Al2O3 catalysts

Applied Catalysis A: General 171 (1998) 333±350

Preparation, reduction, and CO chemisorption properties of cyanide-derived CuxFe/Al2O3 catalysts E. Boellaarda, F.Th. van de Scheura, A.M. van der Kraanb,*, J.W. Geusa a

Department of Inorganic Chemistry, Debije Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands b Interfacultair Reactor Instituut, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received 27 November 1997; received in revised form 19 January 1998; accepted 17 March 1998

Abstract A series of copper±iron catalysts with Cu-to-Fe proportions of 1.0, 1.5, and 2.0 has been prepared by the precipitation of three different copper±iron cyanide complexes, viz. Cu2Fe(CN)6, Cu3[Fe(CN)6]2 and CuFe(CN)5NO, onto a g-Al2O3 support. Oxidative decomposition of these stoichiometric heteronuclear cyanide complexes resulted in the formation of particles of uniform Cu-to-Fe ratio, made up of intimately mixed CuO and Fe2O3 crystallites. The reduction of the oxidic precursors was studied by means of magnetization measurements and MoÈssbauer spectroscopy. The presence of Cu facilitates the reduction of the iron oxide. The distribution of the Cu and Fe atoms in the bimetallic particles appeared to be strongly dependent on the applied reduction procedure. Slow reduction prevents excessive phase separation. Exposure of the reduced catalysts to carbon monoxide results in a rapid segregation of Fe to the particle surface. Infrared spectroscopy indicates the presence of several types of linearly and bridged-bonded CO species on the bimetallic surface. Reversibly adsorbed CO on Cu-like sites decreases the apparent intensity of the absorption band due to irreversibly adsorbed CO on Fe-like sites. Experiments with isotopically labeled C18O suggest that the CO on Fe-like sites is mobile and can be exchanged with CO bonded to adjacent Cu-like sites. # 1998 Elsevier Science B.V. All rights reserved.

1. Introduction Synthesis of hydrocarbons from carbon monoxideand hydrogen-containing gas mixtures via the wellknown Fischer±Tropsch reaction is still a subject of many investigations within both academic and industrial research laboratories. Traditional Fischer± Tropsch catalysts, which are based on cobalt or iron as active components, exhibit under process conditions a rapid decline of activity due to the deposition of carbon. The deactivating carbon deposits can be of amorphous, carbidic or graphitic nature, while the *Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00104-5

occurrence of ®lamentous carbon results in an inadmissible pressure drop over the reactor bed. From a detailed study on the formation of ®lamentous carbon it was expected that dilution of the active iron phase with an inactive component as copper would suppress the growth of these carbon ®laments [1]. Therefore, a study on both copper±iron single crystals [2] and supported copper±iron catalysts was initiated [3]. This paper deals with the preparation and characterization of a series of alumina-supported copper±iron catalysts of different Cu-to-Fe ratios. To study the effect of diluting iron with copper, it is very important that every individual alloy particle has the same composition. Even when a uniform compo-

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sition is achieved, the catalytic properties of the catalyst will depend on the distribution of the metals within an individual particle. Several states of the distribution of the alloying elements can be envisaged: a homogeneous distribution, a complete phase segregation, the cherry model, or a slight surface enrichment of one of the components. The actual distribution within alloy particles will partly be determined by the intrinsic bulk properties of the components. With small particles, however, one has to take into account deviations due to surface effects. In this study, it will be also shown that the reduction procedure and even the composition of the gas phase to which the alloy particles are exposed affect the distribution of the constituting elements within the particles. As there are many possibilities in a bimetallic system, powerful characterization techniques are required to discriminate between them. Magnetic measurements and MoÈssbauer spectroscopy, together with infrared spectroscopy of adsorbed carbon monoxide, have appeared to be an useful combination of techniques for characterization of the bulk and surface of the particles, respectively. To achieve a uniform Cu-to-Fe ratio in the individual particles, catalyst precursors were prepared from stoichiometric copper±iron cyanides, viz. Cu2Fe(CN)6, Cu3[Fe(CN)6]2 and CuFe(CN)5NO, deposited onto a g-alumina support. Hence, oxidative decomposition of these complexes resulted in the formation of particles of uniform Cu-to-Fe ratio, made up of intimately mixed CuO and Fe2O3 crystallites. The CO chemisorption properties of the reduced bimetallic copper± iron catalysts exhibited some interesting phenomena concerning adsorbate-induced segregation, exchange of infrared-absorption intensity, and adsorbate mobility. The results emphasize that the generally accepted idea that the number of bridged-bonded sites decreases on alloying has to be examined with care.

plexes and the applied support was proved by electron microscopy [4] and MoÈssbauer spectroscopy [5]. The general features of this catalyst preparation procedure have been previously described in more details [4]. The quantity of chemicals (Merck, pro analysis) was chosen to provide copper±iron loading of 20 wt.% metal in the ®nal reduced catalyst. The actual copperto-iron ratio in the particles was established by selecting a copper±iron cyanide complex of the proper stoichiometry. For preparing a catalyst with a Cu-to-Fe proportion of 2.0 the following procedure was applied: 4.00 g of the support g-Al2O3 (Aluminium Oxid C, Degussa) was suspended in 900 ml demineralised water and 2.64 g Cu(NO3)2
3H2O, dissolved in 100 ml water was added. The pH was adjusted to 5.0 with HNO3. An aliquot of 2.31 g of K4Fe(CN)6
3H2O dissolved in 100 ml water was slowly injected through a capillary tube ending under the level of the vigorously stirred suspension. The temperature was kept at 295 K. After 57 ks the suspension was allowed to settle, decanted, washed twice with 500 ml water, and dried for 173 ks at room temperature in a vacuum of ca. 1.33 Pa. The batches of several preparations were mixed and subsequently ground, pelletized and crushed. A sieve fraction between 425 and 730 mm was selected for further experiments. Before being reduced and used as a catalyst, the cyanide precursor was converted into an oxidic precursor by a treatment in a helium-1 vol.% oxygen gas mixture at 518 K. A catalyst with a Cu-to-Fe ratio of 1.5 was prepared by the same procedure using 2.18 g of K3Fe(CN)6 and 2.40 g Cu(NO3)2
3H2O. A Cu-to-Fe ratio of 1.0 in the catalyst was obtained by repeating the procedure with 2.50 g Na2Fe(CN)5NO
2H2O and 2.02 g Cu(NO3)2
3H2O, while the oxidation treatment was performed at 543 K.

