MOCVD of iron with [(arene)(diene)Fe(0)] precursors in a fluidized bed reactor
K. Michkova
a,* , A. Schneider
a, H. Gerhard
a, N. Popovska
a, I. Jipa
b, M. Hofmann
b, U. Zenneck
b,**
aLehrstuhl fu¨r Chemische Reaktionstechnik, Universita¨t Erlangen-Nu¨rnberg, Egerlandstraße 3, D-91058 Erlangen, Germany
bInstitut fu¨r Anorganische Chemie, Universita¨t Erlangen-Nu¨rnberg, Egerlandstraße 1, D-91058 Erlangen, Germany Received 30 May 2006; received in revised form 5 September 2006; accepted 6 September 2006
Available online 17 October 2006
Abstract
MOCVD of thin iron films at moderate temperature (T<2008C) was investigated using two metal organic complexes of the type [(arene)(diene)Fe(0)] as precursor. [(1,3-Butadiene)(toluene)Fe(0)] (1) and [(1,3-cyclohexadiene)(toluene)Fe(0)] (2) were synthesized by metal vapor synthesis. They were characterized by their vapor pressure at different temperatures and their decomposition behavior.1and2were used to coat alumina powder with iron through MOCVD in a fluidized bed reactor (FB-MOCVD) to prepare potential supported iron metal catalysts. The new materials were compared to an iron coated sample, which was prepared by conventional wet impregnation technique. Both were characterized by different methods like ICP OES, SEM/EDX, particle size analysis, H2/TPR and CO adsorption. The iron layer of the FB-MOCVD samples forms nanocrystals, which are homogeneously distributed over the surface. They are much smaller in size and show a higher dispersion with respect to the conventionally prepared iron catalysts. As a consequence, the nanocrystals are significantly more reactive with respect to redox processes and adsorb about 40% more carbon monoxide, than the conventionally prepared material, thus indicating a higher catalytic activity.
#2006 Elsevier B.V. All rights reserved.
Keywords:Fe/Al2O3catalyst; MOCVD; Wet impregnation; Fluidized bed reactor; [(1,3-Cyclohexadiene)(toluene)Fe(0)]; [(1,3-Butadiene)(toluene)Fe(0)]; TPR;
CO adsorption
1. Introduction
Iron is widely used as a catalyst for industrial applications, as it combines an excellent availability, a low price, and very useful catalytic properties. Iron is applied in Fischer–Tropsch synthesis [1–4], the Haber–Bosch process [5,6], and for catalytic growth of carbon nanotubes [7–10], for example.
Standard procedures to prepare iron catalysts include wet impregnation of oxidic or metallic support materials, co- precipitation with another component, ion exchange, co- crystallization or chemical vapor deposition at the surface of suitable supports by thermal decomposition of volatile metal organic complexes as precursors (MOCVD). Compared to the other techniques MOCVD has advantages in many aspects.
These include eliminating the solid–liquid separation and the subsequent operations of drying and high-temperature calcina- tion–reduction cycles, thus minimizing the aggregation or the crystalline size-growing problem for the supported metal particles caused by these operations [8]. The MOCVD technique in fluidized bed (FB-MOCVD) turned out to be a very suitable for the production of highly dispersed metal- supported catalysts[8,11].
A wide range of iron complexes have been already tested as precursors for iron MOCVD, but most reports focus on commercially available compounds. On the other hand, an optimal iron precursor for practical MOCVD applications is still missing. Unsolved problems are related to high deposition temperatures, toxicity of the complexes themselves or secondary products which are formed through the process, difficulties to prepare or handle the precursors, or lack of purity of the deposited materials. For example the highly toxic iron pentacarbonyl [(CO)5Fe] is the most often used precursor for the MOCVD deposition of thin magnetic iron films or single crystals on semiconductors, integrated circuits, and insulating
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* Corresponding author. Fax: +49 9131 8527421.
