Mano Mavrikakis, professor of chemical and biological engineering at UW–Madison. Photo: University of Wisconsin–Madison.In an advance they consider a breakthrough in computational chemistry research, chemical engineers at the University of Wisconsin (UW)–Madison have developed a model of how catalytic reactions work at the atomic scale. This understanding could allow engineers and chemists to develop more efficient catalysts and tune industrial processes – potentially with enormous energy savings, given that 90% of the products we encounter in our lives are produced, at least partially, via catalysis.
Catalyst materials accelerate chemical reactions without undergoing changes themselves. They are critical for refining petroleum products and for manufacturing pharmaceuticals, plastics, food additives, fertilizers, green fuels, industrial chemicals and much more.
Scientists and engineers have spent decades fine-tuning catalytic reactions. But because it’s currently impossible to directly observe those reactions at the extreme temperatures and pressures often employed in industrial-scale catalysis, researchers haven’t been able to determine exactly what is taking place at the nano and atomic scales. This new research helps unravel that mystery with potentially major ramifications for industry.
In fact, just three catalytic reactions – steam-methane reforming to produce hydrogen, ammonia synthesis to produce fertilizer and methanol synthesis – use close to 10% of the world’s energy.
“If you decrease the temperatures at which you have to run these reactions by only a few degrees, there will be an enormous decrease in the energy demand that we face as humanity today,” says Manos Mavrikakis, a professor of chemical and biological engineering at UW–Madison who led the research. “By decreasing the energy needs to run all these processes, you are also decreasing their environmental footprint.”
Together with postdoctoral researchers Lang Xu and Konstantinos Papanikolaou and graduate student Lisa Je, Mavrikakis reports this advance in a paper in Science.
In their research, the UW–Madison engineers develop and use powerful modeling techniques to simulate catalytic reactions at the atomic scale. For this study, they looked at reactions involving transition metal catalysts in nanoparticle form, which contain elements like platinum, palladium, rhodium, copper, nickel and others important to industry and green energy.
According to the current rigid-surface model of catalysis, the tightly packed atoms of transition metal catalysts provide a 2D surface where the chemical reactants can adhere and then participate in reactions. When enough pressure and heat or electricity is applied, the bonds between the atoms in the chemical reactants break, allowing their fragments to recombine into new chemical products.
“The prevailing assumption is that these metal atoms are strongly bonded to each other and simply provide ‘landing spots’ for reactants,” says Mavrikakis. “What everybody has assumed is that metal-metal bonds remain intact during the reactions they catalyze. So here, for the first time, we asked the question, ‘Could the energy to break bonds in reactants be of similar amounts to the energy needed to disrupt bonds within the catalyst?’”
According to Mavrikakis’s modeling, the answer is yes. The energy required for many catalytic processes to take place is also sufficient to break bonds in the catalyst, thereby allowing single metal atoms (known as adatoms) to pop loose and start traveling over the surface of the catalyst. These adatoms combine into clusters, which serve as sites on the catalyst where chemical reactions can take place much easier than the original rigid surface of the catalyst.
Using a set of special calculations, the team looked at industrially important interactions of eight transition metal catalysts and 18 reactants. They identified energy levels and temperatures likely to form such small metal clusters, as well as the number of atoms in each cluster, which can also dramatically affect reaction rates.
Their experimental collaborators at the University of California, Berkeley, used atomically resolved scanning tunneling microscopy to look at carbon monoxide adsorption on nickel (111), a stable, crystalline form of nickel that is useful in catalysis. These experiments confirmed the model by showing that various defects in the structure of the catalyst can also influence how single metal atoms pop loose, as well as how reaction sites form.
According to Mavrikakis, this new framework is challenging the foundation of researchers’ understanding of catalysis and how it takes place. It could apply to non-metal catalysts as well, which he will investigate in future work. It is also relevant to understanding other important phenomena, including corrosion and tribology, or the interaction of surfaces in motion.
“We’re revisiting some very well-established assumptions in understanding how catalysts work and, more generally, how molecules interact with solids,” Mavrikakis says.
This story is adapted from material from the University of Wisconsin–Madison, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.