Controlling Selectivity in Unsaturated Aldehyde Hydrogenation Using Single-Site Alloy Catalysts

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.