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X-rays reveal single catalyst nanoparticle at work

X-ray diffraction imaging at ID01 has been used to observe a single catalyst nanoparticle at work, revealing for the first time how the chemical composition of the surface of an individual nanoparticle changes under reaction conditions. This study marks an important step towards a better understanding of industrial catalytic materials.

Catalysts are materials that promote chemical reactions without being consumed themselves. Today, catalysts are used in numerous industrial processes, from fertiliser production to manufacturing plastics. Because of this, catalysts are of huge economic importance. A well-known example is the catalytic converter installed in the exhaust systems of cars. These contain precious metals such as platinum (Pt), rhodium (Rh) and palladium (Pd), which allow highly toxic carbon monoxide (CO) to be converted into carbon dioxide (CO2), and reduce the amount of harmful nitrogen oxides (NOx). Despite their widespread use and great importance, many questions remain regarding how various catalysts work. It is difficult to study real catalysts

while in operation as the catalysts are typically used in the form of tiny nanoparticles in order to make the active surface as large as possible, and the changes that affect their activity occur on their surface.

A new technique for labelling individual nanoparticles and thereby identifying them in a sample has been developed. Nanoparticles of a PtRh alloy were grown on a SrTiO3 substrate, and one specific particle, Pt60Rh40, which was around 100 nanometres in diameter, similar to the particles used in a car s catalytic converter, was selected for characterisation. Coherent X-ray diffraction imaging (CXDI) was carried out at beamline ID01, creating a detailed image of the nanoparticle to measure the mechanical strain within its surface (Figure 73). Since the surface strain is related to the surface composition, in particular the ratio of platinum to rhodium atoms, the strain was computed as a function of surface composition. By comparing the observed and computed facet-dependent strain, conclusions could be drawn concerning the chemical composition at the particle surface.

When the nanoparticle is grown, its surface consists mainly of platinum atoms, as this configuration is energetically

Fig. 73: Nanoparticle shape and surface strain for different gas conditions. a) Top and side views of the reconstructed nanoparticle and (b) strain field ezz at the nanoparticle surface for gas conditions (I to IV). The position of the surface

is defined as a cut at 55% of the reconstructed crystalline electron density from its maximum value. c) Inlet gas composition and mass spectrometer signal during the experiment: CO (blue), O2 (black), and CO2 (red).