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Al2O3 catalyst
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APPLIED CATALYSIS A:GENERAL
Applied Catalysis A: General 147 (1996) 207-227
Preparation, reduction, and CO chemisorption properties of a cyanide-derived Fe/A1203 catalyst E. Boellaard
a
A . M . van der Kraan b,* J.W. G e u s a
a Department of Inorganic Chemistry, Debije Institute, Utrecht Universi~, P.O. Box 80083, 3508 TB Utrecht, The Netherlands b . . . . lnterfacultatr Reactor lnstttuut, Delft Unwerst~ of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Received 30 May 1996; accepted 30 May 1996
Abstract A supported iron catalyst is prepared by precipitation of an iron hexacyanoferrate complex onto y-alumina. Removal of the cyanide ligands by oxidation at 563 K results in the formation of highly dispersed iron(III) oxide particles. The reduction of the oxidic precursor proceeds via a support stabilised Fe~ _ xO phase as revealed by in situ magnetization measurements and M~Sssbauer spectroscopy. Upon exposure of the reduced and subsequently evacuated catalyst to carbon monoxide at room temperature, both linearly and bridged-bonded carbon monoxide species are observed by infrared spectroscopy at wavenumbers of 1985 cm ~ and 1980-1700 cm -~, respectively. A fraction of the carbon monoxide reacts with irreversibly chemisorbed hydrogen to formate and small hydrocarbons, which remain bonded on the surface of the iron particles. Keywords: Iron; Alumina; Cyanide ligands
1. Introduction Iron catalysts are of crucial importance in a number of industrial chemical processes. Consequently, many scientific studies from both a fundamental and applied point of view have been undertaken in order to gain knowledge about the catalytic properties of the catalysts. It appears that the intrinsic properties of the catalyst depend on a variety of parameters. The nature of the precursor used * Corresponding author. i Previously submitted on 30 November 1994 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S O 9 2 6 - 8 6 0 X ( 9 6 ) O 0 1 9 3 - 7
208
E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
in the catalyst preparation process, the type of support, and the added promoter components will each affect the ultimate catalytic performance. One of the problems often encountered in the production of heterogeneous catalysts is the phenomenon of reaction of the metal ion(s) of the catalytically active precursor with the carrier. This reaction can already proceed during the thermal treatment that is normally used to transfer the system into the catalytically desired state: calcination of iron hydroxides and reduction of iron oxides will generate relative high partial pressures of water which, in turn, facilitate formation of iron aluminates, titanates and (hydro)silicates. Due to this metalsupport interaction complete reduction of the oxidic catalyst precursor will be prohibited. If the formation of a compound out of the active precursor and the support is confined to the interface between the active particles and the support, a limited interfacial reaction is attractive as it can stabilize small metallic crystallites and prevent them from sintering. However, reduction of iron oxide particles applied on silica or titania can also result in metallic iron particles which are completely or partly encapsulated by an iron silicate or titanate layer, respectively. Consequently, the surface properties of the metallic particles will be significantly different from that of the bulk metal phase. Additionally, metallic iron particles are very liable to contamination of the metal-gas interface by (sub)monolayers of oxygen. Contamination of the surface will impede the characterization of the iron particle surface in supported catalysts considerably. Infrared spectroscopic studies on carbon monoxide adsorbed on iron surfaces are relatively scarce, and the published spectra are often characterised by broad absorption bands which do not permit a detailed interpretation. This paper deals with an investigation of the properties of alumina-supported iron catalysts prepared by oxidation of small particles of iron cyanides applied on alumina. It has been found that the thus prepared catalysts do not suffer from undesired metal-support interactions when the cyanide precursor is decomposed under oxidative conditions. Reduction of the oxidic precursor has been studied in situ by both bulk and surface sensitive techniques, such as magnetization measurements and M~ssbauer spectroscopy, and infrared spectroscopy of adsorbed carbon monoxide, respectively.
2. Experimental 2.1. Catalyst preparation The catalyst precursor was prepared by means of deposition-precipitation from a homogeneous solution of an iron cyanide complex with a simple iron salt onto an alumina support. The general features of this method have been described more extensively in [1]. The preparation of the catalyst was carded out in a thermostated double-walled glass vessel equipped with a stirring motor and
E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
209
baffles to ensure thoroughly mixing of the contents; pH-electrode, blanket gas and liquids could be introduced via joints in the cover. The quantity of chemicals was chosen to provide an iron loading of 20 wt.-% metal in the reduced catalyst. All water used was previously deoxygenated by boiling; the water was subsequently cooled down, and kept under a flow of nitrogen. 4.00 g of the support material y-A1203 (Aluminium Oxid C, Degussa) was suspended in 900 ml demineralised water and 2.37 g FeCI2 • 4H20 (Merck, pro analysis), dissolved in 100 ml water was added. The pH was adjusted to 5.0 with HC1 (Merck, pro analysis). 2.52 g of K4Fe(CN)6.3H20 (Merck, pro analysis) dissolved in 100 ml water was slowly injected through a capillary tube ending below the level of the vigorously stirred suspension. The temperature was kept at 295 K. After 57 ks the suspension was settled down, decanted, washed twice with 500 ml water and dried for 173 ks at room temperature in a vacuum of about 1.33 Pa. The batches of several preparations were mixed and subsequently ground, pelletized and crushed. A sieve fraction between 425 and 730 /xm was selected for further experiments. The thermal decomposition of the cyanide precursor was investigated in inert, reducing and oxidizing gas atmospheres to develop a pretreatment procedure in which unintended effects of residual cyanide fragments on the catalytic properties of the eventual catalyst could be minimized. A detailed description of the decomposition treatment study has been presented in [1]. Based on the results of these experiments the cyanide precursor used in this study was subjected to an oxidative pretreatment in a flow of helium- 1 vol.-% oxygen at 563 K for 82 ks.
2.2. Catalyst characterization 2.2.1. Transmission electron microscopy (TEM) Samples of the dried cyanide precursor and the reduced catalyst were investigated within a Philips EM320 transmission electron microscope operated at 300 kV. The chemical composition of the samples was analysed by EDX measurements. 2.2.2. Magnetic measurements Magnetic measurements were carried out using self-developed equipment based on the Weiss-extraction method [2]. A prototype of the measuring system has performed well in previous studies on iron and nickel catalysts [3-5]. The present apparatus was constructed by using a standard Bruker B-E10 electromagnet with cylindrical pole caps Z l l C . Magnetic field strengths up to 0.82 MA m - J (1 MA m - 1 47r kOe) could be established with a fixed air gap of 50 mm. At the centre of each pole cap (Q 100 mm), two Helmholtz sensing coils of 25 mm diameter, 10 mm thickness, containing 10,000 turnings of insulated copper wire were mounted side by side. These pick-up coils were connected in series, but in opposite sense to enhance the magnetic signal and minimize =
210
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
DL)---~ Fig. 1. Schematic diagram of apparatus for magnetic measurements: (1) reactor, (2) transportable magnet, (3) Hehnholtz sensing coils, (4) integrating device.
disturbances caused by small field fluctuations. The magnet could be translated along two rails by means of a stepping motor. On the line passing through the centre of the air gap of the magnet and parallel to the direction of the translation of the magnet a specially developed sample holder was placed. A schematic representation of the apparatus is represented in Fig. 1. On moving the energized magnet by translation in a straight line along a ferro- or ferrimagnetic sample, the magnetized sample generates in the Helmholtz coils an induction voltage. The time-integral of the induction voltage corresponds to the net change of magnetic flux at the pick-up coils. The induction voltage was a hundred fold amplified by a Tektronix AM 502 differential amplifier and subsequently sampled at 1.6 kHz by a 12 bits AD-convertor. Data collection was performed for 6 s, whereas the actual signal was build-up within 2 s. The sample was placed in a Heralux ® quartz reactor ( 0 12 mm) on which a bifilar heating coil and chromel-constantan thermocouple had been cemented. The temperature was controlled by a PID-regulator programmed in the system computer. The temperature of the catalyst bed was measured with a separate thermocouple. To facilitate thermal equilibrium and prevent displacement of the catalyst bed during the magnetic measurements the catalyst bed was covered with quartz fragments. The reactor was connected to a high-vacuum system. The base pressure was 0.13 mPa (1 Pa = 7.50 mTorr). The gas phase could be analysed with a quadrupole mass spectrometer of Leybold-Heraeus type Q200 equipped with gauge head QF200. The reactor was also appropriate for atmospheric flow experiments allowing for simultaneous mass-spectroscopic gas analysis. The magnetization of samples could thus be measured in vacuum and in controlled gas atmospheres at temperatures ranging from 300 up to 873 K. The system was controlled by an Apple IIe computer. Reduction experiments were performed with an oxidic sample of 200 mg. The catalyst was heated with 83 m K / s from room temperature to the desired reduction temperature in a flow of 0.83 m l / s argon-10 vol.-% hydrogen gas mixture (HoekLoos, 99.9990%) which was purified over an Oxisorb ® Gas
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
21 l
Purifier (Messer Griessheim GmbH). During the isothermal stage of the reduction process, the magnetization was measured at 0.39 MA m-~ every 0.3 ks. After each reduction step the sample was cooled down to room temperature.
2.2.3. Mi3ssbauer spectroscopy Typically, 200 mg of the oxidic catalyst was placed in a reactor suitable for in situ MiSssbauer measurements in controlled gas atmospheres and at temperatures ranging from room temperature up to 873 K. After a reduction treatment of the catalyst in a flow of 0.83 m l / s argon-10 vol.-% hydrogen, the sample was cooled down to 300 K after which a M~Sssbauer spectrum was recorded. The spectrometer, equipped with a 57Co in Rh source, was operated in the constant acceleration mode, performing a saw-tooth velocity waveform. The equipment has been described in detail previously [6]. The spectra were not corrected for the varying distance between source and detector and, hence, the curved background in the spectra is of experimental origin. The MiSssbauer parameters were determined by fitting the spectra with subspectra consisting of Lorentzian-shaped lines using a non-linear iterative minimisation routine. In the case of quadrupole doublets the line widths as well as the absorption areas were constrained to be equal. Isomer shifts are reported with respect to the NBS standard sodium nitroprusside. The magnetic hyperfine fields were calibrated against the 515 kOe field of c~-Fe20 3. Argon and hydrogen were supplied by HoekLoos and purified over BASF catalysts R 3-11 and molecular sieve Linde 4A.