2. Experimental

2.2.1. Magnetic measurements Magnetic measurements were executed using a modi®ed Weiss-extraction method in an apparatus that has been comprehensively described elsewhere [5]. Reduction experiments were performed with oxidic samples of ca. 450 mg. The catalysts were heated at 83 mK/s from room temperature to the desired reduction temperature in a ¯ow of 0.83 ml/s of an

2.1. Catalyst preparation The catalyst precursors were prepared by means of homogeneous deposition±precipitation of complex iron cyanides with a copper salt onto a support. Interaction between the precipitated cyanide com-

2.2. Catalyst characterization

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argon-10 vol.% hydrogen gas mixture (HoekLoos), which was puri®ed over a palladium catalyst and a molecular-sieve column. During the isothermal stage of the reduction process, the magnetization was measured at 0.39 MA mÿ1 every 0.3 ks. After each reduction step, the catalysts were cooled down to room temperature. 2.2.2. MoÈssbauer spectroscopy MoÈssbauer spectra were recorded with a constant acceleration spectrometer using a 57Co in Rh source (50 mCi). Different types of reactors were used in order to perform MoÈssbauer measurements at room temperature as well as at liquid nitrogen or helium temperature without exposing the reduced catalysts to air. The reactors are described in detail by Van der Kraan and Niemantsverdriet [6]. The spectrometer was operated in a saw-tooth velocity mode; consequently the recorded spectra have a curved background. The MoÈssbauer parameters isomer shift (IS), quadrupole splitting (QS), hyper®ne ®eld (HF), line width (ÿ ) and spectral contribution (SC) were determined by ®tting the collected spectra with one or more sets 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. Samples were made up of ca. 200 mg of the oxidic precursor. 2.2.3. Infrared spectroscopy The surface of the catalyst particles, after oxidation of the cyanide precursor and successive reduction treatments, was monitored by transmission infrared spectroscopic investigation of adsorbed carbon monoxide probe molecules. The equipment, which permits in situ recording of spectra, has been described in more detail in Ref. [5]. Catalyst samples were prepared by pressing the ground cyanide-containing precursor at 32 MPa to a self-supporting wafer ( 15 mm, 55 mg). After an oxidation or a reduction treatment in argon10 vol.% hydrogen gas for 58 ks at the indicated temperature, infrared spectroscopic measurements were performed according to the following procedure: After cooling down to room temperature, the gas ¯ow was stopped and the gas phase was removed from the

335

cell by evacuating the system for 1.8 ks to a pressure of 0.13 mPa. Subsequently, the ®rst (reference) spectrum was recorded in the 1400±2400 cmÿ1 wave number range with a maximum resolution of 5.3 cmÿ1. Next, 1.33 kPa of carbon monoxide was introduced and a second scan was recorded. Additional scans were performed at 0.4, 0.9, 1.8, 3.6, 7.2, and 10.8 ks after the carbon monoxide dose. At last the cell was reconnected to the high-vacuum system and desorption of the adsorbed carbon monoxide was followed by repeated scans in the above mentioned time sequence. Infrared-absorption spectra of CO molecules bonded to the catalyst surface were obtained by computer subtraction of the reference spectrum from the transmission spectrum of interest. In some experiments, isotopically labelled carbon monoxide C18O (Alfa Ventron GmbH) was used without puri®cation. 3. Results The reaction of the aqueous K4Fe(CN)6 solution with the suspension of g-Al2O3 in the copper-nitrate solution resulted in the formation of a dark purplebrown precipitate, which rapidly settled after ®nishing the injection procedure. On injecting a solution of K3Fe(CN)6 or Na2Fe(CN)5NO, a yellow-green or pale blue-green precipitate, respectively, was formed. In the latter case, sedimentation proceeded very slowly. After drying the colour of the precipitated complexes became more intense. Destruction of the cyanide precursor by the oxidation treatment resulted in the formation of a brown product. 3.1. MoÈssbauer spectroscopy and magnetic measurements MoÈssbauer spectra recorded on the cyanide and oxidic precursors are reproduced in Fig. 1. The results of the analysis of the spectra are given in Table 1. From a temperature-programmed reduction experiment with the oxidic CuFe precursor, two reduction stages had been distinguished with maximum reaction rates at 458 and 623 K, respectively [4]. Based on this result a step-wise reduction procedure was applied, using in situ MoÈssbauer spectroscopy for determination of the evolved iron phases. Firstly, a reduction

Fig. 1. MoÈssbauer spectra at 300 K of the dried (CN) and oxidized (OX) copper±iron cyanide catalyst precursors.

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Table 1 MoÈssbauer parameters of the dried (CN) and oxidized (OX) cyanide complexes Cu3Fe2

CuFe CN IS (mm/s) QS (mm/s) ÿ (mm/s) SC (%)

ÿ0.01 1.82 0.26 100

OX 0.59 0.70 0.38 49

Cu2Fe

CN 0.57 1.17 0.46 51

0.10 0.56 0.35 69

experiment was performed with an oxidic CuFe precursor by heating the sample with 30 mK/s to the desired reduction temperature of 473 K and keeping it subsequently isothermal for a period of 0 ks. In addition, analogous experiments are performed at 438 k for 58 ks, and at 373 K for 14 ks, respectively. After

OX 0.18 Ð 0.49 31

0.59 0.70 0.36 43

CN 0.57 1.17 0.46 57

0.17 Ð 0.37 100

OX 0.58 0.73 0.39 54

0.56 1.20 0.44 46

the last series, the reduction was continued for 14, 25 or 58 ks at 373, 473, 523, 623 or 723 K. A MoÈssbauer spectrum was recorded at room temperature after each reduction step. A compilation of the spectra is given in Fig. 2. The reduction of the catalyst at 373 K resulted in the development of a high-spin Fe2‡ doublet,

Fig. 2. MoÈssbauer spectra at 300 K of the oxidic CuFe catalyst after reduction for periods of t ks at a temperature T K.

