Operando XAFS of supported Pd nanoparticles in flowing ethanol/water mixtures: implications for catalysis

Abstract

Ethanol-water, a prototypical 'green' solvent mixture, cannot be considered as inert toward supported Pd nanoparticles. We establish severe size dependencies in Pd redox behavior, persistent gradients in Pd phase in a plug-flow situation, and that Pd nanoparticles are contaminated with hydrogen derived from the solvent. † Electronic supplementary information (ESI) available: Catalyst synthesis, operando EXAFS measurements, linear combination analyses, transmission electron microscopy, comparison of 1, 2, and 5 wt% Pd systems. See

Effects of Cl on the reduction of supported PdO in ethanol/water solvent mixtures

“…metallic Pd nanoparticles; and the bond distance (Å) associated with the PdPd scattering interaction, R PdPd 1 (Figure 6), which is indicative of metallic Pd nanoparticles. In the case of Figure 5 the development of N PdPd 1 as a function of axial position in the bed and temperature is directly comparable to the previous study performed with non-chlorinated samples [11].…”

“…The corresponding EXAFS spectra ( Figure 2) shows that the PdO species present in the 5 and 10 wt-% catalysts, remained essentially unchanged, as was expected based upon our previous observations of the behaviour of their Cl-free analogues [11]. For the 1 wt-% Pd/γ-Al 2 O 3 , exposure to the flowing solvent at room temperature elicits significant change in the habit of the Pd.…”

“…The Pd K-edge EXAFS spectra of the dried catalysts (mounted as a packed bed in the Stokes flow reactor) were recorded ( Figure 1); the structural and statistical parameters derived from analysis of the EXAFS spectra using EXCURV are presented in Table 1. Compared with the data acquired previously of the equivalent catalysts prepared from a Cl-free precursor [11], the 5 and 10 wt% Pd catalysts are found to comprise nanosize PdO without any reduced Pd(0) being present. In contrast, the 1 wt-% Pd/γ-Al 2 O 3 catalyst is very different.…”

“…In all cases, irrespective of the nature of the Pd precursor used, and in common with their Cl-free counterparts, the reduced Pd nanoparticles are found to be significantly contaminated by interstitial hydrogen, which appears to be intrinsic to the chemistry between the Pd and the solvent components [10][11][12].…”

“…To address this problem, we have initiated a programme study, using operando Quick Pd K-edge EXAFS (QEXAFS) to investigate the effect of a flowing ethanol-water mixture on a number of Pd catalysts, containing different phases and size distributions of Pd nanoparticles supported upon different dispersant materials [10][11][12]. Concurrently, We have also investigated the behaviour of polymer encapsulated (ENCAT™) Pd catalysts under the same conditions [13].…”