2.2.4. Infrared spectroscopy The surface of the catalyst particles after oxidation of the cyanide precursor and after successive reduction treatments was monitored by transmission infrared spectroscopic investigation of adsorbed carbon monoxide probe molecules. The ground cyanide precursor was pressed at 32 MPa to a self supporting wafer (Q 15 ram, 55 mg) and placed in a quartz cell with sodium chloride windows, in which the wafer could be moved to a furnace section where the pretreatment was carried out. The cell was connected to a high vacuum system and a gas handling system. Movement of the catalyst into and out of the infrared beam could be done without admission of atmospheric air or breaking the vacuum. After an oxidation in helium-1 vol.-% oxygen gas flow or a reduction in argon-10 vol.-% hydrogen gas flow, which was previously purified over a palladium catalyst and a molecular sieve, infrared spectroscopic measurements were performed according to the following procedure: after cooling down to room temperature, the flow was stopped and the gas phase was removed out of the cell by evacuating the system for 1.8 ks down to a pressure of 0.13 mPa. Subsequently, the first (reference) spectrum was recorded with a resolution of 5.3 cm-1, using a Perkin-Elmer 580B spectrophotometer connected to a 3500 CDS data station. Next, 1.33 kPa of carbon monoxide was introduced and a
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E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
second scan was initiated. Additional scans were performed at 0.4, 0.9, 1.8, 3.6, 7.2, and 10.8 ks after the initial carbon monoxide dose. In some experiments the carbon monoxide pressure was raised to 13.3 kPa to study the pressure dependence of the infrared absorption bands exhibited by the adsorbed CO molecules. Finally, the cell was reconnected to the high vacuum system and desorption of adsorbed carbon monoxide was followed by repeated scans in the above mentioned time sequence. In order to facilitate assignment of molecular group vibrations in which O-atoms are involved, some experiments are performed with isotopically labeled carbon monoxide C ~80 (Alfa Ventron GmbH). This carbon monoxide was used without purification. Infrared absorption spectra of CO molecules bonded to the catalyst surface were obtained by subtracting of the reference spectrum from the transmission spectrum of interest.
3. Results The reaction of the injected Fe(CN) 4- with the Fe2+/A1203 suspension resulted initially in the formation of a white precipitate. As the precipitation process went along the colour of the suspension gradually changed to pale blue. After vacuum drying the colour of the final product was dark blue. The
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E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
213
Mtissbauer spectrum of the dried cyanide complex, is reproduced in Fig. 3 and is analogous to the one presented in [1]. Investigation of the precipitated cyanide by TEM revealed that the catalyst precursor is made up by both very small particles well distributed over the surface of the alumina support and small thin platelets distributed in between the elementary support particles. From EDX measurements it became apparent that exclusively these platelets comprise a large amount of potassium; the atomic Fe to K proportion varies between 3.4 and 3.9. Samples which do not exhibit plate-shaped particles are virtually free of potassium. The MiSssbauer spectrum of the oxidic precursor, represented in Fig. 3, indicates that the cyanide precursor has been converted completely by the oxidation treatment. In addition, infrared spectroscopic investigations revealed that the broad infrared absorption band at 2060 cm-~, which is characteristic for the cyanide ligands, had completely vanished. The spectrum of the oxide, however, exhibited some new absorption bands at 1565 and 1365-1390 cm(poorly resolved) from which both the position and intensity change upon the consecutive reduction treatments, except after reduction at 373 K. The reduction behaviour of the oxidic precursor is studied by in-situ monitoring the magnetization during a series of isothermal treatments in a hydrogenargon gas flow. In Fig. 2 the magnetization-versus-time curves measured at successive reaction temperatures are indicated. At 373 and 473 K the magnetization does not change during the treatment. The first indication for reduction is observed at 548 K by the gradual change in magnetization. Initially the
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E. Boellaard et al. / A p p l i e d Catalysis A: General 147 (1996) 207-227
214 Table 1
Magnetization (a.u.) at 300 K and 0.82 M A m - 1 after reduction at the indicated temperature Catalyst
Oxide
373 K
473 K
548 K
623 K
723 K
Fe 2 Fe
1069
1074
1074
2100
4980
10924
Table 2 M~ssbauer parameters of a Fe2Fe catalyst after different reduction steps Treatment
IS ( m m / s )
F (ram/s)
SC (%)
Cyanide precursor
0.13
-
-
0.31
44
0.69
0.34
-
0.58
56
Oxidic precursor
0.62 0.61
0.66 1.15
-
0.44 0.45
66 34
4 ks, 473 K
0.62 0.61
0.68 1.19
-
0.48 0.44
73 27
4 ks, 548 K
0.64 0.67
0.64 1.18
-
0.55 0.44
78 17
1.01
1.82
-
0.45
5
0.43 1.03
0.12 - 0.03
444 433
0.74 0.78
12 27
1.17 0.59
1.33 0.53
-
1.12 0.61
44 17
58 ks, 548 K
QS ( m m / s )
-0.01
H F (kOe)
4 ks,
0.65
466
0.32
6
623 K
0.95 1.26 0.47 1.34
0.00 1.46 0.40 0.56
431 -
0.88 0.95 0.48 0.53
25 28 15 27
14 ks, 623 K
1.31 1.21 0.49
0.42 1.32 0.44
-
0.51 0.66 0.40
61 26 13
43 ks, 623 K
1.37 1.37 0.54
0.39 1.63 0.39
-
0.60 0.38 0.38
79 7 14
4 ks, 723 K
0.31 1.37 1.33 0.53
0.00 0.29 1.66 0.45
328 -
0.26 0.62 0.52 0.40
19 62 7 12
14 ks, 723 K
0.30 1.36 1.37 0.47
- 0.02 0.26 1.84 0.39
328 -
0.25 0.65 0.45 0.31
54 34 5 7
43 ks, 723 K
0.30 1.34 1.31 0.48
- 0.01 0.26 1.76 0.41
328 -
0.25 0.60 0.61 0.39
72 14 7 7
E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
215
magnetization increased to drop subsequently. At 623 K the treatment causes an initial decrease and a subsequent increase of the magnetization which increase gradually slows down at 723 K, but saturation is not reached. In addition to the dynamic measurements at the reduction temperature, the magnetization of the thermally treated catalyst is also measured at a fixed reference temperature and a fixed magnetic field after each reduction step. The thus measured data are collected in Table 1 and indicate that up to reduction temperatures of 473 K the iron oxide remains unchanged. From the obtained dynamic magnetization data it is obvious that the formation of a ferro- or ferrimagnetic phase is succeeded by the formation of a non-ferro- or non-ferrimagnetic phase, whereas ultimately again a ferro- or ferrimagnetic phase results. A more conclusive identification of the in the reduction process involved phases is derived from the in-situ recorded M5ssbauer spectra, which are shown in Fig. 3. The results of the corresponding spectra analyses are given in Table 2. Reduction of the oxidic precursor at temperatures up to 473 K leads to very small changes in the Mi3ssbauer spectrum. The spectral contribution of the doublet with the larger QS decreases at increasing reduction temperature. Prolonged reduction at 548 K results in the gradual formation of new doublets and a set of sextuplets. The latter can be assigned to small particles of the ferrimagnetically ordered magnetite, Fe304. During the reduction treatment at 623 K, the sextuplets vanish in favour of a new doublet which represents
1600
Fe2Fe
1982 1799 °
a
/, [ a '!~'i 1330
,
0.013
4000
3000
2000
1500
1000
W A V E N U M B E R / cm 1
Fig. 4. Infrared spectra of CO adsorbed on a Fe2Fe catalyst after reduction at (a) 548 K, and (b) 623 K. Spectra recorded 10.8 ks after admission of 1.33 kPa CO.
E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
216
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E. Boellaard et al./ Applied Catalysis A: General 147 (1996) 207-227
217
woestite, Fe~ _xO. After increasingly severe reduction conditions, viz. at 723 K, the characteristic sextuplet of ferromagnetic metallic o~-Fe is formed. The observed changes of the magnetization are in good agreement with the presence of the phases indicated by the Mtissbauer spectra. In Figs. 4 - 8 infrared spectra are shown which are recorded after reduction of the oxidic catalyst precursor and subsequent exposure to carbon monoxide at progressively more severe conditions. Admission of 1.33 kPa carbon monoxide to the evacuated oxidic precursor gives rise to a very weak absorption band at 1575 cm -~, which becomes more pronounced after reduction at 373 and 473 K. After reduction at 548 K the surface properties of the catalyst precursor have drastically changed as illustrated in Fig. 4 by a large number of absorption bands, the most intense ones arising at 1982, 1799, 1750, 1600 and 1330 cm - I Reduction at 623 K results in an overall decrease of the intensity of the absorption bands. After reduction at 723 K the CO chemisorption capacity is restored and also some new surface species appear. In Fig. 5 the time and pressure dependence of the very well developed spectra are shown. In these spectra absorption bands are observed at 2950, 2765, 2677, 2160, 2029, 1985, 1906, 1867, 1804, 1638, 1600, 1349, 1322 and 1050 cm - 1 The intensity of all bands increases, although at different rates, upon prolonged carbon monoxide exposure time and raised pressure. The band maximum in the 1950-1830 cm-J region gradually shifts to a higher wavenumber, viz. 1867 to
1985
Fe2Fe 1600
0.022
1906
/ ~\
4000
0000
1638
2000
1349
1500
j
1000
WAVENUMBER / cm4 Fig. 6. Infrared spectra o f CO adsorbed on a Fe2Fe catalyst after reduction at 723 K. Spectra recorded (a) 7.2
ks after admission of 13.3 kPa CO, and (b) after evacuation for 1.8 ks.
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
218
1985
d
Fe2Fe
0.017
1919
4000
3000
2000 WAVENUMBER
1500
1000
/ c m "1
Fig. 7. Infrared spectra of CO adsorbed on a Fe2Fe catalyst after reduction at 823 K. Spectra recorded (a) immediately, (b) 0.9 ks, and (c) 10.8 ks after admission of 1.33 kPa CO, and (d) 7.2 ks after admission of 13.3 kPa CO.
1985
Fe2Fe
a 0.o17
1919 1800
1868
1349
2765 • A
L~ 4000
J 3000
2033 2677
~ - - ~ _
2160.
i
L
A
I
__J
2000 WAVENUMBER
L
J
1500
10(30
/ cm q
Fig. 8. Infrared spectra of CO adsorbed on a Fe 2 Fe catalyst after reduction at 823 K. Spectra recorded (a) 7.2 ks after admission of 13.3 kPa CO, and (b) after evacuation for 1.8 ks.
E. Boellaard et al./'Applied Catalysis A." General 147 (1996) 207-227
219
1585
Fe2Fe 0.013 1332
1857
1938
2762
4000
3000
a
2000
1500
1000
W A V E N U M B E R / cm -1
Fig. 9. Infrared spectra of isotopically labeled C180 adsorbed on a F%Fe catalyst after reduction at 723 K. Spectra recorded after 0.9 ks exposure at (a) 300 K, (b) 373 K and (c) 423 K.