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Table 2 MoÈssbauer parameters of the CuFe catalyst after reduction Treatment

IS (mm/s)

QS (mm/s)

HF (kOe)

ÿ (mm/s)

SC (%)

Oxide

0.59 0.58

0.85 1.44

Ð Ð

0.49 0.40

85 15

Direct 473 K

0.68 1.32 0.28

0.91 1.89 0.00

Ð Ð 330

0.62 0.93 0.28

26 33 41

Direct 438 K

0.63 1.30 1.12 0.28

0.64 2.06 1.16 0.00

Ð Ð Ð 330

0.67 0.70 0.52 0.61

30 42 14 14

Direct 373 K

0.62 1.44 1.06

0.79 1.95 1.60

Ð Ð Ð

0.59 0.50 0.46

67 21 12

Cont. 473 K

0.62 1.44 1.09

0.71 1.93 1.41

Ð Ð Ð

0.67 0.65 0.67

42 34 24

Cont. 523 K

0.66 1.37 0.28

0.69 2.09 0.00

Ð Ð 330

0.79 0.77 0.34

20 30 50

Cont. 623 K

0.64 1.36 0.28

0.75 2.02 0.00

Ð Ð 330

0.73 0.52 0.32

10 15 75

Cont. 723 K

0.62 1.32 0.28

0.70 2.03 0.00

Ð Ð 330

0.46 0.58 0.34

6 11 83

besides the Fe3‡ doublet initially present. After more severe reduction treatments at temperatures in the 473±723 K range, the doublets were gradually replaced by a sextuplet, as can be seen at the righthand side of Fig. 2. From the experiments shown, at the left-hand side of Fig. 2 it follows that whenever the initial reduction step was performed at temperatures exceeding 373 K, the sextuplet arises at temperatures which are much lower than 523 K. Details of the spectrum analyses are presented in Table 2. In order to determine the actual composition of a partially reduced catalyst, spectra were recorded at 300, 77 and 4.2 K of a CuFe catalyst which has been reduced for a period of 29 ks at 373 and 423 K. In the spectrum measured at 300 K, Fig. 3, doublets of both Fe3‡ and Fe2‡ are present. At 77 K, the spectral

contribution of the latter doublet strongly increased and a magnetically split sextuplet appeared. At 4.2 K the spectrum was dominated by this sextuplet (HF505 kOe). It can be deduced from the results shown in Fig. 2 that for obtaining and maintaining the copper and iron atoms in the metallic particles intimately mixed, the catalyst have to be prevented from having thermal shocks. In further studies, therefore, the oxidic precursors were reduced according to the following mild reduction procedure: Heating rate 83 mK/s, isothermal stages of 29 ks at 373 K, 58 ks at 423, 473, and 548 K. In some experiments, the reduction continued for 58 ks at 623, 723, 823, and 923 K. The suitability of this reduction sequence has been proved by experiments on a Cu2Fe precursor. The resulting MoÈssbauer

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magnetization measurements. The results, which are rescaled to a catalyst mass (oxidic precursor) of 200.0 mg, are represented in Fig. 5. During each isothermal reduction step, the magnetization increased with reduction time. At 548 K, the magnetization of the CuFe catalyst tended to saturate after an initially steep increase and only a slight increase of the magnetization was observed after reduction at 623 and 723 K. The Cu3Fe2 and Cu2Fe catalysts displayed a continuous increase of the magnetization. 3.2. Infrared spectroscopy

Fig. 3. MoÈssbauer spectra recorded at 300, 77, and 4.2 K of a partially reduced CuFe catalyst.

data, which revealed an intermediate doublet, a singlet and sextuplet, are represented in Fig. 4 and Table 3. For the three different copper±iron catalysts, the standard reduction treatment was monitored by in situ Table 3 MoÈssbauer parameters of the Cu2Fe catalyst after reduction Treatment

IS (mm/s)

QS (mm/s)

HF (kOe)

ÿ (mm/s)

SC (%)

Oxide

0.59 0.57

0.72 1.22

Ð Ð

0.41 0.44

57 43

423 K

0.61 1.39 1.07

0.70 2.01 1.78

Ð Ð Ð

0.67 0.54 0.39

30 53 17

548 K

1.48 0.26 0.28

1.67 Ð 0.00

Ð Ð 330

0.87 0.56 1.18

36 33 31

Re-oxidized 0.61 0.62

0.75 1.17

Ð Ð

0.53 0.54

60 40

The surface properties of the copper±iron catalysts are extensively studied by infrared spectroscopy using carbon monoxide as a probe molecule. Admission of 1.33 kPa CO to the oxidic catalysts gave rise to one sharp absorption band at ca. 2105 cmÿ1, whose intensity and wave number gradually increased with the CO exposure time. Upon evacuation of the system, the intensity and wave number gradually decreased. These spectral features are shown in Fig. 6 for all three oxidic catalysts. Infrared spectra recorded with the three copper±iron catalysts after consecutive reduction treatments at temperatures ranging from 373 to 923 K are given in the Figs. 7±11. Spectra recorded immediately after admission of carbon monoxide as well as following a period of 10.8 ks are presented. The spectra exhibit a number of different absorption bands divided over two spectral ranges, viz. 2000±2250 and 1700±2000 cmÿ1. The different catalysts have spectral features, such as position of the bands and intensity effects in common. However, the actual contribution of a speci®c band to the spectrum depends on the reduction temperature, the time of exposure to CO, and the Cu-to-Fe ratio in the catalyst. Reduction of the oxidic precursors at 373 K (Fig. 7) results in a sharp band centred at 2079±2085 cmÿ1, of which the top is chopped-off due to total blocking of the infrared radiation, and a broad, weak composite band initially peaking at ca. 1900±1915 cmÿ1. The wave number and intensity of the latter band increase with time. After reduction at 423, 473 and 523 K recording of spectra was completely prohibited by a further drop of the transmission of the samples for infrared radiation. Upon reduction at 623 K the transmission of the catalyst samples steeply increased,

