Microscopy

Watching catalysts at work

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Solid catalysts speed up many industrial chemical reactions and steer them towards making desired products. A microscopy technique could reveal the changes in composition that catalysts undergo as they perform.

Solid catalysts are used in the production of almost every chemical compound — from plastics and fuels to pharmaceuticals — and for removing environmental pollutants. They typically consist of a porous support onto which are dispersed nanoscale particles of the active catalyst (usually a metal, metal oxide or metal sulphide). Other compounds might also be added to the mix, such as promoters to modify the reactivity of the catalysts, or structural stabilizers1. The size, shape and connectivity of these components affect catalytic activity, stability and selectivity (the ability of the catalyst to make a specific product). Nanoscale imaging of catalysts is therefore required to understand how each of these factors changes during a chemical reaction. Reporting on page 222 of this issue, de Smit et al.2 describe how a highly focused beam of X-rays can be used to acquire chemical maps and images of working heterogeneous catalysts at nanometre spatial resolution.

Many methods have been used to try to achieve high-resolution imaging of catalysts, which is a long-standing goal. Transmission electron microscopy (TEM) enables the structure of catalyst components to be observed with a spatial resolution of 0.1 nanometres, and can yield elemental maps of a catalyst with a resolution of 1 nm (ref. 3). Such information is usually acquired under vacuum conditions, although reactors have been developed that can analyse reactions in situ at pressures of up to 10−2 bar (about one-hundredth of atmospheric pressure) and at temperatures of up to 600 kelvin. But the acquisition of TEM images at pressures of 1 bar and above — conditions more typical of those used in industry for catalysed reactions — remains extremely challenging4. Other techniques for imaging the structure and chemical composition of catalysts, such as a combination of scanning transmission electron microscopy with electron energy-loss spectroscopy, also offer a spatial resolution of about 1 nm, although here too, images are usually taken in a vacuum5.

Another method has recently emerged as a contender in this field. Scanning transmission X-ray microscopy (STXM) offers exciting possibilities for imaging catalysts at nanometre resolution under a broad range of reaction conditions6. In this technique, a beam of monochromatic, low-energy X-rays (soft X-rays) is focused to a spot-size of 10–20 nm, using a zone plate — a device that focuses light using diffraction. The sample under investigation is then scanned by the X-ray beam (in fact, the beam is held stationary and the sample is moved relative to it), and the absorption of the beam by the sample is measured. This is repeated for X-rays of different energies, so that absorption can be plotted as a function of X-ray energy.

Because the whole sample is scanned, the relationship between absorption and beam energy can be mapped out across the sample. When such maps are produced using X-rays at the absorption edge of a particular element — that is, at the particular wavelength at which the element absorbs maximum energy from the X-rays — an image of the spatial distribution of that element is obtained. Alternatively, a plot of absorption against X-ray energy at a given spot provides an X-ray absorption near-edge spectrum; this can be compared with the absorption spectra of standard compounds to deduce the local composition of the material under study. The observed absorption spectrum can thus be thought of as a composite of the individual spectra for components of the catalyst, with the contribution of each component weighted according to its concentration. This means that the spectral map can be used to determine the distribution of pure components throughout the sample.

The STXM technique is ideal for studying heterogeneous catalysts, but has some practical problems associated with it. Soft X-rays are readily absorbed by matter. This means that, for STXM to work, the catalyst particles must be very small, and the distance travelled by the X-rays in the reactor cell — the X-ray gas path length — must be short (less than 100 micrometres). The first STXM study7 of a catalyst was restricted to experiments using diluted gases at temperatures up to only 533 K. Nevertheless, the technique proved its worth by characterizing the reduction and oxidation of tiny particles of copper oxide (CuO) dispersed on silicon dioxide (silica, a commonly used support for catalysts). The report of de Smit et al.2 describes the design, construction and operation of a much improved STXM cell, which has a gas path length of only 50 micrometres, and which can operate at 1 bar and at temperatures of up to 773 K.

The authors used their reactor cell to examine changes in a catalyst for the Fischer–Tropsch synthesis — a reaction in which 'synthesis gas' (a mixture of carbon monoxide and hydrogen) is converted into hydrocarbons. The Fischer–Tropsch synthesis has been known since the 1920s, but is currently attracting renewed interest as a means of producing transportation fuels from coal, natural gas or biomass. The particular catalyst studied by de Smit et al. consists of small iron oxide particles dispersed on silica. However, these are expected to be converted into other compounds after the catalyst has been reduced with hydrogen, and after exposure to synthesis gas.

The authors simulated the reaction conditions of the Fischer–Tropsch synthesis in their cell, and studied the catalyst using X-rays at the absorption edges of carbon, oxygen and iron. In this way, they showed that the iron oxide in the freshly prepared catalyst exists in a single form (known as α-Fe2O3), but that, after exposure to hydrogen under the reaction conditions, the particles are reduced to yield metallic iron and a different iron oxide (Fe3O4); the authors also observe some formation of an iron silicate (Fe2SiO4). After reaction in synthesis gas, the Fe3O4 is further converted to iron and Fe2SiO4, and the iron in turn reacts to form a compound with carbon (iron carbide). The authors also see evidence for the build-up of hydrocarbon compounds. Observing how the composition of a catalyst alters with changes in reaction conditions and reaction time can reveal insight into the factors controlling catalyst activity and stability.

The current findings2 demonstrate the potential of STXM for in situ chemical imaging of catalysts at the nanometre scale. The spatial resolution of the technique (15 nm) is impressive, but is still not high enough to give a truly atomic-scale view of a catalyst's structure. Improvements in X-ray optics and imaging methods, however, should allow higher spatial resolution, opening the way to a deeper understanding of the structure and composition of multi-component catalysts, and the changes they undergo during a reaction.

References

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    Bell, A. T. Science 299, 1688–1691 (2003).

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    de Smit, E. et al. Nature 456, 222–225 (2008).

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    Gai, P. L. Topics Catal. 8(1–2), 97–113 (1999).

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    Creemer, J. F. et al. Ultramicroscopy 108, 993–998 (2008).

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    Hitchcock, A. P. et al. Micron 39, 311–319 (2008).

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    Kilcoyne, A. L. D. et al. J. Synchrotron Radiat. 10, 125–136 (2003).

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    Drake, I. J. et al. Rev. Sci. Instrum. 75, 3242–4247 (2004).

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Bell, A. Watching catalysts at work. Nature 456, 185–186 (2008) doi:10.1038/456185a

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