1906 cm-1, and the well resolved peak at 1981 cm-~ shifts to 1985 c m - I . The previously observed bands at 1799 and 1750 cm-1 are now much less distinct. In Fig. 6 it is shown that after evacuation of the gaseous carbon monoxide for 1.8 ks at 300 K, the intensity of the bands at 1985 and 1906 cm-~ decreases, whereas those at 1600 and 1349 cm -~ slightly increase. The corresponding wavenumbers shift back to their initial values. In Figs. 7 and 8 analogue spectra are shown for the catalyst after reduction at 823 K. Compared with the spectra recorded after reduction at 723 K, the intensity of the bands at 2160 and 1965-1830 cm -~ is increased, whereas the intensity of the other bands is decreased relatively to the band at 1985 cm -~ . Notice that the species exhibiting the 2160 and 2030 c m - 1 bands remain adsorbed after evacuation of the carbon monoxide gas phase. A reduced iron catalyst sample was also exposed to 0.83 kPa isotopically labeled C 180 at room temperature. The recorded spectrum, reproduced in Fig. 9, which should be approximately analogous to the spectrum of Fig. 5, exhibits now absorption bands at 1976, 1938, 1832, 1629 and 1332 cm -1. As this particular sample has been used in several experiments, it can be expected that repeated reduction treatments at 723 K have caused sintering of the metal particles and consequently a decrease of the metal surface. Together with the lower carbon monoxide pressure the smaller metal surface accounts for the relatively low absorbance of the recorded spectra. Increase of the adsorption temperature to 373 and 423 K results in an enhanced absorption intensity.
220
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
Depending on the temperature, absorption bands at 2762, 2672, 2152, 1585 and 1547 cm -1 become also visible.
4. Discussion The formation of a white precipitate upon reaction of Fe(CN) 4- with Fe 2+ is consistent with the intended formation of the ferrous-ferrous hexacyanide Fe2Fe(CN) 6. The observed gradual change of colour of the precipitate from white to pale blue and ultimately dark blue during the stay in the aqueous suspension and the drying step, is due to the formation of a ferri-ferrous cyanide complex. It is well known that the ferrous-ferrous hexacyanide complex is very sensitive to oxidation by air [7]. The M~ssbauer data reveal that the oxidation was limited to the ferrous ions of the iron(II) chloride and that the ferrous cyanide complex remained intact. The presence of potassium containing plate-shaped precipitates in the final product is ascribed to the formation of insoluble salts like KFeFe(CN) 6 or K2FeFe(CN) 6. These compounds can be precipitated when a solution of Fe 3+ or Fe z+, respectively, are brought in contact with K4Fe(CN) 6. Both compounds might be formed in the final stage of the precipitation procedure, when both the colour of the suspension became pale blue and the K + / F e 2+ proportion was increased due to the continuous injection of K4Fe(CN) 6 and the consumption of Fe z+ by the precipitation reaction. Moreover, whenever injection of the cyanide complex into the suspension is not sufficiently homogeneous, the concentration of K + may reach unfavourable high levels. After treatment of the cyanide precursor in helium-oxygen at 563 K, the cyanide complex was completely destructed. The newly formed phase is most likely a highly dispersed FenI-oxide. Since no magnetic ordering was observed in the M~ssbauer spectrum at room temperature, the fully superparamagnetic behaviour of the Fern-oxide indicates a small particle size. In addition, the spectrum has to be described by a set of two doublets of the same IS, but of different QS, representing iron ions in the bulk (QS = 0.66) and in the surface (QS = 1.15) of the iron oxide particles. Based on the results from the spectra the crystallites are assessed to be less than 10 nm in diameter [8]. The different reaction steps involved in the reduction of FeIn-oxide to Fe °, metallic iron, are successfully revealed by the combination of magnetization and M~issbauer measurements. After a prolonged period of time of reduction or after applying more severe reduction conditions most of the initially present FeIn-oxide reacts to a lower oxidation state. During the reduction process several oxidation states and phases of iron coexist. These phases are most likely highly dispersed and intimately mixed and due to the dynamic character of the reaction process they will not posses the ideal (crystallographic) bulk characteristics. The lack of crystallinity is reflected by the large line-width of the successively developed
E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
221
doublets and sextuplets in the MiSssbauer spectra. Moreover, the MiSssbauer parameters of the phases identified as magnetite (the set of sextuplets) and woestite do not exhibit the values reported for the bulk solids. Whereas the IS of woestite phase remains fairly constant at 1.31-1.37 m m / s , the QS gradually decrease from 0.56 to 0.26 m m / s at increasing degree of reduction of the iron oxide particles. The distinct stabilization of the Fe~ _xO phase at temperatures where bulk FeO is metastable is considered to be a proof that this phase is stabilized by interaction with the "y-A1203support [4]. The slower reduction in the MSssbauer experiments compared to the magnetization experiments may be due to a less efficient removal of the produced water vapour since the gas has to diffuse through a perforated aluminium foil, whereas in the magnetic experiments the reaction gas runs through the catalyst bed. The infrared spectra measured with the iron catalyst after successive reduction treatments show a close mutual resemblance of absorption bands. Each reduction treatment, however, brings about a special spectral feature. In the spectra recorded on the catalyst reduced at a moderate temperature, viz. 723 K, several characteristic wavenumber ranges can be distinguished. The major bands of the spectrum arise within the 2100-1700 cm -1 range and are assigned to stretching vibrations of molecularly adsorbed CO. The sharp band at 1981 cm is attributed to CO which is linearly adsorbed on metallic iron sites and the broad band, peaking at 1867 cm -1, to bridged-bonded CO [9]. Upon prolonged exposure time or increasing pressure both bands gain intensity and shift to higher wavenumbers, viz. to 1985 and 1906 cm ~, respectively. Furthermore, at 1804 cm-~ a well resolved band appears. The observed red shift may be due to adsorbate-adsorbate interaction or to occupation of sites of lower binding energy at increasing CO coverage. The weak band at 2025 cm 1 is assigned to CO adsorbed on sites which are made up of metallic surface atoms which are adjacent to charged atoms [8]. The assignment of the mentioned absorption bands to stretching vibrations of molecularly adsorbed CO is supported by the strong shift in wavenumbers by using isotopically labeled C lsO instead of C160. The absorption bands are not attributable to adsorbed molecular iron carbonyl complexes since the model study of Mohana Rao et al. [10] in which Fe(CO)s was adsorbed on 7-A1203 revealed significantly different bands. The wavenumbers of the molecularly adsorbed CO species on the supported iron catalyst prepared from a complex cyanide are low as compared to the wavenumbers generally observed with carbon monoxide adsorbed on supported iron catalysts [8,11-17]. An additional feature of the cyanide derived catalyst is the large number of well-resolved absorption bands in the measured spectra. The occurrence of distinct absorption bands at relatively low wavenumbers (19851700 cm -~) can be rationalized by considering the moderately small size of the particles, which is about 30 to 40 nm in diameter. Particles of such a size expose
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well-ordered low index planes at the surface. Consequently, the catalyst surface contains well-defined sites for adsorption of CO in both on-top and bridged geometries. Spectroscopic studies on vapour deposited iron films and single crystals revealed vibrations of adsorbed CO at even much lower wavenumbers [18,19]. Theoretical studies indicate that the presence of adsorbed hydrogen on the catalyst surface may also account for the low wavenumbers. Formation of a H - F e - C O complex causes an enhanced metal-adsorbate interaction by increasing the 7r-donation from the F e - H complex into the antibonding zr-orbital of carbon monoxide, which weakens the intramolecular CO bonding and, hence, lowers its vibration frequency [20,21]. It will be discussed below that the presence of adsorbed hydrogen after reduction and evacuation of the catalyst is very likely. The band at 2160 cm-1, which is faintly visible in the spectra measured after reduction at 723 K, becomes very distinct after reduction at 823 K. In many studies bands reported in this range are assigned to CO adsorbed on surface iron aluminate [16]. A characteristic of thus adsorbed CO is that it rapidly desorbs upon evacuation of the catalyst. However, in this study, the intensity of the band is not affected by evacuation for even 10.8 ks. Furthermore, in the spectra recorded in the C180 experiments the band appears at 2153 cm -1, a shift of only - 7 cm -1. The bands previously assigned to molecularly adsorbed CO are, on the other hand, shifted by - 4 0 cm-1. Consequently, it is unlikely that the band at 2160 cm -1 represents molecularly adsorbed CO species. The band is not associated with any other band in the spectrum, since its time and pressure dependency is different. An iron hydride F e - H is not probable, as the reference spectrum, recorded just after reduction, does not exhibit any absorption band in this spectral region. However, an analogous band at 2158 cm-1 is observed after reaction of the catalyst with ethylene at 523 K [22]. In that study the band was assigned to an 'ethylidyne' species, CHxCHy, adsorbed on the iron surface. Since iron has the potential to catalyse the hydrogenation of CO, viz. the Fischer-Tropsch reaction, the formation of hydrocarbon fragments on the surface of an iron catalyst can be expected upon co-adsorption of CO and H 2. Whether the hydrogenation reaction proceeds or not, is governed by the catalytic properties of the iron surface. An overall prerequisite for the occurrence of this reaction in the present experiments is, of course, the availability of pre-adsorbed hydrogen at the iron surface. Moreover, the reaction to adsorbed 'ethylidyne' only proceeds on highly reduced iron surfaces. Apparently, the presence of oxygen on the iron surface prohibits the reaction to the adsorbed 'ethylidyne' species. The set bands observed at 2944, 2765, 2677, 1638, 1600, 1349, 1322 and 1055 cm -~ is also a unique feature of the iron catalyst prepared from the cyanide precursor. In literature no such extended spectra have been reported for supported iron catalysts under comparable experimental conditions. The bands at 1600 and 1349 cm -1 are ascribed to the asymmetric and symmetric OCO
