Adsorption of ammonia and methylamines on alumina and copper/alumina studied by dynamic fourier-transform infrared experiments

Abstract

The adsorption of ammonia, monomethylamine, dimethylamine and trimethylamine, on γ-alumina and γ-alumina-supported copper, was investigated using temperature-programmed desorption in the range 300–540 K, combined with time-resolved FTIR spectroscopy. The spectra indicate the simultaneous presence of several surface species. The bands were grouped and assigned to individual molecules by studying the desorption dynamics in either nitrogen or hydrogen/nitrogen carrier gas. Assignments were made for physisorbed molecules as well as for species adsorbed on Lewis-acid sites, Brønsted-acid sites, and species coordinated to copper metal. For all methylamines, species coordinated to oxidized copper were detected. Disproportionation of the methylamines was observed at temperatures above ca. 440 K. At elevated temperatures imine, formate and azide surface species were observed. Trimethylamine was found to be a useful probe for Brønsted- and Lewis-acid sites on surfaces. The role of the observed species for the reaction pathway of the disproportionation of methylamines is discussed.