Adsorption and reaction of nitromethane (CH₃NO₂) on Pt(111)

The adsorption of nitromethane (CH₃NO₂ and CD₃NO₂) on Pt(111) surfaces has been investigated. Temperature programmed desorption (TPD) mass spectrometry, ultraviolet photoelectron spectroscopy (UPS) and high resolution electron energy loss spectroscopy (HREELS) were used to probe the adsorption bond strength and bonding geometry of the chemisorbed state, and also thermal decomposition reactions on the Pt(111) surface, Nitromethane is weakly chemisorbed on Pt(111) with an adsorption energy of ca 11 kcal mol^-1. Adsorption is mostly (>75%) reversible, but some nitromethane decomposition on Pt(111) does occur at low coverages. HCN, H₂O, NO and CO were the primary gas phase products that were detected in TPD as a result of this decomposition. The principal decomposition pathway is via cleavage of N-O bonds, mostly leaving the C-N bond intact, rather than the isomerization of nitromethane to methyl nitrite (CH₃ONO) on the surface. Consistent with the small adsorption energy of molecular nitromethane, UPS and HREELS of the chemisorbed monolayer show only small changes in the electronic structure and vibrational spectra of nitromethane compared to condensed phase results. HREELS data indicate that chemisorbed nitromethane adopts a 'pseudo-bidentate' bonding geometry with C_s(yz) symmetry in which the two oxygens are coordinated inequivalently to the surface with the molecular axis tilted within the molecular plane.