Adsorption and Polymerization of Formaldehyde on Cu(100)

Abstract

The adsorption of formaldehyde (H2CO) on clean Cu(100) at 85 K has been studied using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For coverages up to (1.06 ± 0.22) × 1015 H2CO molecules/cm2, formaldehyde spontaneously polymerized to form a monolayer of disordered poly(oxymethylene) (POM), arranged with the chain directions parallel to the surface plane. Thermal decomposition/desorption of the polymer monolayer occurred by three routes, producing peaks in temperature-programmed desorption (TPD) at approximately 177, 200, and 215 K. The lowest temperature peak was exclusively associated with production of H2 and CO in approximately equal proportions. The two higher temperature peaks were produced by molecular H2CO generated via depolymerization of the polymer. The 200 and 215 K features displayed zero- and first-order desorption kinetics, corresponding to estimated activation energies for depolymerization of 75 ± 10 and 53.9 ± 0.5 kJ/mol, respectively. The presence of two polymer desorption peaks is attributed to chain conformational differences present within the monolayer and has not been previously observed in studies of formaldehyde adsorption on metal surfaces. Large exposures of H2CO on this surface formed multilayers of molecular formaldehyde on top of the first polymer layer. The second layer desorbed at 105 K and subsequent layers at ∼100 K.