Elucidation of structure and nature of the PdO–Pd transformation using in situ PDF and XAS techniques

Abstract

The PdO-Pd phase transformation in a 4 wt% Pd/Al2O3 catalyst has been investigated using in situ X-ray absorption spectroscopy (XAS) and in situ X-ray total scattering (also known as high-energy X-ray diffraction) techniques. Both the partial and total pair distribution functions (PDF) from these respective techniques have been analysed in depth. New information from PDF analysis of total scattering data has been garnered using the differential PDF (d-PDF) approach where only correlations orginating from PdO and metallic Pd are extracted. This method circumvents problems encountered in characerising the catalytically active components due to the diffuse scattering from the disordered γ-Al2O3 support phase. Quantitative analysis of the palladium components within the catalyst allowed for the phase composition to be established at various temperatures. Above 850 °C it was found that PdO had converted to metallic Pd, however, the extent of reduction was of the order ca. 70% Pd metal and 30% PdO. Complementary in situ XANES and EXAFS were performed, with heating to high temperature and subsequent cooling in air, and the results of the analyses support the observations, that residual PdO is detected at elevated temperatures. Hysteresis in the transformation upon cooling is confirmed from XAS studies where reoxidation occurs below 680 °C.

Application of Differential PDF to the Structural Characterization of Supported Catalysts

“…Even with higher noise, the d-PDF is consistent with the PDF of the sample C (pure PdO - Fig. 1c) and with the analysis using synchrotron data reported in the literature [6]. Moreover, from the decay of the d-PDF it is possible to estimate the structural coherence length of PdO particles using the spherical diameter calculation in PDFgui.…”

“…In this contribution we demonstrate the application of this method to investigate the structural properties of nanosized PdO phase distributed on the reducible Fe 2 O 3 support. Unlike d-PDF studies previously reported which were based on synchrotron data [4][5][6], we used a laboratory diffractometer to collect high energy X-ray diffraction data [7][8][9][10].…”

“…Pair distribution function analysis (PDF) based on total scattering data [3] is one of the techniques of choice for the structural characterization of these nanosized catalytic materials, which are often present at very low concentrations and are sometimes amorphous. An advanced application of PDF is the so-called differential PDF (d-PDF), which is based on the subtraction of the PDF of a matrix from the PDF of the system "matrix + guest species", in order to isolate the interatomic correlations of the guest species [4][5][6]. To enable this approach particular care must be taken in both the data collection (especially reducing the statistical noise in the experimental data) and in the subsequent analysis, because of the weakness of the signal to be analyzed, which results from the difference of the PDFs.…”

Methane Catalytic Combustion over Hierarchical Pd@CeO2/Si‐Al2O3: Effect of the Presence of Water

“…Excellent fitting of the Pd EXAFS data was achieved by considering only the contribution of Pd fcc phase with a distance of 0.273 nm (0.271 in bulk Pd) in accordance with a similar study for Pd/PdO on g-Al 2 O 3 (0.274 nm). [32] In the absence of oxygen (N 2 atmosphere), operando XANES experiments demonstrated that PdO almost fully decomposes to Pd at 600 8C, even if in close contact with CeO 2 (Figure 8 b). The equilibrium is reversed under dry conditions leading to the almost immediate reoxidation of 25 % Pd to PdO followed by a further slow increase during the following reaction time (Figure 8 c).…”

Thermal stability of Pd-containing LaAlO3 perovskite as a modern TWC

“…1.2.11d), as shown in Fig. 6 [35]. Note that the curve-fitting of XANES spectra was conducted with the reference materials (PdO and Pd foil) in the range of the absorption edge energy, E 0 between À20 and +30 eV with a typical agreement of R-factors = 0.0002.…”