(751b) A Simple, Generalizable Synthesis of PdAu/SiO2 Single Atom Alloy Catalysts

Single atom alloy (SAA) catalysts have shown initial promise for reactions such as selective hydrogenations [1,2]. The majority of supported SAA catalysts have been prepared in one of two ways; impregnation with extremely dilute ratios of metals [1], or utilizing colloidal synthesis of the diluent metal (which normally gives particles between 7 and 15 nm) and then applying the active metal with galvanic displacement (GD) [2]. Via impregnation, the density of active sites is limited by the extreme dilution needed, and galvanic displacement is limited in practice to metals such as copper [2], which fulfill the thermodynamic requirement of GD without the need for controlled atmosphere apparatus.

In this paper we demonstrate a simple, generalizable method to make supported SAA catalysts based on simultaneous strong electrostatic adsorption (co-SEA) [3]. We have synthesized a series of Pd/Au/SiO₂ catalysts using Pd ammine and Au ethylenediamine complexes adsorbed at high pH over a high surface area amorphous silica (Aerosil 300) at Pd/Au atomic ratios from 1:1 through 1:53, with about 1.5 wt% Au. These have been characterized by powder XRD, aberration corrected STEM, TPR, and CO-FTIR, and have been evaluated for phenylethanol oxidation to acetophenone, which normally requires a homogeneous Pd catalyst.

Because SEA gives ultrasmall particles, SAA catalysts can be synthesized at relatively high ratios of active metal to metal diluent, that is, with a relatively high density of isolated atom sites. Characterization and reactivity measurements suggest that Pd sites are isolated in the small (sub-2 nm) Au nanoparticles at ratios of about 1:35 and below (1:50 etc.). The applicability of the method to other metal pairs will be discussed.

References

1. Pei, G.X., et al., ACS Catal. 7, 2017, 1491.
2. Boucher, M.B., et al., Phys. Chem. Chem Phys. 2013, 15, 12187.
3. Wong, A., et al., Science 2017, 358, 6369.