Coadsorption of Formic Acid and Hydrazine on Cu(110) Single-Crystal Surfaces

The chemistry of coadsorbed formic acid and hydrazine on Cu(110) surfaces was characterized both experimentally, by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS), and theoretically, via density functional theory (DFT) calculations. It was found that the two reactants interact with each other via hydrogen bonds and that this modifies their individual thermal chemistry on the surface in two main ways: by stabilizing a HCOOH:N₂H₄ adduct, which desorbs molecularly at around 240 K, and by slightly delaying the decomposition of the hydrazine to higher temperatures and shifting the selectivity of that step from dehydrogenation and formation of N₂Hₓ(ads) species to scission of the N–N bond and ammonia production. The coadsorbed formic acid was determined to react at higher temperatures than hydrazine, in chemistry not affected by the latter, which is no longer present on the surface at that stage. One interesting aspect of this chemistry revealed by the DFT calculations is that formic acid may preferentially H-bond on top of adsorbed hydrazine rather than directly attach to the copper surface. The implications of these results to atomic layer deposition (ALD) processes are discussed.