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stretching vibrations of formate species adsorbed on the catalyst. Formic acid adsorbed on supported Fe, Co [23] and Ni [22,24] catalysts, gives rise to absorption bands at 1583-1580 and 1350-1345 cm-~. Formate species formed during hydrogenation of CO 2 a n d / o r CO on supported Fe [25], Ru [26,27], and Rh [19] catalysts exhibits absorption bands at 1595, 1395 and 1380 cm-1. In these studies the formate species is reported to be adsorbed exclusively on the support. Gopal et al. [28] provided evidence that formate can only be formed by direct reaction of CO with hydroxyl groups of A120 3 and MgO supports at temperatures of at least 375 K. The frequency of the formate OCO vibrations also depends on the mode of adsorption. The fact that in the present study the absorption bands appear already at room temperature and at deviating wave numbers strongly suggest that the formate species are formed and adsorbed on a metallic iron surface. Although the band at 1322 cm-~ can be assigned to a bending mode of the CH of formate, the growth pattern of this band is different from those assigned to the OCO vibrations of formate. Most probably this band will arise from another species. In conjunction with the weak and broad band at 1640 c m - I , a carbonate species is plausible. The identification of the absorption bands at the exceptionally low wavenumbers 2765 and 2677 cm-1 is greatly hampered by lack of consistent reference data in this frequency range. A set of bands in the spectral range 2900-2700 cm-~ is characteristic for aldehydic CH stretching vibrations of gas phase molecules [29]. In CO-hydrogenation catalysis this functional group can be associated with the adsorbed formyl species, CHO which is supposed to be an intermediate in CO-hydrogenation reactions [30]. Model inorganic compounds containing the CHO functional group clearly demonstrate their existence. Coilman and Winter [31] reported infrared absorption bands at 2690, 2540, and 1577-1610 cm-~. Despite a number of attempts, Yates and Cavanagh [32] have not succeeded in proving the existence of formyl species on a A1203-supported Rh catalyst by spectroscopic techniques. The CHO species is claimed to exist on ZnO [33,34] and CeO 2 [32] after co-adsorption of CO and H 2. The set of bands at 2765 and 2675 cm-~ found in the present study may therefore be due to formyl species. The shift to lower wavenumbers, as compared to the carbonyl model compound, can be explained by the increased mass of the iron substrate to which the functional group is bonded. As the bands at 2765 and 2677 cm-~ show analogous time and pressure dependencies as the 1600 and 1349 cm -~ bands, the former set of bands may also be attributed to the previously discussed formate species. In this view the in-plane-bending of the carboxylate CH bond accounts for the band at 1322 c m - 1, while the stretching vibration of the CH bond is represented by either the 2765 or the 2677 cm -1 band. The other remaining band might represent a combination band involving a stretch vibration of OCO and a CH deformation [26,35]. In the cited literature, however, high-frequency bands of adsorbed
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formate are hardly reported. As Fischer-Tropsch synthesis and formic acid adsorption experiments performed on pure aluminium oxid C revealed well-developed absorption bands at 3007 and 2912 cm-~ [22], a red-shift to 2765 and 2677 cm -1 may indicate that the formate is not adsorbed on the alumina support, but on the iron particles. However, it must be taken into account that a red shift of 240 cm -1 is very large. Shustorovich and Bell [36] speculated guided by BOC-MP approach calculations that decomposition of formic acid on Fe/W(110) surfaces leads to parallel production of formate and formyl species. The formyl species will decompose to CO and atomic hydrogen. Since no hydrogen is admitted during the IR experiment, the H atoms involved in the formate and ethylidyne formation must originate from the previously performed reduction treatment. Apparently, evacuation at room temperature is not sufficient to remove the hydrogen from the metallic iron surface. A strong adsorbate-adsorbent interaction is in agreement with results from temperature-programmed desorption studies [37]. When the surface is covered by hydrogen, the admitted carbon monoxide either displaces the hydrogen or reacts with the hydrogen to formate. Both mechanisms account for the increase of the absorbance in the 2000-1625 cm -1 range, and subsequently of the formate bands, with time. Formate formation from CO implies residual carbon either to remain on the catalyst surface or to react with adsorbed hydrogen to CH species. The observed weak absorptions at 2944 and 1055 cm-~, which are in the range of the aliphatic CH vibrations, nicely support such an interpretation. The infrared spectra recorded after reduction of the catalyst at 823 K exhibit a considerable change in intensity ratio of the 1985 and 1919 cm-~ bands and a decrease of the overall absorbance. These observations may be explained by an increase of the particle size due to thermal sintering, causing an increase of the fraction of smooth, low-index crystal surfaces on which CO can be adsorbed in multicentered modes and a decrease of the area of more reacting iron surfaces. It is remarkable that after low-temperature reduction, Fig. 4, analogous bands are observed as after high-temperature reduction, Figs. 5-8. Because the oxides which constitute the bulk of the oxidic precursor particles do not exhibit infrared absorption bands upon CO exposure, the surface of the catalyst particles after reduction at 548 and 623 K should contain some small patches of metallic iron. On these sites CO is adsorbed molecularly as is indicated by the bands at 1982, 1799 and 1750 c m - 1. Especially after reduction at 548 K, the bands attributed to bridged-bonded CO are very pronounced. After reduction at more severe conditions the spectral contribution of these bands decrease. Probably, these bands are associated with CO adsorbed on rough surfaces. The carbon monoxide also reacts to adsorbed formate species as evident from the 1600 and 1330 c m - 1 bands. From the magnetization curve in Fig. 2 it can be derived that during reduction at 623 K the bulk of the catalyst is reacting to metallic iron. However, the corresponding infrared spectrum in Fig. 4 indicates by the decreased overall
E. Boellaard et a l . / Applied Catalysis A: General 147 (1996) 207-227
225
absorption a reduced ability of the catalyst for CO adsorption. This effect may be due to a considerably disturbed particle surface caused by a poorly defined particle composition. It is likely that the particles comprise a mixture of several reduced intermediates. The in Fig. 4 observed oscillation in the 11500-1450 c m - ~ range is caused by a change in the reference spectrum. As mentioned above, the oxidic precursor exhibits some absorption bands due to remaining surface groups. Upon exposure to 1.33 kPa carbon monoxide the position and intensity of one of these bands changes. Consequently, substraction of the reference spectrum from the spectrum measured in 1.33 kPa carbon monoxide results in a virtual band. An explanation for the difference between the infrared spectroscopic results measured in this work on iron catalysts prepared from cyanide precursors and on those prepared by traditional precipitation and impregnation techniques may be based on the structure of the metallic surface. Since water vapour facilitates the formation of iron aluminate, precautions have been taken to prevent reaction of the iron (oxide) phase with the alumina support. The presence of iron(II) aluminate at the metal-support interface will raise the interaction of the metal particle with the surface of the support. The interfacial interaction brings about that the thermodynamically stable shape of free iron particles, which contains a large fraction of closely packed atomic surfaces, is not established, but a shape containing a much larger fraction of atomically rough iron surfaces. With precipitated iron hydroxides much hydroxyl groups have to be removed by calcination at high temperatures. Fortunately, the oxidation of the cyanide precursor proceeds only after desorption of physisorbed and crystal water, which occurs at moderate temperatures [1]. In addition, the subsequently performed reduction of the oxidic precursor is split up into several isothermal steps to prevent the occurrence of an unfavourable HzO-to-H 2 ratio. Under the described conditions formation of bulk iron aluminate a n d / o r thin surface layers, which influences the metallic chemisorption properties, is prevented.
5. Conclusions
Deposition precipitation of an iron hexacyanoferrate complex onto alumina has been successfully used for the preparation of a supported iron catalyst. The initially obtained cyanide complex can be converted by oxidation into a highly dispersed iron oxide. Reduction of the oxidic catalyst by hydrogen proceeds via Fe304 and an alumina-stabilized Fel_xO phase as demonstrated by in situ performed dynamic magnetization measurements and M~Sssbauer spectroscopy. The ultimately resulting metallic iron catalyst exhibits a remarkable behaviour with respect to the interaction with carbon monoxide. Infrared spectra recorded after exposure of 1.33 kPa CO to the reduced and evacuated catalyst show a number of well resolved absorption bands, which could be assigned to both
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linearly and bridged-bonded carbon monoxide species, as well as to formate and hydrocarbon species. The presence of the latter species is very unusual under the applied experimental conditions, and suggests a reactive iron surface covered with hydrogen. The iron catalysts offer promising properties with respect to the production of hydrocarbons via the Fischer-Tropsch process. The application of in situ performed dynamic magnetization measurements provides an additional dimension to the characterization of catalysts and evaluation of reaction processes.
Acknowledgements We would like to express our sincere thanks to P.R. van der Linde for developing the software for operating the magnetic equipment and C.D. de Haan for performing the T E M / E D X experiments. The investigations were supported by the netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Research (NWO).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
E. Boellaard, A.M. van der Kraan and J.W. Geus, Stud. Surf. Sci. Catal., 91 (1995) 931. P.W. Selwood, Chemisorption and Magnetization, Academic Press, London, 1975, p. 32. P.K. de Bokx, A.J.H.M. Kock, E. Boellaard, W. Klop and J.W. Geus, J. Catal., 96 (1985) 454. A.J.H.M. Kock, P.K. de Bokx, E. Boellaard, W. Klop and J.W. Geus, J. Catal., 96 (1985) 468. A.J.H.M. Kock, H.M. Fortuin and J.W. Geus, J. Catal., 96 (1985) 261. A.M. van der Kraan and J.W. Niemantsverdriet, in G.J. Long and J.G. Stevens (Editors), Industrial Applications of the MiSssbauer Effect, Plenum Press, New York, 1986, p. 609. Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edn., vol. 12, Interscience Publishers, London, 1967, p. 27. A.M. van der Kraan, Phys. Stat. Sol. A, 18 (1973) 215. N. Sheppard and T.T. Nguyen, in R.J.H. Clark and R.E. Hester (Editors), Advances in Infrared and Raman Spectroscopy, vol. 5, Heyden and Son, London, 1978. K. Mohana Rao, G. Spoto, E. Guglielminotti and A. Zecchina, Inorg. Chem., 28 (1989) 243. M.J. Heal, E.C. Leisegang and R.G. Torrington, J. Catal., 51 (1978) 314. V. Perrichon, M. Primer, N. Nahon and P. Turlier, C.R. Acad. Sci. Paris, 289 (1979) C-149. D. Bianchi, H. Batis-Landoulsi, C.O. Bennet, G.M. Pajonk, P. Vergnon and S.J. Teichner, Bull. Soc. Chim. France, 9-10 (1981) 1-345. J.B. Benziger and L.R. Larson, J. Catal., 77 (1982) 550. N. Jorgensen and C.H. Rochester, Appl. Catal., 25 (1986) 69. C. Johnston, N. Jorgensen and C.H. Rochester, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 309. A.F.H. Wielers, A.J.H.M. Kock, C.E.C.A. Hop, J.W. Geus and A.M. van der Kraan, J. Catal., 117 (1989) 1. J.-P. Lu, M.R. Albert and S.L. Bernasek, Surf. Sci., 217 (1989) 55. S.P. Mehandru and A.B. Anderson, Surf. Sci., 210 (1988) 345. F. Solymosi, I. Tombficz and M. Kocsis, J. Catal., 75 (1982) 78. J. Lewell, Chem. Soc. Faraday 2, 84 (1988) 893. E. Boellaard, A.M. van der Kraan and J.W. Geus, Appl. CataL A, 147 (1996) 229. G. Blyholder, D. Shihabi, W.V. Wyatt and R. Bartlett, J. Catal., 43 (1976) 122.
