Using Transmission Geometry PXRD to Observe Solid State Reactions in Real Time

Interview conducted by Jake WilkinsonFeb 23 2016

Dr. Thomas Hartmann and Dr. Sascha Correll, of STOE, are experts in the field of x-ray crystallography. AZoNano spoke to both of them about STOE's new piece of equipment; the INSITU HT2, how it can be used for transmission geometry experiments and why this is exciting news for solid state scientists.

What were the ideas behind the development of the STOE INSITU HT2?

We were often receiving requests from researchers who were interested in catalyst crystallography; these researchers were working with reactive gases flowing over a catalytic active material. A common example of these kinds of catalysts is the platinum catalysts used to reduce the emissions of automobile engines. As noble metals are very expensive, researchers are limited in the amount of catalyst they can use in their experiments. This limitation means researchers have always been looking for an in-situ reaction chamber to follow the catalytic process respective to the particle size of the samples.

The only commercially available product on the market until now was a chamber which uses reflection geometry. Due to the limitations of reflection geometry, a large amount of catalyst has to be used; which is both expensive and doesn’t accurately reflect the true conditions that the catalysis occurs in.

At STOE we produce powder and single-crystal X-ray diffractometers, and we are famous for our equipment that allows transmission mode experiments for powder XRD. We were already supplying capillary furnaces for powder X-ray diffraction (PXRD), though they were not capable for experiments with reactive gases, which was something our customers were longing for. To solve this problem we conducted tests at the Synchrotron in Hamburg where we took a standard furnace and optimized it for experiments involving reactive gases.

Everything went to plan with the result being a new piece of equipment that uses a small gas flow to simulate the reactive conditions of a catalytic active compound.

The INSITU HT2

What are the benefits of being able to measure solid state reactions as they occur?

The main benefit using the STOE INSITU HT2 is that it allows the investigation of crystallographic dynamic processes, such as observing phase transitions as they occur.

Each crystalline compound has its own individual x-ray diffraction pattern, meaning PXRD can be used as a fingerprint method in phase identification. Every temperature and/or gas-reaction related change of the observed compound is associated with a change in its crystallographic structure and therefore results in a different diffraction pattern.

For example, we have observed the annealing of a Carbon coated Platinum-Ruthenium catalyst under oxygen at room temperature, 200° and 250°C. Whilst the catalyst at room temperature showed only the x-ray pattern of the Pt/Ru alloy, additional reflections of the metal oxides appeared at 200°C and became stronger in their intensity at 250°C. At 300°C the pure oxygen flow combined with the catalytic activity of the Pt/Ru alloy resulted in the ignition of the Carbon coating and the loss of the sample in the capillary.

How do results from XRD experiments that use transmission, rather than reflection, differ?

For reflection mode firstly you need a large amount of sample which is sometimes not possible. In transition mode, however, only very small amounts are required.

Secondly, in reflection mode for measurements below approximately 10° 2θ the X-ray beam doesn’t fully enter and exit the sample. This means that the beam over-samples the sample area leading to false reflection intensities at low diffraction angles which have to be corrected later on. This is obviously not ideal, because these corrections introduce error into the measurements which, when you’re performing experiments on catalysts, makes it difficult to determine the exact proportion of gases that bond to the crystal structure.

However, in Transmission mode the measured data consists of true diffracted intensities – which do not have to be corrected over the whole pattern range (which is around 150° due to limitations on the apparatus). This is because the same volume of sample is exposed by the X-ray beam for every experiment which makes structure determination using the STOE INSITU HT2 much simpler.

A phase transition from α to β-quartz during a heating up process )and the reverse as the sample is cooled down), as observed using transmission geometry PXRD

How easy is the STOE INSITU HT2 to use?

It is as easy to use as any other heating chambers. You simply fill the capillary with your sample, fix it between two rock wool pads and use two fittings to connect the capillary to the gas supply.

I carried out some experiments with the STOE INSITU HT2 a few weeks ago. At first it took me half an hour to set up, now it’s only 5 minutes. Once you are used to the preparation procedure, it is very easy to handle and very fast to exchange.

