Determination of Nanoparticle Size by Measuring the Metal–Metal Bond Length: The Case of Palladium Hydride

Abstract

Formation and properties of Pd nanoparticles have been investigated in situ in the temperature range between 186 and 483 K by X-ray absorption fine structure (XAFS) spectroscopy and by ex situ electron microscopy. Results of the experiments conducted under hydrogen and helium atmospheres were compared, in order to illuminate and explain the changes in bond length of Pd particles as a result of hydride formation. By contrasting the data obtained at these two atmospheres we were able to isolate the effects of both the finite particle size and hydrogen absorption on Pd−Pd bond length, obtain hydrogen intake values, and, as a result, estimate the particle size. The calculated average particle size agreed with the electron microscopy data. These results open up new opportunities for in situ size investigations by XAFS of catalytic Pd particles in reaction conditions, when catalyst fragmentation, shape change, and/or coarsening may occur.

New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd−Au Catalysts

“…In the Pd NPs, H/Pd ratio calculated using the EXAFS analysis results was found to be 0.45, which is consistent with the value obtained earlier for Pd NPs of similar sizes . For Pd 25 Au 75 NPs, EXAFS analysis also demonstrated that Pd−Pd bond increased from 2.787±0.008 Å to 2.806±0.008 Å.…”

“…In Pd NPs under He, the coordination number (CN) of first shell Pd−Pd contribution is approximately 9.2±0.6 at a distance of 2.737±0.003 Å. For such a coordination number, a corresponding size of a supported cuboctahedral nanoparticle would be expected to be between 2 and 3 nm, depending on its shape in agreement with the average particle sizes of 3.4±1.7 nm obtained by STEM (Figure S6). In Pd 25 Au 75 NPs under He, the CNs of Pd−Pd and Pd−Au contributions are 1.1±0.2 and 9.6±0.4, respectively, showing that, on average, Pd atoms are predominantly surrounded by Au atoms, in a greater ratio (9.6 : 1.1) than that predicted from the bulk composition (3 : 1).…”

“…We used X‐ray absorption spectroscopy at Pd K‐edge as a probe to detect the formation of Pd hydride at room temperature in both monometallic Pd and Pd 25 Au 75 NPs on RCT SiO 2 . After exposing Pd NPs to flowing H 2 at 298 K and 1 bar, a significant shift was observed in XANES (Figure S3A) and k ‐space EXAFS spectra (Figure S3B) – an effect of Pd hydride formation that has been previously reported . Correspondingly, the R ‐space EXAFS spectrum of the Pd NPs shifted to longer Pd−Pd distance upon H 2 exposure (Figure A).…”

“…In this study, we have taken advantage of the known sensitivity of Pd nanoparticles to form Pd hydrides under hydrogen exposure as a sensor for Pd distributions within the Pd−Au catalysts . Pd hydride formation can only occur on contiguous Pd atom assemblies near the surface within the Pd−Au NPs and can be tracked by the Pd−Pd bond expansion upon hydrogen absorption in X‐ray absorption spectroscopy (XAS) . XAS detection of Pd hydride formation in the alloy open new opportunities for catalysis research, due to the high penetration depth of hard X‐rays and the compatibility of XAS technique with operando characterization requirements.…”

“…After being exposed to flowing H 2 , Pd NPs showed an increase in the Pd−Pd bond distance from 2.737±0.003 Å to 2.810±0.003 Å, which is typical for the lattice expansion from Pd to Pd hydride in NPs . In the earlier work it was shown that an empirical relationship (1) between the Pd−Pd bond length change ΔR and the H/Pd ratio x , known for bulk Pd, can also apply to describe nanoscale Pd [Equation ]: …”