** Corresponding author. Fax: +49 9131 8527367.
E-mail addresses:[email protected](K. Michkova), [email protected](U. Zenneck).
0926-860X/$ – see front matter#2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2006.09.008
substrates in a rage from room temperature to 4008C[12–19].
It is also used as an iron source for the preparation of supported iron nanoparticle catalysts for carbon nanotube (CNTs) synthesis [8,9] and for gas phase catalytic growth of CNTs [20,21]. The reaction temperatures of these processes are very high. They range from 200 to 12008C. The toxicity of iron pentacarbonyl is closely related to its high vapor pressure even at ambient temperature, light sensitivity, and the release of comparable toxic CO during the MOCVD process.
The alternative binuclear species [(C5H5)2(CO)4Fe2] [22]
has a significantly lower vapor pressure, is more robust, but consequently requires much higher deposition temperature of 260–3108C. Ferrocene [(C5H5)2Fe][23,24]and [FeF3][25]are not suitable at all for MOCVD coating processes as they are very stable substances, which need decomposition tempera- tures of even more than 400 and 7508C, respectively.
The iron films deposited from the iron organic substances are in the most cases contaminated with significant amounts of carbon. If a new designed precursor would combine a moderate decomposition temperature below 2008C with a smaller risk of handling toxic compounds and a better control of film purity, it would be helpful for expanding the applicability of iron MOCVD for different purposes, including iron coating of precious materials like integrated circuits.
In this work we present investigations on two new [(arene)(diene)Fe(0)] iron precursors, [(1,3-butadiene)(tolue- ne)Fe(0)] (1) and [(1,3-cyclohexadiene)(toluene)Fe(0)] (2).
The principal suitability of1 for forming thin iron layers by means of MOCVD at moderate temperatures below 2008C was tested with magnesium silicate substrates in a cold wall reactor [26]. 1 is a known compound [27]. To the best of our knowledge, 2 has not been used as precursor for MOCVD before. Thep-bonded ligands of1and2are not very strongly bonded, so that they are designed to dissociate from the metal atom at only slightly elevated temperature. Further on, the oxidation state of the iron atom as well as that of the independently stable ligands of1 and2 is truly zero, thus no coulomb attraction hinders the separation of metal and ligands and gives rise to secondary chemical processes which could lead to carbon formation and incorporation into the iron films.
The main aim of the present work is to investigate the new metal organic complexes as precursors for depositing elemental iron on powder substrate in a fluidized bed reactor (FB- MOCVD). The resulting material is designed as a hetero- geneous catalyst for different applications.
2. Experimental
2.1. General procedure for the synthesis of [(diene)(toluene)Fe(0)] complexes1and 2
A 5–8 g iron and ca. 200 mL of toluene were evaporated within 1–2 h in a 10 L metal vapor reactor which has been reported earlier [28] and have been co-condensed in high vacuum (p<104mbar) to form a rigid matrix at liquid nitrogen temperature. The evaporation of construction steel pieces or chopped nails from a resistant heated alumina crucible
requires a temperature about 18008C, while the toluene evaporates rapidly at room temperature. After the co- condensation step, the reactor was warmed up to about 1008C by an ethanol slush bath. Excess of the diene ligands have been introduced as a gas (1,3-butadiene) or a solution in toluene (1,3-cyclohexadiene) at this stage of the reaction. The vacuum inside the reactor was replaced then by a nitrogen inert gas atmosphere and the stirred reaction mixture was allowed to warm up slowly within 2–3 h to about508C. After reaching 508C, the cooling bath was removed to allow reaching room temperature rapidly. The product solution was then siphoned out of the reactor and the excess of the ligands was removed in vacuum at room temperature. Vacuum distillation at 60–1008C led to analytically pure1 or2, respectively, in 30–50% yield with respect to the evaporated iron metal. Typical runs yielded 5–10 g complex within 1–2 days of laboratory work.
