A combined STM molecular beam study of formic acid oxidation on Cu(110)

In this paper, we describe a powerful combination of methods used to elucidate the mechanisms and kinetics of surface reactions, namely, molecular beam reactor work and scanning tunnelling microscopy (STM). These two combine microkinetic measurements with a technique which has atomic spatial resolution for structural determination during the course of a reaction. This gives the ability to link the worlds of macroscale and nanoscale surface chemistry. A particular application is highlighted here, namely, the adsorption and oxidation of formic acid on Cu(110), in principle, a simple reaction, which turns out to be very complicated. The findings can be summarised as follows: (i) predosed oxygen enhances formic acid sticking by at least an order of magnitude; (ii) oxygen predosed to a coverage below 0.25 monolayers is completely removed as water by the reaction with formic acid to produce formate; (iii) above this oxygen coverage, some oxygen remains on the surface after reaction and is compressed up into the locally higher coverage c(6 x 2) structure, otherwise only obtained by extremely high exposures of oxygen; (iv) the reaction stoichiometry changes from 2 HCOOH: 1 O-(a) to 1:1 at high temperature where the formate is unstable; (v) gross rearrangement and redistribution of Cu atoms occurs during the course of the reaction; and (vi) the formate adsorbate structure varies considerably with substrate temperature, oxygen precoverage and formic acid exposure, five distinct structures being observed with STM. (C) 1998 Elsevier Science B.V.