Surface chemistry of alkylamines. 2. Methylamine and trimethylamine on W(100), W(100)-(5 .times. 1)-C and W(100)-(2 .times. 1)-O

Abstract

The adsorption and reaction of mono- and trimethylamine have been investigated on W(100), W(100)-(5 x 1)-C, and W(100)-(2 x 1)-O, using temperature-programmed reaction spectroscopy. Reactivity and selectivity are dependent on the presence of the oxide or carbide overlayers and the degree of substitution of the amine. The activation energy for irreversible C-H bond cleavage in trimethyl- and methylamine was significantly greater on the -(2 x 1)-O and -(5 x 1)-C surfaces compared to W(100). Most notable was the elevated temperature up to where molecular amine desorption is observed on the -(2 x 1)-O: 650 and 600 K for methyl- and trimethylamine, respectively, corresponding to approximate desorption energies of 39 and 35 kcal/mol. The extent of amine substitution also qualitatively alters the product distribution, with CH/sub 4/ produced only from (CH/sub 3/)/sub 3/N and NH/sub 3/ only from (CH/sub 3/)NH/sub 2/ on W(100) and W(100)-(5 x 1)-C> No C-N bond scission products were observed on W(100)-(2 x 1)-O, consistent with the predominance of reversible molecular adsorption without significant decomposition. Possible reaction mechanisms are discussed in terms of competing C-H, N-H, and C-N bond activation.