The spectroscopic data of [(1,3-cyclohexadiene)(tolue- ne)Fe(0)] (2): MS(FD+): m/z= 228 [M]+ (C13H16Fe)—1H NMR (269.71 MHz, C6D6): d= 4.59 (m, 5H, toluene, aromatic); 4.35 (m, 2H, CHD, olefinic); 2.22 (m, 2H, CHD, olefinic); 1.79 (s, 3H, toluene, methyl), 1.41(m, 2H, CHD, aliphatic); 1.15 (m, 2H, CHD, aliphatic)—13C{1H} NMR (67.81 MHz, C6D6): d= 94.30, 83.19, 82.03, 75.41 (toluene, aromatic C); 80.37, 54.35 (CHD, olefinic C); 26.93 (CHD, aliphatic C); 20.71 (toluene, methyl C). The synthesis of the compounds1 and2 is outlined inScheme 1.
2.2. MOCVD precursor characterization of1and 2
The temperature dependent vapor pressures of1and2were determined. A schematic drawing of the applied apparatus is depicted in Fig. 1. The precursors were syringed into the evaporator which is constructed as a bubbler and whose temperature is controlled by a heated aluminum block. Helium gas at atmospheric pressure bubbles through the liquid inside the evaporator and is thus loaded with the organometallic component. The iron complex saturated helium gas flow reaches a cold trap by heated transfer tubes. A quantitative precursor complex condensation is guaranteed by liquid
Scheme 1. Synthesis of [(diene)(toluene)Fe(0)] complexes1and2.
nitrogen cooling. To analyze the content of the cold trap, aqueous hydrochloric acid (37%) was added at room temperature to oxidize the iron complexes from Fe0 to Fe3+
and form an acidic water solution. The amount of iron was then determined by ICP AES (atom emission spectrometry with inductive coupled plasma, Perkin-Elmer, Plasma 400). With the assumption of a complete conversion of the precursor into iron trichloride n([(arene)(diene)Fe]) =n(Fe3+), the helium volu- metric flow and the general gas equation, the vapor pressure was calculated.
As the accuracy of this method of vapor pressure determina- tion is directly related to a fully precursor saturated helium gas flow, additional experiments were carried out to prove that precondition by determining the relation between volumetric gas flow and the condensed amount of precursor in the cold trap. We found this relation to be precisely linear, which indicates the assumed precursor saturated helium gas flow.
2.3. Iron catalyst preparation 2.3.1. Substrate
Alumina powder (H.C. Starck, Amperit Al2O3740-1) was used as a substrate for the MOCVD experiments in a fluidized bed reactor. Calibrating experiments with 1.5 g of the alumina powder in the available fluidized bed reactor resulted in the determination of a helium gas flow of 265 mL/min for an optimal fluidization of the particles. For each coating run, the alumina powder was calcined under helium atmosphere at 4008C for 2 h. The powder was characterized before and after coating by particle size analysis and scanning electron microscopy (SEM).
2.3.2. Chemical vapor deposition of iron
The MOCVD experiments were carried out under atmo- spheric pressure of helium inert gas with an evaporator temperature of 808C and a fluidized bed reactor temperature of 2008C. The set up for the MOCVD experiments is related to the vapor pressure determination apparatus, however, the
fluidized bed reactor is integrated downstream from the evaporator. In all cases, the optimal helium carrier gas flow of 265 mL/min was saturated with the precursor of choice and introduced into the reactor. The powder was placed on a porous glass plate in a reactor tube with an internal diameter of 16 mm and was fluidized by the helium flow. As designed, the precursor2 decomposed under these conditions quantitatively in the fluidized bed and coated the alumina powder particles with thin iron films. The by-products of the CVD process were transported with the helium flow out of the reactor into the cold trap and condensed there. To prevent the powder to be carried out of the reactor by the helium, the reactor was provided with a sufficient freeboard to decreases the flow velocity due to a higher cross-sectional area. Coating experiments were carried out for run times varied between 0.5 and 8 h.
