Nitromethane and Methyl Nitrite Adsorption on Au(111) Surfaces

Abstract

The adsorption of nitromethane (CH3NO2 and CD3NO2) and d 3-methyl nitrite (CD3ONO) on Au(111) was studied by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). These molecules are only weakly adsorbed on Au(111), and adsorption is completely reversible in both cases. Adsorbed CH3NO2 and CD3ONO in the monolayer each give rise to one thermal desorption peak in TPD with desorption activation energies of 10.5 and 8 kcal/mol, respectively. These desorption energies are close to the values for the heats of adsorption of these molecules, since there is no appreciable activation energy for molecular adsorption. HREELS confirms weak, molecular adsorption in the monolayer for both molecules. Furthermore, nitromethane is bonded on Au(111) in an upright, strongly tilted geometry, suggesting a monodentate coordination to the surface. Methyl nitrite adsorbs on Au(111) with the ONO group in a flat-lying geometry, with evidence for both cis and trans forms. The Au(111) surface does not sufficiently activate nitromethane and methyl nitrite for dissociation or isomerization (CH3NO2 ↔ CH3ONO) to occur under UHV conditions. Thus the activation energies for dissociation and isomerization of nitromethane on Au(111) exceed 10.5 kcal/mol.

The Adsorption of Nitromethane on the Au (111) Surface

“…The geometric parameters for this structure are also reported in Table 4 and part of them reproduced in Figure 4. The structure found in the present calculations is in good agreement with the predictions from the experimental work of Wang et al [ 16]. The high-resolution electron energy loss spectroscopy (HREELS) results ruled out bidentate adsorption forms of the nitromethane molecule on the Au(111) surface.…”

“…The structure of nitromethane is analogous to the structure of the adsorbed formate anion (HCOO -) which is known, from theoretical [18] and experimental work [35], to adsorb on single metal surfaces by its oxygen atoms. Thus, following the previous theoretical work on the adsorption of formate on low index copper surfaces [18] and based also on experimental information [16], two different starting geometries were idealized, see Figure 3. In a first part of our calculations, the plane of the molecule was kept perpendicular to the metal surface, following the previous results for formate.…”

“…Therefore, the major decomposition path is governed by the complete dissociation of the C-N and N-O bonds leaving on the surface N and O atoms combined with surface CH x species. On the gold surface the TPD spectra shows one single peak due to desorption of CH 3 NO 2 and no other peaks associated with the other smaller molecules referred above were observed [16]. Nitromethane is proposed to adsorb on these metal surfaces in a monodentate geometry, with only one oxygen atom approaching the surface and the C-N axis tilted away from the normal to the surface and the adsorption energy is predicted to be close to 10 kcal/mol [15,16].…”

“…Only but a few studies concerning the adsorption of nitromethane on transition metal surfaces can be found in the literature [12][13][14][15][16][17]. Namely, there are several studies concerning the interaction of the simplest nitroalkane with the Ni(111) [12], Rh(111) [13], Pt(111) [14,15] and Au(111) [16] singlemetal surfaces and also with the Pt-Sn alloy [17].…”

“…Namely, there are several studies concerning the interaction of the simplest nitroalkane with the Ni(111) [12], Rh(111) [13], Pt(111) [14,15] and Au(111) [16] singlemetal surfaces and also with the Pt-Sn alloy [17]. From these experimental studies it is concluded that nitromethane is weakly adsorbed and that it desorbs in large quantities upon heating.…”