Microfabrication is a catch-all term for manufacturing processes that involve structures on the micrometre scale. A classic example would be microchips (integrated circuits), the development of which miniaturised and revolutionised computing. A microreactor does the same thing for reaction chambers - allowing researchers to study reactions on a small scale. A paper recently published in Catalysis Science & Technology describes the development of a novel silicon-glass plug-flow microreactor suitable for operando X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) studies. The reactor provides the data required to develop catalyst structure/activity relationships with accurate kinetic expressions and promises to be a useful tool for both catalytic chemists and chemical engineers.
More than 80 percent of all manufactured products involve the use of catalysts at some point, and so catalysis underpins modern life¹. To design and develop new catalytic processes, it is necessary to study the relationship between catalyst structure, activity and product selectivity for a given reaction. This relationship can be explored using a variety of spectroscopic and microscopic techniques, and a combination of two or more methods can offer complementary information.
While in situ experiments allow researchers to study catalysts 'in action', an operando investigation provides additional data about the reaction kinetics, which is essential for improving the design of catalysts and scaling reactions up to a commercial scale.
There has, therefore, been an increase in demand for operando research. However, the cells currently used to house the reactions can be problematic. Issues include non-uniform flow, dead volumes and temperature and concentration gradients that can affect the results.
The project to develop a new microreactor for operando studies was funded by the UK Catalysis Hub at Harwell and led by Professor Asterios Gavriilidis from UCL. Microfabrication allows the development of microreactors with accurate and versatile microchannel geometries, in this case a glass–silicon–glass sandwich-like structure. The advantages offered by silicon microfabricated reactors include high thermal conductivity, chemical inertness, well-defined and adaptable geometry at the microscale. Silicon is transparent to X-rays and IR beams, making glass-bonded silicon microreactors ideal for XAS and DRIFTS studies.
Prof Gavriilidis, explains:
Our microreactor is a new tool for operando research. The motivation behind the design is to provide a reactor for information-rich experiments that produces the data needed for accurate kinetic rate expressions. With this reactor you can perform X-ray experiments, followed by IR experiments, to provide complementary data. It has excellent temperature control, only requires a tiny amount of catalyst, and is perfect for transient experiments, where experimental conditions are changing fast.
The research team tested the microreactor for operando XAS/MS experiments during the catalytic combustion of methane on the B18 beamline. XAS is an element-specific technique, providing information on the geometric, electronic and chemical state of an atom in the catalyst. IR spectroscopy is routinely used to investigate the vibrational state of the catalyst and the adsorbed molecules on the catalyst surface, providing useful information on the reaction and deactivation mechanisms. DRIFTS is suitable for studying powdered catalysts under reaction conditions. Together, XAS, DRIFTS and MS provide a holistic approach to studying the catalyst structure/activity relationships, as well as reaction/deactivation mechanisms. This study demonstrates the advantages of the novel microreactor over conventional cells for operando experiments, particularly those that combine XAS and DRIFTS studies.
Prof Gavriilidis, added:
The research team has expertise in catalysis, synchrotron techniques (XAS) and reactor design, and relied on assistance from the B18 beamline scientists and the technical staff in setting up the experiments. Close collaboration and useful discussions with Diamond were invaluable in achieving these results.
The microreactor can be used with other types of processes, beyond catalysis. The team now hope to partner with other researchers who will use it to explore new reactions.
¹ Catlow CR et al. Catalysis making the world a better place. Philos Trans A Math Phys Eng Sci 374(2061): 20150089 (2016).
Venezia B et al. Silicon microfabricated reactor for operando XAS/ DRIFTS studies of heterogeneous catalytic reactions. Catalysis Science & Technology (2020). DOI:10.1039/d0cy01608j.
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