The mechanical properties of Si3N, based ceramic bodies are largely controlled by the phases formed on the grain boundaries during sintering. For this reason determination of the state and the chemical composition of the surface of the starting Si,N, powder is of prime theoretical and practical importance, especially in the cases when advanced ultrafine powders are used. In this work an ultrafine Si,N, powder, obtained by high temperature plasma nitridation of silicon, has been characterized by XPS. Oxygen, carbon and also a small amount of potassium were detected as surface impurities. The Si 2p and Si (KLL) spectral lines could be decomposed into two components corresponding to silicon in Si,N, and SiO, phases. From angle-resolved and Ar-ion depth-profiling experiments a layer model, consisting of a Si3N, core covered by a relatively thick SiO, layer and a carbon contaminant overlayer, could be elucidated.

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In the vapor phase, the selectivity of the title reaction could be increased to about 90 % by promoting traditional type nickel catalysts with NaOH. The use of strongly basic lanthana as support resulted in 95-99 % selectivity.

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