Thermogravimetry–FTIR study of the surface formate decomposition on Cu, CuCl, Cu2O and CuO. Correlations between reaction selectivity and structural properties

Abstract

In this work three types of the surface formate, ionic, monodentate and bidentate species, are identified to be the main products of formic acid adsorption on Cu, CuCl, Cu₂O and CuO at room temperature. Combined thermal analysis [differential thermal analysis (DTA), thermogravimetry (TG) and evolved gas analysis (EGA)] and FTIR studies have shown that the decomposition selectivity and activity are highly dependent on the electronic and structural properties of the surface formate. The ionic formate which gives the characteristic infrared bands at 1600 and 1320 cm^–1 is found to exist on all the surfaces studied in the lower temperature range (<490 K) and is easily (with an activation energy of 5–19 kcal mol^–1) decomposed at elevated temperatures, forming water as the main product. Bidentate formate with infrared bands at 1630 and 1350 cm^–1 is observed on the Cu, Cu₂O and CuO surfaces and is decomposed at high temperatures (around 500K) with an activation energy of 15–39 kcal mol^–1, producing mainly carbon dioxide. The adsorbed formate on CuCl at high temperatures demonstrates only an asymmetric OCO band at 1610 cm^–1 and is considered to be the monodentate formate, whose pyrolysis favours the production of water, with an observed activation energy of 19 kcal mol^–1. The reaction scheme and the origin of the structural dependence of the surface formate decomposition are discussed.