeO) > Fe3O4 > Fe1-xO. This order is just contrary to that of the catalytic activity, which is as follows: Fe1-xO > Fe3O4 > two-phase region (Fe3O4‚ FeO). For the Fe1-xO-based catalysts (samples 6-9), the desorption activation energy of R1 is only 9.1-30 kJ mol-1. Thus, the H2 adsorbed state R1 is likely attributed to a nonactive adsorbed state or physical adsorbed state on the surface of Fe1-xO-based catalysts. The desorption activity energy of R2 is in the range of 60.8-99.3 kJ mol-1 (except for sample 3). Therefore, R2 is probably a dissociative chemical adsorption state. The desorption activation energy of R3 is in the range of 141-177 kJ mol-1 (samples 6-9 have no R3), and the temperature of desorption reaches 525-605 °C. So, R3 is probably a strong chemisorption state. For the sake of validating
Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 1349
has a higher activity is probably that there is no strong chemisorption state of H2 on the surface of the Fe1-xObased catalyst. Literature Cited
Figure 2. H2-TPD spectra before (a) and after (b) adsorbed H2 reaction with N2.
this view further, the experiment of adding N2 was done. The H2-TPD experiment was directly done according to the above process (see Figure 2a). Another experiment is that the same sample was first reduced by H2, and then it reacted with N2 at 500 °C and at atmospheric pressure for 2 h and was purged with Ar for 2 h to remove the adsorbed nitrogen on the catalyst surface. Eventually, H2-TPD was started (see Figure 2b). As indicated in Figure 2, after adsorbed H2 reacts with N2, the peaks of R1 and R2 become small significantly. This indicates that adsorbed H2 is consumed because of reaction with N2. Therefore, R1 and R2 are the dissociative chemisorption states of H2, and they can react with N2 to form ammonia. However, the peak of R3 cannot or is not easy to react with N2 to form ammonia because the adsorption is too strong. At the same time, we note that the reaction temperature of industrial ammonia synthesis is now commonly below 500 °C. So, the H2 adsorption state of R3 whose desorption temperature is as high as 525-605 °C is difficult to react with N2 to form ammonia. So, R3 is probably a strong chemisorption state of H2 on the surface of both Fe3O4‚FeO- and Fe3O4-based catalysts. Though the strong H2 chemisorbed state has no activity of ammonia synthesis, it can occupy the active site or active center on the surface necessarily. So, it should restrain or hinder the N2 chemical adsorption and reaction of ammonia synthesis. One important reason that the Fe1-xO-based catalyst
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Revised manuscript received December 10, 2002 Resubmitted for review September 27, 2002 Accepted December 16, 2002 IE0202524