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Fig. 4. MoÈssbauer spectra at 300 K of the oxidic Cu2Fe catalyst after reduction for periods of t ks at a temperature T K, and after reoxidation at 300 K.

which enabled us to record spectra. The spectrum of the CuFe catalyst initially comprised bands at 2088, 2001, 1929 and a shoulder at ca. 1895 cmÿ1 (Fig. 8). Upon prolonged exposure to CO the intensity of the band at 2088 cmÿ1 decreased and two shoulders developed at 2127 and 2041 cmÿ1. The increasing intensity in the 1900±2000 range is accompanied by a shift of the band maximum to 1948 cmÿ1 and a decrease in intensity of the shoulder at 1895 cmÿ1. After reduction of the CuFe catalyst at more elevated temperatures, viz. 723 (Fig. 9) and 823 K (Fig. 10), the still present band at 2090 cmÿ1 exhibited initially an increasingly pronounced shoulder at 2070 cmÿ1. The intensity of both bands decreased with the time of exposure to CO in favour of a shoulder at 2128 cmÿ1. The bands in the 1750±2000 cmÿ1 range were hardly in¯uenced by the more severe reduction treatments. The spectra recorded after reduction of the Cu2Fe catalyst at 623 K (Fig. 8) exhibit an intense band at

2086 cmÿ1, a composite band peaking at 1920 and 1890 cmÿ1, and a broad, weak band peaking at ca. 1805 cmÿ1. The intensity of the band at 2086 cmÿ1 decreased with time and a shoulder developed at 2115 cmÿ1. The other bands increased in intensity, which resulted in a new band contour and a maximum at 1932 cmÿ1. Reduction of the Cu2Fe catalyst at ultimately 923 K (Fig. 11) resulted initially in a sharp, intense band at 2092 cmÿ1 with a shoulder at 2068 cmÿ1 and a broad band peaking at 1942 and a shoulder at 1890 cmÿ1. Exposure to CO for 10.8 ks led to a loss of intensity at 2092 and 2068 cmÿ1 and the appearance of a new shoulder at 2120 cmÿ1. The bands below 2000 cmÿ1 exhibited an overall increase of intensity. The Cu3Fe2 catalyst displayed spectra (Figs. 7±11) which are intermediate between those of the previously described CuFe and Cu2Fe catalyst. With increasing reduction temperature a weak band at 2002 cmÿ1 developed.

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Fig. 5. Magnetization vs. time curves for the oxidic copper±iron catalysts during successive reduction at 373, 423, 473, 548, 623, and 723 K.

Fig. 6. Infrared spectra of CO adsorbed on the oxidic copper±iron catalysts. Spectra recorded (a) immediately, (b) 10.8 ks after admission of 1.33 kPa CO, (c) immediately, and (d) 7.2 ks after evacuation.

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Fig. 7. Infrared spectra of CO adsorbed on copper±iron catalysts reduced at 373 K. Spectra recorded (a) immediately, and (b) 10.8 ks after admission of 1.33 kPa CO.

Fig. 8. Infrared spectra of CO adsorbed on copper±iron catalysts reduced at 623 K. Spectra recorded (a) immediately, and (b) 10.8 ks after admission of 1.33 kPa CO.

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Fig. 9. Infrared spectra of CO adsorbed on copper±iron catalysts reduced at 723 K. Spectra recorded (a) immediately, and (b) 10.8 ks after admission of 1.33 kPa CO.

Fig. 10. Infrared spectra of CO adsorbed on copper±iron catalysts reduced at 823 K. Spectra recorded (a) immediately, and (b) 10.8 ks after admission of 1.33 kPa CO.

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Fig. 11. Infrared spectra of CO adsorbed on copper±iron catalysts reduced at 923 K. Spectra recorded (a) immediately, and (b) 10.8 ks after admission of 1.33 kPa CO.

In Fig. 12 a spectrum of a Cu2Fe catalyst which was reduced at 623 K and exposed for 10.8 ks to CO is shown together with the spectrum recorded immediately after subsequent evacuation of the system down to 0.13 mPa. After evacuation the band at 2085 cmÿ1 had almost completely vanished, whereas the band at 1932 cmÿ1 shifted to 1947 cmÿ1 and gained intensity. During subsequent prolonged evacuation of the system, the intensity of the 1947 cmÿ1 band remained nearly constant. The course of the intensities of the absorption bands peaking at 2085 and 1932± 1947 cmÿ1 during the adsorption±desorption experiment is illustrated in Fig. 13. The effect appeared to be reversible, re-admission of carbon monoxide restored exactly the previously measured spectrum. Since fundamentally different principles can account for the observed intensity effect, as will be discussed in the Section 4, an isotope substitution experiment was performed to get information about the origin of the effect. To minimise vibrational coupling effects, the difference between the vibration frequencies of the adsorbed species corresponding

with the bands at 2085 and in the 1900±2000 cmÿ1 range should be increased. This can be achieved by adsorbing C18O on the 1900±2000 cmÿ1 range sites and C16O on the 2085 cmÿ1 sites. Since only the adsorption of the 2085 cmÿ1 sites is reversible, according to Figs. 12 and 13, the desired carbon monoxide occupation can be achieved by exposing C18O to a freshly reduced catalyst, subsequent evacuation of the system and admission of C16O. Such an experiment is performed with a CuFe catalyst. Infrared spectra recorded after 10.8 ks exposure to C18O, subsequent evacuation and directly after admission of C16O are presented in Fig. 14. The bands in the spectra recorded with C18O are located at 2057 and 1915 cmÿ1, and with C16O at 2105 and 1947 cmÿ1. The corresponding bands exhibited identical absorption intensity. In order to enhance vibrational coupling effects the experiment was also performed in the reversed way. The bands in the spectra of Fig. 14 which are observed with C16O are now located at 2101 and 1956 cmÿ1, and with C18O at 2052 and 1930 cmÿ1. The latter band is extremely broadened.

