A vibrational study of borazine adsorbed on Pt(111) and Au(111) surfaces

Abstract

Borazine (B3N3H6) adsorption and reaction on Pt(111) and Au(111) substrates at 100–1000 K has been investigated using thermal desorption mass spectroscopy (TDMS), ultraviolet photoelectron spectroscopy (UPS), and high-resolution electron energy loss spectroscopy (HREELS). Submonolayer coverages of borazine decompose on Pt(111) and only H2 desorption is observed upon heating. Multilayer borazine desorption occurs near 150 K, with an activation energy, Ed = 7.3 kcalmol. The H2 desorption peaks at 300 K are followed by a very broad desorption tail extending to 900 K, which is attributed to dehydrogenation of borazine molecular fragments. In contrast, molecular adsorption of borazine on Au(111) at 110 K is thermally reversible. Molecular borazine desorption from Au(111) occurs at 206 K with Ed = 12 kcalmol. The HREELS data show an unusual vertical orientation of the adsorbed borazine ring on Pt(111) at low temperature (170 K), but indicate a binding geometry for which the plane of the ring is parallel to the Au(111) surface. Possible reasons for the striking orientational difference in adsorption geometry and reactivity on these two metals are discussed.