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favoured. However, the shape of the particle and its surface strain were studied under different gas conditions, including the operating conditions of an automotive catalytic converter, created by heating the particle to around 430 degrees Celsius and allowing carbon monoxide and oxygen molecules to pass over it. Under these reaction conditions, the rhodium inside the particle becomes mobile and migrates to the surface because it interacts more strongly with oxygen than the platinum. This is also predicted by theory.
As a result, the surface strain and the shape of the particle change. A facet-dependent rhodium enrichment takes place, whereby additional corners and edges are formed. The chemical composition of the surface, and the shape and size of the particles have a significant effect on their function and efficiency. It is still not completely understood
how these are connected and how to control the structure and composition of the nanoparticles. However, CXDI made it possible to detect changes of as little as 0.1 in a thousand in the strain, corresponding to a precision of about 0.0003 nanometres in this work.
This work shows, for the first time, that the details of the structural changes in such catalyst nanoparticles can be observed while in operation. This is a major step forward in the understanding of an entire class of catalysts, namely in all reactions that make use of alloy nanoparticles. This investigation is also an important step towards analysing industrial catalytic materials. Until now, model systems have had to be grown in the laboratory in order to conduct such investigations. Future work aims to study individual particles that are ten times smaller, in real catalysts and under reaction conditions.
PRINCIPAL PUBLICATION AND AUTHORS
Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction, Y.Y. Kim (a), T.F. Keller (a,b), T.J. Goncalves (c), M. Abuin (a), H. Runge (a), L. Gelisio (a), J. Carnis (a), V. Vonk (a), P.N. Plessow (c), I.A. Vartanyants (a,d), A. Stierle (a,b), Sci. Adv. 7, 40 (2021); https:/doi.org/10.1126/sciadv.abh0757 (a) Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany) (b) University of Hamburg, Physics Department, Hamburg (Germany) (c) Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Leopoldshafen (Germany) (d) National Research Nuclear University MEPhI, Moscow (Russia)
Use and reactor location-directed heterogeneity development in industrial catalyst bodies
Scanning X-ray diffraction highlights how residence time and location in an industrial reactor affect the structure and composition and, thus, the performance of a heterogeneous catalyst used, for example, in the production of biodegradable polymers.
Increasing environmental regulations, diminishing natural resources, and rising demands for catalytic conversion products encourage the continued development of heterogeneous catalysts of higher productivity, selectivity and longer lifetimes. To be able to meet this demand, and to optimise existing catalysts and develop new ones, an in-depth understanding is required of the catalytic conversion process on the molecular level. It is necessary to understand the structure of the catalyst on the nanometre level, for example, as well as the exposure of active sites in nanoporous and millimetre-sized catalyst bodies or pellets, and how these pellets behave within the industrial setting. The latter is a frequently neglected factor in this optimisation chain.
To iterate the importance of characterising heterogeneous catalysts at all length scales, a comparative study of industrial catalyst pellets as a function of residence time (at the year time scale) and location in an industrial fixed- bed reactor (reactor entrance vs. centre) was undertaken. Specifically, the study examined a catalyst used in the commercial oxidation of n-butane to maleic anhydride, a crucial intermediate in the production of biodegradable polymers, with a current demand of 2 million tonnes per year. The employed vanadium phosphorus oxide (VPO) catalysts were of the porous bulk-type and shaped into Raschig ring-type pellets (Figure 74a).
Using scanning X-ray diffraction at beamline ID13, the cross-section of three catalysts pellets intended for, or extracted from, an industrial fixed-bed reactor after four years of continuous operation were examined [1,2]. A pristine catalyst pellet and two used pellets were studied. One of the used pellets was extracted from the centre of the catalyst bed, the other from the reactor hotspot close to the feed entrance where highest activity is expected (Figure 74a).