A combined QEXAFS/XRD method for on-line, in situ studies of catalysts: Examples of dynamic measurements of Cu-based methanol catalysts

Abstract

The development of a new in situ method which allows high quality XRD and EXAFS to be obtained on-line under ideal catalytic conditions is discussed. The possibility to obtain both types of information simultaneously enables a much better structural description of catalytic materials which typically contain both crystalline and X-ray amorphous structures. The system employs a capillary tube as the microreactor/in situ cell. The high degree of temperature uniformity of the cell and the possibilities of fast changes in reaction conditions make the system ideal for dynamic studies. For this purpose, the EXAFS measurements are carried out in the newly developed quick scanning mode (QEXAFS) which also allows high quality data to be obtained. The XRD is acquired using a position sensitive detector. The application of the setup for time resolved measurements is demonstrated in a study of the calcination and reduction of Cu-based methanol catalysts where the changes take place over a few degrees. The high quality of the data made it possible to obtain important new insight regarding the presence of intermediate phases during these processes. Studies of Cu/SiO₂ catalysts show the advantages of a newly developed theory for a better estimation of coordination numbers (and thus particle sizes) from EXAFS.