Palladium Single-Atom (In)Stability Under Aqueous Reductive Conditions

Abstract

Here, we investigate the stability and performance of single-atom Pd on TiO₂ for the selective dechlorination of 4-chlorophenol. A challenge inherent to single atoms is their high surface free energy, which results in a tendency for the surface migration and aggregation of metal atoms. This work evaluates various factors affecting the stability of Pd single-atoms, including atomic dispersion, coordination environment, and substrate properties, under reductive aqueous conditions. The transition from single atoms to clusters vastly enhanced dechlorination kinetics without diminishing carbon-chlorine bond selectivity. X-ray absorption spectroscopy analysis using both in situ and ex situ conditions followed the dynamic transformation of single atoms into amorphous clusters, which consist of a unique unsaturated coordination environment and few nanometer diameter. The intricate relationship between stability and performance underscores the vital role of detailed characterization to properly determine the true active species for dehalogenation reactions.

Keywords: cluster; hydrodechlorination; in situ XAS; single-atom catalyst; stability.

Figure 1

(a) Schematic illustration of Pd₁/TiO₂ SAC synthesis via electrostatic adsorption and UV-A photoreduction. The gray hemisphere represents the TiO₂ support. (b) Normalized XANES spectra of Pd catalysts and references. (c) EXAFS spectra of Pd catalysts and references (Pd foil ×0.4 and Pd_NP/TiO₂ ×0.6). (d) HAADF-STEM image depicting Pd single-atoms on TiO₂.

Figure 2

(a) Degradation of 4-CP by Pd₁/TiO₂ and Pd_NP/TiO₂ and product generation (phenol and chloride) of Pd₁/TiO₂. (b) Before and after reaction EXAFS spectra of Pd₁/TiO₂, Pd_NP/TiO₂, Pd_1,isolated/TiO₂, Pd_1,APTMS/TiO₂, and Pd₁/Al₂O₃ (Pd_NP/TiO₂ and Pd_NP,rxn/TiO₂ ×0.6). The dashed lines represent before reaction spectra, and shaded peaks represent after reaction spectra.

Figure 3

(a) Time series EXAFS spectra of hydrogen-treated Pd₁/TiO₂ compared to Pd references (PdO and Pd foil spectra ×0.8 and 0.3, respectively). Purple and gray boxes indicate Pd–O and Pd–Pd bonding, respectively. Note that the Pd₁/TiO₂ spectrum is identical to that in Figure 1c and shown here for completeness. (b) CNs extracted from EXAFS spectra in panel (a) compared to Pd_NP (values given in Table S1). (c) Time series XANES spectra of hydrogen-treated Pd₁/TiO₂ compared to Pd references. (d) HR-TEM image of Pd_NP/TiO₂. (e, f) HR-TEM and HAADF-STEM images, respectively, of Pd₁/TiO₂ after 5 min of H₂ treatment.

Figure 4

(a) Schematic of in situ gas delivery/jetting setup. The bulk Pd₁/TiO₂ suspension was continuously cycled through a jet system, and a mixture of H₂/N₂ gas was delivered to the bulk. XAS of the jet was measured using an SDD detector. Scans were taken every 50 s. (b) Time series XANES spectra of in situ Pd₁/TiO₂ treatment with 10% H₂. (c) MCR determined time dependent concentration profile of XANES spectra shown in panel (b).