2.3.3. Conventional preparation of supported iron catalyst by wet impregnation
The wet impregnation method was used in the present work to prepare an iron catalyst by a conventional technique for comparison and determination of the relative performances of both catalysts. Fe(NO3)39H2O was dissolved in distilled water and the alumina powder was conditioned exactly as for the CVD experiments at 4008C under helium gas atmosphere. The conditioned alumina and a freshly prepared iron(III) salt solution were mixed to form a suspension, which was heated and stirred at a heater temperature of 1508C until all water was evaporated. The impregnated powder was afterwards dried in an oven at 1508C for 4 h. The amount of iron on the supported catalyst was determined by ICP OES again.
2.4. Characterization of iron coated alumina particles The iron coated alumina particles produced by MOCVD and wet impregnation were investigated by SEM, ICP OES, temperature programmed reduction (TPR) and CO adsorption.
The SEM micrographs (Phillips XL 30) were used for a qualitative estimation of the iron distribution on the surface of the powder. ICP OES and TPR (Altamira, AMI-100) were used for quantitative determination of the iron mass fraction in the catalysts. For the reduction experiments, the catalysts were calcined at 5508C in air for 1 h in order to oxidize the iron at the surfaces to form iron(III) oxide Fe2O3. The catalyst was heated up to 8008C in a hydrogen–helium atmosphere. To prove the existence of iron on the powder surface energy dispersed X-ray analysis (EDX, Phillips XL 30) was applied. The dispersion of both catalysts (synthesized with wet impregnation and MOCVD) was estimated by CO pulse chemical adsorption (Altamira, AMI-100). The catalysts pre-treatment (calcination of the samples) was carried out in the same way as for TPR.
3. Results and discussion
3.1. Precursor complex preparation
The first report on [(arene)(diene)Fe(0)] derivatives deals with [(benzene)(1,3-cyclohexadiene)Fe(0)]. It is accessible by
Fig. 1. Schematic drawing of the experimental set-up for the vapor pressure determination of liquid MOCVD precursors.
reduction of FeCl3in the presence of 1,3-cyclohexadiene[29], a route which cannot be applied on [(1,3-cyclohexadiene)(to- luene)Fe(0)] (2), for example. However, we prefer toluene as the arene ligand due to its much smaller toxicity[30] and a positive effect on a potential low melting point. [(1,3- Butadiene)(toluene)Fe(0)] (1) has been prepared successfully earlier by a three component metal vapor reaction of iron atoms and the two ligands [27,31]. Related [(arene)(L2)Fe(0)]
derivatives like [(p-xylene)(1,5-COD)Fe(0)] have been used for the preparation of iron nanoparticles[32]. A more rational approach on neutral arene iron complexes utilizes the readily available metal vapor product family [bis(arene)Fe(0)] as reactive intermediates, which have been transformed success- fully with several ligands into [(arene)(L2)Fe(0)] derivatives by a low-temperature substitution reaction[28,33]. We followed this straightforward approach for the preparation of1 and2.
The co-condensation product of iron vapor and toluene at 1968C is the highly reactive intermediate [h6:h4-(tolue- ne)2Fe] [33] (Scheme 1). One of the two toluene ligands is bonded in the classical h6-version where the complete p- system of the arene interacts with the transition metal, but the other one looses its aromaticity to bind in an unconventionalh4- fashion[34–36]. The h4-bonded toluene ligand is very labile and can thus be replaced easily around608C by two two- electron or one four-electron ligands [28,33]. 1,3-Butadiene and 1,3-cyclohexadiene follow this line and form the target complexes1 and2in good yield.