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Fig. 12. Infrared spectra of CO adsorbed on a Cu2Fe catalyst reduced at 623 K. Spectra recorded (a) 10.8 ks after admission of 1.33 kPa CO, and (b) immediately after subsequent evacuation.

Fig. 13. Intensity vs. time curve of the infrared absorption bands observed on a Cu2Fe catalyst at 2085 cmÿ1 and 1932 (1947) cmÿ1 in 1.33 kPa CO and after subsequent evacuation.

4. Discussion

also exhibits an increased contribution of the singlet, indicating reduction to Fe…CN†4ÿ 6 . After oxidation of the copper±iron cyanide complexes at the indicated temperatures, the MoÈssbauer spectra show a complete conversion of the cyanide complex to a non-magnetically ordered Fe(III)-oxide. Following Van der Kraan [8], the two doublets, which were required to arrive at a reasonable ®t of the spectra, are assigned to bulk and surface atoms of the iron-oxide phase. MoÈssbauer measurements at cryogenic temperatures with the CuFe catalyst proved the iron phase to be superparamagnetic a-Fe2O3 with an average particle size of <4 nm [4]. Consequently, the copper phase in the oxidic precursor comprises small CuO crystallites. Heating at 850 K resulted in the formation of a mixed oxide CuFe2O4. From these

The precipitates formed on injecting different iron± cyanides solutions into a Cu2‡ containing Al2O3 suspension exhibit colours and MoÈssbauer spectra which are in agreement with the expected stoichiometric copper±iron cyanides. Analysis of the Cu3Fe2 MoÈssbauer spectrum reveals a superposition of a doublet and a singlet. The parameters of the singlet 3ÿ indicate the presence of Fe…CN†4ÿ 6 beside Fe…CN†6 . 3ÿ Reduction by UV-irradiation of the Fe…CN†6 anion to has been reported to occur in the Fe…CN†4ÿ 6 Cu3[Fe(CN)6]2 complex [7]. However, storage of the Cu3Fe2 cyanide precursor in the dark resulted in a change of the colour from yellow-green to purplegrey, and the MoÈssbauer spectrum of this aged sample

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Fig. 14. Infrared spectra of isotopically labeled CO adsorbed on a CuFe catalyst. (a) first dose: C18O, (b) subsequent evacuation of C18O, (c) second dose: C16O. (d) first dose: C16O, (e) subsequent evacuation of C16O, and (f) second dose: C18O.

results it is derived that the individual particles of the oxidic copper±iron precursors are composites of small, intimately mixed CuO and Fe2O3 crystallites, in a ratio dictated by the Cu±Fe stoichiometry of the initial copper±iron cyanide precursor. In the cyanide complex, the copper and iron ions are mixed on an atomic scale. The intimate contact between both metal ions still exists after oxidation and should be retained during and after reduction. The phase diagram of the Cu±Fe system [9] indicates that the mutual solubility of metallic copper and iron is very low. Therefore, phase separation is to be expected in equilibrated systems with copper-to-iron ratios as present within the catalysts studied. However, metastable systems in which the f.c.c. copper stabilises an unstable f.c.c. iron lattice are generally observed with epitaxially grown ®lms [10], single crystals [11], and also with bulk alloys [12]. For small particle systems single phase alloys can exist despite the prediction of immiscibility from the bulk-phase diagram [13]. Formation of bimetallic copper±iron particles in which copper stabilises small f.c.c iron clusters may therefore be possible by reduction of the oxidic precursors. The reduction, however, will be a delicate process liable to cause phase segregation. It has been found

that the reduction of the iron-oxide crystallites in the copper±iron catalyst proceeds at a much lower temperature than in the analogously prepared monometallic iron catalyst [4,5]. The enhanced reducibility of the iron oxide is due to the presence of CuO, which is reduced much more readily than Fe2O3. Metallic copper containing some iron atoms in the surface facilitates activation of hydrogen which can spill-over to, and subsequently reduce the adjacent Fe2O3 crystallites. It was expected that during the reduction assisted by copper the Fe would remain highly dispersed and even assume the f.c.c. structure of the Cu matrix. However, this is de®nitely not observed. Due to the highly exothermal reduction of CuO, the temperature of the composite particle will rise considerably during reduction. This thermal effect is well known for the activation of Cu/ZnO catalysts, carbon monoxide shift conversion and methanol synthesis. High temperatures and thermal shocks will induce relaxation of unstable f.c.c. to stable b.c.c. Fe and phase separation in copper±iron solid solutions [10]. Such an effect is indeed observed. Starting the reduction of the oxidic CuFe precursor at 473 K resulted in a rapid reduction of the small Fe2O3 crystallites to large metallic b.c.c. Fe particles, as evident from the development of the a-Fe sextuplet at the expense of the oxidic doublet in the MoÈssbauer spectra. No indications are found for the presence of small superparamagnetic b.c.c. or even f.c.c. Fe. By decreasing the initial reduction temperature to 438 K, the degree of reduction is much lower as compared to the previous experiment. However, the metallic Fe is still represented by a sextuplet. To separate the bulk reduction of copper and iron oxide, the ®rst reduction step has been carried out at 373 K. At this temperature Fe2O3 is partly reduced to Fe(II). Formation of the metallic iron sextuplet is now observed after additional reduction at 523 K. Despite the careful reduction procedure still relatively large metallic Fe aggregates with the stable b.c.c. structure result. For the Cu2Fe catalyst, on the other hand, the applied mild reduction treatment resulted at 548 K in the formation of metallic iron which is represented by both a sextuplet and a singlet. The singlet originates from iron which has the f.c.c. structure, or from superparamagnetic iron with the b.c.c. structure. Discrimination between these phases requires measurements at cryogenic temperatures or even better in an external magnetic ®eld. Due to the

