In the field of heterogeneous catalysis. it is convenient to distinguish. in a perfectly unjustified and over..simplified way. bet: leen metal catalysts. 2nd the other catalysts. The fj.J"st are easy to define: they are those in which a reduced metal is the active phase. It is thus easy to circumscribe. by exclusion, the other class namely the "non-metals." We have adopted this definition for the sake of our colleagues working on catalysis by metals, and to avoid a lengthy title like "sm' face pl"operties and catalysts by transi tion metal oxides. sulftdes, carbides, nitriles, etc. Defined in...