The adsorption and decomposition of formic acid on Cu{110}

The reactive adsorption of formic acid (HCOOH) on oxygen dosed Cu(110) has been studied using a molecular beam system, TPD, LEED and STM. At low temperature the reaction is strongly oxygen coverage dependent. All coverages result in high reaction probability (0.8 at room temperature) for formic acid and, for less than 0.25 monolayers of oxygen there is complete oxygen clean-off, leaving formate on the surface in a c(2 X 2) structure, At higher coverages the situation is more complex, with some oxygen remaining coadsorbed with the formate. The two adsorbates are then mainly phase separated into islands of c(6 X 2) oxygen and (3 X 1) formate, The two phases mutually compress each other due to pressure at the phase boundaries. The reaction stoichiometry is 2:1 formic acid:oxygen atoms in this temperature range. At higher temperatures (> 450 K) the formate itself is unstable and decomposes during adsorption which results in a change of stoichiometry of the reaction; one molecule of formic acid removes an oxygen atom as water, and hydrogen evolution ceases. There is a range of temperature between 350 and 420 K for which the reaction becomes very difficult, and the reaction probability drops to similar to 0.1. It is proposed that this is due to rapid compression of much of the oxygen adlayer into the unreactive c(6 x 2) structure by small amounts of formate. The reaction proceeds through a highly mobile, weakly held, ''precursor'' state on the surface, which is able to seek out the active sites on the surface, which are low in coverage at high levels of oxygen, These active sites are the terminal oxygen atoms in the oxygen islands (in the [001] direction), which are only present at step edges or phase boundaries at 0.5 monolayers coverage of oxygen.