SELECTIVE ACTIVATION OF C-H AND C=C BONDS ON METAL CARBIDES - A COMPARISON OF REACTIONS OF N-BUTANE AND 1,3-BUTADIENE ON VANADIUM CARBIDE FILMS ON V(110)

We have investigated the adsorption and decomposition of n-butane and 1,3-butadiene on clean and carbide-modified vanadium (110) surfaces. By using high-resolution electron energy loss spectroscopy and thermal desorption spectrometry, we observe that the formation of carbide significantly modifies the reactivities of vanadium. The 1,3-butadiene molecules interact strongly with clean V(110) via the interaction between the d-band of vanadium and the pi orbitals of the adsorbates; the interaction is much weaker on the carbide-modified surfaces. On the other hand, n-butane interacts very weakly and reversibly with the clean surface; the reactivity is enhanced on carbide-modified surfaces. These experimental results are compared to the existing theories on the activation of C-H bonds of alkanes and C=C bonds of unsaturated hydrocarbons on transition metals. Such a comparison indicates that, although the reactivities of clean vanadium agree very well with the theoretical predictions for early transition metals, the properties of carbide-modified surfaces are more similar to those of Group VIIIB metals. (C) 1995 Academic Press, Inc.