Selectivity in catalysis is key to many industrial processes, yet it is often difficult to control. One promising approach is to use  so-called single-atom catalysts, whereby one catalytic component is isolated within a second phase to add a key but otherwise unavailable functionality. Here, we report the use of metal alloys consisting of Pt single atoms diluted within Cu nanoparticles to selectively promote the hydrogenation of CO bonds in unsaturated aldehydes, a reaction of interest in fine chemical manufacturing. Our rationale, that Cu surfaces may favor CO over CC hydrogenation steps with atomic hydrogen but may require Pt sites to promote the initial activation of molecular hydrogen, was corroborated by kinetic catalytic experiments. However, fundamental surface science studies and quantum mechanics calculations showed that the explanation for the observed catalytic performance is more nuanced. For one, titration experiments using carbon monoxide failed to identify Pt atoms accessible on the surface of the catalysts, suggesting that their catalytic contribution may involve indirect electronic changes on neighboring Cu atoms. In addition, infrared absorption and X-ray photoelectron spectroscopy results identified the existence of a thin Cu oxide layer covering the metallic nanoparticles. Finally, it was determined that hydrogenation selectivity with Cu-based catalysts may be explained in part by their preference for bonding unsaturated aldehydes via the terminal oxygen atom but is also affected by competitive adsorption among the reactants and products. Single-atom alloy catalysts appear to indeed help with selectivity in hydrogenation catalysis, but more in situ (or operando) characterization experiments are needed to better understand how they operate.

A comprehensive study of the kinetics of the catalytic hydrogenation of unsaturated aldehydes, in particular of cinnamaldehyde, promoted by CuPt x /SBA-15 single-atom alloy catalysts was carried out in order to identify trends as a function of the composition of the bimetallic nanoparticles, that is, of the value of x. The optimum performance reported by us recently [ACS Catal2019991509157] in terms of selectivity toward the formation of cinnamyl alcohol, the desired product, by the catalyst with x = 0.005 was corroborated. A rapid decrease was seen in catalytic activity in batch reactors with all catalysts, in particular with pure Cu/SBA-15. This was ascribed, based on DFT calculations and microkinetic simulations, to the relative weak adsorption of the reactant compared to that of the products, which leads to the blocking of catalytic sites by the latter early on in the catalytic runs. It was determined that selectivity is controlled by the relative values of the initial rate constants for hydrogenation to the unsaturated alcohol versus the saturated aldehyde. Those were found to vary by up to an order of magnitude as a function of Pt content in the catalyst, in spite of the fact that the hydrogenation steps are presumed to occur on Cu, not Pt, sites. In general, significant changes in equilibrium and kinetic parameters were seen across the series of catalysts tested (versus x), indicating that the addition of even small amounts of Pt to these Cu single-atom alloy (SAA) catalysts affects the intrinsic performance of the hydrogenation catalytic sites.

Buckypapers made with CNx nanotubes.

Metal amidinates are common compounds with many applications, and are of particular value as precursors for the chemical deposition of thin metal films on solid surfaces. In order to better understand those processes, the surface chemistry of copper(I)−N,N′dimethylacetamidinate on Cu(110) single-crystal surfaces has been studied using first-principles quantum-mechanics calculations. Most metal amidinates exist as dimers (or tetramers) in the gas phase. Here, it was found that the initial steps of the adsorption and dissociation of those dimers on metal surfaces depend on their surface coverage. At low coverages, it was found that the copper(I)−N,N′-dimethylacetamidinate dimer initially binds to the Cu surface by occupying two bridge sites, with the four N atoms on top of adjacent surface Cu atoms. This configuration is, however, not stable, so the adsorbed dimer undergoes dissociation soon after via the shedding of one of the ligands; in this more stable configuration, both Cu atoms from the inorganic precursor occupy hollow sites, and one of the ligand remains coordinated on top of them whereas the other breaks away and binds directly to the surface via its N atoms. At high coverages, the dimer dissociates partially as well, but one of its ligands remains partially attached. It is speculated here, on the basis of the energetics of the different adsorbed species, that molecular desorption in this system may occur with the copper(I)−N,N′-dimethylacetamidinate as either a dimer or a monomer, a conclusion consistent with experimental observations. An analysis of the charge distributions in the adsorbed species shows a reduction of the Cu atoms of the dimer until reaching a metallic state once the ligands are all removed.

The oxygen reduction reaction (ORR) is of paramount interest, in the context of both alternative energy applications in fuel cells and for on‐site hydrogen peroxide (H2O2) production in environmental remediation applications. Using theoretical and experimental methods, the mechanism involved in the ORR is studied on nitrogen‐doped graphitic carbon materials. The two principal reaction pathways involved in the ORR are the four‐electron pathway producing water (H2O), or the two‐electron pathway leading to the formation of H2O2. An atomistic step by step ORR mechanism is proposed to understand the selectivity of the reaction toward the two principal pathways. The results show that graphitic N sites favor the two‐electron pathway, in a similar way to three pyridinic N sites. Meanwhile, the one or two pyridinic N sites lead to the four‐electron pathway. The calculations show the importance of dangling bonds and/or pentagonal C rings in selectivity toward the four‐electron pathway. The results are consistent with recent reports on the importance of topological defects in graphitic carbon materials. The understanding of the ORR mechanism is very important for the design and development of novel ORR electrocatalysts to favor the required pathway, according to the application.