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July 7, 2014
Catalyst Voodoo, Yielding to Spectroscopy?
Catalysts are absolutely vital to almost every field of chemistry. And catalysis, way too often, is voodoo or a close approximation thereof. A lot of progress has been made over the years, and in some systems we have a fairly good idea of what the important factors are. But even in the comparatively well-worked-out areas one finds surprises and hard-to-explain patterns of reactivity, and when it comes to optimizing turnover, stability, side reactions, and substrate scope, there's really no substitute for good old empirical experimentation most of the time.
The heterogeneous catalysts are especially sorcerous, because the reactions are usually taken place on a poorly characterized particle surface. Nanoscale effects (and even downright quantum mechanical effects) can be important, but these things are not at all easy to get a handle on. Think of the differences between a lump of, say, iron and small particles of the same. The surface area involved (and the surface/volume ratio) is extremely different, just for starters. And when you get down to very small particles (or bits of a rough surface), you find very different behaviors because these things are no longer a bulk material. Each atom becomes important, and can perhaps behave differently.
Now imagine dealing with a heterogeneous catalyst that's not a single pure substance, but is perhaps an alloy of two or more metals, or is some metal complex that itself is adsorbed onto the surface of another finely divided solid, or needs small amounts of some other additive to perform well, etc. It's no mystery why so much time and effort goes into finding good catalysts, because there's plenty of mystery built into them already.
Here's a new short review article in Angewandte Chemie on some of the current attempts to lift some of the veils. A paper earlier this year in Science illustrated a new way of characterizing surfaces with X-ray diffraction, and at short time scales (seconds) for such a technique. Another recent report in Nature Communications describes a new X-ray tomography system to try to characterize catalyst particles.
None of these are easy techniques, and at the moment they require substantial computing power, very close attention to sample preparation, and (in many cases) the brightest X-ray synchrotron sources you can round up. But they're providing information that no one has ever had before about (in these examples) palladium surfaces and nanoparticle characteristics, with more on the way.
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