Characterization of Catalytic Materials on Laboratory-Based X-Ray Diffraction Platforms

Introduction

Catalytic materials are important for a wide variety of applications from their use in the chemical industry to the utilization in solar energy production to everyday applications such as automotive catalysts.  There are three main types of catalysts; heterogeneous catalysts, homogeneous catalysts, and biocatalysts (enzymes), with heterogeneous catalysts the most studied via X-ray diffraction (XRD). Typically, these types of catalysts consist of solid-state materials such as metals, inorganic compounds, and zeolites which increase the rate of reaction due to the surface binding of reactants.  Additionally, homogeneous and biocatalysts can be studied to some extent as well.  In the analysis of these substances, it is important to examine structure-performance relationships and identify reaction mechanisms (i.e. the elementary reaction steps, intermediates, and active sites) under technologically relevant conditions.  To do so requires an in-depth knowledge of the atomic structure on both the short- and long-range scales.

X-ray diffraction is a technique that can provide information about the average bulk structure of a material based on the interference pattern resulting from the long-range ordering of scattering centers.  Using Bragg’s law, XRD allows for the determination of the spacing between scattering planes (d) by measuring the scattering angle (θ) at a fixed wavelength (λ), as shown in Figure 1. 

Fig-1-WP201201CharacterizationCatalyticMaterialsXRD.jpg

Figure 1: Illustration of Bragg diffraction where two beams with identical wavelength and phase are scattered from two atoms with spacing (d) to give constructive interference (n = 1, 2, 3…)

The observed peaks relate to the symmetry and unit cell dimensions of the material and can be used as a fingerprint for phase identification as well as to provide a quantitative analysis of the sample.  Furthermore, examination of the peak breadths, low angle scattering, and inverse Fourier transformation of the reciprocal space data can provide information about particle and crystallite size as well as short-range ordering in the material.  Much of this information is obtained via X-ray scattering techniques which do not rely on the presence of long-range order and can be utilized to study nano-sized and amorphous materials.  

Malvern Panalytical’s line of X-ray diffraction instruments can aide in the analysis of catalytic materials by yielding the identification and quantification of both crystalline and amorphous phases as well as providing information about particle and crystallite size, micro strain, thermal stability, and local atomic structure (i.e. short- and intermediate-range order).  With the use of non-ambient chambers, catalytic materials can be examined under various operating conditions which can aide in understanding reaction pathways and identifying the presence of intermediate phases.  Under these conditions it is possible to monitor processes such as oxidation/reduction reactions that contribute to the activation of catalytic materials.  Additionally, many catalysts consist of nano sized materials making information regarding the local structure an important aspect of performance optimization.  As XRD only provides information related to the bulk structure of materials, it is often necessary to perform additional methods such as small angle X-ray scattering (SAXS) and pair distribution function (PDF) analysis.  These techniques allow for the determination of structural information on the nanometer scale (i.e. 1 – 100 nm) which is a pertinent size range for many catalytic materials.  In particular, information such as particle size and shape as well as specific surface area (SSA) can be obtained via SAXS measurements while PDF analysis provides information regarding short-range atomic order (i.e. atomic pair correlations) which can give insight into nanocrystalline and amorphous phases as well as local structure variations.

This white paper highlights several techniques that can be performed on Malvern Panalytical’s floor standing and benchtop XRD platforms to assist in the characterization of catalytic materials. With Malvern Panalytical’s Empyrean instrument, which allows for the easy exchange of stages and optics, basic powder X-ray diffraction (PXRD) measurements (i.e. phase identification and quantification) as well as non-ambient, SAXS and PDF experiments can all be performed without the need for re-alignment of the instrument.  Additionally, both PXRD and non-ambient measurements can also be performed on the benchtop Aeris instrument which has comparable resolution, scan-times, and peak intensities to the floor standing Empyrean.  Data from both instruments can be analyzed via HighScore (Plus) software for phase identification and quantification as well as the analysis of non-ambient and PDF data.  Additionally, Malvern Panalytical’s EasySAXS software provides a straightforward method to obtain relevant information from SAXS data.

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