CH₃Br Structures on Pt(111):  Kinetically Controlled Self-Assembly of Weakly Adsorbed Dipolar Molecules

Abstract

Methyl bromide is a rodlike molecule with a large 1.8 D dipole moment that physisorbs on Pt(111) with its Br end preferentially bound to the surface. The kinetically controlled self-assembly of several ordered CH3Br structures following molecular adsorption at a surface temperature of 20 K and various annealing procedures is revealed by scanning tunneling microscopy (STM). Thermal programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) provide complementary evidence for kinetic control over the CH3Br monolayer structures at temperatures under ∼100 K. A well-ordered (6 × 3) CH3Br monolayer can be formed by dosing CH3Br multilayers at 20 K and slowly annealing the surface to 104 K over several minutes. The unit cell contains four CH3Br molecules assigned to adsorption at Pt(111) top sites and 3-fold hollow sites in equal measure. The saturation coverage is 0.22 ML with respect to the Pt(111) areal density. RAIRS shows that CH3Br within the (6 × 3) monolayer is aligned along the surface normal while at lower coverages some molecules are tilted. The RAIRS signature of the ordered (6 × 3) CH3Br monolayer is the disappearance of the ν5 asymmetric CH3 deformational mode near 1411 cm-1 and the adsorption-site-induced splitting of the ν2 symmetric CH3 deformational mode into a doublet at 1271 and 1277 cm-1 with full width at half maximums (fwhm's) of just 3 cm-1. Other ordered structures and molecular aggregates were observed by STM at submonolayer coverages following particular annealing procedures even though TPD shows evidence for only repulsive molecular interactions. The range of structures observed for the relatively simple CH3Br/ Pt(111) system suggest it may be a useful proving ground for theoretical treatments of the kinetics of self-assembly of dipolar molecules at surfaces.

Spatial proton exchange membrane fuel cell performance under bromomethane poisoning

“…Studies of CH 3 Br adsorption on Pt (111) by reflection absorption infrared, thermal programmed desorption spectroscopies, and scanning tunneling microscopy demonstrated that at low coverages, bromomethane occupies only the top sites while with an increase in coverage, less stable multicoordinate sites start to be taken [31,33]. The preferred orientation of CH 3 Br at the saturation coverage of the 0.22-0.25 monolayer (ML) was aligned along the surface normal [31,33,92], and CH 3 Br bound to Pt through Br with a symmetryof less than C 3v [93]. The adsorption energy of CH 3 Br decreases monotonically from 0.62 eV at the 0.12 ML to 0.33 eV at the 0.22 ML [31].…”

“…Its hydrolytic half-life ranges between 20 and 38 days depending on the temperature and pH [28][29][30]. There are a few studies devoted to the adsorption of bromomethane on Pt [31][32][33], while there isa lack of studies on the electrochemical behavior of CH 3 Br in aqueous solutions. However,the adsorption of related species such as Br on Pt and their impactson hydrogen oxidation and oxygen reduction were previously investigated and published [34][35][36][37][38][39][40][41][42][43][44][45][46].…”

Bromomethane Contamination in the Cathode of Proton Exchange Membrane Fuel Cells

“…Under low temperature, low coverages of physisorbed bromomethane tend to lie on the Pt(111) surface [20]. When bromomethane adsorbs on H-covered Pt(111), the desorbed products include CH 4 and HBr [21].…”