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K. Kishi, T. Ogana and K. Hirota, J. Catal., 5 (1966) 464. V. Perrichon, M. Pijolat and M. Primet, J. Mol. Catal., 25 (1984) 207. R.A. Dalla Betta and M. Shelef, J. Catal., 48 (1977) 111. F. Solymosi, A. ErdShelyi and M. Kocsis, J. Chem. Soc., Faraday Trans. 1, 77, 1003. P.G. Gopal, R.L. Schneider and K.L. Watters, J. Catal., 105 (1987) 366. R.L. Pecsok, L.D. Shields, T. Cairns and I.G. McWilliam, Modern Methods of Chemical Analysis, 2nd edn., John Wiley, New York, 1976, p. 165. E.L. Muetterties and L Stein, Chem. Rev., 79 (1979) 479. J.P. Collman and S.R. Winter, J. Am. Chem. Soc., 95 (1973) 4089. J.T. Yates, jr. and R.R. Cavanagh, J. Catal., 74 (1982) 97. J.C. Lavalley, J. Saussey and T. R~/is, J. Mol. Catal., 17 (1982) 289. J. Saussey, J.C. Lavalley, J. Lamotte and T. Ra'is, J. Chem. Soc,, Chem. Commun., 278 (1982). B.E. Hayden, K. Prince, D.P. Woodruff and A.M. Bradshaw, Surf. Sci., 133 (1983) 58. E. Shustorovich and A.T. Bell, Surf. Sci., 222 (1989) 371. F. Bozso, G. Ertl, M. Grunze and M. Weiss, Appl. Surf. Sci., 1 (1977) 103.
APPLIED CATALYSIS A:GENERAL
Applied Catalysis A: General 147 (1996) 207-227
Preparation, reduction, and CO chemisorption properties of a cyanide-derived Fe/A1203 catalyst E. Boellaard
a
A . M . van der Kraan b,* J.W. G e u s a
a Department of Inorganic Chemistry, Debije Institute, Utrecht Universi~, P.O. Box 80083, 3508 TB Utrecht, The Netherlands b . . . . lnterfacultatr Reactor lnstttuut, Delft Unwerst~ of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Received 30 May 1996; accepted 30 May 1996
Abstract A supported iron catalyst is prepared by precipitation of an iron hexacyanoferrate complex onto y-alumina. Removal of the cyanide ligands by oxidation at 563 K results in the formation of highly dispersed iron(III) oxide particles. The reduction of the oxidic precursor proceeds via a support stabilised Fe~ _ xO phase as revealed by in situ magnetization measurements and M~Sssbauer spectroscopy. Upon exposure of the reduced and subsequently evacuated catalyst to carbon monoxide at room temperature, both linearly and bridged-bonded carbon monoxide species are observed by infrared spectroscopy at wavenumbers of 1985 cm ~ and 1980-1700 cm -~, respectively. A fraction of the carbon monoxide reacts with irreversibly chemisorbed hydrogen to formate and small hydrocarbons, which remain bonded on the surface of the iron particles. Keywords: Iron; Alumina; Cyanide ligands
1. Introduction Iron catalysts are of crucial importance in a number of industrial chemical processes. Consequently, many scientific studies from both a fundamental and applied point of view have been undertaken in order to gain knowledge about the catalytic properties of the catalysts. It appears that the intrinsic properties of the catalyst depend on a variety of parameters. The nature of the precursor used * Corresponding author. i Previously submitted on 30 November 1994 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S O 9 2 6 - 8 6 0 X ( 9 6 ) O 0 1 9 3 - 7
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in the catalyst preparation process, the type of support, and the added promoter components will each affect the ultimate catalytic performance. One of the problems often encountered in the production of heterogeneous catalysts is the phenomenon of reaction of the metal ion(s) of the catalytically active precursor with the carrier. This reaction can already proceed during the thermal treatment that is normally used to transfer the system into the catalytically desired state: calcination of iron hydroxides and reduction of iron oxides will generate relative high partial pressures of water which, in turn, facilitate formation of iron aluminates, titanates and (hydro)silicates. Due to this metalsupport interaction complete reduction of the oxidic catalyst precursor will be prohibited. If the formation of a compound out of the active precursor and the support is confined to the interface between the active particles and the support, a limited interfacial reaction is attractive as it can stabilize small metallic crystallites and prevent them from sintering. However, reduction of iron oxide particles applied on silica or titania can also result in metallic iron particles which are completely or partly encapsulated by an iron silicate or titanate layer, respectively. Consequently, the surface properties of the metallic particles will be significantly different from that of the bulk metal phase. Additionally, metallic iron particles are very liable to contamination of the metal-gas interface by (sub)monolayers of oxygen. Contamination of the surface will impede the characterization of the iron particle surface in supported catalysts considerably. Infrared spectroscopic studies on carbon monoxide adsorbed on iron surfaces are relatively scarce, and the published spectra are often characterised by broad absorption bands which do not permit a detailed interpretation. This paper deals with an investigation of the properties of alumina-supported iron catalysts prepared by oxidation of small particles of iron cyanides applied on alumina. It has been found that the thus prepared catalysts do not suffer from undesired metal-support interactions when the cyanide precursor is decomposed under oxidative conditions. Reduction of the oxidic precursor has been studied in situ by both bulk and surface sensitive techniques, such as magnetization measurements and M~ssbauer spectroscopy, and infrared spectroscopy of adsorbed carbon monoxide, respectively.
2. Experimental 2.1. Catalyst preparation The catalyst precursor was prepared by means of deposition-precipitation from a homogeneous solution of an iron cyanide complex with a simple iron salt onto an alumina support. The general features of this method have been described more extensively in [1]. The preparation of the catalyst was carded out in a thermostated double-walled glass vessel equipped with a stirring motor and
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baffles to ensure thoroughly mixing of the contents; pH-electrode, blanket gas and liquids could be introduced via joints in the cover. The quantity of chemicals was chosen to provide an iron loading of 20 wt.-% metal in the reduced catalyst. All water used was previously deoxygenated by boiling; the water was subsequently cooled down, and kept under a flow of nitrogen. 4.00 g of the support material y-A1203 (Aluminium Oxid C, Degussa) was suspended in 900 ml demineralised water and 2.37 g FeCI2 • 4H20 (Merck, pro analysis), dissolved in 100 ml water was added. The pH was adjusted to 5.0 with HC1 (Merck, pro analysis). 2.52 g of K4Fe(CN)6.3H20 (Merck, pro analysis) dissolved in 100 ml water was slowly injected through a capillary tube ending below the level of the vigorously stirred suspension. The temperature was kept at 295 K. After 57 ks the suspension was settled down, decanted, washed twice with 500 ml water and dried for 173 ks at room temperature in a vacuum of about 1.33 Pa. The batches of several preparations were mixed and subsequently ground, pelletized and crushed. A sieve fraction between 425 and 730 /xm was selected for further experiments. The thermal decomposition of the cyanide precursor was investigated in inert, reducing and oxidizing gas atmospheres to develop a pretreatment procedure in which unintended effects of residual cyanide fragments on the catalytic properties of the eventual catalyst could be minimized. A detailed description of the decomposition treatment study has been presented in [1]. Based on the results of these experiments the cyanide precursor used in this study was subjected to an oxidative pretreatment in a flow of helium- 1 vol.-% oxygen at 563 K for 82 ks.
2.2. Catalyst characterization 2.2.1. Transmission electron microscopy (TEM) Samples of the dried cyanide precursor and the reduced catalyst were investigated within a Philips EM320 transmission electron microscope operated at 300 kV. The chemical composition of the samples was analysed by EDX measurements. 2.2.2. Magnetic measurements Magnetic measurements were carried out using self-developed equipment based on the Weiss-extraction method [2]. A prototype of the measuring system has performed well in previous studies on iron and nickel catalysts [3-5]. The present apparatus was constructed by using a standard Bruker B-E10 electromagnet with cylindrical pole caps Z l l C . Magnetic field strengths up to 0.82 MA m - J (1 MA m - 1 47r kOe) could be established with a fixed air gap of 50 mm. At the centre of each pole cap (Q 100 mm), two Helmholtz sensing coils of 25 mm diameter, 10 mm thickness, containing 10,000 turnings of insulated copper wire were mounted side by side. These pick-up coils were connected in series, but in opposite sense to enhance the magnetic signal and minimize =
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DL)---~ Fig. 1. Schematic diagram of apparatus for magnetic measurements: (1) reactor, (2) transportable magnet, (3) Hehnholtz sensing coils, (4) integrating device.
disturbances caused by small field fluctuations. The magnet could be translated along two rails by means of a stepping motor. On the line passing through the centre of the air gap of the magnet and parallel to the direction of the translation of the magnet a specially developed sample holder was placed. A schematic representation of the apparatus is represented in Fig. 1. On moving the energized magnet by translation in a straight line along a ferro- or ferrimagnetic sample, the magnetized sample generates in the Helmholtz coils an induction voltage. The time-integral of the induction voltage corresponds to the net change of magnetic flux at the pick-up coils. The induction voltage was a hundred fold amplified by a Tektronix AM 502 differential amplifier and subsequently sampled at 1.6 kHz by a 12 bits AD-convertor. Data collection was performed for 6 s, whereas the actual signal was build-up within 2 s. The sample was placed in a Heralux ® quartz reactor ( 0 12 mm) on which a bifilar heating coil and chromel-constantan thermocouple had been cemented. The temperature was controlled by a PID-regulator programmed in the system computer. The temperature of the catalyst bed was measured with a separate thermocouple. To facilitate thermal equilibrium and prevent displacement of the catalyst bed during the magnetic measurements the catalyst bed was covered with quartz fragments. The reactor was connected to a high-vacuum system. The base pressure was 0.13 mPa (1 Pa = 7.50 mTorr). The gas phase could be analysed with a quadrupole mass spectrometer of Leybold-Heraeus type Q200 equipped with gauge head QF200. The reactor was also appropriate for atmospheric flow experiments allowing for simultaneous mass-spectroscopic gas analysis. The magnetization of samples could thus be measured in vacuum and in controlled gas atmospheres at temperatures ranging from 300 up to 873 K. The system was controlled by an Apple IIe computer. Reduction experiments were performed with an oxidic sample of 200 mg. The catalyst was heated with 83 m K / s from room temperature to the desired reduction temperature in a flow of 0.83 m l / s argon-10 vol.-% hydrogen gas mixture (HoekLoos, 99.9990%) which was purified over an Oxisorb ® Gas
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
21 l
Purifier (Messer Griessheim GmbH). During the isothermal stage of the reduction process, the magnetization was measured at 0.39 MA m-~ every 0.3 ks. After each reduction step the sample was cooled down to room temperature.
