Integrated Catalysis-Surface Science-Theory Approach to Understand Selectivity in the Hydrogenation of 1-Hexyne to 1-Hexene on PdAu Single-Atom Alloy Catalysts

Abstract

The selective hydrogenation of alkynes to alkenes is an important industrial process. However, achieving high selectivity and reducing the usage of precious platinum group metals are still challenging for the conventional hydrogenation catalysts. With atomically dispersed active metal atoms on the surface of a host metal, single-atom alloys (SAAs) have shown excellent hydrogenation selectivity and activity, but their hydrogenation mechanism is not fully understood. This work reports on the selective hydrogenation of 1-hexyne to 1-hexene on PdAu SAA catalysts. Au is a highly selective hydrogenation catalyst, but it is not active at low temperatures. Through measurements of reaction kinetics and in operando spectroscopy studies, we follow the much more facile activation of PdAu SAA catalysts and demonstrate the different hydrogenation chemistry of single Pd atoms and Pd nanoparticles (NPs). We further investigate the role of Pd atoms and the mechanism behind the improved hydrogenation selectivity through surface science and density functional theory. These studies indicate that the difference in reactivity stems from the relative energy barrier heights for over-hydrogenating the terminal C atom. The complementary catalysis-surface science-theory investigation described here is a powerful and general approach for understanding and controlling NP performance. The selective hydrogenation on PdAu SAAs is demonstrated and understood fundamentally, which serves as a guide for future designs of this type of catalyst.

Directing reaction pathways via in situ control of active site geometries in PdAu single-atom alloy catalysts

“…Inspired by these theoretical results, we synthesized and tested PdAu/SiO 2 bimetallic catalysts. Pd 0.02 Au 0.98 NPs with a Pd/Au atomic ratio of 1/49 and an average diameter of 7.6 ± 1.9 nm 52 , 53 were prepared by the sequential reduction method and then supported on SiO 2 (Fig. 2a and Supplementary Table S1 ).…”

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

“…[1][2][3] Dilute alloys are a promising class of materials for addressing this challenge, in particular the combination of a selective coinage metal host (Au, Ag, Cu) and a small amount of a more active metal dopant such as Pd. [4][5][6][7][8][9] Optimization of dilute alloys for selective hydrogenation relies on carefully tuning the dopant concentration, since high dopant concentrations typically lead to higher hydrogenation activities but lower selectivity. [6][7][8] Conversely, decreasing the Pd concentration can substantially increase the selectivity, but will lower the catalytic activity.…”

“…Whether hydrogen migration from Pd onto the host metals like Au, Ag, and Cu occurs is also a matter of debate, as these metals bind hydrogen more weakly. Yet, spill-over from Pd has been reported for Au-Pd, 5,10,12 Ag-Pd 15 and Cu-Pd 16 alloys at sufficiently high hydrogen pressure and coverage. 15 Despite extensive work from the surface science community on hydrogen dissociation and migration behavior on dilute alloys, these concepts have yet to be extended to nanoparticle catalysts operating at pressures and temperatures relevant for catalytic applications.…”

When More Is Less: Nonmonotonic Trends in Adsorption on Clusters in Alloy Surfaces

“…Small, localized ensembles of dopant atoms embedded in the surface of a host have recently gained significant attention, as they can be effective catalysts for many reactions, such as selective hydrogenation. [9][10][11] Single-atom alloys have received particular attention, including development of models and correlations for predicting or understanding adsorption energies. [12][13][14] Single-atom alloys can have unusual properties, such as allowing spillover and breaking linear correlations between energies.…”