As designed,1and2are vacuum distillable red oils which do not form solids at ambient temperature. They are completely miscible with common non-polar solvents, are stable in inert atmospheres, but are pyrophoric and oxidize rapidly in contact with air. Chlorinated hydrocarbons catalyze their decomposi- tion. The spectroscopic and analytic data of known 1 are identical within the margins of experimental error with literature reports[27,31]and the spectra of2fit for the (1,3- cyclohexadiene)Fe unit closely to those of [(benzene)(1,3- cyclohexadiene)Fe(0)] [20]. The spectral parts which are generated by the (toluene)Fe moiety causes analogous signals as those reported for other [(L2)(toluene)Fe(0)] derivatives [27,31]. A structural and electronic analogy of all [(arene)(- diene)Fe(0)] derivatives in the context of this paper can thus be assumed: The toluene ligands of 1 and 2 are bonded in the classicalh6-version and there is nothing particular with thep- bonding of the 1,3-diene units of the co-ligands as well.
3.2. Vapor pressure of [(arene)(diene)Fe(0)] complexes As compounds which can be purified by vacuum distillation and as liquids at ambient temperature,1 and2fulfill already two essential conditions for potential useful MOCVD precursors. From orienting experiments we learned about the stability of1in the gas phase up to approximately 1008C[26].
A complete thermal decomposition of the gas phase takes place at surfaces with temperatures between 150 and 2008C.
Unchanged 1,3-butadiene and toluene in the ratio 1:1 have been identified as the only condensable organic decomposition products of 1 according to NMR analysis [26,37]. For
identifying more of the crucial MOCVD precursor properties, the temperature dependence of the vapor pressures of1and2 was determined. These data are of great importance for the application the MOCVD technique for the deposition of thin metal films. A precondition for an accurate determination of the vapor pressure is a complete saturation of the carrier gas with the precursor compounds. A clear indication for a saturated carrier gas is a linear behavior of the transported amount of precursor with the volumetric carrier gas flow. The amount of gas-phase transported precursor complexes 1 and2 from the different experiments was plotted against the helium volu- metric flow for three temperatures (Fig. 2). The measured points have close to linear trend for both, 1 and2. This fact proves a complete saturation of the carrier gas in all cases even at temperatures as low as 508C, respectively, 628C.
A first hint on gas phase or surface decomposition of a CVD precursor is given by an under proportional increase of the gas phase concentration with the temperature. A temperature plot of the vapor pressures of1and2give the details (Fig. 3). The vapor pressure of [(1,3-cyclohexadiene)(toluene)Fe(0)] (2) for exam- ple, follows a logarithmic function of the evaporator temperature between 56 and 808C. For more then 808C the experimental vapor pressures were smaller than the curve trend due to partial decomposition of the precursor. As a consequence, the temperature of the MOCVD evaporator has to be kept below 808C for2to prevent losses. This temperature corresponds to a partial gas pressure of about 19 Pa. In the case of [(1,3- butadiene)(toluene)Fe(0)] (1) the decomposition starts at temperatures about 708C, which allows a maximal partial gas pressure of the precursor of about 17 Pa.
Fig. 2. Plot of gas flow dependent transport of precursors1and2for different temperatures.
The logarithmic part of the vapor pressure curves can be rationalized. The vapor pressure of 2follows Eq. (1). for the temperature range 56–808C:
lnp¼ 84231
Tþ27:4 (1)
The data for1and the temperature range 50–708C are given by the following equation:
lnp¼ 10;2041
Tþ31:9 (2)
From this data we learn about a slightly increased stability of 1,3-cyclohexadiene complex 2 with respect to 1,3-butadiene species1and a little higher vapor pressure of1. The reason for the higher vapor pressure of the smaller complex is its reduced size with a smaller number of possible Van der Waals interactions and its lower molecular weight. The increased stability of2relates on the one hand side to the two additional methylene groups which donate a little extra electron density to the 1,3-diene unit and that stabilizes the metal–ligand bond thermodynamically. On the other hand are 1,3-cyclohexadiene ligands kinetically stabilized, as the central C–C single bond cannot rotate completely due to its cyclic character in case of the ligand dissociation intermediate, when only one of the two double bonds is still coordinated to the metal.
