Hydrogenation of Cinnamaldehyde on Cu(110) Single-Crystal Surfaces

Temperature-programmed desorption experiments in two modalities, namely, using linear temperature ramping (TPD) and under isothermal conditions (IsoTPD), were performed to characterize the chemical reaction between cinnamaldehyde and hydrogen on Cu(110) single-crystal surfaces. It was found that hydrogenation can only take place if atomic hydrogen is used, as reactivity with H-2 was not detectable, and that it occurs selectively at the carbonyl moiety to produce the cinnamyl alcohol, the desired product. That molecule accounts for more than two-thirds of all the products, with the rest corresponding to the more extensively hydrogenated hydrocinnamyl alcohol. Interestingly, the production of hydrocinnamaldehyde (the undesired product) via the hydrogenation of the C=C bond is negligible. The same chemistry was observed on clean and oxygen-pretreated surfaces, indicating that the oxidation state of the Cu atoms may not be significant in determining selectivity for this conversion. Also, both types of experiments, TPD and IsoTPD, led to the same results, an observation that points to a Langmuir-Hinshelwood mechanism between two adsorbed species (the unsaturated aldehyde and atomic hydrogen). These results provide molecular-level confirmation of the intrinsic hydrogenation selectivity of Cu surfaces in single-atom alloy catalysts.