We’ve made the entire piece of equipment, including the furnace chamber, as easy to handle as possible. You can quickly assemble or dissemble the STOE INSITU HT2 as all parts are interconnected with quick couplings.

Do STOE supply their own software for the INSITU HT2?

We provide our own STOE software WINXPOW to control the INSITU HT2. The software allows temperature controlled measurements to be taken; multiple measurements can be taken at set temperatures which can then be increased or decreased for further measurements.

A transition of coelsetin from an orthorhombic phase to a cubic phase at 1,152 °C as observed using transmission geometry PXRD

The software is the most flexible currently on the market; the temperature can go up and down in loops if you want it to!

As the temperature can vary so widely we supply two different types of capillary designed to work optimally at different temperatures. For temperatures below 1,000 °C we supply a quartz capillary tube and for temperatures between 1,000 and 1,600 °C we supply a sapphire capillary tube.

In your opinion, in what scientific fields and disciplines does the STOE INSITU HT2 assist researchers the most?

Of course the whole area of inorganic chemistry benefits from equipment like this and especially researchers who are concerned with observing gas reactions with solid state compounds. The high versatility of the STOE INSITU HT2 means it can be used in a wide range of experiments.

Catalytic chemistry is one of the main sectors. We have built the STOE INSITU HT2 to observe the changes in catalyst structure that occur during catalytic cycles.

It’s also a great tool for R&D researchers in pharmaceutical companies who need their chemical transformations to be extremely controlled and with as little impurities present as possible.

Also the whole field of kinetic chemistry benefits from this equipment, as long as the sample material is crystalline, of course.

How do STOE ensure that the INSITU HT2 makes extremely accurate measurements?

To get the best counting statistics we have implemented a panning mechanism which rotates the capillary with the sample up and down by 90 ° while performing the measurements. This means there is no variation on the recorded data due to different sizes, shapes and orientations of grains in the crystalline material, thus preventing false signals.

Additionally, the inner core of the STOE INSITU HT2 (the heating element) consists of a completely new developed graphite rod which has a highly innovative coiled form ensuring a homogenous temperature distribution in the near environment of the capillary sample. The Accuracy of the temperature control of the sample is guaranteed – certainly in combination with a thermal couple optimized for the respective temperature range to be investigated!

The INSITU HT2's Transmission Chamber

Finally please could you tell our readers about STOE and where they can find out more about you?

As a company we are solely focused on X-ray diffraction. STOE was originally started in 1887 as a spin-off from the University of Heidelberg to produce optical goniometers. Ever since then STOE have continued to champion a hands-on attitude to customer service and has been a pioneer in crystallography.

Almost all of our competitors are large conglomerates who have extensive product portfolios but offer little flexibility for their customers. Whereas STOE only produce single crystal and power x-ray diffractometers – and we do it with 100% focus. From R&D, production to service, we keep all steps of the value chain in house, allowing us to achieve a unique product and service quality. At the same time we are open to integrate any available components, e.g. sources and detectors, which allow us to be at the leading edge of technology.

If you would like to learn more about STOE INSITU HT2 or any other STOE products, please do not hesitate to contact us at [email protected]. We are happy to discuss your needs and projects even at an early stage.

About Dr. Thomas Hartmann

Dr. Thomas Hartmann studied Chemistry at Darmstadt University of Technology to become Degreed Engineer in 1997, and finished his-PhD thesis on Rare Earth Rhenium oxides at the Materials Science Department in 2003.

Over the course of his research into the world of inorganic chemistry he encountered STOE’s XRD equipment.In 2001 he began working in the sales and powder teams for STOE, becoming the leader of the powder group eventually in 2006.

About Dr. Sascha Correll

Dr. Sascha Correll studied Chemistry in his hometown of Munich at the Ludwig Maximilians University of Munich. Following graduation with a diploma in Chemistry he went on to study towards a PhD on Zeolite-like structures composed of phosphorus, nitrogen and oxygen which he completed in 2005.

Like Thomas, Sascha also became familiar with STOE’s equipment over the course of his PhD leading him to become responsible for the powder application lab of the company .