CONTROLLING SURFACE REACTIVITIES OF TRANSITION-METALS BY CARBIDE FORMATION

The surface reactivities of transition metals, including the Groups 4- 6 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overla yer. More importantly, the reactivities of carbide-modified surfaces f requently demonstrate strong similarities to those of the Pt-group (Pt , Pd, Ir, Rh, Ru and Os) metals. In this paper, we will summarize our recent surface science investigations of the electronic, structural, a nd catalytic properties of well-characterized model carbide systems. U sing several characteristic surface probing reactions, we will provide experimental evidence for the similar reactivities of the carbide-mod ified surfaces and the Pt-group metals. We will also discuss the under lying structural and electronic properties that are controlling the re activities of the carbide-modified surfaces, such as the binding sites and chemical nature of the carbon atoms, the ionicity of the metal-ca rbon bonds, and the band structure of the transition metal carbides. W e will also show examples of experimental attempts to correlate findin gs from the single crystal model carbide systems to amorphous powder c arbide catalysts. (C) 1998 Elsevier Science B.V.