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still relatively high iron fraction in the bimetallic system, the f.c.c. phase is less likely to occur. The Fe3‡/Fe2‡ mixture in the partially reduced CuFe catalyst can be more speci®cally characterized from the measurements at cryogenic temperatures. With respect to the spectrum at 300 K, the spectral contribution of the Fe2‡ phase increases, which emphasizes the fact that the corresponding phase is highly dispersed. The observed increase of the QS is in accordance with the generally observed trend with high-spin Fe2‡ compounds. At 77 K also a magnetic ordering becomes apparent. At 4.2 K the magnetically ordered phase exhibiting a sextuplet becomes the major spectral component. From the HF of 505 kOe and the fact that the sextuplet exhibits a wide distribution of HF's, the corresponding phase is identi®ed as highly dispersed Fe3O4. Semi-continuous monitoring of the reduction process of the three copper±iron catalysts by means of in situ magnetization measurements also revealed that the reduction of the iron oxide initiates already at 373 K. The increase of the magnetization is consistent with the formation of ferrimagnetic Fe3O4 as was observed with MoÈssbauer spectroscopy. At increasingly severe reduction treatments the magnetization of the samples increases gradually. In contrast to the monometallic iron catalyst [5], no indications were found for an intermediate Fe1ÿxO phase. This ®nding indicates that the iron-oxide±support interaction is replaced by an iron-oxide±copper interaction and, hence, the iron oxide being embedded in copper (oxide). The increase of the magnetization during the high-temperature reduction steps is due to either an increase of the degree of reduction, a f.c.c. to b.c.c. structure transformation, or due to coalescence of small b.c.c. iron precipitates to large crystallites and thus decreasing the fraction of superparamagnetic iron. The surface of the copper±iron catalysts as studied by infrared spectroscopy with adsorbed CO revealed a single absorption band with the oxidic precursors. This band is assigned to molecularly adsorbed CO which is linearly bonded on CuO. The increase of the intensity and wave number of the absorption band with exposure time indicates that the CO surface coverage gradually increases. Desorption of the adsorbed species also appeared to be a slow process. Since Fe2O3 does not adsorb carbon monoxide, the presence of this

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oxide at the catalyst surface could not be ascertained by this experiment. However, XPS measurements on the CuFe catalyst precursor (unpublished) revealed that the surface composition equals the bulk composition. Reduction of the catalyst at 373 K caused a signi®cant change in the infrared spectra. The two distinguished spectral ranges, viz. 2000±2250 and 1700± 2000 cmÿ1, are both attributed to molecularly adsorbed CO species. The band at 2079±2085 cmÿ1 is ascribed to CO adsorbed linearly on metallic Cu. Since Cu does not give rise to absorption bands below 2000 cmÿ1, the bands in the 1700±2000 cmÿ1 range are assigned to bridged-bonded CO on sites involving metallic Fe. The average wave number of the composite band decreases with increasing Cu content of the catalyst. The fact that the presence of metallic iron in the catalyst, after reduction at 373 K which is not observed by the MoÈssbauer measurements, can be explained by the low Debye temperature of the highly dispersed iron atoms in the particle surface. After reduction at 473 and 523 K the infrared transmission of the samples has dropped below the minimum level for obtaining a signi®cant signal. This phenomenon is most probably due to the presence of highly dispersed Fe3O4 in the partially reduced catalyst, as observed by MoÈssbauer spectroscopy. The electrons originating from the octahedral Fe2‡, which can be transferred to the octahedral Fe3‡ ions, account for the semi-conducting properties of Fe3O4. It is suggested that the activation energy, required for the electron-hopping process, is provided by absorption of radiation during the infrared measurements. After reduction at 623 K, the magnetite (Fe3O4) is reduced and the sample becomes again IR-transparant. In the recorded spectra, the previously marked two spectral ranges are still present. The respective bands, however, exhibit now more details. A clear distinction between the different catalysts exists for the initial intensity ratio of the two constituting bands in the 1700±2000 cmÿ1 range. On Cu2Fe the low-frequency band is dominant over the high-frequency band, whereas this ratio is reversed on the CuFe catalyst. The Cu3Fe2 catalysts exhibit an intermediate ratio. The low-frequency band is assigned to CO adsorbed on Fe-sites with a high Cu coordination, the high-

