Synthesis Of Carbon-coated Alumina By Pyrolysis
Synthesis of Carbon-coated Alumina by Pyrolysis
of Adsorbed Acetylacetone
L.F. Sharanda1, Yu.V. Plyuto1, I.V. Babich1,5, I.V. Plyuto2, A.P. Shpak2, Ya.A. Babich3, J.Stoch4, J.A. Moulijn5
1Institute of Surface Chemistry, Ukrainian National Academy of Sciences, General Naumov st. 17, 03164 Kyiv, Ukraine
2Institute of Metal Physics, Ukrainian National Academy of Sciences, Vernadsky avenue 36, 03680 Kiev, Ukraine
3Institute of Colloid Chemistry and Chemistry of Water, Ukrainian National Academy of Sciences, Vernadsky avenue 42, 03680 Kyiv, Ukraine
4Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,
ul. Niezapominajek 8, 30-239 Cracow, Poland
5Department DeltChemTerch, Delft University of Technology,
Julianalaan 136, 2628 BL Delft, The Netherlands
Carbon-coated silica and alumina supports are promising for application in catalysis and chromatography. Therefore much efforts are being made in order to achieve a desired combination of physico-chemical properties of both components in novel material.
The work aims at the development of a novel method for coating of alumina supports with carbon and physico-chemical characterisation of the designed materials. Carbon coating was synthesised by adsorption of acetylacetone on the alumina support (CK 300, Ketjen) and its subsequent pyrolysis at 700° C within a period of 5-6 h under continuous evacuation. To synthesise the samples with an increased carbon content, the adsorption-pyrolysis cycle was repeated up to three times. The above mentioned synthetic route resulted in the samples with carbon content of 2.6, 4.6 and 6.3 C wt. %. The structure of carbon coating was characterised by IR, XRD, TG/DTG-DTA, TEM, XPS, electrophoretic mobility and adsorption measurements.
IR spectroscopy showed a strong interaction of acetylacetone with the co-ordinatively unsaturated Al3+ sites of the initial alumina support. The distinct IR bands at 1550, 1535 and especially 1295 cm-1, which are typical for surface aluminium acetylacetonate complexes, are observed. It has been found
that the carbon coating on the alumina support completely blocks or shields the co-ordinatively unsaturated Al3+ sites.
The shape of the nitrogen adsorption-desorption isotherms and the hysteresis loops were found to be similar for all samples. The pore volume and the apparent surface area of all carbon-coated samples appeared to be somewhat less than those of the initial alumina support. Moreover, a pore mouth plugging did not occur and carbon appeared to be uniformly distributed over the surface of the alumina support.
The adsorption isotherms of hexane and water were measured in order to evaluate the influence of the carbon coating on hydrophilicity of the initial alumina support. At p/po of 0.05, the hexane-to-water adsorption ratio increases from 1 for the initial alumina to 1.57 for sample C(6.3)/Al2O3. This means a progressive decrease in hydrophilicity of carbon coated samples and hence an increase in the alumina coverage with carbon upon increasing carbon content.
Characterisation of the synthesised samples using XRD did not reveal any peaks which should be attributed to ordered carbon structures. Therefore, the presence of either an amorphous carbon coating or a thin graphitic layer can be assumed. Despite of carbon content, the thermoanalytical characterisation of the synthesised samples in air exhibits the intense weight loss in DTG patterns around 500° C which coincides with the exothermic peak in DTA curves. Oxidation proceeds in one step in a narrow temperature interval. Besides, no separate carbon phase is observed in TEM images of the synthesised samples. Therefore, one may suppose that carbon uniformly coats the alumina surface.
It has been found that the ZPC value decreases with the increase of carbon content in carbon coated alumina samples approaching that of bulk carbon. The obtained results are in a good agreement with the data of XRD and TEM characterisation of carbon coated alumina support and confirm the high dispersion of carbon in the support surface layer.
XPS was used for identification of the structure and for monitoring the distribution of the carbon coating on an alumina surface. Due to a difference in surface steady-state charging of the samples, it appeared possible to discriminate the carbon coating and carbonaceous deposits on the insulating alumina support. Carbonaceous surface species of carboxylate type were identified by detailed analysis of individual components in the experimental C1s envelope. The surface C/Al ratio for the synthesised samples was calculated from the integral intensity of the C1s component related to the carbon coating and the overall Al2p envelope. It has been found that at low loading carbon is preferably concentrated at the internal surface of the alumina support. On increasing the carbon loading, the external surface of the alumina support appeared to be enriched with carbon.
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