The characterization of catalysts involves a knowledge of the physical properties of the catalyst as well as an understanding of structure-reactivity or selectivity relationships during the catalytic process. This is achieved by combining spectroscopic characterization with mass spectrometry to determine catalytic activity and selectivity. This article will outline some of the techniques used for catalyst characterization and their main attributes.
Structural Methods of Catalyst Characterization
The three main structural methods of catalyst characterization are X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy.
X-ray diffraction is well suited to ascertaining the bulk structure and composition of heterogeneous catalysts that have crystalline structures. X-ray absorption spectroscopy is used for local compositional and structural analysis in catalyst characterization. It does not require long-range order in the samples under study, as only the local environment is probed. X-ray absorption can also be used in non-vacuum environments allowing the in-situ research of catalysts and catalyst mechanisms combining local structural information with activity/selectivity measurements provided by mass spectrometry. Electron microscopy is a simple and effective method of catalyst characterization, used for determining the morphology and size of solid catalysts. Electron microscopy works well for studying solids and metal clusters, but not for detecting reaction intermediates on the surface of catalysts.
Adsorption-Desorption and Thermal Methods of Catalyst Characterization
Some examples of adsorption-desorption catalyst characterization are surface area/pore structure, thermogravimetry, and thermal analysis.
Many heterogeneous catalysts are porous materials that have specific surfaces areas that can display complex architecture and size distribution. The surface area, pore volume, and average pore size of catalysts play a significant role in ascertaining the number of sites available for catalysis.
Thermal analysis techniques can be used to quantify the changes that occur during the preparation of catalysts (such as oxidation, calcination, and reduction) by observing variations in sample weight. The combination of mass spectrometry with thermal analysis techniques such as TGA, DTA, DSC can additionally provide important information about desorbed species and decomposition products leading to a fuller understanding of the preparation process.
Temperature Programmed Desorption, Reduction & Oxidation (TPD/TPR/TPO)
Temperature programmed profiles such as desorption, reduction, or oxidation can be used to elucidate many of the physical properties of heterogeneous catalysts.
This usually requires a high sensitivity mass spectrometer, along with a microreactor or vacuum chamber.
Pulse Chemisorption for Catalyst Characterization
Pulse chemisorption is a very useful technique for the characterization of catalysts. Pulse chemisorption allows the sample to be exposed to known, reproducible, quantities of gas allowing uptake measurements to be easily performed. Properties such as adsorption isotherms, metal dispersion, metal surface area and surface acidity can then be easily determined from these measurements.
Catalyst Characterization with Hiden Analytical
Hiden Analytical offers a broad variety of quadrupole mass spectrometers and catalyst characterization systems that have built in TPR, TPO, TPD, pulse chemisorption modes. If you would like to find out more about how we can help with catalyst characterization, get in touch with the team at Hiden Analytical today.