Jul 28, 2014
Hydrogen spills over large distances
Hydrogen spillover is an important phenomenon that occurs in many industrial catalytic processes, but it is still little understood. Now, a team of researchers at the University of California at Berkeley and the Lawrence Berkeley National Lab has shown that hydrogen atoms can travel long distances across a substrate and that it is possible for different parts of the same chemical reaction to occur on different places on a catalyst – even if these different points are separated by an inert material that does not absorb hydrogen. The result will be important for understanding how widely-used catalysts, such as cobalt promoted by platinum, work on the molecular scale. It might even help in the production of next-generation transport biofuels and chemicals as crude oil becomes more and more expensive.
Hydrogen spillover occurs when hydrogen atoms migrate from a metal surface that contains a lot of hydrogen to a metal oxide support that contains only a little. When it happens on a solid catalyst surface (where all the important chemistry is taking place), it can transport hydrogen between different points on the catalyst – even if this catalyst is made up of several components, explains team member Simon Beaumont, now at Durham University in the UK. Better understanding the phenomenon is important for shedding more light on how so-called heterogeneous catalysts work. These catalysts play host to hydrogenation or dehydrogenation reactions and are important in most modern chemical processes.
The researchers, led by Gabor Somorjai, studied platinum and cobalt nanoparticles as heterogeneous catalysts for the “Fischer Tropsch” process (which converts syngas into useful liquid transport fuels and chemicals). Such precious metals promote this important reaction, but how they do so is still not completely understood.
“We measured simple reaction rates that allowed to see whether a process was limited by hydrogen transport or reaction phenomena,” says Beaumont. “We used carefully synthesized, uniformly sized nanoparticles to make sure that the active metal components were identical. We put them next to each or some distance apart on a silica substrate and found that as the spacing between the particles increases, the process changes and begins to rely more on hydrogen transport.”
Travelling between different reaction sites
The result strongly implies that surface hydrogen travels between different reaction sites, he tells nanotechweb.org.
“We have used nanostructures to show that reactive intermediates, like hydrogen atoms, can move long distances on a heterogeneous catalyst’s surface,” he adds. “It appears to be possible for different parts of an overall chemical reaction to take place on different places on the catalyst – even when the material between them (in our case, silica) is quite inert and cannot absorb hydrogen atoms.”
For catalytic reactions involving multicomponent systems, this result offers a new way of understanding how different components work together to help a chemical reaction proceed, he says “Traditionally, we might have thought that platinum and cobalt together would form surface alloys with different electronic and geometric structures, but here we find that the two can be micron distances apart and still both contribute to making a reaction go faster.”
The type of catalyst studied here – cobalt with a small amount of platinum added – is typically used in the Fischer Tropsch reaction. At present, the syngas used in this reaction comes from reformed natural gas (including shale gas). However, one day it might come from bio-based feedstocks so the process could potentially be important for developing next-generation transport fuels and chemicals as crude oil becomes more and more scarce, says Beaumont.
The team describes its work in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org