Thorough insight into reacting systems by combined in-situ XAFS and differential scanning calorimetry

Differential scanning calorimetry (DSC) is a well-established technique to monitor energetic changes in solid or liquid samples (1,2). Ther- mally induced processes can be accurately fol- lowed and temperature-dependent properties be determined. We combined quick in-situ X-ray absorption fine-structure spectroscopy (XAFS) in transmission mode with DSC (3,4). This al- lows to obtain element specific local structural and electronic information as measured by XAFS (5) and to simultaneously monitor enthalpic changes by DSC. The combination provides a promising tool for studying the course of chemi- cal reactions and phase transitions in crystalline, glassy or amorphous as well as in liquid systems because these methods do not require a long- range order in the samples. To uncover the underlying mechanisms in these complex pro- cesses it is highly advantageous to employ comple- mentary experimental techniques in an in-situ manner since ambiguities in data interpretation are generally much reduced. Involving synchro- tron radiation, simultaneous DSC and small- angle X-ray scattering measurements have been used since 1985, mainly in the field of polymers, e.g. to investigate melting and crystallization (6- 10). As another example, a combined quick XAFS (QEXAFS) and X-ray diffraction method has been developed for on-line, in-situ studies of catalysts (1 1-13). This enables obtaining both short-range and long-range order structural in- formation of catalytic materials under operating conditions. Here, the successful in-situ combination of XAFS and DSC will be demonstrated by study- ing two different solid-state elimination reac- tions of metal halides in halogenoacetates (3) [l]. It will be shown that, presently, a time resolution of ca. 10 s corresponding to a tem- perature resolution of about 0.3 K is easily achieved. The extent of reaction a (ranging from 0 to 1) is a common quantity to characterize solid state reactions. It can be individually calcu- lated from both DSC and XAFS measurements, thereby relating enthalpic and structural changes. The combination of XAFS and DSC is believed to allow a thorough quantitative insight into mecha- nism and kinetics of reacting sample systems