3.3. Characterization of the powder substrate
Commercially available alumina powder was chosen as substrate for the MOCVD experiments in a fluidized bed reactor according to the proposed properties of the material:
Little or no tendency for cohesion.
Fluidisable without additional equipment.
Mechanical stability during fluidization.
The true suitability of alumina powder for being used in a fluidized bed reactor was checked by applying the Geldart diagram[38,39]. With a mean density of crystalline alumina of about 4000 kg/m3as the input, an optimal particle size between 20 and 70mm was determined. A particle size analysis of the alumina powder used in the present study indicated a good match with this condition (Fig. 4a) and an SEM micrograph of the powder proved the absence of agglomerates (Fig. 4b). Even after fluidization for 5 h, there is only a small abrasion of the powder particles observable and the particle size distribution curve remains almost unchanged by the fluidization experiment (Fig. 4a).
3.4. Characterization of the iron coated alumina particles The iron coated alumina particles of this paper prepared by MOCVD and wet impregnation techniques were compared with respect to their morphology and dispersion. Their iron content was determined by ICP OES analysis. By utilizing deposition times between 0.5 and 8 h it was possible to prepare samples with iron contents betweenwFe ¼0:05 and 0.88%. The results of the SEM investigation of both iron coated alumina particles catalysts prepared in this study withwFe¼0:88 for the MOCVD and wFe ¼0:57% for wet impregnated sample are presented in Fig. 5. The two components of the supported catalyst, alumina and metallic iron can be clearly distinguished in the micrographs. Two principal types of iron layers are formed through the MOCVD deposition process. At deposition times up to 4 h, almost unstructured thin iron films are formed.
If the deposition time is enhanced to 8 h, cubic nanocrystals of iron are found. Their sizes are in the small range between 60 and 90 nm edge length with a mean value of 75 nm (Fig. 5a and b). A few aggregates with cubic surface elements of a related size are scattered between the cubes. In contrast to that, the iron of the wet impregnated sample is concentrated mainly in severalmm size aggregates of irregular metal clusters (Fig. 5c and d). The significantly higher iron dispersion of the MOCVD catalyst fed the hope for a higher activity of the potential catalyst material, which was confirmed (vide infra).
A related research approach led to an iron single crystal layer on monocrystalline GaAs (1 1 0) by epitactic film growth
Fig. 3. Temperature dependence of the vapor pressures of1and2.
Fig. 4. (a) Particle size distribution of the untreated alumina powder (untreated and after 5 h fluidization) and (b) SEM-pictures of the untreated alumina powder.
from Fe(CO)5as a precursor at temperatures above 2008C[14].
Increasing temperatures enhanced the quality of the crystals, but amorphous films were formed between 135 and 2008C.
Compared to that, precursor two yields separated nanocrystals already at 2008C. The combination Fe(CO)5/alumina led to the deposition of highly dispersed iron nanoparticles of undefined size, whose utility for catalytic carbon nanotube formation has been proven. The amount of iron deposited per time unit in a comparable temperature range was smaller. To reach the maximum value ofwFe¼0:29%, a fluidized bed temperature of 6008C was necessary[8].
The purity of the iron particles at the surface of both samples was qualitatively investigated by EDX. Besides of iron only a small amount of carbon was detected in the MOCVD sample.
The chemical reactivity of the iron at the surface of both samples with respect to redox processes was investigated by temperature programmed reduction (TPR) with hydrogen. Prior to the reduction experiments, the catalysts were calcined at
5508C in air for 2 h in order to oxidize the iron at the surfaces to form iron(III) oxide Fe2O3. The temperature programmed reduction of Fe2O3with hydrogen proceeds either in two steps via Fe3O4for temperatures below 5708C or in three steps via Fe3O4 and FeO for temperatures above 6008C[40–44]. The results on the samples of this paper are depicted inFig. 6. The first peak in the diagram of the wet impregnation sample grows in around 4808C and has been assigned to the reduction Fe2O3!Fe3O4. At 6208C Fe3O4 builds FeO which is completely reduced at 7308C and metallic iron is formed again. The MOCVD sample behaves dramatically different.