2.2.3. Mi3ssbauer spectroscopy Typically, 200 mg of the oxidic catalyst was placed in a reactor suitable for in situ MiSssbauer measurements in controlled gas atmospheres and at temperatures ranging from room temperature up to 873 K. After a reduction treatment of the catalyst in a flow of 0.83 m l / s argon-10 vol.-% hydrogen, the sample was cooled down to 300 K after which a M~Sssbauer spectrum was recorded. The spectrometer, equipped with a 57Co in Rh source, was operated in the constant acceleration mode, performing a saw-tooth velocity waveform. The equipment has been described in detail previously [6]. The spectra were not corrected for the varying distance between source and detector and, hence, the curved background in the spectra is of experimental origin. The MiSssbauer parameters were determined by fitting the spectra with subspectra consisting of Lorentzian-shaped lines using a non-linear iterative minimisation routine. In the case of quadrupole doublets the line widths as well as the absorption areas were constrained to be equal. Isomer shifts are reported with respect to the NBS standard sodium nitroprusside. The magnetic hyperfine fields were calibrated against the 515 kOe field of c~-Fe20 3. Argon and hydrogen were supplied by HoekLoos and purified over BASF catalysts R 3-11 and molecular sieve Linde 4A.
2.2.4. Infrared spectroscopy The surface of the catalyst particles after oxidation of the cyanide precursor and after successive reduction treatments was monitored by transmission infrared spectroscopic investigation of adsorbed carbon monoxide probe molecules. The ground cyanide precursor was pressed at 32 MPa to a self supporting wafer (Q 15 ram, 55 mg) and placed in a quartz cell with sodium chloride windows, in which the wafer could be moved to a furnace section where the pretreatment was carried out. The cell was connected to a high vacuum system and a gas handling system. Movement of the catalyst into and out of the infrared beam could be done without admission of atmospheric air or breaking the vacuum. After an oxidation in helium-1 vol.-% oxygen gas flow or a reduction in argon-10 vol.-% hydrogen gas flow, which was previously purified over a palladium catalyst and a molecular sieve, infrared spectroscopic measurements were performed according to the following procedure: after cooling down to room temperature, the flow was stopped and the gas phase was removed out of the cell by evacuating the system for 1.8 ks down to a pressure of 0.13 mPa. Subsequently, the first (reference) spectrum was recorded with a resolution of 5.3 cm-1, using a Perkin-Elmer 580B spectrophotometer connected to a 3500 CDS data station. Next, 1.33 kPa of carbon monoxide was introduced and a
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second scan was initiated. Additional scans were performed at 0.4, 0.9, 1.8, 3.6, 7.2, and 10.8 ks after the initial carbon monoxide dose. In some experiments the carbon monoxide pressure was raised to 13.3 kPa to study the pressure dependence of the infrared absorption bands exhibited by the adsorbed CO molecules. Finally, the cell was reconnected to the high vacuum system and desorption of adsorbed carbon monoxide was followed by repeated scans in the above mentioned time sequence. In order to facilitate assignment of molecular group vibrations in which O-atoms are involved, some experiments are performed with isotopically labeled carbon monoxide C ~80 (Alfa Ventron GmbH). This carbon monoxide was used without purification. Infrared absorption spectra of CO molecules bonded to the catalyst surface were obtained by subtracting of the reference spectrum from the transmission spectrum of interest.
3. Results The reaction of the injected Fe(CN) 4- with the Fe2+/A1203 suspension resulted initially in the formation of a white precipitate. As the precipitation process went along the colour of the suspension gradually changed to pale blue. After vacuum drying the colour of the final product was dark blue. The
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E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
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Mtissbauer spectrum of the dried cyanide complex, is reproduced in Fig. 3 and is analogous to the one presented in [1]. Investigation of the precipitated cyanide by TEM revealed that the catalyst precursor is made up by both very small particles well distributed over the surface of the alumina support and small thin platelets distributed in between the elementary support particles. From EDX measurements it became apparent that exclusively these platelets comprise a large amount of potassium; the atomic Fe to K proportion varies between 3.4 and 3.9. Samples which do not exhibit plate-shaped particles are virtually free of potassium. The MiSssbauer spectrum of the oxidic precursor, represented in Fig. 3, indicates that the cyanide precursor has been converted completely by the oxidation treatment. In addition, infrared spectroscopic investigations revealed that the broad infrared absorption band at 2060 cm-~, which is characteristic for the cyanide ligands, had completely vanished. The spectrum of the oxide, however, exhibited some new absorption bands at 1565 and 1365-1390 cm(poorly resolved) from which both the position and intensity change upon the consecutive reduction treatments, except after reduction at 373 K. The reduction behaviour of the oxidic precursor is studied by in-situ monitoring the magnetization during a series of isothermal treatments in a hydrogenargon gas flow. In Fig. 2 the magnetization-versus-time curves measured at successive reaction temperatures are indicated. At 373 and 473 K the magnetization does not change during the treatment. The first indication for reduction is observed at 548 K by the gradual change in magnetization. Initially the
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1.16
I.t2
h05
~o
~ z
L,[l
1,58
~.8Z
1.52
1.42
1,53
1.16
2.75
t . Z9
'
-10
-5
0
5
t0
-tO
-5 OOPPLE~
0
VELOCIT~
5
-10
lO (~.~
J
i
I
I
-5
0
5
10
j
1)
Fig. 3. MGssbauer spectra of Fe2Fe catalyst precursors. CN = cyanide precursor; OX = oxidic precursor: t,T = oxidic precursor after reduction for a period of time of t ks at a temperature of T K.
E. Boellaard et al. / A p p l i e d Catalysis A: General 147 (1996) 207-227
214 Table 1
Magnetization (a.u.) at 300 K and 0.82 M A m - 1 after reduction at the indicated temperature Catalyst
Oxide
373 K
473 K
548 K
623 K
723 K
Fe 2 Fe
1069
1074
1074
2100
4980
10924
Table 2 M~ssbauer parameters of a Fe2Fe catalyst after different reduction steps Treatment
IS ( m m / s )
F (ram/s)
SC (%)
Cyanide precursor
0.13
-
-
0.31
44
0.69
0.34
-
0.58
56
Oxidic precursor
0.62 0.61
0.66 1.15
-
0.44 0.45
66 34
4 ks, 473 K
0.62 0.61
0.68 1.19
-
0.48 0.44
73 27
4 ks, 548 K
0.64 0.67
0.64 1.18
-
0.55 0.44
78 17
1.01
1.82
-
0.45
5
0.43 1.03
0.12 - 0.03
444 433
0.74 0.78
12 27
1.17 0.59
1.33 0.53
-
1.12 0.61
44 17
58 ks, 548 K
QS ( m m / s )
-0.01
H F (kOe)
4 ks,
0.65
466
0.32
6
623 K
0.95 1.26 0.47 1.34
0.00 1.46 0.40 0.56
431 -
0.88 0.95 0.48 0.53
25 28 15 27
14 ks, 623 K
1.31 1.21 0.49
0.42 1.32 0.44
-
0.51 0.66 0.40
61 26 13
43 ks, 623 K
1.37 1.37 0.54
0.39 1.63 0.39
-
0.60 0.38 0.38
79 7 14
4 ks, 723 K
0.31 1.37 1.33 0.53
0.00 0.29 1.66 0.45
328 -
0.26 0.62 0.52 0.40
19 62 7 12
14 ks, 723 K
0.30 1.36 1.37 0.47
- 0.02 0.26 1.84 0.39
328 -
0.25 0.65 0.45 0.31
54 34 5 7
43 ks, 723 K
0.30 1.34 1.31 0.48
- 0.01 0.26 1.76 0.41
328 -
0.25 0.60 0.61 0.39
72 14 7 7
E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
215
magnetization increased to drop subsequently. At 623 K the treatment causes an initial decrease and a subsequent increase of the magnetization which increase gradually slows down at 723 K, but saturation is not reached. In addition to the dynamic measurements at the reduction temperature, the magnetization of the thermally treated catalyst is also measured at a fixed reference temperature and a fixed magnetic field after each reduction step. The thus measured data are collected in Table 1 and indicate that up to reduction temperatures of 473 K the iron oxide remains unchanged. From the obtained dynamic magnetization data it is obvious that the formation of a ferro- or ferrimagnetic phase is succeeded by the formation of a non-ferro- or non-ferrimagnetic phase, whereas ultimately again a ferro- or ferrimagnetic phase results. A more conclusive identification of the in the reduction process involved phases is derived from the in-situ recorded M5ssbauer spectra, which are shown in Fig. 3. The results of the corresponding spectra analyses are given in Table 2. Reduction of the oxidic precursor at temperatures up to 473 K leads to very small changes in the Mi3ssbauer spectrum. The spectral contribution of the doublet with the larger QS decreases at increasing reduction temperature. Prolonged reduction at 548 K results in the gradual formation of new doublets and a set of sextuplets. The latter can be assigned to small particles of the ferrimagnetically ordered magnetite, Fe304. During the reduction treatment at 623 K, the sextuplets vanish in favour of a new doublet which represents
1600
Fe2Fe
1982 1799 °
a
/, [ a '!~'i 1330
,
0.013
4000
3000
2000
1500
1000
W A V E N U M B E R / cm 1
Fig. 4. Infrared spectra of CO adsorbed on a Fe2Fe catalyst after reduction at (a) 548 K, and (b) 623 K. Spectra recorded 10.8 ks after admission of 1.33 kPa CO.
E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
216
~e o
o
eq
OA
0
oo
¥ o~ t~
>
~c5
o
o
~5
o~
~ c5
E. Boellaard et al./ Applied Catalysis A: General 147 (1996) 207-227
217
woestite, Fe~ _xO. After increasingly severe reduction conditions, viz. at 723 K, the characteristic sextuplet of ferromagnetic metallic o~-Fe is formed. The observed changes of the magnetization are in good agreement with the presence of the phases indicated by the Mtissbauer spectra. In Figs. 4 - 8 infrared spectra are shown which are recorded after reduction of the oxidic catalyst precursor and subsequent exposure to carbon monoxide at progressively more severe conditions. Admission of 1.33 kPa carbon monoxide to the evacuated oxidic precursor gives rise to a very weak absorption band at 1575 cm -~, which becomes more pronounced after reduction at 373 and 473 K. After reduction at 548 K the surface properties of the catalyst precursor have drastically changed as illustrated in Fig. 4 by a large number of absorption bands, the most intense ones arising at 1982, 1799, 1750, 1600 and 1330 cm - I Reduction at 623 K results in an overall decrease of the intensity of the absorption bands. After reduction at 723 K the CO chemisorption capacity is restored and also some new surface species appear. In Fig. 5 the time and pressure dependence of the very well developed spectra are shown. In these spectra absorption bands are observed at 2950, 2765, 2677, 2160, 2029, 1985, 1906, 1867, 1804, 1638, 1600, 1349, 1322 and 1050 cm - 1 The intensity of all bands increases, although at different rates, upon prolonged carbon monoxide exposure time and raised pressure. The band maximum in the 1950-1830 cm-J region gradually shifts to a higher wavenumber, viz. 1867 to
1985
Fe2Fe 1600
0.022
1906
/ ~\
4000
0000
1638
2000
1349
1500
j
1000
WAVENUMBER / cm4 Fig. 6. Infrared spectra o f CO adsorbed on a Fe2Fe catalyst after reduction at 723 K. Spectra recorded (a) 7.2
ks after admission of 13.3 kPa CO, and (b) after evacuation for 1.8 ks.
