Reduction and Agglomeration of Supported Metal Clusters Induced by High-Flux X-ray Absorption Spectroscopy Measurements

Abstract

Supported metal clusters are widely used in catalysis for many important reactions. To understand the catalytic properties, in situ/operando characterization techniques, such as X-ray absorption spectroscopy (XAS), provide essential details of the size, shape, and chemical composition of the cluster and the nature of the active sites. New-generation synchrotrons combined with focusing beamlines provide high-flux-density X-rays for improved detection sensitivity as well as higher time and spatial resolution. Understanding the effects of a high-flux-density X-ray beam on the catalyst during the actual measurement, whether XAS or another synchrotron-based technique, is crucial. This is especially important for in situ and operando studies where both the high flux density and reaction conditions can affect the catalyst structure. In this work, we investigated the effect of the flux density on rhodium clusters supported on Al2O3 at two different beamlines: National Synchrotron Light Source II beamline 08-ID and Stanford Synchrotron Radiation Light Source (SSRL) beamline 4-1. We show that the higher flux density at beamline 08-ID causes the reduction of the highly dispersed RhO x /Al2O3 catalyst, even at room temperature. Additionally, exposure to the higher flux density X-rays at beamline 08-ID during in situ reduction results in significant agglomeration of the Rh clusters. The final size of the Rh nanoparticles reduced at 310 °C is equivalent to that of particles formed after the reduction at 600–650 °C in the absence of the beam. Significant beam-induced reduction and agglomeration is also shown for Ni supported on beta zeolite during in situ reduction at an intermediate-flux-density beamline 9-3 at SSRL, indicating that beam-induced changes in heterogeneous catalysts could be common at intermediate- and high-flux-density beamlines. We provide precautions and recommendations for detecting and minimizing beam damage during in situ/operando XAS measurements.

Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta-Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst

“…However, it is possible that the high intensity X-ray beam may induce the formation of additional metallic Ni; therefore, this value likely represents an upper limit to the amount of metallic Ni formed. 36 Wavelet transform analysis was performed on the EXAFS (Figure S7 37 Another calculation was carried out to determine the number of atoms in a particle that would represent the CNs extracted from the EXAFS fitting (see details in the Supporting Information). 38 The calculation indicates that the CN values from the EXAFS fit correspond to a particle of 40−100 atoms.…”

Exploring the materials space in the smallest particle size range: from heterogeneous catalysis to electrocatalysis and photocatalysis

“…Significant beam-induced reduction and agglomeration is also shown for Ni supported on beta zeolite during in situ reduction at an intermediate-flux-density, indicating that beam-induced changes in heterogeneous catalysts could be common at intermediate- and high-flux-density beamlines. 166…”

Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments

“…Newton et al 27 investigated a Cu(II)-containing zeolite in heterogeneously catalyzed methane oxidation to methanol and found that Cu(II) may undergo reduction in the presence of methane or pure helium upon X-ray irradiation. Albrahim et al 28 investigated a highly dispersed RhO x /Al 2 O 3 catalyst by in situ XAS and detected that rhodium clusters undergo reduction even at room temperature in the presence of hydrogen and intense X-rays; in situ heating to 310 1C leads to further agglomeration of clusters, comparable to agglomeration typically observed at 650 1C without exposure to the X-ray beam. Van Schooneveld and DeBeer 29 showed that iron in K 3 [Fe(CN) 6 ] and manganese in KMnO 4 can undergo X-ray-induced reduction during XAS measurements; the authors investigated the relationship between the X-ray dose and the extent of metal reduction by measuring these samples at different synchrotron beamlines.…”