Chemistry of Glycine on Pd(111): Temperature-Programmed Desorption and X-ray Photoelectron Spectroscopic Study

The surface chemistry of glycine is studied on clean Pd(111) using a combination of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Glycine adsorbs strongly into second and subsequent layers as well as on the first monolayer, where the first-layer coverage is measured by titrating the bare surface with carbon monoxide. A small portion of glycine adsorbed directly on the Pd(111) surface desorbs as intact molecules, whereas the majority thermally decomposes by C−C bond scission. The COO moiety desorbs as CO and CO 2, whereas the nitrogen-containing fragment yields methylamine and HCN. XPS reveals that glycine adsorbs predominantly in its zwitterionic form on the clean surface, whereas the multilayer contains 70−80% zwitterionic glycine, the remainder adsorbing in the neutral form.