The interaction of ammonia with transition metal carbide surfaces

The interaction of ammonia (NH3) with TiC(1 0 0) and VC(1 0 0) surfaces has been investigated over the temperature range of 100-400 K. Temperature programmed desorption (TPD) has been utilized to determine the desorption states of NH3. High-resolution electron energy loss spectroscopy (HREELS) has probed the adsorption states and adsorption geometry of NH3 on these substrates. Ultraviolet photoelectron spectroscopy (UPS) has been used to examine the valence electronic structure of NH3 interacting with the surface. Ammonia exhibits a similar interaction for both TiC and VC as was expected for the sigma-donor adsorbate. The predominant monolayer TPD feature appears at the same temperature for both substrates. HREELS results show very similar vibrational spectra including an identical blue shift of the umbrella mode on both surfaces as the temperature is increased. Finally, UPS reveals the position of the NH3 3a(1) molecular orbital is identical upon adsorption on both TiC and VC. These experimental results are consistent with results obtained from density functional theory calculations and they are explained in terms of electronic structure effects based on molecular orbital theory. (C) 2002 Elsevier Science B.V. All rights reserved.