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frequency bands to Fe-sites with a low Cu coordination. The increase of the absorbance in the 1800± 2000 cmÿ1 frequency range during exposure to carbon monoxide appeared to be opposite to that in the 2000± 2200 cmÿ1 range. This time-dependent effect is observed with all catalysts over the whole range of reduction temperatures. The wave number of the bridged-bonded CO is higher than observed with a cyanide derived Fe catalyst at 1700±1950 cmÿ1 [5]. The weak band at 2001 cmÿ1 is assigned to linearly adsorbed CO on initially present Fe-sites. This band also exhibits a blue shift as compared to the corresponding band at 1982 cmÿ1 of the monometallic Fe catalyst [5]. After a prolonged exposure time, the band gets lost in the broad absorption band due to bridgedbonded CO with the average wave number gradually shifting to higher values. Reduction at high temperatures results in an increase of the overall fraction metallic Fe and favours the development of the 2001 cmÿ1 band. The development of a low-frequency shoulder at the central 2090 cmÿ1 band is interpreted as arising from distinct Fe perturbed Cu adsorption sites. From Cu±Fe single crystal studies it is well known that in vacuum the surface is enriched in Cu [11]. Theoretical model studies predict segregation of iron to the surface upon exposure to CO [14]. It is therefore legitimate to ascribe the inversely changing intensities of the bands in the 2000±2200 and 1800±2000 cmÿ1 range during exposure to carbon monoxide to a COinduced segregation of iron to the surface of the catalyst particle. The simultaneously increasing wave number of the band below 2000 cmÿ1 can be explained by an increase of the adsorbate±adsorbate repulsion at increasing coverage or to a decrease of the dilution effect by Cu. The high-frequency band at 2127 cmÿ1 which accompanies the segregation of Fe, is ascribed to surface roughening. It is reported that the wave number of CO on Cu strongly depends on the surface structure. With stepped Cu surfaces (211), (311) and (755). Hollins et al. [15] reported CO absorption bands from 2100 to 2110 cmÿ1, whereas with more densely packed (111) and (100) surfaces, the absorption peaks at 2070±2090 cmÿ1. Additional reduction steps cause a redistribution of the Cu and Fe atoms within the particles: The spectra again exhibit analogous time-dependent features in

addition to indications for an increased degree of reduction. Apparently, the CO induced segregation is lost after a thermal treatment in hydrogen. The overall decrease of absorption intensity indicates sintering of the bimetallic particles. Evacuation of the gaseous CO results in an instantaneous disappearance of 2085 cmÿ1 band, indicating that the adsorption of CO on the (perturbed) metallic Cu-sites is reversible. The simultaneously occurring increase in intensity of the 1932±1947 cmÿ1 band due to CO adsorbed on Fe-sites is a remarkable new phenomenon encountered in catalyst characterisation by infrared spectroscopy. As it is unlikely that the CO surface coverage has increased at a decreased CO pressure, the apparent extinction coef®cient of the species previously adsorbed on the Fe-sites is assumed to change abruptly. Furthermore, as the effect is correlated with the intensity of the band at 2085 cmÿ1, it is plausible that the effect is induced by the CO species adsorbed on the Cu-sites. The two different mechanisms can cause the following observed intensity effects: Firstly, Hollins [16] demonstrated that for two species, which are adsorbed on a single substrate and exhibit moderately different vibration frequencies, the apparent intensities are strongly in¯uenced by a vibrational coupling mechanism. The high-frequency band steals intensity from the low-frequency band. In the Cu±Fe system, the intensity of the lowfrequency band due to CO adsorbed on Fe-sites is suppressed in the presence of a high-frequency band due to CO adsorbed on Cu-sites. The frequency difference of the two bands at 2085 and 1947 cmÿ1 amounts to 138 cmÿ1 or 7%, and is within the limits indicated for the occurrence of such an intensityexchange process. Secondly, the angle at which species are adsorbed on metallic surfaces also affects the apparent intensity of the absorption band according to the `surface selection rule'. On a metal surface an adsorbed dipole leads to image charges that reinforce the effective moment of dipoles oriented perpendicular to the surface and that decrease the effective moment of dipoles oriented at an angle on the surface. From this point of view, the extinction effect can be attributed to an electrostatic repulsion by which linearly adsorbed CO on the Cu-sites forces the bridged-bonded CO on the Fe-sites to bend away. Upon desorption of the

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CO from the Cu-sites, the irreversibly bonded CO on the Fe sites will reorientate and take up positions perpendicular to the particle surface. The image dipole moment now reaches a maximum value and, consequently, the vibration transition probability and absorption intensity increase. To discriminate between the above two mechanisms the experiments with isotopically labelled CO are performed. Vibrational coupling effects as described by Hollins [16] will be strongly affected by a variation of the energy difference between the vibrations, whereas the electronic repulsion affecting the orientation of adsorbed CO on the surface will be independent of the vibration frequencies. It turned out that adsorption of C16O on a surface with empty Cu-sites and C18O covered Fe-sites resulted almost immediately in a spectrum exhibiting close resemblance to the spectrum recorded with only C16O adsorbed. Upon prolonged exposure to C16O the frequency of the band at 1915 shifts to 1947 cmÿ1 indicating that the preadsorbed C18O is being exchanged by the admitted C16O. Consequently, this exchange of CO which was supposed to be irreversibly adsorbed on Fe-sites prevents an easy discrimination between the above two mechanisms. However, considering the different time domains required for the coverage of the Cu-like sites and the exchange process at the Fe-like sites, the intensity of the band assigned to adsorption on Culike sites provides still useful information. The intensity of the C16O on the Cu-like sites exhibits an identical intensity as with C18O coverage, despite the still existing C18O coverage of the Fe-like sites. Even in the case of the C18O spectrum of Fig. 14 in which the broad band of low intensity at 1930 cmÿ1 indicate that the C18O±C16O exchange is not completed, the band at 2052 cmÿ1 exhibits the same intensity of the band at 2101 cmÿ1. Therefore, it is highly unlikely that the vibrational coupling mechanism accounts for the observed extinction effect. As carbon monoxide was found to adsorb irreversibly on Fe-sites, at least within the time scale of our experiments, the observed C16O±C18O exchange was unexpected. Therefore, one has to conclude that adsorbed CO molecules on the Fe-sites are highly mobile and exchange with CO reversibly adsorbed on the adjacent Cu-sites. Mere desorption of CO from the Fe sites is apparently not favourable. Though the heat of adsorption of CO on copper is lower than on iron,

