Pd/Si3N4 catalysts: preparation, characterization and catalytic activity for the methane oxidation

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Abstract

Silicon nitride (Si3N4) is a refractory material showing high thermal conductivity at high temperature. We can therefore expect very good stability for both the catalytic phase and the support when using it as a support of active phases, even for very exothermic reactions occurring at high temperature.

Pd supported on silicon nitride catalysts were prepared by impregnation of Si3N4 with Pd(II) acetylacetonate. The catalytic precursors were decomposed under oxygen at 350°C and reduced under hydrogen at 300°C; they were tested for the methane total oxidation reaction. Such catalysts were found to be highly stable, even after ageing at temperatures higher than 800°C under the reactive (oxidizing) mixture.

Introduction

Materials generally used as supports of catalytically active phases are insulators such as alumina (Al2O3), silica (SiO2), and silica–alumina (SiO2–Al2O3). The use of other ceramics such as magnesia (MgO), ceria (CeO2), zirconia (ZrO2) or titanium oxide (TiO2) is much less extensive. These supports show more or less good stability at high temperatures and behave like thermal insulators. In order to improve the stability of catalysts, other refractory materials having high thermal stability and high thermal conductivity can be tentatively employed as supports of catalytically active phases.

Up to now, non-oxide refractory materials have not been used extensively as supports of catalytically active phases. Studies in this direction concern silicon carbide mainly. Transition metals supported on SiC have been tested in the depollution of exhaust gases 1, 2, 3, 4, in carbon monoxide hydrogenation [5]or for methane total oxidation [6]. Some other works mention the activity of molybdenum oxide supported on silicon carbide for some isomerization reactions 4, 7. Nevertheless, SiC is partially oxidized when heated at high temperatures in the presence of water and/or oxygen [6].

Among the ceramics, silicon nitride (Si3N4) is known for its very good mechanical properties and its rather high thermal conductivity even at high temperature. In addition, it shows very low chemical reactivity. Up till now, no relevant studies have been undertaken on the benefit of the use of silicon nitride as the catalyst support. The presence of silicon nitride has only been reported in some patents, but always only as a part of a complex material containing many compounds. Therefore, the stability of metal catalysts deposited on silicon nitride, compared to SiC and some more widely used oxides, will be the test for its future development as a catalyst support.

In this work, we present a way of preparation of Pd supported on Si3N4 catalysts together with their characterization by X-ray diffraction, transmission electron microscopy (TEM) and surface analysis by X-ray photoelectron spectroscopy (XPS). Catalytic properties of such catalysts with respect to methane oxidation is reported, with Pd being considered as a good oxidation catalyst. It is noteworthy that such an oxidation reactionCH4+O2⇒CO2+H2OΔH0298=−802kJmol−1is highly exothermic and occurs at rather high temperatures in an oxygen atmosphere. These demanding conditions are relevant for a valuable test of these unconventional supports which, owing to their high thermal conductivity even at high temperatures, are able to evacuate the heat created by the oxidation reaction. Indeed, the thermal conductivity of silicon nitride at 1000°C is about 20 W m−1 K−1 instead of about 1–2 and 9 W m−1 K−1 for SiO2 and Al2O3, respectively. Moreover, silicon nitride does not decompose with NH3 production in the presence of water, unlike other nitrides such as AIN.

The results will be discussed in terms of the reactivity and thermal stability of Pd on Si3N4 after ageing at high temperatures. A brief comparison will be attempted between palladium on silicon nitride, α-alumina and silicon carbide, the last system having previously been studied [6].

Section snippets

The supports

The silicon nitride support used in this work was purchased from Goodfellow. It was in powder form, non-porous with a stead mean particle diameter of 1 μm. The measured BET surface area was 9 m2 g−1.

For comparison, an α-alumina produced by Rhône Poulenc, which has about the same BET surface area (10.5 m2 g−1) as the Goodfellow silicon nitride was also investigated.

Preparation of the catalysts

The almost total absence of chemical anchoring sites (like OH groups) on silicon nitride does not allow us to use an exchange method for

Results

The X-ray diffraction pattern of the silicon nitride support is presented in Fig. 1. It shows very narrow X-ray lines.

Two main peaks are dominant in the XPS spectrum of the silicon nitride: the Si 2p and N 1s core levels (Fig. 2). The binding energy positions of the Si 2p and N 1s peaks are 101.9 and 397.6 eV, respectively, as expected for Si and N atoms in Si3N4 [10]. Besides Si and N, only a small amount of oxygen is detected, evidenced by the presence of an O 1s peak at 532.4 eV. The [O]/[Si]

Discussion

The very narrow X-ray lines of the silicon nitride support are characteristic of a very well crystallized material. The main lines correspond to the low temperature (<1200°C) α-phase, with a hexagonal structure having the following parameters: a=b=0.7752 nm, c=0.562 nm. Some β-phase is also detected, but its concentration does not exceed 5%. Indeed, silicon nitride exhibits two crystallographic forms: the low temperature (<1200°C) α-Si3N4 phase and the high temperature (>1500°C) β-Si3N4 phase.

Conclusion

In conclusion, Pd/α-Si3N4 catalysts prepared by impregnation and decomposition of Pd bis-acetylacetonate are active catalysts for the methane oxidation. They do not deactivate notably after ageing at 800°C. Silicon nitride can be therefore considered as a new and very stable support for catalysis at elevated temperature in oxidizing conditions [23]. The good stability of the Pd/α-Si3N4 catalysts is firstly explained by the total absence of oxidation of the silicon nitride support which would

Acknowledgements

The authors wish to thank the LACE laboratory for the use of catalytic tests and Michèle Brun for her experimental assistance for XPS measurements.

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