Reduction begins already around 3808C and finishes just over 4808C.
These results give clear evidence of a much higher chemical reactivity of the iron deposited by MOCVD. We relate the contrasting complex reduction behavior of the wet impregnated sample to the dominating big iron cluster aggregates, which are believed to generate even bigger oxidic particles upon calcination. This material cannot be reduced as easily and at the same temperature then the smaller structures of the CVD material.
Another relevant chemical test for the relative reactivity of transition metal catalysts is the carbon monoxide adsorption experiment [44]. Prior to CO adsorption, the samples were reduced (H2/TPR) by hydrogen at 6008C for the MOCVD samplesðwFe¼0:49%Þand at 8008C for the wet impregnated samples ðwFe¼0:57%Þ. The CO pulse chemical adsorption experiments were performed at 358C with five consecutive equivolumetric CO pulses per catalyst sample. In both cases, the majority of the CO adsorption occurs during the first CO pulse, saturation is reached with the second one (Fig. 7). As the iron mass fraction of the MOCVD sample was a little smaller than that of the wet impregnated one, a smaller saturation value was assumed as well. In contrast to that consideration, the
Fig. 5. SEM micrographs of iron coated alumina particles: (a) and (b) MOCVD sample (Treactor= 2008C;wFe¼0:88%); (c) and (d) wet impregnated sample ðwFe¼0:57%Þ.
Fig. 6. Temperature programmed reduction (TPR) of MOCVD (Treac- tor= 2008C;wFe¼0:88%) and wet impregnatedðwFe¼0:73%Þiron coated alumina particles.
MOCVD sample adsorbed significantly more CO. If calculated on a molar basis for iron and CO, the adsorption activity of the MOCVD sample exceeds that of the wet impregnated one by 40% (Table 1).
4. Conclusion
The metal organic complexes ([(1,3-butadiene)(tolue- ne)Fe(0)] (1) and [(1,3-cyclohexadiene)(toluene)Fe(0)] (2)) have been shown to be excellent new MOCVD precursors for the surface deposition of catalytically active metallic iron on alumina powder as a mechanically robust and chemically indifferent oxidic support. Their vapor pressure curves, the temperature ranges of evaporation without decomposition and their iron deposition parameters in a fluidized bed reactor were determined experimentally. Helium as a carrier gas can be saturated without losses with the vapors of1at 708C and with2 at 808C. A bed temperature of 2008C leads to a complete conversion of both precursor complexes into elemental iron. If necessary, the fluidized bed temperature can be lowered to ca.
1508C, to allow an iron deposition on lesser robust supports.
The MOCVD deposited iron layer on alumina forms cubic nanocrystals with a mean size of 75 nm which are homo- geneously distributed on the powder surface. The reactivity with respect to temperature programmed reduction and CO adsorption of the MOCVD prepared iron layers is superior over samples, which have been prepared by a classical wet impregnation technique. This work qualifies the toluene iron complexes 1 and 2 as highly competitive novel MOCVD precursors.
Acknowledgements
Generous financial support of both research groups by the Deutsche Forschungsgemeinschaft DFG through the program SPP 1119 is gratefully acknowledged.
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Fig. 7. CO pulse chemical adsorption on iron coated alumina particles: (a) MOCVD deposited ironðwFe¼0:49%Þand (b) wet impregnated sampleðwFe¼0:57%Þ.
Table 1
CO adsorption on iron coated alumina particles
Preparation MOCVD Wet impregnation
Catalyst (mg) 142.9 146.2
wFe;ICPð%Þ 0.49 0.57
nFe,total(mol) 1.25105 1.49105
nCO,ads.(mol) 1.43106 1.25106
nCO,ads./nFe,total 0.114 0.083
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