Correlative In situ imaging and spectroscopy of a bimetallic PdCu catalyst using Synchrotron and Electron Optical Beam Lines

Abstract

Supported metal nanoparticle catalysts are key materials for diverse conversion processes and are widely used throughout the chemical industry. Bimetallic catalysts frequently offer improvements in activity, selectivity, and stability compared to their monometallic equivalents [1]. The nanoparticle (NPs) distributions are generally not homogeneous and not all NP morphologies show equal activity for a given reaction [2]. Characterisation of the elemental distribution and oxidation state is therefore of great importance for understanding the alloying and segregation phenomena in bimetallic catalysts and hence optimising the performance of such materials. However, such characterisation usually requires the sample to be imaged at high vacuum and room temperature and the chemical reactions of interest rarely occur at such benign conditions. Therefore, we have conducted in situ environmental TEM study [3‐4] with complementary bulk analysis using temperature programmed reduction (TPR) and in situ X‐ray Absorption Near Edge Spectroscopy (XANES) to gain in depth insights into the chemical and morphological changes experienced by TiO 2 supported PdCu nanoparticle catalysts when exposed to relevant elevated temperature in 1 bar reducing atmosphere. Recent progress with environmental cell and microscope design has enabled us to follow the evolution of bimetallic NPs using in situ elemental imaging capabilities [5‐6]. The PdCu catalyst was prepared by incipient wetness co‐impregnation of copper and palladium nitrates on titania followed by calcination to yield the unreduced form of the catalyst. Conventional STEM‐XEDS elemental mapping indicates that the catalyst comprises ~10 nm large Pd containing particles with smaller ~1 nm size Cu clusters dispersed across the titania support. The XANES spectra of the calcined material (Figure 1) indicate that both Pd and Cu are initially present as oxides. The Temperature Control Reduction (TPR)‐XANES of the Pd (Figure 1a) shows complete reduction to Pd(0) after room temperature hydrogen treatment and that the metal remains Pd(0) throughout the remaining temperature points. By contrast, the Cu‐K edge reduction profile (Figure 1b) shows a small degree of reduction at lower temperatures, likely due to Cu associated with the Pd clusters, and that the sample contains both Cu oxide and metallic Cu up to 250 °C. Figure 2 shows XEDS elemental mapping of TiO 2 supported PdCu catalysts. After introducing a reducing H 2 atmosphere and heating to 250 °C, NPs start to be resolvable in the Cu distribution visible as local regions of higher intensity (Figure 2e‐h). At 550 °C, where TPR‐XANES indicates that copper oxides are no longer present, Cu NPs are formed with Pd particles generally containing Cu but at varying concentrations. The heterogeneity of the localized composition is striking and in some cases Janus NPs are formed with various degrees of phase separation between the two metals (Figure 2j‐l). In summary, correlative studies on a bimetallic PdCu catalyst using in situ high temperature gaseous environmental conditions in both synchrotron and electron‐optical instruments has allowed the identification of the processes by which the active phases of these catalysts evolve. This has provided invaluable insights to the complementary bulk XANES and TPR characterisation including the subtle changes in the Cu‐K edge. This correlative approach has enabled an unambiguous identification of the states of the catalysts and helped to elucidate the segregation phenomenon which gives rise to their properties.