Characterising an elusive catalyst

Highly active mineral made in bulk for the first time and studied at Diamond

An incredibly rare and highly active catalyst known as georgeite has recently been characterised at Diamond Light Source. This fascinating mineral, found naturally in only two locations in the world, has been synthesised with high purity in bulk and observed under process conditions to understand its behaviour. The results published in Nature demonstrate its potential role in revolutionising industrial catalysis.
Hydroxycarbonate minerals such as zincian malachite, aurichalcite and rosasite are widely used as catalyst precursors for methanol synthesis and low-temperature water-gas shift (LTS) reactions. Methanol is a valuable feedstock with annual production exceeding 65 million tons/year. The LTS reaction is very important for the chemicals industry as it yields high purity hydrogen, which is needed for a multitude of chemical reactions such as ammonia synthesis.
 
The favoured catalyst is derived from zincian malachite using a co-precipitation method and has been optimised extensively over the last forty years. While the preparation technique is highly successful it has its limitations, such as the production of concentrated nitrate waste streams and catalyst sodium poisoning.  To overcome these limitations, a new methodology was applied to produce low sodium content, nitrate free hydroxycarbonate catalysts. The technique, known as supercritical anti-solvent (SAS) precipitation, surprisingly produced an exceptionally rare mineral catalyst known as georgeite.
 
The new ability to produce bulk quantities of this novel catalyst with high purity meant that it could be studied extensively, and using X-ray Absorption Fine-edge Spectroscopy (XAFS) and X-ray Pair Distribution Function (PDF) at Diamond among a host of other techniques, it was shown that georgeite has the potential to improve the catalysis of LTS and methanol synthesis.
 
 
Georgeite from a copper mine in Snowdonia, Wales. One of only a few locations in the world where the mineral has been reported (courtesy of Amgueddfa Cymru- National Museum of Wales, Cardiff)
 
Synthesising georgeite
The elusive mineral, georgeite, has been largely neglected by the scientific community. Studying this mineral was near impossible as it is only found naturally in two locations in the world in minute quantities. Georgeite is produced fleetingly during the co-precipitation method used to prepare the widely-used catalyst, zincian malachite, but an innovative supercritical anti-solvent (SAS) method by scientists at Cardiff University managed to produce a stable version of this mineral for the first time. Together with scientists from the UK Catalysis Hub, University College London, University of Liverpool, Technical University of Denmark, Lehigh University (PA, USA), Johnson Matthey, and Diamond, they sought to determine its properties and potential applications in catalysis. The study was part-funded by Johnson Matthey, a speciality chemicals company that has a track history in catalysis research in collaboration with academia.
 
Dr Peter Wells, Associate Director of the UK Catalysis Hub and member of the study group, explained, “All the standard characterisation we performed in lab was unable to provide sufficient detail on this elusive mineral. Fortunately, the fantastic facilities at Diamond were able to help us learn more about the system”. Synchrotron X-ray PDF gives access to higher resolution data than possible in a lab, in a fraction of the time. Scientists used Diamond’s Extreme Conditions beamline (I15) for PDF due to its high energies and low backgrounds.
 
The important insights that can be learnt from studies like this have led to the development of a new dedicated X-ray PDF branchline at I15. According to Dr Philip Chater, Beamline Scientist on the XPDF branchline (I15-1), “this type of analysis will become even easier using the new XPDF branchline, where automatic PDF processing will deliver data in real-time."
 

Quartz capillary microreactor on B18 for in situ and in operando studies
 
Observing the structure with XAFS
The team wanted to watch the evolution of the catalyst under process conditions, so they carried out X-ray Absorption Fine-edge Spectroscopy (XAFS) at Diamond’s Core XAFS beamline (B18) during calcination and reduction. As Dr Wells continued, “We used XAFS on B18 because it is fantastic for observing these types of structures; it is element specific and it looks at the local coordination environment around the central scattering”.
 
An in situ micro-reactor, developed by the industrial liaison team at Diamond, was used to recreate industrial conditions during the activation of the catalyst. The quartz chamber allowed the sample to be heated while gas flowed over it, so that the georgeite could be observed during its evolution.
 

More reaction sites increase activity
XAFS showed that the georgeite was incredibly disordered initially, and the final structure at the end of its evolution following calcination and reduction was completely different to that of zincian malachite. Moreover, the catalyst derived from georgeite was composed of much smaller copper and zinc oxide particles than that of zincian malachite, with a greater degree of interaction between the two phases. This observation was supported by environmental transmission electron microscopy performed at the Technical University of Denmark.

Dr Wells summarised the main findings: “We saw that georgeite was more disordered and had smaller particles than zincian malachite, which we believe led to a higher number of reaction sites and therefore more copper and zinc interactions, making it a very active catalyst.” In fact, georgeite proved to be more active than zincian malachite for LTS and methanol synthesis, so this newly-synthesised mineral has a very promising future.
 
Dr Wells concluded, “The reason this is such an important piece of work is because this is a very important industrial reaction that makes an awful lot of material, and we were able to show improvements over the conventional catalyst.” He added, “The hydroxycarbonate minerals make up the lion’s share of catalysts for LTS and methanol synthesis and georgeite could easily become one of those widely-used catalysts.”
 
The team now intend to study georgeite in action during LTS and compare it directly with zincian malachite. They hope to combine XAFS with infrared light to observe how the nanoparticles change over the course of the reaction, and to monitor the LTS reaction on the surface of the mineral.
 
To find out more about using XAFS on B18, or to discuss potential applications, please contact Principal Beamline Scientist Dr Giannantonio Cibin: giannantonio.cibin@diamond.ac.uk.
 
To find out more about using PDF on I15-1, or to discuss potential applications, please contact Principal Beamline Scientist Dr Heribert Wilhelm: heribert.wilhelm@diamond.ac.uk.
 

Related publication:

Kondrat SA et al. Stable amorphous georgeite as a precursor to a high-activity catalyst. Nature 531(7592), 83–87 (2016). DOI: 10.1038/nature16935