Elsevier

Thin Solid Films

Volume 515, Issue 16, 4 June 2007, Pages 6590-6595
Thin Solid Films

Thin cobalt oxide films for catalysis deposited by plasma-enhanced metal–organic chemical vapor deposition

https://doi.org/10.1016/j.tsf.2006.11.056Get rights and content

Abstract

The plasma-enhanced metal–organic chemical vapor deposition was used to prepare thin films of cobalt oxide starting with cyclopentadienyldicarbonyl–cobalt(I) (CpCo(CO)2) mixed with argon and oxygen. The films were characterized by Raman and Fourier transform infrared spectroscopies, electron diffraction, and energy dispersive X-ray microanalysis. Their thickness was estimated by ellipsometric measurements. Catalytic properties of the films were tested in oxidation of n-hexane. It has been found that spinel-type Co3O4 nanoclusters with a crystallite size of 4–6 nm are formed in the deposits. Amorphous carbon and amorphous CoOx phases are also observed in the films. The content of these phases depends on the molar fraction of oxygen in the gas mixture. Preliminary catalytic tests have shown that precalcined Cr–Al steel carrier covered by the plasma-deposited films reveals much higher catalytic effect then the non-deposited substrate.

Introduction

One of the very promising methods for producing catalytic structures is the plasma deposition of very thin films from metalorganic precursors supplied to the plasma reactor as a gaseous phase. This well-known method of thin film deposition, commonly called the plasma polymerization or more precisely speaking in this case, plasma-enhanced metal–organic chemical vapor deposition (PEMOCVD), has been already used to fabricate a lot of thin-film materials for various practical applications [1]. In the late 1980s, a possibility of the use of PEMOCVD for the preparation of potentially catalytic films such as Pd, Rh, Pt was already mentioned [2], [3], [4]; however, the direct involvement of this method in the field of catalysis was rather modest so far and only just recently that it has attracted closer attention. The interest in PEMOCVD is mainly provoked by the search for new effective methods for preparing microchannel structures, in which a catalyst in the form of several-nanometer-clusters should be uniformly dispersed throughout the microstructure. Microchannel reactors have significant advantages over packed bed reactors, including faster heat and mass transfer, which allow for process miniaturization without loss of throughput [5]. It is also important that the application of the cold plasma is especially useful for the preparation of supported nano-sized catalysts, because aggregations caused by thermal treatment might be avoided. For example, as small particles of CoOx in the range of 2–10 nm in diameter were deposited in this way on TiO2 support [6]. If CoOx films were fabricated on a substrate at elevated temperature (150–400 °C), then columnar grains with average diameter size at the film surface of 35–60 nm were formed [7]. Similarly, chemical vapor deposition (without plasma), which originally uses a heated substrate, produces from a mixture of cobalt(II)acetyloacetonate and oxygen, Co3O4 films with a crystalline size of 30–40 nm [8].

A few other materials with potential catalytic properties, such as thin films of Pt, MoOx, TiOx, have been recently prepared with success by the PEMOCVD method [5], [9], [10]. More sophisticated materials, such as, platinum-loaded zirconia films, can also be fabricated using plasma deposition from a mixture of Pt and Zr metalorganic compounds [11].

In many cases the catalytic activity and selectivity of thin films deposited by PEMOCVD are only just the matter under investigation; however, we have already seen that the results are very promising. For example, Karches et al. [10] showed in their exploratory investigations that plasma-deposited thin films of TiO2 (with thickness of 7–120 nm) reveal an efficiency of the photodecomposition of oxalic acid comparable with a commercial catalyst. Koyano et al. [12], in turn, prepared cobalt oxide, using Co(OCOCH3)2 and oxygen plasma, with higher dispersion and higher catalytic activity in the oxidation of CO and propane than those typical of CoOx catalysts prepared by wet-chemical methods.

In our work, we have undertaken an attempt at employing the PEMOCVD method to produce thin CoOx films that could play the role of the catalytic layer in a microstructural reactor for combustion of volatile organic compounds. It is already a recognized fact that Co3O4, especially in nanocrystalline form, is a very active component for hydrocarbons oxidation and combustion [13], [14]. Very recently, the microstructural reactor made of wire gauze has been studied as an alternative to ceramic monoliths [15]. Besides the design problem with “gauze microstructure”, the application of a metallic microconstruction to catalytic reactions requires the development of an efficient method for catalyst layering, which on the one hand, would secure high dispersion and a desired catalyst structure, and would not change the carrier geometry, on the other.

Section snippets

Deposition of the films

The cobalt-based films (25–750 nm thick) were deposited in a parallel-plate plasma reactor (13.56 MHz) using the total pressure of 45–75 Pa and the glow discharge power of 80 W. More details on the reactor are given for example in Tyczkowski et al. [16]. The deposition system can be used both for solid and liquid metalorganic precursors that are supplied to the reactor chamber with a carrier gas. In our case, cyclopentadienyldicarbonyl-cobalt(I) denoted as CpCo(CO)2 (Stream Chemicals, Inc.),

Results and discussion

Investigations of chemical structure of the deposited materials constitute the first step towards an assessment of their potential usefulness in catalysis. In Fig. 1A, a representative Raman spectrum for the first type of films (“pure Ar” plasma) is presented. Two evident regions of bands can be distinguished: the region between 150 and 900 cm 1 characteristic for cobalt oxide structures, on a larger scale shown in Fig. 1B, and the region of bands at 1200–1800 cm 1 assigned to carbon

Conclusion

It has been shown that PEMOCVD is a very useful method to produce thin films containing spinel-type Co3O4 nanoclusters. In our work, the films were plasma-deposited at room temperature starting with a reaction mixture of CpCo(CO)2, argon and oxygen. Investigations performed by Raman spectroscopy and electron diffraction have confirmed that Co3O4 with a crystallite size of 4–6 nm are formed. Apart from Co3O4 nanocrystals, amorphous carbon and amorphous CoOx phases can be created in the films

Acknowledgements

The authors wish to thank Dr M. Kozanecki for his assistance in Raman spectroscopy, Prof. K. Zdunek and Ms. M. Mirkowska for electron diffraction measurements, and Dr A. Kołodziej for the preparation of precalcined Cr–Al steel substrates. This work was partially supported by the KBN project (code 3 T09C 046 27).

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