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
218
1985
d
Fe2Fe
0.017
1919
4000
3000
2000 WAVENUMBER
1500
1000
/ c m "1
Fig. 7. Infrared spectra of CO adsorbed on a Fe2Fe catalyst after reduction at 823 K. Spectra recorded (a) immediately, (b) 0.9 ks, and (c) 10.8 ks after admission of 1.33 kPa CO, and (d) 7.2 ks after admission of 13.3 kPa CO.
1985
Fe2Fe
a 0.o17
1919 1800
1868
1349
2765 • A
L~ 4000
J 3000
2033 2677
~ - - ~ _
2160.
i
L
A
I
__J
2000 WAVENUMBER
L
J
1500
10(30
/ cm q
Fig. 8. Infrared spectra of CO adsorbed on a Fe 2 Fe catalyst after reduction at 823 K. Spectra recorded (a) 7.2 ks after admission of 13.3 kPa CO, and (b) after evacuation for 1.8 ks.
E. Boellaard et al./'Applied Catalysis A." General 147 (1996) 207-227
219
1585
Fe2Fe 0.013 1332
1857
1938
2762
4000
3000
a
2000
1500
1000
W A V E N U M B E R / cm -1
Fig. 9. Infrared spectra of isotopically labeled C180 adsorbed on a F%Fe catalyst after reduction at 723 K. Spectra recorded after 0.9 ks exposure at (a) 300 K, (b) 373 K and (c) 423 K.
1906 cm-1, and the well resolved peak at 1981 cm-~ shifts to 1985 c m - I . The previously observed bands at 1799 and 1750 cm-1 are now much less distinct. In Fig. 6 it is shown that after evacuation of the gaseous carbon monoxide for 1.8 ks at 300 K, the intensity of the bands at 1985 and 1906 cm-~ decreases, whereas those at 1600 and 1349 cm -~ slightly increase. The corresponding wavenumbers shift back to their initial values. In Figs. 7 and 8 analogue spectra are shown for the catalyst after reduction at 823 K. Compared with the spectra recorded after reduction at 723 K, the intensity of the bands at 2160 and 1965-1830 cm -~ is increased, whereas the intensity of the other bands is decreased relatively to the band at 1985 cm -~ . Notice that the species exhibiting the 2160 and 2030 c m - 1 bands remain adsorbed after evacuation of the carbon monoxide gas phase. A reduced iron catalyst sample was also exposed to 0.83 kPa isotopically labeled C 180 at room temperature. The recorded spectrum, reproduced in Fig. 9, which should be approximately analogous to the spectrum of Fig. 5, exhibits now absorption bands at 1976, 1938, 1832, 1629 and 1332 cm -1. As this particular sample has been used in several experiments, it can be expected that repeated reduction treatments at 723 K have caused sintering of the metal particles and consequently a decrease of the metal surface. Together with the lower carbon monoxide pressure the smaller metal surface accounts for the relatively low absorbance of the recorded spectra. Increase of the adsorption temperature to 373 and 423 K results in an enhanced absorption intensity.
220
E. Boellaard et al./Applied Catalysis A: General 147 (1996) 207-227
Depending on the temperature, absorption bands at 2762, 2672, 2152, 1585 and 1547 cm -1 become also visible.
4. Discussion The formation of a white precipitate upon reaction of Fe(CN) 4- with Fe 2+ is consistent with the intended formation of the ferrous-ferrous hexacyanide Fe2Fe(CN) 6. The observed gradual change of colour of the precipitate from white to pale blue and ultimately dark blue during the stay in the aqueous suspension and the drying step, is due to the formation of a ferri-ferrous cyanide complex. It is well known that the ferrous-ferrous hexacyanide complex is very sensitive to oxidation by air [7]. The M~ssbauer data reveal that the oxidation was limited to the ferrous ions of the iron(II) chloride and that the ferrous cyanide complex remained intact. The presence of potassium containing plate-shaped precipitates in the final product is ascribed to the formation of insoluble salts like KFeFe(CN) 6 or K2FeFe(CN) 6. These compounds can be precipitated when a solution of Fe 3+ or Fe z+, respectively, are brought in contact with K4Fe(CN) 6. Both compounds might be formed in the final stage of the precipitation procedure, when both the colour of the suspension became pale blue and the K + / F e 2+ proportion was increased due to the continuous injection of K4Fe(CN) 6 and the consumption of Fe z+ by the precipitation reaction. Moreover, whenever injection of the cyanide complex into the suspension is not sufficiently homogeneous, the concentration of K + may reach unfavourable high levels. After treatment of the cyanide precursor in helium-oxygen at 563 K, the cyanide complex was completely destructed. The newly formed phase is most likely a highly dispersed FenI-oxide. Since no magnetic ordering was observed in the M~ssbauer spectrum at room temperature, the fully superparamagnetic behaviour of the Fern-oxide indicates a small particle size. In addition, the spectrum has to be described by a set of two doublets of the same IS, but of different QS, representing iron ions in the bulk (QS = 0.66) and in the surface (QS = 1.15) of the iron oxide particles. Based on the results from the spectra the crystallites are assessed to be less than 10 nm in diameter [8]. The different reaction steps involved in the reduction of FeIn-oxide to Fe °, metallic iron, are successfully revealed by the combination of magnetization and M~issbauer measurements. After a prolonged period of time of reduction or after applying more severe reduction conditions most of the initially present FeIn-oxide reacts to a lower oxidation state. During the reduction process several oxidation states and phases of iron coexist. These phases are most likely highly dispersed and intimately mixed and due to the dynamic character of the reaction process they will not posses the ideal (crystallographic) bulk characteristics. The lack of crystallinity is reflected by the large line-width of the successively developed
E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
221
doublets and sextuplets in the MiSssbauer spectra. Moreover, the MiSssbauer parameters of the phases identified as magnetite (the set of sextuplets) and woestite do not exhibit the values reported for the bulk solids. Whereas the IS of woestite phase remains fairly constant at 1.31-1.37 m m / s , the QS gradually decrease from 0.56 to 0.26 m m / s at increasing degree of reduction of the iron oxide particles. The distinct stabilization of the Fe~ _xO phase at temperatures where bulk FeO is metastable is considered to be a proof that this phase is stabilized by interaction with the "y-A1203support [4]. The slower reduction in the MSssbauer experiments compared to the magnetization experiments may be due to a less efficient removal of the produced water vapour since the gas has to diffuse through a perforated aluminium foil, whereas in the magnetic experiments the reaction gas runs through the catalyst bed. The infrared spectra measured with the iron catalyst after successive reduction treatments show a close mutual resemblance of absorption bands. Each reduction treatment, however, brings about a special spectral feature. In the spectra recorded on the catalyst reduced at a moderate temperature, viz. 723 K, several characteristic wavenumber ranges can be distinguished. The major bands of the spectrum arise within the 2100-1700 cm -1 range and are assigned to stretching vibrations of molecularly adsorbed CO. The sharp band at 1981 cm is attributed to CO which is linearly adsorbed on metallic iron sites and the broad band, peaking at 1867 cm -1, to bridged-bonded CO [9]. Upon prolonged exposure time or increasing pressure both bands gain intensity and shift to higher wavenumbers, viz. to 1985 and 1906 cm ~, respectively. Furthermore, at 1804 cm-~ a well resolved band appears. The observed red shift may be due to adsorbate-adsorbate interaction or to occupation of sites of lower binding energy at increasing CO coverage. The weak band at 2025 cm 1 is assigned to CO adsorbed on sites which are made up of metallic surface atoms which are adjacent to charged atoms [8]. The assignment of the mentioned absorption bands to stretching vibrations of molecularly adsorbed CO is supported by the strong shift in wavenumbers by using isotopically labeled C lsO instead of C160. The absorption bands are not attributable to adsorbed molecular iron carbonyl complexes since the model study of Mohana Rao et al. [10] in which Fe(CO)s was adsorbed on 7-A1203 revealed significantly different bands. The wavenumbers of the molecularly adsorbed CO species on the supported iron catalyst prepared from a complex cyanide are low as compared to the wavenumbers generally observed with carbon monoxide adsorbed on supported iron catalysts [8,11-17]. An additional feature of the cyanide derived catalyst is the large number of well-resolved absorption bands in the measured spectra. The occurrence of distinct absorption bands at relatively low wavenumbers (19851700 cm -~) can be rationalized by considering the moderately small size of the particles, which is about 30 to 40 nm in diameter. Particles of such a size expose
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E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
well-ordered low index planes at the surface. Consequently, the catalyst surface contains well-defined sites for adsorption of CO in both on-top and bridged geometries. Spectroscopic studies on vapour deposited iron films and single crystals revealed vibrations of adsorbed CO at even much lower wavenumbers [18,19]. Theoretical studies indicate that the presence of adsorbed hydrogen on the catalyst surface may also account for the low wavenumbers. Formation of a H - F e - C O complex causes an enhanced metal-adsorbate interaction by increasing the 7r-donation from the F e - H complex into the antibonding zr-orbital of carbon monoxide, which weakens the intramolecular CO bonding and, hence, lowers its vibration frequency [20,21]. It will be discussed below that the presence of adsorbed hydrogen after reduction and evacuation of the catalyst is very likely. The band at 2160 cm-1, which is faintly visible in the spectra measured after reduction at 723 K, becomes very distinct after reduction at 823 K. In many studies bands reported in this range are assigned to CO adsorbed on surface iron aluminate [16]. A characteristic of thus adsorbed CO is that it rapidly desorbs upon evacuation of the catalyst. However, in this study, the intensity of the band is not affected by evacuation for even 10.8 ks. Furthermore, in the spectra recorded in the C180 experiments the band appears at 2153 cm -1, a shift of only - 7 cm -1. The bands previously assigned to molecularly adsorbed CO are, on the other hand, shifted by - 4 0 cm-1. Consequently, it is unlikely that the band at 2160 cm -1 represents molecularly adsorbed CO species. The band is not associated with any other band in the spectrum, since its time and pressure dependency is different. An iron hydride F e - H is not probable, as the reference spectrum, recorded just after reduction, does not exhibit any absorption band in this spectral region. However, an analogous band at 2158 cm-1 is observed after reaction of the catalyst with ethylene at 523 K [22]. In that study the band was assigned to an 'ethylidyne' species, CHxCHy, adsorbed on the iron surface. Since iron has the potential to catalyse the hydrogenation of CO, viz. the Fischer-Tropsch reaction, the formation of hydrocarbon fragments on the surface of an iron catalyst can be expected upon co-adsorption of CO and H 2. Whether the hydrogenation reaction proceeds or not, is governed by the catalytic properties of the iron surface. An overall prerequisite for the occurrence of this reaction in the present experiments is, of course, the availability of pre-adsorbed hydrogen at the iron surface. Moreover, the reaction to adsorbed 'ethylidyne' only proceeds on highly reduced iron surfaces. Apparently, the presence of oxygen on the iron surface prohibits the reaction to the adsorbed 'ethylidyne' species. The set bands observed at 2944, 2765, 2677, 1638, 1600, 1349, 1322 and 1055 cm -~ is also a unique feature of the iron catalyst prepared from the cyanide precursor. In literature no such extended spectra have been reported for supported iron catalysts under comparable experimental conditions. The bands at 1600 and 1349 cm -1 are ascribed to the asymmetric and symmetric OCO
