Formic acid is a potential hydrogen storage molecule which dehydrogenates to form CO2 and H2 on metal surfaces. However, it can also decompose via a competing dehydration reaction that forms CO and H2O, reducing the amount of H2 produced and poisoning the catalyst with CO. Formic acid re-formation to hydrogen is typically performed by Pt and Pd catalysts, which while highly active for dehydrogenation also catalyze dehydration. Cu is typically not utilized, as it requires prohibitively high temperatures, although Cu surfaces are very selective toward dehydrogenation. We studied the reaction of formic acid on single-atom alloys (SAAs), consisting of single Pt atoms substituted into a Cu lattice. Surface science studies allowed us to relate alloy structure to reactivity and selectivity and visualize reaction intermediates. These experiments revealed that SAAs are able to selectively dehydrogenate formic acid with a 6-fold increase in yield in comparison to Cu. This increase in conversion is due to a more facile dehydrogenation of formic acid to formate on the SAA surface (120 K vs 160 K on Cu(111)). We acquired quantitative desorption and molecular scale imaging data showing spillover of formate from Pt sites to Cu. Increasing the Pt concentration beyond the SAA regime resulted in loss of selectivity. These results prompted us to test SAA nanoparticle (NP) catalysts under realistic conditions. However, only a slight increase in conversion was observed between pure Cu and Pt-Cu SAA NPs. In our surface science studies, dehydrogenation of formate to CO2 and H2 did not occur until above 400 K on both the SAA and pure Cu surfaces, indicating that Pt sites do not catalyze this rate-limiting step. While SAAs do not offer increased reactivity for formic acid dehydrogenation, they do offer significantly lower barriers for O–H bond breaking, which holds promise for other dehydrogenation reactions.

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.

Methanol steam reforming is a promising reaction for on-demand hydrogen production. Copper catalysts have excellent activity and selectivity for methanol conversion to hydrogen and carbon dioxide. This product balance is dictated by the formation and weak binding of formaldehyde, the key reaction intermediate. It is widely accepted that oxygen adatoms or oxidized copper are required to activate methanol. However, we show herein by studying a well-defined metallic copper surface that water alone is capable of catalyzing the conversion of methanol to formaldehyde. Our results indicate that six or more water molecules act in concert to deprotonate methanol to methoxy. Isolated palladium atoms in the copper surface further promote this reaction. This work reveals an unexpected role of water, which is typically considered a bystander in this key chemical transformation.

The Ullmann reaction of bromobenzene, the simplest coupling reagent, to form biphenyl on a Cu surface proceeds via a highly mobile organometallic intermediate in which two phenyl groups extract and bind a single surface Cu atom.

Methanol is a versatile chemical feedstock, fuel source, and energy storage material. Many reactions involving methanol are catalyzed by transition metal surfaces, on which hydrogen-bonded methanol overlayers form. As with water, the structure of these overlayers is expected to depend on a delicate balance of hydrogen bonding and adsorbate-substrate bonding. In contrast to water, however, relatively little is known about the structures methanol overlayers form and how these vary from one substrate to another. To address this issue, herein we analyze the hydrogen bonded networks that methanol forms as a function of coverage on three catalytically important surfaces, Au(111), Cu(111), and Pt(111), using a combination of scanning tunneling microscopy and density functional theory. We investigate the effect of intermolecular interactions, surface coverage, and adsorption energies on molecular assembly and compare the results to more widely studied water networks on the same surfaces. Two main factors are shown to direct the structure of methanol on the surfaces studied: the surface coverage and the competition between the methanol-methanol and methanol-surface interactions. Additionally, we report a new chiral form of buckled hexamer formed by surface bound methanol that maximizes the interactions between methanol monomers by sacrificing interactions with the surface. These results serve as a direct comparison of interaction strength, assembly, and chirality of methanol networks on Au(111), Cu(111), and Pt(111) which are catalytically relevant for methanol oxidation, steam reforming, and direct methanol fuel cells.

Co-Cu nanoparticles have recently been explored for Fischer-Tropsch synthesis (FTS) as a way to combine the long chain selectivity of Co with Cu's activity for alcohol formation in order to synthesize oxygenated transportation fuels. Depending on particle size, hydrogen dissociation can be a rate-determining step in cobalt-catalyzed FTS. To understand the fundamentals of uptake and release of hydrogen from the Co/Cu bimetallic system, we prepared well-defined Co nanoparticles on Cu(111). We demonstrate that hydrogen spills over from dissociation sites on the Co nanoparticles to the Cu(111) surface via the Co-Cu interface and that desorption of H occurs at a temperature that is lower than from Co or Cu alone, which we attribute to the Co-Cu interface sites. From this data, we have constructed an energy landscape for the facile dissociation, spillover, and desorption of hydrogen on the Co-Cu bimetallic system.

The dissociative adsorption of hydrogen on cobalt is central to a number of catalytic reactions, yet to date there are relatively few studies examining this important process. Here we utilize Co nanoparticles grown on Cu(111), instead of the traditional planar Co single crystals, to study a more catalytically relevant form of Co. We present scanning tunneling microscopy images of different phases of H on the close-packed Co nanoparticle surfaces with a range of densities. Our data reveal a so-far unreported high coverage phase of H with a (1 × 1) structure and elucidate the importance of spillover from step edges in H adsorption. We also illustrate that, in contrast to the low density phases, the H-(1 × 1) structure can only be formed at an intermediate temperature, indicating that compression to this higher-density phase is activated. Density functional theory calculations yield energies for each of the H overlayer structures, as well as their preferred geometries. This work is the first to report on higher coverage (>0.75 ML) phases of H on Co, which are undoubtedly important in catalytic systems at elevated pressure. Finally, through the use of epitaxial Co nanoparticle growth on Cu(111), we illustrate the importance of step edges in H 2 activation and the formation of dense H phases.

Water has an incredible ability to form a rich variety of structures, with 16 bulk ice phases identified, for example, as well as numerous distinct structures for water at interfaces or under confinement. Many of these structures are built from hexagonal motifs of water molecules, and indeed, for water on metal surfaces, individual hexamers of just six water molecules have been observed. Here, we report the results of low-temperature scanning tunneling microscopy experiments and density functional theory calculations which reveal a host of new structures for water–ice nanoclusters when adsorbed on an atomically flat Cu surface. The H-bonding networks within the nanoclusters resemble the resonance structures of polycyclic aromatic hydrocarbons, and water–ice analogues of inene, naphthalene, phenalene, anthracene, phenanthrene, and triphenylene have been observed. The specific structures identified and the H-bonding patterns within them reveal new insight about water on metals that allows us to refine the so-called “2D ice rules”, which have so far proved useful in understanding water–ice structures at solid surfaces.