In work funded by the CARENA project, scientists from the Industrial Liaison Team at Diamond have created a capillary-based sample environment; a generic plug-flow reactor used on powder samples, which provides ideal conditions for mimicking catalytic reactions. The sample environment has been successfully commissioned and is now extensively used by users at Diamond Light Source. The dedicated in situ cell covers wide range of catalytic processes and the generic design of the sample environment is not only suitable for time-resolved, in situ experiments using Quick EXAFS on beamline B18, but the modular design of the gas system and integration within the EPICS make it compatible with other sample environments across other beamlines at Diamond. The work has recently been published in conference proceedings here.
X-ray Absorption Spectroscopy (XAS) is a synchrotron-based technique which has been applied extensively in the field of heterogeneous catalysis. A particular strength of this measurement is that the penetration depth of X-rays makes possible studies of materials under the conditions of their use, including high temperatures, pressure and reactive atmospheres. Moreover, XAS can be applied with other complementary techniques such as X-ray Diffraction (XRD), UV-visible and Raman spectroscopy, XRD with Raman spectroscopy, Raman/UV-visible combined with Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS). These technique partnerships open up a wide range of research possibilities; for example, the structure of a catalysts can be determined simultaneously with the measurement of the catalytic performance using mass spectrometry.
The possibility of performing time-resolved, in situ experiments and combining them with other techniques has led to the design of different in situ reaction cells that can mimic conditions close to real industrial environments. In this particular case, the approach was to deliver a generic capillary-based reaction cell, with the temperature and gas distribution capabilities built within the Experimental Physics and Industrial Control system (EPICS) interface commonly used at synchrotron facilities, which enables the user to remotely follow in situ gas-solid chemical reactions.
The gas flow sample environment created allows XAS measurements to be performed in both transmission and fluorescence modes. This is achieved by the use of packed-bed quartz capillary-tube reactor supported on a stainless steel frame (Fig. 1).
Heating the sample
The capillary is heated by a hot air blower with a controlled air flow and the temperature measured by a thermocouple at the heating element in order to control the temperature of the heating ramp. The biggest advantage of using a hot air blower instead of other heating sources is that it provides more space in the vicinity of the sample, giving access to additional complementary probes (XRD, Raman, IR). A ceramic block around the sample position allows for the achievement of isothermal conditions. This set up allows for heating of the reaction cell up to 1123 K.
The gas system
The gas system consists of Alicat mass flow controllers, two VICI microelectric-actuated 8-ports dead-end valves, and one VICI air-actuated 4-ports 2-position valve, which drives the final gas switch before the gas feedstock is injected to the reaction cell. The main reason for introducing an air-actuated valve as a driving gas switch to the reaction is that the response time of the switch for this valve is much quicker than the micro-electric actuated valves (ca. 100 ms). This feature is not so crucial when running Quick EXAFS experiments (time resolution of seconds). However, this sample environment has other applications for time-resolved studies on several beamlines where millisecond data collections are required.
All the gas and heating system devices are integrated with EPICS and all variables can be saved in the experimental file. The gas flow capillary-based cell was successfully commissioned on beamline B18 (core-EXAFS) at Diamond. It has since been widely used on a number of sessions on this beamline. Moreover, the gas distribution system has been effectively applied on various experiments at other beamlines at Diamond including I11, I12, B16 and B22.
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A.B. Kroner, K.M.H. Mohammed, M. Gilbert, G. Duller, L. Cahill, P. Leicester, R. Woolliscroft, 030014 DOI: 10.1063/1.4952837
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