Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption

Abstract

The formation of palladium hydride and carbide phases in palladium-based catalysts is a critical process that changes the catalytic performance and selectivity of the catalysts in important industrial reactions, such as the selective hydrogenation of alkynes or alkadienes. We present a comprehensive study of a 5 wt% carbon supported Pd nanoparticle (NP) catalyst in various environments by using in situ and operando X-ray absorption spectroscopy and diffraction, to determine the structure and evolution of palladium hydride and carbide phases, and their distribution throughout the NPs. We demonstrate how the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) spectra and X-ray powder diffraction (XRPD) patterns allows discrimination between the inner "core" and outer "shell" regions of the NP during hydride phase formation at different temperatures and under different hydrogen pressures, indicating that the amount of hydrogen in the shell region of the NP is lower than that in the core. For palladium carbide, advanced analysis of X-ray absorption near-edge structure (XANES) spectra allows the detection of Pd-C bonds with carbon-containing molecules adsorbed at the surface of the NPs. In addition, H/Pd and C/Pd stoichiometries of PdHx and PdCy phases were obtained by using theoretical modelling and fitting of XANES spectra. Finally, the collection of operando time-resolved XRPD patterns (with a time resolution of 5 s) allowed the detection, during the ethylene hydrogenation reaction, of periodic oscillations in the NPs core lattice parameter, which were in phase with the MS signal of ethane (product) and in antiphase with the MS signal of H2 (reactant), highlighting an interesting direct structure-reactivity relationship. The presented studies show how a careful combination of X-ray absorption and diffraction can differentiate the structure of the core, shell and surface of the palladium NPs under working conditions and prove their relevant roles in catalysis.

Chemical Synthesis and Characterization of Pd/SiO2: The Effect of Chemical Reagent

“…These lattice parameters are also close to those reported for palladium NPs and bulk palladium materials [12,17,24,30,33]. According to the analysis of peaks broadening, the mean crystallite size was 6.1 nm in the case of palladium reduced by NaBH 4 , and in case of reduction by hydrazine using ethanol and ammonia, the crystallite sizes were 8.4 nm and 13.7 nm, respectively, which is reported in Table 1 together with TEM results. The fact that the average particle size of Pd-NPs hydrazine was close to 8 nm proves that the bigger fraction of particles-shown in Figure 1b-does not actually originate from big (15-35 nm) particles, but contains agglomerates of smaller ones with average size close to 8 nm.…”

“…Similar size distributions centered around 8 nm were obtained for the sample reduced by sodium borohydride, Pd-NPs NaBH 4 , and the sample reduced in the vapor of hydrazine Pd-NPs hydrazine . However, the latter includes particle agglomerates with sizes from 15 to 35 nm.…”

“…The higher error in determining this value originates from very weak (220) and (311) reflections, which complicates the analysis. According to XRF analysis, Pd-NPs NaBH 4 and Pd-NPs PEG samples contained 19.4 and 11.6 wt % of palladium, respectively, which is close to the expected value according to the loading of the initial chemicals. For the samples reduced in hydrazine vapors, the experimental values of palladium content were lower than the theoretical ones.…”

“…At the final stage of the synthesis, the obtained suspension was filtered, washed with deionized water, and dried. Because this sample was synthesized using NaBH 4 as a reducing agent, we will refer to it as Pd-NPs NaBH 4 . The calculated reaction efficiency was 99%.…”

“…Noble metal nanoparticles (NPs) find numerous applications in catalysis [1][2][3][4], electrochemistry [5], biology [6,7], medicine [8,9], and other fields. In particular, palladium is one of the best-performing metal catalysts for selective hydrogenation of alkynes [10][11][12][13][14].…”