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

Abstract

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 CO 2 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(111), forming hydrogen-bonded chains in the β 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. CO 2 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.

Adsorbed Formate is the Last Common Intermediate in the Dual-Path Mechanism of the Electrooxidation of Formic Acid

“…Evidence that oxidation of formic acid also proceeds through adsorbed monodentate formate on other metal surfaces has been provided in the case of Au [30][31][40][41] and Pd 42 electrodes, as well as for several metal surfaces at the solid-gas interface. [43][44][45][46][47][48] Cyclic voltammetry and DFT calculations suggested that the stabilization of monodentate adsorbed formate can take place by the presence of adatoms or other adsorbed species on the Pt surface, 21 and a similar effect has been proposed recently for Bi-modified Pd nanoparticles also supported by theoretical calculations. 49 Thus, the mechanism presented here can be considered as general, the differences between different surfaces being due to the ratio between the rate constants for each pathway (as in the case of, e.g., Pd) and/or to a lower CO adsorption energy (as in the case of Au), both of which will depend on the electronic properties of each specific surface.…”

Adsorption and Decomposition of Formic Acid on Cobalt(0001)

“…The observation of formate at this temperature is consistent with this reaction on other cobalt surfaces and other transition metals 33,69 and with the low barrier in the reaction scheme. Formation of the carboxyl species should be expected at the same time as the formate, since the two paths have similar barriers, however the carboxyl species was not observed experimentally either because its C 1s signature could not be resolved (Table S1 lists the calculated peak positions for the C 1s photoelectron for all the fragments) or because it decomposes shortly after being produced via CO formation.…”

Strategic examination of the classical catalysis of formic acid decomposition for intermittent hydrogen production, storage and supply: A review

“…Furthermore, Cu can selectively decompose FA via the dehydrogenation step to give CO 2 and H 2 without CO being produced [58], however, this requires impeding the WGS-reaction step. Other researchers have recorded similar results for temperature programmed reactions of FA on Cu with single crystal surfaces [59][60][61][62], while spectroscopic studies have helped to identify the reason behind this occurrence. However, some researchers have recounted that it is caused by the existence of a stable formate group/intermediate which results from the adsorption of H 2 on the surface of Cu [62].…”