Co-adsorption of Formic Acid and Potassium on Cu{100}: A Temperature Programmed Desorption Study

The adsorption and decomposition of formic acid (HCOOH) co-adsorbed with K on Cu{100} has been studied by the spectroscopic technique of temperature programmed desorption (TPD). For the adsorption of K on Cu{100}, the activation energy for desorption falls drastically with increasing coverage from 214 kJ.mol-1 at the low coverage of 0.01 ML (ML=monolayer) to 139 kJ.mole-1 at close to completion of the first potassium layer (0.35 ML). The pre-adsorption of Cu{100} by a saturation coverage ( 10 ML) of HCOOH instigates a different adsorption-desorption behaviour compared to the K/Cu{100}. Below and at the low K coverage of 0.009 ML on HCOOH pre-adsorbed on Cu{100}, a destabilization of the predominantly strong polar covalent Cu-K bonding occurs indicated by the significant decrease in the desorption peak temperature (Tp) from 800K to 560K. At a coverage of 0.02 ML, the peak temperature is found to split to lower values pertaining the original high Tp. This behaviour can be explained by the formation of K-HCOOH compounds that desorb at different Tp values. At higher K coverages on pure Cu{100}, a weaker Cu-K bonding takes place and a metallic-like potassium behaviour emerges which is attributed to the presence of lateral K-K interactions. At higher K coverages ( 0.025 ML) on the Cu{100} pre-adsorbed with HCOOH, a very complex TPD spectrum is obtained. A complex range of K-HCOOH reaction products might be the only way to explain such a spectrum.