In situ X-Ray Diffraction Analyses of Catalytic Reactions and Moderate-Pressure Geological Processes

摘要

The construction of a dedicated powder X-ray diffraction apparatus enables the measurements under controlled conditions applicable to a broad spectrum of analyses from catalysis to geology. The main parts of the instrument are the position sensitive detector (Inel CPS120), the tubing system with the pressure and mixing controls for gases and vapors, and the heating cell for the samples. The samples are mounted in thin-walled quartz-glass capillaries opened at both ends and connected to the tubing system for gases and vapors. The heat cell supplies around the capillary a homogeneous volume of hot nitrogen from a heated blower and is controlled by a thermocouple. The characteristics of the instrument are a large free space for the various attachments, a possibility of transfer between a sealed-tube and synchrotron radiation source, and a very short measuring time even on a standard sealed-tube generator. This enables measurements on a broad range of temperatures, pressures, gas and vapor mixtures and flows, application in routine as well as specialized studies, performing dynamic studies of reactions or phase transformations, and combinations with other simultaneous measurements (e.g. EXAFS, GC). The presently achieved conditions for in situ measurements are: streaming or stationary mixtures of gases and vapors, with mixing accuracy ca. 1% and flows up to 5 1/g/min; temperatures from room up to ca. 950K; pressures from 1 bar to ca. 100 bars. The theoretical limits for temperature/pressure with smaller adaptations would be 1200K/200 bar. Numerous studies of calcinations under oxidative or reducing conditions, of phase compositions and structure of solid catalysts during catalytic reactions, of stabilities of phases under varying atmospheres, temperatures, and pressures, and of crystallization/recrystallization dynamics have been made. The technique enabled us to study the solid state reaction paths, the nature and structure of metastable crystalline phases, the mass transport and the textural changes in reacting solids.