Formic acid adsorption and oxidation on Cu(110)

The adsorption and reaction of formic acid with oxygen on Cu(110) has been studied using TPD (temperature programmed desorption) and molecular beam reaction measurements. The reaction proceeds in an acid-base fashion with reaction of the acid proton of formic acid with the basic oxygen at the surface. This generates a formate and surface hydroxyl groups. The hydroxyl in turn is a strong base and can deprotonate another acid molecule to produce a second formate and liberate water into the gas phase at ambient temperature. The reaction then stops because the oxygen is used up and twice as many formates are produced compared with oxygen atoms pre-dosed onto the surface. However, if the reaction is carried out above 400 K, the formate is unstable and decomposes during reaction to yield CO2 and H-2 into the gas phase. If the reaction is carried out above similar to 550 K, then the formate is so unstable that the reaction stoichiometry changes from HCOOH:O = 2:1 to 1: 1, since water is now produced by net reaction between oxygen and both acid and methyne hydrogen atoms, rather than by reaction with two acid protons. These data are analysed in detail using computer-aided kinetic modelling techniques. In order to model the adsorption and reaction successfully a number of factors need to be taken into account. The adsorption is strongly precursor mediated, described by a temperature-dependent Kisliuk precursor parameter. There is a transition in reaction stoichiometry at around 450 K which relates to the rate of hydrogen atom dissociation from the carbon atom of the formate during decomposition, which occurs at sufficient rate only above 400 K. The hydrogen predominantly recombines at 420 K to yield hydrogen molecules in the gas phase, whereas above 500 K it is mainly scavenged by surface hydroxyl groups to make water. The kinetics are successfully modelled using a five-step reaction sequence involving formic acid adsorption, decomposition and product formation (water, hydrogen and carbon dioxide). (C) 2008 Elsevier B.V. All rights reserved.