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Discovering new Catalysts in the Cluster-Nanoparticle Transition Regime

Periodic Reporting for period 4 - CLUNATRA (Discovering new Catalysts in the Cluster-Nanoparticle Transition Regime)

Reporting period: 2022-03-01 to 2023-06-30

Clunatra is designed to finding new catalytic activities for particles of metal-oxides, -sulfides, -nitrides, or phosphides, in the transition regime where the size goes from clusters with countable number of atoms to nanoparticles. That is a region where the activity may depend critically of adding or subtracting a single atom. This region is uncharted mainly because it is difficult to manufacture well-defined particles in this regime. We have designed and build a cluster source together with an analysis apparatus that allows us to produce such mass-selected particles and subsequent test them for the catalytic activity. This allows for a search for new catalysts with high activity and selectivity which are of in general interest for the chemical industry, which constitute a pivotal role for our society. The specific goal for this project is aiming at delivering solutions for a future fossil free energy system by finding new processes and catalysts for conversion of sustainable energy in the form of electricity into chemical energy. In such a fossil free scenario, we would have to produce hydrogen by water splitting and subsequent use this for hydrogenation of CO2 and N2 into the chemicals we need, and the fuels for areas that cannot be electrified. In these processes new and more efficient catalysts are mandatory.
We have established the described apparatus where a new cluster source was bought and connected to an existing UHV-apparatus equipped with the most useful Surface Science methods such as XPS, ISS, STM, TPD. This already elaborate instrument has furthermore been equipped with a high-pressure cell so that the activity of the nanoparticles could be tested without being exposed to air which is mandatory for many catalysts. We have lately equipped the instrument with an additional sputter magnetron to be capable of preparing in situ different support materials so a study of the metal-support interaction can also be undertaken without exposure to air. We have also just installed a new electronic for the sputter magnetron, so it is possible to use High-impulse sputtering when generating the mass-selected particles improving intensity of cluster in the above-mentioned transition range. At the same time the theory and testing went on and we have investigated alternative interesting materials such as for the electrochemical ammonia production. Clunatra has resulted in 15 scientific publications of which several are in the absolute top international Journals such as Science (2), Nature Energy (2), Energy and Environmental Science (2), Joule (1), ACS Energy Letters (2), and Nature Communication (1). Beyond this another 5-6 papers are expected to be finalized in 2023 as PhDs are finalizing. We have shown that it is possible to have single Ir atoms adhered to a Ta2O3 entity and investigated it activity from 1,2 nm up to 5 nm. The reason for improving is that the scarcity and price of Iridium is prohibiting its implementation for large scale use for acidic hydrogen production by water splitting. It did not show any magic number behavior, but the Ir atoms were a factor of five more active per weight than the similar sized IrO2 nanoparticles for Oxygen Evolution Reaction (OER) in acid water splitting. Beyond being more active they were also more stable, and this shows that there are routes to enhance the use of the Iridium for acidic water splitting which is the only reasonably stable catalyst for OER in acid known today. This was published in Nature Energy 7 (2022). Before starting an investigation, we always must establish state-of-the-art of the field and since we were complaining about the experimental status and approach in the field, we were invited to write a perspective on the status and appropriate approach for measuring OER in water splitting also in Nature Energy 4 (2019) 430. The nature of the more active but less stable Ruthenium for OER was investigated to see if similar enhancements could be obtained. The fundamental electrochemical investigations were published in Energy and Environmental Science 15 (2022) 1977 and 15 (2022) 1988. The possibilities for more active Hydrogen Evolution Catalysts (HER) for acidic water splitting was also investigated. Here we compared all the inorganic catalysts to arch-type Platinum catalyst for HER and found that not only was Platinum the best, but it also proved to be transport limited, which questions the many papers published claiming that they can make catalysts better than Platinum. On a scientific basis, the Turn-Over-Frequency (TOF), we showed that Platinum is superior by roughly three orders of magnitude over all inorganic compounds. This was published in ACS Energy Letters 6 (2021) 1175.
The establishment of State-of-the-Art and describing benchmarking and correct method of measuring is always progressing the field of science. The papers on state-of-the-art of OER, HER, and CO2 are good examples of such. It cannot be iterated often enough that claiming doing catalysis requires measuring the products. This is a particular problem for OER where often the current is used as a proxy for oxygen evolution. We showed how people often are fooled if not doing so and set a new standard. Similarly, for HER where we clearly established what it would take to go beyond State-of-the-Art.
The fact that the use of Ir can be substantially improved by depositing on TaOx is very significant and certainly took us beyond state-of-the-Art. Whether this is really a breakthrough can first be judged if it is possible to scale it up.
Identifying a new mechanism for promoting spin active elements is taking us beyond State-of-the-art. It allowed us to identify new element combinations which has not been tested before. The effect was surprising and allowed us to explain some old data plus a number of new literature data which use an entirely different (and wrong) explanation.
A five-year perspective allowed us to develop a new type of high-pressure cell that is very sensitive and allows us to study both clusters and support materials activity without exposure to air (Rev. Sci. Instrum. 2023). Such measurements are extremely difficult. For example, have nobody tried to repeat the pioneering work by Somorjai in the seventies where he measured the activity of different Iron crystals for their ammonia activity. We have just done that with this new setup, and it seems that we may now be able to settle a 40 years old dispute concerning the nature of the active sites on Iron. The construction was not planned but grow out of the demand for making testing without exposure to air, which was not possible with the microreactor platform. Finally making a scalable cell and process for the electrochemical ammonia process is beyond any doubt a true breakthrough. This was, however, only part of Clunatra because there was a lack of samples for the PhD student, which was planned to measure electrochemical activity of the mass-selected nanoparticles. Sometimes science works in its own mysterious ways.
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