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the energy of CO adsorbed on copper is still signi®cantly lower than that of gaseous CO. Therefore, it is plausible that some CO molecules adsorbed on Fesites at the interface between iron and copper can spend a short period of time on the Cu-sites under conditions where desorption of CO from Fe-sites is negligible. So, the activation energy for surface migration of CO from iron to copper will be much lower than that for direct desorption from iron. The rather rapid exchange of C16O±C18O on Fe-sites can now be understood in the following way: Whenever a Fe-site at the edge with copper becomes empty, a CO molecule from the copper will occupy the Fe-site. Under vacuum conditions adsorbed CO is no longer present on the copper and the exchange will not proceed. By covering again the copper with adsorbed CO molecules, the exchange can proceed and the empty Fesites will be ®lled by CO freshly adsorbed on the copper. The physical/chemical nature of the extinction effect, however, could unfortunately not be elucidated more directly. The effect itself, however, emphasizes that great care should be taken in linking site occupation/site density and IR-absorption intensity. Therefore, the frequently observed [17±19] intensity of bands, assigned to bridged-bonded species, being lower than that of bands attributed to linearly adsorbed CO species in alloy systems as compared to the ratio recorded with the single metal systems, does not necessarily a priori prove a decrease of the number of bridged-bonded species or adsorption sites when the monometallic system is alloyed! The copper±iron system has the unique property of exhibiting bands due to both linearly and bridged-bonded CO species, which are reversibly and irreversibly adsorbed, respectively. When both bands should have been due to irreversibly adsorbed species, the effect would not have been observed at all. An additional complication is the segregation of a component to the particle surface, which can have a dramatic effect on the recorded spectrum. 5. Conclusions Copper±iron cyanide complexes appeared to be attractive precursors for the preparation of supported bimetallic Cu±Fe catalysts. The copper-to-iron ratio in

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the catalysts could be established by using different iron±cyanide complexes. Removal of the cyanide ligands by oxidation resulted in an oxidic precursor comprising intimately mixed CuO and Fe2O3 crystallites. Reduction of Fe2O3 is greatly enhanced by the presence of CuO, which is rapidly reduced to metallic copper with some iron atoms in the surface. The ultimate distribution of iron in the copper matrix is strongly determined by the reduction procedure. Heating of the oxidic precursor in hydrogen beyond the temperature required for the reduction of the copper oxide results in the formation of relatively large b.c.c. iron clusters in a f.c.c. copper matrix. Sequential reduction of the CuO and Fe2O3 phase facilitates the formation of small superparamagnetic iron precipitates in the copper phase. In hydrogen and in vacuum the surface of the bimetallic particles is depleted of iron. Exposure of the reduced catalysts at room temperature to carbon monoxide results in segregation of iron to the surface. The bimetallic particles expose a number of different CO adsorption sites. On Cu-sites or Fe-perturbed Cu-sites, the CO species are linearly bonded where as on Fe-sites or Cuperturbed Fe-sites, CO is mainly bridged-bonded. Infrared spectroscopy revealed that CO species adsorbed on the Cu-sites lowers the absorption intensity of the species adsorbed on the Fe-sites. The effect arises from an adsorbate±adsorbate repulsion induced tilted adsorption geometry of the bridged-bonded CO species. Adsorption of CO on the Cu-sites appeared to be reversible, and that on the Fe-sites to be irreversible. However, on the latter sites the species are highly mobile and can exchange with CO species adsorbed on adjacent Cu-sites. The adsorbate±adsorbate interaction, which was evident from the decreased extinction coef®cient of the bridged-bonded CO species, indicates that the reduced intensity of absorption bands due to

bridged-bonded species not a priori implies that the number of these adsorption sites has decreased by alloying. References [1] P.K. de Bokx, A.J.H.M. Kock, E. Boellaard, W. Klop, J.W. Geus, J. Catal. 96 (1985) 454. [2] O.P. van Pruissen, PhD-Thesis, Utrecht University, The Netherlands, 1987. [3] A.F.H. Wielers, PhD-Thesis, Utrecht University, The Netherlands, 1986. [4] E. Boellaard, A.M. van der Kraan, J.W. Geus, Studies in Surface Science and Catalysis, Vol. 91, Preparation of Catalysts VI. Scientific Bases for the preparation of Heterogeneous Catalysts, G. Poncelet et al. (Eds.), Elsevier Science, Amsterdam, 1995, p. 931. [5] E. Boellaard, A.M. van der Kraan, J.W. Geus, Appl. Catal. A: General 147 (1996) 207. [6] A.M. van der Kraan, J.W. Niemantsverdriet, in: Industrial Applications of the MoÈssbauer Effect, G.J. Long, J.G. Stevens (Eds.), Plenum Press, New York, 1986, p. 609. [7] Gmelins Handbuch der Anorganischen Chemie, 8. Auflage, Kupfer, System-Nummer 60, Teil B, Verlag Chemie GmbH, Berlin, 1965, p. 1313. [8] A.M. van der Kraan, Phys. Stat. Sol. A 18 (1973) 215. [9] M. Hansen, Constitution of Binary Alloys, McGraw-Hill Book Comp., 2nd ed., New York, 1958, p. 580. [10] S.A. Chambers, T.J. Wagener, J.H. Weaver, Phys. Rev. B 36 (1987) 8992. [11] O.P. van Pruissen, E. Boellaard, O.L.J. Gijzeman, J.W. Geus, Appl. Surface Sci. 27 (1986) 1. [12] S.J. Campbell, P.E. Clark, J. Phys. F: Metal Phys. 4 (1974) 1073. [13] D.F. Ollis, J. Catal. 23 (1971) 131. [14] D. TomaÂnek, S. Mukherjee, V. Kumar, K.H. Bennemann, Surface Sci. 114 (1982) 11. [15] P. Hollins, J. Pritchard, Prog. Surface Sci. 19 (1985) 275. [16] P. Hollins, Spectrochimica Acta 43A (1987) 1539. [17] Y. Soma-Noto, W.M.H. Sachtler, J. Catal. 32 (1974) 315. [18] Y. Soma-Noto, W.M.H. Sachtler, J. Catal. 34 (1974) 162. [19] J.-A. Dalmon, M. Primet, G.-A. Martin, B. Imelik, Surface Sci. 50 (1975) 95.