Microscopic View of the Active Sites for Selective Dehydrogenation of Formic Acid on Cu(111)

Formic acid is an important molecule, due to its potential for hydrogen storage and the role of formate in methanol synthesis. Formic acid can decompose on metals and oxides via dehydrogenation or dehydration, although dehydrogenation is preferred for most applications. These two pathways are linked via the water gas shift reaction (WGSR), making them hard to separate, and debate over the mechanisms still exists. Cu catalysts are known to selectively decompose formic acid via dehydrogenation to produce CO2 and H-2. Formic acid's interaction with Cu(110) has been extensively studied, but despite the (111) facet being predominant in many nanoparticles, Cu(111) has received little attention. Using temperature-programmed desorption/reaction (TPD/R) and scanning tunneling microscopy (STM), we have probed key steps in the decomposition of formic acid on Cu(111) at the atomic scale, observing intact adsorption and surface intermediates, as well as the surface after product desorption. Our model system allows us to investigate the reaction under conditions where WGSR is inactive. We find that Cu(111) decomposes formic acid 100% selectively through dehydrogenation. At 85 K, formic acid adsorbs molecularly on Cu(I11), forming hydrogen-bonded chains in the fi configuration. The acid loses a H atom by 160 K, producing the formate intermediate and surface-bound H atoms, both of which are visualized by STM. All molecules at surface step edges react to formate, but on the Cu(111) terraces desorption of formic acid competes with formate production, which limits formate production to 0.05 monolayer. H atoms formed by O-H bond cleavage recombine to form H-2 in a desorption rate limited process by 360 K CO2 and H-2 desorb from the surface in reaction rate limited processes at 400 and 450 K due to formate decomposition on terraces and steps, respectively.