E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
223
stretching vibrations of formate species adsorbed on the catalyst. Formic acid adsorbed on supported Fe, Co [23] and Ni [22,24] catalysts, gives rise to absorption bands at 1583-1580 and 1350-1345 cm-~. Formate species formed during hydrogenation of CO 2 a n d / o r CO on supported Fe [25], Ru [26,27], and Rh [19] catalysts exhibits absorption bands at 1595, 1395 and 1380 cm-1. In these studies the formate species is reported to be adsorbed exclusively on the support. Gopal et al. [28] provided evidence that formate can only be formed by direct reaction of CO with hydroxyl groups of A120 3 and MgO supports at temperatures of at least 375 K. The frequency of the formate OCO vibrations also depends on the mode of adsorption. The fact that in the present study the absorption bands appear already at room temperature and at deviating wave numbers strongly suggest that the formate species are formed and adsorbed on a metallic iron surface. Although the band at 1322 cm-~ can be assigned to a bending mode of the CH of formate, the growth pattern of this band is different from those assigned to the OCO vibrations of formate. Most probably this band will arise from another species. In conjunction with the weak and broad band at 1640 c m - I , a carbonate species is plausible. The identification of the absorption bands at the exceptionally low wavenumbers 2765 and 2677 cm-1 is greatly hampered by lack of consistent reference data in this frequency range. A set of bands in the spectral range 2900-2700 cm-~ is characteristic for aldehydic CH stretching vibrations of gas phase molecules [29]. In CO-hydrogenation catalysis this functional group can be associated with the adsorbed formyl species, CHO which is supposed to be an intermediate in CO-hydrogenation reactions [30]. Model inorganic compounds containing the CHO functional group clearly demonstrate their existence. Coilman and Winter [31] reported infrared absorption bands at 2690, 2540, and 1577-1610 cm-~. Despite a number of attempts, Yates and Cavanagh [32] have not succeeded in proving the existence of formyl species on a A1203-supported Rh catalyst by spectroscopic techniques. The CHO species is claimed to exist on ZnO [33,34] and CeO 2 [32] after co-adsorption of CO and H 2. The set of bands at 2765 and 2675 cm-~ found in the present study may therefore be due to formyl species. The shift to lower wavenumbers, as compared to the carbonyl model compound, can be explained by the increased mass of the iron substrate to which the functional group is bonded. As the bands at 2765 and 2677 cm-~ show analogous time and pressure dependencies as the 1600 and 1349 cm -~ bands, the former set of bands may also be attributed to the previously discussed formate species. In this view the in-plane-bending of the carboxylate CH bond accounts for the band at 1322 c m - 1, while the stretching vibration of the CH bond is represented by either the 2765 or the 2677 cm -1 band. The other remaining band might represent a combination band involving a stretch vibration of OCO and a CH deformation [26,35]. In the cited literature, however, high-frequency bands of adsorbed
224
E. Boellaard et al. / Applied Catalysis A: General 147 (1996) 207-227
formate are hardly reported. As Fischer-Tropsch synthesis and formic acid adsorption experiments performed on pure aluminium oxid C revealed well-developed absorption bands at 3007 and 2912 cm-~ [22], a red-shift to 2765 and 2677 cm -1 may indicate that the formate is not adsorbed on the alumina support, but on the iron particles. However, it must be taken into account that a red shift of 240 cm -1 is very large. Shustorovich and Bell [36] speculated guided by BOC-MP approach calculations that decomposition of formic acid on Fe/W(110) surfaces leads to parallel production of formate and formyl species. The formyl species will decompose to CO and atomic hydrogen. Since no hydrogen is admitted during the IR experiment, the H atoms involved in the formate and ethylidyne formation must originate from the previously performed reduction treatment. Apparently, evacuation at room temperature is not sufficient to remove the hydrogen from the metallic iron surface. A strong adsorbate-adsorbent interaction is in agreement with results from temperature-programmed desorption studies [37]. When the surface is covered by hydrogen, the admitted carbon monoxide either displaces the hydrogen or reacts with the hydrogen to formate. Both mechanisms account for the increase of the absorbance in the 2000-1625 cm -1 range, and subsequently of the formate bands, with time. Formate formation from CO implies residual carbon either to remain on the catalyst surface or to react with adsorbed hydrogen to CH species. The observed weak absorptions at 2944 and 1055 cm-~, which are in the range of the aliphatic CH vibrations, nicely support such an interpretation. The infrared spectra recorded after reduction of the catalyst at 823 K exhibit a considerable change in intensity ratio of the 1985 and 1919 cm-~ bands and a decrease of the overall absorbance. These observations may be explained by an increase of the particle size due to thermal sintering, causing an increase of the fraction of smooth, low-index crystal surfaces on which CO can be adsorbed in multicentered modes and a decrease of the area of more reacting iron surfaces. It is remarkable that after low-temperature reduction, Fig. 4, analogous bands are observed as after high-temperature reduction, Figs. 5-8. Because the oxides which constitute the bulk of the oxidic precursor particles do not exhibit infrared absorption bands upon CO exposure, the surface of the catalyst particles after reduction at 548 and 623 K should contain some small patches of metallic iron. On these sites CO is adsorbed molecularly as is indicated by the bands at 1982, 1799 and 1750 c m - 1. Especially after reduction at 548 K, the bands attributed to bridged-bonded CO are very pronounced. After reduction at more severe conditions the spectral contribution of these bands decrease. Probably, these bands are associated with CO adsorbed on rough surfaces. The carbon monoxide also reacts to adsorbed formate species as evident from the 1600 and 1330 c m - 1 bands. From the magnetization curve in Fig. 2 it can be derived that during reduction at 623 K the bulk of the catalyst is reacting to metallic iron. However, the corresponding infrared spectrum in Fig. 4 indicates by the decreased overall
E. Boellaard et a l . / Applied Catalysis A: General 147 (1996) 207-227
225
absorption a reduced ability of the catalyst for CO adsorption. This effect may be due to a considerably disturbed particle surface caused by a poorly defined particle composition. It is likely that the particles comprise a mixture of several reduced intermediates. The in Fig. 4 observed oscillation in the 11500-1450 c m - ~ range is caused by a change in the reference spectrum. As mentioned above, the oxidic precursor exhibits some absorption bands due to remaining surface groups. Upon exposure to 1.33 kPa carbon monoxide the position and intensity of one of these bands changes. Consequently, substraction of the reference spectrum from the spectrum measured in 1.33 kPa carbon monoxide results in a virtual band. An explanation for the difference between the infrared spectroscopic results measured in this work on iron catalysts prepared from cyanide precursors and on those prepared by traditional precipitation and impregnation techniques may be based on the structure of the metallic surface. Since water vapour facilitates the formation of iron aluminate, precautions have been taken to prevent reaction of the iron (oxide) phase with the alumina support. The presence of iron(II) aluminate at the metal-support interface will raise the interaction of the metal particle with the surface of the support. The interfacial interaction brings about that the thermodynamically stable shape of free iron particles, which contains a large fraction of closely packed atomic surfaces, is not established, but a shape containing a much larger fraction of atomically rough iron surfaces. With precipitated iron hydroxides much hydroxyl groups have to be removed by calcination at high temperatures. Fortunately, the oxidation of the cyanide precursor proceeds only after desorption of physisorbed and crystal water, which occurs at moderate temperatures [1]. In addition, the subsequently performed reduction of the oxidic precursor is split up into several isothermal steps to prevent the occurrence of an unfavourable HzO-to-H 2 ratio. Under the described conditions formation of bulk iron aluminate a n d / o r thin surface layers, which influences the metallic chemisorption properties, is prevented.
5. Conclusions
Deposition precipitation of an iron hexacyanoferrate complex onto alumina has been successfully used for the preparation of a supported iron catalyst. The initially obtained cyanide complex can be converted by oxidation into a highly dispersed iron oxide. Reduction of the oxidic catalyst by hydrogen proceeds via Fe304 and an alumina-stabilized Fel_xO phase as demonstrated by in situ performed dynamic magnetization measurements and M~Sssbauer spectroscopy. The ultimately resulting metallic iron catalyst exhibits a remarkable behaviour with respect to the interaction with carbon monoxide. Infrared spectra recorded after exposure of 1.33 kPa CO to the reduced and evacuated catalyst show a number of well resolved absorption bands, which could be assigned to both
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E. Boellaard et al. /Applied Catalysis A: General 147 (1996) 207-227
linearly and bridged-bonded carbon monoxide species, as well as to formate and hydrocarbon species. The presence of the latter species is very unusual under the applied experimental conditions, and suggests a reactive iron surface covered with hydrogen. The iron catalysts offer promising properties with respect to the production of hydrocarbons via the Fischer-Tropsch process. The application of in situ performed dynamic magnetization measurements provides an additional dimension to the characterization of catalysts and evaluation of reaction processes.
Acknowledgements We would like to express our sincere thanks to P.R. van der Linde for developing the software for operating the magnetic equipment and C.D. de Haan for performing the T E M / E D X experiments. The investigations were supported by the netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Research (NWO).
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